Predominant Tautomer Research Articles

The structure and properties of Unsymmetrical-Facial 2,3-Bis[(2-aminoethyl)(methyl)-aminoethyliminol-1-methylpiperazinecobalt(III) perchlorate monohydrate, u-fac-[Co(amam)] (ClO4)3,H2O

X-ray crystallographic analysis of the title complex, which was formed spontaneously from aqueous s-fac-[Co(medien)2]3+ [medien = 2,2?- methyliminodi(ethylamine)] in the presence of charcoal and oxygen, shows that it has the u-fac geometry of the [Co(medien)2]3+ moiety. Two coplanar en-type rings of this moiety are bridged by a new planar ring which is fused through a di-amidine grouping to a piperazine ring. The close planarity of this linked ring system, which results from the conjugated unsaturation and resonance in the di-amidine grouping, provides a powerful force in stabilizing the u-fac geometry. The crystals are monoclinic, spacegroup P21/c with a 10.700(6), b 13.38(6), c 20.076(5) Ǻ, β 98.58(13)�. The structure was refined to an R-factor (on F) of 0.077 by blocked-matrix least-squares techniques. The complex has an acidic amidine proton (pKa 10.3 at 20�) located on the piperazine nitrogen. The 13C n.m.r, spectra of the complex and its deprotonated form are interpreted and indicate that the predominant solution tautomer is the same as that in the solid state. The existence of this complex indicates that it might be possible to prepare the unstable u- fac-[Co(medien)2]3+ isomer.

Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N -arylguanidinium chlorides

The (13)C and (15)N chemical shifts of five N-arylguanidinium chlorides carrying polar substituents, ranging in character from 4-methoxy to 4-nitro groups, have been determined by NMR spectroscopy at the natural-abundance level of (13)C and (15)N in dimethyl sulfoxide solution. Comparison of the (13)C shifts of these salts with those of monosubstituted benzenes shows that the guanidinium group induces an average downfield shift of -5.8 ppm of the resonance of the aryl carbon to which it is attached (C1), an average upfield shift of +4.2 ppm for C2 and C6, and a small upfield shift of +1.9 ppm for C4. The shifts of C3 and C5 are small and erratic relative to the corresponding carbons in monosubstituted benzenes. The (15)N resonances of the guanidinium nitrogens are quite sensitive to electric effects resulting from substitution of polar groups at C4. The (15)N shift of the [unk]NAr nitrogen relative to that of the salts suggests that the predominant tautomer for N-arylguanidines is (H(2)N)(2)C[unk]NAr. The (15)N shifts of the (NH(2))(2) nitrogens correlate rather well with sigma(p) (-) parameters, whereas the shifts of the -NHAr nitrogens seem to correlate only with R values derived from the sigma(p) (-) substituent constants.

Novel heterocycles. 4. Synthesis of the pyrido[2,3‐c]‐1,2‐thiazine ring system

AbstractThe reaction of 2‐chloronicotinonitrile with N‐methylmethanesulfonamide and subsequent treatment with base furnished the novel pyrido[2,3‐c]‐1,2‐thiazine ring system 7. Spectrally, it was determined that the predominant tautomer in solution was the 4‐amino form 7a. Acidic hydrolysis of 7 furnished the 4‐oxo tautomer 8b, compound 8b was alkylated on oxygen with 1‐bromopentane in the presence of sodium hydride. Reaction of 8b with isocyanates occurs at the 3 position to produce carboxanilides 10. Spectral characteristics of these compounds are also discussed.

A novel mass-spectroscopic technique for the investigation of vapour-phase tautomeric equilibria

Comparison of collision-induced dissociation–mass analysed ion kinetic energy (CID–MIKE) spectra of cations found by chemical ionisation ethylation with those of chemical ionisation protonation of fixed ethyl derivatives, allows identification of predominant tautomers in vapour phase equilibria.

Crystal and molecular structure of diazoxide, an antihypertensive agent

Diazoxide, C8H7ClN2O2S, is monoclinic:P21/c,a = 7.209(8),b = 6.610(8),c = 19.263(11) A, β = 91.01(6) °,Z = 4. The structure was solved by Patterson and Fourier methods and refined to anR of 4.2% by the use of 1174 independent observed reflections which were collected on a computer-controlled, four-circle Philips diffractometer (MoKα radiation). The molecular structure shows that the 4H form of 1,2,4-benzothiadiazine-7-chloro-3-methyl-1,1-dioxide is the predominant tautomer. The benzothiadiazine ring is roughly planar with the sulfonyl group perpendicular to the ring. The bond lengths and angles are in good agreement with the average values observed in related structures. There is substantial evidence for double-bond character in the two N(2)−C(3) and C(3)−(4) bonds and for delocalization of π electrons. Short N(4)⋯O(2) and N(2)⋯Cl intermolecular distances (2.92 and 3.19 A, respectively), with appropriate geometries, indicate the presence of hydrogen bonds.

Application of long-range 13C,H nuclear spin-spin coupling constants in the study of imidazole tautomerism in L-histidine, and related compounds.

Three-bond 13C,H nuclear spin-spin coupling constants can be used to estimate the relative populations of the two possible tautomers of the imidazole ring in compounds containing this moiety. The mole fraction, XII, of the 1-H tautomer (i.e. the tautomer having a proton bound to the nitrogen atom at position 1 of the ring, where the ring is numbered according to IUPAC-IUB convention) can be calculated from the following equation: XII congruent to 1.705--0.164 3J(C5,H2)obs. Estimates of the predominant tautomer in L-histamine (both the neutral molecule and the monocation), and the dipeptide L-histidyl-L-glutamic acid indicate that the 1-H form is favored in basic aqueous solution. The results are compared with those obtained by other methods, and show that the present technique can provide a simple, rapid means of determining tautomeric equilibrium in imidazole derivatives.

1,2,4-Triazoles. IV. Tautomerism of 3,5-disubstituted 1,2,4-triazoles.

Tautomerism of ten 3, 5-disubstituted 1, 2, 4-triazoles, which can exist in three tautomeric forms, has been studied. The nuclear magnetic resonance (NMR) and ultraviolet (UV) spectra of these 1, 2, 4-triazoles were compared with those of the N-methyl derivatives, which are the model compounds of the three kinds of tautomeric forms. Results indicate that a tautomeric hydrogen atom of the predominant tautomer of 3, 5-disubstituted 1, 2, 4-triazoles is attached to the nitrogen atom at position 1 or 2 which is closer to the more electron-releasing substituents in the 3 or 5 position. For instance, 3-p-nitrophenyl-, 3-p-chlorophenyl-, and 3-γ-pyridyl-5-methylthio-1, 2, 4-triazole exist predominantly in the 1H form, while 3-p-aminophenyl- and 3-p-methoxyphenyl-5-methylthio-1, 2, 4-triazole exist predominantly in the 2H form. These results are explained in terms of the formation of cyclic dimer-type hydrogen bonds involving the N-1 and N-2 atoms of triazole rings with an exchange of a proton between the molecules. However, 5-methylthio-3-α-pyridyl-1, 2, 4-triazole is exceptional in existing in the 2H form, since this compound can form hydrogen bonds of another type involving the nitrogen atom of the α-pyridyl group.

An MO theoretical study on the tautomerism of carcinogenic 4-hydroxyaminoquinoline 1-oxide and related compounds.

Tautomerism of hydroxypyridines is investigated by a version of the modified intermediate neglect of differential overlap method to show that the predominance of tautomerism depends on the total energy of tautomer. The calculations are carried out for 4-hydroxyaminopyridine 1-oxide (I) and 4-hydroxyaminoquinoline 1-oxide (II), leading to a conclusion that the predominant tautomer of I is formulated to be the N-oxide form and that of II is the N-hydroxy form. This structural difference is discussed in relation to difference in the biochemical behaviours of these compounds.

Systemes aromatiques a 10 electrons π derives de l'aza-3a pentalene—XVIII : Recherches dans la serie du s-triazolo[4,3-b] s-triazole

From electric dipole moment measurements, it has been possible to determine the tautomerism of the s-triazolo[4,3-b] s-triazole and the structure of its N-methyl derivatives. The structure of the predominant tautomer N(H) 1 is in agreement with theory and with the UV and NMR spectroscopic data.

The crystal and molecular structure of 3-methyl-3-pyrazolin-5-one

The crystal structure of 3-methyl-3-pyrazolin-5-one (C4H6N20) has been determined by X-ray diffraction as a model for the pyrazolin-5-one compounds. The space group is P2a/a. Cell dimensions are a= 10.520 (3), b=6.499 (2), c= 8.052 (1)/~, fl= 114.45 (1) °, Z=4, D~= 1.299 (1) g.cm-3. Three-dimensional X-ray data were collected using Mo Ka radiation. The structure was solved using statistical methods of phase determination, and refined by the full-matrix least-squares method to R= 0.071 (unit weights). Hydrogen-bonded layers are found parallel to the ab plane. The predominant tautomer is established as 3-methyl-3-pyrazolin-5-one. A comparison of structures based on data gathered by moving-crystal moving-counter and stationary-crystal stationary-counter methods shows no significant differences in bond lengths and angles calculated from atomic parameters refined on either set of data.

Spectroscopic studies of 2,6-dihydroxypyridine and some of its derivatives. Tautomerism in solution, and very strong hydrogen bonding in solid 2,6-dihydroxypyridine and its N-methylated derivative

Ultraviolet, infrared, and proton magnetic resonance spectra were measured for 2,6-dihydroxypyridine, 6-hydroxy-1-methylpyrid-2-one, α,α,γ-trichloro- and α,α,γ-tribromo-glutaconimide, with special reference to the hydroxypyridone-pyridinediol-glutaconimide equilibrium. The ratio [pyridol]/[pyridone] is much smaller for 2,6- dihydroxy-than for 6-methoxy-2-hydroxy-pyridine, especially in hydroxylic solvents. The hydroxypyridone form is the main or predominant tautomer for 2,6-dihydroxypyridine in water, dimethyl sulphoxide, ethanol, and 5% ethanol-95% cyclohexane, and for the 1- methylated derivative in ethanol. The glutaconimide form is the main or predominant tautomer for 2,6-dihydroxy-pyridine in dioxan, and for the 1-methylated derivative in water, dioxan, and chloroform. In the solid state 2,6-dihydroxypyridine and the 1-methylated derivative show a very broad intense infrared OH band with a plateau extending from c. 1300 cm-1 to, respectively, 800 and 450 cm-1, and with some transmission ?windows?; there is very strong hydrogen bonding, various aspects of which will be discussed. The solids probably consist of 6-hydroxypyrid- 2-one molecules linked by very short unsymmetrical hydrogen bonds; though, for the unmethylated compound, an alternative ?macromolecular? structure, consisting of units in which the heavy-atom skeletons have C2v symmetry, and are linked together by symmetrical hydrogen bonds, cannot be ruled out entirely on the present evidence.

Studies on 1,4-benzoxazines—I : Preparation, structure, reactions and spectral data of some 2 H-1,4-benzoxazin-3-thiones

Several 2 H-1,4-benzoxazin-3-thiones have been newly prepared. UV, IR and NMR studies establish that the predominant tautomer in these potentially tautomeric compounds is the thiolactam form. Methylation gives the S-Me and not the N-Me derivative. NMR spectroscopy is useful in distinguishing between isomeric benoxazine structures and between exocyclic and endocyclic double bonds in the hetero ring. Nucleophilic displacement reactions have been carried out on 2 H-1,4-benzoxazin-3-thione and its S-Me and N-Me derivatives using hydrazine hydrate, morpholine, cyclohexylamine and aniline. In each case NMR has been used to ascertain the structure of the reaction product.

N- and N′-aryl substitution in benzamidines

N-Phenyl substitution of amino- and imino-groups in benzamidines is shown to reduce basic strength by factors of ca. 10 and 1000, respectively. The configuration of the imino-N-phenyl ring is discussed. The structure of the predominant tautomer of N-(3-diethylaminopropyl)-N′-phenylbenzamidine is indicated.

Die Amid‐Iminol‐Tautomerie

AbstractThe prevailing uncertainty regarding the predominant tautomer in cases of potential amide‐iminol (lactam‐lactim) tautomerism is attributable mainly to the finding that the ultra violet absorption of certain amides resembles that of corresponding imino ethers more closely than that of N, N‐disubstituted amides.