Borohydrides unsaturated carbonyl compounds

Methylmagnesium N-cyclohexyliso-propylamide, 189 By oxidation at an allylic carbon Selenium dioxide, 272 By reduction of a,0-unsaturated carbonyl compounds Sodium borohydride, 278 Sodium dithionite, 281 Other methods r-Butyllithium, 58 Butyllithium-Potassium f-butoxide,... 

Varma reported a facile and rapid method for the reduction of aldehydes and ketones to the respective alcohols, using alumina-supported sodium borohydride and microwave irradiation under solvent-free conditions. Aldehydes tend to react at room temperature, while for the reduction of ketones, short microwave irradiation of 30-180 s was applied to produce the corresponding alcohols in 62-92% yield. With unsaturated carbonyl compounds, reduction at the conjugated C=C bond might occur as a side reaction under these conditions (Scheme 4.9)26. 

With sodium borohydride and catalytic amounts of titanyl acetoacetonate, a,fi-unsaturated carbonyl compounds give allyl alcohols regioselectively, whereas a-diketones and acyloins are reduced to vicinal diols.325 Enantioselectivities in the reduction of acetophenone, catalysed by 1,3,2-oxazaborolidones, have been examined using the AM1-SCF MO method. The optimized geometries, thermal enthalpies, and entropies of R and S transition states in the stereo-controlling steps of the reduction have been obtained.326... 

In addition, supported reagents have been demonstrated to be effective under reaction conditions when either thermal or microwave heating - is employed. They have also been utilised in traditional batch synthesis, stop-flow methods and continuous flow processes. ° However, one caveat is that the immobilisation of reagents can change their reactivity. For example, polymer-supported borohydride selectively reduces a,P-unsaturated carbonyl compounds to the a,P-unsaturated alcohoF in contrast to the behaviour of the solution-phase counterpart, which additionally causes double bond reduction. 

A Michael addition of thiophenol to an a, j6-unsaturated carbonyl compound, followed by sodium borohydride reduction or addition of a metal-organic reagent, gives a y-phenylthioalcohol electrochemical reduction in dry DMF of the methanesulfonate of this alcohol yields a cyclopropane and a small amount of alkene [81] ... 

Sodium borohydride, NaBH4, does not reduce carbon-carbon double bonds, not even those conjugated with carbonyl groups, and is thus useful for the reduction of such unsaturated carbonyl compounds to unsaturated alcohols. 

Zeynizadeh, B., Shirini, F. Mild and efficient reduction of a,P-unsaturated carbonyl compounds, a-diketones and acyloins with the sodium borohydride/Dowex1-x8 system. Bull. Korean Chem. Soc. 2003, 24, 295-298. 

Sol 9. (c) Sodium borohydride in the presence of CeCls selectively reduces a,P-unsaturated carbonyl compounds into allylic alcohols. It is called Luche s reduction. The selective 1,2-addition is facilitated by the strongly oxophilic Ce " " ion, which coordinates with the carbonyl oxygen. The allylic alcohol I then undergoes mercury catalyzed fran -etherification to give allyl vinyl ether (Claisen substrate), which readily undergoes [3,3] shift on heating to give II. 

Sodium hydrogentelluride looks to be a very useful reagent for the reduction of the olefinic bond in a,/S-unsaturated carbonyl compounds (esters, aldehydes, ketones, coumarins). Yields by g.l.c. are virtually quantitative with this reagent, which is prepared in situ from sodium borohydride and tellurium powder. 

The oxy-Cope rearrangement of hexa-l,5-dien-3-ols to give Se-unsaturated carbonyl compounds often gives low yields because of the high temperatures that are needed to elfect the reaction. However, in the presence of mercury tri-fluoroacetate, the reaction proceeds at room temperature and the required Se-unsaturated carbonyls (93) are obtained bydemercuration of the intermediates (92) with sodium borohydride in good yield ( 60%). Nakai and his co-workers... 

Zinc borohydride has been found to effect very efficient reductive amination in the presence of silica. The amine and carbonyl compound are mixed with silica and the powder is then treated with a solution of Zn(BH4)2. Excellent yields are also obtained for unsaturated aldehydes and ketones.96... 

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

An alternative procedure is to convert the carbonyl compound into the toluene-p-sulphonylhydrazone,12 followed by reduction with either sodium borohydride in acetic acid,13 or with catecholborane, followed by decomposition of the intermediate with sodium acetate or tetrabutylammonium acetate.14 The former method is illustrated by the conversion of undecan-6-one into undecane (Expt 5.6), and the latter by the conversion of acetophenone into ethylbenzene (Expt 6.4, Method B). A feature of these methods is that with a, / -unsaturated ketones, migration of the carbon-carbon multiple bond occurs thus the tosylhydrazone of isophorone gives 3,3,5-trimethylcyclohex-l-ene, and the tosylhydrazone of oct-3-yn-2-one gives octa-2,3-diene. 

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