Boron bonding

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

Step 2 Anion of hydrogen peroxide acts as a nucleophile attacking boron and forming an oxygen-boron bond... 

Step 3 Carbon migrates from boron to oxygen displacing hydroxide ion Carbon migrates with the pair of electrons m the carbon-boron bond these become the electrons m the carbon-oxygen bond... 

The influence of boron-bonded ligands on the kinetics and mechanistic pathways of hydrolysis of amine boranes has been examined (37,38). The stoichiometry of trimetbyl amine azidoborane [61652-29-7] hydrolysis in acidic solution is given in equation 10. It is suggested that protonation occurs at the azide ligand enabling its departure as the relatively labile HN species. 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

The chemistry of compounds which contain boron-boron bonds. R. J. Brotherton, Prog. Boron Chem., 1964,1,1-81 (189). 

Some group-IB and -IIB boranes or carboranes, especially of Cu, possess M — H—B structure units and are not discussed here because no direct metal-boron bond exists. 

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

Dihydro-l,2,5-thiadiboroles (Et2C2B2R2S) have remarkable acceptor properties toward metal-ligand moieties. Dihydrothiadiborole-derived complexes can be achieved thermally or photochemically, e.g., Co and Fe sandwich, triple-decked and even tetradecked complexes are known, all having direct metal-boron bonds. 

The phosphorus-boron bond of BisP -BH3 81 is cleaved by treatment with tri-fluoromethanesulfonic acid in toluene followed by treatment with aqueous KOH or, alternatively, with tetrafluoroboronic acid in dichloromethane and subsequent... 

Examination of this anomaly revealed that the carbon-carbon double bond was also reacting, producing a carbon-boron bond with loss of an equivalent of hydride. 

Boron resonances (Table III) appear in the range expected for sandwich-type complexes with metal-boron bonding (for reference data see Ref. 50). In cases where pairs of B-methyl and B-phenyl derivatives (as, e.g., 9 and 17) are known, the nB resonance of the methyl compound is at lower field by 2.0-3.7 ppm (average, 2.6 ppm). 

Replacement of halogen in dimeric iminoboranes containing tetracoordin-ated boron is not so readily accomplished. For example, bis[(dichloromethyl-eneamino)dichloroborane]reacts with C4H9MgCl in nonetheric solvents to yield dimeric (dichloromethyleneamino)butylchloroborane, whereas C4H9Ii replaces both boron-bonded chlorine atoms by butyl groups 25). 

Remarkable carbon-boron bond-forming reactions are catalyzed by iridium complexes and proceed at room temperature with excellent regioselectivity, governed by steric factors. Heteroarenes are borylated in the 2-position and this reaction is generally tolerant of halide substituents on the arene (Equations (87) and (88)). 

Addition of a boron-boron bond across a carbon-carbon triple bond is known for some 40 years since the finding that diboron tetrahalides add to alkenes and alkynes in the absence of catalysts.36 Although the reaction seemed to be potentially attractive, the instability of diboron tetrahalides was the critical drawback for the practical use in synthesis. In 1993, much more stable pinacol ester derivative of diboron was found to add to alkynes in the presence of platinum catalysts such as Pt(PPh3)4, Pt(CH2=CH2)(PPh3)2, and Pt(CO)2(PPh3)2 (Figure 1, Scheme 2).37,38 Other... 

Stereoselective 1,4-addition of the tin-boron bond to 1,3-diene occurs in the presence of a palladium catalyst, which is prepared in situ from Pd(dba)2 and ETPO, at 80 °C in THF to give (Z)-l-boryl-4-stannyl-2-butenes (Equation (102)).249 For unsymmetrical 1,3-diene like isoprene, highly regioselective 1,4-stannaboration is observed. 

Reaction of an allene with stannylborane 9 in the presence of a palladium catalyst results in the double insertion of 1,1-dimethylallene into the tin-boron bond, affording the 1 2 adduct 18 (74% GC yield) along with the 1 1 adduct (18% GC yield) (Equation (103)) 227... 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

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