RESEARCH
Mechanism Explains Dienone Rearrangement Electron redistribution approach accounts for unusual phenol and bicyclic ketone that form during photolysis A new mechanism explains how unusual molecular rearrangements of some dienones can take place during photochemical reactions. Advanced by University of Wisconsin chemists, the mechanism involves the excited state picture coupled with continuous electron redistribution to account for the rearrangement of 4,4-diphenylcyclohexadienone (JACS, Nov. 5, 1961). The mechanism also solves the puzzle of how the electron-rich pi system of an excited state can lead to reactions that usually characterize an electron-poor system, according to Wisconsin's Dr. Howard E. Zimmerman. An n to pi* type of excited state forms when a nonbonding or unshared electron (n) is promoted by light to an antibonding pi orbital, pi* (C&EN, July 17, page 4 6 ) . This reaction occurs when compounds such as carbonyls, for instance, absorb light in the 270- to 360-millimicron region. Three Products. 4,4-Diphenylcyclohexadienone is a dicyclohexa-2,5-dien-
1-one derivative. The cyclohexa-2,5dien-1-ones are dienones that give a variety of molecular rearrangements. These dienones are interesting from another standpoint, too. Changes in their carbon skeleton upon irradiation point to an electron-deficient pi system. But, Dr. Zimmerman says, irradiating them with 300-millimicron light gives an n to pi* excited state whose pi system is electron-rich. The new mechanism resolves this anomaly. In their study, Dr. Zimmerman and co-worker Dr. David Schuster irradiated the dienone (in aqueous dioxane) with ultraviolet light, obtaining three compounds: an acid, a bicyclic ketone, and a phenol. The acid is still being studied; the other compounds are 6,6-diphenylbicyclo (3.1.0) -hex-3en-2-one and 2,3-diphenylphenol. First clue to how these compounds form came from the length of irradiation. The Wisconsin workers find that short exposure of the dienone to UV gives good yields of the ketone.
Light Puts Carbonyl Into Excited State
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C&EN
NOV. 13, 1 9 6 1
Longer treatment gives mostly the phenol and the acid. Moreover, irradiating the ketone itself gives the phenol and the acid. So the ketone, they conclude, is an intermediate. Therefore, they reason, the reaction must proceed by some continuous redistribution of the electrons when going from the excited state to product. And even though excited states mean high energies, such photochemical reactions do not occur randomly. From this, Dr. Zimmerman and Dr. Schuster theorize four different processes for such rearrangement reactions: • • • •
n to pi* electron excitation. Rebonding (bridging). Pi to n electron demotion. Ground state reaction.
This mechanism, Dr. Zimmerman points out, accounts for electron-poor pi systems in n to pi* photochemical reactions. Even more important, this approach can be used in many other photochemical reactions, he says.
Photoketone and Photophenol Form by a Four-Step Mechanism
o
11 Irradiation
-t-
330 mu
X
^IBilBSi^HBi^BIH
Ketone
4,4-Diphenylcyclohexadienone
i
y Excited state
1. Electronic excitation (n to Pi*)
2. Change in bonding—bridging between C-3 and C-5 to give bicyclic intermediate is likely OO
oo
•Of
Ketone product
4. F i n a l l y , t h e compound reverts to a stable ground state
3. Electron demotion (Pi* to n) gives a zwitterionic species
r >
Phenol formation involves the same steps
1
1. n to Pi*
oo
00
Electrons drawn as circles are 2S unshared electrons; y's are 2Py unshared electrons in a P orbital in the 'plane of the paper. Solid dots and lines for electron pairs represent electrons in Pzorbitalsf which are perpendicular to the plane of the paper
NOV. 13, 1961 C&EN
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