Borane.THF

Several oxygen- and sulfur-substituted boranes have been reported (125—130). 1,3,2-Benzodioxaborole [274-07-7] (catecholborane, CB) (15) is the one best studied. It is commercially available or can be prepared by the reaction of catechol with borane-THF (57,131), or by other procedures (132). The product is a Hquid existing as a monomer, remarkably stable to disproportionation. No... 

The same phosphine-borane used for the synthesis of BisP acted as the starting materials of the construction of MiniPHOS, the next smaller analogue to BisP (Scheme 13). The chirally induced lithium salt was treated with alkylphos-phorus dichloride, methylmagnesium bromide, and borane-THF complex to afford enantiomerically pure MiniPHOS-borane 82a. Recrystallization enabled elimination of a small amount of corresponding raeso-diastereomer formed [29]. Yields were generally low, ranging from 13 to 28%. 

First, oligo(ethylene oxide) monomethylether was treated with excess BH3-THF complex in THF. After solvent and borane-THF were removed under reduced pressure, the resulting hydroborane with an oligo(ethylene oxide) tail was polymerized with triethyleneglycol in THF at room temperature (r.t.). The polymers obtained were purified by reprecipitation into //-hexane or by washing with diethylether to give colorless or translucent gums in 61—76% yield. 

After stirring at room temperature for 1 hour, methylmagnesium chloride (1.0 M THF solution, 112mL) was added at 0°C over 30 minutes. The reaction mixture was stirred at room temperature for 1 hour. To this solution, borane-THF complex (1.0 M, 70 mL) was added at 0 °C, and the mixture was stirred at the same temperature for 1 hour. 

Borane-THF complex was obtained from Toso-Akzo Chemical Company, Ltd. in Japan and should be titrated before use. Vigorous evolution of hydrogen is observed during addition of borane-THF solution to the reaction mixture. 

Readily available a,/3-unsaturated nitro compounds such as 77 undergo facile reduction into aUtylhydroxylamines of type 78 (equation 55) with borane/THF complex in the presence of catalytic amounts of sodium borohydride 74 ... 

The CBS-catalyst [(S)-2-methyl-CBS-oxazaborolidine] (S)-5 (1.0 M in toluene) (The CBS catalyst is named after Corey, Bakshi, and Shibata) and the borane-THF complex (1.0 M in THF) were obtained from Aldrich Chemical Co., Inc. and used as received. THF was distilled from potassium directly before use. 

Methylcryptaustoline iodide (14) was synthesized from phenylacetic acid 47 by Elliott (39) as shown in Scheme 7. Nitration of 47 to the 6-nitro compound 48 and reduction with sodium borohydride afforded lactone 49. Reduction of the aromatic nitro group with iron powder in acetic acid gave ami-nolactone 50, which was converted to tetracyclic lactam 51 with trifluoroacetic acid in dichloromethane. Reduction of the lactam by a borane-THF complex followed by treatment with methyl iodide afforded ( )-0-methylcryptaustoline iodide (14). 

The key sequence in a somewhat involved stereospecihc total synthesis of a carbacephem starts by preparation of a chiral auxiliary. It is interesting to note that nitrogen is the only atom from this molecule retained in the hnal product. Constmction of this moiety starts with the formation of the carbethoxy derivative (37-2) from L(- -)-phenylglycine (37-1). Selective reduction of the free carboxyl group with borane. THF leads to the hydroxycarbamate (37-3). In a one-pot sequence, this is first cyclized to the corresponding oxazolidinone (37-4) by means of sodium hydride and then alkylated with ethyl bromoacetate (37-5). Saponification of the side chain then affords the chiral acetic acid (37-6). The carboxyl group is then activated by conversion to its acid chloride (37-7). 

A solution of 4 (110 mg, 0.26 mmol) in a mixture of dichloromethane (25 mL) and methanol (8 mL), cooled to —78°C, was ozonized until the solution turned blue (about 4 min). Excess ozone was removed by a stream of nitrogen, the solution was allowed to attain room temperature and was concentrated to dryness. The crude product was dissolved in THF (10 mL) and a borane-THF complex (1.04 mL of a -M solution) was added at room temperature under nitrogen. After 24 h the reaction mixture was treated with diluted hydrochloric acid (5 mL) and the solution was extracted with ethyl acetate (4x15 mL). The combined extracts were dried (MgS04) and concentrated to dryness. The residue was dissolved in dichloromethane (20 mL) and acetylated with acetic anhydride (62 p,L, 0.66 mmol), triethylamine (200 p,L, 1.50 mmol), and a crystal of DMAP, under nitrogen atmosphere. After 12 h the reaction mixture was concentrated under diminished pressure, and the crude product was purified on a silica gel column with hexane-ethyl acetate (3 1) to give 5 (73 mg, 65%) mp 103°-105°C (after crystallization from hexane), [a]D — 26.6° (c 0.7, CHC13). 

Some chiral oxazaphospholididine-borane catalysts can be used for enantioselective reduction of prochiral ketones by borane-THF or bor-ane-dimethyl sulfide complex (Scheme 19) (44). 

Treatment of [4.S ]-4-/-butyl-l-mcthylcyclohcxcnc with borane-THF followed by oxidation with F Ch-NaOH gives a mixture of [ 1.S , 2.S ,5.S ]-5-/-butyl-2-methylcyclohcxan-1 -ol and [ 1 A,2A,5.S -5-/-butyl-2-methylcyclohexan-l-ol and no other diastereomers. There are two steps in this process—addition to give an organoborane and oxidation which cleaves the carbon-boron bond to an alcohol. 

Reductive cleavage of dioxepanes with borane-THF complex (THF = tetrahydrofuran) leads to 1,4-diols. This procedure has found application in the synthesis of discrete polyethers (Scheme 27) <2003JOC9166>. 

In an alternative route,2 Boc-protected amino acids are reduced to the protected amino alcohol with borane-THF (45-95% yield) and pyridinium dichromate is used for oxidation to the aldehyde (75-90% yield). The optical rotations of the aldehydes obtained by these two procedures differ considerably, presumably owing to racemization encountered in the PDC oxidation. 

Aryl alkyl ketoxime ethers, Ar-C(R1)=N-OR2, have been reduced with borane-THF at ca 0 °C to give amines, Ar- CH(R )-NH2 88 A chiral BINAP with an O3BN (ee) framework gives up to 98% ee. 

Enantioselective reduction of oxime ethers promoted by chiral spiroborate esters (10) with an O3BN framework is reported. In the presence of (R,S)-10, aralkyloxime ethers are reduced by borane-THF at give (S)-l-aralkylamine in high yield and excellent enatiomeric excess (up to 98% ee). A possible mechanism (Scheme 13) of the catalytic reduction is suggested.310... 

If 1-trimethylsiloxy-l-cyclohexene (193) is treated with borane/THF, hydrogen peroxide/alkali and then hydrolyzed, trans-l,2-cyclohexanediol (203) is obtained137 but trans-1 -hydroxy-2-trimethylsiloxycyclohexane (202) can be isolated without subsequent acid-catalyzed hydrolysis138 whereas the direct hydrolysis of the borane adduct 204 leads directly to cyclohexene (205)139K Very interesting is the use of TiCLt as catalyst. 193 plus benzaldehyde and TCI4 gives after hydrolysis 2-[hydroxy-(phenylmethyl)] cyclohexane-1-one (206)l40 ... 

---