Chemistry Reference
In-Depth Information
7.5 CONCLUSIONS AND OUTLOOK
In this chapter, we have demonstrated that time-resolved resonance Raman spectros-
copy (TR
3
) is a very powerful method to study the structure, properties, and chemical
reactivity of a number of aryl nitrenium ions and their reactions with other species in
mixed aqueous and aqueous solutions. High-quality TR
3
spectra were obtained for the
formation of several aryl nitrenium ions (like the 2-fluorenyl nitrenium ion and the 4-
methoxyphenyl nitrenium ion) by protonation of selected singlet aryl nitrene inter-
mediates via very fast reactions with water molecules in aqueous solutions. This
method of using photolysis of aryl azides to generate a very reactive singlet nitrene that
can react effectively with water molecules in aqueous solutions to form appreciable
amounts of the relevant aryl nitrenium ion is limited to those singlet aryl nitrenes that
are able to react fast enough with water to make the aryl nitrenium ion versus other
reactions. Therefore, this method cannot be used to studymany aryl nitrenium ions that
cannot be made efficiently enough from the reaction of the singlet aryl nitrene with
water or to easily study aryl nitrenium ions in different solvents that do not have
appreciable amounts of water to protonate the aryl nitrenes. Future work could make
use of different precursors either reported in the literature or currently being developed
to generate aryl nitrenium ions via different photochemical routes and these methods
may be applied to generate aryl nitrenium ions that are not available from the method
using photolysis of aryl azides in aqueous solutions. By employing different precursors
and photochemical methods to generate aryl nitrenium ions, further work can be done
on investigating how the solvent affects their properties and chemical reactivity with
various substrates. It will be particularly interesting to study to examine aryl nitrenium
ions and their reactions in nonaqueous solvents and compare these results to corre-
sponding results acquired in aqueous solvents so as to elucidate the role of hydrogen
bonding of the water molecules on the properties and chemical reactivity of various
substrates with aryl nitrenium ions. A recent DFT study found that water hydrogen
bonding on a guanosine substrate can noticeably influence the relative reactivity of the
2-fluorenyl nitrenium ion toward the N7 and C8 sites of the guanosine substrate.
93
This
suggests that water hydrogen bonding effects may significantly alter or influence the
chemical reactions of substrates with aryl nitrenium ions and could be an interesting
area to do further work.
ACKNOWLEDGMENTS
I would like to thank my M.Phil. and Ph.D. students and Postdoctoral Fellows who
worked on the research projects described in this chapter: Dr. W. M. Kwok, Dr. P. Y.
Chan, Dr. J. D. Xue, Dr. W. S. Chan, Dr. P. Zhu, Dr. Y. F. Poon, Dr. K. H. Leung,
Mr. S. Y. Ong, Dr. C. Y. Zhao, Dr. Z. Guo, Miss L. M. Chu, and Dr. X. G. Guan.
REFERENCES
1. Miller, J. A.
Cancer Res
.
1970
,
20
, 559-576.
2. Scribner, J. D.; Naimy, N. K.
Cancer Res
.
1975
,
35
, 1416-1421.
