Eliminations across the peri-gap : peri-substitution as a means to promote kinetically disfavoured reactivity

Abstract

While the ability of peri-substitution to stabilise species that are typically short-lived is well documented, its capacity to promote kinetically disfavoured reactivity is less well known. In this work, we examine the ability of the rigid scaffold to enhance reactions that would typically require the addition of an external catalyst, particularly coupling reactions. The spontaneous dehydrocoupling of a primary arsine to give the first arsanylidene–s⁴–phosphorane was recently published by the Kilian group, but its mechanism was unknown. Based on experimental observations, a stepwise ionic mechanism is proposed, and indirect evidence for this mechanism is obtained by chemically mimicking the proposed pathway through hydride abstraction and deprotonation. In the process, a new route to the previously published phosphanylidene–s⁴–phosphorane is found, and the first examples of arsino-phosphonium and phosphino-phosphonium salts with secondary arsine/phosphine moieties are obtained. A series of peri-substituted phosphine-borane adducts were found to undergo a spontaneous P–B dehydrocoupling reaction, occurring at room temperature and in the absence of a catalyst. This is the first example of this reaction occurring under such mild conditions, and highlights the ability of peri- substitution to substantially lower the kinetic barrier to certain reactions. A thermally induced “dealkacoupling” reaction, eliminating a C–H coupled product with formation of a P–P/P–As bond, has been observed in peri-substituted phosphines and arsines. This reaction appears to be unique in the context of the literature, and was surprisingly facile, occurring under relatively mild conditions and essentially quantitatively with 100% diastereoselectivity in the products. Kinetic studies on a representative reaction reveal it to have a low activation barrier (Δ[super]‡ H° = +57 ± 7 kJ mol⁻¹), and suggest the mechanism is radical in nature. The reaction likely relies on peri-substitution to stabilise the radical intermediates of the reaction. This concept was then extended to C-heteroatom couplings. A selection of peri-substituted substrates were found to undergo P–P coupling, with apparent elimination of C–O and C–S coupled products. GC analysis revealed that C–S coupling does indeed occur with these systems, although there are competing side reactions. A variety of arsenic and antimony coupling precursors were also synthesised and characterised.