Structure and reactivity of tautomeric forms of zwitterionic species from the reaction of phosphorus(iii) compounds with electron deficient alkenes and alkynes

Abstract

Synthesis and reactivity of tautomeric forms of the zwitterions proposed in the phosphine catalysed transformations of electron-deficient alkenes/alkynes using the heterocycles [(t-BuNH)PN-t-Bu]₂ (1) and Ph₂P(NH-t-Bu) (2) are discussed. Thus, compounds (t-BuNH)P(N-t-Bu)₂P(N-t-Bu)CHCH(CO₂Me) (3), (t-BuNH)P(N-t-Bu)₂P(N-t-Bu)CH₂CH₂(CN) (4) and Ph₂P(N-t-Bu){C(Ph)CH(CO₂Et)} (5) are isolated. A novel heterocycle [(t-BuNH)P(N-t-Bu)₂P[C(CO₂Me)–CH(OMe)–C(O)–N(t-Bu)–] (6) obtained by utilizing 1 and MeO₂CCCCO₂Me is described. The structural proof for the second stage intermediate after the addition of phenols/carboxylic acids to the zwitterions formed in the reaction of electron deficient alkenes with P^III compounds [e.g. {(t-BuNH)P(N-t-Bu)₂P(NH-t-Bu)CH₂CH₂(CN)}⁺{X}⁻ where X = PhO⁻ + PhOH (7·Ph–O–H⋯OPh), 4-NO₂–C₆H₄–CO₂⁻ + H₂O (8·H₂O)] is also provided for the first time. X-Ray structural characterisation of 3–8 has also been accomplished.