The C1-symmetric organolanthanide complexes Me2SiCpʺ(R*Cp)LnE(SiMe3)2(Cpʺ = η5-Me4C5; R* = (15, 25, 5/?)-tras-5-methyl-cis-2-(2-propyl)cyclohexyl((+)-neomenthyl) (1R, 2S, 5R)-cis-5-methyl-trans-2-(2-propyl)- cyclohexyl ((-)-menthyl), and (1R, 25, 5R)-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-dimethylpyrrolidine, 2, 4, 4-trimethylpyrrolidine, and 2, 5, 5-trimethylpiperidine, respectively. At 25 °C, enantiomeric excesses as high as 69% (74% at -30 °C) and turnover frequencies as high as 93 Ir293are 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 (R) 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 frans-2, 5-dimethylpyrrolidine in >95% diastereoselectivity at 25 °C. The corresponding hydrocarbyl complexes serve as precatalysts for the efficient asymmetric deuteration and hydrogenation of styrene and 2-phenyl-l-butene, respectively. For the organosamarium-derived catalysts, 2-phenyl-l-butene hydrogenation to yield exclusively 2-phenylbutane-1, 2-d2under D2in a non-mass-transfer-limited reaction regime obeys the rate law v = k[olefin]°[lanthanide]1/2[H2]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 turn over-limiting hydrogenolysis of an intermediate samarium alkyl with kH2/kD2= 1.5-2.3 at 25 °C. Enantiomeric excesses as high as 64% (96% at -80 °C) and turnover frequencies as high as 26 000 h-1are observed at 25 °C, Ph2= 1 atm for the hydrogenation of 2-phenyl-l-butene. The (R) catalyst configuration selects for the (R) product and the (5) catalyst configuration for the (S) product, with no major nonlinear effects evident in studies with (R) + (S) mixtures. Product stereochemistry can be understood in terms of olefin approach along the ring centroid-metal-ring centroid angle bisector. Under the same conditions, the deuteration of styrene proceeds at comparable rates and higher selectivities, 72% (S) and 43% (R) ee with the (70/30) (S)/(R) and (R)-(-)-menthyl samarium hydrocarbyls, respectively. Exclusive formation of ethylbenzene-1, 2-d2under D2 indicates that β-hydride elimination/readdition does not effectively compete with turnover-limiting deuterolysis.
ASJC Scopus subject areas
- Colloid and Surface Chemistry