Switching on and off metalation sites in triarylphosphines by manipulating substrate coordination

[摘要]：The inductive electron-withdrawing effect of the positively charged phosphorus atom in triarylphosphine-borane adducts is largely compensated by the steric congestion caused in its vicinity. Therefore, trihydro(triphenylphosphonio)borate reacts only sluggishly even with superbasic reagents and products derived from ortho-metalated intermediates by their trapping with electrophiles are formed only in poor yield. On the other hand, the borane adducts of (3-anisyl)diphenylphosphine and (3-fluorophenyl)diphenylphosphine can be readily deprotonated at the hetero-adjacent para position and converted into final products in high yields. The phosphorus-remote regioselectivity of these phosphine-borane zwitterions is complementary to that previously observed with the corresponding phosphine oxides.

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