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. / Master of Science

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A highly regio- and diastereoselective approach towards complex 6-membered lactones was developed using allyl/propargyl benzyl ethers and delta keto esters. Crucially, the classical ET reagent SmI2 gave unsatisfactory results and it was necessary to develop and screen new Sm(II) cyclopentadienyl complexes to deliver high selectivity in the transformation. The methodology was extended to a one-pot approach to complex cycloheptanols using SmI2-H2O in a second stage of the process. Samarium(II) folding cascades were developed where simple, linear starting materials are converted to complex polycyclic architectures bearing multiple stereocentres. It was found that, depending on the sidechain in the starting material, it was possible to achieve four different pathways from the common radical intermediate. Crucially, transannular 1,5-HAT from tertiary and benzylic positions was observed to give diverse products. A proposed 1,5-HAT facilitated SmI2-mediated 6-membered lactone radical cyclisations for the first time without an activating proton donor additive. Enantioselective samarium(II) mediated cyclisation cascades were achieved, where simple beta keto esters are converted to complex polycyclic architectures bearing up to five contiguous stereocentres with high diastereo- and enantiocontrol. In the process, a simple and easy to prepare chiral aminodiol was employed which could be recycled after the reaction. Unprecedented, enantioselective transannular radical cascades allowed access to unique 3- dimensional scaffolds inaccessible by other synthetic methods.