Boron planar-tetracoordinate

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

Protonation of 4b leads to the symmetrically substituted 3b (Scheme 3.2-3) and methylation of 4b at temperatures higher than —60 °C gives 3c (Scheme 3.2-5) [19]. In the latter reaction, 6a can be identified as an intermediate at —80 °C by 13C NMR spectroscopy [19]. Its planar-tetracoordinate carbon atom is strongly de-shielded 3 13 C = 144 ppm) as compared with tetrahedrally-coordinated carbon atoms connected to three boron and one silicon center (d 13C = 70-100 ppm). Computations for the model compounds 6A and 6B give 144 and 104 ppm, re-... 

The group 4 benzynemetallocenes combined with boron, aluminum, and gallium budding blocks were successftdly employed in the stabihzation of planar-tetracoordinate carbon compounds (equation 26). ... 

Boulton and Prado have concluded from the similarity in the activation free energies for the exchange process (3) as determined from the coalescence of the benzyl methylene proton NMR signals in (1) and (2) and those for the enantiotopomerization of similar compounds studied earlier by Russian workers, that it was unnecessary to postulate a process involving a planar tetracoordinate boron atom. Boulton and Prado favored planar three-coordinate intermediates instead. 

Reaction of borabenzene with Li2 gives structure (116) in which the tetracoordinate boron atom has a planar bond configuration [MNDO (90ZOR210)]. A structure with the tetrahedral bond configuration has an energy higher by 9.6 kcal/mol compared to structure (116) and represents a transition state in the topomerization of the latter. 

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