Reactions of Dimethoxycarbene with Cyclic Perchlorinated Olefins and Ketones

Reactions of dimethoxycarbene (2), a carbonyl group equivalent, with perchlorinated olefins and ketones were investigated. Thermolysis of 2,2-dimethoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazoline (1) at 110 °C generated 2, which reacted with hexachlorocyclopentadiene (4), octachlorocycloheptatriene (12), octachlorobicyclo[3.2.0]hepta-3,6-diene (24), hexachlorotropone (28), hexachlorobicyclo[3.2.0]hepta-3,6-dien-2-one (32), and tetrachloro-1,4-benzoquinone (35). Reactions of 2 with perchlorinated olefins 4, 12, and 24 led to esters or, in the case of 12, to a ketene acetal. Their formation is rationalized in terms of Michael-like addition and displacement (SN2‘ or SN2‘ ‘, if concerted) of allylic chlorine atoms by 2, yielding ion pairs that either dechloromethylate to esters or dechlorinate to a ketene acetal. In contrast, the reactions of 2 with unsaturated perchloroketones 28, 32, and 35 led to ring contraction, ring expansion, and aromatization, respectively. The products from these reactions are consistent with nucleophilic addition of 2 at the carbonyl moiety rather than Michael-type addition. Dimethoxycarbene-d 3 was used to show that demethylation in the latter reaction was intermolecular. Mechanisms for the different reaction courses are proposed.