The Boron-Carbon Bond

The bonds between the carbon atoms and boron atoms as well as between the boron atoms themselves in the icosahedra are essentially covalent. But like silicon carbide (Sec. 3.3), the bonding of boron carbide is also partially ionic. 1 1 The difference between the atomic spacing of SiC and the sum of the covalent radii of carbon and silicon on one hand and the sum of the ionic radii (m the other hand show that the bonding, although mainly covalent, includes a certain d rce of ionicity. The calculated covalent bond energy E is 9.42 eV and die ionic bond energy Ep is 1.41 eV. 

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Various diboriodilithiomethanes of type 225 were synthesized by Bemdt and coworkers, adding different aryllithium compounds to the boron-carbon bonds of compounds 224a-e (Scheme 77) . The dilithium compounds have been characterized in the solid state by X-ray structural analysis. 225a-e adopt the structure of a 1,3-diborataallene system, where lithium-diethyl ether units are bridging the twisted B-C-B axis from both sides. 

The Lewis acidic organoboranes, on the other hand, are electrophilic in their chemical behavior.22 This is attributed to the high degree of covalent character in the boron-carbon bond, which results in the low reactivity of organoboranes versus organolithiums towards electrophiles. In spite of the low reactivity of the triorganoboranes with simple aldehydes and ketones, formal conjugate addition of alkyl (or aryl)... 

The boracyclobutene embedded in [l,8]naphthaborete 27 reacts with a range of boron electrophiles with cleavage of the boron-carbon bond (Scheme 2). Borane, diethylborane, trihaloborane, and triethylborate all react similarly, returning azadiboracyclic products 28 and 29 <1994AGE1247>. Borane 28a is converted into naphtho[l,8-r 7][l,2,6]azadiborinin 29c upon reaction with ethanol. 

If the face discrimination in the asymmetric hydroboration reaction is high then the optical purity of the chiral molecule produced will also be high. Efficient asymmetric hydroboration reactions followed by stereospecific cleavage of the boron-carbon bonds produced have been used in syntheses of several complex homochiral molecules (see Section B2.1). 

Trialkyl boron was first claimed as a new anionic initiator for the polymerization of vinyl compounds (264), although it was rather improbable in view of the low ionic character of the boron-carbon bond. The error was quickly corrected when it was shown that free radicals were involved (265, 266) and that oxygen, peroxides, silver salts and copper salts were co-catalysts (262, 267). Aluminum alkyls can also initiate radical polymerizations in the presence of oxygen (267,262) but, as in the case of zinc, cadmium or boron alkyls, the products were not stereoregular. Thus, complexing between catalyst and monomer probably does not occur. 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

The boron-carbon bonds of organoboranes and organoborates are replaced by substituents or by carbon-carbon bonds. 

The thermal reactions of the pyridinium borate salts are likely to follow the same electron-transfer path. Experimental evidence for this conclusion is the fact that the 5cc-butyl transfer is substantially faster than methyl transfer although a nucleophilic substitution mechanism would predict the less hindered group to be transferred preferentially. The fast rates of 5cc-butyl transfer can be readily explained on the basis of the electron-transfer mechanism (Eqs. 69-71) by considering the different boron-carbon bond strength [189, 190] for the various alkylborates. The boron-carbon bond cleavage (Eq. 70) is apparently the critical step, and its relative rate [191] as compared to that of the back electron transfer determines the overall rate for thermal alkyl transfers in pyridinium tetraalkylborate salts. 

Cleavage rates for the boron-carbon bond in boranyl radicals have been estimated to exceed A = 10" S- [189, 190]. 

Figure 3. The absorption of cyanine dye (Cy) radicals monitored at 430 nm following excitation of a benzene solution with an 18 ps laser pulse. The time dependence of the absorption changes of cyanine radical for the benzyltriphenylborate case is faster than its decay. For the vinyltriphe-nylborate, back electron transfer and the reaction that follows electron transfer have competitive rates. For the tetraphenylborate salt, the back electron transfer process dominates after electron transfer, therefore the boron-carbon bond cleavage does not occur and almost no cyanine dye radical formation is observed (data adapted from [25]).

Such complexes possess sharply reduced chemical reactivity and consequently they often tend to stabilize the valence state of the acceptor metallic atom. Lithium tetraphenylboronate requires heating in acid solution in order to effect cleavage of the boron-carbon bonds and is quite stable in air toward oxidation. The acceptance of the phenyl anion has satisfied the electronic demands of boron. A direct preparation of analogous alkyl complexes has been realized by heating lithium aluminum hydride with ethylene under pressure 139) ... 

Even a boron isotope effect has been determined (B vs B ) for the cleavage of some boronic acids by mercuric chloride (Matte-son et al., 1964). The boron-carbon bonds cleaved with isotope effects of 2-3%. A noteworthy point is the unusual method used to determine the isotope ratios. The method took advantage of the widely differing cross sections of B " and B to neutron bombard-... 

The oxidative cleavage of the boron-carbon bond with radioiodide/Chloramine-T has been shown to be useftil for the radioiodination of various organic compounds (Kabalka et al. 1981). This method is characterized by the reaction sequence in O Fig. 44.4. 

While boron atoms migrate during thermal isomerization (see Section II.B.), the nature of the boron-carbon bond remains the same during catalytic exchange. 

The B—H bonds of fV) react with water to give off hydrogen only above 100° C likewise, addition to olefins only occurs at elevated temperatures. Under ordinary conditions, the boron-carbon bonds of bis(borolanes) are... 

It is interesting to note that the reaction does not proceed through a direct rupture of the boron-carbon bond, but involves a highly selective free radical substitution of the a-hydrogen of the sec-alkyl group by bromine. Protonolysis of the intermediate by hydrogen bromide gives the final product (Chart 13.1) [8]. 

A n = 6600). The authors suggested that the presence of HC10Et2 may promote the sequential insertion reaction of TeMC between the boron-oxygen bond by the activation of the monomer and/or loosening the boron-carbonate bond by coordination to the carbonate oxygen in the polymer propagating end (Scheme 73). 

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