Reductants alkyl borane

Aldehydes and ketones have been converted to sulfides by treatment with thiols and pyridine-borane, RCOR -I- R"SH —+ RR CHSR", in a reductive alkylation reaction, analogous to 16-6. 

Cabacungan, J.C., Ahmed, A.I., and Feeney, R.E. (1982) Amine boranes as alternative reducing agents for reductive alkylation of proteins. Anal. Biochem. 124, 272-278. 

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

The amino groups of ovomucoid, lysozyme, and ovotransferrin were alkylated extensively (40-100%) with various carbonyl reagents in the presence of sodium borohydride. Monosubstitution was observed with acetone, cyclopentanone, cyclohexanone, and benzaldehyde, while 20-50% disubstitution was observed with 1-butanal and nearly 100% disubstitution was observed with formaldehyde. The methylated and isopropylated derivatives of all three proteins were soluble and retained almost full biochemical activities. Recently amine boranes have been shown to be possible alternative reducing agents for reductive alkylation... 

Figure 11. Alternative reducing agents for reductive alkylation (1) di-methylamine borane (II) trimethylamine borane (27)...

Rogic, M. M. Conformational Analysis and the Transition State in Asymmetric Reductions with Boranes Based on (+)-a-Pinene. 1. Benzaldehyde Reduction with Alpine Borane and Other B-Alkyl-9-borabicyclo[3.3.1]nonanes. ASemiempirical Study. J. Org. Chem. 1996, 61, 1341-1346. 

The reaction of a chiral alkene with borane in the proper stoichiometry may afford alkyl boranes R BH2 or dialkyl boranes R BH, where R is a chiral ligand. Attempts to achieve highly selective reductions of ketones using such reagents have met with little success, however. Trialkyl boranes R3B were first reported to reduce aldehydes and ketones (under forcing conditions) in 1966 by Mikhailov [50]. Mechanistic studies (summarized in ref. [46]) showed that there are two limiting mechanisms for the reduction of a carbonyl compound by a trialkylborane, as shown in Scheme 7.4 a pericyclic process reminiscent of the Meerwein-Pondorf-Verley reaction (Scheme 7.4a), and a two step process that involves dehydro-... 

Two sterically hindered /V-sulfonylated aminobomeols 42 were prepared from camphorquinone 11 and used after esterification for the formation of enolates which are alkylated or oxidized diastereoselectively (Sections D.1.1.1.3.2., D. 1.5.2.1., D. 1.5.2.3. and D.4.I.). The synthesis of these auxiliaries involves the reaction of camphorquinone 11 with 3,5-dimethylaniline to give the mono-imine by reaction with the sterically less hindered carbonyl group, followed by reduction with boranate and reaction with benzenesulfonyl chloride to give the exo.exo-product. or initial reduction with zinc/potassium hydroxide, then sulfonylation, and finally reduction with boranate to give the endo,endo-isomer21. 

The most important synthetic application of borane is for the preparation of alkyl boranes by addition to alkenes, aprocess known as hydroboration (5.1). Borane and its derivatives can also be used for reduction (see Section 7.3). The hydroboration reaction has been applied to a large number of alkenes of widely differing structures. In nearly all cases the addition proceeds rapidly at room temperature, and only the most hindered alkenes do not react. 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

Purpose. The oxidation of an alkene to an alcohol is investigated via the in situ formation of the corresponding trialkylborane, followed by the oxidation of the carbon-boron bond with hydrogen peroxide. The conditions required for hydroboration (a reduction) of unsaturated hydrocarbons are explored. Alkylboranes are particularly useful synthetic intermediates for the preparation of alcohols. The example used in this experiment is the conversion of 1-octene to 1-octanol in which an anti-Markovrukov addition to the double bond is required to yield the intermediate, trioctylborane. Since it is this alkyl borane that subsequently undergoes oxidation to the alcohol, hydroboration offers a synthetic pathway for introducing substituents at centers of unsaturation that are not normally available to the anti-Markovnikov addition reactions that are based on radical intermediates. 

We have examined the applications of IBH2 and I2BH complexes for hydroboration of alkenes to obtain dialkyl and mono alkyl boranes. Unfortunately, these species disproportionate and hence the dialkyl and trialkyl boranes were not formed cleanly. Fortunately, however, the HBI2 and BI3 complexes are found to be useful for iodinations of alcohols, reductive iodinations of carbonyl compounds and hydroiodination of alkenes and alkynes. 

The field of borane chemistry has seen some significant advances during 1982, and several syntheses of natural products have appeared, utilizing the new applications, mostly due to the work of Brown and co-workers. By its reaction with alkenes and subsequent reduction, t-hexylchloroborane is converted into the unstable t-hexyl(alkyl)boranes (53), which then add to 1-halogeno-l-alkynes... 

Alkyl borane as the reductant in redox polymerization is well known. It has been used previously in conjunction with alkyl peroxide and peroxyester of carbonic acid. The mechanism of alkyl borane-peroxyester of carboxylic acid is similar to that previously described. Suspension polymerization or copolymerization of vinyl chloride by the redox system such as monotertiary butyl permaleate-EtsB or BU3B or iso-BusB has been reported in patent literature [198]. 

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