3-[Ethoxy(thiocarbonyl)thio]quinoline explosion

Joseph Laquidara
Merck Research Laboratories
Rahway, N.J.

Chemical & Engineering News (9 July 2001) Vol. 79, No. 28, pp. 6.

I was attempting to prepare 3-[ethoxy(thiocarbonyl)thio]quinoline by reacting the diazonium salt of 3-aminoquinine with potassium ethyl xanthate when an explosion occurred. Thankfully, the injury I received was minor. I was following the procedure described in the Journal of the Chemical Society, Part III [3, 2384 (1959)] on twice their scale. Jerry March's book "Advanced Organic Chemistry: Reactions, Mechanisms, Structure" (New York: John Wiley & Sons, 1992, 4th edition, page 670) lists the procedure as a customary route for the desired aryl xanthate ester and a convenient method for producing thiophenols. The same procedure is also described in "Organic Syntheses, Collective Volume III" (New York: John Wiley & Sons, page 809), which contains a cautionary note about a reported explosion, although the editors found no unusual reactivity.

Since diazo esters were present, the explosion most likely resulted from the diazo ester's tendency to decompose explosively. It is unclear whether the diazo ester is a precursor of the desired product or whether the diazo ester forms through a competitive reaction.

The reaction proceeded without event. But unlike the literature preparations, precipitation occurred during product extraction with diethyl ether and was isolated by filtration. A 1H NMR of the wet cake was consistent with the desired structure. The solid was air-dried overnight and weighed 17 g. The detonation occurred when a metal spatula was inserted into the solid.

Related work in Tetrahedron Letters [26, 6365 (1985)] successfully demonstrated the safe preparation of diaryl sulfides. However, this was done at 25 ºC (mine at 45 ºC) with care taken to avoid heat, since a reference is cited of an explosion at 60 ºC. Also, the disappearance of the suspected diazo sulfide was monitored by thin layer chromatography, and sluggish reactions were not pushed to conversion. Instead, chromatography was used to remove any intermediates.

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