Chemistry Reference
In-Depth Information
TABLE 3.2. Rates of Protonation of Nitrene 21 in Alcohol Solvents
k
H
(s
1
)
k
H
(s
1
)
Alcohol
Alcohol
10
11
10
10
CH
3
OH
2.08
CH
3
(CH
2
)
2
CH
2
OH
8.06
10
11
10
10
CH
3
OD
1.39-1.49
(CH
3
)
2
CHOH
5.26
1.05
10
11
4.27
10
10
CH
3
CH
2
OH
CH
3
(CH
2
)
6
CH
2
OH
solutions have not been measured to date, but presumably would be even faster. Here,
as in the previous examples listed in Table 3.1, collapse of the nitrenium ion pairs is
relatively slow requiring 100s of nanoseconds to microseconds.
Several aspects of these nitrene reactions are worthy of note. The profile for their
protonation closely parallels that observed for the extensively studied insertion of
singlet carbenes into
H bonds.
55,56
The azide
absorbs at visible wave-
lengths, which is a very desirable property for PAL agents, since it allows them to be
selectively excited in a matrix of biomolecules that absorb UV light. However,
transient work
52
clearly shows that excitation in the visible region does not lead to
nitrene formation. Nitrene formation occurs from the second or higher excited state
that is accessed via excitation at wavelengths of about 350 nm or shorter. Further-
more, the electronic structure of singlet nitrenes is currently a topic of debate.
57-59
Singlet nitrenes might exist in either a closed-shell or open-shell electronic configu-
ration as shown in Figure 3.2. In many cases, theoretical evaluation indicates that the
open-shell configuration is at lower energy. However, it is much more difficult to
evaluate the configuration in which the nitrene is born. If that happens to be the
closed-shell configuration, relaxation to the open-shell configuration will be highly
forbidden due to difficulty in conserving angular momentum, lack of spin-orbit
coupling,
56
and consequently, the closed-shell nitrene will be slow to relax to the
lower-energy open-shell configuration.
These same spin-orbit coupling considerations indicate that the closed-shell
singlet nitrene will also undergo intersystem crossing to the triplet nitrene much
O
18
Aryl
N
Closed-shell
singlet nitrene
Triplet nitrene
Open-shell
singlet nitrene
Aryl
N
Aryl
N
FIGURE 3.2.
Relationships between aryl nitrene electronic configurations and inter-
conversion between those configurations.
