Cleavages of 1-Methyl-2-trimethylsilylbenzimidazole and 2-Trimethylsilylbenzothiazole in Methanol and Related Media

Article abstract of DOI:10.1039/P29810001051

First order rate constants k at 25 deg C have been determined for the cleavage of 1-methyl-2-trimethylsilylbenzimidazole (I) and 2-trimethylsilylbenzothiazole (II) in MeOH, alone or containing NaOMe, and in 5:2 v/v MeOH-H2O containing HClO4.Compound (I) is cleaved rapidly by neutral methanol (1E3 k 51 s^-1) (and also by H2O, 1E3 k 110 s^-1 or EtOH, 1E3 k 3.0 s^-1); very small amounts of NaOMe do not affect the rate (which thus refers to a spontaneous process), but larger amounts induce a base-catalysed process (1E2 k 63 l mol^-1s^-1).The data for the neutral cleavage of (I), including a rate isotope effect, r.i.e. , of 3.3, can be interpreted either in terms of the cyclic mechanism involving a solvent molecule previously proposed for cleavage of 2-trimethylsilylpyridine or one in which MeO(1-) attacks the protonated substrate.In base cleavage (I) gives an r.i.e. of 1.1, and this is interpreted in terms of proton transfer to the N of the N=C bond synchronous with breaking of the C-SiMe3 bond, but (II) gives an r.i.e. of 0.46, suggesting that there is little if any such electrophilic assistance.The low product isotope effects (the product ratio RH/RD on cleavage in 1:1 MeOH-MeOD) of 1.2-1.3 for neutral cleavage of (I) and the base cleavages of (I) and (II) are attributed to the fact that the isotopic composition of the product is determined in a step involving transfer of hydrogen to a localized carbanion.Addition of acid greatly raises the rate of cleavage of (II) in 5:2 MeOH-H2O, but reduces it for (I), for which the rate levels off at a value thought to refer to the fully protonated species.The acid-catalysed processes may involve attack of a solvent molecule at silicon in the protonated species.