The interaction of 1,4-diketones with thiazyl chloride (N ≡ SCl)

Thiazyl chloride (N ≡ SCl) thermally derived from the preformed trimer and that made from the interaction of urethane, thionyl chloride and pyridine in benzene (Katz reagent) react somewhat differently with dibenzoyl –ethane, -ethene (E and Z) and –ethyne. In all cases, 3,4-dibenzoyl-1,2,5-thiadiazole is formed but with Katz reagent a by-product is also formed, 5-benzoyl-3-phenylisothiazole from the ethane and ethyne and 1-ethoxycarbonylamino-1,2-dibenzoylethene from the ethenes. With Katz reagent tetraphenylcyclopentadienone gives 3,4,5,6-tetraphenyl-2-pyridone and its O -ethoxycarbonyl derivative.


Introduction
Katz and co-workers 1 showed that a mixture of ethyl carbamate, thionyl chloride and pyridine in a suitable boiling solvent generates thiazyl chloride [N≡SCl], a much more effective and convenient source of this useful reagent than the traditional methods.We have demonstrated elsewhere 2 that this mixture which we refer to as the 'Katz reagent', efficiently converts suitable furans into isothiazoles.The close parallel between the action of this mixture and of thiazyl chloride [which is in equilibrium with its trimer 1 in solution] generated from its trimer is not always observed however, particularly when the substrate can react with thionyl chloride or ethyl carbamate, as the following results show.
1,2-Dibenzoylethyne gave the expected thiadiazole 2 (27%) together with a substantial amount of isothiazole 3 (31%).In the dibenzoylethane reaction we assumed that this isothiazole was formed from 2,5-diphenylfuran, but it is hard to see how this compound could be formed from dibenzoylethyne.Isothiazole 3 was not isolated from treatment of the ethyne with trimer 1, and its mode of formation here is as yet unknown.
The reactions of dibenzoylethenes can be compared with that of another Michael acceptor, tetraphenylcyclopentadienone (tetracyclone) 5.With trimer 1 this gave 3,4,5,6-tetraphenyl-2pyridone 7b (56%) after crystallisation from ethanol. 5 With Katz reagent it gave the same product (37%) together with its O-ethoxycarbonyl derivative 8 (26%); 8 was shown to give 7b under the reaction conditions.A possible pathway for the trimer reaction could be dissociation to the monomer, NηSCl, and addition of this to a carbon-carbon double bond of the cyclopentadienone ring to give the fused aziridine 6a; The addition could be nucleophilic in character via -N=S + -Cl, or nitrenoidal and concerted, via: N-S-Cl.The aziridine could rearrange as shown to give the N-chlorosulfanylpyridone 7a which would hydrolyse to 7b on work-up.With the Katz reagent there could be a similar pathway of nucleophilic addition of ethyl carbamate followed by an oxidation step, possibly via N-chlorination, as proposed for the dibenzoylethenes above, to give the ethoxycarbonylaziridine 6b which could rearrange as before to give the pyridones isolated.
Thus we see that differences are to be expected in the reactions of trimer 1 and Katz reagent where the substrate reacts irreversibly with thionyl chloride, for example by cyclodehydration, or with ethyl carbamate, for example by Michael-type addition.
In conclusion, Katz reagent could well behave differently to trimer-derived thiazyl chloride, particularly with potential Michael acceptors and with substrates that are oxidised or chlorinated by thionyl chloride.

Experimental Section
General Procedures.The general conditions were as described earlier. 2 Dibenzoylethane, Edibenzoylethene and tetraphenylcyclopentadienone were purchased from Lancaster Synthesis or from Aldrich.Dibenzoylethyne was made from dibenzoylethene according to the literature. 6Z-Dibenzoylethene was made from 2,5-diphenylfuran with lead tetraacetate according to the literature. 7