ACS Catalysis
Research Article
Figure 5. Calculated free-energy profile [kcal/mol, M06-2X/6-311+G(2d,2p)-SMD(CH2Cl2)//M06-2X/6-31+G(d,p)-SMD(CH2Cl2)] and
optimized structures of transition states for the tropolonate-catalyzed acylation of benzoyl chloride.
tropolonate 3-K-MeCN to benzoic anhydride via TS-1 to form
intermediate 12, which is a chelating potassium complex. The
activation barrier for the first step is calculated to be 20.2 kcal/
mol relative to 3-K-MeCN. The nucleophilic attack of
cyclohexanol 1l to the benzoyl moiety of 12 takes place via
TS-2, which involves another potassium tropolonate, to
generate product 5l and potassium complexes 13 and 14.
TS-2 is calculated to be 15.3 kcal/mol higher in energy than 3-
K-MeCN. Finally, a proton transfer step from 14 to 13 occurs
to produce benzoic acid, regenerate 3-K-MeCN, and close the
catalytic cycle. It is important to mention that in the second
step, the participation of another potassium tropolonate as a
base catalyst is necessary in order to abstract proton from
cyclohexanol and, thus, enhance the nucleophilicity of
cyclohexanol. We have also considered the possibility that
the benzoate ion acts as a base catalyst instead of potassium
tropolonate (TS-2′). This reaction pathway is also viable,
however, the activation barrier of TS-2′ is calculated to be 4.3
kcal/mol higher in energy than TS-2. Thus, it agrees well with
our earlier belief that the presence of benzoate ion (Figures
3a,b) assists the reaction but is not the driving factor for it to
happen like the tropolonate ion.
Our DFT calculations suggest that, in this reaction,
potassium tropolonate acts as nucleophilic and base catalysts
and that the nucleophilic attack of potassium tropolonate to
benzoic anhydride is the rate-determining step (Figure 4). Our
proposed mechanism is consistent with previous calculations
for the acylation of DMAP derivatives.2a,4d Overall, this
mechanism also agrees well with the experimental results we
collected, as depicted and discussed earlier in Figure 3.
Obviously, this acylation reaction can also take place via a
single-step mechanism with the direct nucleophilic attack of
cyclohexanol to benzoic anhydride (see Figure S1 page S5 in
structure).19 In this mechanistic pathway, potassium tropolo-
nate only participates as a base catalyst. However, the
activation barrier for this step is calculated to be 7.7 kcal/
mol higher in energy than TS-1 and can, therefore, be ruled
out as a feasible or competitive mechanism.
The calculated free-energy profile for the acylation of
benzoyl chloride 6b is shown in Figure 5. It is likely that this
reaction also occurs via a two-step mechanism, that is, the first
step is the nucleophilic attack of potassium tropolonate,
solvated by cyclohexanol instead of the non-coordinating
dichloromethane, to benzoyl chloride, which is followed by the
nucleophilic attack of cyclohexanol to generate the product. It
is interesting to note that the potassium complex cluster (16 +
17) is calculated to be the most stable species. This result
suggests that (16 + 17) is a possible resting state of the
reaction, that is, after the first cycle, the reaction starts from
(16 + 17), which is followed by a proton transfer from 17 to
16 and a ligand exchange giving 3-K-CyOH; after that, two
subsequent nucleophilic additions can then take place giving
the product. The nucleophilic attack of potassium tropolonate
to benzoyl chloride is the rate-determining step with an overall
activation barrier of 22.3 kcal/mol [from (16 + 17) to TS-3,
see Figure 5]. This barrier, although higher than that of the
reaction with benzoic anhydride, is still quite feasible and
consistent with the experimental conditions of this reaction. It
should also be noted here that computational exploration of a
possible role of HCl, such as protonating the carbonyl of 6a′
hence activating it, did not lead to any feasible pathways that
need to be taken into account.
Photochemical Studies. As mentioned in our introduc-
tion, tropolonate ion is known to be a good chromophoric
chelating agent for transition metals12,13 or a sensing probe for
biological systems by enhancing their fluorescent properties.14
Thus, we also wanted to explore if this photoactivity can also
be exploited in the promotion of acyl-transfer reactions. We
started the investigation by looking at the UV−vis absorption
and photoluminescent spectra of tetrabutylammonium tropol-
onate (3-TBA) in acetonitrile. This tropolonate salt was used
in preference over potassium tropolonate (3-K) because of its
superior solubility in acetonitrile and some other organic
solvents. 3-TBA displayed two absorption maxima in
acetonitrile at ∼350 nm and ∼410 nm (Figure 6, top-left).
Excitations at 400 or 420 nm both gave weak luminescence
with emission peaks at ∼460−470 nm.
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ACS Catal. 2020, 10, 12596−12606