Regioselectivity in Hydroboration of Alkenes

Hydroboration of c -4-methylpent-2-ene followed by basic workup yields two isomeric alcohols, the ratio of which depends on the hydroboration reagent, HBR2. 

9-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. 

Calculate activation energies for the preferred addition mode of each reagent. (Data for borane, 9-BBN and cis-4-methylpent-2-ene are available.) Which reaction will be faster Is the faster reaction more or less regioselective than the slower reaction Compare the structures of the two transition states and identify specific interactions that can account for differences in regioselectivity and reactivity. Use space-filling models. 

Finally, examine bond density surfaces for the lower-energy transition state for each reaction. Are all bonds broken and formed to roughly the same extent, or are some bonds made or broken to greater extent  

AE is the energy difference between lower-energy and higher-energy transition states (in an). 

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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 for hydroboration of alkenes with 9-BBN is controlled by steric factors. Selective hydroboration of terminal alkenes in dienes 4, 6, and 8 can be readily achieved in the presence of internal olefins (Scheme 28.2). ... 

SCHEME 28.15 Reversal in regioselectivity in hydroboration of perflorinated alkenes. 

The hydroboration of alkenes, in which the Ca=C is not symmetrically substituted can lead to constitutionally isomeric trialkylhoranes. This is because the new C—B bond can form either at the Ca or at the (A of the Ca=C double bond. In the oxidation/hydrolysis sequence that follows, constitutionally isomeric alcohols are produced. In one of them, the OH group binds to Ca and in the other it binds to C. If only one constitutional isomer of the trialkylbo-rane and consequently only one constitutional isomer of the alcohol is to he produced, the hydroboration step must take place regioselectively. Whether regioselectivity occurs is determined by steric and electronic effects. 

Hydroboration of alkenes.2 Wilkinson s catalyst is effective for catalysis of hydroboration with catecholborane (CB), which can differ from the regioselectivity and stereochemistry of uncatalyzed hydroboration with 9-BBN. In the case of... 

Many metal complexes catalyze the hydroboration. A neutral rhodium-phosphine complex RhCl(PPh3)3 is the most studied catalyst for the hydroboration of alkenes, but the complex is unfortunately highly sensitive to air. Thus, handling the catalyst under argon or in air resulted in different regioselectivity. The in situ preparation of the catalyst... 

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. ... 

Gp2TiCl2/NaBH4 mixtures have been studied as a convenient system for the hydroboration of alkenes. Mechanistic studies for these reactions are reported. These processes provide different regioselectivities and are catalyzed by the isolated Cp2Ti(/x-H)2BEl2 species. Lithium borohydride appears to be involved in the formation of the true catalytically active complex. Extensive nB NMR experiments indicated that the predominant products in the hydroboration reaction of Ph-CH=CH2 are a regiomeric mixture of tetraalkylborates, with minor amounts of trialkylborohydrides.1624,1625... 

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... 

When the alkylboranes formed by reaction with an alkene are oxidized with Na0H/H202, the product is an alcohol. When the vinylborane unit is similarly oxidized in 115 (or 114), replacement of boron with the OH gives an enol product, 117 or 116, which tautomerizes to the corresponding carbonyl derivative (see Section 10.4.5 for the similar reaction in oxymercuration). Enol 116 tautomerizes to 2-pentanone (118) and 117 to butanal (118). Because vinylborane 115 is the most prevalent vinylborane, aldehyde 119 is the major isolated product after oxidation. Note the anti-Markovnikov orientation of the boron and the oxygen, analogous to the regioselectivity shown by hydroboration of alkenes. 

Regioselectivity of DBBS in the hydroboration of alkenes and derivatives is high, approaching 9-Borabicyclo[3.3.1]nonane, e.g. 1-hexene, styrene, 2-methyl-1-pentene, 2-methyl-2-butene and 4-(dimethylphenylsilyl)-2-pentene, react by placing the boron atom at the less hindered position with >99% selectivity. Lower regioselectivity of the hydroboration-oxidation is observed... 

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. 

Hydroboration of alkenes 1 with commercially available 9-borabicyclo[3,3,l]nonane (9-BBN) 2 progresses very smoothly to provide alkyl-9-BBN derivatives 3 (Scheme 28.1). 9-BBN provides superior levels of regioselectivity as compared with BH3.SMe2 and the boron atom is added to less-substituted carbon in the double bond. ... 

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. 

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). 

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