Reactions of Internal and Terminal Alkynes via Insertion

In the construction of the conjugated triene system 177 in vitamin D, the intermolecular insertion of the terminal triple bond of the 1,7-enyne 175 to the alkenylpalladium, formed from 174, occurs at first to form the alkenylpalladium 176. Further intramolecular insertion of the terminal double bond in 176, followed by fi-elimination yielded the triene system 177 in 76% yield [77]. 

Intramolecular version can be extended to polycyclization as a one-pot reaction. In the so-called Pd-catalysed domino carbopalladation of trienediyne 178, the first step is the oxidative addition to alkenyl iodide. Then the intramolecular alkyne insertion generates 179. One alkyne and two alkene insertions are followed. The last step is the elimination of /f-hydrogen. In this way, the steroid skeleton 180 is constructed from the linear trienediyne 178 [78]. 

The dienyne 181 undergoes domino 6-exo-dig, 5-exo-trig and 3-exo-trig cyclizations to give the tetracycle 185 exclusively [79]. As the neopentylpalladium 183, which has no -hydrogen, is formed after the insertion of the disubstituted terminal alkene in 182, cyclopropanation occurs to give 184. 

As mentioned in Section 3.3.1, allenes 169 are not formed from alkenylpalladium 167. However, aryl-substituted allenes 187 are obtained predominantly by the coupling of aryl bromides with dialkylacetylenes 186 [80]. 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles 

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Terminal alkynes react with aryl halides to form arylalkynes and enynes in the presence of Cul as described in section 3.4.2. Insertion of terminal alkynes also occurs in the absence of Cul, and the alkenylpalladium species 2 and 7 are formed and vmdergo further reactions (Scheme 3.6). The reactions of internal and terminal alkynes via insertion are treated in this section. 

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