J. Org. Chem., Vol. 39,No. 2, 1974
Cava, Lakshmikantham, and Behforouz
A. Kirrmann, J. Cantacuzene, and L. Vio, Rev. Chim. Acad. Repub. Pop. Rourn., 7,1011 (1962); Chem. Abstr., 61,8180a (1962). R . G. Hiskey, J . A. Kepler, and B. D. Thomas, J. Ofg. Chem., 29, 3684 (1964), S. F. Birch, R. A. Dean, N. J. Hunter, and E. V . Whitehead, J. Org. Chem., 30,1178 (1965).
(27) V. F. Kucherov, N. Y . Grigor'eva, T. M. Fadeeva, and G . A. Kogan, Izv. Akad. Nauk SSSR, Ofd. Khim. Nauk, 2196 (1962):Chern. Abstr.. 58, 1123501 (1962). (28) I . lchikizaki and A. Arai, Bull. Chem. SOC. Jap., 37,432 (1964). (29) N. L. Goidman, Chem. Ind. (London),1036 (1963). (30)'S.F. Reed, J. Org. Chern., 30, 3258 (1965)
A Photochemical Route to the Thieno[c]cyclobutene System M. P. Cava,* M. V. Lakshmikantham, and M. Behforouz Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19174 Received August 23, 1973 Photolysis of either of the cis or trans dihydrothienothiophene sulfones 15 and 16 affords primarily the thienocyclobutene 6; similarly, the methoxy sulfone isomers 18 yield the methoxythienocyclobutene 19. Both 6 and 19 undergo ready thermolysis to naphtho[c]thiophene derivatives. Evidence is presented which indicates that the thermolysis of 6 does not proceed via a tetravalent sulfur quinodimethane-type intermediate.
The first thieno[c]cyclobutenes have been reported only recently. These include the parent heterocycle l1 and the tetrahalo derivatives 2 and 3,* as well as the stable thienocyclobutadiene 4.3 Compounds 1 and 4 were prepared by constructing the thiophene nucleus by a Wittig synthesis; compounds 2 and 3 were prepared by a Finkelstein-type dehalogenation of an appropriate halo thiophene (5).
R3
Ph
/
Ph 10
Ph
'Ph 12 4
X 5
We now report the synthesis and properties of several 3,4-diphenylthieno[c]cyclobutenes(6, 7 and 8), employing the photochemical decomposition of a sulfone precursor as the key synthetic step.
Ph P h
g
Ph 6
Ph
9
Ph €& = R, = H 9- = C1; R, = Br (cis and trans) 3, rZ, C1; = R, = Br (cis and trans)
\
Ph
2
11
Ph
Ph
Ph
Ph
Ph
R, I
q
Ph. Ph$S
@ WPh
+SO2
ph)Q 1, 2,
Ph
s
Ph 7
;+$ OMe
Ph Ph 13, cis isomer 14, trans isomer
15, cis isomer 16, trans isomer
to give, in good yield, the crystalline trans thienocyclobutene 6. Careful chromatography of the photolysis residues afforded a very small amount of the corresponding cis isomer 7. The nmr spectra of 6 and 7 showed benzylic singlets a t 6 4.47 and 5.20, respectively; the corresponding reported values for trans-1,2-diphenylbenzocyclobutene (1 1)and its cis isomer are 6 4.42and 5.20,respectively.6 A two-step conversion of tetraphenylthien0[3,4-~]thiophene (17) to the methoxy sulfone 18 has been r e p ~ r t e d . ~ We have now found that sulfone 18 can be separated into two stereoisomers, A and B (mp 234" dec and 210" dec, respectively), both of which lose sulfur dioxide upon irradiation to give the same methoxycyclobutene 19. The nmr spectrum of 19, which is probably the trans isomer, shows a- single benzylic hydrogen a t 6 4.72 as well as a methoxyl signal at 6 3.00. Ph
Ph
Ph
Ph
Ph
Ph
Ph 18
Ph
8
Results The photochemical decomposition of the cyclic benzylic sulfones 9 and 10 affords a direct synthesis of the condensed cyclobutane aromatic hydrocarbons 11 and Consequently, we decided to investigate the applicability of this type of reaction in the thiophene series. Thus, peracid oxidation of the known dihydrothienothiophenes 13 and 145gave the corresponding sulfone isomers 15 and 16. Both 15 and 16 lost sulfur dioxide cleanly upon irradiation in benzene-methanol in the presence of barium oxide
Ph 17
Ph 19
The thienocyclobutene 6 is quite stable in solution at temperatures up to about 60". At 75", however, an nmr study showed that it rearranged completely in hexachlorobutadiene solution within 45 min. The product, which was isomeric with 6 and which showed a one-proton singlet at 6 5.42 and a two-proton singlet at 6 3.98,was assigned structure 20. Compound 20 was also obtained directly by the pyrolysis of sulfones 15 and 16 at their melt-
J. Org. Chem., Vol. 39, No. 2, 1974 207
Thieno[c]cyclobutene System via Photolysis ing points (>200"). Palladium dehydrogenation of 20 afforded the bright red naphtho[c]thiophene derivative 21. Ph
ph
Ph
-.
Ph
Ph
ph rii
11
I
27
Ph
9%
30
9
Ph
m,
21
A number of unsuccessful attempts were made to carry out an acid-catalyzed elimination of methanol from the methoxy cyclobutene 19, a reaction which we hoped would yield the thienocyclobutadiene 22. The failure to achieve this conversion appears to be due to the thermal lability of 19, which was completely converted to a complex mixture of products in a neutral deuteriobenzene solution in 30 min at 45". Preparative decomposition of 19 in refluxing benzene, or in benzene containing p-toluenesulfonic acid at 25", gave, after chromatographic separation, three isolable products. The major product was the red naphtho[c]thiophene 21. The second product was also red and had spectral properties similar to those of 21, although it contained a methoxyl group. It was therefore assigned structure 23. The third product was colorless and contained a carbonyl group, and was assigned structure 24, which is the expected acid hydrolysis product of 23. It has been shown7 that the pareht ketone of 24, namely 25, Ph
s$7J Ph 22
Ph
Ph
ph
s+$rJ Ph
OMe 23
R
Ph
12-
28
29
26
sents the first case of any condensed cyclobutane aromatic compound which decomposes thermally without the generation of an 0-quinonoid intermediate. Ph
R 4
I
SyJ
'
Ph
R
O
24, R = Ph
25,R=H
Ph
1
'CH.
I
ph 6
Ph
i Ph
20
32 Ph
exists in the keto form rather than in the phenolic form. The pyrolysis of either of the stereoisomers of methoxy sulfone 18 also gave the naphtho[c]thiophene 21. Discussion The thermal isomerization of the thienocyclobutene 6 to the isomeric dihydronaphtho[c]thiophene 20 is quite analogous to the thermolysis of the benzocyclobutene 11,8 the naphtho[b]cyclobutene 12,9 and the phenanthro[l]cyclobutene 261° to give the corresponding rearrangement products 27, 28, and 29. The rearrangements of 11, 12, and 26 all proceed by way of reactive o-quinodimethane intermediates ( i e . , 30), which can be trapped either by olefinic dienophiles or by sulfur d i ~ x i d es l.o + l~l For example, the intermediate 30 from 11 is intercepted quantitatively by sulfur dioxide in boiling carbon tetrachloride,'l or even in ether,12to give the sulfone 9. If the isomerization of 6 to 20 were to follow a similar pathway, the o-quinodimethane intermediate 31 would be an example of a new type of tetracovalent sulfur structure.13 Attempts to intercept 31 during the thermolysis of 6 with either sulfur dioxide or N-phenylmaleimide were completely unsuccessful; only the isomer 20 was formed in the presence of these reagents. We conclude, therefore, that the tetravalent nature of the sulfur in quinomethane 31 destabilizes this species to a considerable extent relative to the diradical 32, and that 32 is the actual intermediate in the conversion of 6 to 20. Concerted addition of sulfur dioxide or a dienophile to 32 would not be expected on the basis of electrocyclic reaction theory.14 Finally, it may be pointed out that the conversion of 6 to 20 repre-
31
The ultraviolet spectra of both benzocyclobutene and naphtho[ blcyclobutene show practically no effect of ring strain on the positions of the aromatic maxima.16Js In contrast, a p,p-fused four-membered ring causes a considerable bathochromic shift in the thiophene series. Thus, the thieno[c]cyclobutene 6 has as its band of longest wavelength a triplet of maxima centered a t 343 nm. Its thermal rearrangement product 20, in which an unconjugated six-membered ring has replaced the cyclobutene ring, has a corresponding unresolved band centered a t 310 nm. Since the analogous band of sulfide 13 is observed at 317 nm, it is clear that the major electronic distortion of the 2,5-diphenylthiophene chromophore occurs only when a four-membered ring is condensed to the heterocyclic nucleus. Experimental Section General. Melting points are uncorrected. Infrared spectra were determined in KBr, on a Perkin-Elmer Model 137 spectrophotometer. Ultraviolet and visible spectra were determined in cyclohexane solution, unless otherwise noted, using a Perkin-Elmer Model 202 instrument. Nmr spectra were recorded on a Varian HA-100 MHz machine. Photolyses were carried out under nitrogen using a Hanovia medium-pressure lamp with a Vycor filter. Oxidation of Sulfide 13 to Sulfone 15. A solution of 1.6 g of cis-1,3-dihydro-1,3,4,6-tetraphenylthieno~3,4-c]thiophene (13)6 in 180 ml of benzene-methanol (2:l) was treated with 50 ml of 40%
208
Cava, Lakshmikantham, and Behforouz
J.Org. Chem., Vol. 39, No. 2, 1974
peracetic acid. The mixture was refluxed for 1 hr. The crystalline cis sulfone 15 was filtered. The filtrate was cooled, cautiously treated with ammonia, and extracted (chloroform) to yield additional 15. Recrystallization of the total crude 15 from chloroformethanol afforded 1.3 g (76%) of pure 15: mp 290" dec; nmr (CDC13-DMSO) 6 5.86 (s); ir 7.55, 8.80 p (SOz); uv spectrum A,, (EtOH) 310 nm (log t 4.20);mass spectrum m/e (re1 intensity) 478 ( M A ,5),414 (M - 64,100). Anal. Calcd for C30Hz2S202: C, 75.31;H, 4.63.Found: C, 74.82;
H, 4.87. Oxidation of Sulfide 14 to Sulfone 16. The corresponding trans-dihydrothieno[3,4-c]thiophene( 14),5 upon peracetic acid oxidation in a similar manner, yielded sulfone 16, mp 225" dec, in 71% yield: nmr (CDC13-DMSO) 6 5.67 (5); ir 7.55,8.65 /A ( S o n ) ; uv spectrum Amax (EtOH) 312 nm (log e 4.20); mass spectrum m / e (re1intensity) 478 (MA, 6),414 (M- 64,100). Anal. Calcd for C ~ ~ H ~ ~ SC,Z 75.31; O Z : H, 4.63.Found: C, 74.62; H, 4.86. Photolysis of Sulfone 15 to the Thienocyclobutenes 6 and 7. A solution of 140 mg of 15 in 430 ml of benzene and 100 ml of methanol containing 100 mg of barium oxide was irradiated for 4 hr. The pale yellow solution was concentrated under reduced pressure (
