Optimised preparation of 1,5-dialkylsemibullvalenes

H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering 1159

Optimised Preparation of 1,5-Dialkylsernibullvalenes* Helmut Quast*a, Thomas Dietz", Eva-Maria Petersb, Karl Petersb, and Hans Georg von Schneringb

Institut fur Organische Chemie der Universitat Wurzburg", Am Hubland, D-97074 Wurzburg, Germany Max-Planck-Institut fur Festkorperforschungb, HeisenbergstraDe 1, D-70506 Stuttgart, Germany

Received January 27, 1995

Key Words: Semibullvalenes / Tricyclo[3.3.0.02~8]octa-3,7-dienes / Barbaralane I [3.3.3]Propellanes / [4.3.3]Propellanes I Bicyclo[3.3.0]octadienes, dibromo-, configuration of I Allylic bromination I Reductive cyclisation

Askani's synthesis of the 1,5-dialkylsemibullvalenes l a and c was optimised on a gram scale. The as yet unknown tri- methylenesemibullvalene lb was obtained according to the same protocol which also greatly improved a recent synthesis of barbaralane (2). Allylic brominations of the dienes 5 + 6 with N-bromosuccinimide yielded complex mixtures of 7- 10. Depending on the substitution pattern at the apical carbon atoms C-1 and C-5 of the bicyclo[3.3.0]octadiene system, the intermediate allylic radicals are attacked preferentially either

Devising a path to a synthetic target is the first, an ef- ficient and convenient route the ultimate goal in synthesis. Most instructive examples of such developments, taken from dodecahedrane chemistry, have been pointed out re- cently by PrinzbachL21. A similar situation exists with re- spect to many substituted semibullvalenes. Few years after the discovery of the parent semibullvalene by Zimmerman et al.r3I, a breakthrough in the synthesis of substituted semi- bullvalenes came from the work of A ~ k a n i [ ~ , ~ ] , who con- verted 1,5-substituted bicyclo[3.3.0]octane-3,7-diones, con- veniently available by the Weiss reaction@], into 1,5-, 1,3,5-, and 1,3,5,7-substituted semibullvalenes in a sequence of four straightforward steps. Subsequently, Paquette et al. adopted this sequence for the synthesis of further alkylsemi- bullvalenes which were used as precursors of alkylcyclooc- tatetraenesL71. Though experimental details have been dis- closed in full papers it is still by no means a trivial task to obtain substantial, not just tiny amounts of alkylsemi- bullvalenes because only moderate yields were achieved by elaborate isolation and purification procedures involving column chromatography with exclusion of oxygen followed by preparative VPC[5,7a]. This may be one of the reasons why the chemistry of alkylsemibullvalenes has been ex- plored so far only in part, e.g. addition reaction^[^,^,^] and thermal rearrangements to cyclo~ctatetraenes[~]. In order to provide a broader basis for the study of alkylsemibullval- enes, we have now reinvestigated each step of Askani's syn- thesis with the intent of engineering a protocol for the con- venient preparation of gram quantities of the known semi- bullvalenes l a and c. Furthermore, this protocol which is detailed here not only afforded the hitherto unknown 1,5-

from the exo (R = Me) or from the endo face [R, R = (CH2j3, (CH&]. Reductive cyclisations of the dibromides 9 and exo,exo-l5 were carried out with a sonicated suspension of an activated zinc-copper couple in tetraglyme. Sweeping of the volatile products from the reaction mixtures with nitrogen at elevated temperatures allowed the convenient isolation of the semibullvalenes 1 and barbaralane (2) in reasonable pu- rities (92-97 %) and yields (57-82%).

trimethylenesemibullvalene (lb) but also improved the syn- thesis of the parent barbaralane (2) reported recently[lO].

1 2

Substituents are denoted as follows:

a: R = Me, b: R-R = -(CHJ-, c: R-R = -(CH&

Bicyclo[3.3.0]octane-3,7-diones 3 are the starting mate- rials of Askani's semibullvalene synthesis and conveniently available on a large scale by the Weiss reaction of 1,2-dike- tones with dimethyl 3-0xoglutarate[~]. Reductions of 3 with NaBH4L1 - 151, LiA1H4[5,7a*13], or by the Meerwein-Ponndorf procedure" l] afford high yields of the 3,7-diols as mixtures of the diastereomers exo,exo-4, exo,endo-4, and endo,endo- 4 in various ratios.

The deceptively simple dehydrations of the secondary al- cohols 4 affording mixtures of the alkenes 5 and 6 have met with only moderate success, however, despite numerous methods and reagents tried[5*7a,' Therefore, we recently devised a novel one-step dehydration protocol for second- ary alcohols which greatly improves the situation. This pro- cedure involves heating of solutions in high-boiling, water- miscible solvents of the secondary alcohols, triphenyl phos- phate, and quinoline to temperatures well above 200 "C and

Liebigs Ann. 1995,1159- 1168 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1995 0947-3440/95/0707- I159 $10.00+.25/0

1160 H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering

allowing the pure alkenes to distill from the reaction mix- tures. Thus, mixtures of the alkenes 5 and 6 are now avail- able on a molar scale in yields close to

3 4

5 6

Allylic Bromination of Bicyclo[3.3.0]octadienes Allylic bromination with N-bromosuccinimide of mix-

tures of the alkenes 5a + 6a and 5c + 6cL5] and similar bicyclo[3.3.0]octadienes with larger carbocyclic rings in- corporating the apical carbon atoms[7b] have not been stud- ied in detail so far. Instead, the crude mixtures of bromides obtained were immediately used for the next step, i.e. the reductive cyclisation to semibullvalenes. In particular, the actual amounts of dibromides 9 present were not estab- lished let alone their configurations which are expected to influence the ease of conversion to semibullvalenes. Usually, solutions of the alkenes in tetrachloromethane were heated in the presence of two equivalents of N-bromosuccinimide, and the radical chain reactions were initiated by azodiiso- b~tyronitrile[',~]. After allylic bromination of the mixture of 5a + 6a, a single dibromide possessing C, symmetry crys- tallised from the mixture of bromides in 27% yield[lsl. It was identical with the major product of the addition of bro- mine to 1,5-dimethylsernibulIvalene (1 a) at low temperature. The structure exo,exo3a was assigned to this product on the basis of the low-field proton chemical shift of its methyl

Because large amounts of tetrachloromethane present en- vironmental hazards, we carried out the bromination reac- tions in dichloromethane as solvent. Moreover, this solvent is recommended as being superior to tetrachloromethane in many cases because of higher selectivities achieved[191. Monitoring the course of the allylic brominations by proton spectra is difficult because the multitude of brominated al- kenes gives rise to complex spectra. Therefore, we resorted to GC on packed columns whose diagrams show nicely sep- arated clusters of signals for the monobromo (7 + S), di- bromo (9), and higher brominated products (10) which al- low a convenient estimate of the progress of the reactions. Thus, it became immediately obvious that the desired dibro- mides 9 are inevitably accompanied by variable amounts of monobromo (7 + 8) and tribromoalkenes 10. Only the lat- ter can be partially removed from the mixtures by high- vacuum sublimation.

These byproducts of 9 are expected to be reduced by the zinc-copper couple in the next step to afford the alkenes 5 + 6 and bromo-l,5-dialkylsemibullvalenes 19 (19')[9,'81, respectively. Separation of the latter from the desired semi- bullvalenes 1 seemed less difficult than removal of the for- mer. In order to facilitate purification of 1, we therefore allowed the allylic brominations to proceed until the frac- tion of monobromides 7 + 8 was as low as a few percent even though considerable amounts of useless tribromides 10 had already arisen at this point. Purification and partial separation of the mixtures of ally1 bromides on a large scale were conveniently carried out by sublimation of the crude products at Torr. Thus, colourless, low-melting, semi- solid materials were obtained in high yields (Table 1). Recrystallisation from pentane at low temperature of the sublimed mixture of bromination products of 5a + 6a yielded colourless crystals, m.p. 1 13 - 1 15 "C, in which the fraction of exo,exo-9a had increased to almost 80%, while more exo,endo3a was found in the material obtained from the mother liquor after evaporation of the solvent and sub- limation of the residue.

5 6 7 8

l a 9 10

Capillary GC of the sublimed mixtures disclosed the minimum numbers of isomers present (Table 1). The mono- bromides formed from the mixture of 5a + 6a give rise to only two peaks which were assigned to the ex0 (exo-7a + exo-8a) and endo compounds (endo-7a + endo3a) because structural isomers that differ only in the positions of the double bonds, e.g. 5a and 6a, possess very close retention times while diastereomers in the 1,5-dimethyl series, such as exo,exo3a, exo,endo-9a, and endo,endo-9a, are readily distinguished. Repeated recrystallisation from pentane at low temperature of the mixture of the three diastereomeric dibromodienes 9a furnished the predominant diastereomer in pure form. It was identified by m.p., GC, proton and carbon-13 NMR spectra as the known crystalline dibromo- diene exo,ex0-9a[~~~~I. Consequently, the other dibromides were assigned the configurations exo,endo-9a and endo, endo-9a on the basis of symmetry (C, and C,, respectively) deduced from the carbon- 13 spectra (Table 4).

The tribromodienes, which presented a substantial frac- tion of the mixture obtained from 5a + 6a, give rise also to only two peaks in the capillary GC trace. That only two

Liebigs Ann. 1995, 1159-1168

1,5-Dialkylsemibullvalenes 1161

isomers are indeed present was confirmed by the obser- vation of two pairs of methyl signals in the proton and car- bon-13 NMR spectra, the relative intensities of which correspond to those expected on the basis of the GC dia- gram. Elucidation of the structures was not attempted. Be- cause both tribromides apparently arise from 9a, they were tentatively assigned the gross structure 10a. A similar tri- bromide was obtained by bromination of 1,5-dimethylbi- cyclo[3.3.0]octadiene-3,7-dicarbonitriles with N-bromosuc- cinimide[1s~201.

Table 1. Reaction conditions, products, and yields in allyiic bromi- nations of mixtures of the dienes 5 and 6 with N-bromosuccinimide in refluxing dichloromethane solutions. The ratios of mono- (7, 8) and tribromides 10, listed in the order of increasing retention times (in square brackets), were calculated from peak areas of traces in high-resolution GC, those of the diastereomeric dibromides 9 from

integrations of proton signals

Cpd. NBS T i Products[*~ Ykld [mol] [min] ( 7 + 8 ) : 9 : 10 [%I

5a+6a 2.2bl 70 11 74 15 (55 : 45)

2.4W 70 3 78 19

2.4 70 1 67 32

2 80 18 84[dl [33 : 671 [n,n-/x,n-/x~-9a [41 : 591

= 8 : 35 : 571

5b+6b 2.4 45 5 76 19 93cc1 (1 : 1) [46 : 541 [n,n- : x,n-9b

= 59 : 411

5c+& 2.2 20 3 79 18

2.4 33 <1 70 30 (1 : 1)

3 86 11 80[dl [71 : 291 [n,n- : x,n-k

= 55 : 451

[a] Ratios of peak areas of traces in GC recorded from crude pro- ducts. - p] More solvent was used (3.5 1 instead of 3.0 1). - [‘I Azodiisobutyronitrile was added. - Ld] After high-vacuum subli- mation of the combined crude products of two or three experi- ments. - Crude product.

Bromination of the mixture of [3.3.3]propelladienes 5b + 6b under the optimised conditions furnished a mixture of products which, surprisingly, did not sublime at lop5 Torr at temperatures below 100°C. The possibility of decompo- sition deterred us from using higher temperatures. GC showed the presence of at least two monobromo and two dibromo[3.3.3]propelladienes and a single tribromo com- pound (Table 1). The major of the important components, i.e. the dibromides 9b, possesses C2 symmetry according to its proton and carbon-13 NMR spectra. Spectroscopic evi- dence, discussed below, is indicative of the endo position of the bromine atoms. The other dibromide lacks symmetry and, therefore, was assigned the structure exo,endo3b.

Bromination of the mixture of [4.3.3]propelladienes 5c + 6c proceeds very similarly to that of the [3.3.3]propelladi- enes 5b + 6b affording the same types and numbers of products: Two monobromides, about equal amounts of a

dibromide of C2 symmetry and a dibromide devoid of sym- metry (= exo,endo3c), and a single tribromide (Table 1). Recrystallisation of the sublimed mixture from pentane at low temperature furnished the pure symmetrical dibromide as colourless crystals of m.p. 64-65°C. Its structure endo,- endo-9c was unequivocally established by an X-ray diffrac- tion analysis (Figure 1).

Figure 1. Perspective drawing of the dibromo[4.3.3]propelladiene endo, endo-9c showing the numbering of the atoms

c11

c10

0 Br9

These results show that the attack of molecular bromine at ally1 radical moieties that are part of a bicyclo[3.3.0]- octane system is directed by the substituents of the bridge- head atoms (Figure 2). While two methyl groups still allow attack from the ex0 face, this side is more strongly shielded by a trimethylene or a tetramethylene bridge. As a result, endo attack prevails. The same steric effect of these bridges[21,221 was already observed in the allylic bromi- nation of 1,5-substituted bicyclo[3.3.0]octa-2,6-diene-2,6-di- carbonitriles.

Figure 2. ex0 and endo attack at allylic radicals that are part of bicyclo[3.3.0]octane and [n.3.3]propellane systems (n = 3, 4). For the sake of consistency, we deviate from the definition of ex0 and endo for positions of substituents and faces of bicyclic s k e l e t ~ n s [ ~ ~ J and define ex0 and endo with respect to the bicyclo[3.3.0]octane system irrespective of the correct numbering of the [4.3.3]propel-

lane system according to IUPAC rules

ex0

endo

In view of the diastereoselectivities in the allylic bromi- nations of bicyclo[3.3.0]octadiene derivatives described in the foregoing section, it seemed of interest to extend this

Liebigs Ann. 1995, 1159- 1168

1162 H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering

study to the allylic bromination of bicyclo[3.3.l]nona-2,6- diene (11), which is readily available from Meerwein's dike- tone, i.e. bicyclo[3.3.1]nonane-2,6-dione~171. Brominated de- rivatives of ll were already obtained by the addition at low temperature of hydrogen bromide (-+ 13) and molecular bromine to barbaralane (2 + exo,exo-15 + exo,endo- 15)[24,251 and of bromine to 4-bromobarbaralane (16 + 17)[241. Allylic brominations of 13 and 14 have been re- ported to afford the diastereomer exo,exo-15 which gives the tribromide 17 and higher brominated products on con- tinued bromination['0~2s].

When the conditions, optimised for the bicyclo[3.3.0]0~- tadiene derivatives 5 + 6, were employed in the allylic bro- mination of 11 + 12, GC disclosed the initial formation of two monobromides (58 :42) of very similar retention times. Separation by analytical HPLC was even impossible. The structures 13 and 14 may be assigned to these monobro- mides because continued bromination confirmed the exclus- ive formation of exo,exo-15 as shown by capillary GC and HPLC. Hence, allylic radicals that are part of the bicyclo- [3.3. llnonane system suffer attack only from the ex0 face, in marked contrast to the bridgehead substituent-controlled stereoselectivities observed in the bicyclo[3.3.0]octane series.

12 / 2 exo,exo-15 Br exo,endo-15 Br

1

16 BI

17

Cyclisation of Dibromobicyclo(3.3.0]octadienes to Semibullvalenes

Reductive cyclisation of 1,5-substituted dibromobicyclo- [3.3.0]octadienes of unknown purities and structures were carried out with large amounts of lithium amalgam in ether[4,5,7al or tetracarbonylnickel, when [n.3.3]propelladi- ene derivatives (n = 5 , 6, 8, 10) were involved. The latter dibromides do not undergo cyclisation with a host of re- agents tried[7b]. Not surprisingly in view of the ill-defined starting materials, isolation of the semibullvalenes required cumbersome purification procedures, i.e. column chroma- tography on silica gel with pentane as eluent followed by

preparative VPC. Moreover, substantial losses of material are dificult to avoid when the solvent is to be distilled from solutions containing highly volatile semibullvalenes such as la-e. Because lithium amalgam is neither commercially nor conveniently available[26] and handling of large amo- unts of mercury is not very attractive, cyclisation of exo,exo-9a with the zinc-copper couple was a welcome im- provement['*]. Activation of freshly prepared zinc-copper couple during the reaction by ultrasonic ac- celerates the cyclisation considerably and was employed throughout the present work.

In order to circumvent the problems arising from solvent separation and the application of chromatographic tech- niques, we took advantage of the volatility of the semi- bullvalenes la-c. To this end the cyclisations by the zinc- copper couple of the dibromides 9 were carried out not in diethyl ether but in dry tetraglyme as solvent whose high boiling point (275 "C) allowed us to sweep the volatile prod- ucts la-c, which were condensed in very efficient cold traps (Figure 3), with a stream of dry nitrogen. While dimethylsemibullvalene 1 a is completely isolated in this way from the reaction mixture within 10 hours at a temperature as low as 100 "C, temperatures around 150 "C are required for the isolation of the somewhat less volatile semibullval- enes l b and c (Table 2). That removal of the products from the reaction mixtures is complete after the periods of time given in Table 2 could be shown by distillation of the sol- vent in vacuo followed by GC of the distillate. Very small amounts of tetraglyme and traces of moisture were finally removed from the combined contents of the cold traps by distillation in a microdistillation apparatus from sodium hy- dride, a process which simultaneously diminishes the frac- tion of higher boiling byproducts, i.e. 19a-c and 1,5-di- methylcyclooctatetraene (18) (see below). Thus, the semi- bullvalenes l a and c and the yet unknown semibullvalene l b are conveniently obtained on a gram scale in a reasonable purity and in yields considerably higher than those reported originally for l a and ds1. Inspection of Table 2 shows that the product ratios are related to the composition of the starting materials as expected. When the starting materials contain substantial amounts of the tribromides 10, small amounts of bromosemibullvalenes 19 are found in the prod- uct which are missing when pure dibromide endo,endo-9e is cyclised. On the other hand, the crude products inevitably contain a few percent of the dienes 5 + 6, whether or not small amounts of the monobromides 7 + 8 are present in the starting materials. Obviously, the dibromides 9 are re- duced to the alkenes 5 + 6 by the zinc-copper couple in a minor side reaction which may be initiated by traces of a proton source.

Zinc-copper-induced cyclisation of 2,6- and 3,7-substi- tuted 4,8-dibromobicyclo[3.3.0]octa-2,6-dienes to semi- bullvalenes[21~281 reported in the literature started from the exo,exo diastereomers. Significantly, an endo,endo dibro- mide fails to undergo cyclisation under these conditions[22]. Likewise, the dibromides obtained by allylic bromination of [n.3.3]propelladienes (n = 5, 6, 8, 10) do not react with the zinc-copper couple[7b]. These results appeared to indicate

Liebigs Ann. 1995, 1159- 1168

1,s-Dialkylsemibullvalenes 1163

that the exo,exo configuration of the 4,8-dibromides was a prerequisite for the cyclisation reaction which hence was assumed to occur on an extended W or semi-W Therefore, it came as a surprise that the [n.3.3]propelladiene dibromides (n = 3, 4) exo,endo- + endo,endo-9b and exo,endo- + endo,endo-9c do close the cyclopropane ring upon the action of the zinc-copper couple. Furthermore, a mixture enriched in exo,endo-9a gives a reasonable yield of la. This mixture and endo,endo-9c react smoothly even in boiling ether as shown by GC in small-scale experiments (Table 2). Probably, the use of a highly active zinc-copper couple under ultrasonic irradiation, perhaps also the higher temperatures employed, and the prolonged times required for the sweep of the products from the reaction mixtures have afforded these gratifying results.

@ Br

9

\

9b, c _____.*

l b (n = 3) l c (n = 4)

9a

l a ' 18

10 19 19'

Sweeping of 1,5-dimethylsemibullvalene (la) from the re- action mixture was carried out at temperatures (105°C) well below those required for the equilibration with 1,5-di- methylcyclooctatetraene (18) (tli2 of l a at 160°C = 14 hL3'1). Therefore, it was surprising to obtain considerable amounts of 18, when mixtures of bromides were used as starting material, and product isolation was performed by means of the sweep technique. Control experiments showed that l a is stable at 105°C in tetraglyme in the presence of the zinc-copper couple and zinc bromide. Furthermore, the ratio of l a and 18 does not change significantly under these conditions. Cyclisation with the zinc-copper couple of pure dibromide exo,exo-9a or the mixture enriched in exo,endo- 9a (see above) in boiling ether does not furnish any trace of 18[311. Further scrutinity of the crude products by capil- lary GC revealed the presence of traces of o-xylene (0.18%),

Liebigs Ann. 1995, 1159- 1168

m-xylene (0.12%), toluene (0.370/0), and benzene [0.03% relative to the peak areas of C8H6(CH& hydrocarbons = 1 OO%] which were identified by coinjection. Since Paquette et al. have shown that these aromatic products arise in flash vacuum pyrolyses of dimethylcyclooctatetraenes[321, it is reasonable to assume that they result from further de- composition of 18. Because the preparative experiments de- scribed here did not start from pure dibromides 9 , we did not attempt to uncover the origin of the byproducts. Pre- sumably, they arise in metal-mediated processes at elevated temperatures.

Table 2. Reaction conditions, products, and yields in reductive cy- clisations of mixtures of the bromides 7-10 with the zinc-copper couple in tetraglyme or diethyl ether solutions. Product ratios were calculated from peak areas of traces in high-resolution GC and are listed in the order of increasing retention times (in square brackets)

StartingMaterials Temp. T i products yj&$al

(7+8) : 9 : 10 ["C] [h] (5 +6)bl : 1 : 19(193 [%I

b 5 76 19 140 27 3 92 5 80(75) [n,n- : x,n-9b = 59 : 411

c 3 86 11 150 52 4 94 2 61(57) [n,n- : x.n-9c = 55 : 451

endo,endo-9c 150 41 2 98 <0.3 61

35 2 3 97 4.3 Icl

(5a + 6a) :la : 18 : lSa(19a3

a 0.7 98 2 35 1.5 7 91 4.02 2 [cl exaexo-9a

0.4 92 8 105 10 2 81 15 2 79(60) [n,n-lx,n-ixJ-9a

= 4 : 17 : 791

1 78 21 105 10 1 71 17 11 58 [n,n-lx,n-ix~-9a = 16 : 60 : 241

35 1.5 39 32 0.6 28 I4

La] Yields of (la + 18), lb, and c are based on the dibromides 9 and were calculated from the total amounts of volatile products and their ratio (GC). Yields given in brackets refer to distilled semi- bullvalenes of >90% purity (la: 95, lb: 92, lc: 94%). - The ratios of the dienes 5 and 6 were constant in all experiments [5a:6a = 75:25, 5b:6b = 63:37, 5c:6c = 69:31 (GC)]. - Small- scale experiment performed in diethyl ether as solvent. The ratios of products were determined from the reaction mixtures after com- plete conversion (GC).

Recently, Trinks and Miillen have adopted the Askani route to semibullvalenes for the synthesis of barbaralane (2). Reductive cyclisation of the dibromide exo,exo-15 with the zinc-copper couple in boiling ether as solvent gave 2 in 37% yield besides oligomeric components['']. In view of the success of the sweep technique in the isolation of volatile semibullvalenes 1, we put our procedure to the test in this reaction. It was gratifying to obtain an 82% yield of barbar-

1164 H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering

alane (2) of 97% purity, when exo,exo-15 was treated with the zinc-copper couple in tetraglyme under ultrasonic ir- radiation followed by sweeping of the product with nitrogen at 140 "C. With these modifications, Mullen's barbaralane synthesis compares very favourably with earlier pro- cedures[24.*5.331.

Proton and Carbon-13 NMR Spectra

Most compounds studied in the present work existed only as components of more or less complex mixtures. Ac- cordingly, the proton NMR spectra are complicated by sig- nal overlap. In order to minimise this overlap, we preferred [D6]benzene over [Dltrichloromethane as solvent in the case of the dibromides 9.

The assignments of the signals to a particular compound were based on relative intensities which were deduced from the ratios of components determined by capillary GC. In addition, NMR spectra were recorded from mixtures of dif- ferent compositions. The NMR spectra were interpreted with the help of DEPT, 'H,'H COSY, and 13C,'H COSY spectra. The spectral parameters of the allylic protons were obtained by first-order analyses. Chemical shifts and coup- ling constants in proton spectra of the dibromides 9, exo,exo-15 and the semibullvalenes 1 are listed in Table 3, chemical shifts in carbon-13 spectra in Table 4. Inspection of these Tables shows that the diastereomeric dibromides 9 differ most strongly in the chemical shifts of the terminal vinyl protons of the allylic systems. Since the configuration of endo,endo-9c was established unequivocally by X-ray dif- fraction analysis, it is safe to conclude that a low-field shift of the signals of these vinyl protons is a reliable criterion for the endo position of the bromine atom at the other cyclopentene ring of the bicyclo[3.3.0]octadiene system. The deshielding effect of a bromine atom on the chemical shift of a vicinal cis methyl group at a cyclopentene ring was already employed for the assignment of the configuration of exo,ex0-9a[~]. This shift criterion allowed the assignment of the configurations to the other dibromides of this study.

6 = 5.02 - 5.20 6 ~ 5 . 6 1 -5.95

exo,exoJ exo,endo-9

exo,endo-9 endo,endo-9

The proton and carbon-13 NMR spectra of the semi- bullvalenes 1 are consistent with rapidly equilibrating de- generate valence tautomers. The interpretation of the pro- ton NMR spectrum of the bromosemibullvalene 19a[341, which is in equilibrium with 15%) of its valence tautomer 19a' at room temperature[1s], has shown that the allylic pro- tons give rise to a highly coupled spin system involving long-range couplings between the allylic subunits. There-

fore, the allylic protons of the rapidly exchanging degener- ate semibullvalenes la-c are anticipated to form an AA'A"AXX' system the spin Hamiltonian of which can- not be Since further experiments have not been carried out so far, only chemical shifts of the ring protons can therefore be given.

Concluding Remarks Askani's synthesis of the alkylsemibullvalenes la, c was

optimised and extended to 1,5-trimethylenesemibullvalene (lb). The configurations of the intermediate bicyclic or tri- cyclic dibromodienes 9 were elucidated. After reductive cy- clisation of 9 with a highly active zinc-copper couple in a sonicated suspension in tetraglyme, application of the sweep technique using nitrogen allows the isolation of the semi- bullvalenes 1 and barbaralane (2) most conveniently in gram quantities and reasonable purities. Some aspects of the chemistry of these semibullvalenes, e.g. products and activation parameters of the thermolysis, a study of their degenerate Cope rearrangement, and experiments using me- tal cations[36] and reactive transition metal complexes, de- signed with the intent to trap the (delocalised) transition state of the Cope rearrangement in a q6 complex, will be reported in separate papers.

We thank Mrs. E. Ruckdeschel and Dr. D. Scheutzow for re- cording NMR spectra and Dr. G Lunge and Mr. E Dadrich for measuring the mass spectra. We thank the Hoechst AG for a gen- erous gift of tetraglyme. Financial support by the Fonds der Che- rnischen Zndustrie, Frankfurt am Main, is gratefully acknowledged. T. D. thanks the Fonds der Chemischen Industrie for a doctoral fel- lowship.

Experimental Melting points: Sealed capillary tubes, apparatus from Buchi,

Flawil, Switzerland. - 'H NMR: Bruker AC 250, WM 400, DMX 600; Table 3. - I3C NMR: Bruker AC 250, WM 400, DMX 600; Table 4. - IR: Perkin Elmer 283. - 70-eV MS: Finnigan MAT 8200. - Capillary GC: Chrompack 436 gas chromatograph equipped with Shimadzu integrator C-R6A and a Chrompack (50 X 0.11 mm) WCOT-fused silica column coated with CP-Sil 5 CB (film thickness 0.13 pm); 4.0 bar NZ; on-column injection; con- ditions A : column temp. T = 180°C; B: column temp. T = 80°C (22 min), 80-180°C (10"C/min). - GC: Varian 1400 gas chroma- tograph equipped with Shimadzu integrator C-R6A and a 3-m glass column with 10% silicon oil SE 30 on Volaspher A2 (60-80 pm, Merck), T = 170"C, 15 ml/min Nz. Ratios were calculated from peak areas with neglection of specific response factors. - Sonications were performed in an ultrasonic cleaning bath Sonorex RK 102 by Bandelin, D-12207 Berlin, 120 W, 50 kHz.

Aromatic compounds, capillary GC, B: tR [min] = 9.4 (benzene), 11.7 (toluene), 16.4 (m-xylene), 18.1 (o-xylene).

1,5-Dimethyl series, GC: tR [min] = 1.4 (5a, 6a), 3.3 (7a, sa), 10.3 (9a), 22.7 ( loa) . - Capillary GC, A: tR [rnin] = 15.6, 16.1 (7a, Sa), 20.4 (endo,endo-9a), 20.8 (exo,endo-9a), 22.4 (exo,exo-9a), 36.7, 39.7 (10a); B: 19.0 (6a), 19.2 (54, 21.2 ( l a ) , 28.1 (18), 35.8 (19a, 19a').

[3.3.3]Propellane series, GC: tR [rnin] = 1.8 (5b, 6b), 5.4 (7b, Sb), 17.5 (9b), 34.8 ( lob) . - Capillary GC, A : 15.0, 15.2 (7b, Sb), 42.2 (9b), 57.8 (lob); B 29.2, 29.4 (5b, 6b), 30.4 ( l b ) , 40.8 (19b).

[4.3.3]Propellane series, GC: tR [min] = 3.5 (5c, 6c), 8.1 (712, Sc), 27.8 (Sc), 56.9 (1Oc). - Capillary GC, A: t R [min] = 19.5, 19.6 (7c,

Liebigs Ann. 1995, 1159-1168

1,5-Dialkylsemibullvalenes 1165

Sc), 47.0 (9c), 86.7 ( 1 0 ~ ) ; B: 34.7, 35.1 (5c,6c), 36.0 ( l c ) , 47.0 (19c). Bicyclo[3.3.l]nonane series, GC: tR [min] = 2.4 (11, 12), 6.2 (13,

14), 18.3 (exo,exo-15), 38.5 (17). - Capillary GC: A: tR [min] = 13.6, 13.7 (13, 14), 25.6 (exo,exo-15), 45.5 (17); B: 25.6 ( l l ) , 26.2 (12), 28.2 (2).

Sulfolane and quinoline were stirred with powdered CaH, for 1 d, distilled at lo-' Torr from CaH2, and stored under N2. Tetra- glyme was stirred with CaH, for 1 d, distilled at lo-' Torr from CaH,, subsequently from LiAIH4, and, immediately before use, from Nahenzophenone. It was handled under Ar. Dichlorome- thane was distilled from P205, diethyl ether from NaH under N2.

The mixtures of 5a + 6a (55:45) and 11 + 12 (9: 1) were prepared as described[I7]. - The ketones 3b[6b1 and cLZ2] were prepared and purified as described. The diols 4b, c were prepared by reduction of 3b, c with NaBH4 in methanol at 10-30°C according to the procedure reported for 3a + 4a["]. The mixture of diastereomers 4b was recrystallised with the aid of an extraction from petroleum ether (30-70 "C) (17 h) and subsequently from toluene (24 h), yield 49%, m.p. 94- 103 "C; m.p., IR, and 'H NMR of the pure diastereomers: ref.[38]. 4c: m.p. and IR of the pure dia- stereomers: 'H NMR: ref^.[^.^^^].

Tricyclo(3.3.3. 01~5]undeca-2,6-diene (5b) and Tricyclo[3.3.3. OIJ]- undeca-2,T-diene (6b): A 250-ml three-necked flask equipped with heating mantle, reflux condenser connected to a N2 source, mag- netic stirrer, and thermometer was charged with 4b (18.4 g, 100 mmol), triphenyl phosphate (71.8 g, 220 mmol), quinoline (26 ml, 28.4 g, 220 mmol), and sulfolane (100 ml). The stirred mixture was degassed in vacuo, saturated with N2, and heated at reflux under N2 for about 3 h, during which time the temperature fell from 272 to 243 "C. After cooling of the dark red mixture to 20-25 "C, the reflux condenser was replaced by a Claisen still head equipped with a receiver cooled to -20°C. On heating, the alkenes distilled while a vigorous stream of N2 bubbled through the mixture. The distillate was extracted with petroleum ether (3O-5O0C, 3 X 50 ml). The combined petroleum ether solutions were extracted with aqueous Na2C03 (1 M, 2 X 100 ml), &SO4 (1 M, 2 X 75 ml), and a saturated aqueous solution of NaHCO, (2 X 75 ml), and dried with KZC03. The solvent was distilled through a 50-cm Spaltrohr column (Fischer, D-53340 Meckenheim). Distillation of the remaining liquid afforded a pale yellow liquid (10.7 g, 73%, purity 94%, 5b:6b = 1:1, b.p. 89-9O0C155-60 Torr). - 'H NMR (400 MHz, CDCl,): 6 = 1.5-1.7 [m, 2 (CHz)3], 2.24, 2.40 (2 ddd, J = 16.8, 2.3, 2.3 Hz, 2 CHZ), 2.38 (dd, J = 2.3, 2.3, 2 CHz), 5.44, 5.45, 5.56, 5.66 (4 ddd, J = 5.7, 2.3, 2.3 Hz, 8 CH). - I3C NMR (100 MHz, CDCl3): 6 = 25.5, 26.2, 37.9, 41.0, 42.3, 46.0, 48.6 (CH,), 127.2, 127.9, 136.4, 137.8 (CH), 58.2, 67.5, 76.6 (quat. C). - CIIHl4 (146.2): calcd. C 90.35, H 9.65; found C 90.15, H 9.83.

Tricyclo[4. 3.3.O1J/dodeca-7, I0-diene (5c) and Tricycl0(4.3.3.0~~~/- dodeca-7,Il-diene (6c): The procedure described for 5b and 6b was applied. The temperature fell from 277 to 236°C during 2 h. Distil- lation afforded a colourless liquid (12.7 g, 79%, purity 95%, 5c: 6c = 1:1, b.p. 62-65"C/7 Torr). - 'H NMR (60 MHz: ref.L8], 600 MHz, CDC13): 6 = 1.2-1.8 [m, 2 (CH,k], 2.19, 2.21 (2 ddd, J =

2 CH,), 5.45, 5.48, 5.55, 5.62 (4 ddd, J = 5.7, 2.2, 2.2 Hz, 8 CH).

32.98, 42.0, 45.5 (CH,), 126.6, 128.4, 136.6, 141.4 (CH), 50.1, 55.1, 62.0 (quat. C).

16.1, 2.2, 2.2 Hz, 2 CH,), 2.23, 2.36 (2 ddd, J = 16.3, 2.2, 2.2 Hz,

- I3C NMR (63 MHz, CDCl3): 6 = 19.8, 20.4, 21.5, 32.48, 32.77,

Brominations with N-Bromosuccinimide (NBS). - General Pro- cedure: The apparatus consisted of a 4-1 three-necked flask of cylin- drical shape equipped with an efficient condenser (P205 drying tube), magnetic stirrer and two daylight lamps (Radium MRL 250

W) which also served as heat sources. - Dichloromethane (3 1) was distilled into the flask and the mixture of the dimes 5 + 6 or 1 1 + 12 (50 mmol) was added. At the onset of boiling, NBS (21.4 g, 120 mmol) was added, and the irradiation was continued until the fraction of monobromodienes had decreased to 1-5%. Monitoring by GC of the progress of the conversions was performed on 5-ml samples which were extracted with aqueous NaOH (0.5 M, 2 ml) and a saturated aqueous solution of NaHC0, (2 ml), dried with K2CO3 and concentrated in vacuo. - The reaction mixture was extracted with aqueous NaOH (0.5 M, 1 l), saturated aqueous solu- tions of NaHC03 (1 I) and NaCl (1 l), and dried with K2C03. Distillation of the solvent in vacuo yielded yellow to orange viscous oils or semisolid materials which were sublimed at Torr on a cold finger cooled to -20 to -70°C. Starting materials, reaction conditions, product ratios, and yields: Table 1.

Bromination of a Mixture of 5a + 6a (55:45): Sublimation at 70°C bath temp. of the yellow oil obtained from 3 runs yielded a colourless, semisolid material (37 g, 84%). Recrystallisation from pentane (14.3 g/150 ml, -2O"C, 4 d) afforded colourless crystals [3.7 g, m.p. 113-115"C, (7a + Sa): 9a:lOa = 0.4:92:8, endo,endo- : exo,endo: exo,exo-9a = 4: 17:79]. Repeated recrystallisation from pentane yielded colourless crystals of exo,exo-9a, m.p. 120 "C (124°C[91, 117-l19°C['81), (7a + Sa):9a:lOa = 0.7:98:2. - Distil- lation of the solvent in vacuo from the mother liquors and subli- mation of the residue at 75- 100°C bath temp. yielded a pale yellow oil [7.8 g, (7a + Sa):9a:lOa = 1:78:21, endo,endo-:exo,endo-:exo, exo-9a = 16:60:24].

Bromination of a Mixture of 5b + 6b (1 : 1): Drying of the crude product at 20-25°C/10~2 Torr yielded an orange-coloured oil [18 g, 93%, (7b + Sb):9b:lOb = 5:76:19, endo,endo-:exo,endo-9b = 59:41], which did not sublime at bath temp. <100"C/10-5 Torr and frustrated attempts at crystallisation from pentane.

Brornination of a Mixture of 5c + 6c (1:l): Sublimation at 65-90°C bath temp. of the orange-coloured oil obtained from 2 runs yielded a colourless oil [25 g, 80%, (7c + Sc):9c:lOc = 3:86: 11, endo,endo-:exo,endo-9 = 55:45]. Recrystallisation from pentane (20.5 g/60 ml, -2O"C, 10 d) afforded colourless, large prisms (3.5 g) of endo,endo-9c, m.p. 66-65°C. Distillation of the solvent from the mother liquor yielded a pale yellow oil [I6 g, (7c + Sc):9c:lOc = 3:84: 13, endo,endo-:exo,endo-9 = 44:56]. - endo, endo-9c, CI2Hl4Br2 (318.0): calcd. C 45.32, H 4.44; found C 45.09, H 4.73.

Bromination of a Mixture of 11 + 12 (9:l): The general pro- cedure was used. After 2 h an additional portion of NBS (30 mmol) was added, and irradiation was continued for 0.5 h. The yellow, semisolid crude products of 2 runs consisted of (13 + 14), exo,exo- 15 and 17 (14:83:3 and 7:78:15). Sublimation at 80-120°C bath temp. yielded a colourless waxy solid (23 g). Recrystallisation from methanol (160 ml) afforded colourless plates of exo,exo-15, m.p. 118°C (19.4 g, 72%, ref.['']: 70%, m.p. 117"C, 'H and I3C NMR).

Reductive Cyclisations of the Dibromides with the Zinc-Copper Couple Suspended in Tetruglyme. - General Procedure: The appa- ratus consisted of a 50-ml two-necked flask equipped with magnetic stirrer, 25-ml dropping funnel, reflux condenser, and a 300 X 2 mm stainless steel tube (through the condensor) for the introduction of N2. The upper outlet of the condensor was connected via gas-tight 4 X 1 mm PFA tubings (Bohlender, D-97922 Lauda) to 2 efficient cold traps (-78°C) in a row (Figure 3). N2 was dried by passing it through a 1.5-m drying tube containing Pz05 (desiccator quality, Merck). - In the two-necked flask, powdered zinc (5 g, particle diameter ca. 60 pm) was vigorously stirred for 10 s with hydro- chloric acid (1 M, 3 X 8 ml) and for 1 min with water (4 X 8 ml),

Liebigs Ann. 1995, 1159- 1168

1166 H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering

aqueous copper(I1) sulfate (0.2 M, 2 X 8 ml), water (3 X 8 ml), acetone (3 X 8 ml), and ether (3 X 8 ml). The supernatant liquids were decanted each time. Finally, the zinc-copper couple was dried at 6OoC/1Op2 Torr for 0.5 h and kept under Ar. Tetraglyme (ca. 10 ml) was distilled at lo-' Torr from Na/benzophenone to the freshly prepared zinc-copper couple and the suspension was sonicated for 0.5 h, while a vigorous stream of N2 was introduced. The dropping funnel was charged under N2 with the dibromodiene (12 mmol) which was subsequently dissolved with magnetic stirring in tetra- glyme (17 ml, distilled directly into the funnel). This solution was added within 3 h to the sonicated suspension of the zinc-copper couple while the introduction of N2 was continued. After 0.5 h, the ultrasonic cleaning bath was replaced by an oil bath and the suspension was heated in the N2 stream. The crude products were colourless or pale yellow liquids which were distilled from NaH in a microdistillation apparatus into a receiver cooled to -20 to -30°C. For further details see Table 2. Figure 3. Cold traps used in the isolation of the semibullvalenes 1

and barbaralane (2) by means of the sweep technique

KK l,5-Dime~hyItricyclo~3.3.~.~~~]octa-3,6-diene (1 a): Pale yellow

liquid (1.0 g, 60%, purity 95%, b.p. 115"C, 85-87OC1120 Torr). - IR (C2C14): B = 1690.1 (w), 1657.0 (m), 1623.0 (w), 1574.6 (m), 1566.4 (s) cm-I. - 13C NMR: ref.[181.

Tetracycl0[3.3.3.0'~'.@~~]undeca-3,&diene (lb): Pale yellow liquid (1.4 g, 75%. b.p. 105"C/300 Torr, purity 92%). - MS, m/z

115 (69), 103 (15), 91 (13).

Tetracyclo[4.3.3.0'~6.07~12]dodeca-8,10-diene (lc): Colourless liquid (1.15 g, 570/0, b.p. 104"C/50 Torr, purity 94%). - 60-MHz 'H NMR, MS: ref.L5].

Tricycl0[3.3.l.@~~]nona-3.6-diene (2): After 10 h at 130°C, a colourless waxy solid (1.2 g, 82Y0, purity 97%) was obtained from exo,exo-15 (physical and spectral data: ref^.['^.^^]). Three unknown impurities (sum 3%), exhibiting shorter retention times than 2 (KGC, B: tR [min] = 21.3, 21.9 and 27.0), were detected.

Reductive Cyclisations with the Zinc-Copper Couple Suspended in Ether: Small-Scale, General Procedure: To a suspension of freshly prepared zinc-copper couple (ca. 100 mg) in ether (1 ml), contained in a Schlenk tube under Ar, the dibromide (50 mg) was added, and the suspension was sonicated for 1.5-2 h. The solution was de- canted and the zinc-copper couple washed with ether. The com-

(Yo): 144 (100) [M+], 143 (31), 129 (78), 128 (57), 117 (39), 116 (86),

Schlenk tube under Ar, tetraglyme (ca. 0.5 ml) was distilled from Na/benzophenone. A mixture of l a and 18 (91 :9,26 mg, 0.2 mmol) and undecane [l drop, capillary GC (B) tR = 28.9 min] were added [+ ( la + 18):undecane = 75:25] followed by stirring and heat- ing at 105°C for 3 h [+ ( la + 18):undecane = 72:28, la:18 = 89:11]. After addition of 1,2-dibromoethane (37.5 mg, 0.2 mmol) followed by heating at 105°C for 1.5 h and 7.5 h, the la:18 ratios were 89:ll and 85:15, respectively. The ratio of ( l a + 18): undecane remained unchanged.

exo,endo-9a, b exo,end03c

Table 3. Chemical shifts (6 values) and coupling constants (absolute values [Hz] in italics) in proton NMR spectra (250 MHz) of the semibullvalenes 1 and the dibromides 9, exo,exo-15 recorded from solutions in [D6]benzene. Chemical shifts of protons that are equiv- alent in virtue of symmetry (exo,exo-9, endo,endo-9, exo,exo-15) or

rapid degenerate Cope rearrangement (1) are given only once

Cpd 2-H 3J2.1 3-H 3J3.4 4-H 4J24 CH? 6-H 3J67 7-H 3J71( 8-H 4Jl;R ( s )

exo,exo-% 5.02 5.6 5.43 2.4 4.40 1.31 exo,endo3a 5.89 5.8 5.67 2.5 4.61 0.98

5.03 5.8 5.32 1.7 4.33 1.7 1.02 endo.endo-9a 5.84 5.9 5.50 1.7 4.38 1.7 0.70

la 4.57(m) 5.13(m) 0.93

exo,e&-9b 5.61 5.6 5.52 1.8 4.79 1.8 1.0-2.5

endo,endo-9b 5.64 5 5 5.48 22 4.49 0.9 1.0-2.5 5.20 5.6 5.36 2.1 4.46 2.1

lb[4 4.30(m) 5.11(m) 1.4 - 1.5

4CH2)4- 7-H 357.8 8-H 3JR.9 9-H 4J7,9

10-H 3J10.11 11-H ?11,,12 12-H 4J1".12

exo,endo-k 5.95 5.8 5.66 2.3 4.76 1.8 - 2.2 5.04 5.7 5.34 1.8 4.71 1.8

endo,endo-k 5.91 5.8 5.53 1.6 4.66 1.6 0.8- 1.8

lC[B] 4.18(m) 5.13(m) 1.3 - 1.6

2-H 352.3 3-Hbl 'J3.4 4-H 1-H 3.f1,g 9-H

eX0,eX0-15['] 5.07 9.7 5.40 4.6 3.88 2.4- 3.0 2.06 2.5

['I Recorded from solutions in [D]trichloromethane. - cb] 4J1,3 = 1.0, 4J3,5 = 1.0 Hz. - Lc] For a proton NMR spectrum recorded from [D]trichloromethane solution see ref.['O].

bined ether solutions were washed with saturated aqueous KH2P04, dried with K2C03 and a n a b e d by capillary GC. Results: Table 2.

Treatment of l a with the Zinc-Copper Couple and Zinc Bromide: To freshly prepared zinc-copper couple (ca. 90 mg), contained in a

x - ~ a y Diffraction Analysis of endo,endo-9~ was performed from a transparent colourless crystal. The cell parameters were deter- mined on the basis of 22 reflections. The number of reflections reported in Table 5 was obtained with Mo-K, radiation and 20,,, = 55" (graphite monochromator, Wyckoff scan). Measure-

Liebigs Ann. 1995, 1 159- 1 168

1167

Table 4. Chemical shifts (6 values) in carbon-I3 NMR spectra of the semibullvalenes 1 and the dibromides 9, exo,exo-15 recorded from solutions in [D6]benzene. Chemical shifts of carbon atoms that are equivalent in virtue of symmetry (endo,endo-9, exo,exo-15) or rapid degenerate Cope rearrangement (1) are given only once.

The data of exo,ex0-9a['~I are listed for comparison

w. C-1 C-2 C-3 C-4 CH, C-5 C-6 C-7 C-8

exo,em-9a 59.1 142.0 130.2 64.3 20.7 e.w,endo-9a 60.6 143.2 131.27 65.3 21.6

58.6 140.2 130.83 63.0 18.5

endo.endo-9a 60.1 141.1 131.3 64.3 19.4

c-9

exo,exo-15 36.9 129.2 127.9 47.2 19.0

exo,endo3b 71.6 138.9 132.32 64.03 38.6 26.8

endo,endo-9b 71.5 139.5 131.5 63.87 38.8 26.1

lb[a] 69.8 89.9 120.7 31.2 27.1

71.0 137.1 132.61 62.8 39.2

C-1 C-7 C-8 C-9 -(cHd4- C-6 C-10 C-11 (2-12

exo,endo-9c 59.8 143.3 130.91 65.2 29.5 20.4 57.3 140.9 129.8 60.3 33.7 21.5

endo,endo-9c 59.4 141.7 130.8 61.8 30.9 20.1

lc[al 60.1 92.5 119.7 20.3

La] Recorded from solutions in [D]trichloromethane.

ments were carried out with the system Nicolet R3mN. Compu- tations were performed with a computer Micro-Vax 11. The pro- gramme SHELXTL-PLUS[391 was employed. The structure was solved by direct methods and refined anisotropically by the least- squares method. The weighting scheme for R, is l l d . The po- sitions of hydrogen atoms were calculated and included in the re- finements with isotropic

Table 5. Experimental details and results of the X-ray diffraction analysis of endojendo-9c

Molecular formula CI2Hl4Br2, molecular mass 318.05. - Crystal system: orthorhombic, space group Pbca. - a = 1394.1(9), b = 2018(1), c = 829.2(6) qm, V = 2333(3) . lo6 pm3, Z = 8, d(ca1cd.) = 1.811 g . cm- . - Size ofcrystal: 0.7 X 1.0 X 0.55 mm. - Range: h = 0-18, k = 0-26, I = 0-10. - Number of measured reflections: 3071, symmetry-independent reflections: 2685, obser- ved reflections F > 3 0 ( 4 : 1129. - Linear absorption coefficient: p = 6.91 mm-'. Absorption correction: yr-scan. - Ratio Fobs/ parameters = 8.89. - R = 0.112, R = 0.108. - Diff. Four.: Apmax[*] = 1.45, Apmin[**] = 1.18 eA-,:

[*I Maximum and [**I minimum of the remaining electron density in the final differential Fourier synthesis.

* Dedicated to Professor Paul von Raguk Schleyer on the occasion

[ I ] The results are part of the Dissertation by T. Dietz, Universitat

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of his 65th birthday.

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[281 H. Quast, J. Carlsen, R. Janiak, E.-M. Peters. K. Peters, H. G. von Schnering, Chem. Be% 1992, 125, 955-968; H. Quast, R. Janiak, E.-M. Peters, K. Peters, H. G. von Schnering, ibid. 1992, 125. 969-973: H. Ouast. A. Witzel. E.-M. Peters. K. Peters.

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H. G. von Schnering, ibid. 1992, 125; 2613-2623; H. Quast, J: Carlsen, ibid. 1992, 125, 2757-2763.

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r3O] H. Quast, T. Dietz, M. Heubes, A. Witzel, M. Boenke, W. R. Roth, to be submitted for publication.

r3I] Traces of 18 as little as 0.02% would have been detected by the capillary GC used in the present work.

566-586.

Liebigs Ann. 1995, 1159- 1168

1168 H. Quast, T. Dietz, E.-M. Peters, K. Peters, H. G. von Schnering

[321 L. A. Paquette, M. Oku, W. E. Heyd, R. H. Meisinger, J. Am. Chem. Soc. 1974, 96, 5815-5825.

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[341 The assignment of the proton spectrum of 19a (19a') given in ref.['*] has recently been confirmed by homonnclear 'H, H shift correlation of the COSY-45 type: D. Moskau, R. Aydin, W. Leber, H. Giinther, H. Quast, H.-D. Martin, K. Hassenriick, L. S. Miller, K. Grohmann, Chem. Ber. 1989, 122, 925-931.

[351 W. von der Saal, W. Risler, J. Stawitz, H. Quast, J. Org. Chem.

1983, 48, 2374-2379; P. L. Corio in Structure of High Resol- ution N M R Spectra, Academic Press, New York, 1967.

t361 H. Jiao, P. von Raguk Schleyer, Angew. Chem. 1993, 105, 1830-1834; Angew. Chem. Int. Ed. Engl. 1993, 32, 1760.

K3'1 0. Jiibermann, Methoden Org. Chem. (Houben- Weyl) 1958, Thieme, Stuttgart, vol. II1, p. 310.

[381 A. Natrajan, J. D. Ferrara, J. D. Hays, M. Khot, J. Colonell, W. J. Youngs, C. N. Sukenik, J Org. Chem. 1990, 55, 2164-2169.

[3y1 G. M. Sheldrick, Universitat Gottingen, unpublished. I4O1 Further details of the crystal structure investigation are avail-

able on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen, on quoting the depository number CSD-401507, the names of the authors, and the jour- nal citation.

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