Russian Journal of Organic Chemistry, Vol. 40, No. 4, 2004, pp. 462–466. Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 4, 2004, pp. 494–497. Original Russian Text Copyright © 2004 by Filimoshkin, Kosolapova, Petrenko, Aksenov, Poleshchuk. 1070-4280/04/4004-0462 © 2004 MAIK “Nauka/Interperiodica” New Type of Prototropic Tautomerism Involving Carbon, Hydrogen, and Oxygen Atoms A. G. Filimoshkin 1 , V. F. Kosolapova 2 , T. V. Petrenko 2 , V. S. Aksenov 3 , and O. Kh. Poleshchuk 4 1 Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia e-mail: poly@ipc.tsc.ru 2 Institute of Petroleum Chemistry, Siberian Division, Russian Academy of Sciences, Tomsk, Russia 3 Kursk State Technical University, Kursk, Russia 4 Tomsk Pedagogical University, Tomsk, Russia Received April 27, 2003 Abstract—A low-molecular weight analog of the vinyl chloride–maleic anhydride copolymer, chloro- methylsuccinic anhydride gives rise to a new type of prototropic tautomerism, cyclic anhydride–enol, with formation of quasiaromatic enol and dienol derivatives. The ratio of the anhydride and enol tautomers in solution at room temperature is 7 : 1. Scheme 1. 3 4 1 O 2 2 O 1 5 O 3 Cl 3 4 1 O 2 2 O 1 5 O 3 Cl 3 4 1 O 2 2 OH 1 5 O 3 Cl H H Ia Ib Ic While studying the properties of vinyl chloride– maleic anhydride copolymer, we have revealed and described a new type of prototropic reactions, namely cyclic anhydride–enol tautomerism, which spontane- ously occurs in the macromolecules in organic solvents [1]. By physical and chemical methods we have found that the copolymer molecules in the condensed state consist of 67 mol % of alternating monomer units and 33 mol % of the corresponding enol tautomers [2]. Just the presence of such tautomeric ensembles in the polymeric chains is responsible for the anomalous reactivity of the copolymer [3]. It was important to elucidate whether the observed pattern originates from conformation–configuration effects in the macromole- cules or it is also intrinsic to the respective low- molecular analogs. Therefore, the goal of the present study was to synthesize chloromethylsuccinic anhy- dride (I) as a low-molecular model of the regular unit of the vinyl chloride–maleic anhydride copolymer and examine its ability to undergo prototropic tautomerism of the above type. We presumed that, like vinyl chloride–maleic anhydride copolymer [4], compound I in solution gives rise to cyclic anhydride–enol tautomerism and that the tautomeric equilibrium involves anhydride form Ia, enol Ib, and dienol Ic, as shown in Scheme 1. Tautomers Ib and Ic are stabilized via intramolecular hydrogen bond O–H · · · Cl. The second O 1 –H group in tautomers Ic is involved in intermolecular hydrogen bond with the solvent molecules. In order to estimate the possibility for such tautomeric equilibrium, we performed quantum- chemical calculations of the geometric structure and energies of formation of tautomers Ia–Ic (see table). The results showed that the formation of tautomers Ib and Ic is accompanied by reduction of the electronic energy by 9 kJ/mol, which corresponds to the energy of stabilization due to intramolecular hydrogen bonding H · · · Cl. The H · · · Cl distances in Ib and Ic conform to the structural criterion for closure of such bonds [5]: these distances are shorter than the sum of the corresponding van der Waals radii (0.30 nm) [6].