Overall Research Project Description UROP Project Saltiel Laboratory
The Mechanism of Ether Formation from the Photoaddition of Alcohols to p- and m-Methoxystilbenes
The photoaddition of methanol, ethanol and 2,2,2-trifluoroethanol (TFE) to methoxy-substituted stilbene derivatives was described by Roberts and Pincock. [1] The position of the methoxy group was shown to have an enormous effect on the fluorescence efficiency (quantum yield, QYf) of the stilbene. [2] Measured fluorescence yields in acetonitrile are 0.007 and 0.16 for t-p-MeOSt and t-m-MeOSt, respectively, Chart 1. [2] TFE quenches the fluorescence of the trans-isomers and they concluded that this was due to protonation of the trans-stilbene singlet excited states (1t*). The resulting cations react with the alkoxide ion to give ether by coupling, or bibenzyl products by hydride transfer. [2] Ether also forms following rearrangement to a carbene intermediate. [1-4] Our more recent work with the unsubstituted stilbenes established that the major protonation pathway follows torsional relaxation of 1t* to a common zwitterionic twisted intermediate, 1p* that is also accessed from the cis-stilbene singlet excited state, 1c*. [3] The mechanism is shown for deuterated methanol in Scheme 1. Consequently, we have questioned Pincock’s conclusion that protonation precedes torsional relaxation. [3] Studying the initial formation of the ethers starting from the pure trans- and cis-isomers of p-MeOSt and m-MeOSt will allow evaluation of the degree to which ether formation involves 1t* or 1p*.
References
(1) Roberts, J. C.; Pincock, J. A. The Photochemical Addition of 2,2,2-Trifluorethanol to Methoxy-Substituted Stilbenes. J. Org. Chem. 2004, 69, 4279-4282.
(2) Roberts, J. C.; Pincock, J. A. Methoxy-substituted stilbenes, styrenes, and 1-arylpropenes: Photophysical properties and photoadditions of alcohols. J. Org. Chem. 2006, 71, 1480-1492.
(3) Saltiel, J.; Gupta, S. Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State. J. Phys. Chem. A 2018, 122, 6089-6099. Woning, J.; Oudenampsen, A.;
(4) Laarhoven, W. H. Photochemical Addition of Methanol to Stilbenes. J. C. S. Perkin Trans. 2 1989, 2147-2154.