Steric and aromatic impact on intramolecular hydrogen bonds in o‐hydroxyaryl ketones and ketimines

Abstract

X‐ray measurements at 100 K and quantum‐mechanical calculations showed a domination of the enol (OH) form in o‐hydroxyaryl ketones [6‐methoxy‐2‐hydroxyacetophenone (6OMeK), 4‐methoxy‐2‐hydroxyacetophenone (4OMeK), 5‐chloro‐4‐methyl‐2‐hydroxybenzophenone (5Cl4MeK) and 2‐hydroxyacetonaphthone (o‐HAN)], whereas a prevailing proton‐transfer (NH) form was found in o‐hydroxyacetonaphthylimine [2‐(N‐methyl‐α‐iminoethyl)naphthol (o‐HIN)]. The effective mechanism of the reduction in hydrogen bridge length due to steric repulsion in the o‐hydroxyaryl ketones is presented. The following phenomena were found: a decrease in phenol ring aromaticity caused by the proton‐transfer process, dependence of the HOMA index of aromaticity on the HOSE index defining the destabilization of the chelate ring and a dependence of the HOMA index on the lengths of the carbonyl bonds. Moreover, a correlation between phenol ring aromaticity [HOMA(phenol)] and the pseudo‐aromatic ring [HOSE(chelate)] is shown. Correlations between structural parameters of the chelate ring [d(C—O), d(Caryl—Calkyl)] and aromatic rings [HOMA(phenol)] of the o‐hydroxyaryl ketimines and ketones are presented. Copyright © 2005 John Wiley & Sons, Ltd.

Structural and aromatic aspects for tautomerism of (Z)-6-((4-bromophenylamino)methylene)-2,3-dihydroxycyclohexa-2,4-dienone

“…Furthermore, it can be stated that dynamic balance between phenol ring and adjacent chelate ring facilitates obtaining the NH form as previously reported in [1,3,37]. Stabilization of the pseudo-aromatic ring is increased with increasing its HOMA index.…”

“…In other words, while hydroxyl bond elongation causes to dearomatization of the phenol ring, molecular stacking pattern in the crystal structure enforces to hold aromaticity of phenol ring at a relatively high level (HOMA = 0.663). At this point, it should be pointed out that depending on the proton transfer HOMA index of aromatic rings carrying O atom in ortho position may be reduced to 0.310 for o-hydroxyaryl [1] and to 0.5-0.2 for naphthaldimine Schiff-bases [37].…”

“…of the ring involving proton transfer is decreased [1]. HOMA index is a useful tool to describe collectively degree of such bond-length deformations of the ring involving proton transfer.…”

“…The structures of o-hydroxy aromatic Schiff bases have received much attention due to their keto-enol (or NH-OH) tautomerism in recent years [1][2][3][4]. Depending on the type of tautomer, two types of intramolecular hydrogen bond involving a photochemically or thermochemically induced proton are possible, namely OÁÁÁH-N in keto (NH) and O-HÁÁÁN in enol (OH) tautomers.…”

Crystal structure, spectral characterization, molecular modeling studies and structural effects of the proton transfer process for (E)-5-methoxy-2-[(3,4-dimethylphenylimino) methyl]phenol

“…As the names suggest, transfer occurs under the influence of light in photochromic compounds and under the influence of temperature in thermochromic compounds. These interesting properties were discovered by Senier and Shepherd in 1909 and are still being studied today [20,21]. orthohydroxy Schiff base compounds adopt OH (enolimine/benzenoid) [22,23] or NH (keto-amine/ quinoid) [24,25] tautomeric forms in the solid state with reference to the location of the transferred proton.…”