INSERTION OF ACETYLENES INTO TRANS,TRANS-WH(CO)2(NO) (PME3)2

The insertion reactions of trans,trans-WH(CO)2(NO)(PMe3)2 with activated acetylenes of the type RC = CCO2R' (R = H, Me, Ph, R'O2C) were investigated. In all cases selective alpha-metalation occurs, affording tungsten-vinyl compounds, and except for R = Ph, selective trans insertion is observed as well. Thus, from the reaction of RC = CCO2Me (R = H, Me) the products W[Z-C(CO2Me) = CH(R)](CO)2(NO)(PMe3)2, where R = H (1) and Me (2), were isolated in high yield. The reaction with PhC = CCO2Et initially yields the trans- and cis-insertion products W[Z-C(CO2Me) = CH(Ph)](CO)2(NO)(PMe3)2 (3a) and W[E-C-(CO2Me) = CH(R)](CO)2(NO)(PMe3)2 (3b) in a 85:15 ratio. Compound 3a slowly and reversibly loses CO affording the four-membered metallacycle activated W{Z-C[C(O)OEt] = CH(Ph)}(CO)(NO)(PMe3)2 (3c). Heating equilibrium solutions of 3a and 3c and CO results in complete, irreversible conversion of these compounds to 3b. Insertion reactions of RO2CC = CCO2R initially afford W[Z-C(CO2R) = CH(CO2R)](CO)2(NO)(PMe3)2, where R = Me (4a) and tBu (5a). These compounds have half-lives of ca. 10 min at room temperature, as irreversible CO loss produces the isolable five-membered-ring compounds activated W{Z-C(CO2R) = CH[C(O)OR]}(CO)(NO)(PMe3)2 (4b, 5b). Remarkably enough, these compounds are also only metastable, since a PMe3-catalyzed isomerization occurs in which the NO and CO ligands switch positions, affording 4c and 5c, respectively. The X-ray structures of three complexes were determined: 3c, orthorhombic, space group P2(1)2(1)2(1), a = 15.985 (5) angstrom, b = 16.000 (4) angstrom, c = 9.167 (3) angstrom, Z = 4; 5b, monoclinic, space group P2(1)/c, a = 12.282 (5) angstrom, b = 11.238 (4) angstrom, c = 19.463 (8) angstrom, beta = 98.87 (3)-degrees; 5c, monoclinic, space group P2(1)/c, a = 10.537 (4) angstrom, b = 12.319 (4) angstrom, c = 21.606 (8) angstrom, beta = 95.20 (3)-degrees. Comparison of structures 5b and 5c provides direct insight into the difference in electronic effects between CO and NO ligands.