Tetrahydrofurane-borane complex

This 1 1 complex, a white crystalline solid of considerable stability, is prepared either by heterogeneous absorption of diborane by ethylenediamine in a high vacuum apparatus or by reaction of ethylenediamine with the tetrahydrofurane-borane complex ... 

An efficient pathway towards 3-fluoroindoles was proposed starting from isatines 35. Fluorinated 2-indoIinones 36 were obtained in high yields by treatment of isatine derivatives 35 with diethylaminosulfur trifluoride in dichloromethane. In case of electron-donating substituents reduction of 36 with tetrahydrofuran-borane complex led smoothly to 3-fluoroindoles 37 in high yields. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Borane complexes are the most widely used commercial boron compounds, after sodium borobydride. Examples used in organic synthesis are amine borane complexes and borane complexes of tetrahydrofuran and dimethyl sulfide. 

High stereoselectivities (94-100 %) are attained in the reduction of aromatic ketones by use of a new chiral borane complex with (S)-2-amino-3-methyl-l,l-diphenylbutan-l-ol,(S-68) readily prepared in two steps from (S)-valine, in an experimentally convenient procedure961. (S)-Valine methyl ester hydrochloride was converted with excess of phenylmagnesium bromide into (S-68). The same treatment of (R)-valine gave (R-68). In a typical asymmetric reduction the reagent, prepared from (S-68) and borane, and the ketone (69) in tetrahydrofuran were kept at 30 °C for some hours. The corresponding alcohols were obtained in high optical purity. (S-68) could be recovered to more than 80% without racemization 96). 

Enantioselective Ketone Reduction. Following Itsuno s lead for enantioselective reductions using diphenylvalinol, Kraatz was the first to describe the use of a 1 2 mixture of (5)-diphenylprolinol (1) and Borane-Tetrahydrofuran for the stoichiometric enantioselective reduction of ketone (2) to obtain the plant growth regulator triapenthenol (3) (eq 1). Although not characterized at the time, the species responsible for the enantiose-lectivity observed was presumed to be an oxazaborolidine-borane complex. ... 

If the chiral amino alcohol is incorporated into polystyrene, a chiral polymeric reducing agent is obtained. Acetophenone O-methyloxime was reduced by such a polymeric borane complex to give optically active 1-phenylethylamine with 99% ee35 36 With borane-tetrahydrofuran and 1 mol% of the chiral amino alcohol (lS,3S,5S)-(a,a-diphenyl)hydroxymethyl-2-azabicy-clo[3.3.0]octane (7), (7 )-1-phenylethylamine can be synthesized from acetophenone O-methyloxime in 64% yield with 17% ee37. 

Conanine (la) readily yielded the diastereoisomeric borane complexes (2a) on reaction with borane in tetrahydrofuran. The starting material was re-formed when a solution of (2a) in ethanol was refluxed. The utility of borane as a protecting group for a tertiary amine function was demonstrated by the preparation of dihydroconessimine (4) from dihydroconessine (lb). The intermediate mono-borane complex (3) could be prepared by selective boronation of dihydroconessine, but was obtained in higher yield by selective deprotection of the bis-borane complex (2b), as depicted in Scheme 1. Dihydroconessine iV-oxide also was prepared from the borane complex (3) by oxidation with a peracid followed by deprotection as before. ... 

However, the direct and convenient synthesis of (9-BBN)2 has been reported by Knights and Brown [2], and this development opened the door for its application in hydroboration [2-5]. The synthesis involves the cyclic hydroboration of 1,5-cyclooctadiene with a borane-tetrahydrofuran (THF) complex [2, 3] in a 1 1 ratio, followed by refluxing the mixture at 65 °C, thus producing a solution containing (9-BBN)2 in ca. 90% yield. 

A more convenient hydroboratmg agent is the borane-tetrahydrofuran complex (H3B THF) It IS very reactive adding to alkenes within minutes at 0°C and is used m tetrahydrofuran as the solvent... 

Examine the electrostatic potential map of H3B THE (borane-tetrahydrofuran complex) on Learning By Modeling How does the electrostatic potential of the hydrogens bonded to boron dif fer from the potential of the hydrogens of the tetrahydrofuran ring" ... 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Borane is very reactive because the boron atom has only six electrons in its valence shell. In tetrahydrofuran solution, BH3 accepts an electron pair from a solvent molecule in a Lewis acid-base reaction to complete its octet and form a stable BH3-THF complex. 

Scheme 4 outlines the synthesis of key intermediate 7 in its correct absolute stereochemical form from readily available (S)-(-)-malic acid (15). Simultaneous protection of the contiguous carboxyl and secondary hydroxyl groups in the form of an acetonide proceeds smoothly with 2,2 -dimethoxypropane and para-toluene-sulfonic acid and provides intermediate 26 as a crystalline solid in 75-85 % yield. Chemoselective reduction of the terminal carboxyl group in 26 with borane-tetrahydrofuran complex (B H3 THF) affords a primary hydroxyl group that attacks the proximal carbonyl group, upon acidification, to give a hydroxybutyrolactone. Treat-... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Borane dimethyl sulfide complex 2 M solution in tetrahydrofuran, 0.5 mL, 1 mmol, 1 eq... 

M Borane-dimethylsulfide complex (2.0 mL) was added to the resulting solution. The mixture was cooled to 0-5 °C with an ice-bath, and then a solution of 2,3-butadione monoxime trityl ether (1.72 g) in dry tetrahydrofuran (5 mL) was added dropwise via a syringe pump over 1 hour at that temperature. 

In a 250 mL round-bottomed flask with an argon inlet equipped with a magnetic stirring bar the CBS-catalyst (1.85 g) was dissolved in tetrahydrofuran (10 mL) and cooled to 0°C in an ice bath. From a syringe filled with borane dimethyl sulfide-complex (2.00 mL dissolved in 10 mL THF) 20% of the volume (2.40 mL) were added and the solution was stirred for 5 minutes. A solution of the diketone (3.00 g dissolved in 30 mL THF) was added from a second syringe simultaneously with the rest of the borane dimethyl sulfide-complex over 2 hours. The resulting yellow solution was stirred for another... 

Borane-tetrahydrofuran complex Furan, tetrahydro, compd. with borane (1 1) (8,9) (14044-65-6)... 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Opening of a bottle where some particles of lithium aluminum hydride were squeezed between the neck and the stopper caused a fire [68]. Lithium aluminum hydride must not be crushed in a porcelain mortar with a pestle. Fire and even explosion may result from contact of lithium aluminum hydride with small amounts of water or moisture. Sodium bis(2-methoxy-ethoxy)aluminum hydride (Vitride, Red-Al ) delivered in benzene or toluene solutions also may ignite in contact with water. Borane (diborane) ignites in contact with air and is therefore kept in solutions in tetrahydrofuran or in complexes with amines and sulfides. Powdered lithium borohydride may ignite in moist air. Sodium borohydride and sodium cyanoborohydride, on the other hand, are considered safe. ... 

Other reagents used for reduction are boranes and complex borohydrides. Lithium borohydride whose reducing power lies between that of lithium aluminum hydride and that of sodium borohydride reacts with esters sluggishly and requires refluxing for several hours in ether or tetrahydrofuran (in which it is more soluble) [750]. The reduction of esters with lithium borohydride is strongly catalyzed by boranes such as B-methoxy-9-bora-bicyclo[3.3.1]nonane and some other complex lithium borohydrides such as lithium triethylborohydride and lithium 9-borabicyclo[3.3.1]nonane. Addition of 10mol% of such hydrides shortens the time necessary for complete reduction of esters in ether or tetrahydrofuran from 8 hours to 0.5-1 hour [1060],... 

High yields of amines have also been obtained by reduction of amides with an excess of magnesium aluminum hydride (yield 100%) [577], with lithium trimethoxyaluminohydride at 25° (yield 83%) [94] with sodium bis(2-methoxy-ethoxy)aluminum hydride at 80° (yield 84.5%) [544], with alane in tetra-hydrofuran at 0-25° (isolated yields 46-93%) [994, 1117], with sodium boro-hydride and triethoxyoxonium fluoroborates at room temperature (yields 81-94%) [1121], with sodium borohydride in the presence of acetic or trifluoroacetic acid on refluxing (yields 20-92.5%) [1118], with borane in tetrahydrofuran on refluxing (isolated yields 79-84%) [1119], with borane-dimethyl sulflde complex (5 mol) in tetrahydrofuran on refluxing (isolated yields 37-89%) [1064], and by electrolysis in dilute sulfuric acid at 5° using a lead cathode (yields 63-76%) [1120]. 

Reduction of 5,5-dimethyl-2-pyrrolidone with 3 mol of lithium aluminum hydride by refluxing for 8 hours in tetrahydrofuran gave 2,2-dimethylpyrrol-idine in 67-79% yields [1123]. Reduction of e-caprolactam was accomplished by heating with sodium bis(2-methoxyethoxy)aluminum hydride [544], by successive treatment with triethyloxonium fiuoroborate and sodium borohydride [1121], and by refluxing with borane-d ras. )a.y sulfide complex [1064]. 

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