Hydrogenolysis and Elimination of Propargyl Compounds

Allenes 169 and alkynes 170 are prepared by hydrogenolysis of propargyl compounds with several hydrides. Triethylammonium formate is used most conveniently under mild conditions [45]. Chromium tricarbonyl-complexed phenylallene 172 was prepared from the carbonate 171 [46]. The alkyne 174 was obtained selectively from the propargyl formate 173 having an amino group [47]. 

Reaction of propargyl formate 175 affords two products 179 and 181 depending on the kind of intermediates being subjected to hydrogenolysis with formate. When 5-exo cyclization of 176 and 3-exo cyclization of 177 to give the cyclopropane 178 occurred before the hydrogenolysis, the bicyclo[3.1.0]hexane 179 was obtained. On the other hand, the cyclopentane 181 was formed by hydrogenolysis of 176, and the cyclopentane 181 was obtained by Pd-catalyzed cyclization of 180 [3]. 

When propargyl carbonates 195 are treated with a Pd catalyst in the absence of other reactants, (S-H elimination from the propargylpalladium intermediates 196 occurs to give conjugated enynes 197. Formation of cumulative 1,2,3-alkatrienes 199 from the allenylpalladiums 198 does not take place. Preparation of the conjugated ene-yne-ene system 201 in high yield from the propargyl carbonate 200 is an example [51]. 

Reviews (a) J. Tsuji, Angew. Chem. Int. Ed. Engl., 34, 2589 (1995) (b) J. Tsuji and T. Mandai, in Metal-Catalyzed Cross-Coupling Reactions, Eds. F. Diedrich and P. J. Stang, Wiiey-VCH, New York, 1998, p. 455. 

Tanikawa, T. Ishihara, and H. Yamanaka, Chem. Lett, 1360 (2000). 

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