Theoretical study of the molecular mechanism of the domino pathways for squarate ester sequential reactions

Abstract

The PM3 semiempirical method was selected to study the stereoselectivity along the domino pathways corresponding to the twofold addition of the chiral (R)-3-tert-butylcyclopentenyllithium 2 and the achiral cyclopentenyllithium 5 to dimethyl squarate ester 1 as a model for squarate ester sequential reactions. Stationary points, reactants, intermediates, transition structures and products, on the reactive potential energy surface were characterized with analytical gradient techniques. The main steps of this consecutive chemical reaction can be classified as follows: (1) addition of 2 to 1 to obtain the ketoalkoxide products 3 and 4; (ii) a domino sequence corresponding to an initial addition of 5 to these products to yield dialkoxide intermediates IN1 and IN2; subsequent C—C bond breaking processes of IN1 and IN2, associated with electrocyclic four-membered ring cleavage, to yield the endo and exo octatetraene rotamers IN3 and IN4, respectively; the final cyclizations give the cyclooctatriene products 6 and 7, respectively; and (iii) hydrolysis and subsequent intramolecular aldolic condensation of these cyclooctatrienes to obtain the final products. The theoretical results show that the step controlling the product stereochemisty corresponds to the conrotatory cyclization of the exo octatetraene intermediate IN4. The large value of the barrier height associated with the cyclization of the endo octatetraene IN3 together with the easy interconversion between the octatetraene rotamers IN3 and IN4 indicate that the cyclization process takes place only through the exo octatetraene IN4. Copyright © 1999 John Wiley & Sons, Ltd.