Highly Electrophilic Titania Hole as a Versatile and Efficient Photochemical Free Radical Source.

Andrew Hainer,Stefanie Rincon,Nancy Marina,Juan C. Scaiano,Paolo Costa,Anabel E. Lanterna

doi:10.1021/jacs.8b13422

Andrew Hainer, Stefanie Rincon + Show 4 more

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https://doi.org/10.1021/jacs.8b13422

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Abstract

Photogenerated holes in nanometric semiconductors, such as TiO2, constitute remarkable powerful electrophilic centers, capable of capturing an electron from numerous donors such as ethers, or nonactivated substrates like toluene or acetonitrile, and constitute an exceptionally clean and efficient source of free radicals. In contrast with typical free radical precursors, semiconductors generate single radicals (rather than pairs), where the precursors can be readily removed by filtration or centrifugation after use, thus making it a convenient tool in organic chemistry. The process can be described as an example of dystonic proton coupled electron transfer.

Highlights

C–H activation has gained much interest due to its atom and step-economy in the synthesis of functional molecules,[1 15] many of them are of importance in drug development
These mechanisms normally require the presence of a free radical initiator that generates two radicals, leaving initiatorderived debris in the solution along with residual initiator, which can be difficult to remove
We demonstrate that 35 photoactivated TiO2 can be used as a clean free radical initiator, circumventing some of the disadvantages mentioned

Summary

The generation of radicals does not guarantee they are

80 “free”, in the sense that they are mobile and can participate in solution reactions, as in some cases surface-generated intermediates can remain and react on the surface. TEMPO and different M@TiO2 nanocomposites (Fig. 1). 90 order to establish how efficient the process was, we examined 125 Having established that the THF reactivity of various eight different catalysts, including bare TiO2. M@TiO2 is similar among the materials tested, we explored materials were used in earlier reports and their properties are a series of ethers to evaluate the generality of this approach. 50% of the TEMPO to yield radical trapping products upon icals, another highly electrophilic species.[14] The reaction re-. Formation is a good reporter for hole trapping even when ether adduct yields are slightly higher in basic media Yields of TEMPO-ether adducts using Pd@TiO2 pho-

Further confirmation that the radicals are free and mobile

No ether

Corresponding Authors

Substitution on the Structure and Thermochemistry of the Cyanomethyl