Mechanism of Reaction of Geminal Dihalides with Lithium Aluminum Hydride. Evidence for Single Electron Transfer as the Major Reaction Pathway

Article abstract of DOI:10.1021/jo00093a011

The reactions of a sterically hindered geminal dichloride 4a and the corresponding diiodide 4b with LiAlH4 (LAH) were found to involve an electron-transfer mechanism.Whereas the monochloro analog of 4a (8) is inert toward LAH, 4a was more reactive.Observation of radical-derived products in the reaction of 4a with LAH indicates that the presence of two chlorine atoms on the same carbon atom results in a favorable reduction potential for 4a, as compared to 8.These results show that LAH can function as a one-electron donor toward geminal dichlorides.It was found that the diiodo analog 4b is more reactive toward LAH than 4a due to the inherent favorable reduction potentials of alkyl iodides compared to chlorides.The reactivity of 4b toward LAH was also found to be greater than that of the monoiodo analog (1b) in keeping with the more favorable reduction potential of the diiodide (4b) compared to the monoiodide (1b).The rates of cyclization of the corresponding haloalkyl radicals generated from 4a and 4b were also determined and were found to be 7.4E5/s for the chloroalkyl and 5.5E5/s for the iodoalkyl radical at 25 deg C.The formation of small amounts of the carbene-derived product, 5, in the reactions of both 4a and 4b with LAH appeared to be preceded by a radical intermediate.