Solid-State and Solution Structural Studies of 4-{[C(E)]-1H-Azol-1-
ylimino)methyl}pyridin-3-ols
by Dionisia Sanz*
a
), Almudena Perona
a
), Rosa M. Claramunt
a
), Elena Pinilla*
b
), M. Rosario Torres
b
),
and José Elguero
c
)
a
) Departamento de Química Orgánica y Bio-Orgánica, Facultad de Ciencias, UNED, Senda del Rey9,
ES-28040 Madrid (fax: + 34 913988372; e-mail: dsanz@ccia.uned.es)
b
) Departamento de Química Inorgánica, Laboratorio de Difracción de Rayos X, Facultad de Ciencias
Químicas, Universidad Complutense, ES-28040 Madrid (e-mail: epinilla@quim.ucm.es)
c
) Instituto de Química Médica (CSIC), Centro de Química Orgánica ‘Manuel Lora Tamayo’, Juan de la
Cierva 3, ES-28006 Madrid
The new N-salicylideneheteroarenamines 1 – 4 were prepared by reacting the biologically relevant 3-
hydroxy-4-pyridinecarboxaldehyde (5) with 1H-imidazol-1-amine (6), 1H-pyrazol-1-amine (7), 1H-1,2,4-
triazol-1-amine (8), and 1H-1,3,4-triazol-1-amine (9). Solution
1
H-,
13
C-, and
15
N-NMR were used to
establish that the hydroxyimino form A is the predominant tautomer. A combination of
13
C- and
15
N-
CPMAS-NMR with X-ray crystallographic studies confirms that the same form is present in the solid
state. The stabilities and H-bond geometries of the different forms, tautomers and rotamers, are discussed
by using B3LYP/6-31G** calculations.
1. Introduction.– N-Salycilidenearenamines (salicylidene = (2-hydroxyphenyl)-
methylene) show hydroxyimino A/oxoenamino B tautomerism, due to a proton-trans-
fer process both in solution and in the solid state (Scheme 1) [1][2].
It has been established that in aromatic Schiff bases of o-hydroxybenzaldehydes, the
equilibrium, normally favoring A, is shifted toward tautomer B when increasing the
polarity of the solvent or the electron-withdrawing ability of the substituent R [1].
However, the NH form B is rarely found [2] [3].
Recently, we have been involved in the study of aromatic Schiff bases derived from
3-hydroxypyridine-4-carboxaldehyde (5) in an attempt to model the H-bonded
structure of the cofactor pyridoxal-5’-phosphate (= 3-hydroxy-2-methyl-5-[(phos-
phonooxy)ACHTUNGTRENNUNGmethyl]pyridine-4-carboxaldehyde) intervening in various enzymatic trans-
formations of amino acids [4]. As a continuation of that project, we decided to exploit
Scheme 1
© 2006 Verlag Helvetica Chimica Acta AG, Zürich
Helvetica Chimica Acta – Vol. 89 (2006) 1290