Diborane complex with ethers

Boron trifluoride is used as a Lewis acid catalyst in a wide variety of reactions. Like diborane, BF3 is a toxic gas, but BF forms a stable complex with ethers, allowing it to be conveniently stored and measured. The complex of BF3 with diethyl ether is called boron trifluoride etherate. ... 

Hydroboration is the addition of borane, BH3, to an alkene to form a trialkyl-borane. Borane cannot be prepared as a pure compound because it reacts with itself (2BHj B2H0) to form diborane BgHe, a toxic gas that ignites spontaneously in air. However, BH3 forms a stable Lewis acid-base complex with ethers and is most commonly used as a commercially available solution of BH3 in tetrahydrofuran (THF). 

Dimethylenebicyclo[2.2.IJheptene complexes with group VIB metals, 12 236 Dimethyl ether, reaction with diborane, 16 267, 268... 

Reduction of esters, nitriles, and amides. These groups are rapidly reduced by horanc-dimcthyl sulfide in refluxing THF (b.p. 67°) if the dimethyl sulfide (b.p. 38°) is removed as liberated. Under these conditions, the reagent is comparable to uncomplexed diborane. Reduction of secondary and tertiary amides is best effected in the presence of boron trifluoride etherate otherwise, excess reagent is utilized for formation of complexes with the products. 

Diborane is an inconvenient reagent. It is a toxic, flammable, and explosive gas. It is more easily used as a complex with tetrahydrofuran (THF), a cyclic ether. This complex reacts like diborane, yet the solution is easily measured and transferred. 

Complexes with Electrophiles An ether s nonbonding electrons also stabilize borane, BH3. Pure borane exists as a dimer called diborane, B2H6. Diborane is a toxic, flammable, and explosive gas, whose use is both dangerous and inconvenient. Borane forms a stable complex with tetrahydrofuran. The BH3 THF complex is commercially available as a 1 M solution, easily measured and transferred like any other air-sensitive liquid reagent. The availability of BH3 THF has contributed greatly to the convenience of hydroboration (Section 8-7). 

The first studies on the reaction of olefins and acetylenes with diborane met with little success. In the gas phase, olefins react sluggishly over long periods at elevated T to give mixtures . Acetylene gives polymeric organoboranes . However, in ether solvents, the addition is fast and quantitative at 0°C The catalytic effect of ethers is attributed to the formation of weak, reactive borane complexes ... 

The combination of crown ether and porphyrin has recently been extended by Hamilton et al. The macrotetracyclic cryptand 226 was prepared by condensation of the biphenyl-linked bis-crown ether 224 with the di-p-nitrophenyl ester porphyrin 225a, b in pyridine at 55 °C under high dilution conditions (45-54% after chromatography). Reduction to the tetra-amine 227 was effected by treating the zinc complex with diborane... 

Complexes with Electrophiles An ether s nonbonding electrons also stabilize borane, BH3. Pure borane exists as a dimer called diborane, B2H6. Diborane is a toxic, flammable, and explosive gas, whose use is both dangerous and inconvenient. Borane... 

Other reported reductions of alkyl or arylboron halides with ether solutions of complex metal hydrides imply a convenient general route to the 1,2-di- or tetrasubstituted diboranes. 

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 diborane is generated (in situ, or separately, from NaBH4 and Et2Offi—BF3e), and probably complexes, as the monomeric BH3, with the ethereal solvent used for the reaction. BH3 is a Lewis acid and adds to the least substituted carbon atom of the alkene (Markownikov addition), overall addition is completed by hydride transfer to the adjacent, positively polarised carbon atom ... 

Reduction of aromatic carboxylic acids to alcohols can be achieved by hydrides and complex hydrides, e.g. lithium aluminum hydride 968], sodium aluminum hydride [55] and sodium bis 2-methoxyethoxy)aluminum hydride [544, 969, 970], and with borane (diborane) [976] prepared from sodium borohydride and boron trifluoride etherate [971, 977] or aluminum chloride [755, 975] in diglyme. Sodium borohydride alone does not reduce free carboxylic acids. Anthranilic acid was reduced to the corresponding alcohol by electroreduction in sulfuric acid at 20-30° in 69-78% yield [979],... 

Any of these BH3 compounds adds readily to most alkenes at room temperature or lower temperatures. The reactions usually are carried out in ether solvents, although hydrocarbon solvents can be used with the borane-dimethyl sulfide complex. When diborane is the reagent, it can be generated either in situ or externally through the reaction of boron trifluoride with sodium borohydride ... 

Recent methods for the cleavage of allyl ethers that have that have yet to be tested on the anvil of complex target synthesis include (a) diborane generated in situ by reaction of sodium borohydride with iodine in THF at 0 °C (cyanoT ester, nitro, acetonide and tetrahydropyranyl groups survive) 434 (b) cerium(Ill) chloride and sodium iodide in refluxing acetonitrile (benzyl. THP and Boc groups survive) 435 (c) iodotrimethylsilane in acetonitrile at room temperature 436 and (d) DDO in wet dichloromethane (secondary allyl ethers, benzyl, acetate and TBS groups survive).437... 

The presence of boron-boron bonds has been suggested in dimeric complexes formed by reaction of some triarylboron compounds with active metals in ethereal solution (25, 73) although alternative structures have been proposed (44). The salt Na2[(CHj)2BH], formed from tetramethyl-diborane(6) and sodium in liquid ammonia, reacts with trimethylborane in that solvent to form Na2[B2H(CH3)5], which also may contain a boron-boron bond (22). 

For the synthesis of trialkylboranes. hydroboration is carried out with a borane solution in THF or with the borane-methyl sulfide complex (BMS) in THF, diethyl ether, or dichloromethane. Diborane reacts rapidly and quantitatively with alkenes to produce a solution of trialkylborane [16] (eq (13)). The addition of dialkylboranes. such as 9-borabicyclo[3.3.1]nonane (9-BBN. 3). disiamylborane (1), or dicyclohexylb-orane (2), to alkenes or alkynes gives mixed alkylboron compounds. The high regio-. stereo-, or chemoselectivity in the additions of these borane reagents unsaturated C—C bonds have been extensively used in organic syntheses. 

Reduction of 20 with diborane in bis(2-methoxyethyl)ether 3delds the bis(cyclam) derivative (21) 48). Addition of one equivalent of nickeUII) nitrate to an aqueous solution of 21 at pH 7, followed by one equivalent of copperdi) nitrate, is expected to result in a mixture of complexes in the proportion of 1 1 2 [di-Cu(II) complex dirNidl) complex Cu(II)/Ni(II) complex] (provided the two rings behave independently toward both metals). Separation of these three species was achieved by ion-exchange chromatography. Spectral and electrochemical investigations on the above binuclear complexes were undertaken. The visible spectra are very similar to those of the corresponding (mononuclear) cyclam complexes and, for example, the spectrum of the binuclear complex is virtually equal to the sum of the spectra of [Cu(cyclam)] and [Ni(cyclam]. Despite this, electrochemical studies confirm the presence of a weak interaction between the coordinated metals in each dinuclear complex. This is manifested by a shift in the second oxidation potential [M(II) M(IID] to a more positive value in each case. 

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