Hydride donors classes

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

Hydride donors can be subdivided into three classes. They are... 

Generally, these reductions can be divided in two classes vide supra), similarly to the C=0 reductions. The first and more prominent type makes use of silanes as hydride donors and chiral Lewis bases as their activators. The second type is based on the combination of Hantzsch esters and chiral Brpnsted acids. 

In the same study, several ligands variously functional on both the nitrogen and the sulfur atoms have been developed, providing a new class of cyclo-hexylamino sulfide ligands derived from cyclohexene oxide. All the ligands depicted in Scheme 9.7 were evaluated for the Ir-catalysed hydride-transfer reduction of acetophenone in the presence of i-PrOH as the hydrogen donor, providing enantioselectivities of up to 70% ee. 

Oxidoreductases. Oxidation-reduction reactions are very common in biochemical pathways and are catalyzed by a broad class of enzymes called oxidoreductases. Whenever an oxidation-reduction reaction occurs, at least one substrate gains electrons and becomes reduced, and another substrate loses electrons and becomes oxidized. One subset of reactions is catalyzed by dehydrogenases, which accept and donate electrons in the form of hydride ions (H ) or hydrogen atoms. Usually an electron-transferring coenzyme, such as NAD /NADH, acts as an electron donor or acceptor (e.g., see Fig. 8.14 and Fig. 8.15). 

Similarly, for the reductive HAT activation of Jt-systems, the new bond to hydrogen is dramatically destabilized by virtue of being vicinal to an unpaired electron (Fig. lb). As such, few if any HAT donors have been shown to be competent to activate ketones, imines, arenes, or certain classes of olefins [23-26] via HAT. In fact, the weakest, well-characterized metal hydride is Norton s H-V(CO)4(dppb), which features a V-H BDFE of 50 kcal mol [27]. While exceptionally weak, this bond it is still far too strong to serve as an effective H-atom donor to many common organic tt-systems such as those depicted in Fig. lb. 

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