Structure of 2-(N,N-dimethyliminio)-2-phenyldithioacetate, a probable intermediate of the Willgerodt-Kindler reaction

Abstract

C10HllNS2, M,=209.3, orthorhombic, Pbca, a -- 11.662(1), b -- 11.709 (1), c = 15.430 (1) A, V = 2107.0 (3) A3, z = 8, Dx = 1.32 g cm- 3, A(Mo Ka) = 0.71073 A,/z = 4.4 cm- 1, F(000) = 880, T= 293 K, R=0.043 for 1246 unique observed reflections. The structure of the title compound, a probable intermediate of the Willgerodt-Kindler reaction, was solved by direct methods. The observed molecular geometry confirms the postulated zwitter- ionic structure. Introduction. In organic chemistry the Willgerodt- Kindler reaction has been known for a long time and successfully used to prepare co-arylalkane carboxylic acids via their thioamides from aryl alkyl ketones with the same number of C atoms (Krauch & Kunz, 1976). The mechanism of this reaction is rather complicated and despite much experimental work it has not been elucidated until now. As a primary step the formation of a thioketone or a (multiple) thio- lation of the methyl group are discussed in the literature (Wegler, Kfihle & Sch/ifer, 1958; Asinger, Sch/ifer, Halcour, Saus & Triem, 1963). But all attempts to identify, let alone to isolate, correspond- ing intermediates were without success. The function of hydrogen sulfide as a redox mediator is common to most of the mechanistic considerations (Mayer & Funk, 1963). Therefore, the isolation of potential intermediate products of the Willgerodt-Kindler reaction may possibly succeed if the hydrogen sulfide formed by the thiolation of the aryl methyl ketone is quickly removed from the reaction mixture after its formation. This assumption was confirmed by the observation that the thiolation of acetophenone gives a dark-red crystalline product (2) in the presence of dimsulf, i.e. the distillable 1:1 addition compound of dimethylamine and sulfur dioxide which binds * To whom correspondence should be addressed. 0108-2701/92/122161-03506.00 hydrogen sulfide immediately and completely. Because (2) reacts with hydrogen sulfide giving sulfur and the Willgerodt-Kindler product N,N-dimethyl- phenylthioacetamide, it may be justly assumed to be an intermediate. Consequently, the constitution and structure of (2) was of particular interest. Based on the results of elemental analysis, some spectroscopic investigations and chemical intuition the isomeric structures (2a) to (2d) came into consideration. In order to reach an unequivocal decision an X-ray analysis of (2) has been performed. ~.._ 0 Sa, Dimsulf