Novel Polyfunctional Tautomeric Systems Containing Salicylideneamino and Proton Sponge Moieties

Abstract

The first salicylidenearylamines containing a proton sponge residue with 5′‐bromo (6), 5′‐nitro (7), and 3′,5′‐dinitro (8) substituents have been synthesized. X‐ray diffraction and 1H, 13C, and 15N NMR and infrared spectra revealed that 6 and 7 contain very strong OHN bridges typical of o‐hydroxy Schiff bases. Additional π‐electron coupling with the dimethylaminonaphthyl ring is manifested both in geometrical and infrared parameters. In the case of 7, proton transfer to the imine nitrogen atom occurs in the solid state, accompanied by a remarkable shortening of the OHN bridge (2.553 Å). In dinitro derivative 8, the OH proton jumps to the dimethylamino groups, giving a new tautomeric form with the formation of a homoconjugated [NHN]+ cation. Protonation of 6 and 7, affecting the (dimethylamino)naphthyl moiety, disrupts the π‐coupling and makes these molecules, unlike common aromatic Schiff bases, highly sensitive to hydrolysis, while 8, being already self‐protonated, equilibrates in wet DMSO with the corresponding salicylaldehyde and arylamine in a ca. 1:1 ratio at room temperature.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Schiff Bases from TRIS and ortho‐Hydroxyarenecarbaldehydes: Structures and Tautomeric Equilibria in the Solid State and in Solution

“…Most Schiff bases derived from salicylaldehydes and alkyl/arylamines show imine structures, [1][2][3][4] though some derivatives have been reported with enamine skeletons in the solid state. These arise from both aliphatic [1,5] and aromatic amines.…”

“…These arise from both aliphatic [1,5] and aromatic amines. [3,6,7] Structural assignments have been frequently revisited in recent years. Thus, the adduct between levels.…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…[183][184][185][186][187][188] Photochromism and thermochromism of Schiff bases occur due to their ability to undergo protona nd electron transfer from one atom through the RAHB system, that is, reversible tautomerism between enol and keto forms caused by heat or electromagnetic radiation, making these compoundsp otential candidates for optical switchesa nd storage devices. [201,202] The enol form is more stable than the keto and zwitterionic forms, owing to the loss of aromaticity and to RAHB-assisted delocalization, respectively,i nt he latter forms. [193][194][195][196][197][198][199][200] The structures of keto and zwitterionic forms can be regarded as extreme canonical forms of ar eso-nance hybrid, although experimentally derived bond lengths reflect the predominance of one form.…”

“…[193][194][195][196][197][198][199][200] The structures of keto and zwitterionic forms can be regarded as extreme canonical forms of ar eso-nance hybrid, although experimentally derived bond lengths reflect the predominance of one form. [199][200][201][202] Electron delocalization in these systems, which are susceptible to proton transfer,r epresents au seful concept to rationalize the structure and reactivity of av ariety of Schiff bases. In fact, most Schiff bases derived from salicylaldehydes and alkyl/arylamines adopt imine structures (enol), [189-192, 199, 201] but some derivatives have been reported with enamine skeletons (keto and zwitterionic forms)i nt he solid state.…”

A Silylene–Borane Lewis Pair as a Tool for Trapping a Water Molecule: Silanol Formation and Dehydrogenation

“…Similar observations were reported for other compounds exhibitingh ydrogen-bonded OÀH···X moieties (X:h ydrogen-bond acceptor) with stretching frequencies in the range of ñ = 2800-1600 cm À1 . [15] The 1 HNMR spectrum of 3a in [D 8 ]THF exhibits ab roadened resonance at d = 11.77 ppm at room temperature for both protons of the hydrogen bonds indicating ad ynamic process. When the sample was cooled to 203 K, the signal separates into two singlets at d = 11.93 and 11.47 ppm suggesting ad ouble minimum potential with as mall energy barrier for each hydrogen bond.…”