Borane-pyridine adducts

The BH3 adduct of 2-aminopyridine seems to be very stable to internal hydroboration and polymerisation compared to borane pyridine. We attribute the extra stability to hydride-proton interactions these together with those of fluorine-proton were reported for other amine borane compounds16 (Figure 5). The MM models for the two boron complexes shown in Figure 6, present atomic distances... 

A similar reaction performed with P(NMe2)3 and 2-aminopyridine gives trisaminophosphorus compound. Borane addition gives the corresponding phosphorus and pyridinic adducts (Figure 15). 

Borane (BH3) adducts of some nitrogen heterocyclic bases e.g. quinoline, aminopyridines) have been prepared. Halogenoboranes form boronium salts with such heterocycles, and some containing two different bases attached to a single boron atom, of the type (R -pyridine)(R -pyridine)aBH + 2I, have been identified. The syntheses and molecular structure determinations of complexes of phosphoranes with borane have been reported. The molecular structure of MejP.BCls has been determined by gas electron diffraction. ... 

The compounds 93 (R = Me, Et) are extremely sensitive toward oxidation and moisture and were not isolated in a pure state. Their structures were supported by NMR spectroscopy. Treatment of 93 with pyridine also gives rise to pyridine-l-boraadamantane complex 111 and starting acetylene derivative, via 1,1-deorganoboration. Accordingly, the 1-boraadamantane-TMEDA adduct (TMEDA - bis(dimethylamino)ethane) (along with Me3SnC CSnMc() is formed when the tricyclic borane 93a (R=Me) reacts with TMEDA <2003JOM(687)108>. 

The BH3 molecule is not stable as a separate entity. This molecule can be stabilized by combining it with another molecule that can donate a pair of electrons (indicated as ) to the boron atom to complete the octet (see Chapter 9). For example, the reaction between pyridine and B2H6 produces C5H5N BH3. Another stable adduct is carbonyl borane, OC BH3 in which a pair of electrons is donated from carbon monoxide, which stabilizes borane. In CO, the carbon atom has a negative formal charge, so it is the "electron-rich" end of the molecule. Because the stable compound is B2H6 rather than BH3, the bonding in that molecule should be explained. 

In a novel reaction, the ester (18 Scheme 8) was heated with NaBH4 in pyridine to 115 C. The reaction was stopped at the ketal stage (19) so that overreduction to the alcohol was not possible. Overall yields of about 50% were reported but it might be noted that the other product of the reaction was the py-ridine/borane adduct C5H5N BH3, so that only a quarter of the available hydride was used in the reduction. 

Pyridine and methyl(methylthio)boranes, BMe (SMe)3- , form adducts which are more stable than the corresponding NMe3 adducts. The acidity of these boranes decreases with an increased number of SMe groups.246... 

Tetracoordinate Boron in Pyridine/Borane Lewis Base Adducts... 

In comparison with the A-nnsnbstitnted pyridine the hyperpolarizabilities by insertion of the Lewis acidic borane increase by a factor of approximately two. The (3 values determined by EFISH experiments ranged between 30 and 72.5 x 10 ° esn for the pyridine/BFj and BlCgFjlj Lewis adducts. 

In 2001, Su et al. confirmed this tendency by calculation of pyridine, styryl pyridine, and phenyl-ethynyl pyridine/borane adducts using the quantum chemical AMl/Finite Field method. These theoretical data are in good agreement with the experimental values determined by Lesley et al., 1998. 

In addition to tricoordinate boron which acts as a strong electron acceptor because of its vacant p-orbital, negatively charged electron-donating tetracoordinate boron in two variations, zwitterionic molecules and as pyridine/borane Lewis base adducts, shows interesting nonlinear optical properties. 

Theoretical and experimental investigations of Lewis base adducts of boranes with pyridine derivatives with an overall neutral net charge show that they possess large hyperpolarizability values. 

M. J. G. Lesley, A. Woodward, N. J. Taylor, T. B. Marder, I. Cazenobe, I. Ledoux, J. Zyss, A. Thornton, D. W. Bruce, A. K. Kakkar, Lewis acidic borane adducts of pyridines and stilbazoles for nonlinear optics. 

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