Photochemistry of Polymethyl and Hydroxy-Polymethyl Cyclohexadienyl Cations.

Abstract

<p> Fluorosulphonic acid has been employed to generate cyclohexadienyl cations by protonation of weak bases in order to study the affect of uv light on such systems which are variably substituted. Differing substitution has been obtained by selecting polymethylbenzenes, cyclohexadienones and polymethylphenols as cation precursors. </p> <p> The photoisomerization of the 1,2,4,5-te-tramethylcyclohexadienyl cation to the 1,2,4,5-tetramethylbicyclo-[3 .1.O.] hexenyl cation has been studied by nmr methods and is discussed mechanistically in terms of competitive electrocyclic and [ δ2a + π2a] mechanisms of ring closure. Investigations of a similar 1,2,3,5-tetrarnethylcyclohexadienyl cation failed to uncover an analogous photoisomerization. This result is briefly discussed in relation to the demonstrated instability of the supposed photo-product which has been produced by a non photo chemical route. </p> <p> Both the 1-hydroxy-6,6-dimethyl- and 1-hydroxy-4 1 4 dimethylcyclohexadienyl cations have been generated from the corresponding cyclohexadienones by low temperature protonation in FS03H. Photoisornerization of these systems produced the same 2-hydroxy-6,6-dimethylbicyclo[3.l.0.] hexenyl cation which underwent further photorearrangement to various protonated phenols, the nature of which depended upon the frequency of irradiation. </p> <p> A number of protonated polymethylphenols have been investigated with respect to their site(s) of protonation in fluorosulphonic acid and their tendency to undergo photochemical reaction. Several members of this series have been shown to rearrange photochemically to isomeric phenols through a bicyclo[3.l.O.] hexenyl cation intermediate. A necessary criterion for this reaction has been established as protonation of a methyl bearing ring carbon. </p> <p> An exceptional example of the phenol photo- isomerization was the photo-conversion of protonated 2,4,6-trimethy·lphenol·to protonated 1,3,5-trimethyl. bicyclo[3.1.o] hex-3-en-2-one. This, as well as additional evidence led to the conclusion that a significant amount of meta protonation occurred with this phenol. </p>