Borane , compounds with dimeric derivatives

Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65]. 

The boron-11 n.m.r. spectrum of the compound [(CH3)3CCH=N-B(C4H9)2]n exhibits a strong signal at -7,4 ppm and a weak one at -38,8 ppm. At 120 °C the signal intensities reverse thus indicating that the dimer reverts to the monomer with increasing temperature 6>. The chemical shifts of imine-borane derivatives (c.f. Eq. (5)) are observed near +1,0 ppm 7>. 

Reaction of compound 56 with Li20 yields the anhydro dimer 57, which is the N,N-dimethyl derivative of anhydro dimer 53, and this provides another route to the anhydro dimers. In analogy with the reaction to form the 1-azonia-2-boratanaphthalene 56, l-oxonia-2-boratanaphthalenes 58a—fare formed by the reaction of phenylethyne with arylchloro(phenyloxy)boranes (Scheme 23). 

Zwitterionic borane adducts were reacted with trityl cation salts to provide a series of primary borenium cation derivatives (Scheme 12). For example, the A-heterocyclic carbene (NHC) borane (45) was prepared from the corresponding NHC (44). With the addition of the trityl salt, the dicationic dimer (46) was isolated. Product (46) was characterized by X-ray crystallography and trapped by nucleophiles. Its observation by NMR resonance (5 + 10.6) is in accordance with the GIAO calculated value (5 + 0.8). Similar borenium dications were generated from amine nucleophiles, such as compound (47). The authors considered the question why some borenium ion salts... 

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