The Journal of Organic Chemistry
Article
recovery (ΦIC = 30%) in chloroform after 390 nm excitation.
Thus, we can assume the same decay of amplitude of the νNN
bleach takes place at about 2120 cm−1 (Figure 1A) as for νCO at
1630 cm−1. Excitation at 266 nm (Figure 1B), compared to 390
nm (Figure 1A), produces a similar amount of ketene deduced by
comparison of the IR band amplitude ratio of thermalized ketene
to initial νNN bleach.
The occurrence of a RIES process as the major ketene
formation route might be confirmed by unchanged yield of
ketene formation in methanol, a carbene-trapping solvent,
compared to the relatively inert solvent as chloroform.
Measurements performed using the same experimental con-
ditions show that the intensity of the thermalized ketene band in
both solvents is similar (λexc = 390 nm). Thus only a small
amount of methanol-trappable carbene can be formed from
diazoquinone, implying that the major path of ketene formation
is RIES. An alternative scenario excludes RIES, if methanol is
unable to compete efficiently with carbene isomerization.
To investigate the mechanism of the ketene formation in the
ground and the excited state of the precursor, we carried out
further calculations using the Turbomole program. This
mechanism has been investigated computationally in the past,
however, with semiempirical calculations.21 We carried out
relaxed scans on the ground state to estimate the barrier of (1)
nitrogen extrusion and carbene formation (stepwise mechanism)
and (2) ketene formation without forming the carbene
intermediate (concerted mechanism). As shown in Figure S2
(Supporting Information), when the ground state is optimized
with constrained C−N bond lengths, carbene formation begins
around 1.8−1.9 Å. Further elongation of the C−N bond does not
change the energy significantly; however, when the C−N bond
length reaches about 2.6 Å, the carbene rearranges to form the
ketene. This is consistent with previous studies,4,5 which
suggested that carbonyl carbenes lie in a shallow well that can
almost barrierlessly rearrange to form the corresponding ketene
product. Figure S2 (Supporting Information) also suggests that
the carbene formation has a barrier of about 32−35 kcal/mol on
the ground state along the C−N bond length.
To investigate the concerted mechanism, the potential energy
surface along C−C bond length was scanned as shown in Figure
S2 (Supporting Information). As the C−C distance shortens the
C−N bond length increases and the energy rapidly rises. When
the C−C bond length is about 2.0 Å, the energy rises to about 36
kcal/mol above the ground state’s energy, and compression of
the C−C bond any further rapidly drops the energy of the system
to immediately yield the ketene product without the formation of
any carbene intermediate.
In order to characterize the excited states of the diazocyclo-
hexadienone, we obtained the difference density plots for the first
four excited states using a procedure described elsewhere.22
Figure 2 depicts the computed difference density plots of the four
S1−S4 excited states. The red and green color contours indicate
depletion and accumulation of electronic density upon electronic
excitation, respectively. It is clear that the S1 state is a π → π*
(out-of-plane) excited state while S2−S4 states have significant n
→ π* character with a notable contribution from the carbonyl
unit. The electronic density is significantly depleted at C−N2
bond in the S1 state, which suggests that the C−N bond cleavage
may proceed in the S1 excited state and can lead to carbene
formation.
Figure 2. Difference density plots for the S1−S4 excited states of
diazocyclohexadienone as computed at the B3LYP/TZVP level of
theory. The red contours depict depletion and green contours depict
accumulation of electron density in the excited state upon electronic
excitation from the S0 ground state, respectively. S1−S4 plots are plotted
with isocontour values of 0.005 a.u.
optimize the S1 state geometry. As observed earlier for
phenyldiazomethane,22 the diazo unit attains an angular
geometry in the S1 excited state and the C−N2 bond length
elongates significantly when compared to the ground state
geometry (Figure 3).
Figure 3. Optimized (a) ground state and (b) S1 excited-state geometry
of diazocyclohexadienone calculated at the B3LYP/TZVP and TD-
B3LYP/TZVP levels of theory, respectively. The bond lengths are
shown in angstroms.
We carried out the relaxed scans on the S1 surface using this
optimized geometry along C−N and C−C bond lengths to
investigate the stepwise and concerted mechanisms, respectively.
As shown in Figure 4a, the stepwise extrusion of molecular
nitrogen from the diazo precursor has an energy barrier of only 6
kcal/mol compared to 35 kcal/mol on the ground state, which
suggests that the nitrogen extrusion to form carbene is more
facile on the S1 state. On the other hand, the scan for the
concerted mechanism is shown in Figure 4b, where the energy
barrier to nitrogen extrusion and simultaneous ketene formation
is almost the same on the S1 state and S0 state surfaces.
Consequently, we conclude that the diazo precursor extrudes
molecular nitrogen first and then rearranges to form the ketene.
However, it is possible that the C−N bond cleavage generates a
carbene with excess vibrational energy and as a result, ketene
formation proceeds immediately (<1 ps) after the departure of
molecular nitrogen.
o-Phenylene Thioxocarbonate. Vinylene thioxocarbon-
ates have been proposed as sources for the synthesis of ketenes
and for mechanistic study of the Wolff rearrangement (WR).18,23
Photolysis of o-phenylene thioxocarbonate (2) in an argon
Obtaining the same information for the S1 state as discussed
above required the optimized geometry of the precursor on the
S1 surface. We used the ground-state-optimized geometry to
C
dx.doi.org/10.1021/jo302023a | J. Org. Chem. XXXX, XXX, XXX−XXX