<i>C</i>,<i>C</i>-Diacetylenic Phosphaalkenes as Heavy Diethynylethene Analogues

A series of C,C-diacetylenic phosphaalkenes 1b−e has been prepared from 1-chloropenta-1,2-dien-4-ynes 6b−e in a reaction with Mes*PCl2 (Mes* = 2,4,6-(tBu)3Ph) in the presence of LDA. Under identical conditions, isomeric butadiyne-substituted phosphaalkenes 2c−f can be obtained from 3-chloropenta-1,4-diynes 5c−f. The title compounds represent rare examples of diethynylethenes in which a constituting methylene has been replaced by a phosphorus center. The formation of both isomers can be rationalized by a common pathway that involves isomeric allenyllithium species. Spectroscopic, electrochemical, and theoretical investigations show that the phosphorus heteroatoms are an intrinsic part of the compounds’ π−systems and lead to decreased HOMO−LUMO gaps compared to those in all-carbon-based reference compounds.