Rearrangement, allylic boranes

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. 

As a conseqnence of their superior stability, many types of allylic boronates can be isolated and purified. It should be noted that most pinacol allylic boronic esters and other bnlky esters are stable to hydrolysis and can be conveniently pnrified by chromatography on silica gel. A potential pitfall of all allylic boron componnds is their stereochemical integrity, and snbstituted allylic boranes are known to nndergo reversible borotropic rearrangements at temperatures above —45° (see Eq. 12, M = Eor this reason, allylic boranes are nor-... 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

Upon treatment of indoles with allylic borane, [l,3]hydrogen shift from the nitrogen atom to C-3 proceeds, producing the imine complex 8. Further allylboration of the C=N bond formed occurs with rearrangement through the chair-like six-membered transition state 9 allylic group being added mainly or quantitatively in /ram-fashion relative to the 3-R. Hydrolysis of iminoborane 10 formed leads to the 2-allylated indoline 11. 

The allyl-borane (23) is much more stable than other allylborane systems, most of which undergo spontaneous rearrangements.146... 

Simple alkyl boranes do not react with aldehydes and ketones but allyl boranes react rapidly because they can use a six-membered cyclic transition state 68 not unlike that of the Claisen rearrangement. The evidence for this is that the allyl group is rearranged in the product. In this example a crotyl borane 66 adds to an aldehyde to give anti-70 since both R and Me prefer to be equatorial in the transition state 68. 

Two allylic organoboranes might be expected in the hydroboration of unsymmetri-cally substituted allenes. However, only one, possessing the boron atom at the least substituted site, is formed. This may not reflect solely the initial position of boron attack on the allene, because allylic boranes undergo permanent allylic rearrangement . In the equilibrium, the boron atom favors the least substituted position ... 

Also, a prominent feature of such allylic boranes is that reaction is accompanied by an allylic skin rearrangement (78) [Eq. (9)]. Such a reaction may be favored over the direct insertion of the alkyne into the 2-butenyl-boron bond because of the possible pericyclic character of the rearrangement. If no intermediate is involved, the reaction in Eq. (9) can be viewed as an allowed ene-reaction (55), a [,2s -l-, 2s -t- process if an intermediate collapse is involved, its collapse to the product can be viewed as a converted process, analogous to that shown in Eq. (8). 

The isomerizing rearrangements of such allylic boranes vary markedly... 

Allylic boranes generally undergo rapid allylic rearrangement, but hydroboration of cyclohexa-1,3-diene with (+)- or (-)-a-iso-pinocamphenylborane (Ipc BH) gives the first example of an optically active allylic borane (109) that is stable towards rearrange-... 

Dipropyl(but-2-en-l-yl)borane gave the 1,1-allylboration products without rearrangement of allylic substituent likewise 1,2-allylboration.2 The final olefin contains predominantly the allylic substituent that means that 1,1-allylboration is much faster than 1,1-organoboration with nonallylic boranes. It can be explained by the higher Lewis acidity of allylboranes compared to trialkylboranes. 

Allylic rearrangement also provides a means of interconverting propargylic and allenic boranes. ... 

Reaction with alkenyl epoxides. The Lewis acid nature of the boranes is felt by the substrates so that rearrangement often precedes allylation. 

Syntheses with boranes and acetylene derivs. Allyl-type rearrangement a-Alkoxystyrenes from alkoxyacetylenes... 

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