Borane.THF complex

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%. 

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). 

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). 

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>. 

Reaction of borane-THF complex with mono(2,6-diisopropyl)benzoyltartaric acid in dry propionitrile at 0 °C affords the catalyst solution. Condensation of achiral aldehydes with allylsilanes is promoted by this catalyst, 2, (20 mol %) at -78 °C to produce homoallylic alcohols with enantio- and diastereoselectivity (Eq. 66). 

Oxazolidines (53) are readily formed from aldehydes or ketones and ethanolamines they can be hydrolyzed with ease and show reactions that might be expected of the imino alcohol intermediate (54). Among these are the addition of Grignard reagentsand catalytic hydrogenolysis of the C—O Ixjnd (equation 28).This reaction is exothermic over Adam s catalyst in methanol but slower in acetic acid. Nickel and copper chromite are also effective but at higher temperatures and pressures,as is the case with palladium. The same cleavage occurs with LAH (unassisted)and with the borane-THF complex. ... 

Reduction of the free carboxylic acid in an otherwise fully protected uronic acid can be accomplished with borane in THF [ 169]. For example, the borane-THF complex reduces diisopropy-... 

Differently from the secondary enamino sulphoxides, tertiary enamino sulphoxides cannot be r uced by L-selectride, but they are reduced by the borane-THF complex (Scheme 118). 

The 4-0X0 group of 2-benzyl-1,4-dioxoperhydro derivatives of (11) was regioselectively reduced when the 1-oxo group was alkylated with Mc30BF4, the l-methoxy-4-oxo-3,6,7,8,9,9a-hexahydro-47f derivative was treated with the borane-THF complex, and finally the 1-oxo group was regenerated <88S963>. 

This alcohol-forming hydrohoration reaction figures prominently in many natural product syntheses. It is facile and highly regioselective, and sometimes diastereoselective.94 in a synthesis of a usneoidol E analog, Dfez et al. reacted 117 with a borane-THF complex, which led to incorporation of the OH group on the less... 

The reduction of P-enaminosulfoxides (25) with borane-THF complex was also investigated. These reactions clearly involve direct hydroboration of the carbon-carbon double bond and are only moderately diastereoselective (Table 4.5). The potential application of this methodology to the synthesis of non-racemic amines was demonstrated (Scheme 4.17) by Raney nickel desulfinylation of P-aminosulfoxide (28) to furnish (29) in 86% yield. 

Table 4.5 Reduction of p-enaminosulfoxides (25) using borane-THF complex...

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

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