Dimethyl diazomalonate (4) and thiobenzophenone (2a) do not react in toluene even after warming to 50". After addition of catalytic amounts of Rh,(OAc),, a smooth reaction under N, evolution afforded a mixture of thiiranedicarboxylate 5 and (diphenylmethy1idene)malonate 6 (Scheme 2 ) . A reaction mechanism via an intermediate 'thiocarbonyl ylide' 7, formed by the addition of the carbenoid species 8 to the S-atom of 2a, is plausible. Similar reactions were carried out with 9H-xanthene-9-thione (2b), 9H-thioxanthene-9-thione (2c, Scheme 4 ) , and 1,3-thiazole-5(4H)-thione 18 (Scheme 6 ) . In the cases of 2b and 2c, spirocyclic 1,3-dithiolanetetracarboxylates 14a and 14b, respectively, were obtained as the third product. Reaction mechanisms for their formation are proposed in Scheme 5 : S-transfer from intermediate thiirane 12 to the carbenoid species yielded thioxomalonate 15 which underwent a 1,3-dipolar cycloaddition with 'thiocarbonyl ylide' 16. An alternative is the formation of 'thiocar-bony1 ylide' 17 via carbene addition to 15, followed by 1,3-dipolar cycloaddition with 2b and 2c, respectively.
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Stay at the Institute of Organic Chemistry of the University of Zurich (March-June 1995) as a Swiss Federal Scholar (Bundesstipendiat). Recently, the primary, stable cycloadduct of a sterically crowded diazo amide and 2,2,4,4-tetramethyl-3thioxocyclobutanone could be isolated [15].
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Unlike reactions of carbenes and carbenoids with other C=X bonds (X = C, N, 0), reactions with thiocarbonyl groups are very scarcely described (cl: [17]).