Double bonds Hantzsch esters

Combination of the Hantzsch ester mediated transfer hydrogenation together with chlorine (116) or fluorine (117) electrophiles allows for the formal addition of HCl or HF aaoss a double bond in a catalytic asymmetric manner (Scheme 48) [178], Within this paper the reactions were further refined by the use of two cycle-specific secondary amines which effectively operated independently within the same reaction mixture. Impressively, this allowed access to either diastereoisomer of the product depending upon the absolute configuration of the catalyst used in the second step of the sequence. 

The organocatalytic enantioselective reduction of C=C, C=0, and C=N double bonds is a relatively young area for which many new and exciting developments can be expected in the near future. Hantzsch esters are useful organic hydrides, and a recent review has summarized the results obtained to date in organocataly-sis [27]. The case of silicon hydrides is convenient for imine or ketone reductions, as a chiral base can act as an organic catalyst. The asymmetric reductions of ketones catalyzed by oxazaborolidines and pioneered by Itsuno [28] and Corey [29] could not be included in this chapter. 

The 1,4-dihydropyridines produced in this approach, carrying conjugating substituents at each p-position, are stable, and can be easily isolated before dehydrogenation classically the oxidation has been achieved with nitric acid, or nitrous acid, but other oxidants such as ceric ammonium nitrate, cupric nitrate on montmorillonite, and manganese dioxide on bentonite also all achieve this objective smoothlyIt is a corollary that Hantzsch esters can be viewed as reducing agents they have selectivity for the carbon-carbon double bond in a nitroalkene. ... 

In path A, the substrate is protonated on the more electronegative end of the double bond followed by a hydride transfer to the opposite end. An example of this mechanism type is the reduction of double bonds using a Br0nsted acid for protonation and a Hantzsch ester as the hydride source. The addition of a hydride to the more electropositive end of the double bond followed by subsequent protonation of the anion leads to products on path B. In this case, the silane reduction of imines serves as an example for this reaction type, wherein the subsequent protonation usually proceeds during workup. ... 

Double bonds conjugated with aldehydes or ketones can selectively be reduced with two different catalytic systems. Both systems use Hantzsch ester derivatives (HEH, 1) as hydride donor, analogous to nicotinamide adenine dinucleotide (NADH, 2), nature s reducing agent (Figure 32.1). These esters were first prepared by Hantzsch in 1882 from ethyl acetoacetate, formaldehyde, and ammonia. The... 

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