Pyridine ring protonization temperature

In the dihapto mode the pyridine ring can be protonated intermolecularly at nitrogen, or even intramolecularly deprotonated at carbon. The first evidence for metal C—N insertion is the reaction of the metallaaziridine complex (111) with homogeneity LiHBEt3 in THF at low temperature that yields (112) (Scheme 49).251-254 Experiments with carbon nucleophiles (RMgCl, MeLi) in place of LiHBEt3 have provided valuable information to allow discrimination between... 

Rotation barriers have also been looked at (80TL1553) in TV-substituted 2,4,6-trimethyI-pyridinium cations (267). In addition to diastereotopic protons HA and HB, the a-Me and (8-protons are anisochronous as shown by HNMR, as are the a-Me, a-ring and /3-ring carbons, as shown by 13C NMR. This anisochronism appears to be due to hindered rotation about the pyridinium N—C bond (268), the stable conformation being that in which the hydrogen atom on the sp3 C is in the plane of the pyridine ring. Observation of coalescence temperatures in deuterated acetone or pyridine solvent allows calculation of energy barriers to this rotation, and these are as follows (267 R = Me, R = H), 7.1 (267 R = Me, R = COMe) (267 R = Et, R = H), 8.1 kJ mol-1. 

In this same vein, the synthesis of 25m and 2621) has been attempted by initial formation of the cyclic sulfides, then extrusion of sulfur by irradiation in the presence of triethylphosphite. In both cases the limited sample size precluded formation of the desired cyclophanes (25 and 26) by this photolytic procedure. Variable temperature H NMR studies have been performed on 27 and 28 27 has a barrier to conformational inversion of 12.5 kcal/mole, while 28 has a corresponding barrier of 10.4 kcal/mole. Phane 28 exhibits a shift of the aromatic protons to higher field as compared to 29 the most dramatic shift is observed for H-3, which absorbs at 7.72 ppm for 28 and at 8.37 ppm for 29. This shift has been explained in terms of an approximately syn orientation of the pyridine rings in 28. 

In order to contribute to answering the open questions, the influence of hydrogen bonding to and protonation of the pyridine ring of PLP on its enolimine—ketoamine tautomerism has been studied by X-ray crystallography [96] and NMR [97] of solid model systems, and the influence of a polar environment by low-temperature liquid state NMR [98]. In these studies, correlations... 

Conformation in such macrocycles is of most interest when there are interactions between the pyridine nitrogens and other components of the macrocyclic ring. This is demonstrated by the difference in conformational preference between the two isomeric pyridinophanes 55 and 56. In the case of the 2,6-disubstituted pyridinophane 55, the attractive interactions between the pyridine nitrogens and the amino protons of the macrocyclic bridge result in the boat-boat conformation shown. For the isomeric case 56, the chair-boat and boat-boat forms have the same stability as shown by variable temperature NMR spectroscopic studies and ab initio calculations <2002J(P2)393>. 

The reaction does not occur on heating at 310°C for 2 h in the gas phase or in basic medium (e.g., pyridine, aqueous sodium hydroxide) suggesting that the reaction involves the protonation of the double bond. When the hydroxyl group is tertiary, it appears impossible to avoid the competitive dehydration reaction. However, after standing at room temperature for 7 years in a sealed tube, 2-isopropylidene-l-methylcyclobutanol (253) has undergone ring contraction to the extent of 60% (equation 173) " . ... 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

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