SOLUTION PHASE PHOTOCHEMISTRY AND PHOTOPHYSICS OF BENZNORBORNEN-2-YL CHLORIDES AND MESYLATES
Abstract
The photochemistry and photophysics of exo and endo-benznorbornen-2-yl chlorides (EXOClBNB and ENDOClBNB), and exo and endobenznorbornen-2-yl methanesulfonates (EXOMsBNB and ENDOMsBNB) have been investigated in hydrocarbon and alcoholic media. Free radical chemistry is observed in the photolyses (254 nm) of EXOClBNB ((phi)= 0.25), ENDOClBNB ((phi)= 0.013), and EXOMsBNB ((phi)= 0.013) in cyclohexane, while ENDOMsBNB is essentially inert. EXOClBNB yields an admixture of ionic as well as free radical chemistry upon irradiation (254 nm) in alcoholic media ((phi)= 0.38 - 0.55), whereas ENDOClBNB in methanol ((phi)= 0.019), and EXOMsBNB in t-butyl alcohol ((phi)= 0.15), produce ion derived products exclusively. Photochemical bond cleavage is stereoelectronically controlled, with the exo derivatives showing enhanced reactivity relative to the endo derivatives. EXOClBNB and EXOMsBNB undergo a novel 1,2 migration of the C(,9) bridge in alcoholic media to give 1,3 methanonaphthalen-4-yl ethers. This rearrangement is observed in the photolysis of the tosylhydrazone of benznorbornen-2-one in alkaline alcoholic media, a reaction known to produce "free" ions. A "similar "free" ion is implied to be an intermediate in these photosolvolyses. p-Xylene and acetone sensitization experiments in conjunction with xenon perturbation, have shown the radical products derived from EXOClBNB in cyclohexane, to be predominantly singlet state derived. Similarly, in methanol, the ion and radical derived products have been shown by sensitization, and heptene and oxygen quenching experiments, to be predominantly singlet state derived. However, the triplet state of EXOClBNB (and EXOMsBNB) has been shown to be capable of producing an ion which behaves analogous to that formed in ground state solvolyses. That is, the singlet state produces a "free" on, while the triplet state generates a more "normal" ion. The mechanism of product formation in the photolysis of EXOClBNB in methanol has been investigated, to distinguish primarily between: (1) a heterolytic carbon-chlorine bond cleavage, in competition with a homolytic cleavage and (2) a homolytic cleavage followed by electron transfer within a caged diradical intermediate. External reagents (n-propyl mercaptan, oxygen) fail to intercept such a caged diradical species, however by varying the properties of the solvent cage itself, photochemistry is observed from which one must conclude the existence of such a species. Additional information regarding carbon-chlorine bond cleavage has been obtained from solvent effects on the photophysical properties of EXOClBNB. A dramatic fluorescence quenching is observed for EXOClBNB as the polarity of the solvent increases, and a correspondingly large increase in reaction efficiency is observed. This implies that the carbon-chlorine bond cleavage has substantial polar character. However, a heterolytic cleavage cannot be concluded, since homolytic cleavage has been shown to be enhanced dramatically in polar solvents as well.
Degree
Ph.D.
Subject Area
Organic chemistry
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