PREPARATION, PROPERTIES, AND REACTIONS OF METAL-CONTAINING HETEROCYCLES .91. INSERTION OF ACTIVATED ALKYNES AND SULFUR INTO THE P-P BOND OFDIPHOSPHAMOLYBDACYCLOPROPANES

The insertion of the alkynes ZC=CZ (2a-i) [Z = CO(2)R: R = Me (a), Et (b), nPr (c), iPr (d), nBu (e), tBu (f), neo-Pent (g), eye-Hex Fl; Z = CF3 (i)] into the p(1)-p(2) bond of the diphosphamolybdacyclopropane (eta(5)-C5H5) (OC)(2)Mo-P(1)Ph(2)-CZ=ZC-P(2)MeS (1) (Mes = 2,4,6-trime thylphenyl) results in the formation of the diphosphamolytbdacyclopent enes (eta(5)-C5H5)(OC)(2)Mo-P(1)Ph(2)-CZ=ZC-P(2)Mes (3a-i). According to an X-ray structural analysis, 3a crystallizes in the space group P2 (1)/c with the mesityl function at P-2 and the cyclopentadienyl fragme nt at the metal atom on the opposite side of the planar five-membered ring. The same orientation is found in solution by NMR experiments (NO E; 2D H-1, C-13 HETCOR). Kinetic investigations confirm that the rate constants decrease in the sequence R = Me > Et > nPr > isoProp > Cy > nBu > neo-Pent > tBu. The second-order reaction and the strongly negat ive activation entropy which was determined in the case of the reactio n 1 + 2e --> 3e are consistent with a nucleophilic attack of P-2 at on e of the triply bonded carbon atoms of the alkyne. The reaction of 3a, h with sulfur affords the diastereomeric compounds (eta(5)-C5H5(OC)(2 )Mo-P(1)Ph(2)-CZ=CZ-P-2(S)Mes (4a, h). An X-ray structural analysis of the heterocycle 4h, which crystallizes in the space group P ($) over bar 1, shows the presence of the enantiomeric pair SR/RS. The reaction of 1 with alkynes is hindered if this heterocycle is oxidized by sulf ur to give (eta(5)-C5H5)(OC)(2)Mo-P(1)Ph(2)-P-2(S)Mes (5). Further rea ction of 5 with sulfur leads to the ring-expanded heterocyclic interme diate (eta(5)-C5H5)(OC)(2)Mo-P(1)Ph(2)-S-P-2(S)Mes-S (6) which crystal lizes in the space group P ($) over bar 1. The five-membered ring in 6 has ax envelope conformation. Upon thermally induced cleavage of the P(2)S(2)Mes fragment from 6 the three-membered ring (eta(5)-C5H5)(OC)( 2)Mo-P(1)Ph(2)-S (7) is formed as the final product.