REGIOSELECTIVITY PATTERNS FEATURED BY FORMALDEHYDE IN THE ELECTROPHILIC ADDITION TO GEM-DIFLUORO AND GEM-DICHLOROALLYL SYSTEMS - AN AB-INITIO THEORETICAL-STUDY

The addition reactions of formaldehyde with (1,1-difluoroallyl)lithium and (1,1-dichloroallyl)lithium and -potassium (and corresponding free anions) have been investigated at the HF and MP2 levels of theory. Th e two competing pathways leading from the initial electrostatic sigma- complexes to the alpha and gamma addition products have been studied. In (1,1-difluoroallyl)lithium, (1,1-dichloroallyl)potassium, and bath free anions the alpha-pathway is sharply preferred. In contrast, for ( 1,1-dichloroallyl)lithium the difference between the two activation en ergies is smaller and in favor of the gamma-pathway: as a consequence, on the basis of the present calculations, it is expected that in this case varying substituents on the electrophilic carbon could modulate the regioselective preferences. All results appear to be consistent wi th available experimental evidences. A balance between an exothermicit y factor and the different destabilization undergone by the various re actants in the transition structures offers a qualitative rationalizat ion of the opposite regioselective inclinations of (1,1-dichloroallyl) lithium with respect to the other systems.