Two complementary approaches for the synthesis and isolation of stable phosphanylphosphaalkenes

Abstract

The phospha–Wittig (phosphanylphosphinidene titanium(IV) complex) and phospha–Peterson (lithiated diphosphane) reactions were used to obtain phosphanylphosphaalkenes with the general formula XYCP–PtBu₂ (X = alkyl or aryl group or H; Y = alkyl or aryl group). Therefore, two series of reactions with different ketones and aldehydes were performed. Examination revealed that the two methods are complementary. For smaller carbonyl compounds, the phospha–Wittig reaction was shown to be a much better method, while for larger substituents (mainly aromatic), the phospha–Peterson reaction was shown to be a reliable method. These studies led to, among other things, the isolation of seven phosphanylphosphaalkenes in crystalline form after the phospha–Peterson reaction: (Ph)₂CP–PtBu₂ (3a), Z-(Ph)(4-CN-Ph)CP–PtBu₂ (3c), (4-CN-Ph)₂CP–PtBu₂ (3d), (4-MeO-Ph)₂CP–PtBu₂ (3e), E-tBu(Ph)CP–PtBu₂ (3f), E-{(Me)₂N-Ph}(H)CP–PtBu₂ (4d) and E-PhPh(H)CP–PtBu₂ (4e). The corresponding compounds were obtained in high yields under mild conditions, and more importantly, these new species are relatively air- and absolutely moisture-stable, especially those originating from aldehydes. In addition, a comprehensive DFT study helped us to establish not only the key factors crucial for effective phospha–Wittig-based synthesis of CP–P species but also the consecutive steps along the reaction path leading to the formation of these compounds.