Photoreduction of Triplet Benzophenone by Tertiary Amines: Amine molecular structure and ketyl radical yield

Abstract

Dedicated to Professor Fabian Gerson on the occasion of his retirement, in grateful appreciation of his contributions to science and to education (3.11.97)The photoreduction of triplet benzophenone by 14 tertiary amines was investigated. The ketyl radical yields do not correlate with the quenching rate constants, nor with the electron donor propensity of the amines. Individual structural features of the amines seem to determine the photoreduction yields.Introduction. -The photoreduction of triplet benzophenone (3BP) by tertiary amines is a well-known reaction which has been extensively investigated [l -31. Basically, two different reaction mechanisms were proposed as shown in the Scheme: direct hydrogen abstraction (HA) or electron transfer (ET) followed by proton transfer (PT). Both mechanisms lead to the same products: the ketyl radical (BPH) of BP, and a C(a)-centered radical being stabilised by two-center three-electron bonding involving the N-atom lone-pair. While HA dominates quenching by secondary amines [4], the ET-PT mechanism is generally assumed to prevail for tertiary amines with available H-atoms at C(a) [l] [5-71 in a polar environment.

Spectroscopic characterization of photoaccumulated radical anions: a litmus test to evaluate the efficiency of photoinduced electron transfer (PET) processes

“…Likewise, in accordance with the role of amine radical cation deprotonation, is the fact that, of the two further tertiary amines, iPr 3 N causes a somewhat slower accumulation (the conformation of the radical cation is known to be less favorable for deprotonation) [48–50], and DABCO (for which deprotonation is impossible [51–52]) causes no detectable formation of TCB •− . Indeed, previous laser flash photolysis experiments had shown that DABCO formed the TCB radical anion on the nanosecond time scale just as the other tertiary amines [53].…”

Quenching of n,?*-excited azoalkanes by amines: structural and electronic effects on charge transfer

“…While a 'direct' hydrogen abstraction competes in some cases, 1 the dominant photochemical process is assumed to involve electron transfer in the case of ketones. [2][3][4] The primary radical ion pair may undergo proton transfer or back electron transfer which results in an overall hydrogen transfer or deactivation, respectively. In support of this mechanism, quenching rate constants generally increase with decreasing ionization potential of the quencher and increasing solvent polarity unless specific solvation, 3,5,6 steric 4 or stereoelectronic effects 7,8 operate.…”

1H NMR Study of Hydrogen Abstraction in Model Compound Mimics of Polymers

“…Methyl trimethylacetate (6) and butyl isovalerate (9) were chosen to represent poly(tert-butyl acrylate) and poly(n-butyl acrylate), respectively. Hydrocarbons 2,2-dimethylbutane (7) and trans-3-hexene (8) Benzophenone (ca 4 mg) was dissolved in 6 mL of an approximately 90 mM solution of model compound (1)(2)(3)(4)(5)(6)(7)(8)(9) in CD 3 CN. The solution was transferred to a quartz tube capable of being inserted into a standard NMR tube, and inert gas was bubbled through the solution for 5 min to remove oxygen.…”