화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.116, No.22, 10241-10254, 1994
Chiral Organolanthanides Designed for Asymmetric Catalysis - A Kinetic and Mechanistic Study of Enantioselective Olefin Hydroamination/Cyclization and Hydrogenation by C-1-Symmetrical Me(2)Si(Me(4)C(5))(C(5)H(3)R-Asterisk)Ln Complexes Where R-Asterisk=chiral Auxiliary
The C-1-symmetric organolanthanide complexes Me(2)SiCp"(R*Cp)LnE(SiMe(3))(2) (Cp" = eta(5)-Me(4)C(5); R* = (1S,2S,5R)-trans-5-methyl-cis-2-(2-propyl)cyclohexyl ((+)-neomenthyl), (1R,2S,5R)-cis-5-methyl-trans-2-(2-propyl)- cyclohexyl ((-)-menthyl), and (1R,2S,SR)-cis-5-methyl-trans-2-(2-phenyl-2-propyl)-cyclohexyl ((-)-phenylmenthyl); Ln = La, Nd, Sm, Y, Lu; E = N, CH) serve as precatalysts for the efficient regio- and enantioselective hydroamination/ cyclization of the amino olefins 1-aminopent-4-ene, 2-amino-hex-5-ene, 2,2-dimethyl-1-aminopent-5-ene, and 2,2-dimethyl-1-aminohex-5-ene to yield the corresponding heterocycles 2-methylpyrrolidine, 2,5-dimethyipyrrolidine, 2,4,4-trimethylpyrrolidine, and 2,5,5-trimethylpiperidine, respectively. At 25 degrees C, enantiomeric excesses as high as 69% (74% at -30 degrees C) and turnover frequencies as high as 93 h(-1) are observed. Catalyst epimerization is observed in the presence of primary amines; however, equilibrium homochiralities are frequently very high (in some cases >95%), and epimerization is complete in the early stages of preparative scale reactions. The (+)-neomenthyl, (-)-menthyl, and (-)-phenylmenthyl catalysts afford 2-methylpyrrolidines with the (R) catalyst configuration selecting for (Ii) product configuration and (S) catalyst configuration selecting for (S) product configuration. Product stereochemistry can be understood in terms of olefin insertion via a chairlike, seven-membered transition state. The (+)-neomenthyl precatalysts (Ln = Nd, Sm) effect the cyclization of 2-aminohex-5-ene to trans-2,5-dimethylpyrrolidine in >95% diastereoselectivity at 25 degrees C, The corresponding hydrocarbyl complexes serve as precatalysts for the efficient asymmetric deuteration and hydrogenation of styrene and 2-phenyl-1-butene, respectively. For the organosamarium-derived catalysts, 2-phenyl-1-butene hydrogenation to yield exclusively 2-phenylbutane-1,2-d(2) under D-2 in a non-mass-transfer-limited reaction regime obeys the rate law nu = k[olefin](0)[lanthanide](1/2)[H-2](1), suggesting rapid, operationally irreversible olefin insertion(the step in which stereochemistry is fixed), a rapid preequilibrium involving an alkyl or alkyl/hydride dimer, and turnover-limiting hydrogenolysis of an intermediate samarium alkyl with k(H2)/k(D2) = 1.5-2.3 at 25 degrees C.