Nucleophile-Assisted Cleavage of Benzyltrialkylsilane Cation Radicals

Abstract

The cation radicals of benzyltrialkylsilanes have been generated using photoinduced electron transfer and characterized using transient absorption spectroscopy. Absolute rate constants for nucleophile-assisted C-Si bond cleavage have been determined, for different nucleophiles in different solvents and with different substituents on the phenyl ring. The short lifetime (<10 -9 s) of the parent benzyltrimethylsilane cation radical in acetonitrile was unambiguously shown to be due to a rapid nucleophile-assisted bond cleavage, with the solvent acting as the nucleophile. Even in less polar solvents, such as dichloromethane, the lifetime of benzyltrimethylsilane cation radical is quite short (ca. 20 ns) unless trace amounts of water, which acts as an efficient nucleophile, are removed. Consistent with the nucleophile-assisted cleavage mechanism for the benzyltrialkylsilanes, sterically-demanding substituents on silicon decrease the rate constant for cleavage by as much as 4 orders of magnitude, depending upon the nucleophile. Similarly, increasing steric crowding on the nucleophile also decreases the rate constant, although smaller changes in the rate constants are observed. Electron-donating substituents (4-methyl and 4-methoxy) on the phenyl group also lead to a substantial decrease in the rate constant for cleavage of the cation radicals. When measurements are performed in the least nucleophilic solvent and under conditions that minimize contributions from adventitious nucleophiles, the lifetimes of the cation radicals of the benzyltrialkylsilanes can be so long that the rate of pseudofirst-order decay can not be accurately determined. If the cation radicals undergo unimolecular C-Si bond cleavage (i.e., not nucleophile-assisted) under these conditions, the rate constant for this process is estimated to be less than 10 4 s -1 .

Diastereoselective protonation on a radical anion in the photoallylation and photoreduction of 1,1-dicyano-2-methyl-3-phenyl-1-butene by allyltrimethylsilane

“…The best result was obtained by the addition of 1 equiv of L-lactic acid based on 1 to give 4 in 72% de 1 In the presence of MeOH or H 2 O instead of AcOH as a proton source, the reaction proceeded slowly to give a complex mixture. 2 The hydrogenation of compound 1 by Pd/C has been reported to proceed in a syn selective manner [19]. We prepared syn-rich mixtures of 3 and 4 by the hydrogenation of 1 by Pd/C and allylation of the syn-rich mixture of 4 by NaH and allyl bromide.…”

“…The primary step is the generation of radical cation of Phen (Phen ?• ) and the radical anion of 1 (1 -• ) through a one-electron transfer from the excited singlet state of Phen ( 1 Phen*) to 1. The secondary electron transfer from 2 to Phen ?• produces the radical cation of 2 (2 ?• ), which gives the allyl radical via the nucleophile-assisted cleavage of the C-Si bond [18][19][20]. In the presence of a proton source, the protonation at the b-position to the cyano groups of 1 -• predominantly takes place to generate the radical 5.…”

Photoinduced electron transfer reactions in the 10-methylacridinium cation–benzyltrimethylsilane system: steady-state and flash photolysis studies

“…The (CH 3 ) 3 -Si ? cation is known to absorb in the same spectral region (k max = 530 nm) [20]. This cation was not present in the pulse radiolysis experiments.…”

Radical Cation Fragmentation Reactions in Organic Synthesis

“…In principle, the kinetics of this reaction could be improved by utilizing a cationic oxidant and an aromatic cooxidant in accord with Dinnocenzo's studies. 12 When 15 was irradiated in the presence of NMQPF 6 and the cooxidant tert-butylbenzene, shown by Kochi 16 to form a radical cation that stable toward fragmentation, the reaction proceeded to form 16 within 20 min. As illustrated in Scheme 3.10, other examples in the same report demonstrated that five-, seven-, and eight-membered rings could also be formed in good yields.…”