Boranes in Organic Chemistry

2. b-Aminoalkyl- and b-sulfanylalkylboranes in organic synthesis

V.M. Dembitsky1, G.A. Tolstikov2, M. Srebnik1

1Department of Pharmaceutical Chemistry and Natural Products, School of Pharmacy,

P.O. Box 12065, The Hebrew University of Jerusalem, Jerusalem 91120, Israel

2Novosibirsk Institute of Organic Chemistry SB RAS,

9, Lavrentieva Ave., Novosibirsk, 630090, Russia

Abstract

Problems on using of b-aminoalkyl- and b-sulfanylalkylboranes in organic synthesis are considered in this review. The synthesis of boron containing a-aminoacids by Curtius rearrangement draws attention. The use of b-aminoalkylboranes available by enamine hydroboration are described.

Examples of enamine desamination with the formation of alkenes, aminoalcohols and their transformations into allylic alcohol are presented. These conversions have been carried out on steroids and nitrogen containing heterocyclic compounds.

The dihydroboration of N-vinyl-carbamate and N-vinyl-urea have been described. Examples using nitrogen and oxygen containing boron derivatives for introduction of boron functions were presented. The route to borylhydrazones by hydroboration of enehydrazones was envisaged.

The possibility of trialkylamine hydroboration was shown on indole alkaloids and 11-azatricyclo[6.2.11,802,7]2,4,6,9-undecatetraene examples.

The synthesis of b-sulfanyl-alkylboranes by various routes was described.

The synthesis of boronic thioaminoacids was carried out by free radical thiilation of dialkyl-vinyl-boronates. Ethoxyacetylene has been shown smoothly added 1-ethylthioboracyclopentane. Derivatives of 1,4-thiaborinane were readily obtained by divinylboronate hydroboration. Dialkylvinylboronates react with mercaptoethanol with the formation of 1,5,2-oxathioborepane derivatives. Stereochemistry of thiavinyl esters hydroboration leading to stereoisomeric b-sulfanylalkylboranes are discussed.

Examples of radical thiilation of various structural types vinylboronates were presented. In particular, 1,3,2-dioxaborinanes and 1,3,2-dioxaborolanes, containing by boron atom vinyl-, propenyl-, isopropenyl- or isopropylidene substituents have been used. Thiilation has been achieved by use of alkylmercaptanes, as well as mercaptamine derivatives. Alkylmercaptanes were able to replace the bromine substituent in tris-(2-bromoctyl)-borane. Dialkylvinylborates have been added hydrosulfite with the formation of 2-boronoethane sulfuric acids. A lot of examples of radical thiilation of vinylboronic acid dialkyl esters with mercaptoacids are presented. Under the azaisobutyric acid dinitryle conditions thioglycolic, b-mercaptopropionic, 2-mercaptoamberic acids and their esters as well as cysteine were added. Vinyl-, propenyl- and isopropenyl-dioxaborolanes were also participated in the thiilation with the formation of acetic, propionic or amberic acid thioethanoboronates.

The high reactivity of B,B,B-trivinyl-N,N,N-triphenylborazine in the reaction with thiophenol, leading to B-tris-(phenylmercaptoethyl)-N-phenylborazine was shown.

The problems of asymmetric hydroboration leading to chiral b-sulfanylalkylboranes were discussed briefly. In particular, an example, including dihydro-thiophene hydroboration, leading to (+)-R-thiofan-3-yl-diisopinocamphenylborane, and the interaction with acetaldehyde with the formation of (+)-R-3-thiophanyl-diethoxyborane was implemented. The reaction with 3,4-dihydrothiapyrane proceeds analogously.

A synthetic route to sulfono-norbornen-boronic acid esters by Diels-Alder reaction of cyclopentadiene with arylsulfanyl-vinylboronic acid esters has been discussed.