Carbenoid Reactions of Dimethyl Diazomalonate with Aromatic Thioketones and 1,3‐Thiazole‐5(4H)‐thiones

Dimethyl diazomalonate (4) and thiobenzophenone (2a) do not react in toluene even after warming to 50°. After addition of catalytic amounts of Rh2(OAc)4, a smooth reaction under N2 evolution afforded a mixture of thiiranedicarboxylate 5 and (diphenylmethylidene)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 ‘thiocarbonyl ylide’ 17 via carbene addition to 15, followed by 1,3‐dipolar cycloaddition with 2b and 2c, respectively.