Trialkylboranes alcohols

Protonolysis. Simple trialkylboranes are resistant to protonolysis by alcohols, water, aqueous bases, and mineral acids. In contrast, carboxyUc acids react readily with trialkylboranes, removing the first alkyl group at room temperature and the third one at elevated temperatures. Acetic and propionic acids are most often used. The reaction proceeds with retention of configuration of the alkyl group via a cycHc, six-membered transition state (206). 

Primary alkyl groups are more reactive than secondary and tertiary. PivaUc acid accelerates the rate of protonolysis of trialkylboranes with water and alcohols (207,208). The reaction can be controlled to give excellent yields of dialkylbotinic acids and esters. 

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). 

Treatment with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid. This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydro-boration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving a rearrangement from boron to oxygenr ... 

If the reaction between trialkylboranes and carbon monoxide (18-23) is carried out in the presence of water followed by addition of NaOH, the product is a secondary alcohol. If H2O2 is added along with the NaOH, the corresponding ketone is obtained instead. Various functional groups (e.g., OAc, COOR, CN) may be present in R without being affected,though if they are in the a or p position relative to the boron atom, difficulties may be encountered. The use of an equimolar... 

For another conversion of trialkylboranes to ketones, see 18-26. Other conversions of boranes to secondary alcohols are also known. 

Generally, trialkylboranes are useful intermediates in the field of organic synthesis with versatile reactivity. The polymers prepared by polyaddition between diene monomers and thexylborane are polymer homologues of trialkylboranes, which can be converted to poly(alcohol)s, poly(ketone)s, and other polymers having some functional groups (scheme 4).8-12... 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

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]. 

When the reaction between a trialkylborane and carbon monoxide (8-24) is carried out in the presence of a reducing agent such as lithium borohydride or potassium triisopropoxy-borohydride, the reduction agent intercepts the intermediate 73, so that only one boron-to-carbon migration takes place, and the product is hydrolyzed to a primary alcohol or oxidized to an aldehyde.333 This procedure wastes two of the three R groups, but this problem can be avoided by the use of B-alkyl-9-BBN derivatives (p. 785). Since only the 9-alkyl group... 

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],... 

Hydroboration is widely employed to obtain an anti-Markovnikov alcohol from an olefin. Addition of diborane to the double bond produces an organoborane intermediate. Three equivalents of the olefin are needed to consume the BH3 and a trialkylborane is produced. Reaction with basic H202 converts the carbon-boron bond to a carbon oxygen bond. This process is effective and widely used. 

Carbonylation of a trialkylborane in the presence of ethylene glycol promotes migration of both the second and third alkyl groups from the boron atom of intermediate X to the carbon atom derived from carbon monoxide. Subsequent oxidation by hydrogen peroxide in this case produces a tertiary alcohol which bears three substituents derived from the trialkylborane (Figure B3.4). 

Trialkylboranes react rapidly with dichloromethyl methyl ether in the presence of a hindered base. Transfer of all three alkyl groups from the boron atom of the intermediate organoborate occurs and subsequent oxidation produces the tertiary alcohol derived from the three alkyl groups of the alkylborane (Equation B3.15). 

In 2007 the scope of the trialkylborane/water system was extended to the dehalogenation of alkyl iodides and the chemoselective deoxygenation of secondary alcohols in the presence of alkyl and aryl halides [86]. The rate constants for the hydrogen-atom transfer from this reagent to secondary radicals (Scheme 37) are substantially lower than those of the Ti(III) aqua-complex [78, 87]. 

Ca =ClS double bond. In the oxidation/hydrolysis sequence that follows, constitutionally isomeric alcohols are produced. In one of them, the OH group binds to Ca and in the other it binds to C. If only one constitutional isomer of the trialkylborane and consequently only one constitutional isomer of the alcohol shall be produced, the hy-droboration step must take place regioselectively. Whether regioselectivity occurs is determined by steric and electronic effects. 

Typical trialkylboranes are relatively stable toward protonolysis by treatment with alcohols, water, aqueous bases, and mineral acids,481-485 with the exception of anhydrous hydrogen fluoride.486 In contrast, carboxylic acids react with... 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Trialkylboranes react exactly as we have discussed, and they oxidize to give anti-Markovnikov alcohols. Trialkylboranes are quite bulky, further reinforcing the preference for boron to add to the less hindered carbon atom of the double bond. Boranes are often... 

When the anions 287 derived from compounds 285 were allowed to react with tri-alkylboranes, followed by oxidation, the expected ketones were obtained455. A similar process is described in Scheme 40 for bis(phenylsulfanyl)alkyllithiums151,152. Successive treatment of the obtained trialkylborane adducts 288 with mercury(II) chloride and hydrogen peroxide yielded tertiary alcohols (Scheme 75)456. The last reactions failed with bis(phenylsulfanyl)alkyllithiums. 

Treatment of the trialkylborane with H202 forms 3 molecules of an alcohol. 

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 ... 

Trialkylboranes add to propellane to give a zwitterion that can rearrange to give 60 or react with another molecule of la to give 61. Both organoboron compounds were oxidized with hydrogen peroxide and isolated as the corresponding alcohols in 65% and 21% yield, respectively (equation 23). 

HomoallyUc alcohols. The anion (LDAi of ally I phenyl selenide reacts with a trialkylborane in THE at -78° to form an ate complex (a) which rearranges at 0° to the complex b. An allylic rearrangement of b to c occurs at 25°. Reaction of b with an... 

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