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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].