We report the synthesis and reactivity of a very robust iridium catalyst for glycerol to lactate conversion. The high reactivity and selectivity of this catalyst enable a sequence for the conversion of biodiesel waste stream to lactide monomers for the preparation of poly(lactic acid). Furthermore, experimental data collected with this system provide a general understanding of its reactive mechanism.

Selective hydrogen transfer remains a central research focus in catalysis: hydrogenation and dehydrogenation have central roles, both historical and contemporary, in all aspects of fuel, agricultural, pharmaceutical, and fine chemical synthesis. Our lab has been involved in this area by designing homogeneous catalysts for dehydrogenation and hydrogen transfer that fill needs ranging from on-demand hydrogen storage to fine chemical synthesis. A keen eye toward mechanism has enabled us to develop systems with excellent selectivity and longevity and demonstrate these in a diversity of high-value applications. Here we describe recent work from our lab in these areas that are linked by a central mechanistic trichotomy of catalyst initiation pathways that lead highly analogous precursors to a diversity of useful applications.

No abstract

There are few ways to switch a catalyst's reactivity on or off, or change its selectivity, with external radiation; many of these involve photochemical activation of a catalyst. In the case of homogeneous late-transition-metal catalysts, the metal complex itself is frequently the chromophore involved in such reactivity switching. We show here a base-pendant ligand−metal bifunctional scaffold wherein a photobase, a compound that becomes more basic in the excited state (pK a < pK a *), is used to switch the proton acceptor ability on an active site of the complex. The system differs from those with metal-centered chromophores, because the photobase operates independently of the metal. While excellent progress has been made in photoacid chemistry, neither a photoacid nor a photobase has been designed into the structure of a transition-metal catalyst where the metal is not part of the chromophore. We find that quinoline is an efficient photobase that preserves its unique properties in the close proximity of an iridium center: the efficacy of the photobase (9.3 < pK a * < 12.4) in the iridium complex is unhindered relative to the free quinoline. We apply this notion to successful photodriven deprotonation of an aliphatic alcohol, thus showing the first case of metal-orthogonal optical pK a control in a transition-metal complex.