Exchange reactions diboranes

BCl, BBr, and BI undergo exchange reactions to yield mixed boron hahdes. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr or BI (21)-... 

Approximate equilibrium constants for the gas-phase reaction at 300° T have been determined for the trimethylborane-diborane exchange from equilibrium experiments and from thermodynamic data (164,261) with the computed constants listed in parentheses in Eqs. (68)-(71). 

Exchange reactions are common between boron halides and alkyl or aryl snbstitnents, other halides, alkoxy and aryloxy gronps, and diborane. Some examples are shown in Scheme 2. 

Exchange reactions of alkylated diboranes with higher boron hydrides are discussed in Sections II,B>1 and II,D. 

Phosphorus-containing Ring Systems. - A range of new chiral oxazaphospholidine oxides 266 and 267 have been synthesised and used as catalysts in asymmetric reductions of ketones with diborane. Mannich-type cyclisation reactions of 5-amino-3-benzylthio-4-cyano(ethoxycarbonyl)pyrazoles with dichlorophenylphosphine and aromatic aldehydes in the presence of cation exchange resin have been used to prepare a number of 6-oxo-6-phospha-4,5,6-trihydroimidazolo[l,2-b]pyrazoles, e.g. 268. Some of these compounds have herbicidal activity and this report is typical of a number of similar ones in the Chinese literature. A number of metallocycles, e.g. 269, have been reported as products from reactions of transient zirconocene-benzyne intermediates with phosphaimines followed by sulfuration or selenation. ... 

Both diborane and alkyldiboranes serve as B—H catalysts for the redistribution reaction. Very strongly associated B—H dimer compounds such as bisborolanes are not effective catalysts for ligand exchange below 100°. 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

The reactivity of the vinyllead bond is clearly shown in the reaction of Vi4Pb with diborane (113). Even at — 78°C one hydrogen atom is quantitatively exchanged, while with Me4Pb at higher temperatures no reaction takes place (see Section III,A,l,e). 

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