Cleavages of 1-methyl-2-trimethylsilylbenzimidazole and 2-trimethylsilylbenzothiazole in methanol and related media

Abstract

First-order rate constants k at 25 °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 (10³k 51 s^–1)(and also by H₂O, 10³k 110 s^–1 or EtOH, 10³k 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 (10²k₈ 63 l mol^–1 s^–1). The data for the neutral cleavage of (I), including a rate isotope effect, r.i.e. [given by the ratio k(MeOH)/k(MeOD)], 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^– 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 NC 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 products 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.