The photocyclization and photosolvolysis of a series of hydroxy-substituted biaryl
methanols [special characters omitted] have been studied. The proposed mechanism involves
deprotonation of the HOAr moiety and heterolytic cleavage of the C-O bond of the
hydroxymethyl group [special characters omitted] in the first excited singlet state (S1), to give biaryl quinone
methide intermediates, which subsequently cyclize to the corresponding chromene product
and/or react with solvent to give the solvolysis product. The formation of these quinone
methide intermediates is facilitated by the excited state planarization and the subsequent
charge polarization (negative charge transferred from the HOAr ring into the [special characters omitted]
ring) of the biaryl. Although both of these processes are influenced by steric and
electronic factors, the latter turns out to have a more significant effect on reaction
efficiency.
When the geometry of the molecule goes from a twisted conformation to a more
planar form, the molecule becomes more conjugated and thus gains delocalization energy.
This energy is generally larger for the planarization in the S1 state than in the ground state.
When it is large enough to overcome the steric repulsion for twisting, biaryls can planarize
efficiently in the excited state, which is true for most biphenyl systems. However, it is
shown that the deprotonated forms [special characters omitted] of these biaryls have an even larger
driving force for S1 planarization than the neutral forms [special characters omitted]. Thus,
biaryls with naphthalene ring(s) joined at the 1-position which do not planarize efficiently
in the neutral form can still reach a more planar geometry after adiabatic deprotonation of
the phenolic hydroxy group in S1. That is, the photocyclization of these molecules
proceeds via initial adiabatic deprotonation from the twisted S1 state, followed by twisting
(to the planar form) and subsequent charge polarization which expels the hydroxy group at
the benzylic position [special characters omitted], to give the required quinone methide intermediate.
The o,o'-biaryl quinone methides derived from the o,o'-substituted biaryl methanols
are very short-lived due to rapid intramolecular ring closure and are therefore not
detectable by nanosecond laser flash photolysis. The o,p’- and p,p'-biphenyl quinone
methides, however, do not cyclize and as expected, are readily observable by nanosecond
laser flash photolysis. When the benzylic hydroxy group is replaced by other leaving
groups, the reaction can be used, in principle, as a photodeprotecting reaction and also to
photogenerate acid.
These and other results of the Thesis have uncovered many interesting mechanistic
details of this new class of reaction and hence have increased our general knowledge of
the excited state behavior of aromatic molecules. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/9779 |
| Date | 26 July 2018 |
| Creators | Shi, Yijian |
| Contributors | Wan, Peter |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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