C,C-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*PCI2 (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5c—f. The title compounds represent rare examples of diethynylethenes in which a constituting methylene has been replaced by a phosphorus center.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. [ABSTRACT FROM AUTHOR]