MECHANISM OF REACTION OF GEMINAL DIHALIDES WITH MAGNESIUM - EVIDENCE FOR THE FORMATION OF CARBENES FROM RADICAL PRECURSORS - THE SIMILARITYIN REACTIONS OF GEMINAL DIHALIDES WITH MAGNESIUM AND LIALH4

Two 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 ac tivated magnesium (Mg) in THF at 25 degrees C. Both radical and carbe ne intermediates were identified by product analyses, by radical trapp ing experiments using DCPH and THF-d(8), and by selective experiments in which isolated intermediates were shown to form the predicted produ cts. Although carbene products predominated over radical products when the dichloride was allowed to react with Mg, radical products predom inated when the diiodide was allowed to react with Mg. Evidence is pr esented that indicates that the carbene intermediate arises from a rad ical precursor. A mechanistic scheme (Scheme 5) is presented that is c onsistent 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 LiAlH 4 as a one-electron donor, as is the known behavior of Mg.