Borane reaction with alkenes, isomerization

Figure 3.32 showed the reaction of our enantiomerically pure chiral cyclic dialkylborane with (Vi )-3-ethyl- l-methylcyclohexene. ft took place relatively slowly with the rate constant k6 The reaction of the same dialkylborane with the isomeric. S -alkene was shown in Figure 3.33. ft took place considerably faster with the rate constant ky The combination of the two reactions is shown in Figure 3.34. There the same enantiomerically pure borane is reacted simultaneously with both alkene enantiomers (i.e., the racemate). What is happening In the first moment of the reaction the R- and the 5-alkene react in the ratio k6 (small )/ 5 (big). The matched pair thus reacts faster than the mismatched pair. This means that at low conversions (< 50%) the trialkylborane produced is essentially derived from the 5-alkene only, ft has the stereostructure E. Therefore, relative to the main by-product F, compound E is produced...

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, although a few anomalies are known. However, an indirect method of double-bond isomerization us known, leading to migration in the other direction. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol 17 (see 12-31), and dehydration of the alcohol (17-1) to the alkene. The reaction is driven by the fact that with heating the addition of borane is reversible, and the equilibrium favors formation of the less stericaUy hindered borane, which is 16 in this case. 

By far the most widely used method for making boron-carbon bonds is by the addition of boranes to alkenes or alkynes. Much of the work in this area is due to H.C. Brown. This reaction is termed hydroboration . Although it is reversible the equilibrium normally lies in favour of product rather than reactants in ether solvents at room temperature, so that the addition goes to completion. On heating above ca. 100°C organoboranes with a j5-C—H function eliminate alkene, that is, the reverse reaction takes place. Isomerization of alkylboranes occurs by a series of these addition-elimination steps, leading eventually to terminal boranes. The alkene can then be displaced by a less volatile alkene. The direction of this... 

Since the migration reaction is always toward the end of a chain, terminal alkenes can be produced from internal ones, so the migration is often opposite to that with the other methods. Alternatively, the rearranged borane can be converted directly to the alkene by heating with an alkene of molecular weight higher than that of the product (17-14). Photochemical isomerization can also lead to the thermodynamically less stable isomer. ... 

In Fig. 4, reaction A is a highly stereoselective reduction of 1-aryl alkanones with (-)-chlo-ro diisopinocampheylborane [21]. Upon co-ordination of the ketone oxygen with the Lewis acidic chirotopic and non-stereogenic [22] boron atom of the chiral reagent, two diastereo-isomeric complexes arise. The sterically less hindered one is preferentially formed and leads the major (,S)-enantiomer, which is isolated after a work-up that allows recovery of a-pinene, the chiral alkene from which the borane is prepared. 

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