Isopinocampheyl borane

Several alkylboranes are available in enantiomerically enriched or pure form and can be used to prepare enantiomerically enriched alcohols and other compounds available via organoborane intermediates.196 One route to enantiopure boranes is by hydroboration of readily available terpenes that occur naturally in enantiomerically enriched or pure form. The most thoroughly investigated of these is bis-(isopinocampheyl)borane (Ipc)2BH), which can be prepared in 100% enantiomeric purity from the readily available terpene a-pinene.197 Both enantiomers are available. 

P-Allyl-to-(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.40 The stereoselectivity is reagent controlled, in that there is no change in stereoselectivity between the two enantiomeric boranes in reaction with a chiral aldehyde. Rather, the configuration of the product is determined by the borane. Both enantiomers of (Ipc)2BH are available, so either enantiomer can be prepared from a given aldehyde. 

The bis(isopinocampheyl)borane reagents described in the sections on enantioselective additions have found extensive use in the total synthesis of complex, bioactive natural products. A synthesis of the potent anticancer agent epothilone... 

Allyl-bis(isopinocampheyl)borane exhibits high stereoselectivity in reactions with chiral a-substituted aldehydes.39... 

In his recent review of betweenanenes, Marshall (74) reported the synthesis of the [22.10] -compound, which was found to be inert toward complex formation with tetracyanoethylene, and toward hydroboration with isopinocampheyl-borane, suggesting that the [22] -bridge is not long enough to let the [10] -bridge pass through it. 

Di(isopinocampheyl)borane Sec. alcohols from ketones Asym. reduction... 

Methyl-3-keto-l-pentanol tetrahydro-2-pyranyl ether added at 0-3° during 0.5 hr. to a tetrahydrofuran soln. of (+)-di(isopinocampheyl)borane prepared from di-borane and (—)-a-pinene, stirred overnight at 6-7°, excess hydride decomposed with water, 3 N NaOH followed by 30%-H2O2 added with cooling, and stirred 1.5 hrs. at ca. 40° -> (+)-4-methyl-l-(3R)-dihydroxypentane l-tetrahydro-2-pyranyl ether. Y 91.4%. - Also prepn. of the (—) isomer with (+)-a-pinene and the (+)-(3R)-3-Di-compound with deuteriodiborane prepared from NaBD4 s. E. Caspi and K. R. Varma, J. Org. Chem. 33, 2181 (1968). 

Thexylborane Al-compds., organo-R2BH, Bis-(3-methyl-2-butyl)-Dicyclohexyl-Di( isopinocampheyl)-borane, Bis-(3,5-di-methyl)borinane, 3,6-Dimethylborepane,... 

Double stereodifFerentiation was used to achieve enantioselective enolborination of chiral ketones by reaction with chlorobis(isopinocampheyl)borane (DIPCl) in the presence of a chiral amine [69]. As an example, reaction of excess ( )-94 with IPC2BCI and sparteine at —78°C produced 95 regioselectively with a 90% ee (evaluated after oxidation into diadds). 

Di(isopinocampheyl)borane Resolution of racemic ethylene derivatives with optically active boranes s. 19, 720 Lithium tetrahydridoborate s. under (C2H )gN,HCl Sodium tetrahydridoborate/boron fluoride Boranes from ethylene derivatives s. 16, 753... 

Garbodiimides Diisobutylaluminum hydride as reagent 16, 98 —, reductions with — 16, 98/9 Di(isopinocampheyl)borane as reagent 16,196 Diketals... 

Methylpropionitrile added all at once to a soln. of di(isopinocampheyl)borane prepared from borane and (+)-2-pinene in tetrahydrofuran, stirred 0.5 hr., coned, in vacuo to remove excess nitrile, redissolved in tetrahydrofuran, acetone cyanohydrin added, after 15 min. the crude intermediate a-aminonitrile isolated, and heated 10 hrs. at 75° in aq. WCl (R)(-)-valine. Y 45%. F. e. s. U.E. Diner et al., Tetrah. Let. 1972, 3145. 

In contrast to the first- and second-generation methods, the control is now intermolecular. This is obviously an attractive procedure but the range of reactions for which effective chiral reagents exist is somewhat limited at present. An example is provided by the hydroboration of 1-methylcyclohexene, using isopinocampheyl-borane (40) derived from (+)-a-pinene (31), to give alcohol (41) with two adjacent stereogenic centres. 

The hydroboration of cw-but-2-ene and c -hex-3-ene with di-isopinocampheyl-borane and monoisopinocampheylborane (both prepared from (+)-a-pinene) has been studied as has the hydroboration of a/ -unsaturated carbonyl corn-compounds by borane-dimethyl sulphide, ... 

Di(isopinocampheyl)borane Resolution of racemic ethylene derivatives with optically active horanes... 

Bis(isopinocampheyl)borane adopts a conformation which minimizes steric interactions. This conformation can be represented schematically as in F and G, where the S, M, and L substituents are, respectively, the 3—H, 4—CH2, and 2—CHCH3 groups of the carbocyclic structure. The steric environment at boron in this conformation is such that Z-alkenes encounter less steric encumberance in transition state G than in F. 

The degree of enantioselectivity of bis(isopinocampheyl)borane is not high for all simple alkenes. Z-Disubstituted alkenes give good enantioselectivity (75-90%), but -alkenes and simple cycloalkenes give low enantioselectivity (5-30%). 

Bis(isopinocampheyl)borane has been used in the enantioselective preparation of alkynes by the reaction sequence for alkylation of alkynes (see page 452). 

---