Benzylic and Allylic Grignard Reagents

Direct substitution reaction of 125 <1995EJM697> with organozinc reagents derived from allyl, benzyl, phenyl, vinyl, and -butyl Grignard reagents stereoselectively produced C-lO-substituted deoxoartemisinins 142 in good to moderate yields (Scheme 17 Table 12) <2002TL2891>. 

The mechanisms of reactions of benzylmagnesium halides can be either homolytic or concerted. As with allylic Grignard reagents, also reversible addition reactions with benzylic Grignard reagents and carbonyl compounds have been reported. In general, yields of normal alcohols are low, and such alcohols are obtained in much better yields by use of dibenzylcadmium [66] or benzyllithium [67]. However, in 1980, it was demonstrated that, by far, the best and quickest results for the synthesis of these normal addition reaction products are obtained by applying benzylic halides in the one-step Li-Barbier reaction under ultrasonic irradiation [68,69]. 

Benzylic and allylic type Grignard reagents can be difficult to prepare or inaccessible using magnesium powder ind turnings as in the classical... 

Grignard reagents r>f benzylic. and allylic halides, are acce.ssible in high yield using magnesium-anthracene complexes, and pohiticr ind silica... 

Benzyl and allyl halides are both easily reduced by transfer of an electron (Table 2.51. They are also reactive in both Sn2 reactions (comparable to methyl and 30 to over l(K) times luster than 1 ) and SnI (between 2 and 3 in rale) 7-9. They might, therefore, be expected to react readily with Grignard reagents by any of the three mechanisms under consideration, and, in fact, they are generally more reactive than alkyl halides. 

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