Hydroboration 2,3-dimethyl-2-butene

Nor must the reaction stop here. The dialkylborane is also a Lewis acid, and it can do one more hydroboration to give a trialkylborane. Now, however, all the original hydrogens that were on the boron are used up and there can be no further hydroborations. In practice, the number of hydroborations depends on the size of the alkyl groups in the alkene. When the groups are rather large, further reaction is retarded by steric effects—the alkyl groups just get in the way. For example, 2-methyl-2-butene hydroborates only twice, and 2,3-dimethyl-2-butene only once (Fig. 9.65). 

Monosubstituted Boranes. Only a few monoaLkylboranes are directiy available by hydroboration. Tertiary hexylborane, 2,3-dimethyl-2-butylborane [3688-24-2] (thexylborane, Thx) BH2 (5), easily prepared from 2,3-dimethyl-2-butene, is the best studied (3,60—62). It should be... 

Thexylchloroborane-Dimethyl sulHde (1). This boranc can be prepared by treating thexylborane-dimethyl sulfide with hydrogen chloride or by hydroboration of 2,3-di-methyl-2-butene with monochloroborane-dimethyl sulfide. 

Stereospecific synthesis of (2Z,4E,6E)-3,7,11-trimethyl-2,4,6,10-dodecatetraene, trans (Cio)-allofamesene 49), isolated from Perilla fmtscem Makino, was realized by the ptdladium-catalyzed cross-coupling of 4,8-dimethyl-l,3,7-nonatrienyl-l,3,2-benzodioxaborole 48) with (Z)-2-bromo-2-butene. The benzodioxaborole derivative 48) was prepared by hydroboration of 4,8-dimethylnona-3,7-dien-l-yne 47), obtained via two steps from geranial, with 1,3,2-benzodioxaborole (Eq- 112) Bombykol and its geometrical isomers were also synthesized selectively... 

Hydroboration. Thexylborane stabilized as the triethylamine complex is not useful for hydroboration, because 2,3-dimethyl-2-butene is displaced with formation of RBH2-N( 2145)3. However, TBDA is a useful reagent for hydroboration and for various reductions. Thus it reacts with 1-octene to form di- -octylthexylborane in quantitative yield. It is comparable to thexylborane-THF for reduction of aldehydes and ketones. Carboxylic acids are reduced to the corresponding alcohol. 10-Undecenoic acid is reduced selectively to undecanoic acid (90% yield). Tertiary amides are reduced very rapidly to f-amines. Acid chlorides and nitriles are reduced very slowly. 

The reactions of hindered alkenes with thexylborane are generally accompanied by a significant amount of dehydroboration to give 2,3-dimethyl-2-butene. Thus, a very large excess of the latter is needed in order to produce (monomeric) dithexylborane. Hydroboration of a-pinene (1 1 ratio) with thexylborane results in ca. 15% dehydroboration.However, alkenes of somewhat lower steric requirements, such as simple 1,2-disubstituted ethylenes, can be converted fairly cleanly into the corresponding thexylmonoalkylboranes e.g. equation 13). These are mixed dialkylboranes and behave like other fairly hindered dialkylboranes (Section 3.10.4.3). They can hydroborate alkenes of lower steric requirements to give totally mixed trialkylboranes. 

BBN-H is readily prepared (Section 3.10.2.1, equation 8) and is commercially available. It shows considerable stability, even in air for limited periods, and is therefore a very convenient hydroborating agent.Unlike di-primary-alkylboranes it is not prone to disproportionation, but it is substantially less hindered than other di-5-alkylboranes such as dicyclohexylborane and disiamylborane. Thus, it hydroborates hindered alkenes such as 2,3-dimethyl-2-butene slowly. It is less sensitive to steric factors and more sensitive to electronic factors than disiamylborane. Thus, it shows relatively little ability to discriminate between ( )/(Z) pairs but readily discriminates between 4-methoxystyrene and 4-(trifluoro-methyl)styrene. ... 

The reaction of 3,3-dimethyl-1-butene illustrates a particular advantage of the method. Rearrangement does not occur in hydroboration—evidently because carbonium ions are not intermediates— and hence the method can be used without the complications that often accompany other addition reactions. 

Thexylchloroborane, prepared by the reaction of thexylborane with an equimolar amount of hydrogen chloride in ether ° or by hydroboration of 2,3-dimethyl-2-butene with BH2CI SMe2/ is a valuable reagent for the synthesis of mixed thexyl-n-dialkylboranes and for the reduction of cai boxylic acids to the corresponding aldehydes (see Section 4.10). 

Competitive hydroborations with diborane in diglyme established that the reaction is relatively insensitive to the structure of the olefin. The most reactive olefin studied, 2-methy 1-1-butene, is separated by a factor of only 20 or 30 from the least reactive ones, 2,4,4-trimethyl-2-pentene and 2,3-dimethyl-2-butene. In hydrobora-tion with BMB a factor of 10,000 separates reactive 1-octene from cyclohexene, one of the least reactive olefins studied the study could not be extended to still more inert structures such as 2,4,4-trimethyl-2-pentene. 1-Hexyne and 3-hexyne are more reactive than the most reactive olefins studied. 

Hydroboration.2 The reagent is as selective as disiamylborane for hydroboration of olefins. Thus 1 -hexene is converted into 1-hexanol (99%) and 2 hexanol (1%) styrene into 2-phenylethanol (98%) and 1-phenylethanol (2%). The reactions are usually complete within 5 min. except in the case of highly hindered olefins (2.3-dimethyl-2-butene requires 24 hrs. at 25°). 

An alternative approach to monoalkylboranes via hydroboration is based on thexylborane. The 1 1 reaction of di- and more reactive trisubstituted olefins, e.g., 1-methylcyclopentene at —20° to — 25°C stops at the thexylmonoalkylborane stage . Less reactive trisubstituted olefins, e.g., 1-methylcyclohexene and a-pinene, react slowly, and the corresponding thexylmonoalkylboranes are formed only in 80% and 75 % yields, respectively. At 0°C, dehydroboration of 2,3-dimethyl-2-butene occurs owing to the equilibrium ... 

An overall perspective of the data in Table 5.9 and Figure 5.7 can be gained by simply comparing the relative rates of ethene and 2,3-dimethyl-2-butene. These go from very large for protonation (10" ) to small (10 - ) for hydroboration by 9-BBN as the dominance of carbocation stability effects for protonation is replaced by dominant steric effects for hydroboration by 9-BBN. 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

The kinetics for hydroboration of aikenes are conducted in various solvents such as carbon tetrachloride, hexane, cyclohexane, benzene, and THE 9-BBN exists predominantly as the dimer (9-BBN)2 [2]. After the addition of olefins, at 25 °C, the aliquots from the reaction mixture are removed after appropriate intervals of time, quenched with an excess methanol, and analyzed by GLC for residual olefin. All operations are performed under nitrogen until identical rates are observed for more reactive olefins such as 1-hexene, 2-methyl-1-pen-tene, 3,3-dimethyl-1-butene, and cyclopentene, and variation of olefin concentration does not alter the rate. These results establish that the reaction is first order (Eq. 4.1). Typical data for cyclopentene and cyclohexene are presented in Table 4.1 [1]. 

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