RESUMEN
The intricate σ and π-bonding of N-heterocyclic carbenes (NHCs) to metals and the need to quantify their electronic properties to rationalize reactivity of complexes have resulted in the creation of numerous methodologies to understand the NHC-metal interaction which are, as we now show, flawed. Our search for a unified, easily accessible system to gauge these fundamental properties has resulted in the discovery of two systems that highlight the flaws present in existing systems and provide a more accurate measure of the NHC ligand electronic properties.
RESUMEN
The synthesis and isolation of [Pd(NHC)(PhC≡CPh)] complexes are reported. These new 14-electron Pd(0)-complexes are key synthons leading to known palladium(0) and palladium(II) species, as well as permitting access to unprecedented mixed NHC-phosphite palladium(0) complexes. This motif permits the facile catalytic hydrosilylation of allenes. DFT calculations have allowed the characterization of the relatively weak interaction between the metal and the diphenylacetylene ligand, with a comparison with a series of ligands with more or less coordinating power, bearing varied structural and electronic properties.
RESUMEN
We present the use of gold sensitizers [Au(SIPr)(Cbz)] (PhotAu 1) and [Au(IPr)(Cbz)] (PhotAu 2) as attractive alternatives to state-of-the-art iridium-based systems. These novel photocatalysts are deployed in [2 + 2] cycloadditions of diallyl ethers and N-tosylamides. The reactions proceed in short reaction times and in environmentally friendly solvents. [Au(SIPr)Cbz] and [Au(IPr)(Cbz)] have higher triplet energy (E T) values (66.6 and 66.3 kcal mol-1, respectively) compared to commonly used iridium photosensitizers. These E T values permit the use of these gold complexes as sensitizers enabling energy transfer catalysis involving unprotected indole derivatives, a substrate class previously inaccessible with state-of-the-art Ir photocatalysts. The photosynthesis of unprotected tetracyclic spiroindolines via intramolecular [2 + 2] cycloaddition using our simple mononuclear gold sensitizer is readily achieved. Mechanistic studies support the involvement of triplet-triplet energy transfer (TTEnT) for both [2 + 2] photocycloadditions.