Rearrangement boranes

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

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, though a few anomalies are known. However, there is another, indirect, method of double-bond isomerization, by means of which migration in the other direction can often be carried out. 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 (12-28), and dehydration of the alcohol (17-1) ... 

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 another procedure, the addition of a dialkyIborane to a 1-haloalkyne produces an a-halo vinylic borane (82). Treatment of this with NaOMe gives the rearrangement shown, and protonolysis of the product... 

The intramolecular rearrangement of allylic boranes (Eq. 5) clearly involves a multiple boron-carbon bond in the transition state (45), as the boron 2pz-orbital interacts with the 7r-bonding MO. 

However, the pseudo-borole 95 stemmed from another reaction of 96. Unknown to the earlier researchers who reported the synthesis of 95,94 their attempted preparation of 96 instead gave partly 98, a rearrangement observed previously,95 whereupon the reaction of 98 with PhBCl2 yielded phenyl(bis(2,3,4-triphenyl-l-naphthyl)borane, the actual identity of pseudo-borole 95 (Eq. 31). 

Another interesting reaction is the very slow acid-induced rearrangement-hydration of kuomine. This reaction results in the formation of a labile hemiacetal, hydrakouminol (54), and its reduction with a borane complex in... 

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