科研之友
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摘要
Tertiary aminoferrocenes complexed to boron trifluoride (BF3) arc shown to undergo asymmetric titillation with alkyllithiums in the presence of bulky chiral 1,2-diaminocyclohexane ligands This reaction represents the first BF3-activated asymmetric titillation of a prochiral aromatic amine and the first such transformation to be mediated by a chiral diamine other than (-)-sparteine The process provides rapid access to a broad range of enantiomerically enriched 2-substituted-1-aminoferrocenes. Including derivatives with uncommon substitution patterns that are of interest in catalysis. The enantioselectivity of the process is high enough (87:13 to 91:9 er) to allow for isolation of single enantiomers of several products after simple recrystallization as either the free aminoferrocenes or their ammonium fluoroborate salts. Both antipodes of the planar chiral 2-substituted-1-aminoferrocene products are accessible, as confirmed by single crystal X-ray diffraction analysis of two compounds with opposite relative stereochemistry. Single-point calculation of thirty-two different transition states of the reaction at the M06-2X/6-311 + g(2d.2p) level produced a computational model that correctly predicted both the sense and extent of chiral induction. Three factors appeared to play important roles in determining enantioinduction during titillation of BF3-complexed tertiary aminoferrocenes: (i) the maintenance of a highly organized eight-membered ring transition state, (ii) the existence of a strong Li F contact which placed the chiral diamine ligand in close proximity to the ferrocene substrate: (m) the orientation of the sterically demanding N-alkyl groups of the chiral diamine additives, either away or towards, the aminoferrocene and the alkyllithium. The model may serve as a predictive tool for the rational design of new ligands for this and related asymmetric lithiations.
- 出版日期2010-8
