Light-catalysed reduction of tris(trimethylsilyl)methyl iodide by alcohols. A radical-chain process showing a large kinetic hydrogen isotope effect

Abstract

The substituted-alkyl iodide (Me₃Si)₃Cl (Tsil) reacted with MeOH in the presence of AgO₃SCF₃, NaOMe, or (less readily) NaOAc under ordinary laboratory light to give (Me₃Si)₃CH (TsiH). Such reduction also took place readily in the presence of the corresponding alkoxide in EtOH or PrⁱOH (in which a second organosilicon product was formed) and markedly less readily in Bu^tOH (in which the second product dominated). In the presence of AgO₃SCF₃, under conditions in which MeOH and EtOH react readily, reaction was markedly slower with PrⁱOH and did not take place with Bu^tOH, Bu^tCH₂OH or PhCH₂OH, and the (slow) reaction with CF₃CH₂OH gave a different product. The reaction in MeOH in the presence of AgO₃SCF₃, NaOMe, or NaOAc was inhibited by CCl₄, PhMe or PhCH₂OH. In CH₃OD in the presence of NaOMe or AgO₃SCF₃ only undeuteriated TsiH was formed, and in CD₃OD the reaction was very slow. Reactions in CD₃OD–CH₃OD mixtures showed that the kinetic isotope effect for this hydrogen atom abstraction was ca. 19 with NaOMe and ca. 22 with AgO₃SCF₃ present. The reactions with MeOH appear to involve a radical chain process in which Tsi˙ and I˙ are formed by homolytic cleavage of the Tsi–I bond, with the Tsi˙ radicals then abstracting H atoms from the CH₃ group of CH₃OH to give ˙CH₂OH radicals, which abstract I˙ from Tsil to regenerate the Tsi˙ radicals. It is suggested that the function of NaOMe and AgO₃CF₃ is to remove I₂, which otherwise acts as an effective radical trap.