Alcohols organoboranes

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

In addition to the oxymercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H. C. Brown and cailed hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

The most widely used reaction of organoboranes is the oxidation to alcohols, and alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation. The mechanism, which is outlined below, involves a series of B to O migrations of the alkyl groups. The R—O—B bonds are hydrolyzed in the alkaline aqueous solution, generating the alcohol. 

Several alkylboranes are available in enantiomerically enriched or pure form and can be used to prepare enantiomerically enriched alcohols and other compounds available via organoborane intermediates.196 One route to enantiopure boranes is by hydroboration of readily available terpenes that occur naturally in enantiomerically enriched or pure form. The most thoroughly investigated of these is bis-(isopinocampheyl)borane (Ipc)2BH), which can be prepared in 100% enantiomeric purity from the readily available terpene a-pinene.197 Both enantiomers are available. 

The regiospecific functionalization of polyolefins can be achieved in a similar fashion to yield organoboranes, which are then converted into alcohols (Equation (21)).32... 

Alkylzirconocene derivatives can be converted to alcohols with H202/NaOH, /BuOOH, or m-chloroperbenzoic acid (m-CPBA) [98], These reactions appear to involve migratory insertion processes (Pattern 14) similar to those observed with organoboranes (Scheme 1.20). On the other hand, oxidation with 02 may be a radical process. 

ESTERIFICATION OF HINDERED ALCOHOLS tert-BUTYL p-TOLUATE, 51, 96 Esters, from diazoketones and organoboranes, 53, 82 Esters, a-deuterio-, 53, 82 Esters, y [Pg.59]

Organoboranes, ketones and alcohols from, 53, 77 Organolithium compounds, 53,... 

KETONES AND ALCOHOLS FROM ORGANOBORANES 1. PHENYL HEPTYL KETONE 2. 1-HEXANOL ... 

Addition of boranes to olefins, followed by basic oxidation of the organoborane adducts, resulting in alcohols. 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

Halide-metal exchange, 58, 2 Halides, synthesis from alcohols, 34, 2 by chloromethylation, 1, 3 from organoboranes, 33, 1 from primary and secondary alcohols, 29, 1... 

The most widely used reaction of organoboranes is the oxidation to alcohols. Alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation. The mechanism is outlined below. 

Conditions that permit oxidation of organoboranes to alcohols using molecular oxygen,139 sodium peroxycarbonate,140 or amine oxides141 as oxidants have also been developed. The reaction with molecular oxygen is particularly effective in perfluoroalkane solvents.142... 

Scheme 4.8. Alcohols, Ketones, Aldehydes, and Amines from Organoboranes...

If the organoborane is heated with carbon monoxide to 100-125°C, all of the groups migrate and a tertiary alcohol is obtained after workup by oxidation. The presence of water... 

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