Size matters: room temperature P-C bond formation through C-H activation in m-terphenyldiiodophosphines.

m-Terphenyl- and biphenyl-2-diiodophosphines, TerphPI 2 and BiphPI 2, have been obtained by halide exchange from the chloro derivatives TerphPCl 2 and BiphPCl 2 and excess LiI in a benzene solution at room temperature. Whereas BiphPI 2 compounds are stable, the TerphPI 2 species undergo intramolecular C-H activation at room temperature and cyclize to form unsymmetrical 9-iodo-9-phosphafluorenes 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-iodo-9-phosphafluorene, 4; 1-(4-t-butylphenyl)-7-t-butyl-9-iodo-9-phosphafluorene, 5; and 1-(2-methylphenyl)-5-methyl-9-iodo-9-phosphafluorene, 6, albeit the latter reaction is rather slow due to unfavorable steric interactions. Cyclization of the alkyl-substituted 4,4'-di- tert-butyl-biphenyl-2-diiodophosphine, 11, is slow in refluxing benzene solution, but faster than that for the parent biphenyl-2-diiodophosphine.