Hydrolysis of boranes

An indirect method ° of double-bond reduction involves hydrolysis of boranes (prepared by 15-16). Trialkylboranes can be hydrolyzed by refluxing with carboxylic acids,while monoalkylboranes RBH2 can be hydrolyzed with base. ° Triple bonds can be similarly reduced, to cis alkenes. °... 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

An indirect method286 of double-bond reduction involves hydrolysis of boranes (prepared... 

Diborane or aUcylboranes are used for reduaion of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

N HCl) are necessary for the hydrolysis of The [B22H22] anion is the most hydrolytically stable borane anion, withstanding even 3 NUCl... 

The influence of boron-bonded ligands on the kinetics and mechanistic pathways of hydrolysis of amine boranes has been examined (37,38). The stoichiometry of trimetbyl amine azidoborane [61652-29-7] hydrolysis in acidic solution is given in equation 10. It is suggested that protonation occurs at the azide ligand enabling its departure as the relatively labile HN species. 

Boronic acids RB(OH)2 were first made over a century ago by the unlikely route of slow partial oxidation of the spontaneously flammable trialkyl boranes followed by hydrolysis of the ester so formed (E. Frankland, 1862) ... 

Hydrolysis of 1-boronylallylstannanes, available by stannylation of alkenyl and allyl boranes. gives (Z)-allylstannanes stereoselectively34. 

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) ... 

Alkylation and hydrolysis of imines alkylation of aldehydes 10-107 Alkylation and hydrolysis of dithianes 10-108 Alkylation and hydrolysis of oxazines and similar compounds 10-109 Reaction of diazo aldehydes with boranes... 

Also, desymmetrization of prochiral hydroxyalkylphosphine P-boranes was successfully performed using similar reagents and conditions. In the case of bis(hydroxymethyl)phenylphosphine P-borane 87, both its acetylation and hydrolysis of the diacetyl derivative 89 gave good results, although in addition to the expected monoacetate 88, the diol 87 and diacetate 89 were always present in the reaction mixture (Equation 42). °°... 

The first example of allylation of imines using dialkyl 2-allyl-l, 3-dioxaborolane-4,5-dicarboxylates 110 or fi-allyldiisopinocamphcnyl borane 154 was reported in 1995." After hydrolysis of the reaction product, homoallyl primary amine was obtained in 54-90% yield and up to 73% ee (Scheme 3-52). 

Decomposition of the reaction mixtures with water followed by dilute acids applies also to the reductions with boranes and alanes. Modifications are occasionally needed, for example hydrolysis of esters of boric acid and the alcohols formed in the reduction. Heating of the mixture with dilute mineral acid or dilute alkali is sometimes necessary. 

The domain of hydrides and complex hydrides is reduction of carbonyl functions (in aldehydes, ketones, acids and acid derivatives). With the exception of boranes, which add across carbon-carbon multiple bonds and afford, after hydrolysis, hydrogenated products, isolated carbon-carbon double bonds resist reduction with hydrides and complex hydrides. However, a conjugated double bond may be reduced by some hydrides, as well as a triple bond to the double bond (p. 44). Reductions of other functions vary with the hydride reagents. Examples of applications of hydrides are shown in Procedures 14-24 (pp. 207-210). 

Reduction of phenols 5-12 Oxidation of boranes hydrolysis of unsaturated boranes... 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

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