Preparation, Properties, and Reactions of Metal‐Containing Heterocycles, XCI . Insertion of Activated Alkynes and Sulfur into the P‐P Bond of Diphosphamolybdacyclopropanes

The insertion of the alkynes ZC≡CZ ( 2a‐i ) [Z = Co 2 R: R = Me ( a ), Et ( b ), n Pr ( c ), i Pr ( d ), n Bu ( e ), t Bu ( f ), neo ‐Pent ( g ), cyc‐Hex ( h ); Z = CF 3 ( i )] into the P 1 ‐P 2 bond of the diphos‐phamolybdacyclopropane Mes ( 1 ) (Mes = 2,4,6‐trimethylphenyl) results in the formation of the diphosphamolybdacyclopentenes Mes ( 3a‐i ). According to an X‐ray structural analysis, 3a crystallizes in the space group P 2 1 / c with the mesityl function at P 2 and the cyclopentadienyl fragment at the metal atom on the opposite side of the planar five‐membered ring. The same orientation is found in solution by NMR experiments (NOE; 2D 1 H, 13 C HETCOR). Kinetic investigations confirm that the rate constants decrease in the sequence R = Me > Et > n Pr > iso Prop > Cy > n Bu > neo ‐Pent > t Bu. The second‐order reaction and the strongly negative activation entropy which was determined in the case of the reaction 1 + 2e → 3e are consistent with a nucleophilic attack of P 2 at one of the triply bonded carbon atoms of the alkyne. The reaction of 3a, h with sulfur affords the diastereomeric compounds Mes ( 4a, h ). An X‐ray structural analysis of the heterocycle 4h , which crystallizes in the space group P 1, shows the presence of the enantiomeric pair SR/RS. The reaction of 1 with alkynes is hindered if this heterocycle is oxidized by sulfur to give Mes ( 5 ). Further reaction of 5 with sulfur leads to the ring‐expanded heterocyclic intermediate ( 6 ) which crystallizes in the space group P 1. The five‐membered ring in 6 has an envelope conformation. Upon thermally induced cleavage of the P 2 S 2 Mes fragment from 6 the three‐membered ring ( 7 ) is formed as the final product.