STEREOSELECTIVE BINDING OF OLEFINS TO A MOLYBDENUM CENTER AND SYNTHESIS OF A CHIRAL MOLYBDENUM OLEFIN COMPLEX

Synthesis and characterization of the molybdenum-alkene complexes [Cp( P-P)Mo(CO)-(alkene)]+PF6- [P-P = 1,2-bis(dimethylphosphino)ethane (Dmp e), 1,2-bis(dimethoxyphosphino)-ethane (Pom-Pom); alkene = methyl acry late, ethyl acrylate, styrene, dimethyl maleate, p-chlorostyrene, alph a-methylene-gamma-butyrolactonel are reported. The Mo-alkene complexes were synthesized by reaction of [Cp(P-P)Mo(CO)(acetone)]+PF6 with the alkenes at room temperature. X-ray crystallographic analysis of [Cp(P om-Pom)Mo(CO)(ethyl acrylate)]+PF6- (9) was carried out. Crystal data for 9: P2(1)/n, a = 8.825 (2) angstrom, b = 27.834 (8) angstrom, c = 1 0.192 (2) angstrom, beta = 93.03(2)-degrees, V = 2500.0 (11) angstrom3 , Z = 4, R(F) = 4.93%. For the Pom-Pom complexes, high kinetic stereos electivity is observed for binding the olefin to produce only one dias tereomer. In this diastereomer, orbital overlap between the metal and the alkene is maximized; i.e., the olefin ligand is contained in the p lane defined by the metal, the center of the Cp ligand, and the trans- phosphorus atom, and the substituent on the olefin ligand lies syn to the Cp ring, thus minimizing steric hindrance between the phosphine an d the alkene substituent. For the Dmpe complexes, reaction of [Cp(Dmpe )Mo(CO)(acetone)]+PF6- with styrene resulted in similar stereoselectiv ity for olefin binding, as demonstrated by crystallographic data from the structures of [Cp(D;mpe)Mo(CO)(styrene)]+PF6- (17) and [Cp(Dmpe)Mo (CO)(dimethyl maleate)]+PF6- (15). Crystal data for 17: P2(1)/n, a = 8 .925 (3) angstrom, b = 35.824 (9) angstrom, c = 16.180 (5) angstrom, b eta = 102.75(2))-degrees, V = 5045 (2) angstrom3, z = 8, R(F) = 8.29%. Crystal data for 15: P2(1)2(1)2(1), a = 15.821 (4) angstrom, b = 17.6 37 (3) angstrom, c = 8.597 (2) angstrom, V = 2398.8 (9) angstrom3, Z = 4, R(F) = 3.76%. An unusual difference was noted in the formation of [Cp(Dmpe)Mo(CO)(methyl acrylate)]+PF6-, where a 50:50 mixture of two d iastereomeric alkene complexes was obtained. On standing at room tempe rature, the more thermodynamically stable alkene complex is formed exc lusively (>99%). The mechanism of isomerization of diastereomers is in tramolecular; i.e., no dissociation of the olefin ligand occurs during the isomerization process. Synthesis of the chiral molybdenum olefin complex [Cp-(P-P)Mo(CO)(methylacrylate)]+PF6- [P-P = bis[bis[(S)-2-met hyl-1-butanoxylphosphino]-ethane] was also carried out. Again, extreme ly high kinetic selectivity was observed in the formation of the olefi n complexes; however, when the synthesis was carried out under photoly tic conditions, a 60:40 mixture of the isomeric products was obtained. Isomerization did not occur in this system.