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. The 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 both free anions the alpha-pathway is sharply preferred. In contrast, for (1,1-dichloroallyl)lithium the difference between the two activation energies 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 with available experimental evidences. A balance between an exothermicity factor and the different destabilization undergone by the various reactants in the transition structures offers a qualitative rationalization of the opposite regioselective inclinations of (1,1-dichloroallyl)lithium with respect to the other systems.
Canepa
Carlo
C
Dipartimento di Chimica Generale e Organica Applicata dell' Università di Torino, Corso Massimo D'Azeglio, 48 10125 Torino, Italy.
Tonachini
Glauco
G