Borane complexes with Lewis bases

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

Interestingly, homolytic substitution at boron does not proceed with carbon centered radicals [8]. However, many different types of heteroatom centered radicals, for example alkoxyl radicals, react efficiently with the organoboranes (Scheme 2). This difference in reactivity is caused by the Lewis base character of the heteroatom centered radicals. Indeed, the first step of the homolytic substitution is the formation of a Lewis acid-Lewis base complex between the borane and the radical. This complex can then undergo a -fragmentation leading to the alkyl radical. This process is of particular interest for the development of radical chain reactions. 

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

In covalent organolithium compounds and covalent Grignard reagents neither the lithium nor the magnesium possesses a valence electron octet. This is energetically disadvantageous. In principle, the same mechanism can be used to stabilize these metals that monomeric boranes BH3 n Rb use to attain a valence electron octet at the boron atom (Section 3.3.3) the formation either of oligomers or, with suitable electron pair donors, of Lewis acid/Lewis base complexes. 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

Brown, H. C., Chandrasekharan, J. Mechanism of hydroboration of alkenes with borane-Lewis base complexes. Evidence that the mechanism of the hydroboration reaction proceeds through a prior dissociation of such complexes. J. Am. Chem. Soc. 1984,106,1863-1865. 

Because of its similarity to trimethylamine oxide, the ylid may be considered as a Lewis base, and, as such, it should coordinate with various Lewis acids. Complexes with lithium bromide and trimethylbromostannane have already been mentioned and it would be expected that coordination with boranes and alanes would also give stable products. 

Boron-nitrogen and boron-phosphorous compounds are classical textbook examples of donor-acceptor complexes. The chemical bonds of the Lewis-acid Lewis-base complexes are usually explained in terms of frontier orbital interactions and/or quasiclassical electrostatic attraction in the framework of the Hard and Soft Acid and Base (HSAB) model [73]. We were interested in seeing if the differences between the bond strengths of boron-nitrogen and boron-phosphorous complexes for different boranes, amines and phosphanes can be explained with the EDA method. 

Thus we examined the reactions of borane-Lewis base adducts with rhenium polyhydride complexes to synthesize highly fluxional polyhydride(borane) complexes. However, treatment of several boranes with the rhenium complexes in deuterated solvents resulted in an unexpected reaction H-D exchange between the boranes and solvents. This is the first example of H-D exchange between sp carbon and sp boron, and closely associated with metal-catalyzed deuteration of alkanes. ... 

Catalyst (3.115) is often referred to as the CBS catalyst after the names of the original authors (Corey, Bakshi and Shibata). A catalytic cycle was proposed which explains the experimental observations. The oxazaborolidine interacts reversibly with borane, which then allows complexation of the ketone to give the key intermediate (3.122), as depicted in Figure 3.2. In this process the catalyst acts as both a Lewis acid and Lewis base activating the borane towards hydride delivery and the ketone towards reduction by interaction with the boron in the oxazaborolidine. This dual activation and enhanced steric bulk of the pyrrolidine moiety leads to... 

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). 

However, BH3 forms stable Lewis acid-base complexes with ethers. Borane is most often used as a commercially available solution of BH3 in THF. 

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). 

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]. 

Formation of the 1 1 complex of (CH3)3N with (CD3)3B is favored in the gas phase to the extent of 1.25 0.03 over the complex with (CH3)3B.87 This is opposite to the general results presented above, where the deuterated base is stronger, but here the deuterated borane is the stronger Lewis acid. The difference is attributed to better hyperconjugation by H into the vacant p orbital on boron, which is lost on complexation. 

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,... 

Amine-boranes are coordination complexes formed by combination of an amine Lewis base with a tricoordinate Lewis acid borane fragment (equation 1), and as noted in Figure 1, they are analogs of alkanes. 

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