J. Phys. Chem. 1981, 85, 1719-1725
1719
Catalysis by Aliphatic Thiol in Photoreduction of Benzophenone by Amines and Alcohols Paul 0. Stone and Saul G. Cohen" Department of Chemistty, Brandeis University, Waltham, Massachusetts 02254 (Received: September 3, 1980)
The quantum yield of photoreduction of benzophenone by aliphatic thiol is low, -0.03. However, the photoreduction by primary and secondary aliphatic amines, which contain a-C-H (and N-H), is markedly catalyzed by low concentrations M) of aliphatic thiol, while it is retarded by aromatic thiol. The catalysis is greater at high concentrations of ketone and amine and passes through a maximum with increasing concentration of thiol. Photoreduction by tertiary amines is retarded by both aromatic and aliphatic thiols. tert-Butylamine and aniline, which lack a-C-H, lead to ketyl and N-centered aminyl radicals, which disproportionate, regenerating ketone and amine. These amines retard photoreduction by amines which possess a-H, and aliphatic thiol greatly decreases this retardation. Acceleration by thiol is ascribed to a sequence of hydrogen atom abstractions, by N-centered aminyl radical from thiol, with rate constant >lo5 M-l s-l, and by thiyl radical from a-C of amine, with rate constant >lo3 M-' s-l. These reactions convert a disproportionating N-centered radical to a reducing C-centered radical. The tert-butylamine-retarded reduction by benzhydrol, but not that by 2-propanol, is accelerated by aliphatic thiol. Retardation and acceleration by thiols in photoreduction by amines and alcohols are discussed in terms of competing free radical reactions and are related to the S-H bond strengths of aromatic and aliphatic thiols, the reactivities of the related thiyl radicals, the ease of abstraction of H from a-C of alcohols and amines, and the reactivities of a-hydroxyalkyl and a-aminoalkyl radicals.
The photoreduction of benzophenone by aliphatic alcohols is retarded very efficiently by aromatic, and less efficiently by aliphatic, thiols and disulfides.lI2 At low sulfur compound and high alcohol concentrations, the retardation is almost completely due to repeated use of thiol and thiyl radical in hydrogen atom transfer reactions which convert the ketyl radical intermediates back to starting materials, in competition with the product forming hydrogen atom transfer3 and ketyl coupling reactions (eq 1). In this retardation sulfur compounds initially present (Ar2COH)2
Ar2COH
I SA
-
ArZC=O
R2C=O
t ASH
t Ar2dOH
R2CHOH t AS.
as either mercaptan or disulfide are converted to photostationary equilibrium of the two oxidation states.' On the other hand, while the photoreduction by amines is also retarded by aromatic thiols, it may be substantially catalyzed by aliphatic thi01s.~ The quantum yield of photoreduction of 0.14 M benzophenone by 1.3 M 2-aminobutane in benzene was increased from 1.17 to 1.80 by 0.02 M l-pentanethiol, approaching the maximum theoretical value of 2. The quantum yield for reduction by 0.52 M diisopropylamine was increased 2.6-fold by 0.16 M 1pentanethiol, from 0.48 to 1.25. In this catalysis, unlike the retardation, thiol was not converted detectably to disulfide, while any disulfide initially present was slowly reduced to thiol. Catalysis increased with increasing (1)(a) Cohen, S.G.; Orman, S.; Laufer, D. A. Proc. Chem. SOC.,London 1961,301; (b) J.Am. Chem. SOC.1962,84,1061,3905. (2)Cohen, S. G.;Rose, A. W.; Stone, P. G.; Ehret, A. J. Am. Chem. Sac. 1979;101, 1827. (3)Pitts, J. N.;Letsinger, R. L.; Taylor, R. P.; Patterson, J. M.; Recktenwald, G.; Martin, R. B. J. Am. Chem. SOC.1959,81,1068. (4) (a) Cohen, S. G.; Rose, A. W.; Stone, P. G. Tetrahedron Lett. 1976, 3101; (b) Isr. J. Chem. 1977,16,318. (c) Stone, P. G.; Cohen, S. G. J. Am. Chem. SOC. 1980,102,5685.
concentration of thiol up to a point and then decreased. In photoreduction by a tertiary amine, there was no acceleration at 0.01 M thiol, and retardation was observed at 0.1 M.hb Thiol alone is a very inefficient photoreducing agent.4c It has been proposed that photoreduction by amines proceeds via initial electron abstraction or charge transfer (eq 2).5p6 This accounted for high rates (kir), for broad effectiveness in reducing ketones not photoreduced by alcohols, for some loss of quantum yield, possibly by quenching within an exciplex (he),and for preferred abstraction from the least-substituted, most acidic a carbon.' Alcohols, on the other hand, would reduce by direct transfer of H from a-C (hr, eq 1) facilitated by a polar contribution to the transition state. Accordingly, we suggested that aliphatic mercaptans raised quantum yields for reduction by amines by reacting with the exciplex, catalyzing hydrogen transfer ( k h ) and increasing the k h / k , rati0.~*9~ However, this is not since recent experiments show that quenching does not occur substantially in such reactions.8 Benzophenone reacts with common aliphatic amines in benzene to form ketyl radical with quantum yields cp = 0.9-1.0. This is particularly significant in reaction with tert-butylamine, (CH3)3CNH2,where abstraction is from N (eq 2, kh'). The net final reduction by this amine is as it is for reduction by aniline,1° which also lacks a-H, indicating that starting materials are regenerated by disproportionation of ketyl and N-centered radicals (eq 2, kb).8a'10This is a reaction of a heteroatomcentered radical similar to that of thiyl radical (eq 1). Ketyl radical is also formed with cp 1.0 from reaction with triethylamine, by abstraction from a C B aThus, in reaction with primary and secondary amines which contain both N-H and a-C-H, H is probably abstracted from both positions. Hydrogen is generally not abstracted by benzophenone from 0 of alcohols; it may be abstracted from
-
(5)Cohen, S.G.;Cohen, J. I. J.Phys. Chem. 1968,72,3782. (6)Cohen, S.G.;Chao, H. M. J. Am. Chem. SOC.1968,90,165. (7) Cohen, S. G.; Stein, N. J. Am. Chem. SOC.1969,91,3690. (8)(a) Inbar, S.; Linschitz, H.; Cohen, S. G. J. Am. Chem. SOC. 1980, 102,1419; (b) Ibid. 1981,103, 1048. (9) Cohen, S. G.; Baumgarten, R. J. J.Am. Chem. SOC.1965,87,2996. (10)Davidson, R. S.; Lambeth, P. F.; Santhaman, M. J . Chem. SOC., Perkin Trans. 2 1972,2351.
0022-3654/81/2085-1719$01.25/00 1981 American Chemical Society
1720
The Journal of Physical Chemistry, Vol. 85,No. 12, 1981
Stone and Cohen
r Ar 2C
=O* (TI )
t
Ar2kOH
.NH-C
I/ \
H Ar2kOH
t
I
:N-C
I:*
I
A r Z k O H t -N=C
'/
\
AreC=O(So)
:A-
t
H-C *r2\
, 1\
-
Ar2COH
I H-C , 1
:N-
\
