While photoredox catalysis continues to transform modern synthetic chemistry, detailed mechanistic studies involving direct observation of reaction intermediates and rate constants are rare. Using a combination of steady state photochemical measurements, transient laser spectroscopy, and electrochemical methods we rigorously characterize an α-aminoarylation mechanism that is the inspiration for a large number of photoredox reactions. Despite high product yields, the external quantum yield of the reaction remains low (15-30%). Using transient absorption spectroscopy, productive and unproductive reaction pathways were identified and rate constants assigned to develop a comprehensive mechanistic picture of the reaction. The role of the cyanoarene, 1,4-dicyanobenzne, was found to be unexpectedly complex, functioning both as initial proton acceptor in the reaction and as neutral stabilizer for a 1,4-dicyanobenzene radical anion. Finally, we utilize kinetic modeling to analyze the reaction at an unprecedented level of understanding. This modeling demonstrates that the reaction is limited not by the kinetics of the individual steps but instead by scattering losses and parasitic absorption by a photochemically inactive donor-acceptor complex.

Experimental and theoretical 13 C kinetic isotope effects (KIEs) are utilized to obtain atomistic insight into the catalytic mechanism of the Pd(PPh 3 ) 4 -catalyzed Suzuki-Miyaura reaction of aryl halides and aryl boronic acids. Under catalytic conditions, we establish that oxidative addition of aryl bromides occurs to a 12-electron monoligated palladium complex (Pd-(PPh 3 )). This is based on the congruence of the experimental KIE for the carbon attached to bromine (KIE C−Br = 1.020) and predicted KIE C−Br for the transition state for oxidative addition to the Pd(PPh 3 ) complex (1.021). For aryl iodides, the near-unity KIE C−I of ∼1.003 suggests that the first irreversible step in the catalytic cycle precedes oxidative addition and is likely the binding of the iodoarene to Pd(PPh 3 ). Our results suggest that the commonly proposed oxidative addition to the 14-electron Pd(PPh 3 ) 2 complex can occur only in the presence of excess added ligand or under stoichiometric conditions; in both cases, experimental KIE C−Br of 1.031 is measured, which is identical to the predicted KIE C−Br for the transition state for oxidative addition to the Pd(PPh 3 ) 2 complex (1.031). The transmetalation step, under catalytic conditions, is shown to proceed via a tetracoordinate boronate (8B4) intermediate with a Pd−O−B linkage based on the agreement between an experimental KIE for the carbon atom involved in transmetalation (KIE C-Boron = 1.035) and a predicted KIE C-Boron for the 8B4 transmetalation transition state (1.034).

A practical approach is introduced for the rapid determination of 13C kinetic isotope effects that utilizes a “designed” reactant with two identical reaction sites. The mechanism of the Buchwald–Hartwig amination of tert-butylbromobenzene with primary and secondary amines is investigated under synthetically relevant catalytic conditions using traditional intermolecular 13C NMR methodology at natural abundance. Switching to 1,4-dibromobenzene, a symmetric bromoarene as the designed reactant, the same experimental 13C KIEs are determined using an intramolecular KIE approach. This rapid methodology for KIE determination requires substantially less material and time compared to traditional approaches. Details of the Buchwald–Hartwig amination mechanism are investigated under varying synthetic conditions, namely a variety of halides and bases. The enantioselectivity-determining step of the l-proline catalyzed aldol reaction is also evaluated using this approach. We expect this mechanistic methodology to gain traction among synthetic chemists as a practical technique to rapidly obtain high-resolution information regarding the transition structure of synthetically relevant reactions under catalytic conditions.

The demand for safer design and synthesis of gold nanoparticles (AuNPs) is on the increase with the ultimate goal of producing clean nanomaterials for biological applications. We hereby present a rapid, greener, and photochemical synthesis of gold nanoplates with sizes ranging from 10 to 200 nm using water-soluble quercetin diphosphate (QDP) macromolecules. The synthesis was achieved in water without the use of surfactants, reducing agents, or polymers. The edge length of the triangular nanoplates ranged from 50 to 1200 nm. Furthermore, the reduction of methylene blue was used to investigate the catalytic activity of AuNPs. The catalytic activity of triangular AuNPs was three times higher than that of the spherical AuNPs based on kinetic rate constants ( k ). The rate constants were 3.44 × 10 –2 and 1.11 × 10 –2 s –1 for triangular and spherical AuNPs, respectively. The X-ray diffraction data of gold nanoplates synthesized by this method exhibited that the nanocrystals were mainly dominated by (111) facets which are in agreement to the nanoplates synthesized by using thermal and chemical approaches. The calculated relative diffraction peak intensity of (200), (220), and (311) in comparison with (111) was found to be 0.35, 0.17, and 0.15, respectively, which were lower than the corresponding standard values (JCPDS 04-0784). For example, (200)/(111) = 0.35 compared to 0.52 obtained from the standard (JCPDS 04-0784), indicating that the gold nanoplates are dominated by (111) facets. The calculated lattice from selected area electron diffraction data of the as-synthesized and after 1 year nanoplates was 4.060 and 4.088 Å, respectively. Our calculations were found to be in agreement with 4.078 Å for face-centered cubic gold (JCPDS 04-0784) and literature values of 4.07 Å. The computed QDP–Au complex demonstrated that the reduction process took place in the B ring of QDP. This approach contributes immensely to promoting the ideals of sustainable nanotechnology by eradicating the use of hazardous and toxic organic solvents.

Experimental and theoretical 13C kinetic isotope effects are utilized to obtain atomistic insight into the catalytic mechanism of the Pd(PPh3)4 catalyzed Suzuki-Miyaura reaction of aryl halides and aryl boronic acids. Under catalytic conditions, we establish that oxidative addition of aryl bromides occurs to a 12-electron monoligated palladium complex (Pd(PPh3)). For aryl iodides, the first irreversible step in the catalytic cycle precedes oxidative addition and is shown to be binding of the iodoarene to Pd(PPh3). Our results suggest that the commonly proposed oxidative addition to the 14-electron Pd(PPh3)2 complex can occur only in the presence of excess added ligand or under stoichiometric conditions. The transmetalation step, under catalytic conditions, is shown to proceed via a tetracoordinate boronate (8B4) intermediate with a Pd-O-B linkage.

The rational design and implementation of enantiodivergent enamine catalysis is reported. A simple secondary amine catalyst, 2-methyl-l-proline, and its tetrabutylammonium salt function as an enantiodivergent catalyst pair delivering the enantiomers of α-functionalized aldehyde products in excellent enantioselectivities. This novel concept of designed enantiodivergence is applied to the enantioselective α-amination, aldol, and α-aminoxylation/α-hydroxyamination reactions of aldehydes.

The rational design and implementation of enantiodivergent enamine catalysis is reported. As imple secondary amine catalyst, 2-methyl-l-proline,a nd its tetrabutylammonium salt function as an enantiodivergent catalyst pair delivering the enantiomers of a-functionalized aldehyde products in excellent enantioselectivities.T his novel concept of designed enantiodivergence is applied to the enantioselective aamination, aldol, and a-aminoxylation/a-hydroxyamination reactions of aldehydes. Scheme 1. Mechanistic framework for proline-enamine catalysis.

We present time-dependent percent and quantum yield measurements of a photoredox-catalyzed coupling reaction between 1,4-dicyanobenzene and N-phenylpyrrolidine. We also use transient absorption spectroscopy to examine the kinetics within the reaction and use kinetic modeling to extract rate constants and predict how changes in rate constant will impact the quantum yield.