Borane-tetrahydrofuran, reaction with

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). 

The temperature has a significant effect on the selectivity of the reaction, with the optimal temperature being dependent on the borane source. The optimal range of temperature was 25 °C when borane-dimethylsulfide was used and 0-5 °C when borane-tetrahydrofuran was used as the reducing agent (Table 11.5). 

Treatment of cyanoethylcellulose with borane-dimethylsulfide or borane-tetrahydrofuran complexes in tetrahydrofuran has resulted in the quantitative conversion to 3-aminopropylceIlulose. Such aminopropylcellulose derivatives have also been employed as intermediates for acetamido or aryluredo products, and in grafting reactions [119]. 

The rapid reaction between carboxylic acids and borane is related to the electrophilicity of the latter. The carbonyl group of the initially formed acyloxyborane intermediate, which is essentially a mixed anhydride, is activated by the Lewis acidity of the trivalent boron atom. Addition of 1/3 equiv of the Borane-Tetrahydrofuran complex to acrylic acid in dichloromethane followed by addition of a diene at low temperature results in the formation of Diels-Alder adducts in good yield (eq 1). Further, the reaction is successful even with a catalytic amount of borane. 

As for dihydro-1,3-oxazines, perhydro-1,3-oxazine methiodides (125) are ring-opened by reaction with sodium borohydride. The products obtained depend upon the conditions used thus, in anhydrous tetrahydrofuran tertiary amines and their borane derivatives are formed, but in ethanol or methanol transesterification occurs to give the corresponding ethyl or methyl ethers (Scheme 32) <90H(31)2079>. 

The preparations of lithium and sodium (cyclooctane-1,5-diyl)dihydro-borates(l-) in tetrahydrofuran proceed via isolable, stable etherates. These can be made solvent-free simply by heating under vacuum. 9-Borabicyclo[3.3.1 ]-nonane dimer (9-BBN) can easily be prepared from cycloocta-1,5-diene2 by reaction with tetraethyldiborane(6), tetrahydrofuran-borane8,9 or dimethyl sulfide-borane.10 The synthesis of alkali metal (cyclooctane-1,5-diyl)dihydroborates is achieved by addition of 9-BBN to a suspension of the alkali metal hydride in tetrahydrofuran. Lithium hydride reacts more slowly than sodium or potassium hydride. The reactions are brought to completion by heating under reflux. 

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. ... 

Dithietanef Bis(chloromethyl) sulfoxide (1) reacts with aqueous sodium sulfide in the presence of 1 equiv. of this phase-transfer catalyst to form 1,3-thietane 1-oxide (2) in 36% yield. Bis(chloromethyl) sulfide does not undergo this reaction. However, the product (3) that would have been formed can be obtained by reduction of (2) with borane-tetrahydrofurane (5, 48). The microwave spectra... 

Eiiborane is a dimer of borane, BH3. The bonding in diborane is unusual because the hydrogen atoms bridge the two boron atoms with the two monomeric BH3 subunits being bound by two-electron, three-center bonds. Because the boron atom in borane possesses an empty p-orbital, borane is a Lewis acid, and it forms stable complexes upon reaction with tetrahydrofuran (THF) and other ethers, which function as Lewis bases, as illustrated by the formation of a borane-THF complex (Eq. 10.27). 

The course of this reaction depends (1) on the stereospecific reductive addition of diborane (B2H6, introduced as the borane. tetrahydrofuran complex (BH3 -THF)) to an alkene to form an intermediate trialkylborane and (2) on oxidation of the borane with alkaline hydrogen peroxide to yield the corresponding alcohol. 

Preparative Methods three methods of preparation have been reported (a) from the treatment of ethyl bromoacetate with zinc followed by the reaction with chlorotrimethylsUane and subsequent reduction of the resultant ethyl trimethylsUylacetate with lithium aluminum hydride or borane-tetrahydrofuran (eq 1) (b) from the hydroboration/oxidation or oxymercu-ration/ demercuration of vinyltrimethylsUane (eq 2) and (c) most conveniently, by the reaction of the Grignard reagent formed from (chloromethyl)trimethylsilane with paraformaldehyde (eq 3). ... 

A soln. of borane in tetrahydrofuran added dropwise at 0° to cyclohexene and dry tetrahydrofuran, warmed 3 hrs. at 50°, cooled to -78°, stirred and treated with oxygen until Og-absorption ceases after 2 moles have been consumed, then aq. 30%-HgOg added dropwise, and stirred 0.5 hr. at 0° cyclohexyl hydroperoxide. Y 80-95%. - The low-temp, autoxidation of organoboranes provides a rapid and convenient synthesis of alkyl hydroperoxides. F. e. s. H. C. Brown and M. M. Midland, Am. Soc. 93, 4078 (1971) synthetic applications of organoboranes, review, s. Chem. Britain 7, 458 (1971) radical reactions with boranes, review, s. Ang. Ch. 84, 702 (1972). 

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