Neutral nitrogen bases

Ammonia is the simplest nitrogen base and has a piCaH of 9.24. Any substituent that increases the electron density on the nitrogen therefore raises the energy of the lone pair thus making it more available for proto nation and increasing the basicity of the amine (larger pkTaH). Conversely, any substituent that withdraws electron density from the nitrogen makes it less basic (smaller pK ). 

Effects that increase the electron density on nitrogen 

Ifwe look at the pKan values in the gas phase, we can eliminate the hydrogen bonding contribution and we find the basicity increases in the order we expect, that is, tertiary secondary primary. 

Remember that McaSI, tetramethyl silane, in Garb on NMR resonates at 0.0 p.p.m, This is another cOn equence of siltaanf being more electropositive than carbon—the methyl carbons of TMS are more shielded and So resonate at a smalter chemical shift than other saturated carbons. 

Compare this summary with the one for the stabilization of the conjugate base A on p. 190. In both cases, we are considering the same factors. 

We can increase the electron density on nitrogen either by attaching an electron-releas- ----------- 

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Computational studies concerning theoretical approaches to the intrinsic basicity of neutral nitrogen bases have been reported, including those of phos-phoranimines. The non-ionic phosphazene bases BEMP (112), BTPP (113) and (114, R = Ph) appear to be excellent catalysts for the Michael addition reactions. Thus the yield of the coupling reaction of ethyl isocyanoacetate with l,2-bis(4-bromomethylphenyl)ethane is increased by the addition of the phosphazene base BEMP. Polymer-supported BEMP (P-BEMP) has been applied for the allylation of 2H-benzo[d]l,3-dioxolan-5-ol by allyl bromide. " Cyclodehydration of 1,2 diacylhydrazines by tosyl chloride in the presence of P-BEMP leads to excellent yields of 1,3,4,-oxadiazoles. Addition of P-BEMP also improves the yield of the Hofmann elimination step in the synthesis of tertiary mines using REM resin (polymer-bound acrylate ester). ... 

Schwesinger, R. and Schlemper, H. (1987) Peralkylated polyaminophosphazenes - extremely strong, neutral nitrogen bases. Angewandte Chemie - International Edition, 26, 1167-1170. Kaljurand, I., Rodima, T, Pihl, A. et al. (2003) Acid-base equilibria in nonpolar media. 4. Extension of the self-consistent basicity scale in THF medium. Gas phase basicities of phosphazenes. The Journal of Organic Chemistry, 68, 9988-9993. 

Koppel, I.A., Schwesinger, R., Breuer, T. ef a/. (2001) Intrinsic basicity of phosphorus imines and yhdes a theoretical study. Journal of Physical Chemistry A, 105,9575-9586 Maksic, Z.B. and Vianello, R. (2002) Quest for the origin of basicity initial vs final effect in neutral nitrogen bases. Journal of Physical Chemistry A, 106, 419 30. 

Neutral nitrogen bases such as imidazole and pyridine are quite common axial ligands for iron(lll) porphyrin complexes. These complexes usually adopt six-coordinate low-spin state expressed as [Fe(Por)(L)2] [2]. However, the spin state of these complexes could change to the intermediate-spin state as shown in Fig. 10.1 b if the field strengths of nitrogen bases are weakened to a great extent. 

In positive-mode experiments, Mirza and Chait showed that the charge-state distribution was influenced by the type of anion (conjugate base) that was used to acidify the solution. These authors considered that the anion remained associated with the multiply protonated protein in solution and that charge removal could occur via dissociation of the neutral acid (anion plus proton in tow) in the later moments of the droplet lifetime. Shortly afterwards, LeBlanc et al. proposed that neutral nitrogen bases that were noncovalently attached to gramicidin S peptide molecules also could serve to remove charge as the complex underwent collisions in the gas phase. These conclusions were reiterated by Hiraoka et al. in their work with amino acids. 

The ES-MS behavior of peptides and proteins in the presence of small, neutral nitrogen bases was also examined. Beyond the subtle direct effect on initial solution pH, it was proposed that the desorbed peptides enter into the gas phase with the nitrogen bases stUl attached at specihc protonated sites. These complexes could then dissociate in the gas phase with the fate of the central proton determined by the competition to capture it between the nitrogen base and the basic site on the molecule. Departure of the protonated nitrogen base, of course, will lower the peptide s charge state. 

The modulation of the reactivity by the basket-handle superstructures was mostly investigated for reactions involving a one-unit increase of the negative charge of the porphyrin complex Fe(II)(Fe(I)-, Fe(I) (Fe("0")2-redox reactions, complexation of iron(II) by anionic ligands. However, we will see that significant supramolecular effects also exist for the complexation of iron(II) by neutral nitrogen bases. 

Table 8. Specific effect of the NHCO groups on the coordination of iron(II) by neutral nitrogen bases.

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