5-Fluoro- and 5-methyl-1,3-didehydrobenzene—a matrix isolation study

Abstract

The UV photolysis of [2.2]metaparacyclophanediones 4b and 4c leads to α-cleavage and formation of CO, p-xylylene 5, and m-benzynes 2b and c with a fluorine or methyl substituent in the 5-position, respectively. The IR spectra of 2b and 2c were assigned by comparison with that of the parent m-benzyne 2a and with spectra calculated at the BLYP/6-311G(d,p) level of theory. Both band positions and intensities of the experimental spectra are in excellent agreement with the DFT calculations. This method thus provides a reliable tool for the investigation of ground state properties of m-benzynes. The benzynes 2 exhibit a very strong IR absorption at 545 ± 2 cm^–1 which is almost not affected by the substituents and therefore can be used to identify m-benzynes in product mixtures.