3420
J . Org. Chem. 1986,51, 3420-3428
1720 cm-'; 'H NMR (CDClJ 6 2.41 (3 H, s), 5.37 (2 H, AB q, J = 13.9),5.94 (1 H, d, J = 3.9), 6.00 (1H, d, J = 3.9), 7.60-8.24 (8 H, m); UV (CHCl,),,A 270 nm ( e 15927),306 (13230). Anal. Calcd for CZ2H15N307S: C, 56.77; N, 9.03; H, 3.25. Found C, 56.53; N, 8.95; H, 3.31.
2a,103365-75-9;2b,103304-86-5;2c,103304-87-6;3a,103365-74-8; 3c, 103304-88-7;4a,91618-80-3;4b,103321-10-4;4c, 103304-90-1; 5a, 103304-89-8;6a,9168582-4;6c,103365-73-7;7a, 103420-11-7; 712, 103365-78-2;Sa, 103365-76-0;SC, 103365-77-1;9a,103365-79-3; 9c,103365-81-7;loa, 103365-80-6;lOc, 103304-91-2;lla,8699874-5; llb, 103304-92-3;llc, 103304-93-4;12a,103320-89-4;1212, 103304-94-5.
Registry No. la, 86998-70-1;lb, 103304-84-3;IC,103304-85-4;
Photochemical Transformations of l-Imidazolyl-1,2-dibenzoylalkenes. Steady-State and Laser Flash Photolysis Investigations' Rabindra Barik,2a Kankan Bhattacharyya,2bParitosh K. Das,*2band Manapurathu V. George*2ab Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India, and Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received February 7, 1986
The photochemistryof a number of l-imidazolyl-l,2-dibenzoylalkenea and l-benzimidazolyl-l,2-dibenzoylalkenes has been investigated by steady-state photolysis combined with product analysis and laser flash photolysis. In several cases, the intramolecular phenyl group migration leading to ketene-mediated 3-butenoicacids and esters is observed. In addition, depending on the substituents present in the imidazolyl and benzimidazolyl groups, a variety of phototransformations occur; these include electrocyclic ring-closure reactions leading to dihydrophenanthrene and dihydroisoquinolinederivatives and photofragmentation reactions resulting in the loss of the imidazolyl moieties from the parent dibenzoylalkenes. Plausible mechanismsfor these photoreactionsare discussed. Laser flash photolysis in several cases gives rise to transient processes related to ketene and zwitterionic intermediates. Photorearrangements of dibenzoylalkenes are known to give ketene-derived products and lactones, in addition to cis-trans isomerization product^.^-^ As a part of our continuing studies on the photorearrangements of 1,2dibenzoylalkenes, we have recently examined the phototransformations of several substrates containing 1,2-dibenzoylalkene moieties such as 1,4- and 1,2-epoxy comp o u n d ~ , diben~obarrelenes,"-'~ ~,~~ l-pyrazolyl-1,2-dibenzoyialkene~,'~J~ and l-a~iridinyl-l,2-dibenzoylalkenes.~~
Scheme I
I a-e
2
2 0 ) 66%
2 a)
22%
b l 74%
b) I I%
c ) 74%
cl 4% dl 35%
el 6 %
(1) Document No. NDRL-2824 from the Notre Dame Radiation Laboratory. (2) (a) Indian Institute of Technology. (b)University of Notre Dame. (3) Griffin, G. W.; O'Connell, E. J. J. Am. Chem. SOC.1962, 84, 4148-4149. (4) Zimmerman, H. E.; Durr, H. G. C.; Lewis, R. G.; Braun, S. J.Am. Chem. SOC.1962,84, 4149-4150. (5) Padwa, A.; Crumrine, D.; Shubber, A. J. Am. Chem. SOC.1966,88, 3064-3069. (6) Sugiyama, N.; Kashima, C. Bull. Chem. SOC.J p n . 1970, 43, 1875-1877. (7) Zimmerman, H. E.; Diirr, H. G. C.; Givens, R. S.; Lewis, R. G. J. Am. Chem. SOC.1967,89, 1863-1874. (8) Lahiri, S.; Dabral, V.; Chauhan, S. M. S.; Chackkachery, E.; Kumar, C. V.; Scaiano, J. C.; George, M. V. J . Org. Chem. 1980, 45, 3782-3790. (9) Murty, B. A. R. C.; Kumar, C. V.; Dabral, V.; Das, P. K.; George, M. V. J . Org. Chem. 1984,49, 4165-4171. (10) Murty, B. A. R. C. Ph.D. Thesis, Indian Institute of Technology, Kanpur, 1982. (11) Kumar, C. V.; Murty, B. A. R. C.; Lahiri, S.; Chackkachery, E.; Scaiano, J. C.; George, M. V. J. Org. Chem. 1984, 49, 4923-4929. (12) Kumar, C. V. Ph.D. Thesis, Indian Institute of Technology, Kanpur, 1981. (13) Murty, B. A. R. C.; Pratapan, S.; Kumar, C. V.; Das, P. K.; George, M. V. J. Org. Chem. 1985,50, 2533-2538. (14) Lohray, B. B.; Kumar, C. V.; Das, P. K.; George, M. V. J. Org. Chem. 1984,49,4647-4656.
0022-3263/86/1951-3420$01.50/0
In general, it has been observed that photorearrangements of substituted 1,2-dibenzoylalkenes strongly depend on the nature of the substitutes present in them. The object of the present investigation has been to examine the phototransformations of some selected 1imidazolyl-1,2-dibenzoylalkenesand l-benzimidazolyl1,2-dibenzoylalkenes, containing suitably positioned substituents which are capable of undergoing other types of photoreactions, besides the 1,2-dibenzoylalkene rearrangement. In addition, laser flash photolysis studies have (15) Lohray, B. B. Ph.D. Thesis, Indian Institute of Technology, Kanpur, 1983. (16) Barik, R.; Kumar, C. V.; Das, P. K.; George, M. V. J. Org. Chem. 1985,50, 4309-4317.
0 1986 American Chemical Society
J. Org. Chem., Vol. 51, No. 18, 1986 3421
Steady-State and Laser Flash Photolysis Investigations Scheme I1
a) R' =R~=F?=c,H, b) R' = ; R2=R !I '6% C)
R ' =C6Hs; R 2 = R3*CH3
dl R' =C6H5;
-
8)
R2, R3. (CH = CH )2R ' SH;
R2, R = - (C H * C H 12-
8 , R4=H 7 a ) R2.R3*C6H5
9 ,R4=OCH3
-,
R2,R3=-(CH=CHlf
c ) R2.R3*CH5
d ) R2, R x -(CH
I_od)R'=C6%;RS=H;
* CH)p
+
x
N'
e) R ' = # = H ; R2,R3a-(CH= CH),N-H
d ) R ' 'C6H5;R5-CH3;
-
R2, R3= (C H = C H 10-e
-,
been carried out to characterize the transients involved in these photoreactions. The substrates that we have examined in this study include the dibenzoylethylenes 3a-e and 4d.
Results and Discussion (1) Preparation of Starting Materials. The imidazolylethylenes 3a-e and 4a-e were prepared by refluxing equimolar amounts of the imidazole la-e and dibenzoylacetylene (DBA, 2) in acetonitrile (Scheme I). The geometry across the carbon-carbon double bond has been ascertained on the basis of their electronic spectra.'"ls The E isomers 3a-e had A- 251-271 nm (e 27 82240); the Z isomers 4a-e absorb around 260-280 nm with higher extinction coefficients (29 800-53 000). (2) Preparative Photochemistry and Product Identification. Irradiation of 3 s in benzene gave a mixture of the dihydroisoquinoline 7a (18%), dihydrophenanthroimidazolyl derivative 5a (22%), phenanthroimidazolyl derivative 6a (20% ), 2,4,5-triphenylimidazole (la, 17%), and trans-1,2-dibenzoylethylene (8, 6 % ) (Scheme 11). Similarly, the irradiation of 3a in methanol gave a mixture of 7a (lo%), 5a (15%), 6a (25%), la (a%), and trans-l-methoxy-l,2-dibenzoylethylene(9,14%). The 'H NMR spectrum of 5a showed a doublet of doublets at 6 2.45 (1 H, J7e,7b = 18 Hz, J7,7s = 4.5 Hz), assigned to the 7a-H proton and a second doublet of doublets at 6 3.25 (1 H, JTa,,b = 18 HZ, J7b,8 = 6.5 HZ), assigned to the C7b-~ proton. The protons at C7 and C8 positions appeared as two quartets a t 6 5.25 (1 H) and 6.35 (1 H), respectively, whereas the aromatic and vinylic protons appeared as a complex multiplet centered at 6 8.0 (22 H).The 'H NMR spectrum of 6a showed a multiplet spread over the region of 6 6.95-8.95, assigned to the aromatic and vinylic protons. Air-oxidation of 5a in refluxing benzene gave a 819% yield of 6a. The lH NMR spectrum of 7a showed two doublets at 6 3.05 (1H, J = 15 Hz) ((2,-H) and 6 4.35 (1H, J = 15 (17)Dolfini, J. E.J. Org. Chem. 1965,30,1298-1300. a n ,M.P.;Prasad, R.;George,M.V. Tetrahedron (18)Lahiri,5.;m 1977,33,3159-3170.
e) R' - H i RS=CH3 R2, R '= (C H = C H Ip
-
Hz) ((2,-H). The relatively high coupling constant of 15 Hz (H5,6)indicates an anti relationship of C5- and c6protons in 7a. Similarly, the irradiation of 3b in benzene gave a mixture of the dihydrophenanthroimidazolylderivative 5b (20%), phenanthroimidazolyl derivative 6b (24% 1, 4,5-diphenylimidazole (lb, 35%), and trans-1,2-dibenzoylethylene (8,12%), whereas the irradiation of 3b in methanol yielded 16% of 5b, 28% of 6b, 38% of lb, and 17% of 9. Likewise, the irradiation of 3c in benzene gave a 31% yield of the dihydroisoquinoline derivative 7c, a 47% yield of IC, and a 15% yield of 8, whereas the irradiation of 3c in methanol gave a 25% yield of 7c, a 59% yield of la, and a 23% yield of 9. In contrast, the irradiation of 3d in benzene gave a mixture of the dihydroisoquinoline derivative 7d (13%), 2-[1-(2-phenylbenzimidazolyl)]-4-phenoxy-4-phenyl-3-butenoic acid (lOd, 9%), 2-phenylbenzimidazole (la, 58%), and trans-1,2-dibenzoylethylene(8, 18%1, whereas the irradiation of 3d in methanol gave a mixture of the methyl ester l l d (14%), 7d (8%), Id (65%), and 9 (27%). Similarly, the irradiation of 3e in benzene, under analogous conditions, gave the acid 10e (19%), benzimidazole (le, 56%), and truns-1,2-dibenzoylethylene(8, 18%); in methanol, the methyl ester lle, 22%), le (59%), and 9 (21%) were the products. The structures of the butenoic acids 10d and 1Oe and the methyl butenoates l l d and l l e were established on the basis of spectral evidence. The 'H NMR spectrum of 10d, for example, showed a doublet at 6 5.90 (1H, J = 8 Hz, D20-exchangeable),assigned to the methine proton and a second doublet at 6 6.45 (1H, J = 8 Hz),assigned to the vinylic proton. The aromatic protons appeared as a multiplet at 6 7.10-7.65 (19 H, m), whereas the carboxylic acid proton appeared as a broad singlet at 6 7.85 (1 H, D,O-exchangeable). The butenoic acids 10d and 1Oe were converted by treatment with diazomethane to the methyl esters l l d (68%) and l l e (72%), respectively. In the (2)-alkene series, irradiation of 4d in benzene gave a 8% yield of 7d, a 20% yield of the E isomer 3d, a 14% recovery of the unchanged starting material 4d, a 36%
Barik et al.
3422 J . Org. Chem., Vol. 51, No. 18, 1986 S c h e m e I11
,[ s'
)=(
H O
R2F? 7 a) R2=R3=C6%
-4
c) R2=R3=CH3 d) R 2 , R 3 = - ( C H = C H ) ~ S c h e m e IV
2 d) R ' =%Hs e) R' = H
Q d) R'=CgHs e) R' = H
yield of Id, and a 14% yield of 8. The formation of the different products in the photo3a-e can be reactions of l-imidazolyl-l,2-dibenzoylalkenes understood in terms of the reaction pathways shown in Schemes 111-V. It is quite evident that the photoreactions of 3a-e are strongly dependent on the substituents present in the imidazolyl component, and several pathways leading to different products are observed in these cases. One of the photoreactions observed in the case of 3a and 3b, containing phenyl substituents at the C4-and CB-positions of the imidazolyl moiety, for example, is a singlet-statemediated electrocyclic ring closure reaction, leading to the dihydrophenanthrene derivatives 5a and 5b, respectively
(Scheme 111, path a). Such ring closure reactions of substrates containing the cis-stilbene functionality, leading to dihydrophenanthrene derivatives, are well-documented in the 1iterat~re.l~The formation of the phenanthroimidazolyl-1,2-dibenzoylalkenes6a and 6b in the photoreactions of 3a and 3b, however, could be understood in terms of the air-oxidation of the dihydrophenanthrene derivatives 5a and 5b, respectively, under the reaction conditions. A second mode of reaction, observed in the case of those substrates containing a phenyl substituent in the C2-position of the imidazolyl component, as in the case of 3a, 3c, and 3d is again a singlet-statemediated 10 or 14 electron, electrocyclic, conrotatory ring-closure reaction, involving the phenyl substituent and the 1,2-dibenzoylalkenecomponent and leading to the zwitterionic intermediates l%a,c,d,as shown in Scheme I11 (path b). Subsequent proton shifts in 12a,c,dthrough a [1,4] or two successive (19) For some examples of the photocyclizations of stilbene derivatives, see: Starmitz, F. R. Organic Photochemistry; Chapman, 0. L., Ed.; Marcel Dekker: New York, 1967; Vol. 1, pp 247-282.
J. Org. Chem., Vol. 51, No. 18, 1986 3423
Steady-State and Laser Flash Photolysis Investigations [1,2] sigmatropic shifts will lead to the imidazolo[2,1a]-5,6-dihydroisoquinolinederivatives 7a, 7c, and 7d, respectively. I t may be mentioned here that similar electrocyclic ring-closure reactions have been observed earlier in the photoreactions of E-phenyl-substituted 1pyrazolyl-1,2-dibenzoylalkenes,leading to the corresponding pyrazolo[5,1-~]-5,6-dihydroisoquinoline derivatives.14J5 Yet another photoreaction, that has been observed only in the case of 3d and 3e in the present study, is the 1,2dibenzoylalkene proceeding through the ketene intermediates 14d,e and leading to the butenoic acids 10d,e or the butenoic esters 1ld,e, respectively, depending on the reaction conditions (Scheme IV, path d). On the basis of analogy14,15it is assumed that the ketene intermediates 14d,e are formed from the singlet excited state and involving singlet diradical intermediates 15d,e, as shown in Scheme IV (path d). It may be pointed out here that the observed regioselectivity, leading to the ketene intermediates 14d,e (Scheme IV, path d) as against 16d,e (Scheme IV, path c), could be understood in terms of the ground-state conformational preferences of the starting imidazolyl-l,2-dibenzoylalkenes3d,e. The conformers leading to the diradical intermediates 13d,e (path c) will be less populated due to steric crowding, as compared to those leading to the diradical intermediates 15d,e (path d) and hence the observed regioselectivity. It is not very clear why the 1,Zdibenzoylalkene rearrangement has been observed only in the case of 3d and 3e and not in the other substrates (3a,b,c) under study. Perhaps, other reactions such as the photofragmentations (SchemeV) and electrocyclic reactions (Scheme 111)may be predominant in these cases. Also, steric factors, arising through phenyl or methyl substituents at C4- and Cs-positions of the imidazolyl moiety, may be adversely affecting the 1,2-dibenzoylalkene rearrangement occurring in 3a,b,c. The formation of photofragmentation products such as imidazoles la-c or benzimidazoles ld,e, truns-1,2-dibenzoylethylene (8),and trans-l-methoxy-l,2-dibenzoylethylene (9) in these reactions could be understood in terms of pathways shown in Scheme V, involving zwitterionic intermediates 18a-e. A similtrr pathway has been suggested earlier for the formation of pyrazoles from 1pyrazoly1-1,2-dibenz0ylalkenes.~~ I t is also possible that small amounts of la-e and 8 in these reactions could arise through radical intermediates such as 19a-e and 20 (Scheme V). (3) Laser Flash Photolysis Studies. Preliminary studies using 337.1-nm laser excitation showed that the flash photolysis of 3a, 3c, and 4d in benzene and methanol and of 3d and 3e in benzene led to only very weak, featureless transient absorption phenomena at 350-700 nm in the time domains 0.1-100 ps. These systems were not pursued in any more detail. In the case of 3e in benzene (Aex = 337.1 nm), ground-state bleaching due to photochemical loss was noticed at
