Rhodium and Iridium Vinylidenes in Catalysis

When desired vinylidene-mediated pathways are not sufficiently favorable. Group 9 metal catalysts can access a set of typical side-reaction pathways. Alkyne dimerization to give conjugated enynes or higher oligomers is often observed. Polysubstituted benzenes resulting from [2 + 2 + 2] alkyne cyclotrimerization are also common coproducts. Fortunately, the selectivity of rhodium and iridium catalysts can often be modulated by the variation of spectator ligands. 

The authors proposed mechanism for dimerization involves initial formation of metal vinylidene complex 9 via 1,2-H-migration. A second molecule of arylacetylene acts as a dienophUe in a formal [4 + 2] Diels-Alder cycloaddition with 9. A subsequent 

4-H-shift is invoked for the formation of 6 and regeneration of catalyst 8. The proposed mechanism is unusual insofar as the tt-bonds of electroneutral alkynes and arenes seldom participate in Diels-Alder reactions. The intermediacy of metal vinylidenes is supported by the failure of internal alkynes to dimerize under the reported conditions. More importantly, mechanistic restrictions imposed by the porphyrin ligand set severely restrict conceivable alternative mechanisms. 

Dankwardt s vinylidene-mediated reaction apparently operates concurrently with cationic cycloisomerization to give 3-silyl-l-silyloxy naphthalenes (e.g., 13). From his data, a direct comparison can be made of the effect of different metal complexes and silyl groups on selectivity for a vinylidene-mediated reaction pathway (Table 9.2). At least in this instance, Rh(I) is more vinylidene friendly than Pt(II). Iwasawa and coworkers [7], in an isolated related report, also obtained high selectivity for silyl-shifted products in the presence of a Rh(I)-catalyst, albeit one with a substantially different ligand set from that employed by Dankwardt. 

Whereas the thermally allowed Gtt-pericyclic reactions of metal vinylidenes described above engage the vinylidene C=C tt-bond, hope for success in a thermally 

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Rhodium and Iridium Vinylidenes in Catalysis 287 Table 9.5 Rh(l)-catalyzed cycloaromatization of enediynes. 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

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