Reactions via Insertion of Alkenes and Alkynes

Smooth insertion of alkene 19 into the cr-allenylpalladium bond generates 20, and subsequent -H elimination affords 1,2,4-trienes 21. 

Intramolecular domino reactions proceed smoothly to afford cyclic compounds [2]. Domino 5-exo cyclization of the carbonate 22 generated the allenyl intermediate 23, and 3-exo cyclization of 23 gave the cyclopropane 24, which was converted to a dimeric product 25 [3]. 

The allenylpalladium 27, formed from the carbonate 26, undergoes 5-exo cycliza-tion to give 28 and its 3-exo cyclization, without f-E. elimination, generates the bicyclo[3.1.0]hexane system 29, which has no -hydrogen to be eliminated. Thus transmetallation of 29 with the tin reagent 30, followed by reductive elimination affords the azabicyclo[3.1.0]hexane 31 diastereoselectively [4], The synthesis of (—)-a-thujone was achieved by diastereoselective domino cyclization of the chiral carbonate 32, and trapping with dimethylzinc to give 33 as a key reaction [5], 

The Pd-catalyzed reaction of o-alkynylphenol 39 with tertiaryl propargyl carbonate 40 gave the 2-substituted 3-allenylbenzo[i ]furan 43 under neutral conditions. Attack of phenoxide anion to the triple bond from the opposite side of a-allenylpalladium intermediate as shown by 41 generates allenyl(benzofuryl)palladium intermediate 42, and the coupling product 43 is formed by reductive elimination [7]. 

Similarly 2-substituted 3-allenylbenzo[ ]furan 45 was prepared by the Pd-cata-lyzed reaction of the propargyl o-(alkynyl)phenyl ether 44. The isomer 46 was a minor product [8]. 

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