Excited State Intramolecular Proton Transfer (ESIPT) to Aromatic

200 Pages·2007·2.63 MB·English

Excited State Intramolecular Proton Transfer (ESIPT)
to Aromatic Carbon
z '.,
Matthew Joseph Lukeman
B.Sc. (Honours), St. Francis Xavier University, 1999
A Dissertation Submitted in Partial Fulfilment of the
Requirements for the Degree of
Doctor of Philosophy
in the Department of Chemistry
0 Matthew Joseph Lukeman, 2003
University of Victoria
All rights reserved. This dissertation may be reproduced in whole or in part, by
photocopying or by other means, without the permission of the author.
Supervisor: Dr. Peter C. Wan
Abstract
Excited state intramolecular proton transfer (ESIPT) reactions occur when the
acidity and basicity of groups on the same molecule are sufficiently enhanced on
excitation to permit a proton transfer between these sites. Most studies have been carried
out for proton transfers between heteroatoms, though ESIPTs to carbon atoms (alkenyl or
alkynyl) have been reported, and these give rise to new and predictable products. This
Thesis outlines the discovery of a new class of ESIPT reactions in which aromatic carbon
atoms act as the proton accepting group. The reaction either leads to deuterium exchange
of the starting material, or gives new products not readily accessible by thermal means.
The photochemical deuterium incorporation at the 2' and 4'-positions of 2-
phenylphenol(18) and equivalent positions of related compounds has been studied in
D20-CH,CN solutions with varying D,O content. Predominant exchange was observed at
the 2'-position with an efficiency that is independent of D20c ontent. The data are
consistent with an exchange mechanism that involves ESIPT from the phenol to the 2'-
carbon position of the benzene ring not containing the phenol, to generate the
corresponding keto tautomer (an o-quinone methide (QM)). This is the first explicit
example of a new class of ESIPT in which an acidic phenolic proton is transferred to an
sp2-h ybridized carbon of an aromatic ring. Examination of related derivatives led to
postulation of a mechanism of ESIPT that requires an initial hydrogen bonding
interaction between the phenol proton and the benzene n-system. '
The photochemistry of derivatives containing 1- naphthyl proton accepting rings
..
*
111
(59 and 60) was studied. Irradiation of these derivatives in 1: 9 H,O-CH,CN gave rise to
dihydrobenzoxanthene products. Deuterium exchange was observed in both these
products and recovered starting material when D,O was the co-solvent. The reactivity is
rationalized by a mechanism involving solvent-mediated ESIPT processes to the 2'- and
7'-positions of the naphthyl ring for both 59 and 60. Evidence for the intermediacy of
QM intermediates was obtained by laser flash photolysis (LFP) experiments. These
reactions represent the first examples of net product formation (other than simple
deuterium exchange) resulting from ESIPT to an aromatic carbon atom.
The photochemistry of naphthols 10, 62, and 63 has been studied in aqueous
solution with the primary aim of exploring the viability of such compounds for
naphthoquinone methide (NQM) generation. Our results show these derivatives give rise
to three types of NQMs (61,88, and 64) on irradiation. Photolysis of the parent 1-
naphthol (10) in neutral aqueous solution gave 1,5-NQM 61 as well as the non-KekulC
1,s-NQM 88, both via the process of water-mediated ESIPT, based on observation of
deuterium exchange at the 5- and 8-positions, respectively, on photolysis in D,O-CH,CN.
A transient assignable to the 1,5-NQM 61 was observed by LFP experiments. Evidence
is presented that suggests a related ESIPT reaction operates in 1- pyreno1 (89). The more
conjugated I ,5-NQM 64 was formed efficiently via photodehydroxylation of 62; isomeric
63 was unreactive. The efficient photosolvolytic reaction observed for 62 opens the way
to design related naphthol systems for application as photoreleasable protecting groups by
virtue of the long-wavelength absorption of the naphthalene chromophore.

Table of Contents
..
Abstract ............................................................... 11
Tableofcontents ........................................................ v
ListofTables ..........................................................i x
ListofFigures .......................................................... x
..
Listofschemes ....................................................... xvll
...
List of Important Abbreviations ......................................... xvlli
List of Structures ....................................................... xix
Acknowledgements ..................................................... xxi
...
Dedication ..........................................................x xll
1
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Chapter 1 Introduction
1. I Basic Photophysical Processes ........................................ 1
1.2 Changes in Nuclear Configuration .................................... .2
1.2.1General ..................................................... 2
1.2.2 Singlet States of Rigid Aromatic Hydrocarbons ..................... .4
1.2.3 Singlet Excited States of Biaryl Hydrocarbons .......................5
1.2.3.1 Fluorescence of Biaryls with Low Steric Repulsion ...........6
1.2.3.2 Fluorescence of Biaryls with Medium Steric Repulsion ........7
1.2.3.3 Fluorescence of Biaryls with High Steric Repulsion ...........9
1.3 Effects of Substitution of Aromatic Hydrocarbons ....................... 11
1.3.1General .................................................... 11
vi
1.3.2 Reactions Arising from Enhanced Electron Donation in S, ........... 12
1.2.3.1 Photosolvolysis of Benzylic Compounds .................. 12
1.2.3.2 Photoprotonation of Aromatic Rings ..................... 13
1.3.3 Reactions Arising from Enhanced Electron Withdrawal in S, .......... 19
1.3.3.1 Photogeneration of Nitrobenzyl Carbanions ................ 19
1.3.3.2 Photoredox Behaviour of Hydroxyrnethylanthraquinone ......2 0
1.4 Enhanced Acidity of Hydroxyarenes in S, ..............................2 0
1.5 Excited State Intramolecular Proton Transfer (ESIPT) ....................2 7
1.5.1General .................................................... 27
1.5.2 Intrinsic ESIPT ..............................................2 8
1.5.3 Water-Mediated ESIPT ........................................3 0
1.5.4 ESIPT to Carbon Atoms .......................................3 2
1.5.5 ESIPT to Aromatic Carbon .....................................3 8
1.6 Proposed Research ................................................3 9
Chapter 2 .E xcited State Intramolecular Proton Transfer
to Aromatic Carbon in Simple o-Hydroxybiaryls
2.1 Introduction ..................................................... 46
2.2 Materials ....................................................... 47
2.3 Product Studies .................................................. 48
2.4 Fluorescence Measurements ........................................ 62
2.5 Nanosecond Laser Flash Photolysis (LFP) .............................6 5
vii
2.6 Mechanism of Deuterium Incorporation ............................... 68
Chapter 3 .P hotocyclization of o-Naphthyl Hydroxyarenes
3.1 Introduction ..................................................... 74
3.2 Synthesis and Materials ........................................... -75
3.3 Ground State Geometries of Substrates ................................7 6
3.4 UV-vis Studies ................................................... 80
3.5 Product Studies .................................................. 83
3.6 Fluorescence Spectroscopy ......................................... 92
3.7 Laser Flash Photolysis ............................................ 101
3.8 Mechanism of Reaction ........................................... 107
Chapter 4 .E xcited State Intramolecular Proton Transfer
in 1-Naphthol and 1-Pyrenol
4.1 Introduction .................................................... 116
4.2 Materials ...................................................... 119
4.3 Product Studies ................................................. 119
4.3.1 Deuterium Exchange Studies of 10 and 9 ......................... 119
4.3.2 Photodehydration of 62 and 63 ................................. 124
4.3.3 Deuterium Incorporation Studies of 89 ........................... 126
4.4 Fluorescence Studies ............................................. 129
4.5 Laser Flash Photolysis (LFP) Studies ................................ 134
...
Vlll
4.5 Mechanism of Reaction ........................................... 142
4.5.1 ESIPT in 10 ................................................ 142
4.5.2 Photodehydroxylation of 62 and ESIPT of 63 ..................... 144
4.5.3 ESIPT in 89 ................................................ 147
4.6 Summary .................................................... 151
Chapter 5 - Experimental
5.1 General ........................................................ 153
5.2 Materials ...................................................... 154
5.2.1 Common Laboratory Reagents ................................. 154
5.2.2 Synthesis .................................................. 154
5.3 Product Studies ................................................. 161
5.3.1General ...................................................1 61
5.3.2 Results of Product Studies .................................... 162
5.3.3 UV-vis Studies ............................................. 166
5.3.4 Reaction Quantum Yields ..................................... 166
5.4 Steady State Fluorescence and Lifetime Measurements .................. 168
5.5 Laser Flash Photolysis (LFP) .......................................1 70
5.6 X-Ray Crystallography ............................................ 172
References ........................................................... 173
1X
List of Tables
Table 1.1 Excited State pKa Values for 2-Naphthols ........................2 5
Table 1.2 Acidity Data for Phenylphenols ................................2 6
Table 2.1 Deuterium Exchange Data for 18 .............................. -50
Table 2.2 Photophysical and Photochemical Parameters for Hydroxybiphenyls ...6 1
Table 3.1 Calculated Ground State Structural Parameters for Hydroxybiaryls .... 77
Table 3.2 Product Quantum Yields for 58 .6 0 ............................9 1
Table 3.3 Relative Eneries of Chromophores in Hydroxybiaryls .............1 14
Table 4.1 Photochemical and Photophysical Parameters for Naphthols ........ 125
Table 4.2 Summary of LFP Data for 1-Naphthols .........................1 41
Table 5.1 Reaction Quantum Yield of Reactive Compounds ................1 68
Table 5.2 Fluorescence Quantum Yields ................................1 70
Table 5.3 Fluorescence Lifetimes .....................................1 71
Table 5.4 X-Ray Crystallographic Data for 58 ........................... 172
X
List of Figures
Fig. 1.1 Absorption and fluorescence spectra of anthracene in cyclohexane. .... .4
Fig. 1.2 n-Bond orders and relevant valence structures for biphenyl in So
(left) and S, (right). ......................................... .5
Fig. 1.3 Absorption and fluorescence emission spectra of biphenyl. ...........7
Fig. 1.4 Absorption and fluorescence emission spectra of 1,l I-binaphthyl in
cyclohexane ................................................. 8
Fig. 1.5 Absorption and fluorescence emission spectra of 9-phenylanthracene in
cyclohexane ............................................... 10
Fig. 1.6 Orbital diagrams of the HOMO (top) and LUMO (bottom) of
Phenol (left) and 10 (right) as predicted by HMO theory. Lobe
diameters are proportional to the orbital coefficient at that atom,
and the shading represents the phase ............................ 22
Fig. 2.1 Expanded aromatic region of the 'H NMR (360 MHz) of 18 before
(bottom) and after (top) 1 h irradiation in 1 :1 D,O-CH,CN. ......... .49
Fig. 2.2 Plot showing the dependence of deuterium exchange at the
2'-position (,. so lid line) and the 4'-position (0,d ashed line)
of 18 on irradiation time in 1: 1 D20-CH,CN. ..................... 51
Fig. 2.3 Plot showing the dependence of photochemical deuterium
exchange at the 2-position (m, solid line) and 4'-position (*, dashed
line) for 18 on concentration of D20( in CHJN). ................. 54
Fig. 2.4 Plot showing the efficiency of exchange at the 2'-position of 18 solid
(A,