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X-ray Crystal Structure, ab Initio Calculations, and Reactivity of 1,3,2λ5-Diazaphosphetes:  A New Type of 4-π-Electron 4-Membered Heterocycle

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posted on 1996-04-24, 00:00 authored by Gilles Alcaraz, Antoine Baceiredo, Martin Nieger, Wolfgang W. Schoeller, Guy Bertrand
The structure of P,P-bis(diisopropylamino)-4-phenyl-1,3,2λ5-diazaphosphete, 1a, has been determined by a single-crystal X-ray diffraction study (C19H33N4P, monoclinic system, space group P21, a = 9.482(1) Å, b = 11.374(3) Å, c = 9.668(2) Å, β = 97.16(1)°, Z = 2). According to quantum chemical calculations at an RHF level of optimization utilizing the 6-31g(d,p) basis set, 1a has a zwitterionic structure with the negative charge delocalized on the NCN allylic fragment and the positive charge localized at the phosphorus. Heterocycle 1a reacts with water and benzaldehyde affording N-phosphoranylbenzamidine 3 (95% yield) and the expected aza-Wittig adduct 4 (85% yield), respectively. Addition of 1 equiv of methyl trifluoromethanesulfonate and of 2 equiv of BH3·THF to 1a affords cyclic phosphonium salt 5 (94% yield) and the bis(borane) adduct 6a (90% yield), respectively. Dimethyl acetylenedicarboxylate slowly reacts with 1a giving rise to 1,3,4λ5-diazaphosphinine, 9, in 70% yield. The X-ray crystal structures of products 2, 3, and 6a are reported (2:  C26H38N5P, monoclinic system, space group C2/c, a = 16.337(8) Å, b = 19.810(2) Å, c = 8.800(2) Å, β = 117.68(2)°, Z = 4. 3:  C19H35N4OP, orthorhombic system, space group P212121, a = 9.090(1) Å, b = 12.955(2) Å, c = 17.860(3) Å, Z = 4. 6a:  C19H39B2N4P, orthorhombic system, space group P212121, a = 10.340(1) Å, b = 13.247(1) Å, c = 16.996(1) Å, Z = 4).

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