Borane preparation

An indirect method ° of double-bond reduction involves hydrolysis of boranes (prepared by 15-16). Trialkylboranes can be hydrolyzed by refluxing with carboxylic acids,while monoalkylboranes RBH2 can be hydrolyzed with base. ° Triple bonds can be similarly reduced, to cis alkenes. °... 

Sodium borohydride does not reduce the free carboxylic group, but borane prepared from sodium borohydride and boron trifluoride etherate in tetrahydrofuran converts aliphatic acids to alcohols at 0-25° in 89-100% yields... 

Borane prepared by adding aluminum chloride to a solution of sodium borohydride in diethylene glycol dimethyl ether (diglyme) reduced aliphatic and aromatic esters to alcohols in quantitative yields in 3 hours at 25° using a 100% excess, or in 1 hour at 75° using a 25% excess of lithium borohydride over 2 mol of the hydride per mol of the ester [738] Procedure 20, p. 209). 

An indirect method286 of double-bond reduction involves hydrolysis of boranes (prepared... 

Fig. 10.24. Asymmetric carbonyl group reduction with Alpine-Borane (preparation Figure 3.27 for the "parachute-like" notation of the 9-BBN part of this reagent see Figure 3.21). The hydrogen atom that is in the cis-position to the boron atom (which applies to both ft- and /T-H) and that after removal of the reducing agent leaves behind a tri- instead of a disubstituted C=C double bond (which applies to ft-, but not / -H) is transferred as a hydride equivalent. In regard to the reduction product depicted in the top row, the designation S of the configuration relates to the aryl-substituted and R to the Rtert-substituted propargylic alcohol.

Fig. 8.19. Asymmetric carbonyl group reduction with Alpine-Borane (preparation Figure 3.21).

Similarly, bis(benzoyloxy)borane prepared easily from benzoic acid and diborane (Eq. 137) reduces the tosylhydrazones in excellent yield, e.g., the tosylhydrazone of 6-oxo-15-hexadecenoic acid affords 15-hexadecenoic acid (Eq. 138) 225). 

Commercially available solutions of H3B-THF in THF (Aldrich-Boranes, Inc., 2371 N. 30th St., Milwaukee, WI 53210) which are used in this synthesis, contain approximately 5% of sodium tetrahydroboratefl —) as a stabilizer. Tri(sec-butyl)borane prepared from these solutions should be distilled. An alternative commercial (Aldrich-Boranes, Inc.) hydroborating agent free of sodium tetrahydroborate(l —) is borane-methyl sulfide. 

Alpine-Borane,prepared by hydroborationofa-pinenewith9-borabicyclo[3.3. Ijnonane (9-BBN), reduces aldehydes, a-keto esters and acetylenic ketones with excellent enantioselectivity. The reduction proceeds via a cyclic process similar to the MPV reaction. 

Trialkylborohydrides prepared from optically active alkenes may be used for asymmetric-reduction. For example, reaction of fcrt-butyllithium with Alpine-borane, prepared by hy-droboration of ( + )- or ( —)-a-pinene with 9-borabicyclo[3.3.1]nonane (see Section D.2.3.5.1.), provides Alpine-hydride. 1 his reagent reduces 1-phenylethanone in 17% ee72. A more effective reagent is NB-Enantride, prepared in a similar manner by treatment of the product from hydroboration of the benzyl ether of 2-(2-hydroxyethyl)-6,6-diinethylbicyclo[3.3.1]hept-2-ene (nopol. see Section D.2.3.5.1.) with toV-butyllithium73. 

Because diborane (B2Hg), the source of borane, is a flammable, toxic, explosive gas, a solution of borane—prepared by dissolving diborane in an ether such as THF— is a more convenient and less dangerous reagent. One of the ether oxygen s lone pairs satisfies boron s requirement for an additional two electrons The ether is a Lewis base... 

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). 

Table 2.6 Mono- and diphosphine boranes prepared by the dynamic resolution of rac-21 (Scheme 2.6).

Scheme 2.40 Phospholane boranes prepared from cyclic sulfates 114.

Scheme 2.44 Dihydrobenzazaphosphole boranes prepared from enantiomerically pure amines.

Table 4.6 P-stereogenic AMPP boranes prepared to date.

Alkylchloro(thexyl)boranes prepared from terminal alkenes can be hydrolyzed with one equivalent of water and then oxidized with pyridinium chlorochromate (PCC) to the corresponding aldehyde in moderate yields (eq 6). ... 

The following model (Fig. 26.1) explains the delivery of deuterium for chiral reduction of aldehydes from Alpine-Borane prepared from (+)-a-pinene. 

The stereochemical outcome of the reduction is predicted by the following model [14] for Alpine-Borane prepared from (-i-)-a-pinene. The hydride transfer occurs from the boat-like transition state in which the acetylene occupies the axial position (Eig. 26.3). 

The reagent Eapine-Borane, prepared (Eq. 26.15) from (-)-2-ethylapopinene by hydroboration with 9-BBN have been studied for its reaction with representative ketones and is found to react at faster rate than NB-Enantrane, but at slightly slower rate than Alpine-Borane (Table 26.15) [1]. 

It is found that incorporation of an oxygen into the chiral ligand provides a fixed coordination site for lithium and hence to a more rigid and thus more ste-rically demanding transition state. The NB-Enantride reduces acetophenone to (S)-phenylethanol (Eq. 26.22), whereas the Alpine-Borane prepared from (-)-a-pinene gives the (7 )-enantiomer [4]. 

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. 

R)-Propargylic alcohols have also been produced, this time in 73—100% e.e., by asymmetric reduction of a,/3-acetylenic ketones with the chiral borane prepared from (+)-a-pinene and 9-borabicyclo[3.3.1]nonane (9-BBN) (Scheme 10), a system already known to reduce aldehydes to chiral alcohols cf. 2,115). These results compare favourably with those from the LiAlH4- Darvon complex above, and the availability of both (+)-and (-)-o -pinene means that either (R)-or (5)-propargylic alcohols may be produced. 

C6H5-C = CHC4Hg-n from a-diazoketones. Ethereal methyllithium added dropwise at 0 to a soln. of di-n-butyl-(l-phenyl-l-hexenyloxy)borane prepared from diazoacetophenone and tri-n-butylborane in tetrahydrofuran, the ice bath removed, stirred 1 hr. at room temp., methyl iodide added, and stirring continued 1.5 hrs. 2-hexyl phenyl ketone. Y 69%. Via vinyloxyboranes, a,a- and ,/5-dialkylated ketones can be obtained in good yield from a-diazoketones and methyl vinyl ketone respectively. F. e. s. D. J. Pasto and P. W. Wojtkowski, J. Org. Chem. 36, 1790 (1971). 

Parmee ER, Tempkin O, Masamune S, Abiko A. New catalysts for the asymmetric aldol reaction chiral boranes prepared from a, a-disubstituted glycine arenesulfonamides. J.Am. Chem. Soc. 1991 113 9365-9366. 

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