In hydroboration of alkenes

Dibromoborane-dimethyl sulfide exhibits regioselectivity in hydroboration of alkenes which is comparable to that exhibited by dialkylboranes. Excess alkene should be avoided in reactions of trisubstituted ethylenes, otherwise hydrogen bromide liberated during work-up may add to the excess ethylene and cause problems. 

Fig. 6.3 Most frequently used achiral and chiral btuanes in hydroboration of alkenes...

Other convenient routes to carboranes, selected from the growing number of recently reported syntheses, are as follows. Monocarbon carboranes can be prepared in good yield by the transition-metal catalysed hydroboration of alkenes followed by thermal rearrangement of the intermediate product, c.gP" ... 

Organoboranes react with a mixture of aqueous NH3 and NaOCl to produce primary amines. It is likely that the actual reagent is chloramine NH2CI. Chloramine itself,hydroxylamine-O-sulfonic acid in diglyme, and trimethyl-silyl azide " also give the reaction. Since the boranes can be prepared by the hydroboration of alkenes (15-16), this is an indirect method for the addition of NH3 to a double bond with anti-Markovnikov orientation. Secondary amines can be prepared by the treatment of alkyl- or aryldichloroboranes or dialkylchlorobor-anes with alkyl or aryl azides. 

Hydroboration of alkenes or alkynes followed by cross-coupling with organic electrophiles provides a straightforward method for the carbon-carbon bond formation (Scheme 1-19). The hydroboration of thioalkynes with catecholborane in the presence of a nickel or palladium catalyst yields P-(aLkylthio)-l-alkenylboronates (72a)... 

Hydroboration of alkenes with chiral hydroboration reagents such as di-isopinocamphenylborane succeeds in various syntheses of chiral organoboron com-... 

Subtle ligand changes have pronounced effects in hydroboration of 1,1-disubstituted alkenes. Addition of HBcat across the C=C bond in 2,3,3-trimethyl-l-butene is catalyzed by [Rh(COD)Cl]2/DIOP at —5°C affording 2-t-butyl-l-propanol in 69% ee upon oxidation. In the case of 2-phenylpropene, [Rh(C2H4)2Cl]2 combines with either DIOP or BINAP to provide higher enantiomeric excesses than with [Rh(COD)Cl]2.46... 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

Hydroboration of alkenes generates organoboranes, which react with sodium azide in the presence of an aqueous acid to give primary amines (equation 58). Thus 1-nonene... 

Hydroboration of alkenes in non-ethereal solvent has been reported using diborane generated in situ from a quaternary ammonium borohydride and bromoethane (see Section 11.5). Almost quantitative yields of the alcohols are reported [e.g. 1 ]. As an alternative to the haloalkane, trimethylsilyl chloride has also been used in conjunction with the ammonium borohydride [2]. Reduction of the alkene to the alkane also occurs as a side reaction (<20%) and diphenylethyne is converted into 1,2-diphenylethanol (70%), via the intermediate /ra 5-stilbene. 

Phase-transfer generated diborane has been used for the hydroboration of alkenes and their conversion into alcohols [1,2] and the procedure has also been employed for the cleavage of formamido compounds to yield the amines [3]. Cyclododecan-1,3- and 1,4-diones have been obtained in a 3 1 ratio and overall yield of 59% via... 

While the introduction of aryl groups has been well documented, the use of alkylboronic acids to decorate the pyridazine core is hitherto not well explored. Wermuth and co-workers nicely showed that hydroboration of alkenes with 9-BBN followed by Suzuki coupling with 3-iodopyridazines 180 and 182 yielded the corresponding 3-alkylpyr-idazines 181 and 183 in good yield (Schemes 43 and 44) <2002SL1123>. 

Guided by Marks s report of the samarium-catalyzed hydroboration of alkenes, Molander has developed a samarium-catalyzed protocol for the cyclization/hydroboration of unfunctionalized 1,6-dienes. In an optimized procedure, reaction of 1,5-hexadiene and l,3-dimethyl-l,3-diaza-2-boracyclopentane catalyzed by Gp 2Sm(THF) in toluene at room temperature for 18 h followed by oxidation gave hydroxymethylcyclopentane in 86% yield (Equation (70) R = H, n — ). The transformation was stereoselective, and Sm-catalyzed cyclization/hydroboration of 2-phenyl-1,5-hexadiene followed by oxidation formed /ra/ i--l-hydroxymethyl-2-phenylcyclopentane in 64% yield (Equation (70) R = Ph, n = ). The samarium-catalyzed reactions was also applicable to the synthesis of hydroxymethylcyclohexanes (Equation (70), n=X) but tolerated neither polar functionality nor substitution on the alkenyl carbon atoms. 

RCH—CHi - RCHiCHO. This reaction can be carried out by hydroboration followed by oxidation of the resulting organoborane with pyridinium chlorochro-mate (PCC). In first reports of the transformation BH3 S(CII3)2 was used, but this hydroboration is not regiospecific in the case of terminal alkenes. Use of disiamylborane is superior since this borane is highly selective in hydroboration of both alkenes and dienes.1... 

The syn steroselectivity was based on the assumption that the oxidation step—the transformation of the carbon-boron bond to carbon-oxygen bond—took place with retention of configuration. More recent NMR studies of alkylboranes formed in hydroborating labeled alkenes indeed confirmed the validity of the earlier conclusion.351... 

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