Boron complexes pyridine oxide

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Mass spectra of numerous boron-chelate complexes of pyridines and quinolines (five-membered chelate rings) and their N-oxides (six-membered chelate rings) have been recorded.309... 

The empty 2/j,-orbital on boron interacting with the ji-electrons of the butadienylidene system in borole (5) is proved by the pronounced bathochromic shift in the visible spectrum compared with that of its pyridine adduct. This Ti-electron delocalization in boroles is destabilizing and produces a 471-electron antiaromatic system. The antiaromaticity is further evidenced by the rapidity of reactions that remove the 2p ,-orbital on boron from conjugation. These reactions include Lewis complexation, transition metal complexation, or borole ring opening reactions such as Diels-Alder addition, oxidation, and protodeboration. This removal of the antiaromaticity from borole results in transforming the compound to a typical, low reacting, sterically hindered arylborane. 

Figure 11.1.25 shows data from a microdroplet liquid-liquid voltammetry experiment in which lactate anions A are transferred from the aqueous phase into the organic microdroplet phase (here composed of the organic solvent 4-(3-phenylpropyl)-pyridine or PPP containing a Mn(III/II) redox system and naphthyl-2-boronic acid B [120]). Schematically, the oxidation of each metal complex Mn(II)TPP (with TPP = tetraphenylporphyrinato) is generating a positive charge within the microdroplet and this is coupled to the reversible transfer of the anion A ... 

Fig. II.1.25 (a) Schematic representation of the transfer of anion from the aqueous into the organic phase upon oxidation of Mn(II)TPP to Mn(III)TPP . (b) In the presence of the boronic acid B as a facilitator the tiansfer of the anion A leads to the formation of the complex AB in the oiganic phase, (c) Cyclic voltammograms [120] (scan rate 10 mVs ) for the oxidation and le-ieduction of 75 mM Mn(II)TPP dissolved in PPP (4-(3-phenylpropyl)-pyridine, 75 nL) and immobilised in the form of microdroplets onto a 4.9-mm diameter graphite electrode immersed in aqueous 0.1 M sodium lactate pH = 7.34. The presence of (i) 0 and (ii) 973 mM naphthyl-2-borDnic add is shown to cause a negative shift in the voltammetric response, (d) Plot of the midpoint potential versus the natural logarithm of the naphthyl-2-boronic add concentration in the microdroplets. Lines indicate calculated data [120] for reversible lactate-4)oronic add complex formation for three equilibrium constants...

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