Diborane Lewis base complexes

Free borane (2) exists as gaseous dimer—the diborane BaHg. In addition Lewis acid/Lewis base-complexes, as for example formed in an ethereal solvent, e.g. 4, are commercially available ... 

Early reports of hydroboration reactions often recommended in situ generation of diborane from sodium tetrahydroborate (borohydride) and trifluoroborane etherate or some similar mixture. Indeed, it is still reasonable to use this method for simple hydroborations, provided that nothing more complicated than oxidation of the resultant organoborane is intended. Otherwise, the approach should be avoided, particularly since borane is now commercially available in the form of several Lewis base complexes. 

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). 

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]. 

Boranes are strong reducing agents and the neutral molecules, inflame spontaneously in air, although the anions [BnHn]2- have remarkable kinetic stability. Diborane itself reacts with Lewis bases to give donor-acceptor complexes with BH3, which is a soft Lewis acid and forms adducts with soft bases such as CO (1). More complex products often result from unsymmetrical cleavage of B2H6, for example,... 

Although the bond angles are known to vary considerably depending on the metal and halogen, the arrangement is approximately tetrahedral around each metal atom. When the trihalides are dissolved in solvents that are Lewis bases, the dimers separate and complexes containing the monomer and the solvent are formed as a result of Lewis acid-base interactions. Such behavior is similar to that of borane and diborane in that the monomer, BH3, is not stable but adducts of it are. This type of behavior is illustrated in the following equations ... 

Borane (BH3) is a reactive gas that exists mostly as the dimer, diborane (B2H6). Borane is a strong Lewis acid that reacts readily with Lewis bases. For ease in handling in the laboratory, it is commonly used as a complex with tetrahydrofuran (THF). 

In. addition 10 terminal and bridging B—H bonds, this compound contains a direct B—B bond. Tetmborane undergoes both symmetric and unsymmetne cleavage (Fig. 16 J4). Larger Lewis bases tend to split ofl BH3 moieties, which are either complexed or allowed to dimerize to form diborane. 

Similarly, the parent hydride, diborane(4), has been prepared only in the form of complexes with Lewis bases 30, 43, 72). Attempted reduction of B2CI4 with various metallic hydrides leads to the formation of diborane(6) and other boron hydrides 104, 109). 

The reaction between alkenes and boranes forms the basis of hydroboration (see Chap. 5). Borane (B2H6, formally known as diborane) is a soft Lewis base and will complex with soft bases such as alkenes. The boron... 

Hydroboration can be accomplished with diborane (B2H6), which is a gaseous dimer of borane (BH3), or more conveniently with a reagent prepared by dissolving diborane in THE When diborane is introduced to THE it reacts to form a Lewis acid—base complex of borane (the Lewis acid) and THE. The complex is represented as BHsrTHF. 

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). 

In the reaction of 1-hexene to give 56 as the major product, an essential component is easy to overlook. The ether solvent is important, and the solvent glytne is commonly used. The formal name of glyme is 1,2-dimethoxyethane and it has the structure CH3OCH2CH2OCH3. If borane is simply mixed with 1-hexene with no solvent, no reaction occru-s unless the mixture is heated to about 180-200°C. In ether solvents, however, the reaction occurs rapidly at ambient temperatures (i.e., room temperatru-e). In fact, ethers catalyze the reaction of borane with an alkene and, although the ether structure does not appear in the product, the ether is not simply a solvent. This is probably because the oxygen atom of an ether is a Lewis base (remember the ate complex formed from borane and diethyl ether) and reacts with diborane to form a highly reactive complex that reacts with the alkene. All reactions of alkenes with borane in this chapter will use an ether solvent. 

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). 

Carbon monoxide is an extremely weak Lewis base towards conventional Lewis acids. It does not complex with the boron trihalides, although it does yield a weak adduct H3B CO with diborane. On the other hand it forms numerous complexes with transition elements. The source of this difference is that in the latter complexes not only is the weakly donating a-orbital of CO involved, but also the 7C orbitals which can function as acceptors. The conventional explanation is that a synergic effect exists in which the n interaction removes electron density from the metal, allowing a donation from the ligand to be enhanced. 

Borane (BH3) and its organic derivatives (RBH2 or R2BH) will add to an alkene (Scheme 10.4). BH3 dimerizes in the gas phase to make diborane. In solution reactions, however, this dimer is easily broken up (Eq. 10.25), especially if the reaction is occurring in a donor solvent such as THE, which produces a Lewis acid-base complex, BHs THF. 

Hydroboration reactions are frequently done using BHsiTHF as a complex in solution. BH3 in pure form is a gas, and in the absence of other Lewis bases it exists as the dimer diborane, B2H6. Open the molecular model at the book s website for the BH3 THF complex and display its LUMO. Does the LUMO have lobes suitably disposed to allow the BH3 portion of the BH3 THF complex to interact with other Lewis bases, e.g., an alkene tt bond in the... 

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