Acyl-Transfer Mechanisms Involving Various Acyl Functional Groups: >XY with X = C, S, P and Y = O, S

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Abstract

Mechanisms of the gas-phase acyl group transfers, Cl- + R(XY)Cl, involving various acyl functional groups, >XY with X = C, S, or P and Y = O or S, are investigated theoretically at the MP2/6-31+G* and B3LYP/6-31+G* levels (additionally with extended basis sets of B3LYP/6-311+G(3df,2p)), and the effects of solvent (ε = 78.5) are calculated with the SCIPCM model at the isodensity level of 0.0004 au. The tetrahedral adducts formed in the carbonyl (RCO) and thiocarbonyl (RCS) group transfers are either transition states (double-well PES) or intermediates (single- or triple-well PES) depending on R, a stronger electron acceptor R favoring the intermediate. However, all of the sulfonyl (RSO2) and phosphoryl ((RO)2PO) transfers proceed with trigonal bipyramid (TBP)-type transition states, in contrast to the stepwise mechanism through TBP-type intermediates for the sulfinyl (RSO) (and sulfonyl transfers between F-) transfers. The most important factor determining whether an adduct in an acyl-group-transfer reaction is the transition state or intermediate is the energy gap between the and orbitals. The possibility of reacting through an intermediate is greater for lower and higher levels. The backside σ-attack pathway is favored over the π-attack pathway only when a low-lying orbital, preferably below the level, is available. In general, the results are in good agreement with those of experiments. The solvent effect elevates the barrier height almost uniformly so that the relative orders of gas-phase activation barriers between different R groups are maintained in solution.

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