金属有机化学第五章.pptVIP

Migratory insertion into M-H bonds occurs at much higher rates than insertion into M-R bonds. As in CO migratory insertion, alkene insertion into M-R is thermodynamically favored over M-H insertion. Therefore, the preference for M-H insertion must be due to kinetic factors, not thermodynamic factors. D. Insertion into M-H versus M-R: J. Am. Chem. Soc. 1992, 114, 10394 24.7 15.0 PMe3 Cp 23.4 15.0 P(OMe)3 Cp 23.4 12.1 PMe3 Cp* 22.4 12.2 P(OMe)3 Cp* ?G? Et-Mig (kcal/mol) ?G? H-Mig (kcal/mol) L Cp’ The difference in activation energies suggests kH/kEt = 106~108. Migratory Elimination A. ?-Hydride Elimination Elimination is the microscopic reverse of 1,2-insertion, therefore the mechanisms must be the same. In particular, ?-H elimination requires an open coordination site, and that the M-C-C-H group can become coplanar. For 18 electron complexes, a ligand must always dissociate to form an open coordination site cis to the alkyl group undergoing ?-H elimination. Ligand dissociation may, or may not, be rate limiting. D. L. Reger, J. Am. Chem. Soc. 1976, 98, 2789 Experimental observations 1. Only alkene products were isolated. 2. Decompositions of 1,1-d2 or 2,2-d2 butyl complexes resulted in complete scrambling of deuterium in the 1-butene product. 3. First order kinetics with a very small kinetic isotope effect for 1,1-d2 or 2,2-d2 butyl complexes. 4. Reaction is completely retarded in the presence of excess PPh3. Other Lewis bases (THF or dioxane) also slow down the reaction, but to a lesser extent. 5. Starting material recovered from reactions stopped after less than one half-life showed extensive scrambling of deuterium throughout the alkyl chain. For 16 electron complexes, prior ligand dissociation is not required with the exception of Pd(II) and particularly Pt(II), which do not like to become 18e complexes. (J. Am. Chem. Soc., 1972, 94, 5258; ibid., 1973, 95, 4491, ibid, 1976, 98, 6521; Organometallics, 1986, 5, 1473) The platinacyclopentane analog undergoes decomp