THE PHOTOCHEMICAL ACTIVATION OF A REMOTE CARBON-CHLORINE BOND BY A CARBONYL CHROMOPHORE
Abstract
The solution phase photochemistry and photophysics of several chloroketones has been investigated. Initially, molecular orbital calculations at the STO-3G level were performed to determine the C-Cl * degree of involvement in the LUMO of representative chloroketones. Considerable C-Cl * interactions were calculated to be present in the LUMO of -2-chlorocyclohexanone, with less involvement for its equatorial counterpart. The interaction was not limited to -chloroketones, as -6-chloro-2-norbornanone and the 90, 180(DEGREES) rotamer of 4-chloro-2-butanone showed extensive */* mixing in their LUMOs. Experimentally, the postulated but never proven photochemical consequences of the --halogen effect were successfully tested. Upon irradiation in cyclopentane, the isomer of 4-t-butyl-2-chlorocyclohexanone has a quantum yield of disappearance of 0.67 compared to 0.18 for its equatorial counterpart. In both cases, the major photochemical process involved is the homolytic cleavage of the carbon-chlorine bond to produce 4-t-butylcyclohexanone, 4-t-butyl-2-cyclohexenone, and 4-t-butyl-2-cyclopentylcyclohexanone. The photochemistry and photophysics of exo-6-chloro-2-norbornanone (EXO6) were studied. The effects of orbital mixing were observed in the absorption spectrum of EXO6 in the form of bathochromic and hyperchromic shifts relative to other chloronorbornanones. Electron Transmission Spectroscopy (ETS) studies, facile electrochemical reduction and the ease of hydride attack on EXO6 support the extensive (sigma)*/(pi)* mixing calculated for this compound. However, the photochemical consequences of this mixing, as evidenced by C-Cl cleavage, were masked by an extremely facile Norrish Type I cleavage of the norbornanone skeleton. The previously reported photochemistry of 4-chloro-2-butanone was reexamined in detail. The major process in methanol is an acid-catalyzed ground state ketalization to produce 1-chloro-3,3-dimethoxybutane which is in turn converted to 4-methoxy-2-butanone (4MB) by ground state aqueous acid catalysis, not the previously proposed photochemical solvent substitution to produce 4MB. Irradiation of methanolic solutions to promote acid formation which catalyzes the ground state reactions. An analogous behavior is observed in isopropanol and involves the ground state acid-catalyzed formation of 4-isopropoxy-2-butanone. This transformation is thought to occur via the intermediacy of 1-chloro-3,3-diisopropoxybutane but this species could not be detected. Thus, the photochemical consequences of orbital mixing in this simple acyclic (beta)-chloroketone were not observed, due to conformational mobility and ground state reactivity.
Degree
Ph.D.
Subject Area
Organic chemistry
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