Mechanistic studies on the reactions of trimethylsilanide and trimethylstannylide ions with haloarenes in hexamethylphosphoramide

The large positive ρ values obtained from Me3M− ions reacting with appropriately-substituted chloroarenes (4.1±0.1, and 2.94±0.02 for M=Sn and Si, respectively) can be accounted for the presence of a strong negative charge in the transition state of the substitution reaction. These ρ values for Me3Sn− and Me3Si− nucleophilic attack on ArX provide evidence for the overall larger selectivity of the former, compared with the more reactive, and less selective Me3Si− ions. When 1-allyloxy-2-halobenzenes (X=Cl, I) are allowed to react with Me3Sn− ions in HMPA, an ipso substitution product 6 is obtained, in addition to stannylated products on the allylic moiety. This reaction proceeds by a HME process, as opposed to the same reaction carried out in liquid ammonia, where (2,3-dihydro-benzofuran-3-ylmethyl)-trimethylstannane (4) is furnished in high yield through an SRN1 mechanism. Reaction of Me3Si− ions with 1-allyloxy-2-iodobenzene in HMPA, affords only the ipso-substituted product, namely 2-(allyloxy-phenyl)-trimethylsilane (11), through the intermediacy of a hypervalent silicon species.