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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.

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(a) The reaction scheme for the photocatalyst selectivity investigation, (b) scatter plot of the log(Z : E) as a function of the emissive energy of the labeled photocatalysts which were taken from the literature and correspond to the emission spectrum λmax,8 and (c) scatter plot of the log(Z : E) as a function of the effective radius of labeled photocatalysts, colored by the measured emissive energy. Conversion to the strained Z-isomer is increasingly less effective with increasing catalyst size, though catalysts with high emissive energies can deviate from this trend. Cationic catalysts are PF6– salts.
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fig2: (a) The reaction scheme for the photocatalyst selectivity investigation, (b) scatter plot of the log(Z : E) as a function of the emissive energy of the labeled photocatalysts which were taken from the literature and correspond to the emission spectrum λmax,8 and (c) scatter plot of the log(Z : E) as a function of the effective radius of labeled photocatalysts, colored by the measured emissive energy. Conversion to the strained Z-isomer is increasingly less effective with increasing catalyst size, though catalysts with high emissive energies can deviate from this trend. Cationic catalysts are PF6– salts.

Mentions: Using the optimal conditions found in Table 1 (entry 14) we sought to evaluate the effect of the catalyst on the E : Z selectivity. We chose t-butylacetylene because it was expected to have a strong kinetic preference for the E-isomer (Fig. 2a),12 which would allow us to determine whether isomerization occurs. Additionally, we postulated that the steric bulkiness of the t-butyl group would make the substrate more sensitive to changes in volume of the photocatalyst. Using our library of photocatalysts8 which met two of three criteria, we evaluated the ability to facilitate the electron transfer and isomerization. The criteria included a demonstrated ability in C–F functionalization, had a reduction potential of –1.5 V (vs. SCE) or more negative from either their excited state or reduced ground state, and an emissive energy that was at least 51 kcal mol–1. While the conversions varied depending on which catalyst was used,12 the ratio of the isomers at the photostationary state were recorded.


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
(a) The reaction scheme for the photocatalyst selectivity investigation, (b) scatter plot of the log(Z : E) as a function of the emissive energy of the labeled photocatalysts which were taken from the literature and correspond to the emission spectrum λmax,8 and (c) scatter plot of the log(Z : E) as a function of the effective radius of labeled photocatalysts, colored by the measured emissive energy. Conversion to the strained Z-isomer is increasingly less effective with increasing catalyst size, though catalysts with high emissive energies can deviate from this trend. Cationic catalysts are PF6– salts.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig2: (a) The reaction scheme for the photocatalyst selectivity investigation, (b) scatter plot of the log(Z : E) as a function of the emissive energy of the labeled photocatalysts which were taken from the literature and correspond to the emission spectrum λmax,8 and (c) scatter plot of the log(Z : E) as a function of the effective radius of labeled photocatalysts, colored by the measured emissive energy. Conversion to the strained Z-isomer is increasingly less effective with increasing catalyst size, though catalysts with high emissive energies can deviate from this trend. Cationic catalysts are PF6– salts.
Mentions: Using the optimal conditions found in Table 1 (entry 14) we sought to evaluate the effect of the catalyst on the E : Z selectivity. We chose t-butylacetylene because it was expected to have a strong kinetic preference for the E-isomer (Fig. 2a),12 which would allow us to determine whether isomerization occurs. Additionally, we postulated that the steric bulkiness of the t-butyl group would make the substrate more sensitive to changes in volume of the photocatalyst. Using our library of photocatalysts8 which met two of three criteria, we evaluated the ability to facilitate the electron transfer and isomerization. The criteria included a demonstrated ability in C–F functionalization, had a reduction potential of –1.5 V (vs. SCE) or more negative from either their excited state or reduced ground state, and an emissive energy that was at least 51 kcal mol–1. While the conversions varied depending on which catalyst was used,12 the ratio of the isomers at the photostationary state were recorded.

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.


Related in: MedlinePlus