Photochemistry. Volume 42

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DaSilva, R. Nagao, Y. Camacho, M. Harley-Mason, J. Stang, P. Sowan, D. Khimich, N. Dissertation , Leningrad, ; Shevehenko, V. Kaplan, F. Meldrum, A. Regitz, M. Properties and Synthesis , New York: Academic, , p. Popik, V. Hochrainer, A. Illger, W.


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Photochemistry of alkenes (continued)

Excited states. Metal ions. Controlling the reactive state through cation binding: Photochemistry of enones within zeolites. Tetrahedron Letters , 42 11 , In: Tetrahedron Letters , Vol. Tetrahedron Letters. In: Tetrahedron Letters. Lakshminarasimhan and J. However, implementation of photochemistry at multikilogram scale has been hindered by several inherent challenges associated with the attenuation of light as dictated by the Beer—Lambert law.

The primary challenge in scaling photochemical reactions is addressing the Beer—Lambert law, which dictates the depth to which light can penetrate a solution. In photochemical reactor design, flow chemistry has been universally identified as the solution to overcome the attenuation of light by providing reactor geometries with increased surface-area-to-volume ratios, allowing increased illumination of the reaction solution relative to conventional batch reactors. One approach that has been pursued involves increasing the number of identical reactors operating in parallel, thereby increasing the overall throughput.

Efforts by the Stephenson group and others have demonstrated some limited scale-up with this design; however, a key challenge remains the translation of results between bench-scale reactors used for reaction optimization to production-scale reactors. Chemical actinometry, a method for relating the rate of reaction to the rate of photon absorption in a reactor, can correlate reactor performance across scales, but requires time, effort, and equivalent amounts of material to generate accurate calibrations.

As a result, the heat that the LEDs generate may negatively influence the reaction, requiring additional engineering solutions at scale. In general, the engineering challenges inherent to building an LED-based tubular flow reactor capable of multikilogram throughput necessitate construction of a fixed volume reactor and a fixed light source, thereby significantly limiting the modularity and adaptability benefits of flow chemistry.

After studying the scalability of plug flow photoreactors, we concluded that these reactors would not meet our adaptability requirements and accordingly directed our efforts to developing a more modular system. Our studies indicated that higher-intensity light sources would lead to increased rates of reaction, thereby increasing throughput and yield. Our efforts to find higher-intensity monochromatic light sources led us to employ laser diodes in photochemical reactions. We compared lasers to other light sources in relevant photochemical reactions and generated several broad observations that informed our efforts to develop a versatile platform for visible light photocatalysis.

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Similar results have also been reported by MacMillan and co-workers across a separate set of reactions. To explore the potential of higher-powered light sources, a 25 W nm fiber coupled laser system was fit with an adjustable beam expander. The rate of reaction was not affected by variations in concentration of DABCO or pyrrolidine, but did depend on photocatalyst, Ni, and aryl bromide concentrations.

In preparation for eventual scale-up, the optimal reaction concentration was determined to be 0. A C—N coupling reaction used with the optimized conditions. B Plot of the initial rates of reaction at several different catalyst concentrations with the proposed exponential relationship shown as a dotted line. C Time course plot of the C—N coupling reaction showing the conversion of aryl bromide in red and the solution darkening effect in blue. At concentrations lower than the optimal concentration, traditional kinetic behavior is observed, implying that catalyst quenching is the rate-limiting step.

At catalyst concentrations higher than the optimal point, the rate of reaction declines exponentially, a relationship which appears to be counterintuitive to the general principles of catalysis where increased catalyst loadings should result in faster rates.

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However, the Beer—Lambert law states that light attenuates exponentially as the catalyst concentration the absorbing species increases. At the highest catalyst concentrations, This correlation to the Beer—Lambert law can be powerfully applied to design photochemical systems where the solution depth path length and catalyst concentration can be adjusted relative to each other and adapted to process constraints, whether those constraints are on catalyst loading or reactor geometry.

Beyond the context of the high-intensity laser, we have observed this same trend where decreasing catalyst concentration up to a certain point leads to increased rate of reaction, independent of the light source used. Initially surprised by this counterintuitive trend, we sought to explore the generality of this principle in other reactions. These combined results strongly indicate that optimal reaction performance in terms of rate is directly correlated to photocatalyst concentration which can be determined entirely from the Beer—Lambert law based on the vessel used and not the associated chemical transformation.

KEYWORDS/PHRASES

A Initial rates as a function of catalyst concentration demonstrating a Beer—Lambert law relationship in decarboxylative C—C bond formation. B Initial rates as a function of catalyst concentration as another demonstration of the Beer—Lambert law in the anti-Markovnikov addition of carboxylic acids to alkenes. This demonstration of the impact of the Beer—Lambert law on the rate of reaction led us to consider alternate flow reactor designs.

The absolute molar concentrations for common photocatalysts result in complete extinction of light within millimeters of liquid depth, especially for catalysts with molar extinction coefficients orders of magnitude higher than Catalyst 1 M —1 cm —1 as used in the C—N coupling. Perhaps due to its counterintuitive nature, little consideration has been given to optimizing reactors with larger cross-sectional areas and decreased photocatalyst concentrations; however, our results suggested a reactor of this type would be ideal when coupled to a high-intensity light source.

Another key variable we explored was the effect of laser output power on the reaction rate by varying the output from 10 to 26 W. Applied to a flow reactor, this relationship is extraordinarily powerful because it implies that the reaction throughput can only be increased by increasing both the reaction volume as a function of surface area and the power of the light, if the reactor depth and catalyst concentration are already optimized using the Beer—Lambert law.

To test this relationship as a design principle, identical conditions were evaluated in larger and smaller diameter reactors resulting in two different power densities, while maintaining the optimal 5 cm depth. A Correlation between initial rates in the C—N coupling and power density of the laser source where the standard reactor employed was 6. Equipment power limitations prohibited further exploration in the C—N coupling; however, such a deviation in linearity was observed in our exploration of the anti-Markovnikov addition to alkenes reported by Nicewicz and co-workers.

This type of relationship can also inform reactor design and indicates the optimal power density where increasing the power output from the laser no longer improves throughput. Where this type of behavior is observed, the reactor size can be increased as long as this power density is maintained, providing increased throughput.

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With these fundamental studies as guides, the optimal flow reactor was designed to implement the high-powered laser as the light source. However, to incorporate the laser as a light source and to take advantage of the concepts detailed above, a CSTR became an obvious choice.


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CSTRs are better suited to handling solids as opposed to tubular or plug flow reactors. Most importantly, a simple CSTR design enables the easy adaptation of our laser light source and provides modularity in reaction vessel choice, enabling the use of common laboratory equipment.

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