Benzylic directing group

The anti stereospecificity of epoxidation by the peracid is interpreted as due to association of the reagent with the allylic hydroxyl group which directs the entering oxygen atom to the same face of the molecule. The stereospecificity of bromohydrin formation is explicable in terms of steric approach control involving initial attack of the bulky bromine atom on the face opposite to the benzylic hydroxyl group (7). 

Lateral lithiation is the lithiation of the benzylic position aUtyl groups which are themselves ortho to a directing group . A general scheme for a lateral lithiation directed by a group G is shown in Scheme 186. 

Generating tin hydride in situ by using a catalytic amount of tributyltin chloride and sodium cyanoborohydride [24] did not solve the problem. Although this method inherently produces the desired low concentration of n-BujSnH to promote cyclization over direct reduction of the starting material and reduces drastically the amount of tin residue, a displacement reaction between solvent and the benzylic hydroxyl group takes place. Eventually, the best method was the use of high dilution conditions at approximately 70 °C in benzene followed by the KF workup. 

If the arenes are very electron-deficient direct attack of the nucleophile at the arene might also compete with displacement of the benzylic leaving group, to yield complex structures such as that shown in Scheme 4.28. 2-Nitrobenzyl halides can also react with amines to yield, instead of simple products of nucleophilic substitution, 2fi-indazoles [122] (Scheme4.28). 4-Nitrobenzyl halides, however, yield the expected benzyl amines on treatment with amines [123]. 

Silylation of benzyl C-H bonds using hydrosilanes can also be performed with the aid of Ru3(CO)12-catalyst (Table 1) [9]. This silylation occurs only at benzylic CH3 groups. Pyridine, pyrazole, and hydrazones function as good directing groups. Benzylamines, oxime ethers, dimethylanilides, and aryl pyridyl ethers have no activity in this silylation. 

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