The Frustrated Lewis Pair Pathway to Methylene Phosphonium Systems

Reaction of HB(C6F5)(2) with ((Bu3C6H2)-Bu-t)P(vinyl)(2) does not yield the expected ethylene-bridged frustrated Lewis pair (FLP) but its cyclic methylene phosphonium isomer which is formed in a unique way by subsequent internal 1,2-P/B addition to the vinylphosphane unit. The product is a rare example of a stable phosphorus analogue of the ubiquitous iminium salts. It features a short P=C bond (1.637(3) angstrom). Subsequent reactions with pyridine and with dihydrogen probably proceed by equilibration with the "invisible" ethylene-bridged FLP isomer. The reaction of ((Bu3C6H2)-Bu-t)P(vinyl)(2) with B(C6F5)(3) gave the zwitterionic acyclic methylene phosphonium product that was crystallized at low temperature and isolated. The X-ray crystal structure analysis also revealed a short P=C bond (1.635(5) angstrom).