Boranes alcohols from

Hydrogen peroxide sodium hydroxide Alcohols from boranes... 

Hydrogen peroxide sodium hydroxide Alcohols from boranes s. 12, 697 HiOjNaOH RjB 3 R-OH... 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Borane and aluminum hydrides modified by chiral diols or amino alcohols are well-known, effective reagents for the stoichiometric enan-tioselective reduction of prochiral ketones and related compounds (34). Reduction of prochiral aromatic ketones with the Itsuno reagent, which is prepared from a chiral, sterically congested /3-amino alcohol and borane, yields the corresponding secondary alcohols in 94-100% ee... 

Reduction of carbonyl compounds with metal hydrides or boranes a. primary alcohols from aldehydes, acids, acid halides, and esters... 

The electrophile (E) which attacks the dienamine system could be a proton from the solvent (water or alcohol) or borane formed from the... 

Radical Epoxide Reduction Borane-free Two-step Synthesis of anti-Markownikow Alcohols from Alkenes... 

As we saw in Chapter 7, one of the goals of synthetic organic chemistry is to develop methods that produce only a single enantiomer of a desired chiral compound rather than the usual racemic mixture that is the result of most reactions. Recently, a method has been developed that employs one enantiomer of a chiral borane to prepare a single enantiomer of a chiral alcohol from an achiral alkene. The chiral borane that is used is /rans-2,5-dimethylborolane. 

In situ formation of the 1,3,2-oxazaborolidine from borane and the 1,2-amino alcohol with 2 equiv of H2 uptake ... 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

The isolation and characterization of the reagent derived from the reaction of p-amino alcohols and borane accompanied the first report of a truly catalytic procedure for the enantioselective reduction of ketones. - A representative example is shown in Figure Based on H and NMR spec-... 

One of the most useful methods for preparing alcohols from alkenes is the hydroboration reaction reported in 1959 by H. C. Brown. Hydroboration involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2 ... 

Synthesis of secondary alcohols from 1-aOcynes.2 Dihydroboration at room temperature of a terminal alkyne with either (1) or 9-BBN gives a 1,1-diborylalkane (3) this is treated at 0-5° with I eq. of methyllithium in ether. The product (4) rearranges to (5). An alkyl halide (100% excess) is then added, and the resultant secondary organo-borane (6) is oxidized with alkaline hydrogen peroxide. Secondary alcohols (7) are obtained in 70-85% yield. 

The other alkylborohydrides, 9-BBN and Sia2BH, also transform tertiary amides to alcohols [BK5, PSl]. Alcohols are obtained via the action of LiBH4 in MeOH-hot diglyme on some tertiary amides [S03] or by the controlled reduction with Li pyrrolidinoborohydride in THF [FFl] (Figure 3.69). This latter method, however, has some limitations. The reduction of 3.183 by Li pyrrolidinoborohydride or better by LiNH2BH3 obtained from borane ammonia and n-BuLi does not promote any racemization [MY2, MY4] (Figure 3.69). 

The establishment of natural artemisia alcohol from sage as the (5)-isomer was announced by Zydowsky and Hill in 1982, and an elegant synthesis of both enantiomers by H.C. Brown utilizes the chiral borane 219, which contains a prenyl group. Reaction of the illustrated enantiomer with senecio aldehyde (84) gave the addition product from which (- )-(S)-artemisia alcohol [(- )-209] was obtained after alkaline peroxide oxidation in 96% ee. ... 

The structure of enantiopure 1,3,2-oxazaborolidine was first characterized by Corey [22]. The oxazaborolidine is prepared from an amino alcohol and borane or substituted boronic acids (Scheme 1) [23,24,25]. B-Methyl derivatives of oxazaborolidine were prepared by the reaction of an amino alcohol with trimeth-ylboroxine [26,27]. 

Di(isopinocampheyl)borane Sec. alcohols from ketones Asym. reduction... 

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