Deoxygenation of isocyanates via transient electrophilic terminal phosphinidene complexes: Are strained P‐heterocycles involved?

Abstract

AbstractSynthesis of 1,3,2,4σ3λ3−dioxadiphosphetane complexes 3a,b and 4a,b was achieved via thermal decomposition of 2H‐azaphosphirene complexes of chromium (1a) and tungsten (1b) in the presence of tert‐butyl isocyanate; as reactive intermediates 3‐imino‐oxaphosphirane complexes 8a,b are proposed, which decompose to give phosphinidene oxide complexes 9a,b. The intermediacy of the latter is inferred from a cross‐dimerization experiment (decomposition of a 1:1 mixture of 2H‐azaphosphirene complexes 1a,b with tert‐butyl isocyanate), which furnished the mixed metal dinuclear cis and trans complexes 5a/b and 6a/b. 31P NMR spectroscopy revealed the formation of [bis(trimethylsilyl)methyl]cyanophosphane complexes 11a,b as by‐products. Thermal reaction of complex 1b with the less bulky ethyl isocyanate furnished the novel 1,3,4σ3λ3‐diazaphospholidin‐5‐imin‐2‐one complex 7 together with complexes 3b and 4b. For the latter case, 3‐imino‐oxaphosphirane (12) and azaphosphiridin‐3‐one (13) complexes are proposed as reactive intermediates. All final products were characterized by multinuclear NMR spectroscopy, IR, MS, and single‐crystal X‐ray crystallography in the cases of complexes 3b and 4a,b. DFT calculations strongly support the transient formation and decomposition of 3‐imino‐oxaphosphirane complex 12 to form 9b and isonitrile. Azaphosphiridin‐3‐one complex 13 was characterized computationally as the methyl model. In accordance with the calculations, final products 1,3,2,4σ3λ3‐dioxadiphosphetane complexes 3a,b and 4a,b form by dimerization of transient terminal phosphinidene oxide complexes 9a,b. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:275–286, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/hc.20677