Tetra(2,6-dichlorophenyl)porphyrin — a superior sensitiser for the singlet-oxygen ene reaction (Schenck reaction)

13. Quast, T. Dietz, A. Witzel 1495

‘~etra(2,6-dichlorophenyl)porphyrin - a Singlet-Oxygen Ene Reaction (Schenck Helmut Quast”, Thomas Dietz, and Alexander Witzel

lnstitut fur Organische Chemie der Universitat Wurzburg, Am Hubland, D-97074 Wiirzburg, Germany

Received February 17, 1995

Superior Sensitiser for the Reaction)‘’]

Key Words: Singlet-oxygen ene reaction I Ally1 hydroperoxides, a,p-unsaturated ketones from J Tetraarylporphyrins as singlet-oxygen sensitisers, persistence of I 1,2-Carbonyl transposition with concomitant dehydrogenation

~~ ~~~ ~

The very useful singlet-oxygen ene reaction of reluctant alkenes, e.g. l , 5 , 7, 8, and, in particular, the mono(ally1 hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet-oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP < TPFPP < TDCPP, rendenng TDCPP the sensitiser of choice for the generation of singlet-oxygen in non-polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3 ) this decrease strongly depends on

~ ~

the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate-denved species which induce oxidative destruction of the tetraarylporphynns Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate-dependent photobleaching. - Because the singlet-oxygen ene reaction is the key step of a 1,2-carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improved

Recently, we have devised a novel method for the 1,2- carbonyl transposition with concomitant dehydrogen- ationL21 of symmetrical ketonesL31 involving their conversion into alkenes which react with singlet-oxygen to afford ally1 hydroper~xides[~I. These were dehydrated in the next step to afford unsaturated carbonyl compounds[5]. When this sequence was applied to 1,5-dimethylbicyclo[3.3.0]octane-3,7- dione, problems were encountered in the twofold photooxygenation of the dienes 7a f Sa, because the mono(ally1 hydroperoxides) 10a and l l a react only very slowly with singlet-oxygen. The reluctance of 10a and l l a , apparently caused by electronic and steric effects, necessitated a high excess of singlet-oxygen and prolonged reaction times. Therefore, the only moderate persistence of the sensitiser 5,10,15,20-tetraphenylporphyrin (TPP) under the vigorous reaction conditions became the critical factor, even when small amounts of pyridine were added as stabiliser and the immersion well was cooled with an aqueous solution of potassium dichromate as light filterL31. Thus, for complete conversion of the readily formed hydroperoxides 10a and l l a , often repeated renewal of TPP was necessary, whose accumulating decomposition products were detrimental to the progress of the reaction, however. Furthermore, the unknown decomposition products appeared to enhance the rate of photobleaching of TPP[4c,6]. Results obtained with Rose Bengal bis(triethy1ammonium) in dichloromethane as solvent were even worse[*]. Clearly, the search for i I more stable singlet-oxygen sensitiser, soluble in aprotic solvents, was advisable. To this end, we tested 5,10,15,20- tetra(pentafluoropheny1)porphyrin (TPFPP) and 5,10, 1 5,20-tetra(2,6-dichlorophenyl)porphyrin (TDCPP). We

now report that TDCPP is the sensitiser of choice for the generation of singlet-oxygen in aprotic organic solvents.

0

Ar

N HN

kr

A host of sensitisers have been used for the generation of s ing le t -o~ygen[~~~,~~I . Surprisingly, only little attention has been paid so far to the problem of limited sensitiser stability during preparative photooxygenations except that it has been recommended to balance bleaching by renewal of the sensi- t i~ers [~~I . Though “physical” quenching of singlet-oxygen by TPP is substantial in chlorinated solvents[’ ‘I, TPP became the favoured sensitiser in perhalogenated solvents which are preferred because the lifetime of singlet-oxygen is longest therein (in the ms range)[10a,’21. Such and similar aprotic solvents are indispensable when the Schenck reaction is a

Liebigs Ann. 1995,1495- 1501 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1995 0947-3440/95/0808- 1495 $10.00+.25/0

1496 H. Quast, T. Dietz, A. Witzel

step in the 1,2-carbonyl transposition mentioned above because protic solvents, even when employed as cosolvents to bring about a sufficient solubility of ionic sensitisers, e.g. Rose Bengal, are incompatible with the in situ or sub- sequent dehydration of the allyl hydroperoxides. This conversion, to be performed with concentrated solutions of hydroperoxides or bishydroperoxides on a 0.2-mol scale, and experimental difficulties deterred us from using poly- mer-bound sensitisers because very large amounts (on a weight basis) are required[l3lI. Because photobleaching of TPP is probably due to oxidative destruction, we set out to investigate tetraarylporphyrins that exhibit enhanced stability when their metal complexes are used as catalysts in cytochrom P,,,-type oxidation reaction^"^]. Both electron- withdrawing and steric effects of substituents at the phenyl rings contribute to higher ~tability[’~,’~]. Accordingly, TPFPP and TDCPP[l7] were chosen for the present study.

Several recent investigations were concerned with kinetics and quantum yields of the photooxidation of tetraarylporphyrins and their metal derivatives in polar media related to biological systems[’8], because such compounds are of inter- est in the photodynamic therapy of cancer[19]. Tetraphe- nylbiladienone derivatives were recognised as final products of the singlet-oxygen addition to the dianion and metal complexes of TPP, which is followed by ring opening and addition of a solvent molecule (ROH) to the y-position[20]. Only very slow if any bleaching of TPP and Zn(TPP) in dichloromethane solution was observed in early studies[’”]. To the best of our knowledge, the applicability of TPFPP and TDCPP as sensitisers in singlet-oxygen photooxygenation reactions has not been tested as yet, let alone their stability under these conditions.

Materials and Methods An optimised preparation from pyrrole and benzal-

dehyde has been for the long-known TPP[221. Removal of a few percent of tetraphenylchlorin, present as an impurity in the crude product, has been achieved with several methods, e.g. subl imat i~n[~’ .~~I , chromatogra- p h ~ [ ~ ~ , ~ ~ , ~ ~ ] , oxidation to TPP with dimethyl su l fo~ ide [~~] or 4,5-dichloro-2,3-dicyanobenzoquinone(DDQ)~26~, or photooxidation of the zinc-chlorin ~omplex[~~,~’ ] . We removed tetraphenylchlorin with DDQ and purified the product after adsorption on aluminium oxide by extraction with hot methanol[28]. - After the first synthesis of TPFPP by Long0[~~1 in a (one-pot) Rothemund reaction, Rocha Gon- salves found that, for optimum yields, the condensation of pyrrole with an aldehyde affording the porphyrinogen had to be separated from the oxidation step to the porphyrin level[30]. Using this strategy, Lindsey developed a procedure for TPFPP involving acid-catalysed formation of the porphyrinogen which was subsequently oxidised with DDQL3l1. This method was significantly improved by Drenth[281 and V O ~ Z [ ~ ~ ] who employed an optimised amount of boron trifluoride-diethyl ether as catalyst in the first step[32] and inexpensive tetrdchloro-l,4-benzoquinone instead of DDQ in the second[2x]. We now combined both improvements and obtained very pure TPFPP in 50% yield on a three-

gram scale. - The tiny yield of TDCPP, obtained by the Rothemund reaction in the first synthesis[33], was increased by the application of the two-step procedure[31] and boron trifluoride-diethyl ether as catalyst in the first which was followed by oxidation with hydrogen peroxide[34], tetrachloro-l,4-benzoq~inone[~~1, or DDQ[35,361. Again, the combination of the recent improvements by Drenth[28] and V O ~ Z [ ~ ~ ] resulted in a reliable protocol for the preparation of very pure TDCPP in 30% yield.

From the beginning of the present study, the quest for a more persistent singlet-oxygen sensitiser useful in difficult preparative reactions was the major objective. Therefore, all experiments were carried out under the conditions of large- scale runs, i.e. in an immersion well apparatus[37], and under highly intense irradiation with a 400-W sodium vapour lamp. This equipment was also used in tests of sensitiser stabilities in the absence of photooxygenation substrates. Furthermore, the concentrations of the sensitiser, pyri-

being added as a stabiliser to prevent acid-induced decompositions, and of the alkene was maintained in the range usually employed in preparative experirnent~[~,~~].

Though the lifetime of singlet-oxygen is longest in carbon di~ulfide[~~1 and perhalogenated solvents, e.g. carbon tetrachloride, trichlorofluoromethane (Freon 1 l), hexa- fluorobenzene[l21, or 1,1,2-trichlorotrifluoroethane (Freon 1 1 3)[loaI, we preferred the more polar solvent dichloromethane in order to avoid precipitation of allyl hydroperoxides, e.g. 14a and 15a, during the photooxygenation reactionr3]. - Bleaching of the tetraarylporphyrins can be readily monitored by Vis spectra because, in the concentration range of the experiments, decomposition products do not exhibit absorptions in the visible spectrum that might interfere. The formation of allyl hydroperoxides was monitored by GC after dehydration to a,P-unsaturated ketones with acetic anhydride, pyridine and 4-(dimethylamino)- pyridiner51. - Comparative studies of the reactivity of alkenes in the singlet-oxygen ene reaction have ranged cyclohexene (1) among the least reactive alkenes, reacting slower by a factor of about lop4 than 2,3-dimethyl-2-b~tene[~~%~~I. Consequently, we initially employed cyclohexene in the search for stable sensitisers.

Results

In accordance with the results of previous qualitative studies[lga1, bleaching of TPP is slow in irradiated, oxygen- saturated dichloromethane solution in the absence of cyclohexene. TPFPP and TDCPP remain almost unchanged under these conditions (Figure 1). Surprisingly, the presence of a large excess of cyclohexene causes a dramatic acceler- ation of the decomposition of TPP and TPFPP until their concentrations have dropped to 5- 10% of the initial values. Subsequently, the concentrations decrease only very slowly during several hours. While TPFPP is only slightly less un- stable than TPP, it was gratifying to observe a significantly higher persistence of TDCPP under these conditions.

Monitoring the formation of cyclohexenyl hydroperoxide 2, we were struck by a second surprise. Up to about 40%

Liebigs Ann. 1995, 1495-1501

Tetra(2,6-dichlorophenyl)porphyrin 1497

Figure 1. Bleaching of TPP (O), TPFPP (*), and TDCPP (A) in irradiated solutions in dichloromethane (5 . lop4 M), saturated with oxygen and 0.09 M in pyridine, in the absence (---, upper straight lines) and in the presence of cyclohexene (1 M, -, curves with negative slope) and simultaneous formation of the allyl hydroperoxide

2 (determined as 3, -, curves with positive slope)

100

50

0 1 2 3 4 5 6 7 8 9 10

conversion of cyclohexene, 2 is formed with almost the same rates, virtually independent of the nature of the sensitisers. Only thereafter, the rates begin to differ corresponding to the amounts of sensitiser still present. Accordingly, the initial rate of formation of 2 slows down only very little when TDCPP is used as sensitiser, in contrast to the formation of 2 in experiments with TPFPP and - in particular

~~ TPP as sensitisers (Figure 1). Apparently, TDCPP is the sensitiser of choice when reluctant substrates are to be pho- tooxygenated with singlet-oxygen.

1 2 3

While the reactivity of 3,3,5,5-tetramethylcyclohexene (5) toward singlet-oxygen has not been investigated so far, the methyl groups of 5 most probably retard the singlet-oxygen ene reactionL41 rendering 5 less reactive than the parent cyclohexene. Therefore, the stability of the sensitiser was expected to be even more important for the former than in the photooxygenation of the latter. This expectation was not horn out by the experiment, however. Surprisingly, TPFPP persists for the time required for complete conversion of 5 to the corresponding allyl hydroperoxide (ca. 10 h). This persistence of TPFPP is in marked contrast to its instability in the photooxygenation of cyclohexene (Figure 1). - The allyl hydroperoxide derived from 5 was dehydrated to tetra- methylcyclohexenone 6 which was obtained in 70% (iso- lated) yield. Thus, this step completes the 1,2-carbonyl transposition in 4. The overall yield of the present sequence

is 60%, i.e. three times higher than that of a procedure in which an enamine derived from 4 is hydroborated in the key

#& ---c--, 86% --t, 10% ao

4 5 6

The photooxygenation experiments carried out on a preparative scale indicate similar reactivities toward singlet- oxygen of the cycloalkenes 1, 5, and 7a, 8a: About 50% conversion is observed after 3.5-4 hours in all cases. While the efficiency of TPP as sensitiser is still acceptable in the singlet-oxygen ene reaction of cyclohexene, it is unsatisfac- tory in the conversion of the very reluctant bicyclic allyl hydroperoxides 10a and l l a into the bis(ally1 hydroperoxides) 14a and Mac3]. The photooxygenation of the bi- cyclo[3.3.0]octadienes 7a and 8a offered, therefore, a strin- gent test for the persistence of the tetraarylporphyrins. As expected, their stabilities increase in the order TPP TPFPP < TDCPP when the sensitiser concentrations are monitored during the formation of the mono(ally1 hydroperoxides) 10a and l l a (Figure 2). The differences in stabilities appear even more pronounced in the second photooxygenation affording 14a and 15a: TPP has to be re- newed up to fifteen times, TPFPP once after 40-45 h, and TDCPP not at all before the experiment can be terminated after ca. 75 h. Thus, permanent supervision is obviated when TPFPP or, in particular, TDCPP are employed as sensitisers. Furthermore, decomposition products of the sensitiser are not formed any longer which could interfere in the purification of the crude products.

Monitoring of the product formation revealed the differ- ence in reactivities toward singlet-oxygen between the dienes 7a, 8a and the allyl hydroperoxides 10a, l l a . After ten hours, during which time more than 80% of the former originated, only about 10% of the latter made their appear- ance. As in the photooxygenation of cyclohexene, the rates of product formation do not correlate strongly with the sensitiser stabilities. Thus, notwithstanding bleaching, TPP is still reasonably efficient in the first step leading to 7a and Sa, albeit less efficient than TPFPP and TDCPP which do not differ significantly (Figure 2). It is the second photooxygenation which clearly demonstrates the superiority of TPFPP and, in particular, TDCPP over TPP.

The gratifying results obtained in experiments with TDCPP as sensitiser encouraged us to attempt photooxygenation of the [4.3.3]propelladienes 7b and 8b whose allylic subunits are more strongly encumbered than those of 7a and 8a as may be inferred on the basis of stereoselectivities observed in allylic bromination~[~~1. Indeed, the usual vigorous reaction conditions had to be maintained for no less than nine days before most of the monohydroperoxides 10b and l l b had disappeared. Nevertheless, only a single renewal of TDCPP was necessary after three days. Dehy- dration of the hydroperoxides and flash chromatography al-

Liebigs Ann. 1995, 1495-1501


Discussion a: R = M e

b: R-R = The remarkable dependence of the sensitiser stability on the different parameters presents intriguing puzzles. The slow bleaching in the absence of any photooxygenation substrate (Figure 1) is certainly due to the well-known “self-

caused by the action of singlet-oxygen. The dependence on the type of substrates, i.e. cyclohexene vs. 5 and 7a, 8a, and the remarkable concentration vs. time diagrainmes for both, sensitisers and allyl hydroperoxides, require interpretation (Figures 1 and 2). The formation of allyl hydroperoxides, though the concentrations of the sensitisers have decreased to 5-10% of the initial values, and the persistence of TPP and TPFPP at these low concentrations may be explained by assuming that, at low but more or less suficient levels of the sensitisers, their accumu- lated decomposition products may act as a preservative which protect the sensitisers against further destruction.

O R H O O R R R Identification of the substrate-dependent process, which destroys the sensitiser much more effectively than singlet- oxygen itself, poses a problem which is difficult to solve in

-(q)4- R

&)& &+& &+& @+#“W+&$

7 “ I 8

/ R R R R

12 9 10 I l1

13 14 15 16

lowed us to isolate the [4.3.3]propelladiene ketones as mixtures, i.e. 9b + 12b (2: l), or pure compounds 13b and 16b, albeit in low yields. This experiment discloses a limitation of the singlet-oxygen ene reaction and hence of the present 1,2-carbonyl transposition with concomitant dehydrogenation. While the allylic oxidation at one side of 7b and 8b affording the [4.3.3]propelladienones 9b and 12b may well be optimised on a preparative scale, an efficient oxidation at both sides remains an unsolved problem.

Figure 2. Stability of TDCPP ( 5 . M, Q) and bleaching of TPP (1 . M, *) in irradiated dichloromethane solutions, saturated with oxygen and 0.09 M in pyridine, in the presence of the dienes 7a and 8a (55:45, 1 M, curves with negative slope) and formation of the allyl hydroperoxides 10a + l l a + 14a + 1Sa (determined as 9a + 12a + 13a + 16a, curves with positive slope). The lower curve refers to the formation of the

M, 0) and TPFPP (1

bis(afiyl hydroperoxides) 14a + 15a in the experiments with TPFPP and TDCPP

0 1 2 3 4 5 6 7 8 9 10

view of the low concentrations involved. The following results are not compatible with the hypothesis that a sensitiser-destructing species is derived from or even identical with the allyl hydroperoxide formed. First, a solution of TPP in dichloromethane containing a large excess of tert- butyl hydroperoxide remains almost unchanged for days when kept in the dark. Second, the increase of the allyl hydroperoxide concentration during the photooxygenation is not accompanied by an enhanced bleaching of the sensitiser. Rather, the contrary is true. Third, when the hydroperoxide 2 is trapped in situ by dehydration with acetic anhydride, pyridine, and 4-(dimethylamino)pyridine, the period of time allowing survival of half of the TPP increases only little, that is from about 0.8 to 1.7 hours. Whatever the sensitiser-destructing species may be, it is much more eficient when it is derived from cyclohexene than from the methyl- substituted substrates 5 and 7a, 8a. This may be interpreted in terms of steric effects exerted by the methyl groups which moderate the reactivity of the unknown species in question. An alternative explanation of the substrate dependence of the observed photobleaching may be based on the hypothesis that allylic hydrogen atoms of the alkenes are ab- stracted by the excited tetraarylphorphyrins with rates de- pending on number and accessibility of the hydrogen atoms and on the shielding steric effects of the aryl

The results described in the foregoing section demon- strate the superior persistence and hence usefulness of TDCPP as sensitiser in singlet-oxygen ene reactions. These welcome properties parallel the unsurpassed stability of metal complexes of TDCPP as catalysts in cytochrom P450- type oxidation reaction~[’~.’~]. Apparently, the steric effect of ortho substituents, resulting in an almost perpendicular arrangement of the aryl groups of the porphyrin ring, is more important than electron-withdrawing effects for the stability of a tetraarylporphyrin in very different functions, i.e. as singlet-oxygen sensitiser and as metal complex cata- lysing oxidation reactions. This generalisation is supported

Liebigs Ann. 1995, 1495-1501


by the results of the present comparison between TPP, TPFPP, and TDCPP and of the search for oxidation-resist- ant metal c~rnplexes~ '~-*~I . It remains to be seen whether higher chlorinated tetra(2,6-dichlorophenyl)porphyrins[34,451 will be even better singlet-oxygen sen~itisers[~~]. As an obvious offspring of the present findings, water-soluble derivatives of TDCPP may be envisaged as promising can- didates in the photodynamic therapy of cancer[l91.

Finally, we note that the superiority of TDCPP over TPFPP occasions a fortunate consequence for the budget because 2,6-dichlorobenzaldehyde costs only less than one tenth of the current price of pentafluorobenzaldehyde. This may become particularly important when such tetraarylporphyrins will be tested as sensitisers on a very large scale, e.g. in solar energy

We thank Mrs. E. Ruckdeschel and Dr. D. Scheutzow for re- cording NMR spectra and Dr. G Lange and Mr. F. Dadrich for measuring the mass spectra. Financial support by the Fonds der Chemischen Industrie, Frankfurt am Main, is gratefully acknowl- edged. T. D. thanks the Fonds der Chemischen Industrie for a doc- toral fellowship.

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

Flawil, Switzerland. - 'H and I3C NMR: Bruker AC 250, WM 400. - IR: Perkin Elmer 1420. - 70-eV MS: Finnigan MAT 8200. -- UV/Vis: Hitachi U 3200 ([el = [I . mol-' . cm-'I), 1-mm cells. -- GC: Varian 1400 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); 35 mumin N2; conditions A: column temp. T = 40-170°C (4"C/min), t~ [min] = 2.6 (11, 7.8 (3), 11.5 (7a + 8a), 20.7, 21.2 (9a, 12a), 24.5 (13a), 31.4 (16a); B: T = 170°C, tR

[min] = 2.2 (7b + 8b), 4.3 (9b + 12b), 5.8 (13b), 11.0 (16b). Ratios were calculated from peak areas with neglection of specific re- sponse factors. - Flash chromatography: ( 5 5 X 5)-cm glass column packed with silica gel (32-63 pm) or neutral aluminium oxide (activity I) (both from ICN-Biomedicals), 1.8 bar N2. - HPLC: Waters M-6000A equipped with UV detector 440 (h = 254 nm); (250 X 4.6)-mm stainless steel column (Knauer) packed with Li- Chrosorb Si60, 5 pm (Merck); 1.5 ml/min petroleum ether (b.p. SO-70°C) (PE)/ethyl acetate (EA) (80:20), tR [min] = 3.4 (9b + 12b), 5.6 (13b), 13.8 (16b). - TLC: Silica gel or A1203 on aluminium sheets equipped with a concentration area (Merck).

Photooxygenation experiments and the investigation of the stability of the tetraarylporphyrins were carried out in a 200-ml immersion well apparatus equipped with an efficient condenser (kept at -2O"C), a glass sinter plate at the bottom for passing through of oxygen[37] and a 400-W sodium vapour lamp (OSRAM VIA- LOX NAV-TS 400 W). The immersion well was cooled with tap water, the apparatus in a cooling bath of - 12 "C. The oxygen used was dried by passing through a large drying tube filled with P205.

Petroleum ether (PE) was distilled through an efficient 2.5-m column. Acetic anhydride was distilled through a 1.2-m column. Ethyl acetate (EA) and CH2Clz were distilled under N2 from P20s, pyridine was distilled from CaH2. Cyclohexene (1) was extracted with a saturated aqueous solution of FeS04, dried with K2C03, distilled through a 30-cm Spaltrohr column (Fischer, D-53340 Mecken- heim), and stored at 0°C in the dark. Neutral aluminium oxide (activity 1) by ICN Biomedicals was used throughout. The alkenes 5[481, 7a + 8aL4*1 (55:45) and 7b + 8b[431 (1:l) were prepared as described.

5, I0,I 5,20- Tetra (pentafuorophenyl)porphyrin (TPFPP) : Ac- cording to procedures reported in the l i t e r a t ~ r e [ ~ ~ , ~ ~ ] . boron trifluoride-diethyl ether (1.0 ml, 0.95 g, 4.8 mm0l)[~~1 was added to a degassed and N2-saturated solution of freshly distilled pyrrole (1.68 g, 25 mmol) and pentafluorobenzaldehyde (4.9 g. 25 mmol) in CH2ClZ (1.5 1). The mixture was protected against daylight and stirred for 23 h. Tetrachloro-l,4-benzoquinone (6.15 g, 25 mmol) was added and the solution was heated at reflux for 2 h. The solvent was distilled i.vac. until the final volume was about 0.5 1. A1203 (40 g) was added and the rest of the solvent was distilled i.vac. The black solid material was placed on top of a dry flash chromatography column filled with A1203 (0.5-0.6 kg) and eluted with PE (30-70°C)/CHC13 (7:3, ca. 2 1). Distillation of the solvent i.vac. from the first fraction (detection at 366 nm) and drying of the residue at Torr yielded shiny blue-violet crystals (3.0 g, 50%, ref.: 27%[281, 54Y0[~~1) which were pure as indicated by TLC (A1203, PEIEA, 95:5) and UVNis. - UVNis (CHCI3): h,,, [nm] (lg E ) : 413 (5.494), 506 (4.337), 584 (3.834), 636 (3.01 l), 657 (2.966); ref.[28]: 41 1 (5.49), 505 (4.32), 582 (3.83), 636 (2.95).

5,l0,15,20-Tetra(2,6-dichlorophenyl)porphyrin (TDCPP) : The crude product, obtained from 2,6-dichlorobenzaldehyde according to the procedure described for TPFPP, was adsorbed on A1203 (40 g) which was extracted with hot methanol (0.5 1) in an extractor[49] until the refluxing solvent was colourless (24 h). Subsequently, the prepurified crude product was extracted with hot CHC13 (0.5 1, 5 h). A1203 (20 g) was added and the solvent was distilled i.vac. Flash chromatography (silica gellCHC1,) yielded brown-violet crystals (1.67 g, 30%, ref. 30%[28]) which were pure as revealed by TLC [silica gel, PE/CHC13 (7:3)] and UV/Vis. - UVNis (CHCI?): h,,, [nm] (lg E): 418 (5.428), 513 (4.154), 588 (3.666), 658 (2.818); 418 (3.54), 514 (4.38), 590 (3.97).

5,10,15,20-Tet~uphenylporphyrin (TPP): TPP was prepared according to a literature procedure[21,26] and purified with the method developed by Drenth et al. for TDCPPKZ81. The crude product (3.0 g) was adsorbed on A1203 by dissolving in CH2C12 (0.5 1) and removal of the solvent i.vac. after the addition of A1203 (40 g). The impurities were extracted from this material with hot methanol (0.5 1) in an extractor[49] until the refluxing solvent was colourless (24 h). Extraction with CH2CI2 (0.5 1, 7 h) followed by distillation of the solvent i.vac. yielded blue-violet crystals (2.5 g). Tetraphen- ylchlorin was absent as shown by the UV/Vis spectrum recorded from a solution in CH2C12 to which a few drops of a saturated solution of zinc acetate in methanol had been added[26].

Rates of Bleaching of the Tetraarylporphyrins and Formation of the Ally1 Hydroperoxides: Solutions of TPP, TPFPP, or TDCPP (0.10-0.21 mmol) and pyridine (1.5 ml, 18 mmol) in CH2C12 (190 ml) were irradiated in the absence or presence of the alkenes 1 or 7a + 8a (190 mmol) while a stream of dry oxygen bubbled through the reaction mixture. In a single experiment, 2 formed from 1 was dehydrated in situ to afford 3. In this case, acetic anhydride (18.8 ml, 196 mmol), pyridine (7.7 ml, 95 mmol), and 4-(dimethylamino)- pyridine (0.45 g, 3.8 mmol) were added before the irradiation was started. - Bleaching of the tetraarylporphyrins and formation of the ally1 hydroperoxides were monitored by taking small aliquots of the reaction mixtures. The decrease of the tetraarylporphyrins was calculated from the absorbance of a band in the Vis spectrum (1-mm cells, A,, [nm] = 514 (TPP), 506 (TPFPP), 513 (TDCPP)}. - To 1 ml of the reaction mixture, a mixture (0.2 ml) of acetic anhydride, pyridine, and 4-(dimethy1amino)pyridine (2: 1 :0.04) was added. After 0.5 h, the solution was extracted step by step with aqueous H2S04 (1 M, 1 ml), a saturated aqueous solution of NaHC03 and NaCl (1 nil), then dried with K2C03 and analysed

Liebigs Ann. 1995, 1495-1501


by GC. As an approximation, the sum of the peak areas of all compounds (alkenes and the unsaturated ketones) was taken as 100% in each case. Results: Figures 1 and 2.

Persistence of' TPP in the Presence of tert- Butyl Hydroperoxide: tert-Butyl hydroperoxide (2.37 g, 26 mmol) was added to a solution of TPP (8.3 mg, 14 pmol) in CH2C12 (25 ml) and the mixture was kept at 20-25°C in the dark for 52 h. During this time, the absorbance at 514 nm had decreased by no more than 9%.

4,4,6,6-Tetrumethylcyclohex-2-en-1-one (6): A solution of 5 (26.3 g, 190 mmol), TPFPP (200 mg, 0.21 mmol), and pyridine (1.5 ml, 18 mmol) in CH2C12 (190 ml) was irradiated for 10 h while a gentle stream of 0 2 bubbled through the reaction mixture. This was added dropwise to a stirred, freshly prepared mixture of acetic anhydride (22 ml, 0.23 mol), pyridine (9 ml, 0.12 mol), and 4-(dimethylamino- )pyridine (0.6 g. 4.6 mmol), cooled in an ice bath. After the addition, the mixture was stirred for 1 h without cooling. It was subsequently extracted twice with 200-ml portions of aqueous H2S04 (1 M) and saturated aqueous solutions of NaHCO,, CuS04, and NaCl. The aqueous layers were in turn extracted with CHzClz (4 X 50 ml). The combined organic layers were dried with K2C03. Distillation of the solvent and subsequently of the residue i.vac. afforded a pale yellow liquid (23.5 g, 70o/u), b.p. 75-85"C/15 Torr (ref.[42] 83-84"C/12 Torr). - ' H NMR: ref.[501. - I3C NMR:

Cyclohex-2-en-I-one (3): According to the procedure given for 6, a pale yellow liquid (13.5 g, 73%, b.p. 74"C/25 Torr, ref.[5] 78Y0) was obtained from 1 (15.6 g, 190 mmol).

I,5-Dimethylbicyclo[3.3.O]octu-3,7-diene-2,6-dione (13a) and 1,s- Dimethylbicyclo[3.3.O]octu-3,6-diene-2,8-dione (16a): A solution of 7a + 8a (25.5 g, 190 mmol, 55:45), TDCPP (89 mg, 0.1 mmol), and pyridine (1.5 ml, 18 mmol) in CH2CI2 (190 ml) was irradiated for 72 h while a gentle stream of O2 bubbled through the reaction mixture. This was added dropwise to a stirred, freshly prepared mixture of acetic anhydride (43 ml, 0.46 mol), pyridine (18 ml, 0.23 mol) and 4-(dimethy1amino)pyridine (1.1 g, 9.2 mmol), cooled in an ice bath. After the addition, the mixture was stirred for 1 h without cooling. Workup of the reaction mixture was performed as described for 6. The crude product (red-brown oil, 18 g) was dis- solved in CH2C12. The solution was filtered through a pad of silica gel (50 g, 32-64 pm) which was eluted with CH2C12 (300 ml). The solvent was distilled i.vac. until the final volume was about 50 ml. Flash chromatography on silica gel with PE/EA (85:15) yielded a pale green semisolid material (8 g) as the first fraction which was sublimed at 65-75 "C bath tern~. / lO-~ Torr. The sublimed product was recrystallised twice from ethanol to afford 13a as a colourless waxy solid (5.2 g, 37% based on 7a, m.p. 157-159"C, ref.L3] 158-160°C). - 'H, I3C NMR: ref.[52].

The flash chromatography column was eluted with EA to afford 16a as a yellow oil (7.6 g, %%I based on 8a) which slowly solidified. - 'H, I3C NMR: ref.c3].

Photooxygenation of 76 und 8b: A solution of 7b + 8b (30.5 g, 190 mmol, 1 :l), TDCPP (89 mg, 0.1 mmol), and pyridine (1.5 ml, 18 mmol) in CH2Clz (190 ml) was irradiated for 3 d while a gentle stream of O2 bubbled through the reaction mixture. TDCPP (89 mg, 0.1 mmol) was added again and the solution was irradiated for further 6 d. The reaction mixture was added dropwise to a stirred, freshly prepared mixture of acetic anhydride (43 ml, 0.46 mol), pyridine (18 ml, 0.23 mol) and 4-(dimethylamino)pyridine (1.1 g, 9.2 mmol), cooled in an ice bath. After the addition, the mixture was stirred for 1 h without cooling. Workup of the reaction mixture was carried out as described for 6 and 13a. Flash chromatography

of the dark red, almost black oil (20 g) in two portions on silica gel with PE/EA (85:15) yielded 3 fractions:

Tricych[4.3.3.0' 6]dodeca-8,10-dien-7-one (9b) and Tricyclo(4.3.- 3.0',6]dodeca-8, 11-dien-7-one (12b): Distillation of the first fraction (0.7 g, yellow oil) afforded a mixture of 9b + 12b (2: 1, 'H NMR) as a pale yellow oil (0.3 g), b.p. 80-86"C/8. Torr. - IR (film): 3 [cm-'1 = 1725, 1705 ( G O ) , 1580 (C=C). - MS, m/z (YO): 174 (100) [M+], 146 (39), 145 (45), 132 (48), 131 (71), 118 (38), 117 (59), 115 (30), 105 (22), 104 (29), 103 (26), 91 (73), 77 (30). - 9b: 'H NMR (400 MHz, CDCI,): 6 = 0.8-2.1 (4 CHJ, 2.35 (d, J = 17.9 Hz, 12-H*), 2.57 (ddd, J = 17.9, 2.1, 2.1 Hz, 12-HB), 5.55-5.61 (m, lO-H, 11-H), 6.04 (d, J = 5.7 Hz, 8-H), 7.54 (d, J = 5.7 Hz, 9-H). - 13C NMR (100 MHz, CDC13): 6 = 18.31, 18.39 (C-3, C-4), 28.6, 29.4 (C-2, C-5), 41.1 (C-12), 55.0 (C-l), 62.4 (C- 6), 129.275 (C-ll), 130.5 (C-8), 134.8 (C-lo), 168.9 (C-9), 214.9 (C- 7). - 12b: 'H NMR (400 MHz, CDC13): 6 = 0.8-2.1 (4 CHZ), 2.43, 2.50 (2 ddd, J = 18.8, 2.3, 2.1 Hz, 10-HA, 10-HB), 5.46 (ddd, J = 5.7, 2.1, 2.1 Hz, 11-H), 5.59 (ddd, J = 5.7, 2.3, 2.3 Hz, 12-H), 6.06 (d, J = 5.7, 8-H), 7.39 (d, J = 5.7 Hz, 9-H). - I3C NMR (100 MHz, CDCI,): 6 17.90, 18.03 (C-3, C-4), 26.5, 30.6 (C-2, C-5), 41.7 (C-lo), 53.2 (C-l), 63.9 (C-6), 129.275 (C-ll), 130.1 (C-8), 134.6 (C-12), 169.6 (C-9), 212.0 (C-7). - CIZHI4O: calcd. 174.1045, found 174.1045 (MS).

Tri~yclo[4.3.3.0'~~]dode~a-8,1l-diene-7,10-dione (13b): In the second fraction (2.8 g, yellow oil), crystals appeared after 10 d at - 15 "C which were recrystallised twice from ethanol to afford colourless crystals, m.p. 100°C. - 'H NMR (400 MHz, CDCI,): 6 = 1.4-2.1 (4 CH3, 6.05 (d, J = 5.7 Hz, 8-H, 11-H), 7.46 (d, J = 5.7 Hz, 9-H, 12-H). - I3C NMR (63 MHz, CDC13): 17.4 (C-3, C- 4), 26.9 (C-2, C - 9 , 61.2 (C-1, C-6), 131.3 (C-8, C-111, 163.6 (C-9, C-12), 206.7 (C-7, C-10). - IR (KBr): 3 [cm-'1 = 1695 (C=O), 1575 (C=C). - MS, m/z (%): 188 (95) [M+], 160 (loo), 146 (62), 132 (33), 104 (62), 91 (40), 77 (17), 51 (28), 39 (15).

Tricyclo(4.3 3.0',6]clodeca-8, 10-diene-7.12-dione (16b): The flash chromatography column was eluted with EA to afford a black oil (12.5 g) after evaporation of the solvent i.vac. Crystals appeared in the black oil after 7 d at - 15 "C. Repeated recrystallisation from ethanol yielded colourless crystals, m.p. 155-156°C. - 'H NMR (400 MHz, CDC13): 6 = 1.4-2.1 (4 CH2), 5.98 (d, J = 5.7, 8-H, 11-H), 7.60 (d, J = 5.7, 9-H, 12-H). - 13C NMR (63 MHz, CDC13): 17.74, 18.12 (C-3, C-4), 27.8, 28.5 (C-2, C-5), 59.6 (C-l), 64.3 (C-6), 130.4 (C-8, C-ll), 164.8 (C-9, C-lo), 202.4 (C-7, C-12). - IR (KBr): 3 [cm-'1 = 1720, 1680 (C=O), 1585 (C=C). - MS, m/z (%): 188 (72) [M+], 160 (loo), 146 (60), 132 (39, 104 (60), 91 (43), 77 (21), 63 (14), 51 (26), 39 (17). - CI2Hl2O2 (188.2): calcd. C 76.57, H 6.43; 13b: found C 76.88, H 6.42, 16b: found C 76.55, H 6.23.

"1 The results are part of the Dissertations by T. Dietz (1995) and A. Witzel (1994), both University of Wurzburg.

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Metal complexes of higher halogenated tetraarylporphyrins are less stable and efficient than those of TDCPP as catalysts in hypochlorite e oxidation reactions[34]. P. Esser, B. Potlmann, H.-D. Scharf, Angew. Chem. 1994, 106, 2093-2108; Angew. Chem. Int. Ed. Engl. 1994,33, 2009-2023. H. Quast, T. Dietz, Synthesis, in press. 0. Jiibermann, Methoden Org. Chem. (Houben- Weyl) 4th ed. 1958, vol. 111, p. 310. C. Paris, G. Torri, L. Elegant, M. Azzaro, Bull SOC. Chim. I+ 1974, 1449- 1452. J. Torr, M. Azzaro, Bull. Sac. Chim. Fr. 1974, 1633-1637. H. Quast, J. Christ, Liebigs Ann. Chem. 1984, 1180- 1192.

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Documents

Tetra(2,6-dichlorophenyl)porphyrin — a superior sensitiser for the singlet-oxygen ene reaction (Schenck reaction)