Reductions hydride reagents

The reduction of (aLkylarnino)haloboranes using hydride reagents can provide a convenient route to (aLkylamino)boranes for example, LiAlH has been utilized to prepare bis (dimethyl amino)borane [23884-11-9] from chi orobis (dimethyl amino)borane [6562-41-0] (68). When this same strategy is appHed to (bis(trimethylsi1y1)amino)ch1oro((trimethylsi1y1)amino)borane [10078-93-0] the expected compound is obtained along with the formation of two... 

Reductive cleavage to alcohols by catalytic hydrogenation (64HC(19-1)188) has largely given way to reduction with hydride reagents (Section 5.05.3.4.3(t)u)). 

Recently reductions by a new hydride reagent, sodium bis(2-methoxy-ethoxy)aluminum hydride, have been investigated. This compound is similar to LiAlH4 in its reducing properties but because it is soluble in aromatic hydrocarbons and more stable in air than LiAlH4, it may be more convenient to use. 

The reduction of carbonyl compounds by reaction with hydride reagents (H -) and the Grignard addition by reaction with organomagnesium halides (R - +MgBr) are examples of nucleophilic carbonyl addition reactions. What analogous product do you think might result from reaction of cyanide ion with a ketone ... 

Formation of an Alcohol The simplest reaction of a tetrahedral alkoxide intermediate is protonation to yield an alcohol. We ve already seen two examples of this kind of process during reduction of aldehydes and ketones with hydride reagents such as NaBH4 and LiAlH4 (Section 17.4) and during Grignard reactions (Section 17.5). During a reduction, the nucleophile that adds to the carbonyl... 

Reduction to sulphides with nucleophilic hydride reagents. ... 

D. C. Wigeield, Stereochemistry and Mechanism of Ketone Reductions by Hydride Reagents, Tetrahedron 35,449 (1979). 

By an alternative approach, the enantioselective reduction of 1,2-diketimines was achieved by using hydride reagents in the presence of stoichiometric amounts or an excess of enantiopure carboxylic acids. An... 

Reduction to sulphides with electrophilic hydride reagents. 940... 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

The solid bases CaO and hydroxyapatite catalyze the hydrosilylation of benz-aldehyde by triethoxysilane at 90° in yields of 59% and 72% within one and two hours, respectively.323,324 These reductions also very likely involve activation by valence expansion of the silicon hydride reagent. 

A series of nonracemic triorganotin hydrides (37-39) was prepared by reduction of racemic halostannane precursors with a chiral alkoxy hydride reagent (Scheme 14)21. 

One approach to enantioselective reduction of prochiral carbonyl compounds is to utilize chiral ligand-modified metal hydride reagents. In these reagents, the number of reactive hydride species is minimized in order to get high chemo-selectivity. Enantiofacial differentiation is due to the introduced chiral ligand. 

Hydrazone reduction, 73 572 Hydrazones, 73 576, 580 hydrazone tautomeric form, 9 365-367 Hydride ions, 73 767 Hydride reagents, functional group behavior toward, 73 615-616t... 

C. Reduction of Imines with Chiral Metal Hydride Reagents. 112... 

In an attempt to prepare alkylamines by asymmetric reduction of imines with chiral hydride reagents, diphenylphosphinyl imines (38), prepared by reaction of ketoximes (39) with chlorodiphenylphosphine [(Cg 115)2 PCI], were reduced in the presence of a variety of chiral aluminum and boron hydride reagents43. Among the most promising reagents was BINAHL-H44 (40), a chiral hydride compound prepared by the modification of lithium... 

Reduction of amides without hydride reagents Asymmetric hydrogenation of un functionalized olefins/enamines/imines... 

The domain of hydrides and complex hydrides is reduction of carbonyl functions (in aldehydes, ketones, acids and acid derivatives). With the exception of boranes, which add across carbon-carbon multiple bonds and afford, after hydrolysis, hydrogenated products, isolated carbon-carbon double bonds resist reduction with hydrides and complex hydrides. However, a conjugated double bond may be reduced by some hydrides, as well as a triple bond to the double bond (p. 44). Reductions of other functions vary with the hydride reagents. Examples of applications of hydrides are shown in Procedures 14-24 (pp. 207-210). 

As a consequence of the wide choice of hydride reagents the classical methods such as reduction with sodium in ethanol almost fell into oblivion [579, 520]. Nevertheless some old reductions were resuscitated. Sodium di-thionite was found to be an effective reducing agent [262], and the reduction by alcohols [309] was modified to cut down on the temperature [755] or the time required [527], or to furnish chiral alcohols ( in good yields and excellent optical purity ) by using optically active pentyl alcohol and its aluminum salt [522]. Formation of chiral alcohols by reduction of pro-chiral ketones is... 

A large amount of data has been accumulated on the stereoselectivity of reduction of cyclic ketones.81 Table 5.4 compares the stereochemistry of reduction of several ketones by hydride donors of increasing seric bulk. The trends in the table illustrate the increasing importance of steric approach control as both the hydride reagent and the ketone become more highly substituted. The alkyl-substituted borohydrides have especially high selectivity for the least hindered direction of approach. 

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