Regioselectivity in hydroboration

It is clear from these representative results that regioselectivity in hydroboration is controlled by steric effects. As a result, nonsymmetric internal olefins usually yield a mixture of regioisomeric alkylboranes when they react with borane. Several hindered mono- and diakylboranes with sterically demanding alkyl groups, however, have been developed for use in selective hydroboration. Disiamylborane [bis(3-methyl-2-butyl)borane, 34], thexylborane (2,3-dimethyl-2-butylborane, 35), and 9-BBN (9-borabicyclo[3.3.1]nonane) are the most frequently used reagents. Improvements of regioselectivities in hydroboration of both... 

Due to the considerably greater steric demands of 9-BBN, it is more regioselective in hydroboration reactions than borane as demonstrated by the examples given in Figure B1.3. 

Dibromoborane-dimethyl sulfide exhibits regioselectivity in hydroboration of alkenes which is comparable to that exhibited by dialkylboranes. Excess alkene should be avoided in reactions of trisubstituted ethylenes, otherwise hydrogen bromide liberated during work-up may add to the excess ethylene and cause problems. 

The regioselectivity in hydroboration of functionalized olefins and acetylenes with borane is influenced by substituents located in the vinylic and allylic positions. For... 

The reactivity and regioselectivity in hydroboration is controlled by polarization of the B—H bond and by a combination of electronic and steric substituent effects in the unsaturated system. The favored orientation is shown in Fig. 3. 

The bulky alkyl groups impose a high degree of regioselectivity in hydroboration, which is controlled primarily by steric factors e.g., in the hydroboration of aliphatic monosubstituted 1-alkenes and styrene, 99 % and 98 % of boron is placed at the terminal positions ", respectively, compared to 94% and 81 % for H3B-THF The reactivities of structurally different alkenes vary over 10 (see Table 3), and advantage can be taken of this in the synthesis of alkenylboranes and selective hydroboration of difunctional systems... 

Since the 9-methoxy derivative of 9-BBN is a common by-product of several reactions of 9-BBN,11 its efficient conversion back to 9-BBN has been described.12 Such a process enables one to recycle 9-BBN in reactions which require its high regioselectivity in hydroboration reactions and the related organoborane conversions. 

When alkenes with varying degrees of substitution undergo hydroboration, the boron ends up on the least substituted sp carbon atom. While it migjit appear from the products that the regioselectivity is controlled by steric factors, this assumption is probably too simplistic. Steric and electronic factors both favor, and are both likely responsible for, the observed regioselectivity in hydroboration reactions. 

SCHEME 28.3 Reversal in regioselectivity in hydroboration with 9-BBN and Sia2BH. 

The observed regioselectivity of hydroboration results in part from steric factors — the bulky boron-containing group can approach the less substituted carbon atom more easily. 

The electron-poor biphenyl diphosphine (15) displays poorer regioselectivity for hydroboration of styrene (78% chemical yield) but higher enantioselectivity (78%).51 The [l,2-(diphenylphos-phino)ferrocenyl]ethyldicyclohexylphosphine (JOSIPHOS) ligand52 (16) in combination with... 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

In all cases the boron goes to the side of the double bond that has more hydrogens, whether the substituents are aryl or alkyl.35B Thus the reaction of 43 with BH3 gives 98% 44 and only 2% of the other product. This actually follows Markovnikov s rule, since boron is more positive than hydrogen. However, the regioselectivity is caused mostly by steric factors, though electronic factors also play a part. Studies of the effect of ring substituents on rates and on the direction of attack in hydroboration of substituted styrenes showed that the attack by boron has electrophilic character.359 When both sides of the double bond are... 

SAMPLE SOLUTION (a) In hydroboration-oxidation H and OH are introduced with a regioselectivity opposite to that of Markovnikov s rule. In the case of 2-methylpropene, this leads to 2-methyl-1-propanol as the product. 

The rate of hydroboration with catecholborane and pinacolborane can be tremendously increased by the addition of transition metal catalysts. Hydroboration of pinacolborane 49 with alkenes 50 <1996JA909> and terminal alkynes 52 <19950M3127> proceeds with high regioselectivity in the presence of catalytic HZrCp2Cl furnishing the terminal boronates 51 and vinylboronates 53, respectively (Scheme 8). 

Alkylidene borane 360 undergoes reaction with BH3-THF 361 to yield a dimerized cycloaddition product 362. BH3 hydroborates the B—C bond in two molecules of methylidene borane 360. The regioselectivity of hydroboration is governed by the electronic factors which facilitate the attack of boron (from BH3) on the two terminal carbon atoms, thus generating the B-C-B-C-B chain. The final product is obtained by the binding of two boryl ends via two B-H-B three-center, two-electron bridged bonds (Equation 20) <2004ZFA508>. 

In the case of unsymmetrical alkynes the regioselectivity is only modest. Use of excess borane-THF results in dihydroboration. Many of the problems encountered in hydroboration reactions of borane-THF can be overcome by the use of more selective reagents such as dialkylboranes or dihaloboranes. 

Thexylborane is somewhat more regioselective than borane-THF in hydroborations of simple alkenes (Figure 4 compare Figure 1). The differences can be almost wholly attributed to the greater relative importance of the final hydroboration step, which involves a highly regioselective dialkylborane. ... 

Monohaloboranes generally exhibit significantly greater regioselectivities than borane-THF in hydroboration reactions (Figure 5), though there may be some exceptions to the rule. 

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