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cis ‐ trans ‐and Intramolecular enol‐enolic equilibrium of β‐ketoaldehydes
Author(s) -
Noy R. S.,
Gindin V. A.,
Ershov B. A.,
Kol'tsov A. I.,
Zubkov V. A.
Publication year - 1975
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270070302
Subject(s) - chemistry , enol , intramolecular force , tautomer , solvent , medicinal chemistry , pyridine , hydrogen bond , photochemistry , stereochemistry , molecule , organic chemistry , catalysis
Abstract Tautomerism of aromatic β‐ketoaldehydes p ‐XPhCOCH 2 CHO ( 1 , X = NMe 2 , OMe, Me, H, Br, NO 2 ), aliphatic β‐ketoaldehydes and benzoylacetaldehyde RCOCH 2 CHO ( 2 , R = Me, i ‐Bu, t ‐Bu, Ph), RCOCH(Me)CHO ( 3 , R = Me, Et, i ‐Pr) and methyl 2‐formylpropionate MeOCOCH(Me)CHO ( 4 ) has been studied by the 1 H NMR technique. In basic solvents both cis ‐ and trans ‐enol forms of these compounds co‐exist. trans ‐Enolisation, which occurs exclusively at the formyl group, is most favoured in compound ( 4 ) and least favoured in compounds ( 1 ) and ( 2 ). The increasing electron‐attracting property of the substituent X in the aromatic β‐ketoaldehydes ( 1 ), as well as increasing solvent basicity in the series propanediol‐1, 2‐carbonate, acetone < dimethylformamide < dimethylacetamide < pyridine, also shifts the equilibrium towards the trans ‐enol form. The trans ‐enol form is absent in aprotic solvents of low basicity such as CCl 4 , C 2 HCl 3 and toluene. The thermodynamic parameters of the cis ‐ trans ‐enol (C ⇌ T) and cis ‐enol‐enolic (C ⇌ C') equilibria have been estimated from the temperature dependences. The transition from the cis ‐to the trans ‐enol form is accompanied by an entropy decrease of about 10 cal mol −1 degree −1 . Nevertheless the trans ‐enol form is stabilised due to its lower enthalpy. The cis ‐ trans ‐enol equilibrium is determined by the relative strength of the intramolecular hydrogen bond in the cis ‐enol form and the intermolecular hydrogen bonds with basic solvent molecules of the trans ‐enol form. The enthalpy difference of the two cis ‐enolic forms does not exceed 1.0 kcal/mol, in rough agreement with the data calculated by the CNDO/2 approximation. Polar solvents favour the hydroxymethyleneketone form (C) for both groups of compounds 2 and 3 . The content of the hydroxymethyleneketone form is about the same within series 2 where R = Me, i ‐Bu, Ph and is a little higher for the t ‐Bu derivative. A decrease of temperature only slightly shifts the equilibrium of compounds 1 and 2 to the hydroxymethyleneketone form, while in the case of 2‐methyl‐β‐ketoaldehydes (3) this effect is markedly pronounced.