Tetrahydrofuran ether-borane complex

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). 

When diborane (B2Hg) dissodates in ether solvents, such as tetrahydrofuran (IHF), a complex between borane (BH3) and the ether is formed. For example. 

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... 

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. 

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],... 

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). 

Tris(N-methylanilino)borane has previously been prepared by the reaction of boron trifluoride-ether complexes with three equivalents each of N-methylaniline and a suitable Grignard reagent,1,2 by the reaction of (N-methylanilino)potassium with boron trifluoride-ether complexes,2 and by aminolysis of boron trichloride by N-methylaniline.3 The present general procedure describes a convenient preparation of tris(N-methylanilino)-borane by the reaction of (N-methylanilino)lithium and boron trifluoride-diethyl ether in tetrahydrofuran-hexane as solvent. 

Borane, which is used as a complex with tetrahydrofuran [992] or dimethyl sulfide [611, 992] or generated in situ from lithium borohydride with boron trifluoride etherate [646] or sodium borohydride with aluminum chloride [184], reacts with 3 mol of an alkene to form a tertiary borane. The oxidation with alkaline hydrogen peroxide [183, 992, 1201] or with trimethylamine oxide [991, 992] yields an alcohol (equations 598 and 599). 

Hydroboration is usually carried out in the ether tetrahydrofuran, in which borane exists as a complex 50, from which BH3 is added to an alkene, e.g. 2-methylpropene (44) in Scheme 4.11. Addition takes place at a face of the alkene by means of a four-centre transition state, as shown in 51. The partial bonds in 51 represent progressive formation of bonds between C and H, and between C and B, together with simultaneous weakening of the 7i bond and the B-H bond. In Scheme 4.11 the reaction of borane 52 is detailed this borane has two remaining B-H bonds, and a similar reaction of these two bonds with two further molecules of alkene results in exhaustive alkylation, with formation of the trialkylborane 53. The nature of the transition state 51 implies that H and B are delivered syn (to the same face), and simultaneously, to the double bond. 

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). 

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). 

Borane is a useful and selective reducing agent. It is prepared by the reaction of boron trifluoride etherate with sodium borohydride. The borane produced, as the etherate, may be distilled as the dimer, which is a colorless, toxic gas (B2Hg). Collection of the dimer distillate in tetrahydrofuran (THF) again forms the monomer, in this case as the BHs- THF complex. The latter is commercially available as a 1.0 M solution. 

Borane (which by itself exists as a dimer, B2H6) is commercially available in ether and tetrahydrofuran (THF). In these solutions, borane exists as a Lewis acid-base complex with the ether oxygen (see Sections 2-3 and 9-5), an aggregate that allows the boron to have an electron octet (for the molecular-orbital picture of BH3, see Figure 1-17). 

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