2-Methyl-2-butene hydroboration-oxidation

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). ... 

What product would be obtained from hydroboration-oxidation of the following alkenes a. 2-methyl-2-butene b. 1-methylcyclohexene... 

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... 

PROBLEM 15.74 2-Methyl-2 -butene can be hydrated in acid, or under hydroboration-oxidation conditions to give two different alcohols. Give structures for these alcohols and assign the following NMR spectra (1 and 2) to these isomers. Explain your answers. 

Now let s draw the forward scheme. Addition of HBr to irai/is-2-butene produces 2-bromobutane, which is subsequently treated with terf-butoxide to give 1-butene. /ift-Markovnrkov addition of water (via hydroboration/oxidation) followed by PCC oxidation gives the aldehyde. Reaction with the Gngnard reagent (produced by awn-Markovnikov addition of HBr to 2-methylpropene, then magnesium, as shown) gives 2-methyl-4-heptanol after water workup. Oxidation with PCC produces the target ketone. 

Later, Brown and co-workers developed the method described above for the preparation of enantiomerically pure Ipc2BH (>99% ee) and applied the reagent in the asymmetric hydroboration of prochiral alkenes. Oxidation of the trialkylboranes provided optically active alcohols. In the case of cis-alkenes, secondary alcohols were obtained in excellent enantiomeric purity (Figure 1). The reaction is general for most types of cw-alkene, e.g. C(S-2-butene forms (R)-2-butanol in 98.4% ee, and c(s-3-hexene is converted to (R)-3-hexanol in 93% ee. However, the reagent is somewhat limited in reactions with unsymmetrical alkenes e.g. c/s-4-methyl-2-pentene yields 4-methyl-2-pentanol with 96% regioselectivity but only 76% ee (Figure 1). ... 

A similar example is seen in the [Pd2(dba)3]-catalyzed hydroboration of 2-methyl-l-buten-3-ynes [274]. While PPhj and PPh2(CgF5) favor the 1,4-addition product allenylborane 100 all diphosphines yield the 1,2-addition product ( )-dienylborane 102 exclusively (Table 1-13). This remarkable difference in selectivity is explained based on an 1,3-enyne monophosphine complex 103 and an alkynyl diphosphine complex 104 as intermediates. Dppf exhibits the best product yield among the phosphines tested. Similar observation was noted in the asymmetric hydroboration (Scheme 1-44) [275]. The action of catecholborane on 1-phenyl-1,3-butadiene also proceeds regioselectively to give, after oxidation, anti-l-phenyl-l,3-butanediol... 

The hydroboration reaction is generally highly, but not completely, regioselective. For example, reaction of 1-hexene (47) with diborane, followed by oxidation, produces 1-hexanol (48) in high yield, with only a small amoimt of 2-hexanol (49, equation 9.46). Brown determined that the preference for the boron atom to add to the less-substituted carbon atom is about 94% for monosubstituted alkenes such as 1-pentene, 99% for em-disubstituted al-kenes such as 2-methyl-l-butene, and 98% for trisubstituted alkenes such as 2-methyl-2-butene. ... 

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