PHOSPHAALKENES AS BUILDING-BLOCKS IN ENE REACTIONS - SYNTHESIS AND REACTIVITY OF 3-AMINO-1,2-DIHYDRO-1,2-DIPHOSPHETES

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 functionali zed diphosphanes. Thus, the reaction of the methylidenephosphane 1 wit h 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 activati on of Delta G(double dagger)(PN) = 16.3 kcal.mol(-1) (major diastereom er) and 15.2 kcal . mol(-1) (minor diastereomer) were determined by va riable temperature H-1 NMR spectroscopy; In contrast, reactions of the ethylidenephosphanes 3a-c with the chlorophosphane 2 as enophile foll ow an unusual course involving a regiospecific ene reaction of the typ e II and a subsequent intramolecular ring closure reaction to furnish the 3-amino-1,2-dihydro-1,2-diphosphetes 7a-e in good yields (53-79%). The reactivity of this new class of heterocyclic compounds has been s tudied exemplarily for product 7a. The trans form of 7a obtained by sy nthesis can be converted to the corresponding cis form photochemically . Complexation of both phosphorus centers in 7a is possible by reactio n with 2 equiv of diiron nonacarbonyl and gives rise to complex 8, the constitution of which has been confirmed by X-ray crystallography. Un expectedly, hydrolysis of ?a does not proceed by attack at the endocyc lic 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 P2C2 sk eleton: a selective increase in coordination at P-1 results in the for mation of the 1-oxo-1,2-dihydrodiphosphete 10, the structure of which was confirmed by X-ray crystallography. The reactions of 7a with the e lectron-poor acetylenes 11a-d also follow an unusual course. Nucleophi lic attack of P-1 at the C=C triple bond and proton shift lead stereos pecifically to the 1-methylene-1,2-dihydro-1,2-diphosphetes 12a-d as t hermolabile addition products. The constitution and configuration of 1 2c were unambiguously confirmed by X-ray crystallography.