Versatility of Cyclooctadiene Ligands in Iridium Chemistry and Catalysis

Abstract

In the presence of reactants such as acetonitrile, trimethylphosphine, and diphenylacetylene, the 1,5-cyclooctadiene iridium(I) complex [Ir(1,2,5,6-η-C 8 H 12 )(NCCH 3 )(PMe 3 )]BF 4 (1) has been found to transform into compounds containing cyclooctadiene or cyclooctadienyl ligands in η 3 ,η 2 -; κ,η 3 -; κ 2 ,η 2 -; and η 3 -coordination modes. All these reactions are initiated by an intramolecular C-H activation of the COD ligand and followed by either inter-or intramolecular insertion, or reductive elimination and further C-H activation elementary steps. Compound 1 has also been observed to undergo facile intermolecular oxidative additions of dihydrogen, hydrosilanes, and phenylacetylene to afford iridium(III) hydride complexes. Evidence for the insertion of COD into the Ir-H bonds of these new complexes has been obtained from the isolation of a monohydride complex containing a κ,η 2 -cyclooctenyl ligand, from the isomerization of a silyl derivative into analogues containing 1,4-and 1,3cycloctadiene ligands, and from the occurrence of H/D scrambling among Ir-H and COD C-H sites in the product of DCtCPh oxidative addition. Si-Si coupling reactions to give disilanes and C-C coupling reactions to give an iridacyclopentadiene complex and 1,2,4triphenylbenzene have also been observed in silane and phenylacetylene excess, respectively. Competition of all these intra-and intermolecular reactions under the conditions of phenylacetylene hydrosilylation has been found to result in catalytic reactions, the selectivity of which depends on the presence of introduced acetonitrile and its concentration.

The σ‐CAM Mechanism: σ Complexes as the Basis of σ‐Bond Metathesis at Late‐Transition‐Metal Centers

“…Similar principles could apply to Si À Si bond formation mediated by late transition metals but again lack mechanistic support. [112][113][114] In 1992, Tilley et al had already suggested that "a transient intermediate which may form prior to a four-center transition state is a complex with coordinated SiÀH, HÀH, or Si À Si s bonds." [115] There are now a few examples of C À C agostic complexes, [65,[116][117][118][119][120] but intermolecular s-C À C complexes are not known as far as we are aware.…”

Iridium Catalyzed Carbocyclizations: Efficient (5+2) Cycloadditions of Vinylcyclopropanes and Alkynes

“…When using DCE as the solvent, addition of fluorinated co-solvents was necessary to retain high yields and reaction rates (not shown). [20] Importantly, nonchlorinated solvents, such as 2-Me-THF (with or without TFE added) and propylene carbonate (PC), [21] also allowed for efficient catalysis (Table 1, entries [11][12][13].…”

Solvent promoted reversible cyclometalation in a tethered NHC iridium complex

“…While 5 is stable under CO atmosphere, removal of solvent and addition of CD 2 Cl 2 results in retro-cyclometalation and predominately a mixture of 2 55 and 6 (no 5 observed after 12 h) -the later formed by reaction of 2 with liberated CO. To help gain insight into the mechanism of the cyclometalation 60 reaction, we targeted the reaction of 2 with diphenylacetylene, as an external hydride acceptor. This alkyne has previously been used to trap out C-H bond activation reactions of COD coordinated to iridium 10 and we wished to explore if such reactivity was important in our system. Heating 2 with a large 65 excess of diphenylacetylene at 318 K resulted in the gradual, but irreversible, formation of the new cyclometalated complex [Ir(COD)(py-I t Bu'){κ 1 -C(Ph)=CHPh}][BAr F 4 ] 7, which was isolated in 61% yield after 11 days (Scheme 3).…”