Functional groups sodium cyanoborohydride

An interesting procedure has been proposed for the synthesis of amylose-b-PS block copolymers through the combination of anionic and enzymatic polymerization [131]. PS end-functionalized with primary amine or dimethylsilyl, -SiMe2H groups were prepared by anionic polymerization techniques, as shown in Scheme 56. The PS chains represented by the curved lines in Scheme 56 were further functionalized with maltoheptaose oligomer either through reductive amination (Scheme 57) or hydrosilyla-tion reactions (Scheme 58). In the first case sodium cyanoborohydride was used to couple the saccharide moiety with the PS primary amine group. 

Derivatives of hydrazine, especially the hydrazide compounds formed from carboxylate groups, can react specifically with aldehyde or ketone functional groups in target molecules. Reaction with either group creates a hydrazone linkage (Reaction 44)—a type of Schiff base. This bond is relatively stable if it is formed with a ketone, but somewhat labile if the reaction is with an aldehyde group. However, the reaction rate of hydrazine derivatives with aldehydes typically is faster than the rate with ketones. Hydrazone formation with aldehydes, however, results in much more stable bonds than the easily reversible Schiff base interaction of an amine with an aldehyde. To further stabilize the bond between a hydrazide and an aldehyde, the hydrazone may be reacted with sodium cyanoborohydride to reduce the double bond and form a secure covalent linkage. 

Sodium cyanoborohydride is the most commonly used reagent for reduction of oximes and oxime ethers. Although this reaction is highly versatile, and does not interfere with a majority of functional groups, careful control of reaction conditions is necessary. A considerable problem in the reduction, especially for aldoximes 80 (equation 57), is the reaction of initially formed A-alkylhydroxylamine 81 with the starting oxime 80. The obtained nitrone 82 is subsequently reduced to A,A-dialkylhydroxylamine 83, which was found to be a major reaction product at pH = 4 and above. This side reaction can be avoided by adjusting the pH of the reaction mixture to 3 or below. 

C.F. Lane, Synthesis 1975, 135—146 . .Sodium Cyanoborohydride - A Highly Selective Reducing Agent for Organic Functional Groups". 

Catalytic procedures (introduced by Kuivila and Menapace92) are easier to conduct and the tin hydride concentration is more easily controlled. A catalytic amount of tributyltin hydride or tributyltin chloride is mixed with the radical precursor, the alkene acceptor and a stoichiometric quantity of a coreductant such as sodium borohydride93 or sodium cyanoborohydride.29 Over the course of the reaction, the borohydride continuously converts the tin halide to tin hydride. The use of the catalytic procedure is probably restricted to halide precursors (tin products derived from other precursors may not be reduced to tin hydrides). This method has several advantages over the standard procedures (i) it is simple to conduct (ii) most functional groups are stable to the coreductants (especially sodium cyanoborohydride) (iii) the tin hydride concentration is known, is stationary (assuming that the tin halide is rapidly reduced to tin hydride), and can be varied by either changing the concentration of the reaction or the quantity of the tin reagent (10% is a typical value, but lower quantities can be used) and finally, (iv) the amount of tin hydride precursor that is added limits the amount of tin by-product that must be removed at the end of the reaction. 

Sodium cyanoborohydride was used as received from Aldrich Chemical Company, Inc. If other sensitive functional groups are present, it is advisable to purify the commercial reagent by the method of Purcell.4... 

These preparations that illustrate the use of sodium cyanoborohydride in hexamethylphosphoramide as an effective, selective, and convenient procedure for the reduction of alkyl halides and tosylates is essentially the same as previously described.8 The very mild reducing ability of sodium cyanoborohydride makes the method particularly valuable when other functional groups are present in the molecule... 

Selective reductions. Borch et af. have recenlly reported a study of the reduction of various organic functional groups with sodium cyanoborohydride. Under neutral conditions, carbonyl groups are reduced to a negligible extent, but reduction is rapid at pH 3 4. Ketoximes are reduced smoothly at pH 4 to the corresponding alkyl-hydroxylamines. Reduction of aldoximes results mainly in the dialkylhydroxylaminc. 

Cyanoborohydride and its modified reagents have been used for reductive dehalogenations. Thus, the combination of sodium or tetrabutylammonium cyanoborohydride, sodium or potassium 9-cyano-9-hydro-9-borabicyclo[3.3.1]nonanate [9-BBNCN] (2) or polymeric cyanoborane (3) in HMPA furnishes an efficient and mild system for the reduction of alkyl halides. The reagents are selective in that other functional groups, including ester, carboxylic acid, amide, cyano, alkene, nitro, sulfone, ketone, aldehyde and epoxide, are essentially inert under the reduction conditions thus, the reduction procedure is attractive for synthetic schemes which demand minimum damage to sensitive portions of the molecule. 

In the laboratory of J. Kobayashi, the biomimetic one-pot transformation of serratinine into serratezomine A was accomplished using the Polonovski-Potier reaction Serratinine was first treated with m-chloroperbenzoic acid to obtain the A/-oxide, and then excess TFAA was added. The iminium ion was formed in the following fashion the C13 hydroxyl group formed a hemiacetal with the C5 carbonyl group and simultaneously with the formation of the C5-C13 lactone the C4-C5 bond was broken. The iminium ion was then reduced with sodium cyanoborohydride to afford the tertiary amine functionality. Besides serratezomine A, another lactone (between the C8 hydroxyl and C5 carbonyl) was formed in 27% yield. 

Lane, C. F. (1975). Sodium cyanoborohydride—A highly selective reducing agent for organic functional groups. Synthesis 135-146. 

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