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.