Hydroborating reagents

Hydroboration of c -4-methylpent-2-ene followed by basic workup yields two isomeric alcohols, the ratio of which depends on the hydroboration reagent, HBR2. 

By reaction of borane with two equivalents of a-pinene 15, the chiral hydroboration reagent diisopinocampheylborane 16 (Ipc2BH) is formed ... 

This reagent can be used for the enantioselective hydroboration of Z-alkenes with enantiomeric excess of up to 98%. Other chiral hydroboration reagents have been developed. ... 

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

Catecholborane and pinacolborane, in which the boron has two oxygen substituents, are much less reactive hydroborating reagents than alkyl or haloboranes because the boron electron deficiency is attenuated by the oxygen atoms. Nevertheless, they are useful reagents for certain applications.161 The reactivity of catecholborane has been found to be substantially enhanced by addition of 10-20% of N,N-dimethylacetamide to CH2C12.162... 

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. 

Unlike the selectivity of the hydroboration of terminal and internal alkenes, which depends on the metal catalysts and hydroboration reagents [20], the boron... 

Hydroborating reagent 1-Hexene 2-Methyl-1-butene 4-Methyl-2-pentene Styrene... 

The haloboranes BH2C1, BH2Br, BHC12, and BHBr2 are also useful hydroborating reagents.124 These compounds are somewhat more regioselective than borane itself but otherwise show similar reactivity. The most useful aspects of the chemistry of the haloboranes is their application in sequential introduction of substituents at boron. The... 

Hydroborating reagent 3-Methylcyclopentene 3 -Methy lcyclohexene 7,7-Dimethylnorbomene ... 

Functionalised 2,3-dihydro-l,4-dioxins can be synthesised in a three step-sequence from P-keto esters. The key step is the insertion of a Rh-carbenoid derived from an a-diazo-p-keto ester into an 0-H bond of a 13-diol <99H(51)1073>. The reaction of 2-(l,4-dioxenyl)alkanols with silyl enol ethers yields 23-disubstituted 1,4-dioxanes. When 13-bis(trimethylsilyloxy)-cyclobut-l-ene is used, the expected cyclobutanone products are accompanied by a spirocyclopropane derivative <99TL863>. 1,4-Dioxane-monochloroborane 57 is a highly reactive hydroborating reagent <990L315>. 

WitlUiindered alkenes, it is more difficult to add three a.kenes to borane. This becomes the basis for unique, borane derivatives. Sw Hydroboration Reagents. 

Notes 1. Use as other hydroborating reagents. Its value is in the increased reagent stability and solvent solubility. Like other hydroborating agents, it is stable to an array of functional groups. It is useful for the reduction of ozonides. 

Alkynes. The selectivity of hydroboration of alkynes depends on the hydroborating reagents. Diborane usually forms complex mixtures of organoboranes, including polymers.372,373 Other reagents, however, may be applied to perform selective transformations. 

Hydroxy-6-methyl-5,6-dihydropyran-2-one (597) is brominated by NBS at C-3 (78JHC1153). More than one product is often obtained in this type of reaction, for example from 2,3-dihydro-4//-pyran (593), but in acetic acid this reaction gives 2-acetoxy-3-bromotetrahydropyran (599) (58JOC1128). 2,3-Dihydro-4H-pyran reacts normally with hydroboration reagents to give tetrahydropyran-3-ol in 80% yield (70JOC2282). 

An especially selective hydroborating reagent is prepared from 1,5-cyclooctadiene and borane. The product is a bicyclic compound of structure 1 (often abbreviated as 9-BBN), in which the residual B-H bond adds to unhindered alkenes with much greater selectivity than is observed with other hydroborating reagents. It is also one of the few boranes that reacts sufficiently slowly with oxygen that it can be manipulated in air. 

The principal disadvantage of this procedure resides in its application to terminal olefins. Since the hydroboration step produces ca. 94% primary boron-bound alkyl groups, the maximum purity of primary carbinol is obviously limited to ca. 94%. Isolation of primary alcohol free of the contaminant secondary alcohol requires a tedious, yield-lowering fractionation procedure. This difficulty may be circumvented by employing a more selective hydroborating reagent, disiamylborane, as illustrated in the synthesis of 1-octanol. 

Hydroboration.1 The usual hydroboration reagents, BH3THF and BH3-S(CH3)2, are sensitive to oxygen and moisture and require special handling. 1 he complexes of BH3 and phosphorus compounds are generally stable, but much less reactive. The complex of BH3 and triphenylphosphine, m.p. 189°, can be used for hydroboration if activated by addition of methyl iodide (to form a phosphonium iodide) or sulfur (to form a triphenylphosphine sulfoxide). The complex of borane and triphenyl phosphite does not require activation and hydroborates alkenes in a reasonable time in refluxing DME or THF. Trialkyl phosphite complexes are not useful. 

Borane transforms a wide range of alkenes into trialkylboranes under mild conditions but the trifunctional nature of borane and its trialkylborane products imposes some limitations on its use. Many of the synthetically useful reactions of the trialkylboranes (see Chapters B.2 and B.3) use all three alkyl substituents, but some reactions only utilize either two or even one of the alkyl substituents. This sets a maximum yield (based on the alkene starting material) for these latter transformations of 66% and 33% respectively which is clearly undesirable especially if the alkene involved is the product of a multi-step synthetic sequence. To overcome this problem, and others such as the production of intractable polymers on addition of borane to dienes and alkynes, monoalkylborane and dialkylborane hydroborating reagents were introduced. Some commonly used reagents are depicted in Figure B 1.2 and two are described in more detail below. 

The hydroborating reagents described in Section B1.2 are generated from achiral alkenes. Addition of a source of borane to a homochiral alkene derived... 

Consider hydroboration of the prochiral alkene 2-methylbut-l-ene by the homochiral hydroborating reagent (+)-Ipc BH (Figure B1.5). 

The efficiency of asymmetric hydroboration is high if one approach trajectory leads to severe steric interactions between the hydroborating reagent and the alkene and the approach trajectory to the other face of the alkene involves relatively insignificant steric interactions, i.e. the energy difference between the two transition states is large. It should be noted, however, that if both approaches involve major steric interactions then a decrease in overall reactivity will be observed. 

The two transition states for the addition of a homochiral hydroborating reagent to the two faces of a prochiral alkene are diastereoisomeric and of different energy. 

Asymmetric hydroboration followed by oxidation is used to give optically active alcohols. For example, addition of (+)-IpcBH2 to 1-phenylcyclopentene followed by oxidation gives S,2R)-trans-2-phenylcyclopentanol in 100% e.e. (Equation B2.9). The structure of the product alcohol reveals that the homochiral hydroborating reagent encounters fewer unfavourable steric interactions with alkene substituents if it approaches the lower face of the alkene as drawn in Equation B2.9. This preference determines the absolute stereochemistry of the product. (The regiochemistry and relative stereochemistry of the product are determined by fundamental hydroboration characteristics.)... 

Due to its boron-oxgen bonds it is a less reactive hydroborating reagent than H3B, H2BR, or HBR2. It is often used for hydroboration of alkynes. 

If the hydroboration reaction is to be used to convert 1-alkynes into aldehydes, some way to stop the addition at the vinylborane stage is needed. The problem is that there is not enough steric hindrance at the end carbon of the vinylborane. The solution is to build extra steric hindrance into the other alkyl groups attached to the boron of the vinylborane. A borane, R2BH, with two bulky R groups already attached to the boron is used as the hydroboration reagent. One such reagent is prepared by the reaction of two equivalents of 2-methyl-2-butene (also known by the common name of isoamylene) with borane to produce a dialkylborane called di si amyl borane (a shortened version of diisoamylborane) ... 

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