504 1
J . Am. Chem. SOC. 1983, 105, 5041-5046
Photochemical and Thermal Stereomutations of 3-Aryl-2-propenylideniminiumSalts’ Ronald F. Childs* and Brian D. Dickie Contribution from the Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M1 Canada. Received February 14, 1983
Abstract: The photochemical and thermal stereomutations of some 3-aryl propenylideniminium salts have been examined. Salts 1-7 in trifluoroacetic acid solution undergo efficient cis/trans isomerization about the C=C bonds on irradiation. These reactions involve singlet excited states of the iminium salts. Photoisomerization also occurs about the C=N bonds of 6 and 7, although the quantum efficiencies of these isomerizations were very much less than those about the C=C bonds. The thermally induced isomerizations of 9-13 to 1-5, respectively, were examined in trifluoroacetic acid and trifluoroacetic/sulfuric acid mixtures. A Hammett u’ correlation of the rate constants of these isomerizations as a function of the aryl substituent was markedly curved, indicating that at least two mechanisms were involved in these reactions. With variation of acid strength and the use of deuterated acids it was shown that the stereomutations take place by a Michael type nucleophilic addition when the aryl substituents are electron withdrawing and by protonation at nitrogen with electron-donating aryl substituents. This latter mechanism was confirmed by examination of deuterium exchange at nitrogen of some related n-butyliminium salts.
Unsaturated iminium salts are important compounds. They have commercial significance, for example in the cyanine dyes, and biological importance in the visual pigments. The chromophore of the visual pigment rhodopsin consists of 1 1-cis-retinal that is bound by a Schiff base linkage to the apoprotein opsins2 It is generally accepted that the Schiff base nitrogen is protonated3 and that the primary photochemical event in the natural pigment involves the cis to trans isomerization of the C11C12bond of the chr~mophore.~ In contrast to the extensive work on the chemistry of the visual pigments, comparatively little work has been reported on the photochemistry of simpler unsaturated iminium salts. Cis/trans isomerizations about C=C bonds conjugated with the iminium function have been o b ~ e r v e d . ~In , ~ addition we have recently shown that photoisomerization also takes place about the C=N bonds of these Salk6 The photocyclization of protonated benzalaniline to protonated phenanthridine presumably also involves an initial isomerization about the C=N bond.7 Mariano and co-workers have recently reported a series of photoreactions of iminium salts that involve electron transfer as a key step.* The mechanisms of the thermally induced stereomutations about the C=C bonds of unsaturated iminium salts have also received little a t t e n t i ~ n . ~This is surprising in view of the importance of such reactions in iminium salts. For example, bac(1) This work was supported by a grant from the Natural Science and Engineering Research Council of Canada. (2) Honig, B. Annu. Rev.Phys. Chem. 1978, 29, 31. (3) Ebrey, T. G.; Honig, B. Q.Rev.Biophys. 1975, 8, 139. Eyring, G.; Mathies, R. Proc. Natl. Acad. Sri. U.S.A.1979, 76, 33. (4) Rosenfeld, T.; Honig, B.; Ottolenghi, M. Pure Appl. Chem. 1977, 49, 341. (5) Scheike, G.; Heiss, J.; Feldmann, K. Ber. Bunsenges, Phys. Chem. 1966, 70, 52; Angew. Chem., Int. Ed. Engl. 1965, 4, 525. Guesten, H.; Schulte-Frohlinde, D. Chem. Ber. 1971, 104, 402. (6) Childs, R. F.; Dickie, B. D. J . Chem. Sot., Chem. Commun. 1981, 1268. (7) Badger, G. M.; Joshua, C. P.; Lewis, G. E. Tetrahedron Lett. 1964, 3711. El’tsov, A. V.; Studzinskii, 0. P.; Ogol’tsova, N. V. Zh. Org. Khim. 1970, 6 , 405. Perkampus, H.-H.; Behjati, B. J . Heterocycl. Chem. 1974, 11, 511.
(8) Stavinoha, J. L.; Mariano, P. S.; Leone-Bay,A,; Swanson, R.; Brachen, C. J. Am. Chem. Soc. 1981,103, 3148. Stavinoha, J. L.; Mariano, P. S. Zbid. 1981, 103, 3136. Mariano, P. S.; Stavinoha, J. L.; Bay, E . Tetrahedron 1981, 37, 3385. Mariano, P. S.; Leone-Bay, A. Tetrahedron Lett. 1980, 4581. Mariano, P. S.; Stavinoha, J. L.; Pepe, A,; Meyer, E. F., Jr., J . Am. Chem. SOC.1978, 100, 7114. Mariano, P. S.; Stavinoha, J. L.; Swanson, R. Ibid. 1977, 99, 678 1. (9) Schulte-Frohlinde, D.; Guesten, H.; Liebigs Ann. Chem. 1971, 749, 49. Guesten, H.; Schulte-Frohlinde, D.; 2. Naturforsch., B 1979, 34B, 1556. Le Botlan, D.; Filleux-Blanch, M.-L.; Le Coustumer, G.; Mollier, Y . Org. Magn. Reson. 1974, 6, 454. Jennings, W. B.; AI-Showiman, S.; Tolley, M. S.; Boyd, D. R. J . Chem. Soc., Perkin Trans. 2 1975, 1535.
0002-7863/83/1505-5041$01 S O / O
teriorhodopsin, the light-harvesting pigment of Halobacterium halobium, in the dark-adapted state contains the 13-cis and all-trans forms of the retinal iminium salt chromophore in thermal equilibrium.10 Solution studies of this system have been reported, but the results are difficult to interpret.” In this paper we report a quantitative study of the photochemically induced &/trans isomerizations of some 3-arylpropenylideniminium salts. This photoisomerization allows the ready generation of solutions of the salts that contain substantial amounts of the less-stable cis isomers. Taking advantage of this simple preparative method, we have undertaken a study of the ground-state stereomutations of these unsaturated iminium salts. By combining substituent effects with solvent dependencies, it has been possible to define the mechanisms of these cis to trans isomerizations. l 2
Results and Discussion The salts 1-5 were prepared by reacting the appropriately substituted cinnamaldehyde with dimethylammonium perchlorate.
\;/
I‘ II
l,X=H 2 , X = OCH, 3, X = CH, 4, x = CI 5, X = NO,
6,X=H 7, X = NO, 8, X = OCH,
Solutions of 6, 7, and 8 were obtained by dissolution of the corresponding imines in trifluoroacetic or sulfuric acid. The lH NMR spectra of these iminium salts are given in Table I. The structures of 1 and 2 have been obtained by X-ray ~rystallography.~~ Both cations were shown to exist in s-trans conformations and with little delocalization of the positive charge from nitrogen onto the carbon framework. The s-trans conformation was also shown to be preferred for these cations in solution. ( I O ) Honig, B.; Annu. Rev.Phys. Chem., 1978, 29, 31. Birge, R. R.; Annu. Rev. Biophys. Bioeng., 1981, 10, 315. ( 1 1 ) Mowery, P. C.; Stoeckenius, W.;J. Am. Chem.Soc. 1978,101,414. (12) This work is taken from: Dickie, B. D. Ph.D. Thesis, McMaster University, Hamilton, Ontario, 1982. (13) Childs, R. F.; Dickie, B. D.; Faggiani, R.; Fyfe, C. A,; Lock, C. J. L.; Wasylishen, R. E. J . Org. Chem., submitted for publication.
0 1983 American Chemical Society
Vol. 105, No. 15, 1983 5042 J . Am. Chem. SOC.,
Childs and Dickie
Table I. 'H NMR Data for Iminium Salts chemical shifty, ppm compd H, 1 8.20d 9 8.26 d 2 8.09d 10 8.25 d 3 8.13 d 11 8.26 d 4 8.20 d 1 2 8.25 d 5 8.36 d 13 8.30 d 6 8.25 dd 11 8.25 16 8.42 dd 7 8.43 dd 15 8.43 dd 17 8.45 dd 18 8.45 dd 8 8.12dd
H, 7.08 dd 6.26 t 6.92 dd 6.41 t 7.01 dd 6.47 t 7.05 dd 6.56 t 7.29 dd 6.79 t 7.06 dd 7.20 dd 6.58 t 7.23 dd 7.39 dd 6.80 t 6.93 t 6.92dd
H3 7.79 d 7.97 d 7.71 d 7.86 d 7.74 d 7.92d 7.73 d 7.89 d 7.86 d 7.97 d 7.75 d 7.88 d 7.90 d 7.83 d 7.98 d 7.92d 8.12 d 7.67 d
aryl H 7.34 dd, 7.43 d , 7 . 6 0 d 7.27 dd, 7.35 d , 7.44 d 6.92 d, 7.61 d 6.98 d, 7.13 d 7.16 d, 7.44 d 7.18 s 7.30 d , 7.52 d 7.31 d, 7.34 d 7.81 d , 8.20 d 7.50 d, 8.21 d 7.32 d d , 7.42 d , 7.52 d 7.32 dd, 7.42 d, 7.61 d 7.24 dd, 7.35 d, 7.42 d 7.73 d, 8.20 d 7.82 d, 8.23 d 7.48 d, 8.22 d 7.5 d, 8.2 d 7.54 d , 6 . 9 2 d
NH
9.2 bd 9.2 bd 9.2 bd 9.2 bd
9.2 bd
coupling constant, Hz N group 3.50 s, 3.59 s 3.53 s, 3.61 s 3.45 s, 3.54 s 3.48 s, 3.60 s 3.47 s, 3.56 s 3.50 s , 3.60 s 3.50 s, 3.59 s 3.53 s, 3.62 s 3.60 s, 3.67 s 3.60 s, 3.67 s 3.70 q, 1.68 m , 1.32 m, 0.85 t 3.75 q , 1.68 m, 1 . 3 2 m , 0.85 t 3.70 q , 1.68 m , 1.32 m , 0.85 t 3.77 q , 1.71 m, 1.33 m,0.85 t 3.84 t, 1.71 m, 1.33 m , 0.85 t 3.8 t , 1.71 m, 1.33 m, 0.85 t 3.8 t , 1.71 m, 1.33 m, 0.85 t 3.64 t , 1.66 m, 1.31 m , 0.85 t
Other
3.82 s 3.83 s 2.26 s 2.26 s
'1.1
'1.3
10.6 10.4 10.8 10.8 10.8 10.8 10.8 10.5 10.2 10.3 10.3 11.0 10.3 9.8 10 11 11 9.8
15.3 11.1 14.8 10.8 14.8 10.8 14.9 11.0 15.5 11.4 15.5 15.3 10.3 15.9 14.6 11 11 14.7
JN,~
17.2 17.2 16.5 11 15.9
'bd =broad doublet, d = doublet, dd =doublet of doublets, q. = quartet, dq = doublet of quartets, s = singlet, t = triplet, m = multiplet. In ppm from (CH,),N+BF,- (3.10 ppm) in TFA solution.
Table 111. Photostationary-State Compositions
Table 11. UV Spectra of Iminium Salts
1 2 3 4
5 6 7
TFA TFA TFA TFA TFA %SO, H2SO4
34 1 384 359 350 323 340 335
4.4 4.6 4.6 4.5 4.4 4.9 4.4
The UV spectra of these salts are given in Table 11. Absorption maxima are intense and range from 323 to 384 nm depending on the ring substituent. The high intensity of these bands suggest they are of T,R* character. Attempts were made to observe fluorescence emission of 1 at room temperature in 98% H2SO4 and at 77 K in an acid glass (3:1, v/v, methanesulfoniclnpropanesulfonic acids14). No emission could be detected in either medium. Photoisomerization: Qualitative Observations. Irradiation of salts 1-5 in TFA solutions at 313 and/or 366 nm led to the formation of a single photoproduct in each case. The reactions were monitored by 'H N M R spectroscopy at 250 or 400 MHz, and the products were identified as the corresponding cis isomers 9-13. This identification was based on the very close similarity
a
starting salt
ir rad iation A, nm
1 2 3 4 5 6 7
313 366 313 313 313 313 313
products. %
1 (431, 9 (57) 2 (481, 1 0 (52) 3 (53), 11 (47) 4 (38), 12 (62) 5 (591, 1 3 (41) 6 ( 5 3 ) , 6 (47)' 7 (35), 1 7 (401, 18 (101, 15 (15)
This is not a true photostationary state; see text.
I/
L
\;/
I'
a
C.pm
7
Figure 1. 'HNMR spectra of (A) salt 7, (B) salt 7 after heating, and (C) salt 7 after irradiation (note: spectrum C is obtained in TFA-d).
9,X=H 10, X = OCH, 11, X = CH, 12,
x = c1
13, X = NO,
of the 'H N M R spectra of the photoproducts to those of the starting salts and the magnitudes of the coupling constant between H2 and H3, Table I. The products were shown to be thermally stable under the conditions used for the irradiations. A photostationary state was set up in each case after some 4-24 h irradiation, Table 111. (14) Tarassoff, P. Ph.D. Thesis, The George Washington University, Washington, D.C.,1974. Filipescu, N.; Chakrabarti, S. K.; Tarassoff, P. G. J . Phys. Chem. 1973, 77,2276. Tarassoff, P. G.; Filipescu, N. J . Chem. SOC., Chem. Commun. 1975, 208.
In the cases of 6 and 7 the photoisomerizations are potentially more complex as isomerism about both the C==C and C=N bonds could lead to mixtures containing up to four isomers. The corresponding C=N isomers of 6 and 7 could be produced by heating solutions of these cations in acid solution, Figure 1. In each case an equilibrium was established at 100 OC with some 20% of an additional isomer present in solution. Identification of these isomers as 14 and 15, respectively, was made on the basis of their 'H N M R spectra, which displayed a characteristic coupling constant of 11 Hz between H I and the N H protons, Table I. At room temperature the thermal interconversions of these C=N stereoisomers was slow and could be neglected in comparison with the rates of the photochemical reactions. Irradiation of a solution of 6 in TFA appeared initially to lead to the production of a single photoisomer. This photoproduct had a different 'H N M R spectrum to that of 14. This new product
J . Am. Chem. SOC.,Vol. 105, No. 15, 1983 5043
Stereomutations of 3- Aryl-2-propenylideniminium Salts
Table IV. Quantum Yields for Isomerization of Iminium Salts ~~
6,X=H 7, X = NO,
14, X = H 15, X = NO,
reaction
excitation h, nm
quantum yieldasb
6’16 1’9 1’9 9’1‘ 2’10 3’11 4’12 5’13 13 ’5‘ 7+17
313 313 366 313 366 313 313 313 313 313
0.58 t 0.06 0.60 i 0.06 0.75 i 0.08 0.45 ?- 0.05‘ 0.59 i 0.04 0.52 i 0.05 0.58 f 0.09 0.27 i 0.02 0.33 i 0.03‘ 0.47 i 0.05
Measured relative Errors are standard deviation of 3-4 runs. to the photodecomposition of potassium ferrioxalate, reactions Calculated value, see have been corrected for back reaction. text.
‘
16, X = H 17, X = NO,
18, X = NO,
was identified as 16 on the basis of the coupling constant across the C=C bond, Table I. The situation with 7 was more complicated. As can be seen from Figure l C , a complex ’H N M R spectrum was obtained on irradiation. Two of the materials present could be identified readily as 7 and its C=N isomer 15. Two additional cations also appeared to be present that were closely related to 7 and 15. This is evident, for example, in the 6-7 ppm region of the ‘H N M R spectrum (Figure lC), where two additional resonances associated with H2 proton resonances can be seen. By systematic decoupling of the spectrum it was possible to identify all the vinyl proton resonances of these two additional products and obtain the various coupling constants. On the basis of coupling constants the additional products were identified as 17 and 18, Table I. The results obtained with 7 prompted us to return and examine the irradiation of 6 for extended periods of time. On irradiation of 6 for periods exceeding 1 week it was possible to detect small amounts of the corresponding cis C=N isomers. A photostationary state did not seem to have been reached in this time. Photoisomerization: Quantitative Aspects. The quantum yields for the photoisomerizations of 1-7 to +13, 16, and 17, respectively, were measured. Quantitative analyses were carried out by ‘H N M R spectroscopy directly on the irradiated acid solutions. In all cases the reactions were carried out to less than 10% conversion, and the values obtained corrected for back reaction. The quantum yields obtained are summarized in Table IV. All the quantum yields reported are for aerated solutions. Degassing with a nitrogen purge had no effect on the efficiency of these reactions. As it has not yet proved possible to synthesize the pure cis isomers 9-13, no direct measurement of the quantum yield for the reverse cis to trans photoisomerizations could be made. Values 1 and 13 for the quantum efficiency of the conversion of 9 5 were estimated from the composition of the photostationary state, Table 111, the quantum yields for the forward reactions, and the extinction coefficients of the various isomers. The extinction coefficients of 9 and 13 at 3 13 nm were obtained by difference spectroscopy of solutions of 1 and 5 with photostationary state mixtures of 1 and 9 and 5 and 13, respectively. In the case of the photoisomerization of 7, where both C=N and C=C bond isomerization is observed at the photostationary state, attempts were made to measure the quantum yields for isomerization about the C=N bond. Even after conversion of some 20% of 7 to 17, it was not possible to detect any of the C=N isomers 15 and 18 in the solutions (detection limit by ‘H NMR spectroscopy 2%). This means that the quantum yield for the conversion of 7 to 15 must be
