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12 H. GÜSTEN, L. KLASINC AND 0. VOLKERT

Molecular Orbital Calculation on 1.2-Difury 1- and 1.2-Dithienyl-ethylenes and Their Photocyclisation Products

the Benzodifuranes and BenzodithiophenesH. G ü st en , L. K la s in c * and 0. V o l k e r t * *

Institut für Strahlenchemie, Kernforschungszentrum Karlsruhe

(Z. Naturforschg. 24 b, 12— 15 [1969] ; eingegangen am 26. März 1968)

Es werden die HMO-Berechnungen der 1.2-Difuryl- und 1.2-Dithienyl-äthylene sowie die bei der photochemischen Cyclodehydrierung dieser Verbindungen entstehenden Benzodifurane und Benzo- dithiophene mitgeteilt. Die chemische Reaktivität dieser Verbindungen wird an Hand der berech­neten Reaktivitätsindices vorausgesagt.

During the last year a series of publications has

dealt with the photochemistry of 1 ,2-difuryl- and

1,2-dithienyl-ethylenes 1 3. These compounds under­

go photochemical cis-trans isomerization 1 as well as

1,2-Di- (2,3'-f uryl) -ethylene

.493(.526)526)

,450(.435)

t+198 -94(+,97) .519 ,478 (*62) ,0 . (.710)0615) ,11,

■95 " 5 9----+192

r » U / a ,*’69(-97)

.464 (.469) -34 -20

(̂-133) (-123)̂

.540 .541(.716) («63)

cis - 1

hv

.4970620)

1(*53)

Ew =18,5557

Et =17,2507

Benzo [1,2-b :3,4-b'] -difuran,501

(.566)

+ 166 (+245)

-100 (+3)

.11 >

,501 (.595)

p . & ™ \ sl

,5017̂,70)

/.504

(.621)

6-̂24101

(-62) -51f

(-68)\p .697

.597 w

ft-% ",58)2

(-56)̂,420 ,421

(,731) (.555)

£„ = 20,0037 E* =18,4859

En = total 7r-energy for the ground state. E,T* = total .T-energy for the excited state.

1,2-Di (2,3/-thienyl) -ethylene

.451(.443

Benzo [1,2-b :3,4-b'] -dithiophene

.496(.567)

,498(.565)

\

+190 -73(+215) ,523 ,480 (**7)

^ S v C692)(£01) 11 v.

4

.503(«61)

6-̂ 75-74(-74)

.465(.465)

5-69VJ *.9 #/-18

-4 -16 ^(-92) (-73)

,543 ,M5(.668) (.609)

C/5 - 1a

.333- S+183(+147)

En =15.1340

E t=13.8908

/

6-^< 76 -

:-6D

,490(.591)

/(-119)

.425(.671)

\-0/-7

2-2-38(+14)̂

.413(497)

+150(+197)

1 aEh =16.6068

Ej; =15.1915

* On leave from institute “Ruder Boskovic”, Zagreb, Yugo­slavia.

** Present address: Badische Anilin- und Soda-Fabrik, Lud­wigshafen.

1 C. M. O klando , A. A. Z im m erm a n , M. H. G ia n n i, and K. W e iss , J. org. Chemistry, in press.

2 R. M. K ellogg , M. B. G ro e n , and H. W yn b erg , J. org. Che­mistry 32, 3093 [1967] ; H. W yn berg and M. B. G roen , J. Amer. chem. Soc. 90, 5339 [1968].

3 C. E. L o ader and C. J. T im m o n s , J. chem. Soc. [London] (C) 1967, 1677.

MOLECULAR ORBITAL CALCULATION 13

1,2-Di- (3-furyI) -ethylene Benzo [1,2-b :4,3-b'] -difuran

t 534) +198 +198 (*534) (*778) +166 +166 /(+195) ,519 ,519 (+195) (,7Z8) (+144) (+144) (>

\ (.746)(.746) (K / \ /\ (,74§H746) CK s X ,C ^ /

iS » W A § § I

/™ » 335 fa ? ) «\ A m # s■46* n,_935_ 2 ,464 ,503 N -™ -2 ,503

Ue8) ^33) (-1̂ U68) (K*> ^ i ) (45K <■»*>

(.717) (.7T7) (it? 2) (A72)

. „ EK= 18,5549 , Ew = 20.0005

C/S" E 51=17,2076 Eft=18,4450

1,3-Di- (2-thienyl) -ethylene

,498 (.584)

/ J!>vq-18 J<̂ ?(0)

('584) 4190 +19°'• y ’ (+213) .523 ,523 (*213)

Y (.733) (.733)

6̂ 375 .-69\j (-9)

(-75)

.465(.463)

^ '% j l .1 1-*-74 J?/-G9 V (-9)

^ .935 y

•13 -13

/,542

(.660)

■(-84) (-84̂.542

(.660)

•74(-75)

\.465

(.463)

c/5- 3aEjt=15.1286

E j =13.8420

Benzo [2,1-b :3,4-b'] -dithiophene

.512(.683)

V

+150(+335)

(-172).,7 ^

/.497(.579)

(-57)

+ 150 (+335)

Ar* .620 V §

¥ 4

.512(.683)

1:-i72)

.697(-134)\̂

.-26 -26 /(+50) (+50)\̂

yi-134)

77(-57)

.497(.579)

,421(.518)

.421(.518)

3aEn=16,6051

E j =15.2007

1,2-Di- (2-furyl) -ethylene

451

T.450(.437)

(+51)

.450 (.437)

-94 -94(-65) .478 .478 (-65)

G I R/Ytt t fi/Vtt 11

-8/(-124)

,541(£68)

Benzo [1,2-b :4,3-b'] -difuran

.500 .500(.557) (.557)

tI

-102 -102(-16) (-16)

^ .694

-38 -38 /(-96) (-96)-y

.421 .421(£14) (.614)

EK = 20.0013

Ek =18.5587

1,2-Di- (2-thienyl) -ethylene Benzo [1,2-b :4,3-b'] -dithiophene

.451(442)

.451(.442)

.502(.635) -73 -73

(-46) .480 .480 (-46) ,5 . (.588) (.588) 11.

.502 (.635)

____ /

(+20^ % 4 \ A > % J f ^ 0)^Ä-56IK6rL Z j

333_y \j£L-S — 7 9 ^+ 1 8 3

-18Vi <g/-10 (+153)(+9)'@>___$ / (+9)

S~:+183(+153)

:17—A-80)

,536(.619)

.536(.619)

(-60)̂

c is-2aE-^=15.1392

E£=13,9338

,513(.690)

\(-43)f

.494 .494(.555) (.555)

t 1-77 -77(-16) (-16)

.96) (-9^

,6~59T

.513 (.690)

.1̂ /

+148(+220)

.-26yt-60)

,416(.567)

\j0i.>̂/-17 $/ ( -6)

(-43)

s+148(+220)

—2-26

(-60)\

.416(.567)

2 aE* =16,6066

E * =15,2575

14 H. GÜSTEN, L. KLASINC AND O. VOLKERT

photocyclisation 2’ 3. In presence of oxidizing agents

1 .2-dithienyl-ethyIenes yield benzodithiophenes2.

Similarly l,2-di-(2-furyl)-ethylene results benzo[l,2-

^4,3-^] -difuran 3.

By means of this photocyclodehydrogenation the

unsubstituted compounds of benzodifuranes and

benzodithiophenes which besides the benzo[l,2-b:

4.3-^] -dithiophene4 are unknown, are now avail­

able.

The present communication reports a theoretical

study of some 1 ,2-difuryl- and 1 ,2-dithienyl-ethy-

lenes in order to get some information about the

mechanism of the photocyclisation. Furthermore we

report the reactivity indices of the benzodifuranes

and benzodithiophenes regarding substitution reac­

tions as to predict the positions of electrophilic,

nucleophilic and radical attack.

Theoretical Approach

In general the photocyclisation of 1 ,2-difuryl-

and 1 ,2-dithienyl-ethylenes produces six possible

isomers of benzodifuran as well as benzodithio­

phenes. From chemical argumentations accompany­

ing the photocyclisation only the following isomers

should be stable compounds: benzo[l,2-b:3,4-b ]-

difuran (T) and -dithiophene ( la ) , benzo[l,2-b:

4.3-b ] -difuran (2) and -dithiophene (2 a) and the

benzo[2,l-b:3,4-b/]-difuran (3) and -dithiophene

(3 a ) . To these six molecules and the corresponding

three difuryl- and dithienyl-ethylenes we applied the

MO-theory within the framework of the H ü c k e 1

method 5. The assumption was made that all investi­

gated molecules are planar. The empirical para­

meters used in the calculation are shown in table I.

A similar choise of parameter has been given by

S a p p e n f i e l d and K r e e v o y 6 for furan and thio-

phene. We calculated the rc-bond orders, the ^-elec­

tron densities, the free valence indices and the total

n-energy of the molecules for the ground and

the excited state. The results are presented in the

molecular diagrams showing the starting 1 ,2-disub-

stituted ethylene and the resulting photocyclodehy­

drogenation product. All numbers in brackets refer

to the excited state. The charge densities are multi­

plied by 1000. The numbers at the end of the ar­

rows represent the free valence indices. For the seek

of simplicity the numbering of the atoms in our ske­

letal structures is somewhat different. We always

count the ethylenic carbon atoms in the difuryl- and

dithienyl-ethylenes as 1 and 2, the heteroatom in

the rings as 3 and 8 respectively, and the numbering

of the remaining ring carbon atoms is going clock­

wise upward. The same numbering is applied to the

photocyclodehydrogenation product.

Discussion

The mechanism of the photocyclisation reaction

of cz's-stilbenes is not completely cleared as yet7_9.

Several authors assume that the photochemical part

of the reaction proceeds from the excited state of

the czs-compound to the ground state of the cyclisa-

tion product7-9. An other discussed mechanism

suggests that the photochemical ring closure pro­

ceeds via vibrationally excited ground state levels

of the cw-compound which are readied in a fast

Si —> S0-deactivation process from the short living

excited singlet state of the czs-compound (Lewi s-

mechanism)10. In a study of the photocyclisation

Ethylene- 1,2-di- Benzodi--furanes -thiophenes -furanes -thiophenes

ai a + 0.02 ß a + 0.02 ß a aax* a + 1.12 ß a + 0.93 ß a+ 1.72 ß a + 0.93 ßa4 a a a a«5 a - 0.04 ß a - 0.02 ß a - 0.04 ß a - 0.02 ßßl2 1.02 ß 1.02 ß All resonance integrals in theßl (ring) 0.54 ß 0.60 ß resulting benzene ring are equal to ßßii 0.76 ß 0.50 ß 0.76 ß 0.50 ß/?56 0.59 ß 0.64 ß 0.59 ß 0.64/3

Table I. Resonance and Coulomb Integrals. * x=heteroatom.

4 D. S. Rao and B. D. Tilak, J. Sei. ind. Res. [New Delhi] Sect. B 16, 65 [1957]; C. A. 51,13841 [1957].

5 E. Hückel, Z. Physik 70, 204 [1931].6 D. S. S a ppen f ield and M. K reev o y , Tetrahedron [London]

19 (Suppl. 2), 157 [1963],7 F. R. S t erm it z , in : O. L. C h apm an ’s “Organic Photoche­

mistry I ”, M. Dekker Inc., New York 1967, p. 247.

8 K . A. M u szk a t and E. F isch e r , J. diem. Soc. [London] (B) 1967, 662.

9 M . S ch o lz , F . D ie tz , and M . M ü h ls t ä d t , Z. Chem. 7, 329[1967].

10 H. G üsten and L. K lasinc , Tetrahedron [London] 24, 5499[1968].

MOLECULAR ORBITAL CALCULATION 15

of para- and meta-substituted cis-stilbenes we were

able to show that the quantum yield of the photocycli-

sation 11 is going linear with a reactivity index qTS,

being the product of the electron densities of the

reacting o,(/-positions in the ground state 10. Apply­

ing this criterion to the photocyclisation of the di-

furyl- and dithienyl-ethylenes i. e. multiplying the

excited and ground state .T-electron densities of the

reacting positions we obtain the following results.

cis-1* cis-la cis-2 cis-2 a cis-3 cis-3 a

q t , 1.041 1.087 1.132 1.115 0.970 1.092

?rs 1.085 1.086 1.184 1.145 0.994 1.032

Table II. grs-values for difuryl- and dithienyl-ethylenes.

The q*s- and grs-values predict a photocyclisation

probability in the order of cis-2 > cis-2 a > cis-

3 ä > cis-1 a > czs-1 > cis-3 and cis-2 > cis-

2 a > cz's-1 a > cts-1 > cis-3 a > cis-3, respec-

tivly. According to the experimental data 2’ 3 the cis-

2-, ci's-1 a, cis-2 a- and cis-3 a-compounds undergo

photocyclisation. With the exception of 3 a the oxi­

dation products 2a (90%), 2 (24%) and l a (47%)

have been isolated 2’ 3 (yield in parenthesis). In cis-

3 a the abstraction of hydrogen leeds to a ß-thio- substituted radical which undergoes very rapid eli­

mination of the sulfur as thiyl radical **. From our

theoretical data we can expect that 1 can be ob­

tained by photocyclodehydrogenation and 3 pro­

bably not. On the contrary to the HMO-calculations

for the photocyclisation of mono-substituted cis-

stilbenes10 the results of the HMO-calculation for

the photocyclisation of 1 ,2-difuryl- and dithienyl-

ethylenes give no distinction whether the photo­

cyclisation proceeds via the excited or the ground

state of the czs-compound. Although the predicted

11 H. J ungmann , H. G üsten , and D. S chulte-Fro h l in d e , Chem. Ber. 101, 2690 [1968].

* cj's- 1 is the corresponding 1,2-disubstituted ethylene yield­ing 1 on photocyclodehydrogenation.

** see 2 and papers cited there.12 J . N . M u r r e l l , S . F. A. K ettle , and J. M . T e d d e r , “Valence

Theory”, John Wiley, London 1965, p. 317.13 Derivatives of benzo[l,2-b :3,4-b']-difuran: T. T o m in a g a , J.

pharmac. Soc. Japan [Yakugakuzasshi] 76, 1385 [1956] ;G. R. K elk a r and D. B. L im a y e , Rasayanam [J. Progr. ehem. Sei.] 1, 228 [1943] ; C. A. 36, 1037 [1942] ; D. B. L im a y e and V. V. N a g a r k a r , ibid. 1, 255 [1943] ; C. A. 38,

4264 [1944].14 Derivatives of benzo[l,2-b:4,3-b']-difuran: R. Royer, E.

B is a g n i, A. M . L a v a l-Jeantet , and J. R. M a rq u et , Bull. Soc. chim. France 1965, 2607; R. Royer, E. B is a g n i, C.

order of the ^ r8-values (ground state) fits better

the experimental data than the q * s -values, we feel

that the small differences between the qrs- and q*s-

values do not permit a discrimination on the me­

chanism of the photocyclisation.

In predicting the chemical reactivities shown in

table I I I we have followed the isolated molecule ap­

proximation 12 from which we can expect a correla-

Compound Eleetrophilic Nueleophilic Radical

1 6, 11 > 2 * 12, 7 > 1 6 > 7 , 11, 122 5, 11 > 1, 2 4, 12 > 1, 2 4, 12 ^ 5 , 113 6, 10 > 1, 2 7, 9 > 1, 2 6, 10 ^ 7, 9

la 6, 11 > 2 12 7 > 1 12, 7 > 62a 5, 11 > 1, 2 4, 12 > 1, 2 4, 12 > 5, 113a 6, 10 > 1, 2 7, 9 > 1,2 7, 9 > 6, 10

cis-1 6 > 11 > 10 4 > 12 > 7 1, 2 > 4cis-2 5, 11 > 6,10 4, 12 > 1,2 1,2 > 4 , 12cis-3 6, 10 > 1, 2 4, 12 > 7, 9 1,2 > 4 , 12

cis-la 6 ^ 11 > 10 1 > 4 > 2 , 12 1 > 2 > 4cis-2a 5, 11 > 6, 10 1,2 > 4 , 12 1, 2 > 4, 12cis-3 a 6, 10 > 7, 9 1,2 > 4 , 12 1,2 > 4 , 12

Table III. The Prediction of Chemical Reactivity Based on Charge Densities and Free Valence Indices. * The numbers

refer to the numbers in the skeletal structures.

tion between the electron densities and the attacking

charged species. For radicals the correlation with

the free valence proved to be successful. The most

reactive position in benzodifuranes and benzo-

dithiophenes is generally in the five membered rings.

In the difuryl- and dithienyl-ethylenes this position

tends to be on the ethylenic double bond (especially

for radical attack). Although not much synthesis has

been done on these molecules as yet the available

results support our predictions 4> 13_17.

We thank Priv.Doz. Dr. D. Schulte-Frohlinde for his support of this work.

H u d r y , A. C hen tin , and M. L. D esv o ye , Bull. Soc. chim. France 1963, 1003; R. P u m m e r e r , E. B uchta , E. D e im l e r ,

and E. S in g e r , Chem. Ber. 75 B, 1976 [1942] ; O . D ischen-

d o r f e r , Mh. Chem. 66, 201 [1935].13 Derivatives of benzo [2,1-b :3,4-b'] -difuran: R . R o y er , J. L.

D erocqu e , P . D em ersem a n , and A. C h e n t in , C . R . Ser. C ,

262, 1286 [1966] ; W . L imontschew and E. W ie se n b e r g e r ,

Mh. Chem. 83, 137 [1952] ; O. D isc h e n do rfer and W . L i ­

m ontschew , ibid. 80, 741 [1949].16 Derivatives of benzo[l,2-b:3,4-b']-dithiophene: D. S. R ao

and B. D. T il a k , J. sei. ind. Res. [New Delhi] Sect. B 13, 829 [1954] ; C. A. 50, 934 [1956].

17 Derivatives of benzo[2,l-b:3,4-b']-dithiophene: D. S. R ao

and B. D. T il a k , J. sei. ind. Res. [New Delhi] Sect. B 17, 260 [1958]; C. A. 55, 22282 [1961].