Coordination-Induced Radical Generation: Selective Hydrogen Atom Abstraction via Controlled Ti-C σ-Bond Homolysis

Asymmetric photooxidation of glycerol to hydroxypyruvic acid over Rb-Ir catalytic pairs on poly(heptazine imides)

Selective asymmetric oxidation of glycerol (GLY) to hydroxypyruvic acid (HPA) offers an attractive approach for chiral drug synthesis, but this process is highly challenging. Here we develop a photocatalytic method to achieve heterogeneous selective photooxidation of GLY to HPA over rubidium (Rb) and iridium (Ir) catalytic pairs decorated on a poly(heptazine imide) framework. The Rb sites effectively adsorb GLY molecules through the terminal -OH groups, thus inhibiting their oxidation during photoreaction, while the Ir sites enhance the oxygen reduction reaction and the in situ generated surficial oxygen-reduction radicals on Ir can protect the reactive C-centred radical intermediates produced during photooxidation. The spatial arrangement of Rb and Ir sites facilitates hydrogen extraction-an essential rate-determining step for GLY photooxidation-and protects C3 radical intermediates from overoxidation. This photocatalytic system achieves a remarkable productivity for HPA synthesis (~8,000 μmol of HPA per gram of photocatalyst per hour) under visible-light illumination.

Coordination-induced reductive elimination from a titanium(IV) complex

Diamidopyridine-supported titanium dibenzyl complexes undergo coordination-induced C-C reductive elimination upon addition of alkynes and quantitative formation of titanacyclopentadienes. The distinct radical mechanism of this reductive mechanism gives new insights into C-C bond formation with titanium.

Thermal Formation of Metathesis-Active Tungsten Alkylidene Complexes from Cyclohexene

A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh3)2(C2H4), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh3)2(C2H4), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WCα position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.