Chiral organolanthanides designed for asymmetric catalysis. A kinetic and mechanistic study of enantioselective olefin hydroamination/cyclization and hydrogenation by C1-Symmetric Me2Si(Me4C5)(C5H 3R*)Ln complexes where R* = chiral auxiliary

Michael A. Giardello, Vincent P. Conticello, Laurent Brard, Michel R. Gagné, Tobin J Marks

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The C1-symmetric organolanthanide complexes Me2SiCp″(R*Cp)LnE(SiMe3)2 (Cp″ = η5-Me4C5; 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,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 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 (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 trarns-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-1-butene, respectively. For the organosamarium-derived catalysts, 2-phenyl-1-butene hydrogenation to yield exclusively 2-phenylbutane-1,2-d2 under D2 in a non-mass-transfer-limited reaction regime obeys the rate law v = k[olefin]0[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 turnoverlimiting 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-1 are observed at 25 °C, PH2 = 1 atm for the hydrogenation of 2-phenyl-1-butene. The (R) catalyst configuration selects for the (R) product and the (S) 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-d2 under D2 indicates that β-hydride elimination/readdition does not effectively compete with turnover-limiting deuterolysis.

Original languageEnglish
Pages (from-to)10241-10254
Number of pages14
JournalJournal of the American Chemical Society
Issue number22
Publication statusPublished - Nov 2 1994


ASJC Scopus subject areas

  • Chemistry(all)

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