Regioselectivity alkene reaction with borane

With 1,2-disubstituted alkenes, however, there is little discrimination in reactions with borane itself. In addition, the regioselectivity in the hydroboration of terminal alkenes, although high, is not complete. Further, there is little difference in the rate... 

Borane is unstable but can easily be made from NaBH4 and BF3. In this synthesis of hexan-l-ol from hex-l-ene, a water molecule has been added to the alkene, but with the opposite regioselectivity to reactions with H2O in acid or HBr. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

Hydroboration is highly regioselective and is stereospecilic. The boron becomes bonded primarily to the less substituted carbon atom of the alkene. A combination of steric and electronic effects work together to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value... 

Hydroboration-oxidation of alkenes preparation of alcohols Addition of water to alkenes by hydroboration-oxidation gives alcohols via anti-Markovnikov addition. This addition is opposite to the acid-catalysed addition of water. Hydrohoration is regioselective and syn stereospecific. In the addition reaction, borane bonds to the less substituted carbon, and hydrogen to the more substituted carbon of the double bond. For example, propene reacts with borane and THF complex, followed by oxidation with basic hydrogen peroxide (H2O2), to yield propanol. 

The regioselectivity comes from the first step. The boron s empty p orbital bonds to the more nucleophilic end of the alkene and hydride is transferred to give a borane. Reaction with alkaline H2O2 leads to migration of an alkyl group from boron to oxygen and eventually to the alcohol. 

Reactions that probe these disconnections are presented in the homework problems. The disconnection diagrams should be read as follows. Alkyl halides are prepared from alkenes, with regioselectivity. Similarly, alcohols are prepared from alkenes with regioselectivity. A chemical reaction involves treatment of an alkene with an acid HX to give the alkyl halide. Another chemical reaction is treatment of an alkene with aqueous acid to give the more substituted alcohol, or with borane and then basic hydrogen peroxide to give the less substituted alcohol. 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Asymmetric hydroboration.1 The key step in a synthesis of natural (+ )-hir-sutic add-C (1), based on an earlier synthesis of racemic 1, is an efficient asymmetric hydroboration of the meso-alkene 2. Reaction of 2 with (+ )-diisopinocampheyl-borane (90% ee) followed by oxidation provides the exo-alcohol 3 in 73% yield and in 92% optical purity. Ring expansion of the corresponding ketone with ethyl diazoacetate is not regioselective even in the presence of BF3 etherate or (C2H5)30+ BF4, but does afford the desired a-keto ester in the presence of SbCl5 (8, 500-501). Decarboxylation of the crude product gives (— )-4 in 90% ee after chromatography. 

Borane, as a solution in tetrahydrofuran or generated in situ by the reaction of a metal hydride with boron trifluoride etherate, adds readily to alkenes to yield trialkylboranes. With a terminal alkene the reaction is highly (though not completely) regioselective and gives a primary trialkylborane, since the mode of addition results from the electrophilic character of the boron atom. 

Hydroboration-oxidation occurs by syn addition. The reagents are borane or an alkyl or dialkyl derivative, followed by oxidation, usually with HjOj and "OH. The oxidation occurs with retention of configuration of the alkyl group. The regioselectivity favors addition of the boron at the less-substituted carbon of the double bond. As a result, the reaction sequence provides a stereospecific syn, anti-Markovnikov hydration of alkenes. 

As is true for most addition reactions, there is a preference for approach of the borane from the less hindered face of the double bond. Since diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered molecules. Table 5.8 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result from both the low regioselectivity and from addition to both faces of the double bond. Even 7,7-dimethylnorbornene shows only a modest preference for endo addition with diborane. The selectivity is much enhanced with the bulkier reagent 9-BBN. 

Hydroboration-Oxidadon (Section 6.4) BH3 followed by basic HjOj converts alkenes into alcohols with non-Markovnikov regioselectivity (the OH adds to the less substituted alkene carbon) and syn stereoselectivily without rearrangement. The reaction is stereospedfic because as-alkenes give different products than do frans-alkenes.The mechanism involves coordination of the alkene tt bond to the vacant 2p orbital of borane followed by a four-membered ting transition state, which simultaneously adds H to the mote substituted carbon and boron to the less substituted alkene carbon. The basic peroxide replaces the boron with OH. 

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