Insight into the Mechanism of the Asymmetric Addition of Alkyl Groups to Aldehydes Catalyzed by Titanium-BINOLate Species
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- 作者：Jaume Balsells ; Timothy J. Davis ; Patrick Carroll ; and Patrick J. Walsh
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：September 4, 2002
- 年：2002
- 卷：124
- 期：35
- 页码：10336 - 10348
- 全文大小：392K
- 年卷期：v.124,no.35(September 4, 2002)
- ISSN：1520-5126
文摘
The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexeshas become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigatedthe mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst,(BINOLate)[Ti(O-i-Pr)3]2, based on mono-oxygen-alkylated BINOL ligands have been examined. Comparisonof the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicatethat the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)3]2, is not active in the asymmetric addition reaction.Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography.These include the dinuclear (BINOLate)Ti(O-i-Pr)2·Ti(O-i-Pr)4, which contains a bridging naphtholate andisopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)2·[Ti(O-i-Pr)4]2, and trimeric [(BINOL)Ti(O-i-Pr)2]3. The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)2 prefers to bind to titaniumtetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalyticreaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsiblefor the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposedintermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactionswere examined using different sources of the alkyl group [ZnMe2 or MeTi(O-i-Pr)3]. Under similar conditions,it was found that the product ee's were the same, independent of whether ZnMe2 or Me-Ti(O-i-Pr)3 wasused as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkylgroup to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, wehypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)2(aldehyde)·MeTi(O-i-Pr)3.