Benzylic magnesium reagents

Very recently, a dry-stir method for the mechanical activation of commercial magnesium turnings was reported16. The procedure is also excellently suited for the preparation of allylic and benzylic Grignard reagents (Table 1, method B). 

Carbon-13 shifts of Grignard reagents (Table 4.73) [479]) in ether solutions provide evidence that the Schlenck equilibrium is shifted in favor of the dialkylmagnesium compound There is practically no shift difference between dibenzylmagnesium and benzyl-magnesium chloride. 

Benzotelluropyrylium salts react with a variety of nucleophiles via distinct mechanistic pathways. 2-Benzotelluropyrylium salts 30 react with common Grignard reagents to afford products of reductive dimerization 31 (Equation 6) <1998J(P1)2123>. Under similar conditions, the reactions of the 2-benzotelluropyrylium salts with benzyl magnesium bromide take a completely different course leading to addition of the benzyl group to the... 

Yields of 2-substituted tetrahydrothiophenes (thiolanes) obtained by treating thietane 1-oxide with a two-molar excess of Grignard reagent, RCHjMgX, vary from 18-51%(R=H, CH3,Ph, o-CH3CgH)4. The best yields came from the benzyl-magnesium halides. 

Figure 3 Formation of the benzyl Grignard reagent using magnesium activated by the dry stir method.

Among the benzyl magnesium chlorides prepared, the synthesis of the benzyl Grignard reagent shown in Figure 3 was straightforward, whereas the formation of its derivative (Fig. 4) had to be carried out with careful control of the rate of addition to avoid coupling side products. 

An arbitrary selection of other (li-Grignard reagents 101-105 is given in Scheme I 1.30. While 101-104 (1611 164]. respectively) were obtained more or loss by the classical direct approach, 105 65a,b and other benzylic Grignard reagents 16.5c—e require the use of the magnesium-anthracene adduct (vide infra) which, in a way, may be considered as a soluble and activated form of magnesium, in order to obtain satisfactory results. 

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