Mechanistic Studies of Formal Thioboration Reactions of Alkynes

Abstract

Several formal heteroborylative cyclization reactions have been recently reported, but little physical-organic and mechanistic data are known. We now investigate the catalyst-free formal thioboration reaction of alkynes to gain mechanistic insight into B-chlorocatecholborane (ClBcat) in its new role as an alkynophilic Lewis acid in electrophilic cyclization/dealkylation reactions. In kinetic studies, the reaction is second-order globally and first-order with respect to both the 2-alkynylthioanisole substrate and the ClBcat electrophile, with activation parameters of ΔG^‡ = 27.1 ± 0.1 kcal mol^-1 at 90 °C, ΔH^‡ = 13.8 ± 1.0 kcal mol^-1, and ΔS^‡ = -37 ± 3 cal mol^-1 K^-1, measured over the range 70-90 °C. Carbon kinetic isotope effects supported a rate-determining Ad_E3 mechanism wherein alkyne activation by neutral ClBcat is concerted with cyclative attack by nucleophilic sulfur. A Hammett study found a ρ⁺ of -1.7, suggesting cationic charge buildup during the cyclization and supporting rate-determining concerted cyclization. Studies of the reaction with tris(pentafluorophenyl)borane (B(C₆F₅)₃), an activating agent capable of cyclization but not dealkylation, resulted in the isolation of a postcyclization zwitterionic intermediate. Kinetic studies via UV-vis spectroscopy with this boron reagent found second-order kinetics, supporting the likely relevancy of intermediates in this system to the ClBcat system. Computational studies comparing ClBcat with BCl₃ as an activating agent showed why BCl₃, in contrast to ClBcat, failed to mediate the complete the cyclization/demethylation reaction sequence by itself. Overall, the results support a mechanism in which the ClBcat reagent serves a bifunctional role by sequentially activating the alkyne, despite being less electrophilic than other known alkyne-activating reagents and then providing chloride for post-rate-determining demethylation/neutralization of the resulting zwitterionic intermediate.