Pyridines protonation shifts

Small shift values for CH or CHr protons may indicate cyclopropane units. Proton shifts distinguish between alkyne CH (generally Sh = 2.5 - 3.2), alkene CH (generally 4, = 4.5-6) and aro-matic/heteroaromatic CH (Sh = 6 - 9.5), and also between rr-electron-rich (pyrrole, fiiran, thiophene, 4/ = d - 7) and Tt-electron-deficient heteroaromatic compounds (pyridine, Sh= 7.5 - 9.5). 

BioH14 is a mdo-polyborane, therefore, it is expected that it forms adducts with Lewis bases. This is indeed the case but with concomitant loss of hydrogen as shown in Eq. (51). These adducts of the type BioHi2-2L (L = amines, pyridine, phosphines, nitriles, dialkylsulfides inter alia) proved to be versatile reagents. For instance, when triethylamine is used to replace acetonitrile from the adduct not only does the expected replacement occur but in preference also a proton shift (most likely prior to the base displacement reaction) with cluster closure to the decahydro-doso-decaborate(2—) (Eq. 52) ... 

In nitrogen heteroaromatics, upfield protonation shifts are found for carbons a to nitrogen, while those in / and y positions are deshielded on protonation [94, 99,100]. This is shown in Fig. 3.5 for quinoline [94]. The protonation shifts for C-/1 and C-y can be rationalized in terms of the cannonical formulae of protonated pyridine [73 d], while the upfield shifts for C-a are probably due to the lower n character of the N — C-a bond. The curves in Fig. 3.5 representing the pH dependence of 13C shifts resemble titration curves. pK values and, in the case of amino acids, the isoelectric points pi can be obtained from the point of inflection of the (5 versus pH plot for each individual carbon [84, 94, 98]. 

Protonation shifts of pyridine (Table 4.67) are much stronger but follow the trend of dilution shifts portrayed in Fig. 4.12. Shielding in the a position is attributed to a change of the N —C-a bond order. Deshieldings at C-3 and particularly at C-4 arise from an increased electron withdrawal of the positively charged nitrogen. Shift changes induced... 

Methyl groups attached to the pyridine ring are not usually involved in reactions with MP, but a minor product from 4-methylpyridine is the dihydropyridine 102.291 It could be formed via a zwitterion corresponding to 95, proton transfer giving 101, addition to another molecule of MP, and a further proton shift. 

Agreement between theoretical and experimental TJT2 values is much poorer for X = Br and I and indicates a breakdown of the Redfield theory. (32) Isotropic proton shifts for pyridine-N-oxide and y-picoline-A-oxide protons have been reported in the 5-coordinated adducts of these bases with bis(di-p-tolyl-dithiophosphinato)-Co(ii) and -Ni(ii). (70,71) Dipolar shifts have been evaluated and indicate that the Co(ii)-pyridine-N-oxide adducts have a bent structure in solution with Co-O-N angle of 125°. Results indicate that a rr-spin delocalization mechanism is operating, and INDO calculations suggest that the highest bonding orbital is involved in the spin transfer process. 

For comparison with amphotericin B, Brown and Sidebottom [14] recorded the proton NMR spectrum of natamycin in d -dimethyl sulphoxide and of N-acetylnatamycin in pyridine. Chemical shifts of assigned protons are given. 

The yields of the pyrimidines 147 and the pyridines 151 are not high and vary greatly, depending on the structure of the starting materials. The latter can also determine the nature of the product. Thus, a mixture of 2,4-dimethyl-6-phenylpyrimidine, 2,4-dimethyl-4,6-diphenyl-4//-l,3-oxazine and 1-chloro-l-phenylethene was obtained by refluxing a 1 3 3 ratio of the acetophenone-benzonitrile-phosphorus oxychloride mixture, followed by treatment of the crude product with sodium carbonate solution .It has been shown that addition of an additional ketone molecule to the iminium intermediate is an alternative to the formation of the A -acylenamines and this route is catalyzed by aluminum chloride. Auricchio and coworkers believe also that the reaction of ketones with nitriles catalyzed with both protic and Lewis acids must be considered as a Ritter reaction (see, however, Section V.C). The cationic intermediate 152 thus formed can undergo either a proton shift giving enamide 153 or addition of another ketone molecule... 

A The fact that 20 reacts faster than 21 or 22 suggests that the first step may be the abstraction of a proton from the carboxylic acid group by the nitrogen atom to give the zwitterion 23 (reaction 4.40). This could be followed by loss of carbon dioxide from the car boxy late group to give 24. which would rapidly be converted into pyridine by a proton shift. 

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