title: Probing the limits of ligand steric bulk: backbone C-H activation in a saturated N-heterocyclic carbene.
abstract: The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, % Vbur , of 50.8%) leads to C-H and C-N bond activation processes driven by attack at the backbone β-CH2 unit. In the presence of Ir(I) (or indeed H(+) ) the net result is the formation of an allyl formamidine fragment, while Au(I) brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au](+) represents to our knowledge the first example of backbone C-H activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au(I) centre.
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR (UK).
authors.forename: Nicholas
authors.initials: N
authors.lastname: Phillips
authors.forename: Remi
authors.initials: R
authors.lastname: Tirfoin
authors.forename: Simon
authors.initials: S
authors.lastname: Aldridge
keywords: CH activation; N-heterocyclic carbenes; gold; iridium; steric loading