R R Me R R O R R H 1 2 Ca Y + a R = H b R = Me c R = NMe 2 80 60 40 20 0 360 400 500 600 700 λ / nm Reflectance (a.u.) CH 3 D CH 2 D D CH 2 D H CH 3 H Ca Y (D 2 O) C 6 H 12 –CH 2 Cl 2 C 6 D 12 –CD 2 Cl 2 Ca Y (H 2 O) C 6 D 12 –CD 2 Cl 2 C 6 H 12 –CH 2 Cl 2 Generation, entrapment and reactivity of long-lived organic carbocations and radical cations within a supramolecular assembly: Ca Y zeolite K. Pitchumani, a P. H. Lakshminarasimhan, a Nicolette Prevost, a D. R. Corbin b and V. Ramamurthy* b a Department of Chemistry, Tulane University, New Orleans, LA 70118, USA b Central Research and Development, The Du Pont Company, Wilmington, DE 19880, USA Diarylethenes spontaneously form the corresponding radical cations and carbocations upon inclusion within activated Ca Y zeolite; oxygen plays an important role in the generation of the radical cations. By virtue of their ability to generate carbocations and radical cations, zeolites have been used in a number of catalytic processes. 1 Although the mechanism of carbocation formation is fairly well understood, the potential of such intermediates in routine organic transformations has not been fully realized. On the other hand, no agreement on the mechanism of cation radical generation within zeolites has been reached. 2 This communica- tion addresses these concerns. When Ca Y (300 mg) activated at 500 °C under aerated conditions for about 12 h, and cooled to room temperature, was dropped into a cyclohexane solution of 1,1-diphenylethene 1a, (20 mg 3 ml) a bright green colour developed which, in the slurry, persisted for several weeks.† Extraction of the cyclohex- ane–zeolite slurry with dichloromethane gave 1,1-diphenyl- ethane 2a as the single product (Scheme 1). The zeolite residue after extraction was still coloured and remained so for at least 12 h under aerated conditions. When the zeolite turned colourless, it was extracted for a second time with dichloro- methane to yield benzophenone. The overall yield of the two products was 90% diphenylethane and 10% benzophenone. The diffuse reflectance spectra of the zeolite–1a complex as formed in cyclohexane, showed two strong absorptions with l max at 432 and 615 nm (Fig. 1). The zeolite residue after the first extraction with dichloromethane showed only a peak at 615 nm. On the basis of literature reports, we assign the 432 nm peak to the Ph 2 C + Me carbocation and the one at 615 nm to the radical cation of 1a 3,4 The presence of a radical cation in the Ca Y sample is also indicated by EPR spectroscopy. 5 A strong EPR signal with a g-value of 2.00235 was obtained. Ca Y activated in the temperature range 300–600 °C under aerated conditions was found to be ideal for carbocation generation. It has been established in the literature that such conditions of activation lead to the generation of Brønsted acid sites. 6 Through extensive deuteriation experiments, we have established that one of the hydrogens in diphenylethane 2a (Scheme 1) comes from the water present within the zeolite and the other from dichloromethane which is used to extract the product from the zeolite. Results of the deuteriation experi- ments are summarized in Scheme 2. Of the various activation conditions we attempted, Ca Y activated > 400 °C on a vacuum line under reduced pressure ( < 10 23 Torr) was found to be ideal for the generation of radical cations with minimum interference from carbocations. Two remarkable observations made by us in the case of 1,1-diaryl- ethenes lead us to conclude that oxygen plays an important role in the generation of the radical cation within Ca Y. Inclusion of diphenylethene 1a into Ca Y activated at 450 °C on a vacuum line and handled under nitrogen atmosphere generated only a very light green colour.‡ However, when oxygen was bubbled into such a solution, a dark green colour resulted and the benzophenone yield was enhanced from 1 to 10%. Enhance- ment of the yield of the radical cation was also confirmed by the diffuse reflectance spectral data. This observation suggested to us that oxygen is essential for radical cation generation. The following observation made with 4,4A-dimethylaminodiphenyl- ethene 1c further strengthened our view that oxygen plays a Scheme 1 Fig. 1 Diffuse reflectance spectra of diphenylethene included within activated (500 °C aerated) Ca Y. (—) Soon after the addition of Ca Y to a cyclohexane solution of diphenylethene and (---) after extracting the above solution with CH 2 Cl 2 . Scheme 2 Chem. Commun., 1997 181 Published on 01 January 1997. Downloaded on 25/10/2014 11:27:15. View Article Online / Journal Homepage / Table of Contents for this issue