Hydride abstraction, boranes

Since complete hydride abstraction by B(C6F5)3 from Bu3SnH has been observed,260 it is not surprising that the borane mediates hydrostannation reactions as well. Y. Yamamoto also reported the use of BfCfiFs) in the hydrostannation of alkynes the hydrostannane is generated in situ from silane in the presence of BtCgFsF.261 Furthermore, Maruoka et al. have reported B(C6F5)3 mediated hydrostannation of ketones, chemistry which is discussed in more detail in the next section. 

Hydride abstraction, an approach developed by S. G. Shore and his coworkers, can also be applied to larger anions to generate very pure higher boranes, in most cases as the only volatile product. Thus, treatment of B3Hg salts (readily obtained in reactions of NaBH,) with boron trihalides provides the best available synthesis of tetraborane(lO) ... 

Hydride abstraction from tetrahydroborate anions is the most convenient route for the preparation of boranes - . In these processes an unstable intermediate transfers a BH3 moiety and in so doing forms new BHB bridge bonds. The simplest of these reactions involves the formation of B H from NaBH, and BF3 at 25°C. In addition to forming new B—H—B bonds in BjH, 1 mol of [HBF3] is formed per mol of [BH,]- ... 

Hydride abstraction from IPr-borane with tris(pentafluorophenyl)borane was attempted by In s et al. It was found that only half an equivalent of the borane reacted, producing a two-electron three-center bond of the form H2B-H-BH2, which is stabilized at each boron center by an NHC ligand, giving an overall cationic species with [HB(C6Fs)3] as the anion [93]. 

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... 

Alternative high-yield syntheses of these various boranes via hydride-ion abstraction from borane anions by BBra and other Lewis acids have recently been devised l (see p. 162). 

The proposed mechanism starts with a methyl group abstraction on platinum complex 416 with the borane reagent in the presence of diyne 414 (Scheme 105). The square-planar cationic diyne-platinum(n) complex 417 is converted to the octahedral platinum(rv) hydride intermediate 418 through oxidative addition of the hydrosilane. This complex decomposes rapidly with methane release to form another tetracoordinated platinum(n) species 419, followed by platinasilylation of the triple bond. The resulting vinylplatinum 420 undergoes an intramolecular carboplatination to... 

In the above preparations of BioHm, B5H11, and Bi+Hio, abstraction of H from a borane anion gives an unstable neutral fragment which is believed to obtain a BH3 unit from another like fragment to form the desired borane product. One hundred per cent conversion of the starting material to the desired boron hydride cannot be achieved. On the other hand if BH3 could be "funneled" into the reaction mixture to react with the unstable intermediate, then 100% conversion could be achieved in principle. 

Atkah metal hydrides too abstract protons from boranes. While water is produced with basic hydroxides, hydrogen is liberated with hydrides. Except diborane, all other boron hydrides undergo similar reactions, liberating hydrogen ... 

Tris(pentafluorophenyl)borane (35) is a powerful Lewis acid with a hard boron center. It interacts strongly with hard Lewis basic centers and abstracts hard anion such as methyl anion and hydride to initiate selective reactions that are otherwise difficult to achieve. The B(CgF5)3-promoted early transition-metal-catalyzed olefin polymerizations are typical reactions [16]. The B(C6F5)3-Ph3SiH system is useful for hydro-silylation of the carbonyl group, in which the boron of 35 activates the Si—H bond rather than the carbonyl group (see 40 in Scheme 3.39) and reduces even the ester carbonyl group effectively [17]. A number... 

Quite recently, Prokof)evs et al. [106] have shown that, while NHC-boranes themselves resist hydroboration (in this case with alkenes), if a hydride is abstracted, to form the corresponding NHC-borenium cation, hydroboration does indeed occur. Tf2NH and triphenylmethyl tetrakis(pentafluorophenyl)... 

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