ABINITIO COMPARATIVE-STUDY OF THE ADDITIONS TO THE CARBON AND OXYGEN OF ACETALDEHYDE LITHIUM ENOLATE BY FORMALDEHYDE AND KETENE

Ab initio (3-21G) calculations of the reaction of ketene (1) and formaldehyde (4) with the lithium enolate of acetaldehyde (5) show that in both cases the lithium cation coordinates both oxygens in the complex, the transition structures, and the products. Reaction at carbon of 5 by 1 and 4 proceeds from the initial complexes via half-chair conformations, with activation energies of 6.7 and 1.9 kcal/mol, respectively, and with overall exothermicities of 41.9 and 40.2 kcal/mol, respectively, consistent with a recent thermochemical study. Reaction at oxygen of 5 by 1 and 4 proceeds from the initial complexes via planar four-centered transition structures with activation energies of 5.5 and 5.0 kcal/mol, respectively, and with overall exothermicities of 27.7 and 29.5 kcal/mol, respectively. Thus CH2=O (4) has a kinetic preference for reaction at carbon of CH2=CHOLi (5), whereas CH2=C=O (1) is less reactive than 4 but has a slight kinetic preference for reaction at oxygen of 5 although this does not lead to the most stable product, in agreement with experiment. Single-point energy calculations at higher levels do not change the qualitative conclusions. © 1990, American Chemical Society. All rights reserved.