Oxidation of Trialkylboranes

The boron atom accepts an electron pair from the hydroperoxide ion to form an unstable intermediate. 

Hydroxide anion attacks the boron atom of the borate ester. 

An alkoxide anion departs from the borate anion, reducing the formal change on boron to zero. 

An alkyl group migrates from boron to the adjacent oxygen atom as a hydroxide ion departs. The configuration at the migrating carbon remains unchanged. 

Proton transfer completes the formation of one alcohol molecule. The sequence repeats until all three alkoxy groups are released as alcohols and inorganic borate remains. 

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Arylboronic esters (Figure 5.48) can be oxidized with H202 in acetic acid to give aryl borates, which can then be hydrolyzed to provide phenols. The mechanism for this oxidation is similar to that for the OOH oxidation of trialkylboranes and is discussed in Section 14.4.3. In combination with the frequently simple synthesis of arylboronic esters one can thus achieve the overall conversions Ar—H —> Ar—OH or Ar—Br —> Ar-OH in two or three steps. 

Rearrangements also are involved in the oxidations of trialkylboranes with H202/ NaOH (Figure 11.36) and of arylboronic acid esters with H202/H0Ac (Figure 11.37). 

Some other important aspects of boric acid chemistry are summarized in Fig. 5-26. Among these is the formation of borate esters [B(OR)3, R = alkyl or aryl], usually obtained as colorless liquids, on treatment with alcohols and H2S04. The vast literature on these compounds falls generally within the purview of organic chemistry, and will not be developed here however, it will be noted that a well-known qualitative test for boron involves treatment of the sample with methanol to form B(OMe)3, which produces a bright green color in a Bunsen burner flame. A major early discovery in this area was the synthesis of boronic acids by E. Frankland in 1862, via partial oxidation of trialkylboranes, with subsequent hydrolysis of the ester ... 

As an alternative, organoboranes can be oxidized with Na-perborate (NaB03 4 H20)" or with trimethylamine N-oxide (MejNO). Oxidation of trialkylboranes with pyridinium chlorochromate (PCC) leads directly to the corresponding carbonyl compounds. ... 

Oxidation of trialkylboranes.1 Trialkylboranes react with oxygen2 in THF to give alcohols in practically quantitative yield ... 

The related direct oxidation of trialkylboranes has been studied 178), as well as the brominolysis and iodinolysis of benzeneboronic acid in aqueous acetic acid (50%) and nt-chlorobenzeneboronic acid in aqueous solution. The latter reveals a pattern involving tetracovalent boron 179). In water solutions, and acetate buffers at constant ionic strength with the w-chloro derivative, plots of log k vs. pH were linear with unit slope (from pH 2 to 5), suggesting specific lyate ion catalysis. In both cases catalysis by fluoride ion was observable, and catalysis by hydroxy acids or diols, which form coordination complexes with boron, was seen in the latter case. Indeed in water solvent, the catalytic constant for fluoride ion is some 6000 times that of the uncatalyzed case. 

Oxidation of trialkylboranes with MoOgjHMPA followed by hydrolysis has been reported to give alcohols in 73—78 % yields, while oxidation of organo-boranes derived from cyclic alkenes with an excess of pyridinium chlorochromate provided ketones in yields of 81—92%, ... 

A very mild means of oxidation of trialkylboranes to the corresponding alcohols is provided in the stoicheiometrically controlled reaction with oxygen at least a portion of the reaction is free-radical in nature, iodine acting as an inhibitor this leads to some loss of stereospecificity in cases leading to chiral alcohols. If the oxidation is performed at low temperature in dilute solution, alkyl hydroperoxides are obtained. ... 

Sequential oxidation of trialkylboranes takes place with trimethylamine N-oxide and oxidation of a chiral boronic acid by flavoenzyme cyclohexanone oxygenase proceeds with retention of configuration at the migrating centre in an analogous manner to peroxide oxidation. One electron oxidation of alkyltriphenyl borate anions leads to carbon-boron bond cleavage and the formation of free alkyl radicals. ... 

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). 

The N-oxides of isoquinolines have proved to be excellent intermediates for the preparation of many compounds. Trialkylboranes give 1-alkyl derivatives (147). With cyanogen bromide in ethanol, ethyl N-(l- and 4-isoquinolyl)carbamates are formed (148). A compHcated but potentially important reaction is the formation of 1-acetonyLisoquinoline and 1-cyanoisoquinoline [1198-30-7] when isoquinoline N-oxide reacts with metbacrylonitrile in the presence of hydroquinone (149). Isoquinoline N-oxide undergoes direct acylamination with /V-benzoylanilinoisoquinoline salts to form 1-/V-benzoylanilinoisoquinoline [53112-20-4] in 55% yield (150). A similar reaction of AJ-sulfinyl- -toluenesulfonamide leads to l-(tos5larriino)isoquinoline [25770-51-8] which is readily hydrolyzed to 1-aminoisoquinoline (151). 

Since the catalytic effect of Cu ions with Fenton s reagent has been recognized, continuing interest has been focused on the role of the Cu species in oxidizing systems. The electrochemical alkyltransfer reaction of trialkylboranes to carbonyl compounds has been performed in an undivided cell by using the sacrificial Cu anode in a DMF-BU4NI system, giving the alkylated products in 62 77% yields [580]. 

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

Ketones and tertiary alcohols were also available in good yields under mild conditions through interaction of trialkylboranes with lithium tris(phenylthio)methanide followed by oxidation [285],... 

The overall reaction is quite complex but involves a rearrangement similar to that described for the hydroboration-oxidation of alkenes (Section 11-6E). The first step is hydroboration of the alkene to a trialkylborane. When the trialkylborane is exposed to carbon monoxide, it reacts (carbonylates) to form a tetracovalent boron, 9 ... 

The present preparation illustrates the procedure for the hydroboration18 of a terminal olefin and the oxidation of the resultant trialkylborane. 

The oxidation of a trialkylborane may be effected by per-benzoic acid or by aqueous hydrogen peroxide in the presence of alkali.20 A detailed systematic study of the reaction parameters (oxidation temperature, base concentration, hydrogen peroxide concentration) of the latter method has led to the development3 of a standard and common procedure for oxidizing organoboranes, and is illustrated in the present procedure. 

Fig. 11.36. H202/Na0H oxidation of a trialkylborane (see Figure 3.17 for the preparation of trialkylboranes D and E and for the mechanism of the hydrolysis of the resulting boric acid ester). Deuterium labeling studies show that the conversion of the C-B into the C-O bonds occurs with retention of configuration.

The oxidation of primary and secondary trialkylboranes with pyridinium chlorochromate (PGG) provided aldehydes or ketones.504-507 An oxidative conversion of alkenes into a carbonyl compound was conducted by tandem hydroboration and oxidation with excess A-methylmorpholine-A-oxide (NMO) in the presence of Pr4NRu04 (TPAP) (Equation (105)).508... 

The stereochemistry of the hydroboration-oxidation of 1-methylcyclopentene is shown next. Boron and hydrogen add to the same face of the double bond (syn) to form a trialkylborane. Oxidation of the trialkylborane replaces boron with a hydroxyl group in the same stereochemical position. The product is frracemic mixture is expected because a chiral product is formed from achiral reagents. 

Sodium hypochlorite oxidizes aryldihydroxyboranes to phenols and trialkylboranes to the cone-sponding alcohols. Use of sodium hypobromite for the oxidation of phenyldihydroxyborane gave... 

The chlorination of trialkylboranes has not been well studied. The chlorination of MesB at -95 C yields ClCH2BMe2. The oxidation of dialkoxy(a-phenylthio)alkylboranes with NCS has been referred to in Section 4.1.8 (equation 50). The reaction of thionyl chloride with PhSCH2B(OR)2 yields PhSCH2Cl.ioi... 

Later, Brown and co-workers developed the method described above for the preparation of enantiomerically pure Ipc2BH (>99% ee) and applied the reagent in the asymmetric hydroboration of prochiral alkenes. Oxidation of the trialkylboranes provided optically active alcohols. In the case of cis-alkenes, secondary alcohols were obtained in excellent enantiomeric purity (Figure 1). The reaction is general for most types of cw-alkene, e.g. C(S-2-butene forms (R)-2-butanol in 98.4% ee, and c(s-3-hexene is converted to (R)-3-hexanol in 93% ee. However, the reagent is somewhat limited in reactions with unsymmetrical alkenes e.g. c/s-4-methyl-2-pentene yields 4-methyl-2-pentanol with 96% regioselectivity but only 76% ee (Figure 1). ... 

TriaikykarUnols from trialkylhoraites. Trialkylcarbinols have been prepared by the reaction of trialkylboranes with carbon monoxide in diglyme followed by oxidation with hydrogen peroxide (equation 1). See 2. 60. The method has the disadvantage that at the temperature required isomerization of organoboranes can be significant. 

Carbonylation of trialkylboranes in the presence of ethylene glycol results in migration of a second and a third alkyl group to give, after oxidation, the corresponding tert-alcohols. ... 

Earlier in this chapter we discussed the oxidation of boranes with hydrogen peroxide. When borane itself was used, the reaction was quite unambiguous. The trialkylborane 225 is the first product and when it is oxidised any of the three identical alkyl groups may migrate first to oxygen. 

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