Hydroboration-oxidation reaction

Problem 8.6 What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction ... 

The hydroboration oxidation reaction proceeds to give cis hydration from the less hindered side of the molecule. 

From A17(20)-Olefins The addition of an oxygen function to C-20 is best accomplished by the hydroboration-oxidation reaction on a A17(20) olefin. The ready preparation of such olefins has been discussed earlier predominantly m-olefins are obtained via the Wittig reaction on 17-keto steroids, and the /ra/w-olefins via dehydration reactions. Hydroboration-oxidation of m-A17(20)-olefins gives the 20a-alcohol in high yield, presumably by attack of diborane on the a-side of the double bond 137... 

In the hydroboration/oxidation reaction of 2-cyclohexen-l-ol derivatives the regio- as well as the stereoselectivities can be steered in either of two directions by use of different reagents and reaction conditions. Dialkylboranes, e.g., 9-BBN, give mainly tn rs-l,2-diols, whereas with 1,3,2-benzodioxaborole ( catecholborane") in the presence of the rhodium hydrogenation catalyst Rh(PPh3)jCl the trans-1,3-diols become predominant. The cis- 1,2-diols are usually only formed in trace amounts, but the cis-l,3-diols are always produced as byproducts in 10 — 20% yield (D.A. Evans, 1988). 

The anti-Markovnikov s addition results from a hydroboration-oxidation reaction. 

This reaction is similar to a hydroboration-oxidation reaction. The elements of water are added to 1-butyne (C4H6) to give butanol (C4H80). 

The hydroboration-oxidation reaction initially provides the following enol, which tautomerizes to give the product. 

Although the hydroboration-oxidation reaction gives a product with a regiochemistry opposite to that predicted by Markovnikov s rule, the regiochemistry is in accord with the mechanistic version of this rule—that is, the electrophile adds to the less substituted carbon. Let s look at the mechanism of this reaction. 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

Thus, to draw the product of a hydroboration-oxidation reaction, keep in mind two stereochemical facts ... 

Reactions can be run to give the opposite of the expected product, yielding what is called anti-Markovnikov addition. That is, hydrogen ends up on the more substituted carbon of the double bond. The hydroboration/oxidation reaction yields this, as do reactions that are conducted in peroxides. 

Hydroboration/oxidation reactions take place in H2O2 and HO and yield a product with an anti-... 

The addition is exclusively cis , as demonstrated in hydroboration-oxidation reactions, assuming retention of configuration in the oxidation step. Cis addition of the B—H bond is demonstrated in the NMR study of organoboranes derived from deuterated 1-hexene and dialkylboranes ... 

The connector chain is PS, whereas the side branches are PEO. Using anionic polymerization techniques and potassium naphthalenide as initiator, a triblock copolymer, poly(B-fr-S-fr-PB) with short polybutadiene chains, was prepared. The PB blocks were subjected to hydroboration-oxidation reaction for the addition of H2O to the pendant double-bonds of the 1,2-PB units, according to the reactions (Scheme 98). 

We have seen that, in the overall hydroboration-oxidation reaction, 1 mole of BH3 reacts with 3 moles of alkene to form 3 moles of alcohol. The OH ends up on the sp carbon that was bonded to the greater number of hydrogens because it replaces boron, which was the original electrophile in the reaction. 

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. 

Consequently, the following hydroboration-oxidation reaction is performed ... 

Adoption of an iterative approach of these two strategies helped us prepare some novel bis-spirocyclopentyl derivatives of carbohydrates. For this endeavor, compound 106 was subjected to a hydroboration-oxidation reaction, which led to the formation of a complex mixture of diastereomers (Scheme 30.18). The mixture was oxidized under Swem oxidation conditions to give two products (108 and 109), separated by silica gel chromatography. The correct assignment of structures 108 and 109 was achieved by NOE experiments. 

Although KMnO treatment allowed the hydroxylation of about half of the pendant vinyl groups present in the PHA, the reaction performed with 9-borobicyclononane (a hydroboration-oxidation reaction) hydroxylated most of the vinyl groups (almost 100%) present in poly[(/ )-3-undecenoate] (Eroglu et al. 2005). 

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