UV-vis and IR spectral characterization of persistent carbenium ions, generated upon incorporation of cinnamyl alcohols in the acid zeolites HZSM-5 and HMor

Cinnamyl alcohol (1) and two derivatives 2 and 3 have been incorporated in dehydrated HMor and HZSM-5 zeolites with, the aim to characterize spectrosc opically the corresponding carbocations generated within the solids. Produc t studies of the supernatant liquid phase combined with diffuse reflectance UV-vis and IR spectroscopy provide unequivocal evidence for the carbocatio ns. Thus, cinnamyl alcohol (1) affords the 1,5-diphenylpentadienyl cation i n HMor and HZSM-5 as a persistent species. In the case of HMor with larger pore dimensions the bulkier 1-(2'-cinnamyl)-3-phenylpropenyl cation was als o spectroscopically detected. No persistent carbocation was observed when t he alpha-methylcinnamyl alcohol (2) was incorporated in the acid zeolites, wherein a complete cyclization to 2-methylindene takes place. Finally, inco rporation of 2-methyl-4-tolyl-3-buten-2-ol (3) in HZSM-5 allowed detection of the gem-dimethyl-subsitutedp-methylcinnamyl cation, with a lifetime of h ours. This cation is not persistent enough in HMor to be characterized. The present study illustrates how structurally related allylic substrates may give distinct carbenium ions whose persistence depends on the host-guest fi t in the interior of the acid zeolites.