A novel case of atom-efficient C-C bond formation of small molecules catalyzed by the facile organoaluminum compound

Article abstract of DOI:10.1016/j.jorganchem.2021.121879

The environmentally friendly and inexpensive organoaluminum compound, diisobutylaluminum hydride (HAlⁱBu₂), acts as an efficient pre-catalyst for C-C bond formation by the addition of terminal alkynes to carbodiimides, which is a rare sample that the group 13 complexes to be used for catalyzing such reactions. Preliminary mechanistic studies revealed that the reaction is mainly carried out by initial hydroalumination of carbodiimide and followed by protonolysis with acetylene. A carbodiimide molecule was inserted into the Al-C bond of the generated alkynyl aluminum compound. The final product was obtained by the second protonolysis with acetylene.

Full text of DOI:10.1016/j.jorganchem.2021.121879

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A novel case of atom-efficient C−C bond formation of small
molecules catalyzed by the facile organoaluminum compound
Yunzhou Zhao , Xiaoli Ma , Ben Yan , Congjian Ni , Xing He ,
Yangfan Peng , Zhi Yang
PII:
S0022-328X(21)00200-X
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https://doi.org/10.1016/j.jorganchem.2021.121879
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24 April 2021
6 May 2021
Please cite this article as: Yunzhou Zhao , Xiaoli Ma , Ben Yan , Congjian Ni , Xing He ,
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https://doi.org/10.1016/j.jorganchem.2021.121879
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A novel case of atom-efficient C−C bond formation of small molecules catalyzed
by the facile organoaluminum compound
Yunzhou Zhao, Xiaoli Ma*, Ben Yan, Congjian Ni, Xing He, Yangfan Peng and Zhi Yang*
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, 100081 Beijing,
People's Republic of China
* Corresponding author.
E-mail: maxiaoli@bit.edu.cn (X. Ma), zhiyang@bit.edu.cn (Z. Yang).
Highlights
1. HAliBu2 catalyzed the hydroacetylenation of carbodiimides with C–C bond formation.
2. Aluminum complexes have not been reported as catalysts for such reaction.
3. The catalyst is cheap, non-toxic and environmentally friendly.
4. Stoichiometric reactions were carried out for the preliminary mechanistic studies.
Abstract
The environmentally friendly and inexpensive organoaluminum compound, diisobutylaluminum
hydride (HAliBu2), acts as an efficient pre-catalyst for C−C bond formation by the addition of
terminal alkynes to carbodiimides, which is a rare sample that the group 13 complexes to be used for
catalyzing such reactions. Preliminary mechanistic studies revealed that the reaction is mainly
carried out by initial hydroalumination of carbodiimide and followed by protonolysis with acetylene.
A carbodiimide molecule was inserted into the Al−C bond of the generated alkynyl aluminum
compound. The final product was obtained by the second protonolysis with acetylene.
1

Keywords
Carbodiimide, Hydroacetylenation, Organoaluminum, Catalysts
1. Introduction
Transition metal complexes have been extensively investigated and applied for several decades,
owing to their diversity in structure and high reactivity [1-3]. However, researches on
transition-metal-like behavior of main-group metals in recent time have made it possible to achieve
the replacement of transition metals in some fields [4-9]. Main-group metals, especially aluminum,
as the most abundant metal in the Earth‟s crust, have inexpensive and environmentally benign
characteristics [10]. In contrast, lanthanide, actinide and transition metal complexes, which were
widely used in catalytic reactions, have poor biocompatibility, high prices and heavy pollution [11,
12]. All these shortcomings mean additional expensive purification costs, especially in reactions
involving drug intermediates or products. Hence, sustainable development awareness and the
increasing risk of environmental pollution have motivated researchers to take full advantage of
aluminum complexes with great effort [13, 14]. So far, as depicted in Scheme 1, organoaluminum
complexes as catalysts have played a significant role in B−E (E = N, O, S) bonds formation by
dehydrocoupling reactions [15], B−E (E = C [6], N [16-18], O [19]) bonds formation by
hydroboration reactions, Si−E (E = N [16], C [20]) bonds formation by hydrosilylation reactions, and
C−N double bond formation by hydroamination reactions [21, 22], among which the hydroboration
reactions were diffusely explored due to their excellent atom economy and broad applications
[23-25]. Whereas, an aluminum complex has not been reported as a catalyst for C−C bond formation
to obtain small molecular compounds. In view of this, we tried to find a reaction that could
potentially be catalyzed by organoaluminum compounds to give amidines.
Because of unique structures and properties, amidines have diverse applications in medicinal
chemistry [26, 27], materials science [28, 29], and homogeneous catalysis [30-32]. As a kind of
amidines, propiolamidines [R−C≡C−C(＝NR')(NHR')] introduce a C−C triple bond to the central
carbon atom, which means a new potential active group. This feature allows them to be used to
synthesize heterocyclic molecules such as azole derivatives, which possess a variety of biological
properties, including anticancer, antibacterial, anti‐inflammatory, anti-nociceptive, antiviral, anti-HIV,
2

etc [33-38]. Moreover, propiolamidines are used as ancillary ligands for transition and lanthanide
metal complexes, which have been used as efficient catalysts for various organic synthesis [39-42].
More recently, these compounds have been used to capture carbon dioxide and emit efficient blue
phosphorescence [43, 44].
Unlike the method using alkynyllithium derivatives as the nucleophilic reagent in large amount,
a straightforward and atom-economical method by catalytic addition of terminal alkynes to
carbodiimides using rare-earth metal complexes as catalysts was reported by Hou et al. in 2005 for
the first time [39]. Since then, numerous rare-earth metal and actinide metal complexes involving Y
[39, 45-50], La [48], Nd [42, 46, 48, 50], Sm [42, 45, 46, 49-51], Eu [49], Gd [48], Dy [46, 48, 50],
Ho [42], Er [49, 50], Yb [39, 45, 46], Lu [39, 48], Th [52], and U [52] have been documented as
efficient catalysts for this reaction (Scheme 1). In addition, Richeson et al.'s work on the catalytic
performance of simple lithium bis(trimethylsilyl)amide (LiN(SiMe3)2) proved that alkali metal
reagents also have good catalytic activity [53]. The research on the catalysis of alkaline earth metal
complexes for the hydroacetylenation of carbodiimides began in 2008, when the
β-diketiminate-stabilized calcium amide was synthesized and used for catalysis by Hill et al [54].
Thereafter, related work was done by Coles et al. with a series of magnesium guanidinate, amidinate,
and phosphaguanidinate compounds [55, 56]. Moreover, Hill et al. reported that the readily available
alkaline earth metal complexes, [M{N(SiMe3)2}2(THF)2] (M = Mg; Ca; Sr), showed increased yield
with increasing central metal atom size [57-59]. Recently, the work of Martínez et al. with ZnEt2 and
the use of dinuclear titanium complexes by Bhattacharjee et al. are good complements for the
research on catalysts for this efficient C−C bond formation reaction [60, 61].
3

Scheme 1. Overview of reactions catalyzed by organoaluminum catalysts and selected examples of catalysts for the
hydroacetylenation of carbodiimides.
To the best of our knowledge, there are no reports on the catalysis of such reactions by group 13
elements in the literature. To this end, we have tried different organoaluminum compounds that are
inexpensive and commercially available. It is observed that diisobutylaluminum hydride (HAliBu2)
has good catalytic performance for this straightforward and atom-economical C−C bond formation
reaction by hydroacetylenation of carbodiimides.
4

2. Results and discussion
Initially, we carried out the reaction between 1.0 mmol of phenylacetylene (PhC≡CH) and 1.0
mmol of N,N'-diisopropylcarbodiimide (DIC) in the presence of various organoaluminum
pre-catalysts (5 mol %) (Table 1, entries 2–6). All the experiments were carried out at 80 ℃ for 36
hours with toluene as the solvent. No desirable product was found under the blank experimental
conditions (Table 1, entry 1). It turned out that diisobutylaluminum hydride (HAliBu2) is the most
active pre-catalyst for the preparation of the propiolamidine [iPrN＝C(C≡CPh)(NHiPr)] with a yield
of 55 % (Table 1, entry 6). AlH3·NMe3, AlMe3, AlEt3, and Al(iBu)3 were used as pre-catalysts at the
same condition and 0%, 21%, 28%, and 41% yield, respectively, were observed (Table 1, entries 2–5).
The results showed that the yield increases with the enhancement of the alkyl group's
electron-donating ability on the aluminum alkyl compounds; moreover, organoaluminum compounds
with branches exhibit better catalytic activity. However, when the same reaction was carried out with
a higher loading of the HAliBu2 (10 mol %) at 80 °C, a yield of 73% product was obtained (Table 1,
entry 7). A yield of 19% product was obtained when the loading of the HAliBu2 was reduced to 3
mol % at 80 °C (Table 1, entry 8). When we extended the reaction time to 48 hours, the yield
increased to 67% (Table 1, entry 9). However, there was no significant increase in yield with the
reaction time extended to 54 hours (Table 1, entry 10).
5

Interestingly, when the solvent was changed to THF, the yield was greatly increased to 86%
(Table 1, entry 11). This may be due to the formation of catalytically-active monomeric
THF-solvated aluminum intermediates in the presence of THF which could probably hinder the
formation of inactive multimers that is similar to what were reported in the previous literature [58].
Compared with those in the entry 11, when the reaction temperature was adjusted to 60 °C, the yield
of the reaction was decreased to 51% (Table 1, entry 12). However, when the temperature went down
Table 1
Optimization table for hydroacetylenation of carbodiimidesa.
Entry
Catalyst [mol %]
T [℃]
t [h]
solvent
Yeildb [%]
1
None
80
80
80
80
80
80
80
80
80
80
80
60
r. t.
36
36
36
36
36
36
36
36
48
54
48
48
48
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
Tol
THF
THF
THF
0
2
AlH3·NMe3 (5 %)
AlMe3 (5 %)
0
3
21
28
41
55
73
19
67
69
86
51
5
4
AlEt3 (5 %)
5
Al(iBu)3 (5 %)
HAl(iBu)2 (5 %)
HAl(iBu)2 (10 %)
HAl(iBu)2 (2 %)
HAl(iBu)2 (5 %)
HAl(iBu)2 (5 %)
HAl(iBu)2 (5 %)
HAl(iBu)2 (5 %)
HAl(iBu)2 (5 %)
6
7
8
9
10
11
12
13
a Reactions carried out using 0.5 mL of solvent, 1.0 mmol of PhC≡CH, 1.0 mmol of DIC.
b Yields were determined by 1H NMR from the crude reaction mixture using 1,1,2,2-tetrachloroethane as an
internal standard.
to room temperature, only 5% yield of product was obtained (Table 1, entry 13). Thus, it is
determined that the optimal reaction condition was 5 mol% catalyst loading of the HAliBu2 at 80 °C
with THF as the solvent.
6

With the optimized reaction conditions in hand, we evaluated the versatility of
Table 2
a
Hydroacetylenation of carbodiimides catalyzed by HAliBu2 .
a
Reactions carried out using 0.5 mL of THF as solvent, 1.0 mmol of alkyne, 1.0 mmol of
carbodiimide, and 0.05 mmol of catalyst (5 mol%), at 80 °C, for 48 h. Yields were determined
1
by H NMR from the crude reaction mixture using 1,1,2,2-tetrachloroethane as an internal
standard.
7

HAliBu2-catalyzed hydroacetylenation of carbodiimides. N,N′-diisopropylcarbodiimide (DIC) and
N,N′-dicyclohexylcarbodiimide (DCC), which have low steric hindrance substituents, react with
phenylacetylene smoothly (Table 2, 3aa and 3ab). However, the increase in steric demands of the
carbodiimide N-substituents affects the yield so significantly that no desirable product was found in
the reaction of phenylacetylene and N,N'-bis(2,6-diisopropylphenyl)carbodiimide (Table 2, 3ac). The
reactions
using
4-ethynyltoluene,
4-ethynylanisole,
4-fluorophenylacetylene,
and
4-bromophenylacetylene as substrates obtained 83%, 81%, 82%, and 83% yields, respectively (Table
2, 3ba, 3ca, 3da, and 3ea). It turned out that the reactions using alkynes with substituents at the para
position of the phenyl group occurred with good yields, regardless of whether the substituent is
electron-withdrawing or electron-donating. Moreover, the positions of the substituents at the phenyl
ring of an aromatic alkyne also have no significant effect on the reaction yield. All meta- and
ortho-substituted phenylacetylenes proceeded smoothly to the corresponding propiolamidines (Table
2, 3fa, 3ga, 3ha, and 3ia). Furthermore, probably owing to its weaker acidity, the substrate of
3-ethynylthiophene, whose aromaticity is slightly weaker than that of the phenyl ring, only
underwent a conversion of 50% (Table 2, 3ja). No desirable product was found when the alkyl
substituted terminal alkynes were used, which could also be attributed to the weak acidity. In a word,
the HAliBu2 as the pre-catalysts demonstrated good functional group tolerance for groups on the
phenyl ring. However, both the large sterically hindered group of carbodiimides and the weak acidity
of terminal alkynes could cause a decrease in reaction yield.
In all cases of previously published literatures, the reaction was carried out by initial metalation
of the terminal carbon of the alkyne followed by insertion of a carbodiimide. However, compared
with alkali metal complexes and alkaline earth metal complexes, organoaluminum complexes, as a
kind of Lewis acid, have weaker Lewis basicity, which makes it difficult to form the metal–acetylide
bonds. As reported in the previous literatures, stoichiometric reaction of HAliBu2 with one equivalent
of PhC≡CH gives the corresponding (E)-diisobutyl(styryl)aluminum, which proceeds very slowly at
room temperature (Scheme 2) [62-64]. However, the stoichiometric reaction of HAliBu2 with one
equivalent of DCC was carried out (Scheme 2), which proceeded rapidly and compound 5 was
obtained in a yield of 80% by recrystallization from hexane after three hours of reaction at room
temperature. 1H NMR spectroscopy showed the emergence of a new single signal at δ 7.75 ppm of
8

the formamidine (NCHN) [See Supporting Information]. The difference in rate may be due to the
fact that DCC is more basic than that of phenylacetylene and thus more readily reacts with HAliBu2
as a Lewis acid.
Scheme 2. Stoichiometric reactions of HAliBu2 with DCC and PhC≡CH, respectively.
More information on the catalytic mechanism was obtained by investigating the catalytic
activity of compound 5 and the freshly prepared solution of (E)-diisobutyl(styryl)aluminum. When
DIC reacted with one equivalent of PhC≡CH in the presence of 5 mol% of compound 5 and
(E)-diisobutyl(styryl)aluminum at 80 °C with THF as the solvent, 82% and 59% yield were observed
respectively after 48 hours. The result showed that compound 5 displayed higher catalytic activity
similar to that of HAliBu2. This confirms that compound 5 is the primary intermediate at the
beginning of the reaction. The stoichiometric reaction of compound 5 with one equivalent of
PhC≡CH was carried out at room temperature to explore whether compound 5 can complete the
initial metalation of the alkyne's terminal carbon, which is similar to alkali metals and alkaline earth
1
metals. As expected, by H NMR monitoring of the reaction, it indicated the single signal's
disappearance at δ 3.00 ppm assigned to the CH proton of the free alkyne, showing the completion of
the metalation of the terminal carbon of the alkyne [See Supporting Information]. In addition, the 1H
NMR spectrum showed the expected high-field doublet at δ -0.14 and -0.16 ppm assigned to the
methylene protons, which indicated that the two isobutyl groups were still attached to the central
9

aluminum atom. The diisobutyl(phenylethynyl)aluminum was obtained after heating with the
1
liberation of the formamidine recrystallized from hexane, which was detected by H NMR [See
Supporting Information].
Scheme 3. The proposed mechanism for hydroacetylenation of carbodiimides catalyzed by HAliBu2.
Based on our findings and the previously published literature, a plausible mechanism was
proposed (Scheme 3). As shown in Scheme 3, there are two possible initiation pathways to produce
intermediate C. The first pathway rapidly affords intermediate A through the reaction of HAliBu2 and
carbodiimide. Subsequently, intermediate A is able to react with acetylene to afford B through
deprotonation of the alkyne−CH bond, which may be caused by the Lewis basicity of the
non-coordinated N atom of the formamidine. Then, intermediate C is obtained from B with the
liberation of the formamidine, R2N＝CHNHR2. The second pathway begins with the slow
hydroalumination of acetylene, affording the alkenyl alane F. Then, species G is obtained through
the insertion of a carbodiimide into the Al−C bond of F. Similar insertion of DCC into the Al−C
10

bond to the α-carbon atom of the vinyl group has been reported [65]. Then, intermediate C is
obtained by the reaction of species G with one equivalent of acetylene. Subsequently, the
propiolamidine D is generated from intermediate C in the same way that species G is obtained from
the alkenyl alane F. As mentioned above, the Lewis basicity of the N atom of the amidinate ligand of
the propiolamidine D could prompt deprotonation of the alkyne–CH bond, affording the species E.
Finally, the product 3 is obtained from species E with the liberation of the intermediate C, which can
undergo the next catalytic cycle.
3. Conclusion
In summary, we have disclosed that the straightforward and 100% atom-efficient
hydroacetylenation of carbodiimides was catalyzed using a low-cost, environmentally friendly, and
commercially available aluminum alkyl compound, HAliBu2, as an efficient pre-catalyst. This is the
first time that aluminum complexes have been reported as catalysts for C−C bond formation to afford
small molecular compounds. Except for the examples of carbodiimides containing large hindered
groups and terminal alkynes with lower acidity, most of the reactions proceeded successfully. Finally,
a series of stoichiometric reactions have shown that HAliBu2 mainly reacts with carbodiimide to
generate the amidinate compound of aluminum, HC(NR2)2AliBu2. This amidinate compound of
aluminum continues to react through deprotonation of the alkyne−CH bond followed by insertion of
a carbodiimide. The final product was obtained with the liberation of the alkynyl aluminum
compound, which continued to participate in the reaction cycle.
4. Experimental section
All manipulations were carried out under a purified nitrogen atmosphere using Schlenk
techniques or inside an Etelux MB 200G glovebox. All solvents were refluxed over the appropriate
drying agent and distilled prior to use. Carbodiimides and terminal alkynes were purchased from
Meryer and were used as received. AlMe3 (1 M in hexane), AlEt3 (1 M in toluene), AliBu3 (1 M in
toluene), and HAliBu2 (1 M in hexane) were purchased from Energy Chemical and were used as
received. The 1H and 13C NMR spectra were provided by the Bruker Ascend II 400 spectrometer,
using chloroform-d (CDCl3) as the solvent. The chemical shifts (δ) are given in ppm relative to TMS.
11

Deuterated solvent was purchased from Meryer (99.8 atom% D) and used as received. Melting points
were measured in sealed glass tubes.
4.1. Synthesis of [HC(NCy)2AliBu2] 5.
To a HAliBu2 (1.0 mL, 1 mmol) solution in hexane (5 mL), DCC (0.206 g, 1 mmol) was added.
After 3 h stirring at room temperature, the solution was concentrated and stored at -20 °C overnight
affording colorless needle crystals of compound 5. Yield: 0.279 g (80%). M. p.: 114-116℃. 1H NMR
(400 MHz, CDCl3, 298 K) δ 7.75 (s, 1H, NCHN), 3.14-3.18 (m, 2H, Cy), 1.14-1.82 (m, 20H, Cy),
1.02-1.07 (m, 2H, CHMe2), 0.82 (d, J = 6.5 Hz, 12H, CH3), -0.15 (d, J = 7.0 Hz, 4H, Al−CH2). 13C
NMR (101 MHz, CDCl3, 298 K) δ 163.81 (NCN), 56.15 (N-C of cyclohexyl), 33.73 (Al−CH2),
27.51, 25.72, 25.45 (Cy), 24.77 (Al−CH2−CH), 24.03 (Al−CH2−CH), 23.69 , 21.15
(Al−CH2−CH−CH3).
4.2. Isolation of the leaving formamidine from the step B to C.
The mixture of 1 mmol of compound 5 and one equivalent of PhC≡CH was heated at 80 ℃ for
12 h, followed by removal of volatiles in vacuo. 2 mL of hexane was added and the solution was
obtained by filtration. The formamidine was recrystallized from the filtrate and detected by 1H NMR.
1H NMR (400 MHz, CDCl3, 298 K) δ 7.33 (s, 1H, NCHN), 2.94 (s, 2H, Cy), 1.74-1.48 (m, 10H, Cy),
1.27-1.06 (m, 10H, Cy).
4.3. General catalytic procedure for the hydroacetylenation of carbodiimides.
In a glovebox, to a 10 mL Schlenk tube equipped with a magnetic stir bar were added
carbodiimide (1.0 mmol), terminal alkyne (1.0 mmol), HAliBu2 (5 mol%), and THF (0.5 mL). The
tube was sealed, removed from the glovebox and heated at 80 ˚C for 48 hours. The reaction yield
1
was measured by H NMR spectroscopy using 1.0 mmol 1,1,2,2-tetrachloroethane as an internal
standard. The mixture was purified directly by aluminum oxide column chromatography (Petroleum
ether/ EtOAc = 30/1) to afford the corresponding propiolamidine.
4.3.1. Spectral data for (E)-N,N'-diisopropyl-3-phenylpropiolamidine 3aa.
Colorless solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.38-7.47 (m, 2H, Ar), 7.25-7.35 (m, 3H,
Ar), 3.83-3.96 (m, 2H, CHMe2), 1.10 (d, J = 6.4 Hz, 12H, CH(CH3)2). 13C NMR (101 MHz, CDCl3,
12

298 K) δ 140.30 (NCN), 130.97, 128.32, 127.46, 120.47 (Ar), 90.72 (ArC≡C), 78.30 (ArC≡C), 46.07
(iPr-CH), 22.90, 22.06 (iPr-CH3).
4.3.2. Spectral data for (E)-N,N'-dicyclohexyl-3-phenylpropiolamidine 3ab.
Colorless solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.39-7.44 (m, 2H, Ar), 7.27-7.34 (m, 3H,
Ar), 3.49-3.59 (m, 2H, Cy), 1.87-1.06 (m, 20H, Cy). 13C NMR (101 MHz, CDCl3, 298 K) δ 140.73
(NCN), 131.00, 128.36, 127.50, 120.54 (Ar), 91.18 (ArC≡C), 78.29 (ArC≡C), 33.29, 24.80, 24.10
(Cy).
4.3.3. Spectral data for (E)-N,N'- diisopropyl -3-p-tolylpropiolamidine 3ba.
Colorless liquid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.32 (d, J = 8.1 Hz, 2H, Ar), 7.10 (d, J =
7.9 Hz, 2H, Ar), 3.83-3.94 (m, 2H, CHMe2), 2.31 (s, 3H, p-MeC6H4), 1.10 (d, J = 6.4 Hz, 12H,
CH(CH3)2).13C NMR (101 MHz, CDCl3, 298 K) δ 141.55 (NCN), 139.71, 131.92, 129.25, 118.41
(Ar), 92.11 (ArC≡C), 78.82 (ArC≡C), 47.79 (iPr-CH), 23.94, 21.59 (iPr-CH3).
4.3.4. Spectral data for (E)-N,N'-diisopropyl-3-(4-methoxyphenyl)propiolamidine 3ca.
Light yellow solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.37 (d, J = 8.8 Hz, 2H, Ar), 6.81 (d, J
= 8.7 Hz, 2H, Ar), 3.83-3.94 (m, 2H, CHMe2), 3.76 (s, 3H, OCH3), 1.10 (d, J = 6.4 Hz, 12H,
CH(CH3)2). 13C NMR (101 MHz, CDCl3, 298 K) δ 160.54 (Ar), 141.95 (NCN), 133.64, 114.17,
113.34 (Ar), 92.66 (ArC≡C), 78.13 (ArC≡C), 58.18 (iPr-CH), 55.37 (OMe), 23.91, 18.41 (iPr-CH3).
4.3.5. Spectral data for (E)-N,N'- diisopropyl-3-(4-Fluorophenyl)propiolamidine 3da.
Light yellow solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.38-7.45 (m, 2H, Ar), 6.95-7.02 (m,
13
2H, Ar), 3.82-3.93 (m, 2H, CHMe2), 1.10 (d, J = 6.4 Hz, 12H, CH(CH3)2). C NMR (101 MHz,
CDCl3, 298 K) δ 161.94 (d, J = 251.5 Hz, Ar), 141.39 (NCN), 134.03 (d, J = 8.6 Hz, Ar), 117.46,
116.04, 115.82 (d, J = 22.2 Hz, Ar), 78.91 (ArC≡C), 58.25 (iPr-CH), 23.88, 18.40 (iPr-CH3).
4.3.6. Spectral data for (E)-3-(4-bromophenyl)-N,N'-diisopropylpropiolamidine 3ea.
White solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.43 (d, J = 8.4 Hz, 2H, Ar), 7.28 (d, J = 8.4
Hz, 2H, Ar), 3.81-3.92 (m, 2H, CHMe2), 1.09 (d, J = 6.4 Hz, 12H, CH(CH3)2). 13C NMR (101 MHz,
CDCl3, 298 K) δ 139.90 (NCN), 132.34, 130.79, 122.81, 119.37 (Ar), 89.44 (ArC≡C), 79.33
13

(ArC≡C), 57.15 (iPr-CH), 22.87, 17.38 (iPr-CH3).
4.3.7. Spectral data for (E)-N,N'- diisopropyl-3-(3-fluorophenyl)propiolamidine 3fa.
Light yellow solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.20-7.31 (m, 2H, Ar), 7.09-7.15 (m,
1H, Ar), 6.98-7.06 (m, 1H, Ar), 3.81-3.93 (m, 2H, CHMe2), 1.11 (d, J = 6.4 Hz, 12H, CH(CH3)2).
13C NMR (101 MHz, CDCl3, 298 K) δ 161.10 (d, J = 247.5 Hz, Ar), 141.02 (NCN), 130.22 (d, J =
8.6 Hz, Ar), 127.91, 123.14 (d, J = 9.4 Hz, Ar), 118.91 (d, J = 23.0 Hz, Ar), 116.74 (d, J = 21.0 Hz,
Ar), 90.48 (ArC≡C), 79.87 (ArC≡C), 58.17 (iPr-CH), 23.88, 18.41 (iPr-CH3).
4.3.8. Spectral data for (E)-N,N'-diisopropyl-3-(3-methoxyphenyl)propiolamidine 3ga.
Light yellow solid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.25-7.29 (m, 1H, Ar), 7.08-7.10 (m,
1H, Ar), 6.99-7.03 (m, 1H, Ar), 6.91-6.97 (m, 1H, Ar), 3.90-4.00 (m, 2H, CHMe2), 3.82 (s, 3H,
OCH3), 1.17 (d, J = 6.4 Hz, 12H, CH(CH3)2).13C NMR (101 MHz, CDCl3, 298 K) δ 159.39 (Ar),
141.22 (NCN), 129.59, 124.54, 122.46, 116.88, 115.78 (Ar), 91.58 (ArC≡C), 79.12 (ArC≡C), 58.22
(iPr-CH), 55.36 (OMe), 23.93, 23.09 (iPr-CH3).
4.3.9. Spectral data for (E)-N,N'-diisopropyl-3-(2-fluorophenyl)propiolamidine 3ha.
Colorless liquid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.38-7.45 (m, 1H, Ar), 7.26-7.34 (m, 1H,
Ar), 6.99-7.12 (m, 2H, Ar), 3.85-3.96 (m, 2H, CHMe2), 1.10 (d, J = 6.4 Hz, 12H, CH(CH3)2). 13C
NMR (101 MHz, CDCl3, 298 K) δ 160.69 (d, J = 253.3 Hz, Ar), 140.01 (NCN), 132.67, 130.24 (d, J
= 8.0 Hz, Ar), 123.06, 114.79 (d, J = 20.6 Hz, Ar), 109.30 (d, J = 15.7 Hz, Ar), 84.17 (ArC≡C),
83.10 (ArC≡C), 22.86 (iPr-CH3).
4.3.10. Spectral data for (E)-N,N'-diisopropyl-3-(2-methoxyphenyl)propiolamidine 3ia.
Colorless liquid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.35-7.38 (m, 1H, Ar), 7.25-7.31 (m, 1H,
Ar), 6.80-6.89 (m, 2H, Ar), 3.89-3.99 (m, 2H, CHMe2), 3.82 (s, 3H,OCH3), 1.10 (d, J = 6.4 Hz, 12H,
CH(CH3)2). 13C NMR (101 MHz, CDCl3, 298 K) δ 160.63 (Ar), 141.98 (NCN), 133.75, 130.96,
120.48, 110.78, 110.44 (Ar), 83.27 (ArC≡C), 55.77 (iPr-CH), 47.70 (OMe), 23.95, 23.49 (iPr-CH3).
4.3.11. Spectral data for thiophenyl-3-(N,N'-di-isopropyl)propargylamidine 3ja.
14

Colorless liquid. 1H NMR (400 MHz, CDCl3, 298 K) δ 7.45-7.53 (m, 1H, SCHC), 7.22-7.27 (m,
1H, SCH=CH), 7.06-7.12 (m, 1H, SCH=CH), 3.81-3.92 (m, 2H, CHMe2), 1.09 (d, J = 6.4 Hz, 12H,
CH(CH3)2). 13C NMR (101 MHz, CDCl3, 298 K) δ 140.30 (NCN), 129.41(SCHC), 128.79
(SCH=CH), 124.79 (SCH=CH), 119.48 (SCHC), 85.99 (ArC≡C), 78.04 (ArC≡C), 57.18 (iPr-CH),
22.89, 17.37 (iPr-CH3).
Declaration of competing interest
The authors declare no competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Nature Science Foundation of China (21872005 and
21671018).
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