Hydroboration-oxidation reactions regioselectivity

A reaction in which one direction of bond making or bond breaking occurs preferentially over all other directions. For example, the addition of HC1 is regioselective, predicted by Markovnikov s rule. Hydroboration-oxidation is regioselective because it consistently gives anti-Markovnikov orientation, (p. 332)... 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

Hydroboration—oxidation reactions are regioselective the net result of hydroboration—oxidation is anti-Markovnikov addition of water to an alkene. 

Note that other textbooks will refer to the regioselectivity of the hydroboration-oxidation reaction as anti-Markovnikov, while we prefer the term non-Markovnikov. We do not favor the use of anti-Markovnikov because it can be confusing to students due to the syn, not anti, stereochemistry of addition observed with this reaction. 

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... 

The mechanistic complexity of hydroboration-oxidation stands in contrast to the simplicity with which these reactions aie caiiied out experimentally. Both the hydrobo-ration and oxidation steps are extremely rapid reactions and are performed at room temperature with conventional laboratory equipment. Ease of operation, along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regioselectivity opposite to Maikovnikov s rule, makes this procedure one of great value to the synthetic chemist. 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

The overall result of the sequence hydroboration + oxidation is a regioselective anh-Markownikoff-addition of water to an alkene. This reaction is an important method in organic synthesis, since it can be made stereoselective and even enantioselective. 

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. 

Hydroboration-oxidation of alkynes preparation of aldehydes and ketones Hydroboration-oxidation of terminal alkynes gives syn addition of water across the triple bond. The reaction is regioselective and follows anti-Markovnikov addition. Terminal alkynes are converted to aldehydes, and all other alkynes are converted to ketones. A sterically hindered dialkylborane must be used to prevent the addition of two borane molecules. A vinyl borane is produced with anU-Markovnikov orientation, which is oxidized by basic hydrogen peroxide to an enol. This enol tautomerizes readily to the more stable keto form. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

Hydroboration-oxidation of tabersonine (78) gives mainly 14/3-hydroxyvincadifformine (307), together with a small amount of the 14a-epimer 224). The regioselectivity of this reaction is presumably the consequence of quaternization of Ny, by borane. If this reaction is accompanied by inversion of Ny, as occurs in the quaternization of aspidospermine, the... 

Similarly, 4-homoisotwist-2-ene (282) is hydroborated to give 283 and 284 with low regioselectivity (283 284 = 23 77) and high stereoselectivity (253a, 253b). These results may be compared to the reaction of l-terr-butyl-2,2-dimethylethene (257b), which upon hydroboration-oxidation yields 285 and 286 in a ratio of 98 2 (253c). 

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. 

Hydroboration-oxidation (Sections 6.8-6.9) This two-step sequence converts aikenes to alcohols with a regioselectivity opposite to Markovnikovs s rule. Addition of H and OH is stereospecific and syn. The reaction involves electrophilic addition of a boron hydride to the double bond, followed by oxidation of the intermediate organoborane with hydrogen peroxides. Carbocations are not intermediates and rearrangements do not occur. 

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