Mariappan Periasamy

Methods of enhancement of reactivity and selectivity of sodium borohydride for applications in organic synthesis

NaBH4 does not reduce carboxylic acids, esters, amides and nitriles under ambient conditions. However, the reactivity of NaBH4 can be enhanced by the addition of certain additives. For example, addition of iodine to NaBH4 in THF provides H3B–THF that is useful for hydroborations and reductions of various functional groups. The aldehydes and ketones are reduced in a fast manner by the NaBH4 reagent. Even so, the selectivities realised in such reductions can be enhanced using NaBH4 along with another additive. In this article, various methods used for the enhancement of reactivity and selectivity of NaBH4 in organic synthesis are described.

Convenient methods for the reduction of amides, nitriles, carboxylic esters, acids and hydroboration of alkenes using NaBH4/I2system

Abstract Reaction of amides with NaBH4-I2 system in THF gives the corresponding amines in 70–76% yields. Reduction of nitriles yields the corresponding amines in 70–75% yields. The I2/NaBH4 system is useful in the hydrocarboration of olefins and the corresponding alcohols are obtained in 78–92% yields after H2O2/OH− oxidation. The reagent system is also useful for the reduction of carboxylic esters and acids to the corresponding alcohols in 60–90% yields.

Convenient method for the synthesis of chiral α,α-diphenyl-2-pyrrolidinemethanol

Abstract A simplified, convenient synthesis of chiral α,α-diphenyl-2-pyrrolidine-methanol involving one step N-and O-protection of S-proline using ethylchloroformate followed by Grignard addition-alkaline hydrolysis (KOH/CH 3 OH) is described.

Highly Enantioselective Synthesis of Chiral Allenes by Sequential Creation of Stereogenic Center and Chirality Transfer in a Single Pot Operation

Chiral allenes are readily accessed in a single pot operation in the reaction of terminal alkynes, aldehydes, chiral secondary amines, and zinc halides in good yields (up to 77% yield) and excellent enantioselectivities (up to 99% ee) in toluene at 120 °C. The reaction proceeds through initial formation of chiral propargylamine intermediates with creation of a new stereogenic center and subsequent chirality transfer via an intramolecular hydride shift to produce chiral allenes with high enantiomeric purities.

Reductive coupling of aromatic aldehydes and imines by the low valent titanium species generated in the reaction of TiCl4 with Et3N

Abstract Aromatic aldehydes and imines are converted to the corresponding diols and diamines using the low valent titanium species generated by the reaction of TiCl 4 with triethylamine.

Hydroboration or hydrogenation of alkenes with CoCl2-NaBH4

Abstract The CoCl 2 -NaBH 4 reagent hydroborates or hydrogenates alkenes under appropriate conditions.

A simple convenient method for the generation of diborane from NaBH4 and I2

Abstract Treatment of NaBH4 with I2 in diglyme yields diborane which can be utilized conveniently for the preparation of a borane-N,N-diethylaniline complex and other borane-Lewis base complexes.

A SIMPLE CONVENIENT METHOD FOR THE RESOLUTION OF RACEMIC 2,2'-DIHYDROXY-1,1'-BINAPHTHYL USING (S)-PROLINE

Abstract The racemic mixture of 2,2′-dihydroxy-1,1′-binaphthyl has been resolved to obtain the R(+) and S(−) enantiomers in essentially pure forms by refluxing with (S)-proline (1eq.) in benzene in three successive operations.

Copper(I) Halide Promoted Diastereoselective Synthesis of Chiral Propargylamines and Chiral Allenes using 2-Dialkylaminomethylpyrrolidine, Aldehydes, and 1-Alkynes

Copper bromide promoted reactions of aldehydes, 1-alkynes, and chiral 2-dialkylaminomethylpyrrolidine at 25 °C give the corresponding chiral propargylamine derivatives in up to 96% yield and 99:1 dr that are readily converted to the corresponding disubstitued chiral allenes in up to 81% yield and 99% ee upon reaction with CuI in dioxane at 100 °C.

Acetoxyborohydride: A Simple Selective Hydroborating Agent

Abstract Acetoxyborohydride (CH 3 COO B H 3 ) hydroborates olefins in the presence of some reducible functional groups.