AN AB INITIO MOLECULAR ORBITAL STUDY ON THE AMINO ALCOHOL-PROMOTED REACTION OF DIALKYLZINCS AND ALDEHYDES

摘要

The beta-dialkylamino alcohol-promoted reaction of dialkylzincs and aldehydes has been studied by ab initio molecular orbital calculations using a model system consisting of 2-aminoethanol, dimethylzinc, and formaldehyde. In the organometallic addition reaction, methylzinc alkoxide 1 formed from dimethylzinc and 2-aminoethanol by elimination of methane acts as an actual catalyst, which exists in equilibration with stereoisomeric dimers 2. Sterically less congested anti-2 is more stable than syn-2 by 3.1 kcal/mol. The tricoordinate Zn compound 1, acting as a bifunctional catalyst, assembles dimethylzinc and formaldehyde via 3 or 4 to form the product-forming complex 5. The frontier MOs and structure of 5 indicate that the formation of the mixed-ligand Zn complexes considerably increases the nucleophilic character of the Zn-CH3 group and electrophilic property of the aldehyde. As a consequence, 5 undergoes intramolecular alkyl migration to produce zinc ethoxide 6 which has a bridged structure. This turnover-limiting reaction occurs via the 4/4-bicyclic transition structure, anti- or syn-10, where the methyl group migrates with retention of configuration. The six-membered cyclic transition state that causes the alkyl migration with inversion of configuration is of higher energy. The final product 6 which is viewed as a complex of 1 and Zn alkoxide 7, upon interaction with dimethylzinc or formaldehyde, collapses into 3 or 4, respectively, and 9 (a tetramer of 7). The structural characteristics of the intermediates, products, and transition states are described. This calculation also clarifies the origin of the ligand acceleration; why the reaction is effected only when a few mol %, but not 0% or 100%, of an amino alcohol is employed. The high stability of the final tetrameric Zn alkoxide 9 prevents the undesired product inhibition of the catalytic reaction. The relative stability of the catalyst dimers, syn- and anti-2, is consistent with the chiral amplification phenomena experimentally observed with (2S)-3-exo-(dimethylamino)-borneol. The presence of the amino moiety plays a significant role in the dissociation of the dimers. [References: 60]