Catalytic Enantioselective Preparation of α-Substituted Allylboronates: One-Pot Addition to Functionalized Aldehydes and a Route to Chiral Allylic Trifluoroborate Reagents

摘要

Additions of allylic boron reagents to aldehydes have evolved into one of the most popular methods for stereoselective CC bond formation.[1] Compared to dialkyl allylic boranes, allylic boronic esters are often more advantageous as a class of reagents because of their superior stability. Three strategies have been developed for the control of enantiofacial selectivity in additions of allylic boronates to achiral aldehydes: 1) the use of a chiral diol or a diamine auxiliary as the two nonallylic substituents on the boron center;[2] 2) the use of chiral Lewis and Bronsted acid catalysis with achiral boronates;[3] and 3) the use of optically pure α-substituted reagents (so-called α-chiral allylboronates).[4] The preparation of chiral α-substituted allylboronates 1 and their additions to aldehydes were pioneered by Hoffmann and co-workers.[4] Regrettably, these reagents have remained underused in part because of their stepwise preparation based on a Matteson asymmetric homologation of chiral alkenylboronates.[5], [6] The reagent-controlled additions of 1 to aldehydes proceed with near-complete transfer of chirality to give two diastereomeric products 4 and 5 (Scheme 1). These Z and E allylic alcohols are epimeric, and their ratio is highly dependent on the nature of the substituent R1 and the nature of the boronic ester.[4] The ratio of 4 and 5 can be explained in terms of steric and dipolar effects on the two competing transition structures 2 and 3. With a nonpolar alkyl substituent R1, steric interactions play a dominant role. The chairlike transition structure 2 can be destabilized by a steric interaction between a large boronic ester and the pseudoequatorial substituent R1. On the other hand, the transition structure 3 features unfavorable allylic interactions that result from the pseudoaxial position of the R1 substituent. The common use of a hindered ester, such as pinacolate, aggravates the interactions between R1 and the methyl groups of the pinacol moiety in 2. Thus, in this case transition structure 3 is more probable and leads to mixtures of products 4 and 5 in modest selectivities.[7]