Boranes hydrogenations with

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

Bis(a-aminomethyl)phosphines having a mobile hydrogen atom could be expected to undergo a borylation reaction with borane, hydrogen being evolved. In fact, bis(Af-phenylaminomethyl)-phenylphosphine (199) and its sulfide (200) appeared to interact with borane under mild conditions, yielding a new type of phosphorus-boron-containing heterocycle—2-... 

Belgrano also mentions liquid proplnt mixtures of hydrogen with light substances such as boranes of lithium or beryllium... 

In the first of these reactions (Equation 11-2), a hydrocarbon is produced by the cleavage of a borane, R3B, with aqueous acid, or better, with anhydrous propanoic acid, CH3CH2C02H. The overall sequence of hydroboration-acid hydrolysis achieves the reduction of a carbon-carbon multiple bond without using hydrogen and a metal catalyst or diimide (Table 11-3) ... 

The low yield observed in the isomerization of the 2,4,4-trimethylpent-2-ene (28) is because of the facile cyclization of the borane 30, with evolution of hydrogen, to produce a boron heterocycle 31. Oxidation of the reaction mixture provides... 

The boron atom in BH3 is sf hybridized with a vacant p orbital perpendicular to the plane of the three boron-hydrogen bonds. Thus borane and its derivatives are electrophilic (Lewis acidic) and combine readily with electron-rich species. For example, borane interacts with one of the lone pairs on the oxygen atom of tetrahydrofuran as shown below. 

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

For example, (Z)-but-2-ene (37) reacts with the chiral dialkylborane 36 to produce 38 as the result of the addition of the borane to the upper (Re) face of C(2), or the equivalent lower (Re) face of C(l). Conventional work-up of the borane 38 with alkaline hydrogen peroxide (Scheme 8.3)... 

A different style of reduction comes with the use of trialkyl boranes derived from a-pinene. Hydroboration of a-pinene 22 occurs from the less hindered face away from the gem-dimethyl groups to give the tertiary borane 23 with the usual regioselectivity (chapter 17). The marked hydrogen atom is transferred from boron to carbon.3... 

The reactions may be run on a large scale but is advisable to cool the reaction to 0 "C or 25 "C since the reduction may become exothermic. Usually an excess of organoborane is used since the bimolecular process becomes very slow towards the end of the reduction. Excess organoborane is destroyed by the addition of a low-boiling aldehyde such as acetaldehyde. This helps prevent contamination of the desired product with isopinocampheol. The borane component may be removed by precipitation as an ethanolamine complex or by an oxidative workup using hydrogen peroxide and base. Alternatively, these two steps may be combined, with the majority of the borane removed with ethanolamine and any remaining boron components removed by oxidation. 

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