Regioselectivity hydroboration

Hydroboration-regioselective and stereoselective (syn) addition of BH3 (RBH2, R2BH) to olefins. Synthesis of alcohols or amines including optically active ones from olefins. Also useful in synthesis of ketones by "stitching" of olefins with CO (see 1st edition). 

Hydroboration. Regioselective and enantioselective hydroboration of 3-alkenamides is accomplished with a Rh-catalyzed process. Formation of C—B bond at C-3 is due to amide group direction, and asymmetric induction originates from ligand 13. ... 

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... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

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. 

BBN+4-methylpent-2-ene C2 and 9-BBN+4-methylpent-2-ene C3 are transition states for hydroboration by 9-BBN at the C2 and C3 positions, respectively. Which transition state is the lower energy Calculate the ratio of major to minor regioproducts at room temperature. Is this reaction likely to be more or less regioselective than the corresponding reaction involving BH3 Explain your reasoning. 

Borane 2 adds to carbon-carbon double bonds without the need of catalytic activation. This reaction has been discovered and thoroughly investigated by H. C. Browm, and is called hydroboration It permits a regioselective and stereospecific conversion of alkenes to a variety of functionalized products. 

The hydroboration is a regioselective reaction. In general the addition will lead to a product, where the boron is connected to the less substituted or less sterically hindered carbon center. If the olefinic carbons do not differ much in reactivity or their sterical environment, the regioselectivity may be low. It can be enhanced by use of a less reactive alkylborane—e.g. disiamylborane 8 ... 

The overall result of the sequence hydroboration -I- oxidation is a regioselective and-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. 

Clark and coworkers utilized enyne RCM for constructing the AB ring fragment of the manzamine alkaloids (Scheme 83) [177]. Exposing metathesis precursor 423 and ethylene gas to catalyst A provided bicycle 424 in near quantitative yield. Regioselective hydroboration of the vinyl group in 424, followed... 

Another reagent with high regioselectivity is 9-borabicyclo[3.3.1]nonane (9-BBN), which is prepared by hydroboration of 1,5-cyclooctadiene ... 

Hydroboration of acyclic and cyclic aryl alkenes with (Bpin) (1.1 equiv.) in the presence of NaO Bn (1-100 mol%) and MeOH (2 equiv.) is catalysed by [CuCl(NHC)], (0.5-5 mol%) NHC = Dries, SDries and ICy, and proceeds with very good conversions and regioselectivity (Scheme 2.11). 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

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... 

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