Carbon-boron bonds reactions with

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. 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

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 ... 

Boronates are more stable since they are stabilized owing to the donor effect of oxygen lone-pairs to the empty orbital of the boron. The two different carbon-metal bonds afford particnlar reactivity. For example, addition of propargylbromide on (94) in presence of a catalytic amount of copper cyanide nndergoes a carbon-carbon bond formation with exclusive cleavage of the C-Zr bond. The subsequent borylallene, by treatment with a ,/3-unsaturated aldehydes, affords two trienes, depending on the reaction conditions (Scheme 20). 

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... 

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]).

The reaction of 161 with lithium metal involves a reduction, a trimethylsilyl migration, and formation of a boron-carbon bond to afford 162. Subsequent reaction with trimethylsilyl chloride gives 1,2-diborolane 146 (Scheme 14) <2002AGE1526, 2004EJI3063>. 

Stereochemistry. Brown4 has shown that the carbonylation reaction and the carboethoxymethylation reaction are highly stereospecific in that the original boron-carbon bond is converted into a carbon-carbon bond with retention of configuration. These reactions thus are similar to oxidation of organoboranes with alkaline hydrogen peroxide and animation with hydroxylamine-O-sulfonic acid. 

A number of reactions of the boranes lead to synthesis of boron-carbon bonds. With boron alkyls the boron hydrides undergo important alkylation reactions, but only in the case of diborane have these reactions been described in detail. When diborane is mixed with trimethylborane at room... 

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