Imino shift

Intramolecular additions of carbon-centered radicals to the A-terminus of imines are reported. Frey studied the mechanism of the interconversion of L-a-lysine to L-) -lysine catalyzed by lysine 2,3-aminomutase [69]. They proved the feasibility of a radical mechanism involving a 3-exo cyclization of a primary radical to the N=C bond followed by ring opening to the most stable radical by performing the reaction under tin hydride conditions (Scheme 24). This overall process corresponds to a 1,2-imino shift. 

Nuclear magnetic resonance spectra of 2-aminothiazole and of 2-imino-4-thiazoline were reported during the studies related to protomeric equilibria (125-127) ring protons in the former are centered at 6.48 and 7.14 ppm (internal Me4Si), while those in the latter are shifted upheld to 5.8 and 6.5 ppm (125). 

Characteristic NMR shifts for 2-amino and 2-imino derivatives are given in Table V]-5. 

Table 2.3 shows very obvious parallels with the TMS scale of C shifts. Thus, the shifts (Table 2.3) decrease in size in the sequence nitroso, nitro, imino, amino, following the corresponding behaviour of the shifts of carbonyl, carboxy, alkenyl and alkyl carbon atoms (Table 2.2). 

The tautomeric equilibria of 2-aminoimidazoline-5-thiones 238 are shifted to the imino forms 238b when R is an electron-withdrawing group (Scheme 82) [96CHEC-II(3)77],... 

A detailed study of spectra of compounds 1, 2, and 3 has been published as part of a general study of azolopyridines (84OMR209). The shifts are shown in Table III. The N shifts have been used to determine the structure of 7-amino-triazolopyridines 128 and 129 (89T7041). The shifts recorded were 56.8, 56.2 (Nl), 245.4, 246.3 (N2), 320.6, 316.8 (N7a), all from nitromethane as standard at 380 ppm the absorption for the amine was at 345.5, 350 ppm in accordance with the amino structure shown, rather than the imino forms 128a and 129a. 

Aminoquinoxaline exists predominantly as such rather than in the tautomeric imino form. This is indicated by a comparison of the basic strength of the 2-amino compound (pKo 3.90) and those of its fixed methylated tautomers, 2-dimethylaminoquinoxaline pKa 3.72) and l-methylquinoxalin-2-oneimine (pfCn 8.70). The ultraviolet spectrum of the neutral molecule of 2-dimethylaminoquinoxaline shows the expected bathochromic shifts compared to that of 2-aminoquinoxaline these spectra differ from the ultraviolet spectrum of the neutral molecule of l-methylquinoxalin-2-oneimine (Fig. 1). The mono-cations (68) and (69) derived from 2-aminoquinoxaline and l-methylquinoxalin-2-oneimine have a similar chromophoric system and show almost identical ultraviolet absorption (Fig. 2). 

Synthesis of complex 1. The pentadentate salen catalyst 1 was synthesized as described (9). In short, the tosylated 2-[2-(2-methoxyethoxy)-ethoxy]-ethanol 2 (10) was reacted with 2,4-dihydroxybenzaldehyde 3 to yield 4-alkoxy salicylaldehyde 4 after chromatographic purification (eq. 1). Subsequent condensation of 4 with 1,3-diaminopropanol yielded water-soluble salen ligand 5 in sufficient purity and 89% yield (11). The formation of an azomethine bond is indicated by a shift of the NMR signal for the carbonyl carbon from 194.4 ppm in aldehyde 4 to 166.4 ppm for the imino carbon in 5. The pentadentate ligand 5 was then treated with copper(ll) acetate in methanol to obtain the dinuclear copper(ll) complex 1 as a green solid (eq. 2) (11). 

Like most dyes, substituents on di- and triphenylmethane dyes have a significant effect on the absorption spectra. Acetylation of the imino nitrogen of auramine O (32) results in shift to longer wavelength (Eq. 3). A... 

Recently, Bafquiren and Eddine have studied hindered Schiff base derivatives of fra/ s-l,2-diaminocyclohexane23 The differences in chemical shifts found for imino carbons have indicated the presence of anti- and syn-geometric configurations. The intense NOESY effects between protons of naphthyl and alkyl units have proved the anti-configuration of the major stereoisomer. 

It is apparent that the B-N bond in imine-borane adducts is not very strong and is influenced by the donor and acceptor properties of the components as well as by steric factors. In the infrared spectra of the adducts the ON stretching bands show only slight shifts as compared with those of the free imino derivatives 23>44). 

Finally, we note that Karelson et al. [295] have used the D02 model with small cavity radii to consider aqueous solvation effects on other tautomeric equilibria of substituted pyridines. In particular, they examined methyllmethylene, amino/imino, hydroxy/oxo, and mercapto/thiono substitution at the 2-, 3, and 4-positions of pyridine. They observed methyl/methylene equilibria to be only slightly perturbed by aqueous solvation. Amino/imino equilibria were slightly more perturbed, followed by hydroxy/oxo equilibria. Mercapto/thiono equilibria were very significantly affected by aqueous solvation Karelson et al. predicted pK shifts of up to 16 units. This sensitivity of the thiono group to solvation is also discussed in the next section. Overall, the tautomeric equilibria of 3- and 4-substituted pyridines were more sensitive to aqueous solvation than were those of 2-substituted pyridines. 

Tab. 6.2 Calculated principle values nucleic acid bases and base pairs and orientations of chemical shift tensors of the imino nitrogens in...

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