Specific Molecular Orbital Contributions to Nucleophilicity. The Thiocarbonyl Group as Privileged Monitor To Pinpoint Active and Less Active Molecular Orbitals in Reactions with Methylating Agents

Article abstract of DOI:10.1021/jo00113a010

The rate constants for 41 compounds bearing a C=S function reacting with MeX (X = I, Tos) span 7 orders of magnitude.The PES spectra of these compounds display two very low energy peaks, which stand clearly apart from the other peaks.These two peaks correspond to the ? orbitals of the C-S group; one is its CS ? bonding orbital oriented out of the molecular plane (?_CS) and the other its p-type in-plane lone pair orbital (?_S).For some of the compounds, the HOMO is the ?_CS orbital and for others the HOMO is the ?_S lone pair orbital.The best correlation (R = 0.96) between rate constants k and PES data is obtained when ln(k) is plotted against the inverse of PES energy of the ?_S lone pair orbital.Whether this lone pair orbital is the HOMO or the next lower HOMO has no importance.A modest correlation (R = 0.78) is obtained when ln(k) is plotted against the inverse of PES energy of the ?_CS bonding orbital.An attempt to correlate the calculated energy of the third highest occupied orbital (from AM1 calculations) with ln(k) provides a complete scattering of data (R < 0.1), but the calculated energy of the second lone pair orbital ?_S (ca. 90 kcal mol^-1 deeper than the HOMO) correlates reasonably with ln(k) (R = 0.88).The energies of the S 2s and 2p core orbitals (calculated for 13 cyclic compounds with the HF/3-21G technique to be 4000 to 5500 kcal mol^-1 deeper than HOMO) correlate with ln(k) (R = 0.86) as well as does that of the second lone pair orbital ?_S.These results are the first where both frontier orbitals and core orbitals display correlation with overall reactivity.They are discussed in terms of direct (perturbational) versus indirect (nonperturbational) concepts.