Chemistry Reference
In-Depth Information
Numerous aryl nitrenes have been studied extensively using nanosecond laser
flash photolysis (ns-LFP), nanosecond time-resolved infrared (ns-TRIR), matrix
isolation spectroscopy and with the tools of computational chemistry. Gritsan and
Platz have recently presented a very comprehensive review of this subject.
9
Photolysis of phenyl azide (PhN
3
, Scheme 1.1, R
H) leads to the production of
singlet phenyl nitrene (
1
PhN), benzazirine (PhAZ), ketenimine (PhK), and triplet
nitrene (
3
PhN). The lifetime of
1
PhN is
1 ns in organic solvents at ambient
temperature, and is controlled by intersystem crossing to its lower-energy triplet
state,
3
PhN, intramolecular rearrangements and intermolecular acid-base reactions
(Scheme 1.1). Some singlet aryl nitrenes have even shorter lifetimes in solution, such
as singlet
o
-biphenylyl nitrene and 1- and 2-naphthyl nitrenes, due to their rapid
intramolecular rearrangements. These reactive intermediates cannot be observed by
nanosecond time-resolved spectroscopies at room temperature. Matrix spectroscopic
methods utilize very low temperature to suppress chemical reactions of reactive
intermediates, but cannot prevent relaxation by intersystem crossing unless inter-
system crossing (ISC) is accompanied by a large geometry change.
19
Most aryl
nitrenes have triplet ground states and thus, short-lived singlet aryl nitrenes cannot be
characterized by matrix isolation spectroscopic methods. The development of
ultrafast time-resolved spectroscopy provides the first opportunity to study these
very short-lived reactive intermediates by direct observational techniques.
Nitrenium ions are the conjugate acids of nitrenes. Falvey has reviewed recent
developments in the field of nitrenium ion chemistry.
20
McClelland's group pio-
neered the field of producing nitrenium ions by protonating nitrenes.
21-23
This
method works particularly well when the singlet nitrene to be intercepted has a
relatively long lifetime (
¼
10 ns) in an aprotic solvent at ambient temperatures. In this
manner,
p
-biphenylyl nitrenium cation (
p
-BpNH
þ
), produced by protonation of
singlet
p
-biphenylyl nitrene (
1
p
-BpN), was readily detected by nanosecond transient
UV-Vis spectroscopy.
21-24
Phillips et al. subsequently studied this nitrenium cation
in water using time-resolved resonance Raman spectroscopy and assigned the
spectra with the aid of density functional theory (DFT) calculations.
25
The intrinsic
drawback of McClelland's method is that the protonation of the singlet nitrene has to
be very rapid to compete with other deactivation channels, such as intersystem
crossing to the lower-energy triplet state, and intramolecular rearrangement.
o
-Biphenylyl nitrene (
o
-BpN) and 1-naphthyl nitrene (1-NpN) are well-known
short-lived singlet nitrenes, whose lifetimes in CH
3
CN (16 and 12 ps, respec-
tively)
26,27
are controlled by intramolecular cyclizations. Thus, even when proto-
nation can compete with the other decay channels, ultrafast spectroscopic methods
will still be required to resolve the formation of these nitrenium cations.
In this chapter, we will describe the application of ultrafast transient absorption
spectroscopy in the study of the photochemistry of
para
- and
ortho
-biphenylyl
azides (
p
-BpN
3
and
o
-BpN
3
, respectively) and 1-naphthyl and 2-naphthyl azides
(1-NpN
3
and 2-NpN
3
, respectively) and report the observation of the S
2
azide excited
states and lifetimes, the spectra and lifetimes of the corresponding singlet aryl
nitrenes in acetonitrile solution at ambient temperature and the formation of their
corresponding nitrenium ions in protic solvents. This chapter will mainly focus on
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