Real-Time Monitoring of Aqueous Organic Reduction Reactions Using Ex Situ Fiber Optic Raman Spectroscopy

Abstract

Here, we demonstrate the use of Raman spectroscopy for real-time monitoring and analysis of the reduction of nitroarenes and ketones in an aqueous medium using an external fiber optic probe. In one case, the catalytic reduction of nitroarenes using hydrazine hydrate and a nickel boron composite catalyst was studied by monitoring the disappearance of the distinct nitro stretching frequency at 1350 cm–1. Quantitative Raman spectroscopic techniques showed that the aqueous reduction reaction proceeds with pseudo-first-order kinetics. The biphasic reduction of cyclic ketones by aqueous dimethylamine borane (DMAB) was also examined. A nonpolar carbonyl compound and aqueous DMAB were mixed and an external Raman probe monitored the loss of the starting material via the carbonyl stretch at 1713 cm–1 in the top organic phase. We also found that saturated aliphatic carbonyl compounds were reduced much faster than α,β-unsaturated and aromatic carbonyl compounds. Additionally, DMAB was found to be unreactive toward epoxides and esters, allowing for selective reduction of aliphatic ketones. Density functional theory analysis suggests that carbonyl reduction occurred through a dissociation mechanism followed by a concerted hydride transfer pathway.

In situ Reaction Monitoring in Photocatalytic Organic Synthesis

“…Raman spectroscopy allows to monitor gases, substances in solution and undissolved particles using fiber optic probes and is therefore broadly applicable in chemical analysis [25b–d] . Raman probes are either immersed into the reaction mixture (invasive), or placed outside of a transparent reaction vessel (non‐invasive) [25f] . In general, invasive probes, including the ATR‐FTIR and FTIR‐UV/Vis devices described above, impact the irradiation characteristics (smaller path length, reflection phenomena, etc.).…”

“…Thus, non‐invasive Raman probes are advantageous, because reaction monitoring can be carried out using completely unaltered setups. Another advantage of Raman spectroscopy over other techniques is that signals do not typically interfere with water vibrational modes, allowing its use in aqueous reactions [25f] . However, for photocatalytic applications the light source and the fluorescence of a chromophore may interfere, resulting in a lowering of the signal‐to‐noise ratio, which limits its application in photochemistry [25e] …”