Full Paper
in a concerted manner. In the structure of TS(8–9), the CꢀCl
and PdꢀN bond lengths are lengthened to 1.989 and 2.032 ꢁ,
respectively, whereas the Pd-Cl and CꢀN bond lengths are
shortened to 2.409 and 2.063 ꢁ, respectively (Figure 5). A typi-
cal four-center s-metathesis feature is seen at the TS structure.
The predicted free energy barrier for the s-bond metathesis
shown in Figure S2 in the Supporting Information). The RE
°
ꢀ1
reaction of 4 is predicted to have DG =36 kcalmol and to
RE
0
ꢀ1
lead to an endergonic Pd product with DG =8 kcalmol
RE
(see the Supporting Information, Figure S1, path a).
As for the s-bond metathesis mechanism, a four-centered TS
structure is located (see the Supporting Information, Figure S2,
TS(14–15)). Theory shows that the free energy barrier associ-
ꢀ
1
route is approximately 27 kcalmol relative to the separated 3
ꢀ
1
ꢀ1
plus PhCl, which is approximately 14 kcalmol lower than that
for the reductive elimination of 3. Formation of the amination
product complex is computed to be very exothermic, by
ated with this pathway is 30 kcalmol relative to separated 4
and PhCl. The resulting product complex is significantly exo-
ꢀ
1
thermic, by 31 kcalmol (see the Supporting Information, Fig-
ure S1, path b). Therefore, the RE pathway of 4 is unfavorable
from both kinetic and thermodynamic perspectives compared
with the corresponding s-bond metathesis mechanism. In
ꢀ
1
4
1 kcalmol (Figure 4, path b). The complex 1h can be regen-
erated after diphenylamine is replaced by aniline. Interestingly,
the s-bond metathesis pathway leading to the Ph NH product
2
0
II
IV
directly is much more favorable than the generation of Pd via
addition, similar to 3, the Pd /Pd , SET, and HAT routes of 4 are
energetically unfavorable in comparison with the corres-
ponding s-bond metathesis mechanism (see the Supporting
Information for details).
the RE pathway, both kinetically and thermodynamically.
IV
The oxidative addition of PhCl to 3, forming a Pd inter-
mediate, is computed to have a free energy barrier of 49 kcal
ꢀ
1
IV
mol . This step results in an endergonic Pd intermediate
ꢀ
1
with DG=32 kcalmol (Figure 4, path c). The computational
results show that the oxidative addition of PhCl to 3 is highly
unfavorable.
Possible reaction routes of the [(NHC)Pd(NHPh) ] intermediate
2
For [(NHC)Pd(NHPh) ] intermediate 6, five possible routes were
2
III
Intermolecular SET from 3 to PhCl can yield a Pd radical
also investigated. The RE of 6 was computed to have a barrier
ꢀ
1
cation (11) and an aryl chloride radical anion. Subsequently,
dissociation of the CꢀCl bond of the aryl chloride radical anion
yields a free aryl radical and a chloride anion. Finally, the free
height of 33 kcalmol (the TS structure of this step, TS(6–19),
is shown in Figure S4 in the Supporting Information). The gen-
0
ꢀ1
erated Pd product complex 19 is endothermic by 3 kcalmol
III
aryl radical reacts with the Pd radical cation to produce the
(see the Supporting Information, Figure S3, path a). Compared
with the RE of intermediates 3 and 4, the RE of 6 has the
lowest energy barrier and the smallest endothermicity.
desired product. However, the theoretical results demonstrate
that the free energy barrier of the SET pathway is prohibitively
high, suggesting that the SET mechanism is very unlikely to
occur (Figure 4, path d).
°
The s-bond metathesis pathway for 6 has a computed DG
ꢀ
1
of 24 kcalmol
four-centered
relative to separated 6 and PhCl. The
TS results in product complex,
The chlorine atom transfer pathway involves homolytic
cleavage of the CꢀCl bond of PhCl and simultaneous formation
a
[(NHC)Pd(Cl)(NHPh)(NHPh )] (21), the generation of which is ex-
2
III
ꢀ1
of the PdꢀCl bond to afford a Pd radical (12) and an aryl radi-
ergonic by 44 kcalmol (see the Supporting Information, Fig-
cal. The generated aryl radical favors reaction with the anilido
ure S3, path b). Similar to 3 and 4, intermediate 6 also favors
the s-bond metathesis pathway in the presence of PhCl, in-
ligand of 12 to afford the coupling product, rather than com-
III
IV
0
bining with the Pd radical to form a Pd intermediate. This is
because the formation of the coupling product is dramatically
stead of the corresponding RE route leading to Pd . Computa-
tional results also suggest that the s-bond metathesis pathway
proceeding through 6 is more favorable than the analogous
IV
exothermic, whereas the formation of the Pd intermediate is
I
II
IV
endothermic (Figure 4, path e). Similar to the Cu -catalyzed Ull-
route via 3 and 4. The other three routes applied to 6, Pd /Pd ,
SET, and HAT, are less competitive than the corresponding s-
bond metathesis mechanism (see the Supporting Information
for details).
mann reactions via the HAT mechanism, the PdꢀCl bond for-
mation can compensate the energy needed to break the CꢀCl
[
18]
bond of PhCl. Nevertheless, the predicted energy resulting
in formation of 12 and an aryl radical is approximately 27 kcal
Overall, the present computational results indicate that the
ꢀ
1
II
mol higher than the energy of separated 3 and PhCl. Al-
though the TS of the HAT pathway has not been located, the
energy barrier height of HAT mechanism must be higher than
three plausible deprotonated aniline-containing Pd species
0
may not undergo RE reactions to form Pd easily in the 1f-cat-
alyzed amination of chlorobenzene with aniline. Instead, the s-
bond metathesis mechanism is more facile for all three inter-
mediates than the corresponding RE reactions, both kinetically
and thermodynamically. Other proposed reaction pathways,
ꢀ
1
2
7 kcalmol and, therefore, higher than that of the s-bond
metathesis mechanism, so the HAT pathway is less feasible
[
21]
than the s-bond metathesis route.
II
IV
such as the Pd /Pd mechanism, the SET mechanism, and the
HAT mechanism, cannot compete with the s-bond metathesis
mechanism either.
Possible reaction routes of the [(NHC)Pd(OtBu)(NHPh)]
intermediate
Similarly, the five reaction pathways in Scheme 2 can also
Frontier orbital analysis of the s-bond metathesis mechanism
apply to the [(NHC)Pd(OtBu)(NHPh)] intermediate 4. Of primary
0
interest is the possibility of the RE of 4 to afford the Pd
A frontier orbital analysis provides more insight into the
species (the optimized TS structure of this step, TS(4–13), is
amination reaction via the s-bond metathesis mechanism. The
&
&
Chem. Eur. J. 2015, 21, 1 – 10
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!