Asymmetric hydrogenation of N-alkyl and N-aryl ketimines using chiral cationic Ru(diamine) complexes as catalysts: the counteranion and solvent effects, and substrate scope
作者:Fei Chen、Ziyuan Ding、Yanmei He、Jie Qin、Tianli Wang、Qing-Hua Fan
DOI:10.1016/j.tet.2012.03.019
日期:2012.7
Asymmetrichydrogenation of N-alkyl and N-aryl ketimines catalyzed by chiralcationic η6-arene-(N-monosulfonylated diamine) Ru(II) complexes has been investigated. Strong counteranion and solvent effects on the enantioselectivity were observed. The ruthenium catalyst bearing non-coordinating BArF− anion was found to be particularly effective for the hydrogenation of acyclic and exocyclic N-alkyl ketimines
The Paternò-Büchi Reaction of α-Alkyl-Substituted Enecarbamates and Benzaldehyde
作者:Thorsten Bach、Jürgen Schröder
DOI:10.1055/s-2001-15075
日期:——
The α-substituted enecarbamates and enamides 5a-d, 5f and 6 were prepared in two steps from the corresponding ketone, N-benzylamine and an appropriate acylating agent (Boc2O, Ac2O). The [2+2] photocycloaddition reactions of benzaldehyde to the alkenes 5a-d which bear a primary or secondary alkyl substituent proceeded smoothly and gave the 3-aminooxetanes 8a-d in moderate to good yields (46-71 %). The α-phenyl-substituted enecarbamate 5f did not produce a photocycloaddition product presumably due to rapid energy transfer (triplet sensitization) from the photoexcited aldehyde. For less obvious reasons the tert-butyl-substituted enamide 6 did not react in the Paternò-Büchi reaction either. The 3-alkyl-3-aminooxetanes 8 were obtained as a mixture of cis- and trans-diastereoisomers. An increase in the steric bulk of the alkyl substituent R shifted the diastereomeric ratio (cis-8/trans-8) in the direction of the thermodynamically more stable cis-product (29:71 for R = CH3 up to 57:43 for R = cyclohexyl). The separated oxetane diastereoisomers cis-8a and trans-8a (R = CH3) underwent a smooth ring opening/cyclization reaction upon treatment with trifluoroacetic acid. Oxetane trans-8a yielded the oxazolidinones 9 and trans-10 (92 %), oxetane cis-8a gave exclusively the oxazolidinone cis-10 (54 %).
The influence of the alcohol, as the hydrogen donor, on the efficiency and selectivity of the asymmetrictransferhydrogenation (ATH) of imines is reported for the first time. This discovery not only leads to a highly enantioselective access to N‐aryl and N‐alkyl amines, but also provides new insight into the mechanism of the ATH of imines. Both experimental and computational studies provide support