Electron-Transfer-Photosensitized Conjugate Alkylation

Abstract

Photoinduced electron transfer (PET) from an aliphatic donor to a sensitizer and fragmentation of the radical cation leads to alkyl radicals. Radical alkylation of electron-withdrawing substituted alkenes and alkynes has been obtained in this way, and its scope has been explored. Effective sensitizers are tetramethyl pyromellitate (TMPM), 1,4-dicyanonaphthalene (in combination with biphenyl, DCN/BP), and 1,2,4,5-tetracyanobenzene. Radical precursors are tetraalkylstannanes, 2,2-dialkyldioxolanes, and, less efficiently, carboxylic acids. Steady-state and flash photolysis experiments show that escape out of cage of radical ions is the main factor determining the yield of radical formation. This is efficient with triplet sensitizers such as TMPM, while with singlet sensitizers, the use of a “cosensitizer” is required, as in the DCN/BP system. Radical cations containing primary alkyl radicals escape and fragment more efficiently than those containing tertiary radicals. The thus-formed radicals are trapped by electron-withdrawing substituted alkenes, and the relative efficiency is determined by the rate of radical addition, in accord with the proposed mechanism. Among the alkynes tested, only dimethyl acetylenedicarboxylate reacts, and the order of radical reactivity is different. It is suggested that a different mechanism operates in this case and involves assistance by the alkyne to the radical cation fragmentation.

Organic Photoredox Catalysis

“…In addition to methyl 2-cyanocrotonate, electron-deficient alkenes such as dimethyl maleate and substituted maleimides ( 20.7 ) also were competent Michael acceptors. In related work, it has also been shown that organostannanes can be utilized for nearly identical radical conjugate addition reactions …”

“…In related work, it has also been shown that organostannanes can be utilized for nearly identical radical conjugate addition reactions. 168 In a mechanism reminiscent of their α-trimethylsilyl amide work (Scheme 20), Steckhan and Blechert propose that the αoxy radicals (20.9) are derived from single electron oxidation of 20.2 by biphenyl cation radical (Ph−Ph• + ), in turn, generated via oxidation of Ph−Ph by DCA*. Addition of 20.9 to the electron deficient alkene, furnishes the electron-poor radical 20.10, which can be easily reduced by DCA• − and after protonation yields the final adduct (20.3).…”

Dual Catalysis Strategies in Photochemical Synthesis

“…The redox mediator is a particular type of quencher that undergoes redox processes with both the photocatalyst and the substrate in separate electron-transfer events (Figure ). These cocatalysts are commonly referred to in the photochemistry literature as cosensitizers, and occasionally as electron mediators, redox photosensitizers, redox catalysts, electron relays, or electron-transfer agents. − There are three main mechanistic scenarios in which a redox mediator may increase the rate of a reaction. For a hypothetical oxidative reaction, these are as follows:…”

“…Fagnoni and Albini reported that the photooxidation of organostannane 41 followed by mesolytic fragmentation of the C–Sn bond results in the formation of alkyl radical 44 , which subsequently can react with electron-poor acceptors such as 42 to give 43 (Scheme ). , Although the process can be conducted using single photocatalysts such as 1,4-dicyanonapthalene (DCN) or phenanthrene, the rates were increased in the presence of biphenyl as a redox mediator. Enantioselective, chiral auxiliary controlled versions of this reaction have also been reported …”

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

“…However, when the D–X •+ intermediate contained a good electrofugal group (such as a silyl, stannyl, t -Bu group or a hydrogen [12–27]), unimolecular fragmentation was possible and gave neutral alkyl radicals (D • ) with a reasonable (0.1 or higher) quantum yield (Scheme 1, step b) [5]. Addition to the aromatic radical anion and re-aromatization gave an alkylated aromatic (A–D, path c → d), while trapping of the radical by an electron-withdrawing substituted alkene T (step e) [28–29] followed by BET from the radical anion (step f) of the acceptor (that was thus recovered) led to photocatalyzed alkylation of the alkene [5,30–31]. Both processes have found some application in synthesis for the mild and selective activation of aliphatic derivatives, in particular of a C–H bond [32].…”