Single electron transfer (SET) mechanisms are becoming ubiquitous in modern organic chemistry. However, it is often difficult to distinguish SET mechanisms from polar mechanisms. Kinetics, products and product distributions, and response to perturbation in solvent and substituents are often identical between the two mechanisms. Detection techniques such as EPR, CIDNP, and UV absorption can often detect "blind" pathways and thus cannot provide unambiguous evidence regarding the true mechanism of interest. In recent years mechanistic probes have been developed which can test for single electron transfer in the mechanism of interest in a more unambiguous manner, although a given probe is often applicable to a narrower range of reactions.

In this work 1,1-dimethyl-5,7-di-t-butylspiro[2.5]octa-4,7-dien-6-one (6) is presented as a new "hypersensitive" probe for single electron transfer to conjugated carbonyl compounds. This new probe functions in a rather unique fashion, allowing interpretation of the mechanism at work on the basis of the regiochemistry of spirocyclic ring opening. This "regiodifferentiation" based probe was studied with a variety of nucleophiles (particularly Grignard reagents) and has been found to be effective in differentiating SET from polar processes, although surprising results indicative of polar pathways in the case of reaction of 6 with Grignard reagents other than methyl Grignard were found. Additional insight into the mechanism of the reaction of Grignard reagents with conjugated ketones is also presented.