Phosphaalkenes as Building Blocks in Ene Reactions:  Synthesis and Reactivity of 3-Amino-1,2-dihydro-1,2-diphosphetes

Abstract

Phospha−ene reactions of the type II between methylidenephosphanes as enophiles and C -aminophosphaalkenes possessing allylic hydrogen atoms proceed by P−P bond formation to furnish the corresponding functionalized diphosphanes. Thus, the reaction of the methylidenephosphane 1 with the C -amino-substituted ethylidenephosphane 3a runs smoothly at room temperature to afford the unsymmetrical 1,2-diphosphane 4 as a 60:40 mixture of two diastereomers in 64% yield. Rotation about the P−N bond in 4 is hindered at room temperature, but free enthalpies of activation of Δ G ⧧ (PN) = 16.3 kcal·mol -1 (major diastereomer) and 15.2 kcal·mol -1 (minor diastereomer) were determined by variable temperature 1 H NMR spectroscopy. In contrast, reactions of the ethylidenephosphanes 3a − c with the chlorophosphane 2 as enophile follow an unusual course involving a regiospecific ene reaction of the type II and a subsequent intramolecular ring closure reaction to furnish the 3-amino-1,2-dihydro-1,2-diphosphetes 7a − c in good yields (53−79%). The reactivity of this new class of heterocyclic compounds has been studied exemplarily for product 7a . The trans form of 7a obtained by synthesis can be converted to the corresponding cis form photochemically. Complexation of both phosphorus centers in 7a is possible by reaction with 2 equiv of diiron nonacarbonyl and gives rise to complex 8, the constitution of which has been confirmed by X-ray crystallography. Unexpectedly, hydrolysis of 7a does not proceed by attack at the endocyclic enamine unit but rather by way of P−P bond cleavage to afford the functionalized phosphaalkene 9 . On the other hand, oxidation of 7a by bis(trimethylsilyl) peroxide takes place with retention of the P 2 C 2 skeleton: a selective increase in coordination at P-1 results in the formation of the 1-oxo-1,2-dihydrodiphosphete 10, the structure of which was confirmed by X-ray crystallography. The reactions of 7a with the electron-poor acetylenes 11a − d also follow an unusual course. Nucleophilic attack of P-1 at the C⋮C triple bond and proton shift lead stereospecifically to the 1-methylene-1,2-dihydro-1,2-diphosphetes 12a − d as thermolabile addition products. The constitution and configuration of 12c were unambiguously confirmed by X-ray crystallography.