Boron—oxygen bonds hydrogen

Zirconium-aryne complexes have also found application in the regiose-lective synthesis of halophenols.62 Reaction of 101 with methyl diethylbori-nate or triethylborate leads to insertion of a boron-oxygen bond into a Zr-benzyne bond with formation of 102 and 104, respectively (Scheme 12). Further reactions of 102 and 104 with bromine or iodine followed by excess NaOH and 30% hydrogen peroxide gives halomethyl(methoxy)phe-nols 103 and 105 in good yields. In most cases only one regioisomer of the halophenol was observed. 

Step 1 Hydride (hydrogen + two electrons) is transferred from boron to the positively polarized carbon of the carbonyl group The carbonyl oxygen bonds to boron... 

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

Oxidation of alkylboranes by alkaline hydrogen peroxide produces alcohols. The reaction is essentially quantitative and has been successfully applied to a wide variety of alkylboranes (Equations B2.3-5). It is important to note that the stereochemistry of the carbon atom attached to the boron atom is retained in this conversion of a carbon-boron bond to a carbon-oxygen bond (Equation B2.5). 

The chemical reactivity of several heterocyclic derivatives is closely associated with a reactivity one might expect for open-chain unsaturated derivatives. The boron heterocycles are sensitive to oxygen or hydrogen peroxide 111, 169) and so apparently is 1,1,2,5-tetraphenylsilacyclo-pentadiene 17, 246). These reactions probably lead to ring cleavage but the products have not been characterized. Vinyl-metal bonds are frequently cleaved by electrophilic reagents and the heterocycles are no exception. 

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