Mechanism-Based Enantiodivergence in Manganese Reduction Catalysis: A Chiral Pincer Complex for the Highly Enantioselective Hydroboration of Ketones
作者:Vladislav Vasilenko、Clemens K. Blasius、Hubert Wadepohl、Lutz H. Gade
DOI:10.1002/anie.201704184
日期:2017.7.10
A manganese alkyl complex containing a chiralbis(oxazolinyl‐methylidene)isoindoline pincer ligand is a precatalyst for a catalytic system of unprecedented activity and selectivity in the enantioselective hydroboration of ketones, thus producing preparatively useful chiral alcohols in excellent yields with up to greater than 99 % ee. It is applicable for both aryl alkyl and dialkyl ketone reduction
含有手性双(恶唑啉基-亚甲基)异二氢吲哚钳位配体的锰烷基络合物是酮对映选择性氢硼化反应中空前的活性和选择性的催化体系的预催化剂,从而以高达99的高收率生产了制备有用的手性醇。 %ee。它适用于在温和的反应条件下(TOF> 450 h -1在-40°C下)。富含地球的贱金属催化剂在极低的催化剂负载量(低至0.1 mol%)下运行,并且具有很高的官能团耐受性。有证据表明存在两种不同的锰催化氢化物转移机理,并介绍了它们在选择性确定步骤中对映体控制的作用。
Asymmetric Hydrogenation of 1-Alkyl and 1-Aryl Vinyl Benzoates: A Broad Scope Procedure for the Highly Enantioselective Synthesis of 1-Substituted Ethyl Benzoates
作者:Patryk Kleman、Pedro J. González-Liste、Sergio E. García-Garrido、Victorio Cadierno、Antonio Pizzano
DOI:10.1021/cs501402z
日期:2014.12.5
The enantioselective hydrogenation of enol esters of formula CH2=C(OBz)R with rhodium catalysts based on phosphine phosphite ligands (P-OP) has been studied. The reaction has a broad scope, and it is suitable for the preparation of products possessing a wide variety of R substituents. For the cases where R is a primary alkyl, high catalyst activity (S/C = 500) and enantioselectivities (95-99% ee) were obtained with a catalyst characterized by an ethane backbone and a PPh2 fragment. In contrast, for R = t-Bu, a catalyst possessing a benzene backbone provided the best results (97% ee). Derivatives with a cycloalkyl R substituent were particularly difficult substrates for this reaction. A broader catalyst screening was required for these substrates, which identified a catalyst possessing a P(m-xylyl)(2) fragment as the most appropriate one, affording enantioselectivities between 90 and 95% ee. Outstanding enantioselectivities (99% ee) and high catalyst activity (S/C = 500-1000) were also obtained in the case of substrates bearing a Ph or a fluoroaryl R substituent. In addition, the system is also appropriate for the preparation of other synthetically useful esters as those for R = benzyl, 2-phenylethyl or N-phthalimido alkyl chains. Likewise, the hydrogenation of divinyl dibenzoates proceeded with very high diastero- and enantioselectivity, generating rather low amounts of the meso isomer (3-6%). On the other hand, substrates with Br and MeO substituents at the phenyl benzoate ring, suitable for further functionalization, have also been examined. The results obtained indicate no detrimental effect of these substituents in the hydrogenation. Alternatively, the methodology has been applied to the highly enantioselective synthesis of deuterium isotopomers of 1-octyl benzoate bearing CDH2, CD2H, or CD3 fragments. Finally, as a practical advantage of the present system, it has been observed that the high performance of the catalysts is retained in highly concentrated solutions or even in the neat substrate, minimizing both the amount of solvent added and the volume of the reaction.
The Influence of Branched Chains on Optical Activity. The Configuration of Methyl-tertiary-butylcarbinol