Metallatropic -shift

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Scheme 9.25 Integration of RRCM with metallatropic shift in the syntheses of tricholomenyn A (106), asperpentyn (109), a nd harveynone (110).

Lee et al. have used a relay metathesis trigger to further enable their studies of metallatropic [l,3]-shifts. For example, both of the relay-activated diene-diynes 98 underwent CM with (Z)-l,4-diacetoxy-2-butene (99) to give ene-diyne 100 (Scheme 9.24) [30]. This process was designed to involve insertion of ruthenium onto the relay subunit in 98 to give 101, followed by rapid migration of [Ru] to the isomeric species 102 and 103 and drainage of the most reactive of these, 103, from the equilibrium manifold by final CM with 99. 

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