![]() Structure of (+)-Diisopinocampheylborane dimer | |
Names | |
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IUPAC name Di[(1S,2R,3S,5S)-pinan-3-yl]borane | |
Systematic IUPAC name Bis[(1S,2S,3S,5R)-2,6,6-trimethylbicyclo[3.1.1]heptan-3-yl]borane | |
Other names (+)-Di-3-pinanylborane; Diisopinocampheylborane; Ipc2BH dimer | |
Identifiers | |
3D model (JSmol) | |
Abbreviations | Ipc2BH |
ChemSpider | |
PubChem CID | |
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Properties | |
C20H35B | |
Molar mass | 286.31 g·mol−1 |
Appearance | Colorless solid |
Density | 1.044 g/cm3 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Diisopinocampheylborane is an organoborane that is useful for asymmetric synthesis. This colourless solid is the precursor to a range of related reagents. The compound was reported in 1961 by Zweifel and Brown in a pioneering demonstration of asymmetric synthesis using boranes. The reagent is mainly used for the synthesis of chiral secondary alcohols. The reagent is often depicted as a monomer but like most hydroboranes, it is dimeric with B-H-B bridges. [1]
Diisopinocampheylborane was originally prepared by hydroboration of excess α-pinene with borane, [2] but it is now more commonly generated from borane-methyl sulfide (BMS). [3]
The compound can be isolated as a solid, but because it is quite sensitive to water and air, it is often generated in situ and used as a solution. The synthesis is complicated by a number of factors, including the tendency of the compound to eliminate pinene. [1]
Diisopinocampheylborane is often represented as a monomer (including in this article), but X-ray crystallography establishes a dimeric structure. [1]
Oxidation of diisopinocampheylborane with basic hydrogen peroxide gives isopincampheol. Methanolysis gives methoxydiisopinocampheylborane
Because of the large size of the α-pinenyl substituents, diisopinocampheylborane only hydroborates unhindered alkenes. These reactions proceed with high enantioselectivity. 2-Butene, 2-pentene, 3-hexene are converted to the respective chiral alcohols in high ee's. [4] Norbornene under the same conditions gave an 83% ee. Heterocycles (dihydrofuran, dihydrothiophene, dihydropyrrole, tetrahydropyran) give the alcohols in ≥99% ee; the high ee's reflect their constrained conformations. [5]
It adds to alkynes to form the corresponding vinyldiisopinocampheylboranes
In a highly stereoselective reaction, allyldiisopinocampheylboranes converts aldehydes to the homologated alcohols, rapidly even at -100 °C. [6] The alkyldiisopinocampheylboranes, which result from the addition to alkenes, usefully react with a range of different reagents. Hydroxylamine-O-sulfonic acid provides 3-pinanamine. [7]
Also useful is the reaction of diisopinocampheylborane with an aldehyde (RCHO) to give the chiral boronic ester, (isopinocampheyl)2BOCH2(R), which can be further used is a number of reactions e.g. Suzuki reaction. [4]
Treatment of diisopinocampheylborane with TMEDA give the crystalline adduct of monoisopinocampheylborane. This adduct reacts with boron trifluoride to liberate the monoisopinocampheylborane (IpcBH2) in 100% ee. [8] Monoisopinocampheylborane reacts with a variety of alkenes. [4] Two other reagents have been developed for the hydroboration of ketones:
In the above mechanism where G=O and R is Ipc and Cl or 9-Borabicyclononane. Diisopinocampheylchloroborane (Ipc2BCl) is produced by treating diisopinocampheylborane with hydrogen chloride. The chloride is reported to be more stable that the trialkyl boranes, [4] it works well with aryl alkyl ketones and tert-butyl alkyl ketones. Diisopinocampheylchloroborane is often complementary with diisopinocampheylborane, where one provides the R enantiomer and the other the S, the enantioselectivity is typically very high. [9] [10]
Alpine-borane is produced by hydroborating α-pinene with 9-borabicyclononane. [4] Both of these reagents can be improved upon by using 2-ethylapopinene in place of α-pinene, 2-ethylapopinene has an ethyl group in place of the methyl in α-pinene. The additional steric bulk improves the stereoselectivity of the reduction.
Diisopinocampheylborane reacts with methanol to give diisopinocampheylmethoxyborane, which in turn reacts with an allyl or crotyl Grignard reagent to give B-allyldiisopinocampheylborane. This can then undergo an asymmetric allylboration to give a chiral homologated alcohol, which is a useful building block in a chiral synthesis.