Group IV Hydride Donors

Both Si-H and C-H compounds can fimction as hydride donors under certain circumstances. The silicon-hydrogen bond is capable of transferring a hydride to carbocations. Alcohols that can be ionized in trifluoroacetic acid are reduced to hydrocarbons in the presence of a silane. 

Aliphatic ketones can be reduced to hydrocarbons by triethylsilane and gaseous The BF3 is a sufficiently strong Lewis acid to promote formation of a carbocation from the 

Aryl ketones have also been reduced with triethylsilane and TiCl4. This method was used to prepare 7-aryl amino acids. 

All of these reactions involve formation of oxonium and carbocation intermediates that can abstract hydride from the silane donor. 

There is also a group of reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor so especially favorable circumstances are required to observe this reactivity. Frequently, these reactions proceed through a cyclic transition state in which a new C-H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equlibrium process, and the reaction can be driven to completion if the ketone is removed from the system by distillation, for example. This process is called the Meerwein-Pondorff-Verley reduction. 

Aromatic aldehydes and ketones are reduced to alkyl derivatives.  

Aromatic carboxylic acids and esters are reduced to methyl groups in a process in which trichlorosilane is the reductant.  

The mechanism of this reaction has not been delineated in detail. 

Though not nearly so widely useful as the boron and aluminum hydrides, silicon and carbon can both act as hydride donors under certain circumstances. The silicon-hydrogen bond is reactive toward carbonium ions, resulting in reduction of the carbonium ion to the hydrocarbon. This reaction is preparatively useful for reduction of alcohols that can be converted to carbonium ions in trifluoroacetic 

CHAPTER 5 REDUCTION OF CARBONYL AND OTHER FUNCTIONAL GROUPS 

When the reaction is carried out in alcoholic solvents with controlled amounts of acid, ethers are formed as a result of partial reduction of the hemiacetal. This reaction is applicable to aliphatic as well as aromatic compounds since the remaining alkoxy group provides the requisite stabilization for the carbonium ion intermediate. 

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Reduction of Other Functional Groups by Hydride Donors Group IV Hydride Donors... 

C. Chatgihaloglu and M. Newcomb, Hydrogen donor abilities of the group 14 hydrides, Adv. Organometal Chem. 1999, 44, 67 (rate constants for radical reactions with Group 14 (IV A) metal hydrides). 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

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