Piperidines shifts 274

Each ring contains 6 rr electrons. 20. (a) Via an aryne (b) direct displacement accompanies elimination-addition. Fluoride least reactive toward benzyne formation (p. 838), most reactive toward direct displacement (Sec. 25.12). Piperidine shifts equilibrium (1) toward left, tends to inhibit benzyne formation. 21. UU is aromatic, with 14 n electrons. Methyl protons are insii/e aromatic ring see Fig. 13.4, p. 419. 

A nitrogen atom at X results in a variable downfield shift of the a carbons, depending in its extent on what else is attached to the nitrogen. In piperidine (45 X = NH) the a carbon signal is shifted by about 20 p.p.m., to ca. S 47.7, while in A-methylpiperidine (45 X = Me) it appears at S 56.7. Quaternization at nitrogen produces further effects similar to replacement of NH by A-alkyl, but simple protonation has only a small effect. A-Acylpiperidines show two distinct a carbon atoms, because of restricted rotation about the amide bond. The chemical shift separation is about 6 p.p.m., and the mean shift is close to that of the unsubstituted amine (45 X=NH). The nitroso compound (45 X = N—NO) is similar, but the shift separation of the two a carbons is somewhat greater (ca. 12 p.p.m.). The (3 and y carbon atoms of piperidines. A- acylpiperidines and piperidinium salts are all upfield of the cyclohexane resonance, by 0-7 p.p.m. 

C chemical shifts, 2, 15 Piperidine, 2-aIkyI-JV-nitro-conformation, 2, 160 Piperidine, 4-amino-synthesis... 

Piperidine, l-(2-hydroxythiobenzoyI)-neutron diffraction, 2, 116 Piperidine, 4-hydroxy-2,2,6-trimethyI-as local anaesthetic, 1, 179 Piperidine, JV-methoxycarbonyl-electrolytic oxidation, 2, 374 Piperidine, 2-methyl-synthesis, 2, 524 Piperidine, 3-methyI-mass spectrometry, 2, 130 Piperidine, C-methyl-NMR, 2, 160 Piperidine, JV-methyl- C chemical shifts, 2, 15 catalyst... 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

Absorption maxima for a wide range of heterocyclic systems are shown in Figure 1.5.2 When the indolyl residue 8a is replaced by other heterocyclic residue, a somewhat small shift in the Xmax occurs. Replacement with a benzothiazoline residue, 8c, results in a bathochromic shift. Comparison between saturated heterocycles 8d-8f and the corresponding benzoderiv-atives 8a-8c shows that the conjugation produced by the benzene nuclei causes a bathochromic shift (ca. 20-50 nm). Replacement of saturated five-membered heterocycles by saturated six-membered heterocycles results in a hypsochromic shift. In the case of the piperidine series (8g) a significant hypsochromic shift occurs, due to steric hindrance in the colored form. 

Extension of -conjugation from naphthalene to anthracene and phenanthrene has a small effect on the tmax of the photomerocyanine form. Replacement of the indoline ring with piperidine, benzoxazole, or benzo-thiazole83 has resulted in hypsochromic shift by ca. 10 nm.72... 

In this way piperidine is converted into the hydrocarbon pipery-lene , a-methylbutadiene. The cause of the shift in the double bond is the same as in the rearrangement of eugenol to isoeugenol and of /S- to a-dihydromuconic acid. Formulate this degradation reaction with f -methylpyrrolidine. 

The structure of bis(piperidine)-aPy5-tetraphenylporphinatoiron(ii) shows the complex to have an octahedral [FeN ] core. The 4 1 complex between 4-methylpyridine and iron(ii) phthalocyanine has only two 4-Mepy groups co-ordinated to the metal in a rrfl/j.< -octahedral arrangement.The dimerization of iron(ii) phthalocyanine in DM SO has been studied and iron(ii) phthalocyanines have been used as n.m.r. shift reagents for amines. Although... 

Comparison with axial ligand complexes formed by Ni porphyrins in coordinating solvents (e. g. pyrrolidine, piperidine, and pyridine), however, shows a clear distinction between the frequencies of the "form 5" marker lines of the reconstituted proteins and those of the 6-coordinate models (9). The shifts of the 6-coordinate species relative to the 4-coordinate species are larger than for the form labeled 5 (Figure 1) of the proteins. The shifts for the 6-coordjnate models are about -41 cm ( iq)> i cm (i/g), and -41 cm (i/ ) for the core-size markers ana -12 cm... 

C-NMR spectral data have been recorded for vochysine (7) (5) and the Buchenavia alkaloids (8-16) (6) (Table V). In both cases comparison of the spectra with those of the parent flavonoid enabled the identity of this part of the molecule to be established. The chemical shifts of the A ring carbon atoms in some of the Buchenavia alkaloids have also been used to determine the point of substitution of the piperidine ring as described above, e.g., for buchenavianine (8). 

These conclusions are reinforced by measurement of natural abundance 15N chemical shifts in piperidines and decahydroquinolines (77JA8406,78JA3882,78JA3889). Lack of correlation between 13C shifts of cyclohexanes and 1SN shifts of piperidines bearing the same methyl substituents are attributed to, among other factors, solvent effects and the difference between H-lone pair and H-H interactions. Protonation served to cancel these stereoelec-tronic effects. Correspondence between 1SN shifts in N- and C- methyl substituted piperidines and decahydroquinoline hydrochlorides and the analogous 13C values were, however, generally much closer than for saturated aliphatic amines. 

The orientation of lone electron pairs at heteroatoms can affect H and 13C chemical shifts substantially. This has been studied in conformationally rigid cyclic amines. Protons in piperidines are shielded significantly if they are antiperiplanar to the nitrogen lone pair70-72. 

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