Reducing agents group 1 metals

Group 14 metal hydrides, especially those of silicon and tin, are satisfactory nonreactive hydride donors, as in the absence of a catalyst they are, generally, poor reducing agents. Transition metal complexes are attractive transfer agents because they insert readily into Si—H or Sn—bonds and they also bind specifically to various functional groups. 

Carbonyl groups are easily reduced by metal hydrides such as sodium borohydride (NaBH4) or lithium aluminum hydride. The actual reducing agent in metal-hydride reductions is hydride ion (H ). Hydride ion adds to the carbonyl carbon, and the alkoxide ion that is formed is subsequently protonated. In other words, the carbonyl group is reduced by adding an H followed by an H. The mechanisms for reduction by these reagents are discussed in Section 18.5. 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

The reaction of an alkyl halide with lithium is an oxidation-reduction reac tion Group I metals are powerful reducing agents... 

Calcium is an excellent reducing agent and is widely used for this purpose. At elevated temperatures it reacts with the oxides or haUdes of almost all metallic elements to form the corresponding metal. It also combines with many metals forming a wide range of alloys and intermetaUic compounds. Among the phase systems that have been better characterized are those with Ag, Al, Au, Bi, Cd, Co, Cu, Hg, Li, Na, Ni, Pb, Sb, Si, Sn, Tl, Zn, and the other Group 2 (IIA) metals (13). 

An isolated acetoxyl function would be expected to be converted into the alkoxide of the corresponding steroidal alcohol in the course of a metal-ammonia reduction. Curiously, this conversion is not complete, even in the presence of excess metal. When a completely deacetylated product is desired, the crude reduction product is commonly hydrolyzed with alkali. This incomplete reduction of an acetoxyl function does not appear to interfere with a desired reduction elsewhere in a molecule, but the amount of metal to be consumed by the ester must be known in order to calculate the quantity of reducing agent to be used. In several cases, an isolated acetoxyl group appears to consume approximately 2 g-atoms of lithium, even though a portion of the acetate remains unreduced. Presumably, the unchanged acetate escapes reduction because of precipitation of the steroid from solution or because of conversion of the acetate function to its lithium enolate by lithium amide. 

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