Bond cleavage boron-carbon

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). 

Cleavage of boron-carbon bonds by halogens does not proceed efficiently. The elements of HI, HBr, and HC1 can, however, be added to alkenes in an a/jfr -Markovnikov fashion by hydroboration and subsequent addition of either 12/NaOMe, Br2/NaOMe, or NCI3 (Equations B2.15-17). 

Reactions of this type are somewhat less satisfactory for preparation of mixed diboron derivatives containing boron-carbon bonds. Thus, while tetrakis(dimethylamino)diborane(4) is readily hydrolyzed to tetrahydroxy-diborane(4) in aqueous acid, l,2-bis(dimethylamino)-l,2-diethylborane(4) reacts with formation of hydrogen and cleavage of the boron-boron bond 14). Reaction of the dibutyl analog with a twofold excess of water gave the dimethylamine adduct of the boroxole... 

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]. 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

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]).

Tertiary phosphines substituted at the -carbon by electronegative groups, e.g. (56), react with boron trihalides to give products derived from carbon-phosphorus bond cleavage. Phosphines containing only hydrocarbon groups do not react. 

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) ... 

Figure 7,14 The boronic ester formation can be used to prearrange appropriate building blocks. By ring closing metathesis, the non-macrocyclic precursor is turned into a bimacrocycle. When the norbornane template is removed (by ester hydrolysis or carbon-boron bond cleavage), er do-functionaHzed macrocycles are produced ...

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-... 

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