Chiral organoboranes

One of the newer and more fmitful developments in this area is asymmetric hydroboration giving chiral organoboranes, which can be transformed into chiral carbon compounds of high optical purity. Other new directions focus on catalytic hydroboration, asymmetric aHylboration, cross-coupling reactions, and appHcations in biomedical research. This article gives an account of the most important aspects of the hydroboration reaction and transformations of its products. For more detail, monographs and reviews are available (1—13). 

There have been several reviews of asymmetric synthesis via chiral organoboranes (6,8,378,382,467—472). Asymmetric induction in the hydroboration reaction may result from the chiraHty present in the olefin (asymmetric substrate), in the reagent (asymmetric hydroboration), or in the catalyst (catalytic asymmetric hydroboration). 

Other examples of chiral organoboranes derived from terpenes are E, F, and G, which are derived from longifolene,198 2-carene,199 and limonene,200 respectively. 

Two additional syntheses of 12 employed organostannane and organobo-rane chemistry. Marshall s synthesis of 12 was based on chiral organostannane A as shown in Scheme 20 [30], while Oehlschlager s one was based on chiral organoborane A (Scheme 21) [31]. 

Asymmetric substrate, 13 664-665 Asymmetric synthesis, via chiral organoboranes, 13 664-671 ATI receptor, 5 158 Atacama Desert, sodium nitrate in, 22 844-845 Atacamite, 7 769... 

Sulfonylhydroxylamines and hydroxylamine O-sulfonic acid have found wide apph-cation in synthesis of amines from achiral or chiral organoboranes and boronate esters and the hydroboration-amination methodology is successfully used for direct amination of alkenes. 0-Sulfonyloximes were also found to be good reagents for synthesis of amines from organomagnesium, -copper and -zinc reagents. 

H. C. Brown, Enantiowerically Pure Compounds via Chiral Organoboranes in Modem Synthetic Methods 1986, R. Scheffold, Ed., p 315, Springer, Heidelberg 1986. 

Brown, H. C. and Ramachandran, P. V. Asymmetric Reduction with Chiral Organoboranes Based on a-Pinene. Acc. Chem. Res. 1992, 25, 16-24. 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

Distillation from lithium aluminum hydride serves to remove completely all traces of water and amine as is required for preparation of chiral organoboranes. 

Matteson, D.S. (1989). The use of chiral organoboranes in organic synthesis. Synthesis, 973-985. 

Ramachandran, P.V. and Srebnik, M. (1987). The utility of chiral organoboranes in the preparation of optically active compounds. Aldrichimica Acta, 20, 9-24. 

H. C. Brown and B. Singaram, Development of a Simple Procedure for Synthesis of Pure Enantiomers via Chiral Organoboranes, Acc. Chem. Res. 1988, 21, 287. 

Synthesis of pure enantiomers. Brown and Singaram1 have reviewed the chiral organoboranes obtained from (+)- and (- )-pinene and their use for synthesis of optically pure amines, halides, hydrocarbons, lithium alkylborohydrides, ketones, aldehydes, a-chiral acids, esters, nitriles, alkynes, and alkenes. 

Review "The Use of Chiral Organoboranes in Organic Synthesis" Matteso, D.S. Synthesis, (1986), 973... 

H. C. Brown and P. V. Ramachandra, Asymmetric Syntheses via Chiral Organoboranes Based on a-Pinenein Advances in Asymmetric Synthesis (A. Hassner, Ed.), 1995,1, JAI, Greenwich, CT. 

E. Block, Olefin synthesis via deoxygenation of vicinal diols, Org. React. (N.Y.) 1984, 30, 457. M. M. Midland, Asymmetric reduction with organoborane reagents, Chem. Rev. 1989,89, 1553. H. C. Brown and P. V. Ramachandran, Asymmetric reduction with chiral organoboranes based... 

P. V. Ramachandran and H. C. Brown, Asymmetric Syntheses of Fluoroorganic Compounds via Chiral Organoboranes, in Asymmetric Fluoroorganic Chemistry , ACS Symposium Series 746, eds. P. V. Ramachandran, and H. C. Brown, American Chemical Society, Washington, DC, 2000, p. 22. 

Asymmetric Hydroboration. Dilongifolylborane (Lgf2BH) is a chiral dialkylborane intermediate in steric requirement between the two widely investigated chiral organoboranes derived from a-pinene Monoisopinocampheylborane (IpcBH2) and Di-isopinocampheylborane (Ipc2BH). ... 

Brown, H. C., Singaram, B. The development of a simple general procedure for synthesis of pure enantiomers via chiral organoboranes. 

Chiralphosphines. These important ligands can be obtained from chiral organoboranes by consecutive reaction with EtjZn and various chlorophosphines. 

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