Chemistry Reference
In-Depth Information
and 4-methoxyphenyl nitrenium ions in water. The barrier to reaction with water for
oxocarbocations are smaller for those with larger charge on the oxocarbo group
compared to those with a smaller amount of charge,
69
and the oxocarbocation
character of the 4-methoxyphenyl nitrenium ion was determined to be large from the
transient Raman spectra and DFT calculated structure.
41
The reaction of the
4-methoxyphenyl nitrenium ion with water is far faster (
k
w
¼
10
6
s
1
)
than for 4-(
N
-methylacetamido)phenyl nitrenium ion (
k
w
¼
2
10
2
s
1
).
64
,
70
The
charge on the oxocarbocation portion of these types of aryl nitrenium ions can be
made smaller by inclusion of a spacer phenyl moiety between the methoxy moiety
and the phenyl nitrenium ion as is the case in the 4
0
-methoxybiphenyl-4-yl nitrenium
ion and this results in to substantially longer lifetime on the order of milliseconds
31
and therefore the reactivity of the nitrene with a
para
-substituent with water can
greatly alter the lifetimes of
para
-substituted phenyl nitrenium ions by enhancing
their reactivity toward nucleophiles, like water, make their lifetimes shorter.
The changes in the lifetimes of aryl nitrenium ions can also affect the selectivity of
the substituted phenyl nitrenium ion for reactions with 2
0
-deoxyguanosine (denoted
here as
k
dG
:
k
w
selectivity). The 4-acetamidophenyl nitrenium, 4-(
N
-methylaceta-
mido)phenyl nitrenium and 4-methoxyphenyl nitrenium ions all have approximately
the same iminocyclohexadienyl character for their phenyl nitrenium portion of their
molecules and they also have similar reaction rates with 2
0
-deoxyguanosine, with
k
dG
¼
2.7
10
6
M
1
s
1
for the 4-(
N
-methylacetamido)phenyl nitrenium ion
41
and
9
10
7
M
1
s
1
for the 4-methoxyphenyl nitrenium ion,
63,64
but their reaction
rates with water are very different and can change by more than 10,000 times:
k
w
¼
k
dG
¼
2
10
2
s
1
for the
4-(
N
-methylacetamido)phenyl nitrenium ion
63,64,70
which results in a
k
dG
:
k
w
selec-
tivity of about 4.5
10
6
s
1
for the 4-methoxyphenyl nitrenium ion to only 2
2.7
10
4
M
1
for the 4-(
N
-methylacetamido)phenyl nitrenium ion
10
4
M
1
variation of values observed for some 4-
biphenylyl derivatives of aryl nitrenium ions.
32,68,70
The 4-(
N
-methylacetamido)
phenyl nitrenium ion displays substantial selectivity toward 2
0
-deoxyguanosine, but
the 4-methoxyphenyl nitrenium ion has a low degree of selectivity with a
k
dG
:
k
w
value of only about 7.4M
1
and this shows how a very reactive
para
-substituent can
dramatically change the lifetime and the 2
0
-deoxyguanosine selectivity of
para
-
substituted phenyl nitrenium ions.
and this is similar to the 2-4
7.4 TIME-RESOLVED RESONANCE RAMAN (TR
3
) SPECTROSCOPY
TO STUDY SELECTED CHEMICAL REACTIONS OF ARYL
NITRENIUM IONS
7.4.1 Aryl Nitrenium Ion Reactions with Aryl Azides
Figure 7.14 (see also Figure 1S in Ref. 47) display TR
3
spectra obtained at a number
of time-delays (displayed to the right of each spectrum and the Raman shifts of
selected bands are presented at the top of 100
s and 10 ns spectra in these figures)
after photoexcitation of 1 mM (Fig. 7.14) and 2mM (Figure 1S in Ref. 47)
m
