Cycloaddition Behavior of 1,2‐Thiaphospholes: Reactions with Diazocumulenes and with Cyclopentadiene

Abstract

Abstract1,2‐Thiaphospholes 3a,b react with (1‐diazo‐2‐oxoalkyl)silanes 1a−c to form [1,2]thiaphospholo[2′,3′:3,4][1,3]diphospholo[1,5‐b][1,2]thiaphosphole systems 4 with cis‐anti‐cis configuration of the tricyclic framework. They are accompanied by small amounts of compounds 5 that are presumably the cis‐syn‐cis isomers of 4, and 6‐alkylidene‐1‐phospha‐2‐thiabicyclo[3.1.0]hex‐3‐enes 6. It is likely that these reactions proceed by [3+2] cycloaddition of diazocumulenes, which coexist with diazo compounds as minor equilibrium components, at the P=C bond of the heterophospholes, followed by N2 elimination and formation of short‐lived 2‐alkylidene‐1,2(λ5)thiaphospholes. The latter can either add to excess thiaphosphole to form the tricyclic products or undergo electrocyclization to form bicyclic alkylidenephosphiranes. Thiaphosphole 3a does not seem to react directly with cyclopentadiene in a [4+2] or [2+4] cycloaddition. Reaction with excess cyclopentadiene at 120 °C yields the polycyclic compounds 15 and 16, which are likely to arise from a Diels−Alder reaction of 3a, reacting as a heterodiene, with the cyclopentadiene dimer. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)