Bond stability, boron-carbon

The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

The relatively high stability of carbon-silicon/-boron/-tin bonds allows the metaT to be carried through many heterocyclic syntheses as an inert substitutent some examples are shown below. ... 

Such complexes possess sharply reduced chemical reactivity and consequently they often tend to stabilize the valence state of the acceptor metallic atom. Lithium tetraphenylboronate requires heating in acid solution in order to effect cleavage of the boron-carbon bonds and is quite stable in air toward oxidation. The acceptance of the phenyl anion has satisfied the electronic demands of boron. A direct preparation of analogous alkyl complexes has been realized by heating lithium aluminum hydride with ethylene under pressure 139) ... 

Bauer J, Boucekkine G, Frapper G, Halet J F, Saillard J Y, Zouchoune B, Small boron carbon chains stabilized in rare earth metallic frameworks a bonding analysis , J Solid State Chem, 1997 133 190-194. 

In contrast to this generally high preference for. tyn-products for boron trifluoride mediated reactions between 3-a//c v/-Substituted allylstannanes and aldehydes, //-products are preferred for reactions involving 3-p/ cn>7-substituted allylstannanes. This stereoselectivity was observed for a range of aldehydes, and was explained in terms of the increased propensity for the tin-allylie carbon bond to be polarized when the -substituent is able to stabilize a positive charge so favoring a cyclic transition state73. 

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. 

The stability of the liquid carboranes depends on the substituents R at carbon and boron. The axial (endo) hydrogen atom is acidic and involved in 3c2e bonding to one of the basal boron atoms. In the 1H NMR it exhibits a high field shift near S = —1.4. Deprotonation of 55 with potassium or Bu Li leads to the anion (55-H)-, which is isolobal with C5H5. Reactions of 55 and (55-H) with appropriate metal complexes lead to metallacarboranes with sandwich structures [67, 69],... 

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