The 1,1-Carboboration of Bis(alkynyl)phosphanes as a Route to Phosphole Compounds

摘要

Phospholes have been receiving increasing interest as a class of compounds. They possess a planar five-membered heterocyclic framework that has the heavy Group 15 element incorporated in a markedly nonplanar coordination geometry. Consequently, in contrast to the other first-row five-membered heteroarenes, the phospholes are only weakly aromatic. There are far fewer established synthetic pathways to phospholes compared to their first-row-element relatives. The classical elimination pathway described by Mathey et al. has been augmented by some Group 4 metallocene alkyne coupling/phosphorylation pathways and related syntheses. Nevertheless, the phosphole framework is increasingly being used, for example, in materials science, especially in conjunction with boron-based acceptor substituents in conjugation with the P donor. Also rigid conjugated zwitter-ionic phosphonium/borate frameworks have recently been studied and found considerable interest. We have now found a new very simple synthesis that directly makes 3-boryl substituted phospholes available from suitable bis-(alkynyl)phosphanes in a one-pot reaction. Several representative examples are described and characterized herein and their synthetic utilization explored.