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 ButOH (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 ButOH, ButCH2OH 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.