Hydroboration of representative alkenes

Brown, H. C., Chandrasekharan, J., Nelson, D. J. Hydroboration kinetics. 10. Kinetics, mechanism, and selectivity for hydroboration of representative alkenes with borinane. J. Am. Chem. Soc. 1984,106, 3768-3771. 

Borane-amine complexes react similarly " . Kinetics of hydroboration of representative alkenes with well-characterized dialkylborane dimers show that the dimer initially dissociates into the monomer in an equilibrium ... 

Table 4.5 First-order rate constants for the hydroboration of representative alkenes with (9-BBN)2 in various solvents at 25 °C [3]...

Studies on the rate of hydroboration of 1-octene by several borane-Lewis base adducts indicate that the mechanism proceeds via prior dissociation of the adduct which is contrary to the direct attack mechanism proposed on the basis of ab initio calculations. Hydroboration of representative alkenes by borinane dimer in n-heptane proceeds through dissociation of the dimer followed by reaction of the monomer and alkene. Borinane shows similar behaviour to 9-BBN, both molecules differing markedly from other common monofunctional reagents. The factors that affect the stereoselectivities achieved by various hydroborating reagents have been investigated theoretically and the analysis developed into a predictive tool. The reduction of ketones and hydroboration of alkenes by catecholborane is catalysed by lithium bromide. 

Figure 3. The asymmetric hydroboration of representative alkenes with 3. Chemicai yields were determined by GC analysis employing an internal standard. With the exception of the a-deuteriostyrene example ( optical rotation), all product ee determinations were performed by NMR analysis of the Mosher esters through comparisons with authentic samples of each alcohol in both racemic and optically pure forms.

Hydroboration of isoprene with catecholborane at 1 1 molar ratio in tetrahydrofliran in the presence of 1 molar % of NiCbCdppe) at room temperature was completed in 6 h. An interesting observation is a lower reactivity of 1-decene compared with isoprene under these conditions. It is the first case of higher reactivity of 1,3-diene than 1-alkene in the hydroboration reaction. It is also an indirect indication that borane is not involved in the reaction, since acyclic conjugated dienes are less reactive than 1-alkenes toward borane. Hydroboration of representative conjugated dienes in the presence of NiCUCdppe) was carried out on a preparative scale and the results are shown in Table 2.9 The mono-hydroboration products were isolated by distillation and oxidized to the corresponding unsaturated alcohols. 

The reported levels of asymmetric induction achieved with this reagent in the hydroboration-oxidation of representative alkenes are in the range of 59-78% ee for cis and 45-75% ee for trisubsti-tuted alkenes. The highest levels of asymmetric induction have been recorded for cw-2-butene (eq 2) and 2-methyl-2-pentene (eq 3). ... 

Dhokte, U. P., Brown, H. C. 2-lsopropylapoisopinocampheylborane, an improved reagent for the asymmetric hydroboration of representative prochiral alkenes. Tetrahedron Lett. 1994, 35, 4715-4718. 

The procedure represents one of the most convenient methods for the synthesis of optically active alkylboronates. However, the procedure can be of limited use for the hydroboration of styrene derivatives. The optically active organoborates are also obtained by hydroboration of internal alkenes with diisopinocampheylborane, followed by dealkylation of two diisopinocampheyl groups with acetaldehyde [58]. 

Studies relating to the stereoselective hydroboration of polyfunctional alkenes continue to appear. Various vinyl substituted pyridines, thiophenes and furans have been reacted with representative hydroborating agents,11" and further aspects of the hydroboration of cyclic dienes have been disclosed.11 The asymmetric hydroboration of cyclic enol ethers and enamines (heterocyclic alkenes) provides access... 

Hydroboration of an alkene is the starting point for a number of useful synthetic procedures, including the anti-Markovnikov syn hydration procedure we have just mentioned. Hydroboration was discovered by Herbert C. Brown (Purdue University), and it can be represented in its simplest terms as follows ... 

Armed with direct NMR information cm the diastereomeric excess achieved from the hydroboration of several representative alkenes with ( )-3, we examined the asynunetric hydroboration of various alkene types. As anticipated, 3 is particularly effective for trans alkenes ich prcxluce the alcohols in 96-97% ee. This is ccmsistent with the absence of an observable amount of lg-R R derived from the hydroboration of trans-2- mitnt with ( )-3. As expected, the c posite absolute conftguration is observed in the alccdiols derived from the enantiomeric reagents, 3R and 3S. The selectivities observed for the cw-alkenes is lower (82-... 

Asymmetric Hydroboration. Hydroboration—oxidation of (Z)-2-butene with diisopinocampheylborane was the first highly enantioselective asymmetric synthesis (496) the product was R(—)2-butanol in 87% ee. Since then several asymmetric hydroborating agents have been developed. Enantioselectivity in the hydroboration of significant classes of prochiral alkenes with representative asymmetric hydroborating agents is shown in Table 3. 

Steps a-c (hydroboration-protonolysis-oxidation) represent a method for the selective reduction of a terminal alkyne in the presence of an alkene. 

Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercuri-um ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH4 undergoes reduction. Replacement of the caiton-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process. 

Hydroboration is usually carried out in the ether tetrahydrofuran, in which borane exists as a complex 50, from which BH3 is added to an alkene, e.g. 2-methylpropene (44) in Scheme 4.11. Addition takes place at a face of the alkene by means of a four-centre transition state, as shown in 51. The partial bonds in 51 represent progressive formation of bonds between C and H, and between C and B, together with simultaneous weakening of the 7i bond and the B-H bond. In Scheme 4.11 the reaction of borane 52 is detailed this borane has two remaining B-H bonds, and a similar reaction of these two bonds with two further molecules of alkene results in exhaustive alkylation, with formation of the trialkylborane 53. The nature of the transition state 51 implies that H and B are delivered syn (to the same face), and simultaneously, to the double bond. 

Methyl iodide and dimethyl sulfate were chosen since they react readily and result in a gaseous product either diglyme or THF was used as solvent. The method was shown to be satisfactory by hydroboration of some representative alkenes. 

The first pathway represents hydroboration-oxidation of the alkene to achieve irwri-Markovnikov addition of water. The second pathway represents a radical ((r ft-Markovnikov) addition of HBr followed by an Si,j2 reaction to replace the halogen with a hydroxyl group. Each of these pathways represents a valid synthesis. As we learn more reactions, it will become more common to encounter synthesis problems with several perfectly correct answers. The goal should always be efficiency. A 3-step synthesis will generally be more efficient than a 10-step synthesis. 

The use of the NaBH4/l2 reagent system in THF for the hydroboration of alkenes has been examined using representative olefins (29,31). The corresponding anti-Markovnikov alcohols (13-15) have been isolated after H202/Na0H or H202/NaOAc oxidation. 

Figure 4, Model for the relevant steric features of3 which lead to enantiofacial selectivity in the asymmetric hydroboration of alkenes. The preferred orientations are shown for several representative alkene substrates.

The boron and hydrogen atom add to the same face of the double bond. The resulting product has the groups cis to each other. The dotted lines represent bonds formed and broken in the transition state of the concerted reaction. In summation, two properties of BH3, the electrophilic character of the boron atom and the hydride character of the hydrogen atom, account for Markovnikov addition of BH3 to alkenes. The regioselectivity also reflects some steric control in which the boron adds to the less substimted carbon atom. Figure 15.1 shows the transition state for hydroboration in greater detail. 

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