Oppenauer-type synthesis of α , β-acetylenic ketones from tetra ( phenylethynyl ) tin and aromatic aldehydes

Tetra(phenylethynyl)tin reacts with aromatic aldehydes in the presence of a Lewis acid followed by the Oppenauer-type in situ oxidation of propargyl alcohols to give acetylenic ketones in a very good yield.

α-Acetylenic ketones are highly reactive compounds widely used for the synthesis of various carbo-and heterocycles such as pyrroles [1], chromones [2,3], indenones [4], quinolones [5], benzodiazepines [6], spirocycles [7], etc.One of the most general methods for the synthesis of α-acetylenic ketones is based on the reaction of metal acetylides with aldehydes, followed by oxidation of propargyl alcohols formed [2,5,[8][9][10][11].Trialkylstannylacetylenes Alk3Sn-C≡C-R are tolerant with respect to many functional groups, therefore they are actively used as alkynylating reagents as well as reagents in crosscoupling reactions [12,13].Prior we have developed effective methods for the preparation of tin tetraacetylides (RC≡C)4Sn starting from terminal alkynes and tin tetrachloride [14,15] or tin tetra(N,N-dialkylcarbamates) [16].One may expect that the resulting tetraalkynylstannanes (RC≡C)4Sn will have certain advantages over trialkylstannylacetylenes Alk3Sn-C≡C-R, due to atomefficiency and lower toxicity.To date, the properties of tin tetraacetylides (RC≡C)4Sn have been studied insufficiently and their application in organic synthesis is limited to several reactions with organometallic or organoboron compounds [17,18].
Recently we found that tetraalkynylstannanes (RC≡C)4Sn are useful in the Stille cross-coupling reaction with aryl halides [19].Noteworthy that all four alkynyl fragments are involved into this process, i.e., the reaction has a high E-factor, and gave relatively non-toxic inorganic tin(IV) halides as a byproducts.
Here we studied the reactivity of (RC≡C)4Sn with respect to ArCHO.Thus, it was found that the reaction of tetra(phenylethynyl)tin 1 with aromatic aldehydes 2 occurred in the presence of ZnCl2 at 60°C.However, αacetylenic ketones 3 were obtained as the main products instead of expected propargyl alcohols 4.
Presumably, the reaction proceeds through the Oppenauer oxidation step, similar to the oxidative addition of alkynes to RCHO in the presence of InBr3 [20].The tin alkoxides formed in the first stage are rapidly oxidized by Oppenauer reaction with the second aldehyde molecule to form ketone 3 and the corresponding alcohol (detected by LCMS).Target α-acetylenic ketones 3 were readily isolated by column chromatography eluting with toluene.The structure of the obtained compounds was confirmed by 1 H, 13 C NMR, IR spectroscopy, and LCMS data.The structure of 1-(2,3-dihydro-1,4-benzodioxin-6-yl)-3-phenylprop-2-yn-1one was additionally confirmed by X-ray diffraction analysis.In summary, tetra(phenylethynyl)tin reacted with aromatic aldehydes in the presence of ZnCl2 to form acetylenic ketones.The proposed reaction mechanism involves the Oppenauer oxidation of tin propargyl alcoholates.

Experimental
Tetra(phenylethynyl)tin was prepared according to the procedure [14].