Organoboron species

Tetracoordinate Boron. A number of tetracoordmate organoboron species that are photo- and electroluminescent, and thus of interest for device applications have been developed. Characteristic of complexes (168-170) is the presence of chelating nitrogen ligands such as 8-hydroxyquinoline in (168). The qumolate complexes (168) were reported to show superior stabihfy over more commonly used aluminum complexes. Moreover, their photophysical properties were shown to be tunable through ligand variation. ... 

Trialkylboranes add across the triple bond of bis(trimethylstannyl)ethyne, to give organoboron species which look promising as synthetic reagents (reaction... 

The key challenge for computational chemistry with respect to the transmeta-lation step of the Suzuki-Miyaura cross-coupling was to understand the role of the external base. It is certainly well known from experiment that the presence of a base in solution is required for the reaction to take place. Several proposals have been put forward for the role of this base from experimental studies. Most of this information was summarized by Miyaura in a very clarilying paper [43], Two main pathways are proposed (Fig. 11.5) either the base binds the boronic acid to from the organoboronate species (path A), or the base substitutes the halide ligand in the coordination sphere of the catalyst (path B). These two proposed pathways were theoretically evaluated by us on a model system, using trans-PdBr(CH2 = CH)(PH3)2, CH2 = CH-B(OH)2 and OH species as reactants [44]. 

A related experimental result is that transmetalation takes place in absence of the base if the halide is replaced by a hydroxo or alkoxo group in the starting palladium complex [46], The reaction mechanism for this process (labeled as path C) was computationally studied, and the energy profile was found to be smooth. The highest energy barrier is quite low, around 80 kJ/mol. Path C is in fact identical to path A from the point where the organoboronate species is coordinated to the catalyst. Hence, computational results reproduce the experimental observation of... 

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