Theoretical Study of Chemo-, Regio-, and Stereoselectivity in 1,3-Dipolar Cycloadditions of Nitrones and Nitrile Oxides to Free and Pt-Bound Bifunctional Dipolarophiles

Abstract

1,3-Dipolar cycloadditions of nitrones RCH=N(CH3)O and the nitrile oxide CH3CNO to the bifunctional cyanoalkynes NC-CCR' and cyanoalkenes E-NCCH=CHR' (R=H, Ph; R'=H, Ph)--both free and ligated to PtII and PtIV--were investigated by theoretical methods at B3LYP and, for some reactions, CCSD(T) and CBS-Q levels of theory. Chemo-, regio-, and stereoselectivity of the processes and factors, which affect the reactivity and selectivity, were analyzed, and verified trends are discussed in details. Coordination of dipolarophiles to PtII and, particularly, to PtIV facilitates the CN relative to the CC additions of nitrones due to higher activation of the CN group in comparison to the CC group. The bonding of the ligands to platinum also favors the meta versus ortho pathways and endo versus exo pathways that sometimes lead to a switch of the reaction direction. Introduction of Ph groups into the reactant(s) molecules also leads to the promotion of the CN versus CC routes, and this effect is especially strong when both reactants are Ph-substituted. The substituent effect is accounted for by steric repulsions imposed by the Ph groups in transition states (TSs) and by the loss of a conjugation in phenylnitrone and phenylcyanoalkene molecules upon the TS formation. Solvation inhibits the CN and meta-CC additions and, hence, generally favors the CC versus CN pathway, the ortho versus meta pathway, and the exo versus endo pathway. All reactions except one proceed concertedly via a nearly synchronous mechanism for the CN and meta-CC additions to free ligands and asynchronous mechanism for the other processes. For the reaction CH2=N(CH3)O + PtIV-1, a stepwise route is realized.