Benzyl halides, reaction patterns

Reaction Patterns of Aryl, Alkenyl and Benzyl Halides and Pseudohalides... 

Reaction patterns of aryl, alkenyl and benzyl halides and pseudohalides... 

Alkylation solves the awkward problem of how to make aryl ketones of the pattern 151, awkward because the Friedel-Crafts reaction doesn t work with this substitution pattern. Alkylation of a nitroalkane with the benzylic halide 149 does work well and the product 150 can be oxidised to the ketone32151. Reactions with aldehydes work even with masked aldehydes33 like dihydropyran to give 154 and hence the ketone 155 with a 1,2-substitution pattern after hydrolysis.34 More vigorous conditions give nitroalkenes 145 from aldehydes and these will be very useful later as a2 reagents. 

Reactions between alkyl halides and polar organoalkali compounds often proceed sluggishly. They frequently do not follow SAr2-like mechanistic patterns but rather radical or radical pair pathways.As a corollary, optically active sec-sAky halides such as 2-bromobutane racemize completely when exposed to phenylsodium or -butyllithium. ° To the contrary, predominant inversion is observed with organometallic reagents stabilized either by allylic, propargylic, or benzylic u-delocalization e.g., benzylsodium ) or second-row elements (e.g., l,3-dithian-2-yllithium ). 

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