Asymmetric catalytic variants of sunlight-driven photochemical processes hold extraordinary potential for the sustainable preparation of chiral molecules. However, the involvement of short-lived electronically excited states inherent to any photochemical reaction makes it challenging for a chiral catalyst to dictate the stereochemistry of the products. Here, we report that readily available chiral organic catalysts, with well-known utility in thermal asymmetric processes, can also confer a high level of stereocontrol in synthetically relevant intermolecular carbon-carbon bond-forming reactions driven by visible light. A unique mechanism of catalysis is proposed, wherein the catalyst is involved actively in both the photochemical activation of the substrates (by inducing the transient formation of chiral electron donor-acceptor complexes) and the stereoselectivity-defining event. We use this approach to enable transformations that are extremely difficult under thermal conditions, such as the asymmetric α-alkylation of aldehydes with alkyl halides, the formation of all-carbon quaternary stereocentres and the control of remote stereochemistry.

In this work, the role of nanoparticle surface charge in surface-enhanced Raman scattering (SERS) is examined for the common case of measurements made in colloidal solutions of Ag and Au. Average SERS intensities obtained for several analytes (salicylic acid, pyridine, and 2-naphthalenethiol) on Ag and Au colloids are correlated with the pH and zeta potential (zeta) values of the nanoparticle solutions from which they were recorded. The consequence of the electrostatic interaction between the analyte and the metallic nanoparticle is stressed. The zeta potentials of three commonly used colloidal solutions are reported as a function of pH, and a discussion is given on how these influence SERS intensity. Also examined is the importance of nanoparticle aggregation (and colloidal solution collapse) in determining SERS intensities, and how this varies with the pH of the solution. The results show that SERS enhancement is highest at zeta potential values where the colloidal nanoparticle solutions are most stable and where the electrostatic repulsion between the particles and the analyte molecules is minimized. These results suggest some important criteria for consideration in all SERS measurements and also provide important insights into the problem of predicting SERS activities for different molecular systems.

We have found that an organic molecule as simple as p-anisaldehyde efficiently catalyzes the intermolecular atom-transfer radical addition (ATRA) of a variety of haloalkanes onto olefins, one of the fundamental carbon-carbon bond-forming transformations in organic chemistry. The reaction requires exceptionally mild reaction conditions to proceed, as it occurs at ambient temperature and under illumination by a readily available fluorescent light bulb. Initial investigations support a mechanism whereby the aldehydic catalyst photochemically generates the reactive radical species by sensitization of the organic halides by an energy-transfer pathway.

Abstract:A new method for the catalytic didehydroxylation of vicinal diols is described. Employing a readily available low-valent rhenium carbonyl complex and a simple alcohol as a reducing agent, both terminal and internal vicinal diols are deoxygenated to olefins in good yield. The optional addition of acid (TsOH, H 2 SO 4 ) provides access to lower reaction temperatures. This new system enables the transformation of a four-carbon sugar polyol into an oxygen-reduced compound, providing promising evidence for its practical application to produce unsaturated compounds from biomass-derived materials.The development of new synthetic technologies for the production of reduced oxygen-content materials from renewable biomass resources is a key target in chemistry and chemical engineering research. 1 Our group is exploring new routes to deoxygenate biomass-derived polyols, 2 and the didehydroxylation of diol moieties is an important goal in this context. Various stoichiometric methods for carrying out diol didehydroxylation are synthetically valuable, 3 but catalytic methods are rare. 4 In particular, didehydroxylation has been achieved using Cp*ReO 3 as a catalyst, although triphenylphosphine was needed as a sacrificial reductant. 4a We report herein a catalytic method for the didehydroxylation of vicinal diols to alkenes that employs a readily available low-valent rhenium carbonyl complex and a simple alcohol as an environmentally friendly reducing agent.During preliminary experiments, we observed that heating (4S*,4S*)-octane-4,5-diol (1) at 180°C in solvent-free conditions, under air and in the presence of dirhenium decacarbonyl (3a, 2.5 mol %), resulted in complete consumption of 1 in 3.5 h to yield the olefin 2 in 50% yield as determined by NMR spectroscopy (Scheme 1). A droplet of phase-separated water was also detected in the reaction mixture.Aliquots of the mixture from the reaction of diol 1 and 3a at intermediate conversion contained the vicinal diketone 4 (as identified by 1 H, 13 C NMR spectroscopy and MS). Although at the early stages of the reaction the amount of 4 increased along with the formation of 2, its depletion was subsequently observed leading to its disappearance when complete conversion of 1 was achieved (vicinal diketones are unstable under the reaction conditions; see Supporting Information (SI)). These results suggested that the transformation might occur through a disproportionation reaction in which the diol (1) is reduced to generate an alkene (2) and concurrently acts as a reductant to give oxidized products (4).Similar results were observed when the terminal diol 1,2-tetradecanediol (5; see Table 1) was submitted to the same reaction conditions, in which the yield of 1-tetradecene (6) was determined to be 49% (isolated). The diols remained unchanged when heated in the presence of 3a under air at temperatures lower than 170°C. When the experiment was conducted in the absence of O 2 (N 2 atmosphere) no reaction was observed after heating at 180°C for extended periods (24 h).As this sy...