Despite well-known thermodynamic and kinetic challenges associated with phosphine oxide reduction, recent developments have demonstrated the viability of the P(III)-P(V)=O redox couple to drive O-atom atom transfer reactions in a catalytic way. In this presentation, recent developments in the use of small-ring phosphacycles as catalysts for a diverse range of synthetic O-atom transfer reactions will be described. Synthetic reactions focusing on carbon-heteroatom bond forming methods will be highlighted. Physical and mechanistic studies will be described to illustrate important design principles in organophosphorus redox catalysis.
The use of N-heterocyclic carbenes to modify homogeneous metal catalysts is widespread since the high metal–NHC bond strength renders high oxidative and chemical stability to NHC–ligated metal complexes. Despite this fact, the use of NHCs to modify metal surfaces received little attention until recently.
Our group and others have shown that NHCs can be excellent ligands for transition metal surfaces, including diverse metals such as Au, Ag, Cu, Ru, Pt, Co and Mg. The impact of the type of metal, type of NHC and NHC structure on bonding on planar surfaces will be described.
NHCs have also been shown to stabilize metals in atomically precise nanoclusters. The synthesis of these clusters relies heavily on organometallic principles, and has enabled our group to prepare clusters as small as Au10 and as large as Au25, with many examples in between. Chiral NHC-stabilized clusters, catalytically active clusters, and mixed NHC/hydride clusters will be presented.
Many organic and main group compounds can catalyze chemical reactions. In contrast, very few are able to activate small molecules, such as carbon monoxide, and, until now, none of them catalyze its chemical transformation, a classical task for transition metals. We will report that a stable singlet ambiphilic carbene activates CO and catalytically promotes the carbonylation of an ortho-quinone into a cyclic carbonate. These findings pave the way for the discovery of metal-free catalyzed carbonylation reactions.
H/D exchange at formyl groups is the most direct approach for the synthesis of deuterated aldehydes. Until now, only platinum-group metal complexes were known to catalyze this transformation, with significant substrate scope limitations. We have found that mesoionic carbenes (MICs) catalyze the H/D exchange of aryl, alkenyl and alkyl aldehydes in high yields and deuterium incorporation levels using deuterated methanol as an affordable D source.
Lastly, we will report that Breslow Intermediates derived from MICs, are among the most potent organic reducing agents reported to date. They are reductive enough to undergo SET with iodoarenes, which allows for the highly efficient inter- and intra-molecular MIC-catalyzed arylacylation of styrenes and alkenes, respectively.
The alkaline earth metals (Be, Mg, Ca, Sr, Ba) are experiencing a surge of interest due to newly discovered reagents that facilitate bond activation, and the stabilization of novel low-valent organometallic compounds. While magnesium(I) dimers have served as exceptional reducing agents, the reactivity of molecular beryllium compounds is still emerging. Our laboratory recently demonstrated that a beryllium(0) complex could serve as a reducing agent, which led to the synthesis of the first carbene-bismuthinidene [L-BiPh, L = cyclic(alkyl)(amino) carbene]. Despite the interesting structure, carbene-bismuthinidene is thermally unstable in solution, preventing extensive reactivity studies. Consequently, we are pursued low-coordinate bismuth cations using carbodicarbene, an exceptionally strong carbon(0) donor ligand. Mono-, di-, and tri-positive bismuthenium ions were isolated, which possess short bismuth-carbon bonds. This award lecture will cover our recent progress on these research topics. The utility of carbodicarbene in organoboron materials chemistry will also be discussed.