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Production of Optically Pure Organoboranes

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Production of Optically Pure Organoboranes Europaisches Patentamt European Patent Office © Publication number: 0188 235 B1 Office europeen des brevets © EUROPEAN PATENT SPECIFICATION © Date of publication of patent specification: 11.12.91 © Int. CI.5: C07F 5/02, C07C 29/50 © Application number: 86100239.2 © Date of filing: 10.01.86 ® Production of optically pure organoboranes. © Priority: 16.01.85 US 692046 substituted tertiary olefins. Synthesis of al- cohols approaching 100% enantiomeric ex- © Date of publication of application: cess" 23.07.86 Bulletin 86/30 J. AM. CHEM. SOC, vol. 107, 1985, pages © Publication of the grant of the patent: 2564-2565, American Chemical Society, 11.12.91 Bulletin 91/50 Washington, DC, US; H.C. BROWN et al.: "Asymmetric synthesis of the dia- © Designated Contracting States: stereomeric 1-(2-cyclohexenyl)-1-alkanols in AT BE CH DE FR GB IT LI LU NL SE high optical purity via a stereochemicallly stable ailylic borane, B- © References cited: 2-cyclohexen-1-yldiisopinocampheylborane" J. AM. CHEM. SOC, vol. 106, no. 6, 1984, @ Proprietor: Aldrich Chemical Company, Inc. pages 1797-1800, American Chemical Soci- 940 West St. Paul Avenue ety, Washington, DC, US; H.C. BROWN et al.: Milwaukee, Wisconsin 53233(US) "Chiral synthesis via organoboranes. 1.1 A simple procedure to achieve products of @ Inventor: Brown, Herbert C, Ph.D. essentially 100% optical purity in hydrobora- 1840 Garden Street tion of alkenes with monoisopinocampheyl- West Lafayette Indiana 47906(US) borane. Synthesis of boronic esters and de- rived products of very high enantiomeric purities" © Representative: Modiano, Guido et al MODIANO, JOSIF, PISANTY & STAUB Baader- J. ORG. CHEM., vol. 45, no. 17, 1980, pages strasse 3 in Chemical W-8000 Munchen CO 3543-3544, American Society, 5(DE) CM Washington, DC, US; A.K. MANDAL et al.: "Monoisopinocampheylborane: an excellent GO chiral for OP hydroborating agent phenyl- O Note: Within nine months from the publication of the mention of the grant of the European patent, any person ft may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition in shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid (Art. 99(1 ) European patent convention). Rank Xerox (UK) Business Services EP 0 188 235 B1 Description Background of the Invention 5 (I) held of invention The present invention relates to an improved method for converting organoboranes containing a partially optically active organyl group attached to boron, and more specifically relates to methods for obtaining organoboranes containing an essentially optically pure organyl group. w Asymmetric hydroboration of aikenes, with diisopinocampheylborane or in many cases also with monoisopinocampheylborane typically provides the corresponding chiral organoborane containing the new alkyl group, FT, in from 50 to 90% enantiomeric excess (ee), and occasionally in purities of from 90 to 100% ee. Because of the importance of hydroboration to, for example, the pharmaceutical industry, there has been a long standing need for a simple, reliable process which provides the alkyl group, Ff, in 100% ee 75 in all cases. The present invention fulfills that need in cases involving cis-alkenes and the hydroborating agent diisopinylcampheylborane, whether the organoborane formed is liquid or solid, and furthermore in cases involving trans-alkenes or tertiary-alkenes and the hydroborating agent monoisopinocampheylborane whenever the organoborane formed is liquid rather than a solid. 20 (II) Description of the Prior Art The first successful asymmetric synthesis occured in 1961 with the hydroboration of cis-2-butene in diglyme (DG) by the chiral dialkylborane, diisopinocampheylborane to yield 2-butanol of 87% ee. Brown, H.C. et al., J. Am. Chem. Soc. 86, 397 (1964). Diisopinocampheylborane (Ipc BH) was prepared by the 25 hydroboration of ( + )- and (-)-a-pinene. See Brown, H.C. and Zweifel, G., J. Am. Chem. Soc, 83. 486 (1961). (FIG. 1). That landmark achievement not only provided a remarkably high asymmetric synthesis, the first of its kind, but further provided a reagent which appeared generally applicable to the asymmetric hydroboration of cis-alkenes. Brown, H.C. et al, JACS 86, 397 (1964) and Partridge, J.J., et al, JACS 95, 532 (1973). (FIG. 2) ~ — 30 The chiral intermediate, 2-butyldiisopinocampheylborane, was subsequently converted into optically active 2-aminobutane with complete retention of configuration and into 2-iodobutane with complete inversion of configuration. Verbit, L. et al, JOC 32, 3199 (1967) and Brown, H.C. et al, JASC 98, 1290 (1976). (FIG. 3). In the original study of hydroborations with optically active diisopinocampheylborane, the reagent employed was prepared from commercial a-pinene of relatively low enantiomeric purity ( ~ 93%). 35 Subsequent advances resulted in the preparation of reagent of high enantiomeric purity from such a-pinene. The reagent is equilibrated at 0° C with 15% excess a-pinene. The major isomer becomes incorporated into the crystalline reagent, leaving the minor isomer in solution. Brown, H.C. et al, Israel J. Chem. 15, 12 (1977). (FIG. 4). Treatment of the diisopinocampheylborane with benzaldehyde liberated a-pinene of approximately 100 % ee. Thus, two reactions were developed which provided a convenient procedure for 40 upgrading the commercial a-pinene to an enantiomeric purity of essentially 100% ee. Brown, H.C. et al, JOC 47, 4583 (1982). (FIG. 5). Improved asymmetric results were achieved in the hydroboration of cis-alkenes with this improved reagent and a somewhat lower hydroboration temperature (-25° C). Brown, HTC. et al, JOC 47, 5065 (1982). (FIG. 6). — 45 It has been found that while diisopinocampheylborane [(lpc2)BH] handles cis-alkenes very effectively, it is not an effective asymmetric hydroborating agent for trans-alkenes and trisubstituted aikenes and that monoisopinocampheylborane (lpcBH2 )is a more effective hydroborating agent for the latter types of aikenes. It is difficult to halt the hydroboration of a -pinene at the monoalkylborane stage. Consequently, it was so found that the monoisopinocampheylborane must be prepared by an indirect route, such as by treating diisopinocampheylborane with one-half molar equivalent of N,N,N,N-tetramethylethylenediamine (TMED) to obtain 2(lpcBH2)*TMED. The diastereomeric adduct crystalfizeYout in enantiomerically pure form, and the pure monoisopinocampheylborane is readily liberated by treating the adduct with boron trifluoride etherate according to the method of Brown, H.C. et al, JOC 43, 4395 (1978). 55 Monoisopinocampheylborane has been found tcTbe very effective for the asymmetric hydroboration of trans -aikenes (See Brown, H.C. et al, JOC 46, 5047 (1981)(FIG. 10). Similarly, the hydroboration of trisubstituted aikenes with monoisopinocampheylborane, followed by oxidation of the intermediate or- ganoboranes, provides the corresponding alcohols in 53-72% ee, Brown, H.C. et al, JACS 99, 5514 (1977); EP 0 188 235 B1 Brown, H.C. et al., JOC 47, 5074 (1982) (FIG. 10). For reasons that are not understood, the asymmetric hydroboration of the phenyl derivatives provides considerably improved hydroboration products of, for example 82% ee, 85% ee, 100% ee and 88 % ee- (FIG. 11) as compared to the 53% ee, 62% ee., 66% ee and 72% ee respectively for the corresponding 5 parent compounds (FIG. 10), see Mandal, A.K. et al, Joe 45, 3543 (1980). Monoisopinocampheylborane and diisopinocampheylborane are complementary to each other, and are capable of handling three of the four major classes of aikenes. Diisopinocampheylborane, a reagent with larger steric requirements, is more suited for use in the case of unhindered cis-olefins. Initially, the application of chiral organoboranes was limited primarily to alcohols because of the 10 presence of isopinocampheyl groups on boron in the product. Recently, it was discovered that these groups can be selectively eliminated by treatment of the mixed chiral organoboranes with acetaldehyde, regenerat- ing the a-pinene, and providing the optically active boronate as the product In this way, for example, 2- butyldiisopinocampheylborane is readily converted into diethyl 2-butylboranate in 97% ee. Brown, H.C. et al, JACS, 104, 4303 (1982). (FIG. 12) 15 Similarly, diethyl trans-2-phenylcyclopentylboronate can be obtained in 100% ee by the method of Brown, H.C. et al, JACS104, 4303(1982). (FIG. 13). Hydroboration of prochiral aikenes with diisopinocampheylborane and monoisopinocampheylborane typically results in products in which approximately 60 to 90% optical activity is generally induced in the R*Blpc2 and R*BHIpc This means that the major isomer is present in large amounts, 80-95%, and the minor 20 isomer in much smaller amounts, 20-50%. The problem is how to separate the major isomer from the small amount of minor isomer present. The usual organic synthesis of enantiomers produces a 50:50 mixture of the two optical isomers. Normally, these are separated by combining them with a naturally occurring optically active acid or base (chiral auxiliary) to form a pair of diastereoisomers. Laborious fractional crystallization then separates the 25 two diastereoisomers. The chiral auxiliary is then removed to regenerate the desired optically active compound. However, it is not always possible to resolve optical mixtures once the final products have been obtained without destroying the molecule, and there has been a longstanding need for a reliable method of resolving the
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