Boron—carbon bonds reactions with hydrogen

Reactions of this type are somewhat less satisfactory for preparation of mixed diboron derivatives containing boron-carbon bonds. Thus, while tetrakis(dimethylamino)diborane(4) is readily hydrolyzed to tetrahydroxy-diborane(4) in aqueous acid, l,2-bis(dimethylamino)-l,2-diethylborane(4) reacts with formation of hydrogen and cleavage of the boron-boron bond 14). Reaction of the dibutyl analog with a twofold excess of water gave the dimethylamine adduct of the boroxole... 

Stereochemistry. Brown4 has shown that the carbonylation reaction and the carboethoxymethylation reaction are highly stereospecific in that the original boron-carbon bond is converted into a carbon-carbon bond with retention of configuration. These reactions thus are similar to oxidation of organoboranes with alkaline hydrogen peroxide and animation with hydroxylamine-O-sulfonic acid. 

When the alkylborane is oxidized by reaction with hydrogen peroxide and hydroxide ion, the OH group ends up in the same position as the boron group it replaces. Consequently, the overall hydroboration-oxidation reaction amounts to a syn addition of water to a carbon-carbon double bond. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

The regiochemistry of the reaction is in accord with the rule that the electrophile— the boron, in this case—adds to the carbon that is bonded to more hydrogens. The reason given previously for this orientation was that the electrophile adds so as to produce the more stable carbocation. As shown in Figure 11.7, no carbocation is involved in the... 

Hexyne has the triple bond in the middle of a carbon chain and is termed an internal alkyne. If, instead, an alkyne with the triple bond at the end of the carbon chain, a 1-alkyne or a terminal alkyne, were used in this reaction, then the reaction might be useful for the synthesis of aldehydes. The boron is expected to add to the terminal carbon of a 1-alkyne. Reaction with basic hydrogen peroxide would produce the enol resulting from anti-Markovnikov addition of water to the alkyne. Tautomerization of this enol would produce an aldehyde. Unfortunately, the vinylborane produced from a 1-alkyne reacts with a second equivalent of boron as shown in the following reaction. The product, with two borons bonded to the end carbon, does not produce an aldehyde when treated with basic hydrogen peroxide. 

---