Chemistry Reference
In-Depth Information
C11 of 1.4732A
;C11
nitrenium ion whose first phenyl ring has C12
C10 of
1.3701 A
;C10
C7 of 1.4373A
;andC12
C13 of 1.4696A
;C13
C8 of 1.3685A
;
C7 of 1.4497A
C8
(see Figure 4 in Ref. 41 for the numbering system) compared to
C8 of 1.4710A
;C8
C7 of 1.3688A
;C7
C6 of 1.4413A
C3
; and C3
C4 of
1.4705 A
;C4
C6 of 1.4486A
for the singlet 4-methoxyphenyl
nitreniumion. These results showthat thealkoxyphenyl nitreniumionshavea substantial
amount of iminocyclohexadienyl character similar to that observed for the 2-fluorenyl
nitrenium ion. The experimental resonance Raman vibrational frequencies of the
symmetric aromatic C
C5 of 1.3641A
;C5
C stretch mode seen at 1633 cm
1
in the 4-methoxyphenyl
nitrenium ion and at 1636 cm
1
in the 4-ethoxyphenyl nitrenium ion are similar to the
2-fluorenyl nitrenium ion stretches 1633 or 1637 cm
1
.
39,40
We note that the symmetric
aromatic C
C stretch vibrational mode in the 1568-1650 cm
1
region has previously
been shown to display a good correlation with the amount of iminocyclohexadienyl
character of a number of aryl nitrenium ions
37,39,40
and the alkoxy nitrenium ions
discussed here also exhibit this kind of correlation.
The greater amount of iminocyclohexadienyl character observed in the 2-fluorenyl
nitrenium ion was thought to be mainly due to the substitution of a second phenyl
ring
para
to the nitrenium ion and the tendency of the biphenyl-like rings to have
quinoidal-like structure with charge being delocalized into the phenyl rings which
may also be helped by the fluorene bridge that further connect and constrain the two
phenyl rings.
40
The alkoxyphenyl nitrenium ions do not have this second phenyl ring,
and it is the alkoxy substitution
para
to the nitrenium ion group that induces a similar
effect as a
para
-substituted phenyl group by having the charge in the alkoxyphenyl
nitrenium ion move to the C
O group resulting in a noticeable charge on the C
O
group and significant imine and oxo cyclohexadienyl character.
The 4-alkoxyphenyl nitrenium ions have interesting properties and chemical
reactivity compared to
para
-phenyl-substituted aryl nitrenium ions. One of these
properties is that the 4-alkoxyphenyl nitrenium ions have lifetimes similar to
para
-
phenyl-substituted aryl nitrenium ions such as the biphenyl-4-yl nitrenium ion
63,64,68
although the 4-alkoxy substitution has much greater cation stabilizing power on the
s
รพ
scale than the 4-phenyl substitution. Another interesting aspect is that adding an
additional phenyl group between the methoxy group and the nitrenium ion group in
the 4
0
-methoxybiphenyl-4-yl nitrenium ion results in an about three orders of
magnitude longer lifetime that are on the order of milliseconds.
31
McClelland and
Ramlall
64
proposed that this could be attributed to the 4-alkoxyphenyl nitrenium ions
being like oxocarbocations and more charge delocalization in the biphenylyl systems
gives a lower oxocarbocation character and hence a longer lifetime. Our TR
3
and DFT
results indicate that 4-alkoxyphenyl nitrenium ions have substantial oxocarbocation
character and in agreement with previous studies on
para
-phenyl-substituted aryl
nitrenium ions that show charge delocalization into the
para
-substituted phenyl
ring
39,40
support this hypothesis and it interesting to note that the barriers to reaction
with water for oxocarbocations are likely lower for those with more charge on the
oxocarbo group compared to those where the charge is more delocalized.
69
The oxocarbocation character of the 4-alkoxyphenyl nitrenium ions also affects
the chemical reactions of these ions with nucleosides like 2
0
-deoxyguanosine. The
