H. Duan et al.
Applied Catalysis A, General 611 (2021) 117943
Table 2
Comparison of raw materials consumption and solid waste generation between
typical Friedel-Crafts method and our method when preparing 1 mol DCPPa.
Entry
Benzene (mol)
AlCl3 (mol)
Et3N⋅HCl (mol)
Solid waste (g)
1b
1.23
1.30
1.18
0.18
1.18
–
344
36
2c
a
Reaction conditions : benzene (1 mol), PCl3 (1.9 equiv.), AlCl3, 80 ◦C, 6 h.
Typical Friedel-Crafts method: Et3N.HCl (1 mol) was used as the decm-
b
plexation reagent.
c
Our method: recovery of benzene (0.79 mol, 94.2 %), the yield of DCPP (77
%).
Fig. 3. Proposed coordination forms of AlCl3 with PCl3, DCPP, and DPC.
2.3. Substrate scope
potential was found above the Al atom in AlCl3 (61.85 kcal molꢀ 1).
Therefore, for each of the three molecules (PCl3, DCPP, and DPC), the Al
atom attacked the region with a minimum value of the electrostatic
potential to form a coordination compound due to the electrostatic
attraction. As shown in Table 3, the Vs, min, p and Vs, min, Cl values in
DCPP and DPC were both negative, so that two types of coordination
compounds can be formed. This validated the proposed coordination
mode. The two configurations were also confirmed by the 31P NMR of
DCPP-AlCl3 and DPC-AlCl3 (Fig. S3). The presence of three strong
electron-withdrawing chlorine atoms gave rise to the positive value of
Based on these promising results, the universality and limitations of
the reaction between arenes and PCl3 were explored with the results
summarized in Scheme 2. It was found that more than one mole of
products was obtained per mole AlCl3 for all substrates when the cata-
lytic amount of AlCl3 was used in this reaction. However, for other
substrates, both with electron-withdrawing (-F, -Cl) and electron-
donating groups (-Me, -Et), the catalytic efficiency of AlCl3 was far
lower than that in the preparation of 2a. The decrease in the AlCl3
catalytic efficiency was attributed to the low reactivity of the arene
caused by the electron-withdrawing groups (Scheme 2, b, and c). While
electron-donating groups endowed the arene with higher reactivity, the
formation of a more stable coordination compound between the product
and AlCl3 also resulted in the low catalytic ability of AlCl3, as demon-
strated by the decrease in the catalytic efficiency of AlCl3 with
increasing number of electron-donating groups (Scheme 2, a, d, and f).
N-methylpyrrole also gave the target product as shown in Fig. S2.
V
s, min, p in PCl3. Therefore, PCl3 only coordinated with AlCl3 in the Type
II configuration. It was also found that both Vs, min, p and Vs, min, Cl
decreased greatly when the Cl atoms in PCl3 were replaced by benzene
rings. This indicated that the coordination ability of AlCl3 with the three
molecules was in the order of DPC > DCPP > PCl3.
Based on the above coordination modes, the mechanism of the re-
action of benzene and PCl3 catalyzed by the catalytic amount of AlCl3
was proposed. Since PCl3 only coordinated with AlCl3 in the Type II
configuration, PCl3 was endowed with electrophilic ability due to the
effect of AlCl3 (Scheme 3, I). Then, the generated electrophilic species
underwent an electrophilic substitution reaction with benzene to form
DCPP-AlCl3 (Scheme 3, III). Different from the common view that DCPP
forms a stable compound with AlCl3 leading to the deactivation of AlCl3,
our results indicate that DCPP and AlCl3 maintain a coordination/
dissociation equilibrium in the catalyst layer. More importantly, the
immiscibility of DCPP-AlCl3 with the raw materials greatly promoted
the dissociation process. The Friedel-Crafts reaction still proceeded due
to the continuous AlCl3 dissociation from DCPP-AlCl3. DCPP was ob-
tained when DCPP-AlCl3 was extracted by PCl3, proving the existence of
the dissociation process (Fig. S4).
2.4. Mechanistic study
Based on the above results and previous reports on the reaction of
benzene and PCl3 in the presence of AlCl3, the reaction mechanism in the
presence of a catalytic amount of AlCl3 was studied in detail. It is
reasonable to suppose that AlCl3 coordinates with PCl3, DCPP, and DPC,
in two configurations, namely either through the coordination of the Al
atom with the P atom (Fig. 3, Type I) or through the coordination of the
Al atom with the Cl atom (Fig. 3, Type II). Analysis of the electrostatic
potential on the molecular surface can be used to predict the coordi-
nation points between the two molecules, and was used to verify the
proposed coordination modes [11]. It was found that the local electro-
static potential in PCl3, DCPP and DPC had two local minima. The first
The proposed mechanism for the reaction of benzene and PCl3 in the
presence of AlCl3 implies that AlCl3 can be recycled. However, it was
found that the catalytic efficiency decreased significantly in the subse-
quent three runs when the catalyst layer was recycled directly (Table 4,
entries 1–3). The obtained catalyst layer was not as transparent as DCPP-
AlCl3 and its appearance was similar to that of DPC-AlCl3 (Fig. S5). This
suggested that DPC was generated in the reaction of benzene and PCl3
under the action of AlCl3. Then, DPC was obtained by the addition of
triethylamine hydrochloride into the catalyst layer, confirming our hy-
pothesis (Fig. S6). Additionally, the fact that DPC was only obtained in
the catalyst layer indicated that DPC formed a stable coordination
compound with AlCl3 that was insoluble in the raw materials. Therefore,
this suggested that the generation of DPC rather than DCPP led to the
deactivation of AlCl3.
minimum (Vs,
p) was located directly above the P atom, corre-
sponding to themloinn, e pair electrons of the P atom. The second minimum
(Vs, min, Cl) corresponded to the negative electrostatic potential around
the Cl atom [12]. Additionally, a local maximum of the electrostatic
Table 3
Local minimum values of electrostatic potential in PCl3, DCPP, and DPC.
Entry
Monomer
Vs, min, Cl (kcal∙molꢀ 1
)
Vs, min, p (kcal∙molꢀ 1
)
Scheme 2. Substrate scope a,b
.
a Reaction conditions: arene (1 mol), PCl3 (1.9 mol) and AlCl3 (0.03 mol), 80 ◦C,
1
2
3
PCl3
DCPP
DPC
ꢀ 5.642
6.775
ꢀ 15.326
ꢀ 21.645
ꢀ 8.376
ꢀ 18.629
6 h.
b Catalytic efficiency of AlCl3.
3