For hydroboration of alkene

The reactions can be made enantioselective by using enantiomerically pure IpcBH2 for hydroboration of alkenes and then transforming the products to enantiomerically pure derivatives of 9-BBN by reaction with 1,5-cyclooctadiene.22... 

Stereochemistry. The four-center transition state for hydroboration of alkenes discussed above implies that addition of the B-H bond to a double bond proceeds in a syn-mannev. Moreover, hydroboration of a bicyclic alkene, such as a-pinene, results in the addition of B-H from the less hindered face of the molecule to give IpCjBH. 

The reagent itself, IpcBH2 57 is released from the crystalline complex 56 by treatment with BF3 in THF. TMEDA prefers to form a complex with the more electron-deficient BF3 this crystallises leaving pure IpcBH2 57 in solution ready for hydroboration of alkenes. Reactions with simple inms -disubstituted alkenes 58 are reasonable trans butene gives (+)-(R)-2-butanol of 73% ee in 73% yield.11... 

Hydroboration and hydrosilylation. A mixture of EtjSiH and BCI3 is used for hydroboration of alkenes, leading to alkyldichloroboranes. Conjugated dienes give (Zj-alkenyltriethylsilanes on reaction with EtjSiH photochemically in the presence of Cr(CO),. 

Hydroboration. Pinacolborane is a stable, easily prepared and stored hydroborating agent. Unlike catecholborane which requires harsh reaction conditions for hydroboration of alkenes (100 °C) and alkynes (70 °C), hydroboration with pinacolborane proceeds under mild conditions furnishing the boronates. JCnochel and co-workers observed an excellent level of regioselectivity for hydroboration of alkynes with pinacolborane at room temperature (eq 2). Alkenes, however, react slowly with pinacolborane and often require heating for 2-3 days to furnish the terminal pinacolboronates as the major regioisomer (>98%) (eq 3). 

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 regioselectivity for hydroboration of alkenes with 9-BBN is controlled by steric factors. Selective hydroboration of terminal alkenes in dienes 4, 6, and 8 can be readily achieved in the presence of internal olefins (Scheme 28.2). ... 

In 1985, Mannig and Noth reported the discovery that Wilkinson s catalyst ([RhlPPhjljCl]) is an effective catalyst for hydroboration of alkenes with catecholborane, a reaction that otherwise requires elevated temperatures [32], Extensive subsequent studies on metal-catalyzed hydroborations [8, 11, 33-35] revealed a range of Interesting results. Firstly, hydroboration of cyclohex-2-enols affords regioisomeric products that are complementary to those isolated from the uncatalyzed reaction with 9-BBN (Equation 2) [33, 34). Secondly, the hydroboration of 91 with 9-BBN affords anti product 93, but the Rh-catalyzed reaction leads to preferential formation of syn product 92. This feature of the metal-catalyzed process was used by Evans in the stereoselective synthesis of polyketide building blocks 92 and 93 (Equation 3) [33]. 

Molander has recently reported the preparation of various 3-functionalized propyl trifluoroborates from the corresponding allyl derivatives following various literature protocols for hydroboration of alkenes [127]. The 3-heteropropyl trifluoroborates were obtained as crystalline air-stable solids by treatment of hydroboronated products with KHFj. Palladium-catalyzed cross-coupling of these 3-heteropropyl trifluoroborates with 4-acetylphenyl triflate gave functionalized alkylarenes in good yields (Scheme 3.82). 

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. 

Tab. 1-2 Catalysts for Hydroboration of Aliphatic 1-Alkenes 1. HBXj/Rh catalyst/r.t. R OH...

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)... 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

B-alkylcatecholboranes, prepared by rhodium(I)-catalyzed hydroboration of alkenes, are suitable radical precursors for conjugate addition to activated olefins. This procedure is particularly useful for the control of the regio- and... 

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... 

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. 

Alkylboranes formed in the hydroboration of alkenes and alkynes seldom are isolated for the most part they are used as reactive intermediates for the synthesis of other substances. In the reactions of alkylboranes, the B-C bond... 

Alkylboronates can be obtained by the hydroboration of alkenes. This reaction is frequently applicable when the addition of an anion to a trialkyl boronate is not feasible. Cat-echolborane is an effective hydroboronating agent. 12 In addition to reacting with unsubstituted alkenes, it is effective in the hydroboronation of 1-haloalkenes at elevated temperatures 21 and at room temperature in the presence of Wilkinsons catalyst. 22 For example, 1,3-dibromopropene reacts with catecholborane to give 1,3-dibromopropylboronate... 

Dicarbonyltitanocene [Cp2Ti(CO)2] is also active for the hydroboration of alkynes at room temperature. Dimethyltitanocene [Cp2TiMe2] catalyses the hydroboration of alkene with CBH at room temperature [203],... 

Catalysis with LiBH4 Hydroboration of alkenes with catecholborane is generally slow, but can be affected at room temperature in about an hour when catalyzed by a small amount of LiBH4. The catecholborane can be generated in situ by reaction of BHj with catechol after evolution of H2 stops, the alkene (1 equiv.) and LiBH4 (0.1 equiv.) are added. Hydroboration is usually complete after stirring for 1 hour at 25°. Yields (after oxidation) are generally almost quantitative. 

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