Neutral Ligands with Exceptional Donor Ability for Palladium‐Catalyzed Alkene Hydrogenation

Abstract

The metal-mediated (catalytic) activation of strong and typically unreactive bonds under mild conditions requires the development of powerful ligand sets. Two particularly useful strategies have been developed during the last few years. [1] The first relies on electron-deficient systems with high-valent early transition metals, often in a d 0 configuration. [2] Such metal centers have been shown to activate unreactive bonds through agostic interactions and subsequent s-bond metathesis. In a second strategy, electronrich metal centers are used to promote bond activation through oxidative addition, and late transition metals such as the platinum group metals typically accommodate the required electron density for these reactions. [3] The relative basicity can be further increased by using acidic media [4] and by installing strongly donating nontransferable ligands in the metal coordination sphere. [5]

C4-bound imidazolylidenes: from curiosities to high-impact carbene ligands

“…22 Single-crystal structure analysis of the adduct 33 (R = iPr) revealed a Pd-Ag distance of 2.8701(6) Å , which is one of the shortest contacts known so far. 44 The propensity of the Pd(dicarbene) unit to bind effectively to a Lewis acid has been used to underline the high electron density of the palladium center in a C4-bound dicarbene ligand environment.…”

“…Substitution effectively blocks the C2 position and directs metallation to the C4 position.w This approach, developed by Crabtree and co-workers, has been extended by us and others for preparing different complexes containing C4-bound w For reasons of consistency, the atom numbering of Scheme 1 is preserved throughout the entire article and the metal-bound carbon is referred to as C4 carbon, even though the metal-bound carbon actually constitutes the C2, C3 or C5 carbon depending on the substituents at the nitrogen atoms and the location of annelated rings. mono- [18][19][20][21] and dicarbene [22][23][24][25] ligands. Metallation has been reported to occur predominantly via C-H bond activation [19][20][21][22][23][24][25] though oxidative C-H bond addition is viable as well (Scheme 5).…”

“…mono- [18][19][20][21] and dicarbene [22][23][24][25] ligands. Metallation has been reported to occur predominantly via C-H bond activation [19][20][21][22][23][24][25] though oxidative C-H bond addition is viable as well (Scheme 5). 18 When the substitutents R 1 and R 3 at the nitrogen heteroatoms are different, C4-and C5-metallation becomes distinguishable (R 5 = H) and can lead to the formation of isomeric product mixtures.…”

“…4). 22 The weaker electron bonding in 5a reflects the stronger donor ability of the C4-bound carbene. Notably, similar binding energy differences have been measured for the sterically identical complexes 19 and 20 ( Fig.…”

“…We have therefore probed the catalytic activity of the Pd(dicarbene) complex 34 (Scheme 14) in the direct hydrogenation of alkenes (Scheme 16), 22 which involves as a key step the activation of dihydrogen.…”