Phosphenium‐Cation‐Mediated Formal Cycloaddition Approach to Benzophospholes

Abstract

A phosphenium-cation-mediated formal [3+2]-cycloaddition reaction of internal alkynes is reported. The reaction proceeds under metal-free conditions to give the corresponding C-P rearranged benzophospholes regioselectively, even when ortho- and para-substituted arylphosphine oxides are starting substrates. Mechanistic studies by P{ H} NMR analysis suggest an involvement of three-membered phosphirenium cation species and C-P rearrangement prior to a ring closure for benzophosphole skeleton formation.

Alkyne Insertion into P–C(sp2) Bonds as a Route to Fused Phospholes: Transition-Metal-Like Reactivity at Phosphorus

“…1,3-Aryl migrations, which are effectively net alkyne insertion reactions, have been observed as minor pathways in P-radical mediated annulation reactions forming benzophospholes . More recently, a mechanistic proposal similar to ours was made for the formation of benzophospholes from metal-free phosphine triflates . However, in the metal free case, the alkyne insertion into P–C bonds takes place through the reductive elimination from a formally P­(V) phosphirenyl cation.…”

“…Similar 1,3-aryl migration steps have been proposed in P-centered radical-induced annulation reactions, to explain the regiochemistry of a minor benzophosphole product, 22 and in the formation of benzophospholes from metal-free phosphine triflates. 16 In our electrophilic P-centered cationic annulation reactions, migration is the predominant pathway leading to the only observed product.…”

“…22 More recently, a mechanistic proposal similar to ours was made for the formation of benzophospholes from metal-free phosphine triflates. 16 However, in the metal free case, the alkyne insertion into P−C bonds takes place through the reductive elimination from a formally P(V) phosphirenyl cation. In our annulation reactions, as the P lone pair is protected through tungsten complexation, the +III oxidation state is maintained in every step of the reaction mechanism and the P−C bond forming reaction is a migration.…”

“…The alkyne annulation reactions, whether metal-mediated or photocatalyzed, offer very efficient routes to benzophospholes; however, regioselectivity remains a challenge, with mixtures of direct and rearranged products resulting. This tendency to rearrange may result from the radical mechanisms. ,, Very recently, a new alkyne annulation route to benzophospholes that involves phosphenium cation intermediates, rather than radical intermediates, was reported, and shows better regioselectivity …”

“…9, 14,15 Very recently, a new alkyne annulation route to benzophospholes that involves phosphenium cation intermediates, rather than radical intermediates, was reported, and shows better regioselectivity. 16 As part of our ongoing research into the reactivity of electrophilic phosphenium ions, we have described facile P−C bond formation via electrophilic substitution reactions of W(CO) 5 coordinated phosphenium ions and phosphine triflates with arenes, heteroarenes, alkenes, and alkynes. 17,18 These reactions can also be used for controlled sequential substitution of different aromatic groups onto one P center.…”