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Photocatalyst size controls electron and energy transfer: selectable E / Z isomer synthesis via C – F alkenylation † † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sc02422j

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ABSTRACT

Photocatalytic alkene synthesis can involve electron and energy transfer processes. The structure of the photocatalyst can be used to control the rate of the energy transfer, providing a mechanistic handle over the two processes. Jointly considering catalyst volume and emissive energy provides a highly sensitive strategy for predicting which mechanistic pathway will dominate. This model was developed en route to a photocatalytic Caryl–F alkenylation reaction of alkynes and highly-fluorinated arenes as partners. By judicious choice of photocatalyst, access to E- or Z-olefins was accomplished, even in the case of synthetically challenging trisubstituted alkenes. The generality and transferability of this model was tested by evaluating established photocatalytic reactions, resulting in shortened reaction times and access to complimentary Z-cinnamylamines in the photocatalytic [2 + 2] and C–H vinylation of amines, respectively. These results show that taking into account the size of the photocatalyst provides predictive ability and control in photochemical quenching events.

No MeSH data available.


Strategies used to control energy transfer.
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Mentions: A survey of the literature for processes that might involve both electron and energy transfer provides some insight into how to control the olefin geometry. For instance, MacMillan showed that the rate of the isomerization of cinnamyl amines could be substantially reduced by changing from dimethylacetamide (DMA) to toluene as the solvent (eqn (1), Scheme 1).6 Alternatively, Qing5c showed that electron rich styrenes could undergo an oxidative β-trifluoromethylation and isomerization (eqn (2)). In this case, the thermodynamic E-isomer is likely also the kinetic product. Consequently, the E/Z selectivity could be controlled by the choice of photocatalyst. Use of Ru(bpy)3Cl2·6H2O whose excited state emissive energy is 46.5 kcal mol (ref. 1a) makes the energy transfer process substantially endergonic (53.2 kcal mol–1 for trans-β-methylstyrene)7 and sufficiently slow that the isolation of the kinetic product is possible. In contrast, use of Ir(ppy)3 whose emissive energy is 55.2 kcal mol–1 (ref. 8) is of sufficient triplet state energy to isomerize the trans-styrenyl product selectively leading to an enrichment of the Z-isomer at the photostationary state.


Photocatalyst size controls electron and energy transfer: selectable E / Z isomer synthesis via C – F alkenylation † † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sc02422j
Strategies used to control energy transfer.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5134729&req=5

sch1: Strategies used to control energy transfer.
Mentions: A survey of the literature for processes that might involve both electron and energy transfer provides some insight into how to control the olefin geometry. For instance, MacMillan showed that the rate of the isomerization of cinnamyl amines could be substantially reduced by changing from dimethylacetamide (DMA) to toluene as the solvent (eqn (1), Scheme 1).6 Alternatively, Qing5c showed that electron rich styrenes could undergo an oxidative β-trifluoromethylation and isomerization (eqn (2)). In this case, the thermodynamic E-isomer is likely also the kinetic product. Consequently, the E/Z selectivity could be controlled by the choice of photocatalyst. Use of Ru(bpy)3Cl2·6H2O whose excited state emissive energy is 46.5 kcal mol (ref. 1a) makes the energy transfer process substantially endergonic (53.2 kcal mol–1 for trans-β-methylstyrene)7 and sufficiently slow that the isolation of the kinetic product is possible. In contrast, use of Ir(ppy)3 whose emissive energy is 55.2 kcal mol–1 (ref. 8) is of sufficient triplet state energy to isomerize the trans-styrenyl product selectively leading to an enrichment of the Z-isomer at the photostationary state.

View Article: PubMed Central - PubMed

ABSTRACT

Photocatalytic alkene synthesis can involve electron and energy transfer processes. The structure of the photocatalyst can be used to control the rate of the energy transfer, providing a mechanistic handle over the two processes. Jointly considering catalyst volume and emissive energy provides a highly sensitive strategy for predicting which mechanistic pathway will dominate. This model was developed en route to a photocatalytic Caryl–F alkenylation reaction of alkynes and highly-fluorinated arenes as partners. By judicious choice of photocatalyst, access to E- or Z-olefins was accomplished, even in the case of synthetically challenging trisubstituted alkenes. The generality and transferability of this model was tested by evaluating established photocatalytic reactions, resulting in shortened reaction times and access to complimentary Z-cinnamylamines in the photocatalytic [2 + 2] and C–H vinylation of amines, respectively. These results show that taking into account the size of the photocatalyst provides predictive ability and control in photochemical quenching events.

No MeSH data available.