Nucleophilic Addition to (η⁶-Alkylbenzene)Cr(CO)₃ Complexes. Dependence of Regloselectivlty on the Size of the Alkyl Group and the Reactivity of the Nucleophile

The regioselectivity of addition of carbanions to (η6-alkylbenzene)Cr(CO)3 complexes correlates well with a frontier orbital picture using the lowest energy arene-centered molecular orbital in the (arene)Cr(CO)3 complex. However, the selectivity is sensitive to both the size of the alkyl substituent on the arene and the reactivity of the carbanion. Para substitution can become important with large alkyl groups and more stabilized carbanions. Systematic variation of anion reactivity using 2-aryH,3-dithiane derivatives shows the effect clearly. A rationale based on a balance of charge control and orbital control is presented, employing extended Hückel theory calculations on the complexes.