Water trialkylborane complexes

Brown proposed a mechanism where the enolate radical resulting from the radical addition reacts with the trialkylborane to give a boron enolate and a new alkyl radical that can propagate the chain (Scheme 24) [61]. The formation of the intermediate boron enolate was confirmed by H NMR spectroscopy [66,67]. The role of water present in the system is to hydrolyze the boron enolate and to prevent its degradation by undesired free-radical processes. This hydrolysis step is essential when alkynones [68] and acrylonitrile [58] are used as radical traps since the resulting allenes or keteneimines respectively, react readily with radical species. Maillard and Walton have shown by nB NMR, ll NMR und IR spectroscopy, that tri-ethylborane does complex methyl vinyl ketone, acrolein and 3-methylbut-3-en-2-one. They proposed that the reaction of triethylborane with these traps involves complexation of the trap by the Lewis acidic borane prior to conjugate addition [69]. 

In 2007 the scope of the trialkylborane/water system was extended to the dehalogenation of alkyl iodides and the chemoselective deoxygenation of secondary alcohols in the presence of alkyl and aryl halides [86]. The rate constants for the hydrogen-atom transfer from this reagent to secondary radicals (Scheme 37) are substantially lower than those of the Ti(III) aqua-complex [78, 87]. 

The reaction of trialkylboranes with A-chloro- or A-(benzoyloxy)alkylamines afforded secondary amines via an anisotropic 1,2-shift of the alkyl group from boron atom to nitrogen in the B-N complex intermediate.519-521 Alkylation of A-chlorodimethylamine with primary trialkylboranes to give A,A-dimethylalkylamines was conducted in the presence of galvinoxyl to avoid the formation of alkyl chlorides via free radical process.522,523 A convenient approach to mixed secondary amines is alkylation of alkyl azides with relatively unhindered trialkylboranes in refluxing xylene followed by hydrolysis with water. The reaction smoothly took place at low temperature when trialkylboranes were replaced by alkyl(dichloro)boranes (Equation (108)).524 Intramolecular amination furnished cyclic amines (Equation (109)).400,525-528... 

The related direct oxidation of trialkylboranes has been studied 178), as well as the brominolysis and iodinolysis of benzeneboronic acid in aqueous acetic acid (50%) and nt-chlorobenzeneboronic acid in aqueous solution. The latter reveals a pattern involving tetracovalent boron 179). In water solutions, and acetate buffers at constant ionic strength with the w-chloro derivative, plots of log k vs. pH were linear with unit slope (from pH 2 to 5), suggesting specific lyate ion catalysis. In both cases catalysis by fluoride ion was observable, and catalysis by hydroxy acids or diols, which form coordination complexes with boron, was seen in the latter case. Indeed in water solvent, the catalytic constant for fluoride ion is some 6000 times that of the uncatalyzed case. 

Bimolecular Lewis Acid-Base Complexation under Non-aqueous Conditions As evidenced by the high pH required in the formation of boronate anions, boronic acids and most dialkyl esters are weak Lewis acids. This behavior contrasts sharply with trialkylboranes, which form strong adducts with phosphines, amines, and other Lewis bases [66]. Aside from the formation of boronate anions, discussed in the previous section, very few stable intermolecular acid-base adducts of boronic acids (esters) exist. Long ago, aliphatic amines and pyridine were found to form complexes in a 1 3 amine boronic acid stoichiometry [67]. Cbmbustion analyses of these air-stable solids suggested that two molecules of water are lost in the process, which led the authors to propose structure 18 (Equation 7, Figure 1.8). Subsequently, Snyder... 

---

See also in sourсe #XX -- [ , ]

---