Lithium aluminum hydride benzylic halides

A thio-substituted, quaternary ammonium salt can be synthesized by the Michael addition of an alkyl thiol to acrylamide in the presence of benzyl trimethyl ammonium hydroxide as a catalyst [793-795]. The reaction leads to the crystallization of the adducts in essentially quantitative yield. Reduction of the amides by lithium aluminum hydride in tetrahydrofuran solution produces the desired amines, which are converted to desired halide by reaction of the methyl iodide with the amines. The inhibitor is useful in controlling corrosion such as that caused by CO2 and H2S. 

The difference in the reactivity of benzylic versus aromatic halogens makes it possible to reduce the former ones preferentially. Lithium aluminum hydride replaced only the benzylic bromine by hydrogen in 2-bromomethyl-3-chloro-naphthalene (yield 75%) [540]. Sodium borohydride in diglyme reduces, as a rule, benzylic halides but not aromatic halides (except for some iodo derivatives) [505, 541]. Lithium aluminum hydride hydrogenolyzes benzyl halides and aryl bromides and iodides. Aryl chlorides and especially fluorides are quite resistant [540,542], However, in polyfluorinated aromatics, because of the very low electron density of the ring, even fluorine was replaced by hydrogen using lithium aluminum hydride [543]. 

After a SMA has been reacted with a benzyl halide, SMA might be recovered by reaction of the salt with an excess of lithium aluminum hydride (LAH) in THF at reflux. However, the elimination of toluene was a minor feature in this reaction (2-6% yields) that cannot be considered of synthetic value.248... 

Halides and tosylates. Trevoy and W. G. Brown found that lithium aluminum hydride reduces benzyl iodide and benzyl bromide in high yield at 35° in either diethyl ether or tetrahydrofurane. In tetrahydrofurane at 65°, benzyl chloride and 1 -bromodecane are reduced to toluene and to n-decane, both in 72% yield. Johnson, Blizzard, and Carhart found that lithium aluminum hydride reacts more sluggishly than in other reductions and presented experimental evidence that not all four hydrogen atoms possess adquate reactivity toward alkyl halides. Thus the reaction probably proceeds in at least two steps, the first of which is much more rapid than... 

PROBLEM 16.44 As we saw on page 798, benzyl halides can be reduced by metal hydrides. Why can compound A not be reduced successfully with lithium aluminum hydride What reaction will complicate matters ... 

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