Mechanism of Reaction of Geminal Dihalides with Magnesium. Evidence for the Formation of Carbenes from Radical Precursors. The Similarity in Reactions of Geminal Dihalides with Magnesium and LiAlH4

Article abstract of DOI:10.1021/jo00100a024

The geminal dihalides, 6,6-dichloro-5,5-dimethyl-1-hexene (2a) and 6,6-diiodo-5,5-dimethyl-1-hexene (2b) were allowed to react with Rieke activated magnesium (Mg*) in THF at 25 deg C.Both radical and carbene intermediates were identified by product analyses, by radical trapping experiments using DCPH and THF-d8, and by selective experiments in which isolated intermediates were shown to form the predicted products.Although carbene products predominated over radical products when the dichloride was allowed to react with Mg*, radical products predominated when the diiodide was allowed to react with Mg*.Evidence is presented that indicates that the carbene intermediate arises from a radical precursor.A mechanistic scheme (Scheme 5) is presented that is consistent with all of the observed data.A comparison of the reactions of 2b with Mg* and 2b with LiAlH4 shows that most of the products are common in both reactions thereby providing further evidence for LiAlH4 as a one-electron donor, as is known behavior of Mg*.