Alkylboranes isomerization

On heating, alkylboranes isomerize and the boron atom moves to a position where steric interactions are minimized (Equations B3.1 and B3.2). 

The second desired product is a little more tricky because the isomerization of 6 to 7 cannot be stopped at the alkylborane, 8 ... 

Thexylborane slowly isomerizes to a primary alkylborane at ambient temperature, so it is usual to prepare it freshly and to use it, at 0 °C, when it is required. However, addition of an equimolar quantity of NA -diethylaniline produces a reagent which appears significantly more stable yet retains the hydrobora-ting and reducing capabilities. ... 

When hydroboration of an olefin is carried out with the intention of oxidizing an initially formed secondary alcohol to a ketone, hydrogen peroxide oxidation can be dispensed with and the alkylborane oxidized directly with chromic acid. Thus Pappo converted a hydroxyl-free A -steroid into a mixture of stereoisomeric alkyl-boranes comparable to (1) and (2), oxidized the mixture with chromic acid, and obtained the 6-keto-5a-steroid in good yield the initially formed 6-keto-5i3-steroid underwent isomerism in the process. H. C. Brown developed an efficient two-phase system for oxidation of either alkylboranes or free alcohols to the ketones using aqueous chromic acid and ether. 

Typical RjB s isomerize at useful rates at ca. 150°C, providing equilibrium mixtures of products. Usually, the thermodynamically most stable alkylboranes are those in which... 

The ease of dehydroboration of alkylboranes decreases in the order tertiary > secondary > primary. The order of the ease of hydroboration is opposite to this. Both factors contribute to the order of isomerization decreasing tertiary > secondary > primary. Isomerization of sterically hindered or strained alkylboranes may occur even at RT e.g., oxidation of freshly prepared 2,3-dimethyl-2-butylborane (thexylborane) gives a 99 1 mixture of 2,3-dimethyl-2-butanol and 2,3-dimethyl-1-butanol, whereas oxidation... 

In the thermal isomerization of boracyclanes, the equilibrium positions of the borons are often dictated by their ring size. Given a choice, borinane (boracyclohexane) is formed preferentially. Other factors not encountered in the isomerization of acyclic alkylboranes may affect the isomerization of boracyclanes, as discussed below. 

A.iii. Isomerization of Alkylboranes. One of the early discoveries of organoboranes was their tendency to undergo isomerization if the temperature approached 160°C. This isomerization involves migration of the boron from a more to a less substituted position, usually giving the terminal alkylborane as the predom-... 

Use of alkylM (Sect. in.2.11) Satisfactory if Me, primary, homoaUyl, hompropargyl, and homobenzyl derivatives are used secondary and tertiary aUcyl may undergo isomerization Mg should be considered, when RZnX is prepared via RMgX B is often competitive especially if alkylboranes are available via hydroboration the scope with Sn is limited... 

Numerous other reactions include the synthesis of trialkylcarbinols from trialkylboranes (Brown, H. C. Acc. Chem. Res. 1969, 2, 65) and hydroboration-protonolysis leading to alkanes (Brown, H. C. Murray, K. /. Am. Chem. Soc. 1959,81,4108). Isomerization and disproportionation of the alkylboranes was also seen (reference 230b). 

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... 

The product (CXI) of the displacement reaction between 8(1-04119)3 and 8(CH2CH—0112)3 ( 9)t on the basis of present knowledge about thermal isomerization of alkylboranes, cannot be solely the 1,5-diboracyclooctane derivative suggested. At high temperatures, it readily forms polymers and boron heterocyclics of other ring sizes. 

Trialkylboranes (B-R-9-BBN) derived from hydroboration of alkenes with 9-BBN are remarkably stable to thermal isomerization [6]. The alkylborane, derived from (Z)-hex-3-ene by hydroboration with 9-BBN, requires the heating at 150 °C for 163 h to attain the equilibrium distribution of boron along the hexyl chain. At the same temperature the isomerization of the alkylborane derived from (Z)-hex-3-ene using borane-THF is complete in 1 h. 

Hydroboration of an internal olefin followed by thermal isomerization of the organoboron product accordingly gives an alkylborane in which boron is attached to a terminal carbon. The terminal olefin can then be isolated by displacement from the alkylborane using an involatile olefin ... 

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