Palladium-Catalyzed Diarylation of Isocyanides with Tetraarylleads for the Selective Synthesis of Imines and α-Diimines

Article abstract of DOI:10.1021/acs.joc.9b01639

Using tetraaryllead compounds (PbAr₄) as arylating reagents, isocyanides undergo selective diarylation in the presence of palladium catalysts such as Pd(OAc)₂ or Pd(PPh₃)₄ to afford imines and/or α-diimines based on the isocyanide employed. With aliphatic isocyanides, imines are obtained preferentially, whereas α-diimines are formed in the case of electron-rich aromatic isocyanides. The differences in imine/α-diimine selectivity can be attributed to the stability of imidoylpalladium intermediates formed in this catalytic reaction. Compared with other arylating reagents, tetraaryllead compounds are excellent candidates for use in the selective transformations to imines and/or α-diimines, especially in terms of inhibiting the oligomerization of isocyanides, which results in a lower product selectivity in many transition-metal-catalyzed reactions of isocyanides.

Full text of DOI:10.1021/acs.joc.9b01639

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Article
Palladium-Catalyzed Diarylation of Isocyanides with Tetraarylleads
for the Selective Synthesis of Imines and #-Diimines
Cong Chi Tran, Shin-ichi Kawaguchi, Yohsuke Kobiki, Hitomi Matsubara,
Dat Phuc Tran, Shintaro Kodama, Akihiro Nomoto, and Akiya Ogawa
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01639 • Publication Date (Web): 21 Aug 2019
Downloaded from pubs.acs.org on August 21, 2019
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The Journal of Organic Chemistry
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Palladium-Catalyzed Diarylation of Isocyanides with
Tetraarylleads for the Selective Synthesis of Imines and
α-Diimines
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Cong Chi Tran†, Shin-ichi Kawaguchi§,*, Yohsuke Kobiki†, Hitomi Matsubara†, Dat Phuc Tran†,
Shintaro Kodama†, Akihiro Nomoto†, and Akiya Ogawa†,*
†Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1
Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
§Center for Education and Research in Agricultural Innovation, Faculty of Agriculture, Saga University,
152-1 Shonan-cho Karatsu, Saga 847-0021, Japan
ogawa@chem.osakafu-u.ac.jp ; skawa@cc.saga-u.ac.jp
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
CORRESPONDING AUTHOR FOOTNOTE. *,§Phone: +81-955-77-4484, fax: +81-955-77-4486, and
e-mail: skawa@cc.saga-u.ac.jp ; *,†Phone: +81-72-254-9290, fax: +81-72-254-9290, and e-mail:
ogawa@chem.osakafu-u.ac.jp
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R
N
1
2
3
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R
N
Ar
R
Pd catalyst (5 mol%)
Ar
PbAr4
+
RNC
and/or
toluene, 100 °C, 2 h
Ar
Ar
N
R =
aryl, alkyl
Catalytic Diarylation.
No Tri- or Oligo-
merization was
observed.
Up to 91% Yield
14 Examples
Up to 86% Yield
13 Examples
Product selectivity is dependent on the
isocyanides substituent
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ABSTRACT
Using tetraaryllead compounds (PbAr4) as arylating reagents, isocyanides undergo selective diarylation
in the presence of palladium catalysts such as Pd(OAc)2 or Pd(PPh3)4 to afford imines and/or α-diimines
based on the isocyanide employed. With aliphatic isocyanides, imines are obtained preferentially, whereas
α-diimines are formed in the case of electron-rich aromatic isocyanides. The differences in imines/α-
diimines selectivity can be attributed to the stability of imidoylpalladium intermediates formed in this
catalytic reaction. Compared with other arylating reagents, tetraaryllead compounds are excellent
candidates for use in the selective transformations to imines and/or α-diimines, especially in terms of
inhibiting the oligomerization of isocyanides which results in a lower product selectivity in many
transition-metal-catalyzed reactions of isocyanides.
KEYWORDS. Tetraaryllead, Isocyanide, Diarylation, Imine, α-diimines, Palladium catalyst
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The Journal of Organic Chemistry
INTRODUCTION
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Isocyanides, which contain a moiety whose structure is isoelectronic with that of carbon monoxide, are
useful C1 units in organic synthesis in addition to being excellent building blocks for nitrogen-containing
heterocyclic compounds.1 In transition-metal-catalyzed reactions, carbon monoxide inserts into carbon-
metal (C-M) bonds, generating acyl-metal species as key intermediates, and usually, monocarbonylation
takes place selectively. In contrast, in the case of isocyanides, not only one molecule but two or more
molecules of isocyanides can insert into C-M bonds, resulting in the formation of a mixture of mono-, di-,
tri-, and oligo-isocyanide containing compounds.2-5 Thus, to develop novel synthetic reactions using
isocyanides, control of this “multi-insertion” process is necessary. In this context, we recently developed
a selective reaction between isocyanides and triarylbismuth species in the presence of a Pd(OAc)2 catalyst,
to selectively afford α-diimines, through the incorporation of two isocyanides molecules (Scheme 1, eq
1).6 The same reaction failed using arylating reagents of early elements such as PhB(OH)2 (eq 2). We
therefore turned our focus to rate element arylating reagents, and examined the reaction of t-butyl
isocyanide with PhI, Ph3Sb, Ph4Sn, and Ph4Pb under similar conditions as those employed for the
diarylation reaction using triarylbismuths species (eq 2). Interestingly, the use of tetraphenyllead7,8
resulted in the novel diphenylation of one isocyanide molecule to selectively give the corresponding
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benzophenone imine in good yield. Thus, we report our investigation into the Pd-catalyzed diarylation of
isocyanides using tetraaryllead (eq 3)
.
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Scheme 1. Palladium-catalyzed arylation of isocyanides
Previous work:
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R
N
Pd(OAc)2 (5 mol%)
Ph
RNC + BiAr3
(1)
Ph
MeCN, air, 70 °C
N
R
t-Bu
N
9
t-Bu
N
Pd(OAc)2 (20 mol%)
phenyl
source
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Ph
t-BuNC
+
+
(2)
Ph
bezene (2 mL)
N2, 70 °C, 18 h
Ph
Ph
N
0.2 mmol
0.2 mmol
PbAr4
t-Bu
72%
trace
ND
28%
3%
PhB(OH)2
PhI
ND
SbPh3
SbPh4
0%
18%
ND
15%
*Determined by 1H NMR (ND = not detected).
This work:
R
N
R
Pd cat. (5 mol%)
toluene, N2, 100 °C
N
Ph
(3)
RNC + PbAr4
and/or
Ph
Ph
Ph
N
R
RESULTS AND DISCUSSION
Initially, we optimized the reaction conditions for imines synthesis using t-butylisocyanide 1a and
tetraphenyllead 2a (Table 1). When the reaction of 1a (0.2 mmol) with 2a (1.5 equiv) was carried out in
the presence of Pd(OAc)2 (2.5 mol%) in toluene under N2 atmosphere for 4 h, imine 3aa was obtained in
a good yield (75%) along with α-diimine 4aa (11%, entry 1). Increasing the quantity of palladium catalyst
employed (i.e., 5 or 10 mol %) led to 85 and 72% yields of 3aa, respectively (entries 2 and 3). The yield
dramatically decreased when the reaction was conducted under air (45% yield, entry 4), while the use of
acetonitrile (MeCN) as a solvent inplace of toluene also gave a low yield (42%) of 3aa (entry 5).
Increasing the quantity of 2a to 2.0 equiv. resulted in a 77% yield of 3aa (entry 6). Increase the reaction
temperature with shorten time result in the similar result to entry 2 (entry 7).
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Table 1. Optimization of the catalytic diarylation reaction conditionsa.b
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t-Bu
N
t-Bu
palladium catalyst
N
Ph
t-BuNC
PbPh4
+
+
Ph
toluene, temp., time, N2
Ph
3aa
Ph
N
t-Bu
1a
2a
4aa
(0.2 mmol)
(1.5 equiv.)
8
9
temp.,
°C
yield of
3aa, %
yield of
4aa, %
time,
h
entry
Catalyst
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2
Pd(OAc)2 2.5 mol%
Pd(OAc)2 5 mol%
Pd(OAc)2 10 mol%
Pd(OAc)2 5 mol%
Pd(OAc)2 5 mol%
Pd(OAc)2 5 mol%
Pd(OAc)2 5 mol%
Pd(OAc)2 5 mol%
Pd(PPh3)4 5 mol%
PdCl2(PPh3)2 5 mol%
70
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2
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2
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2
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8
70
3
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72
4c
5d
6e
7
70
45
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43
70
77
100
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100
100
100
85
8f
9
92 (72)
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26
22
15
53
10
41
11f,g Pd(OAc)2 5 mol%
12f
Catalyst A 5 mol%
75
28
aReaction conditions: Isocyanide (1a, 0.2 mmol), tetraphenyllead (2a, 1.5
equiv.), toluene (2 mL), and the transition metal catalyst were added in this
order. bDetermined by 1H NMR spectroscopy (the single isolated yield is
indicated in parenthesis), ND = not detected. cThe reaction was conducted
d
e
under air. MeCN (2 mL) was used. PbAr4 (2.0 equiv.) was used.
fIsocyanide 1a was added last. gBINAP 20 mol% was added.
Catalyst A:
i-Pr
i-Pr
N
N
i-Pr
i-Pr
Cl Pd Cl
N
Cl
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Fortunately, imine 3aa was obtained in 92% yield (72% isolated yield, entry 8) when isocyanide 1a was
added last to a mixture of PbPh4 and Pd(OAc)2 in toluene. The influence of the palladium catalyst was
also investigated (entries 9 and 10), and it was found that zero- and divalent palladium complexes
exhibited catalytic activity toward the diarylation of isocyanides, but the product selectivity (3aa vs. 4aa)
was lower compared to that obtained using Pd(OAc)2. The addition of (±)-BINAP ((±)-2,2’-
bis(diphenylphosphino)-1,1’-binaphthyl) ligand slightly decreased the reaction yield and imine/α-diimine
selectivity (entry 11). Coordination mode of phosphine ligands toward palladium center might lead to
minor change in the reaction yield and product selectivity. When NHC-Pd catalyst was used, interestingly,
α-diimine 4aa was obtained in preference to imine 3aa (entry 12).9
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We then investigated the scope of isocyanides and tetraaryllead compounds using the optimized
conditions (Table 1, entry 8), and the results are presented in Table 2. Various para-substituted
tetraaryllead species were investigated for the catalytic diarylation of t-BuNC. When electron-rich
tetraaryllead compounds 2b–2d (cf. Hammet  values: -0.170 (X = Me); -0197 (tBu); -2.68 (MeO)) were
employed, the corresponding imines 3ab–3ad were obtained selectively in higher yields, compared with
Ph4Pb 2a ( = 0.00) (entries 2–4 vs. entry 1). On the other hand, p-Cl organolead compound 2e ( =
+0.227) afforded imine 3ae exclusively in lower yields (entry 5). In the case of p-F organolead 2f ( =
+0.062), the yield of 3af is similar to that of 3aa (entry 6). These results suggest that the electronic nature
of the aryl group on Pb affects the yields of imines 3. In terms of the isocyanides, long-chain aliphatic
isocyanides 1b and 1c underwent the catalytic diarylation process to give imines 3ba and 3ca as the major
products (entries 7 and 8). However, compare to the case of t-BuNC, the imine/α-diimine selectivity is
lower. Furthermore, isocyanides 1d and 1e bearing cyclohexyl and benzyl groups preferentially afforded
imines 3da and 3ea in good yields along with α-diimines 4da and 4ea, respectively, as minor products
(entries 9 and 10). As can be seen from the results of entries 7-10, the present diarylation is sensitive to
the substituents on isocyanides. Moreover, Table 1 clearly indicates that reaction conditions also influence
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The Journal of Organic Chemistry
the imine/-diimine selectivity. Therefore, to get an excellent product selectivity, further optimization is
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3
required for each isocyanide substrate.
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a
Table 2. Substrate scope for the palladium-catalyzed diarylation of alkylisocyanides with PbAr4
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R
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N
Pd(OAc)2 5 mol %
Ar
R
PbAr4
RNC
+
+
Ar
toluene, N2,100 °C, 2 h
Ar
Ar
R = alkyl
N
1
2
3
4
entry
isocyanide 1
tetraaryllead 2
yield of 3, %b
yield of 4, %b
X
Pb
N
N
NC
X
4
N
X
X
X
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
X = H (2a)
3aa, 72
3ab, 86
3ac, 91
3ad, 88
3ae, 50
3af, 74
4aa, (8)
4ab, 12
4ac, (6)
4ad, (5)
4ae, ND
4af, ND
Me (2b)
t-C4H9 (2c)
OMe (2d)
Cl (2e)
F (2f)
Pb
4
N
N
NC
N
7
8
1b
2a
2a
3ba, (52)
4ba, 36
N
N
NC
N
1c
3ca, (31)
4ca, 23
N
N
NC
N
9
1d
2a
3da, 47
4da, 33
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Ph
Ph
N
N
1
2
Ph
NC
N
Ph
3
4
5
10c
1e
2a
3ea, 70
4ea, 20
6
7
8
9
aReaction conditions: Isocyanide 1 (0.2 mmol) was added to a mixture of 2 (0.3 mmol)
and Pd(OAc)2 (0.01 mmol) in toluene (2 mL), 100 C, 2 h. Isolated yield (yields in
parentheses were determined by 1H NMR spectroscopy, difficult to purify from reaction
mixture), ND = not detected. cReaction time: 4 h.
o
b
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Interestingly, when an aromatic isocyanide 1f was employed for the Pd(OAc)2-catalyzed diarylation
with PbPh4, the product selectivity (imine 3 vs. α-diimine 4) was altered: α-diimines 4fa was obtained in
preference to imine 3fa (Table 3, entry 1). The reaction conditions were therefore investigated once again
for the diarylation of aryl isocyanides with PbPh4. As indicated in entry 2, the use of Pd(PPh3)4 instead of
Pd(OAc)2 for the diarylation of 1f with PbPh4 increased the selectivity of α-diimines 4fa, in sharp contrast
to the Pd(PPh3)4-catalyzed diarylation of t-BuNC 1a, which afforded imine 3aa preferentially (see Table
1, entry 9). Since unreacted PbPh4 was recovered after the reaction, the ratio of isocyanide 1 to
tetraphenyllead 2 was optimized. More specifically, when a 2:1 ratio of isocyanide 1 to 2 was employed,
α-diimine 4 were obtained in excellent yields using both palladium catalysts (entries 3 and 4). Lowering
the temperature (i.e., 70 °C) or using MeCN as a solvent also resulted in the selective synthesis of α-
diimine 4, but no further improvement was observed in the yield or selectivity (entries 5 and 6). When
palladium catalyst 2.5 mol% was used, the reaction yield is lower (entry 7). The reaction using NHC-Pd
catalyst proceeded quantitatively, but the α-diimine/imine selectivity was not improved anymore (entry
8). The use of PPh3 reduced the yields of 4fa and 3fa with slight increase in the α-diimine/imine selectivity
(entry 9). Lower amount of tetraaryllead also results in decrease of the yields (entry 10). Entry 4 was
chosen as the optimized reaction conditions for further investigation.
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The Journal of Organic Chemistry
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Table 3. Reaction conditions for the diarylation of an arylisocyanide using PbPh4
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5
6
7
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9
N
conditions
Ph
PbPh4
+
+
N
Ph
NC
N
Ph
Ph
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1f
2a
3fa
4fa
entry
1
Conditions
1f (0.2 mmol), 2a (0.3 mmol),
yield of 3fa, %a yield of 4fa, %a
26
13
60
87
Pd(OAc)2 5 mol%, toluene, 100 °C, 2 h
1f (0.2 mmol), 2a (0.3 mmol),
Pd(PPh3)4 5 mol %, toluene, 100 °C, 2 h
1f (0.4 mmol), 2a (0.2 mmol),
Pd(OAc)2 5 mol%, toluene, 100 °C, 4 h
1f (0.4 mmol), 2a (0.2 mmol),
Pd(PPh3)4 5 mol%, toluene, 100 °C, 4 h
1f (0.4 mmol), 2a (0.2 mmol),
2
3
4
5
6
7
8
16(14)
13(12)
8
84(85)
87(86)
84
Pd(OAc)2 5 mol%, toluene, 70 °C, 4 h
1f (0.4 mmol), 2a (0.2 mmol),
13
69
Pd(OAc)2 5 mol%, MeCN, 70 oC, 4 h
1f (0.4 mmol), 2a (0.2 mmol),
9
76
Pd(PPh3)4 2.5 mol%, toluene, 100 °C, 4 h
1f (0.4 mmol), 2a (0.2 mmol),
Catalyst A 5 mol%, toluene, 100 °C, 4 h
23
77
1f (0.4 mmol), 2a (0.2 mmol),
Pd(PPh3)4 5 mol%, PPh3 (20 mol %), toluene,
100 °C, 4 h
9
9
66
75
1f (0.4 mmol), 2a (0.05 mmol),
10
8
Pd(PPh3)4 5 mol%, toluene, 100 °C, 4 h
aDetermined by 1H NMR spectroscopy (isolated yield are indicated in parentheses).
Table 4 shows the results obtained for the catalytic diarylation of aromatic isocyanides. Similar to
aliphatic isocyanies, the electron and steric properties of aromatic isocyanides strongly influence the yield
and selectivity of products. The catalytic diarylation of o-substituted isocyanides 1f, 1g, 1h, 1q
successfully afforded the corresponding -diimines 4fa, 4ga, 4ha, and 4qa in moderate to good yields
and high -diimine/imine selectivity (Table 4, entries 1-3 and 10). It can be deduced that steric effect of
o-substituents stabilize imidoyl-Pd complex leading to high yield and selectivity toward formation of α-
diimines. In the case of m- and p-isocyanides, the yields of -diimines and selectivity of products vary
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upon electron properties of isocyanides (entries 4-9). When electron rich isocyanides were used (1i-l), the
-diimines were obtained in moderate to good yield and selectivity (4ia-la, entries 4-7). In sharp contrast,
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2
3
4
5
6
7
8
the electron poor isocyanides (1m and 1n) afforded corresponding imines and -diimines in low yields
and product selectivity (entries 8 and 9). We speculate that electron properties of isocyanides dramatically
interfere the stability of imidoyl-Palladium intermediate which is crucial for the formation of imines 3
and α-dimines 4. Moreover, m- and p-isocyanides are unstable and should be used for the diarylation
quickly after synthesis of them; in addition, the corresponding -diimines, especially 4ma and 4na, are
unstable and partially lost during separation. For example, in the case of p-MeO-NC, the product 4la was
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1
formed in 90% yield (determined by H NMR), but the isolated yield was 72%. When 1.0 gram of
isocyanide 1g was used, 4ga was obtained in 50% yield (0.85 gram). p-Nitrophenyl isocyanide and p-
acetylphenyl isocyanide (1o and 1p) did not apply for the present reaction (1o and 1p).
a
Table 4. Substrate scope for the palladium-catalyzed diarylation of arylisocyanides with PbAr4
R
N
R
Pd(PPh3)4 5 mol%
N
Ph
R
PbPh4
RNC
R = Aryl
1
+
+
Ph
toluene, N2,100 °C, 4 h
Ph
Ph
N
2a
3
4
entry
isocyanide 1
yield of 3, %b
yield of 4, %b
N
N
NC
N
Ph
Ph
1
2
1f
3fa, 12
4fa, 86c (i.r.: 5.3/1)
N
N
NC
N
Ph
Ph
4ga, 67c(i.r.: 5/1),
1g
3ga, (2)
50d
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i-Pr
1
2
3
4
5
6
i-Pr
N
i-Pr
NC
i-Pr
N
i-Pr
i-Pr
N
Ph
Ph
i-Pr
i-Pr
7
8
3
1h
3ha, (14)
4ha, 52c(i.r.: 3.5/1)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
N
N
NC
Ph
Ph
4
1i
3ia, 15
4ia, 60
O
O
O
N
N
N
NC
Ph
Ph
O
5
1j
3ja, 19
4ja, 44
Y
Y
N
Y
N
NC
N
Ph
Ph
Y
6
7
1k, Y = Me
3ka, 5
4ka, 40
4la, 72 (90)
4ma, (46), (17) e
4na, (26), (32) e
4oa, ND
1l,
OMe
Cl
3la (8）
8
1m,
1n,
1o,
3ma, 12, 31e
3na, trace, 20 e
3oa, ND
9
F
10
11
NO2
1p, C(O)CH3
3pa, ND
4pa, ND
N
N
N
NC
Ph
Ph
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12
1q
3qa, (17)
4qa, 68
1
2
3
4
5
6
7
8
9
aReaction conditions: Isocyanide 1 (0.4 mmol) was added to a mixture
of 2a (0.2 mmol) and Pd(PPh3)4 (0.02 mmol) in toluene (2 mL), 100
oC, 4 h. bIsolated yield (yields in parentheses were determined by 1H
c
NMR spectroscopy, difficult to purify from reaction mixture). The
product was obtained as a mixture of geometric isomers. dIsocyanide
1g (1.0 gram, 6.9 mmol) was used.ePd(OAc)2 (0.02 mmol) was used
instead of Pd(PPh3)4 (0.02 mmol). ND = Not detected.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To gain an insight into the reaction mechanism involved in this process, several control experiments
were carried out (Scheme 2). When the reaction of t-butyl isocyanide 1a with 0.25 equiv. of
tetraphenyllead 2a was attempted, the corresponding imine 3aa and α-diimine 4aa were obtained in yields
of 90 and 10%, respectively, based on 2a (eq 4). Using Pb2Ph6 2g instead of PbAr4 2a resulted in 93%
and 5% of 3aa and 4aa, respectively, based on 2g (eq 5). In the case of the reaction of 1a with 0.5 equiv.
of tetra(p-tolyl)lead 2b, 60% yield of 3ab and 20% yield of 4ab were obtained along with 20% yield of
biaryl 5 as a byproduct (eq 6). These results of eqs 4-6 indicate that all aryl groups of 2a or 2g could be
used for the synthesis of imine 3, α-diimine 4, and biaryl 5. Competition experiments using aliphatic
isocyanide 1a and aromatic isocyanide 1f resulted in a similar tendency of product selectivity as those of
the catalytic diarylation using 1a or 1f independently. On the other hand, when a reduced quantity of
tetraphenyllead 2a was employed, imine 3aa and α-diimine 4fa were preferentially obtained with similar
tendency, respectively (eq 7). The result of eq 7 supports the idea that the product selectivity of imine 3/α-
diimine 4 depends on the substituents of isocyanides. Intermolecular competitive reactions of two
different tetraaryllead were also carried out (eq 8 and 9). Very interestingly, α-diimines 4aa’ and 4af’
with both phenyl and p-tolyl substituents were not detected at all. The results strongly suggest that α-
diimine 4 is not produced via intermolecular ligand-exchange process of intermediate C each other to
form intermediate E and Pd(PbAr3)2L2 (see, Scheme 3).
Scheme 2. Control experiments.
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t-Bu
t-Bu
N
N
1
2
3
Pd(OAc)2 ( 5 mol%)
PbPh4
+
+
t-BuNC
(4)
toluene (2 mL),
N2,100 °C, 2 h
N
4
t-Bu
5
6
7
8
1a
2a
3aa
4aa
0.01 mmol (10%)*
2a
0.2 mmol
0.05 mmol
(0.25 equiv.)
0.09 mmol (90%)*
(*Based on
)
9
t-Bu
t-Bu
N
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Pd(OAc)2 ( 5 mol%)
Pb2Ph6
+
+
t-BuNC
(5)
toluene (2 mL),
N2,100 °C, 2 h
N
t-Bu
1a
2g
3aa
4aa
0.007 mmol (5%)*
2g
0.2 mmol
0.05 mmol
(0.25 equiv.)
0.14 mmol (93%)*
(*Based on
)
t-Bu
t-Bu
N
N
Pb
Pd(OAc)2 ( 5 mol%)
+
+
t-BuNC
toluene (2 mL),
N2,100 °C, 2 h
N
4
t-Bu
1a
2b
3ab
4ab
0.04 mmol (20%)*
0.12 mmol (60%)*
0.2 mmol
0.1 mmol
(0.5 equiv.)
+
(6)
5
0.04 mmol (20%)*
2b
(*Based on
)
Pd(OAc)2 ( 5 mol%)
PbPh4
(7)
+
XyNC
+
t-BuNC
toluene (2 mL),
N2,100 °C, 2 h
1a
1f
2a
(Xy = 2,6-Me2C6H3)
0.2 mmol
0.2 mmol
x mmol
Xy
t-Bu
t-Bu
N
N
Xy
Ph
N
N
+
+
Ph
Xy
Ph
+
Ph
Ph
Ph
Ph
Ph
3fa
N
N
t-Bu
3aa
4aa
4fa
2a
2a
= 0.2 mmol
= 0.1 mmol
70%
41%
7%
5%
23%
18%
50% (Based on XyNC)
(Based on t-BuNC)
(Based on t-BuNC)
(Based on XyNC)
57%
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t-Bu
N
1
2
3
4
t-Bu
Pd(OAc)2 ( 5 mol%)
N
Ph
PbPh4
Pb(p-tolyl)4
+
+
+
(8)
t-BuNC
Ph
toluene (2 mL),
N2,100 °C, 2 h
Ph
Ph
N
1a
2a
2b
t-Bu
5
6
0.2 mmol
0.1 mmol
0.1 mmol
3aa
4aa
7
8
35%
13%
9
t-Bu
t-Bu
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
N
t-Bu
t-Bu
N
N
p-tolyl
t-Bu
Ph
+
+
+
+
p-tolyl
p-tolyl
p-tolyl
p-tolyl
p-tolyl
Ph
N
N
t-Bu
3ab
29%
4ab
12%
3aa'
ND
4aa'
ND
Xy
Ph
N
Xy
N
Pd(PPh3)4 ( 5 mol%)
Ph
+
PbPh4
Pb(p-tolyl)4
XyNC
+
+
(9)
toluene (2 mL),
N2,100 °C, 4 h
Ph
Ph
N
Xy
1f
2a
2b
0.4 mmol
(Xy = 2,6-Me2C6H3)
0.2 mmol
0.2 mmol
3fa
4fa
43%
10%
Xy
N
Xy
N
Xy
Xy
N
N
p-tolyl
Ph
Xy
+
+
+
p-tolyl
p-tolyl
+
p-tolyl
p-tolyl
p-tolyl
Ph
N
N
Xy
3fb
4fb
39%
3fa'
4fa'
7%
ND
ND
Expecting that imidoylpaladium intermediate plays crucial role in catalytic cycle and imine/α-diimine
selectivity, we prepared Pd complex 6, which is an analog of complex C in Scheme 3, according to the
procedure reported by Amii10 et al. and examined the catalytic diarylation of aromatic isocyanide 1f with
tetraphenyllead 2a. Interestingly, the Pd complex 6 efficiently catalyzed the diarylation of 1f with 2a
affording 4fa selectively in good yield (eq 10). This Pd complex also catalyzed the diarylation of
aliphatic isocyanide 1a to give imine 3aa preferentially (eq 11). These results indicate that Pd complex
6 and its analog play important roles in the catalytic diarylation of isocyanides The key point that can
explain the different imine/α-diimines selectivity between aliphatic and aromatic isocyanides is the
stability of arylimidoylpalladium intermediate D. If the arylimidoyl-Pd complex D is stable enough, D
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The Journal of Organic Chemistry
can undergo another isocyanide insertion to form E; the following reductive elimination leads to α-
diimines 4. In contrast, if D is unstable, D tends to undergo reductive elimination to form imine 3. In
general, the presence or absence of resonance effect of the substituents of isocyanides contributes to the
stability of arylimidoylpalladium complex D. In the case of aromatic isocyanides, the conjugation of
C-N double bond of D with aryl group on N stabilizes D to afford -diimine 4. Aliphatic isocyanides
lack such conjugation, resulting in destabilizing D to give imine 3, preferentially. It is noteworthy that
the model Pd complex 6 exhibits a similar product selectivity as those of other palladium catalysts (see,
Table 3 and eq 8). In the case of equation 9, however, the imine 3aa/α-diimine 4aa selectivity was
somewhat lower compared with the high imine/α-diimine selectivity of Table 1, entry 1. This is most
probably because the higher stability of imidoyl-Pd complex 6 compared with D (R = aliphatic group)
favors the double isocyanie insertion reaction, resulting in lower imine/α-diimine selectivity.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
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Xy
N
1
2
CN Xy
Ph
Xy NC
Pd
3
4
5
6
7
8
I
cat.
Xy
Ph
N
Xy
Ph
6
(complex , 2.5 mol%)
N
(10)
Ph
Xy
+
XyNC
+
PbPh4
toluene (2 mL), N2, 100 °C, 4 h
Ph
N
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1f
2a
3fa
4fa
0.4 mmol
0.2 mmol
(0.5 equiv.)
15%
68%
conversion: 83%
Xy
N
CN Xy
Ph
Pd
I
cat.
Xy NC
t-Bu
N
t-Bu
6
(complex , 2.5 mol%)
N
Ph
PbPh4
(11)
t-BuNC
+
+
Ph
toluene (2 mL), N2, 100 °C, 2 h
Ph
Ph
N
t-Bu
1a
2a
3aa
51%
conversion: 86%
4aa
0.2 mmol
0.3 mmol
35%
Based on the above results, a plausible reaction pathway was proposed in Scheme 3. As can be seen
from the scheme, initially, palladium(0) complex A (generated from Pd(PPh3)4 (or Pd(OAc)2) and the
isocyanide) undergoes oxidative addition with PbAr4 to form complex B bearing a Pd-Ar bond.11 Complex
B is then converted into imidoyl complex C by insertion of the isocyanide into the Pd-Ar bond. The
following decomposition of complex C leads to arylimidoyl-Pd species D, the stability of which
determines the reaction outcome. If complex D is not stable, the reductive elimination affords imine 3
preferentially. In contrast, if complex D is stable, D undergoes a second isocyanide insertion to produce
complex E. Product 4 is then formed by reductive elimination from complex E. Subsequently, formed
PbAr2 gives tetraphenyllead and zero-valent lead is precipitated as distinct black solid.12 When complex
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The Journal of Organic Chemistry
B undergoes reductive elimination, biphenyl is formed as a byproduct in addition to PbAr2 and complex
A.
1
2
3
4
5
6
7
8
Scheme 3. A plausible reaction pathway.
9
NR
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ar
Ar
NR
4
PbAr4
(RNC)nPd
Oxidative
addition
A
NR
Ar
Ar
Reductive
elimination
RNC
RNC
Ar-Ar
+
Ar
RNC
RNC
Pd
Pd
PbIVAr3
E
- PdL2
PbAr2
B
NR
(L = PPh3 or RNC)
Insertion
RNC
RNC
Insertion
RN
RN
Reductive
elimination
NR
Ar
Ar
Ar
RNC
RNC
RNC
RNC
Pd
Pd
PbIVAr3
Ar
- (RNC)nPd
Ar
3
D
C
A
PbAr2
PbAr4
+ Pb
CONCLUSIONS
In summary, we successfully developed novel, divergent transformations of isocyanides to yield imines
and α-diimines via a palladium-catalyzed diarylation with process using tetraaryllead species. It is
noteworthy that, the diarylation reaction proceeded without the requirement of any additives or bases.
More specifically, this reaction afforded imines from the corresponding aliphatic isocyanides, while α-
diimines were formed primarily when electron-rich aromatic isocyanides were employed. Because α-
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Page 18 of 35
diimines are precursor of N-heterocyclic carbenes (NHC)13, it is expected that the present method will be
1
2
3
used for the synthesis of NHCs.
4
5
6
7
8
9
EXPERIMENTAL SECTION
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
General Remarks Unless otherwise stated, all starting materials and catalysts were purchased from
commercial sources and used without further purification. All solvents were distilled and degassed with
nitrogen before use. Isocyanide 1a-f, tetraphenyllead 2a and hexaphenyldilead 2g were purchased from
commercial supplies. Isocyanides 1g-1o were prepared according to the procedure that we previously
reported.6 Tetraaryllead 2b-f were prepared according to the procedure.14 1H NMR spectra were recorded
on JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JNM-ECX400 (400 MHz) FT NMR system
in CDCl3 and C6D6 with Me4Si as an internal standard. 13C NMR spectra were recorded on JEOL JNM-
ECX400 (100 MHz) FT NMR or JEOL JNM-ECS400 (100 MHz) FT NMR in CDCl3 and C6D6. 19F NMR
spectra were recorded on Variant NMR System 400 (376 MHz). IR spectra were reported in wave
numbers (cm-1). ESI and EI mass spectra were obtained by employing double focusing mass
spectrometers. High resolution mass spectra (HRMS) were obtained on JEOL JMS-AX500 or MALDI
TOFMS were obtained on Applied Biosystems 4800 MALDI TOF/TOF Analyzer.
Representative procedure for the synthesis of 1-Isocyano-4-methoxybenzene (1l). 4-
Methoxyformanilide. A solution of 4-methoxyaniline (1.85 g, 15 mmol) and formic acid (85% aq.) (1.0
mL, 22.5 mmol) in toluene (15 mL) is refluxed under N2 atmosphere. The reaction was monitor by TLC
(hexane/ AcOEt : 3/1). After the reaction, volatile materials were evaporated under reduced pressure. The
resulting residue containing the 4-methoxyformanilide was taken on to the next step without additional
purification.
1-Isocyano-4-methoxybenzene (1l). In 100 mL two-necked flask after heating for drying, 4-
methoxyformanilide (1.51 g, 10 mmol) was dissolved in CHCl3 (15 mL) and NEt3 (4.2 mL, 30 mml)
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o
under N2 atmosphere and cooled to 0 C by ice bath. To the mixture, phosphoryl chloride (1.2 mL, 12
mmol, 5 mL CHCl3 solution) was added dropwise slowly. After stirring for 1 h, the reaction temperature
was allowed to room temperature and the mixture was stirred for more 1 h. After the reaction was
completed, the resulting mixture was cooled to 0 oC and aqueous saturated solution of sodium carbonate
(10 mL) was added dropwise to quench the reaction. After vigorous stirring for 1 h, more water (ca. 20
mL) was added and organic layer was separated. Aqueous layer was extracted by CHCl3 (20 mL) twice.
Combined organic layer was dried over sodium sulfate, and solvent was removed by evaporation. The
residue was purified by silica gel column chromatography (eluent hexane/ AcOEt = 9/1). 1-Isocyano-4-
methoxybenzene (1l) was obtained in 90% yield (1.20 g) as green liquid. Only in the case of 2-
Biphenylisocyanide (1p) CH2Cl2 was used as solvent instead of CHCl3.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Representative procedure for the synthesis of tetraaryllead species (2b). Into a 100 mL dried round-
bottom flask was added magnesium turnings (1,21 g, 0.05 mol) and dry ether (20 mL). 4-Bromotoluene
(8,55 g, 0.05 mol) was then added dropwise, and the reaction mixture was allowed to stir until the solid
magnesium had disappeared. Dry benzene (20 mL) was then added to the reaction mixture followed by
lead(II) chloride (6.3 g, 0.023 mol), and the mixture was heated under reflux for 8 h in oil bath. After this
time, the mixture was cooled to room temperature and hydrolyzed by pouring into iced hydrochloric acid.
The resulting suspension was filtered through a Buchner funnel, the obtained solid was then transferred
to a beaker using benzene (50 mL), and the mixture was heated until benzene is boiling. Upon cooling of
the hot extract benzene, crystals of the title compounds were formed and obtained by filtration. The filtrate
was extracted with ethyl acetate; the volatile was evaporated and procedure was repeated twice to obtain
tetra-p-tolylplumbane (1,86g, 26%) as white solid. In the case of 2e and 2f further purification by silica
gel column (eluent: hexane) followed GPC is needed due to low reaction yield.
General procedure for the Pd-catalyzed arylation of alkylisocyanides (1a–e) with tetraaryllead
species. In a two-neck 10 mL flask equipped with a magnetic stirring bar under a N2 atmosphere were
placed Pd(OAc)2 (2.2 mg, 0.01 mmol), tetraaryllead 4 (0.3 mmol), distilled toluene (2 mL) and isocyanide
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3 (0.2 mmol), in that order. The reaction was conducted at 100 oC for 2 h in oil bath, and then the resulting
solution was filtered through Celite with ethyl acetate as an eluent. All volatiles were evaporated under
reduced pressure, and the crude product was purified by recycle GPC (eluent: CHCl3) to give the imine 3
and α-diimine 4.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
General procedure for the Pd-catalyzed arylation of arylisocyanides (1f–o) with tetraaryllead
species. In a two-neck 10 mL flask equipped with a magnetic stirring bar under a N2 atmosphere were
placed Pd(PPh3)4 (23.1 mg, 0.02 mmol), tetraaryllead 4 (0.2 mmol), distilled toluene (2 mL) and
isocyanide 3 (0.4 mmol), in that order. The reaction was conducted at 100 oC for 4 h in oil bath, and then
the resulting solution was filtered through Celite with ethyl acetate as an eluent. All volatiles were
evaporated under reduced pressure, and the crude product was purified by recycle GPC (eluent: CHCl3)
to give the imine 3 and α-diimine 4.
Procedure for gram-scale synthesis of 1-isocyano-2,4,6-trimethylbenzene (1g) with tetraphenyllead
(2a). In a two-neck 200 mL flash equipped with a magnetic stirring bar under a N2 atmosphere were placed
Pd(PPh3)4 (392.9 mg, 0.34 mmol), tetraaryllead 2a (1.78 gram, 3.45 mmol), distilled toluene (50 mL), and
o
isocyanide 1g (1.00 gram, 6.89 mmol). The reaction was conducted at 100 C for 24 h in oil bath, and
then all the volatiles were evaporated under reduced pressure. The crude product was subjected to silica-
gel column chromatography (eluent: Hexane/ EtOAc = 10/1) to give α-diimine 4ga (850.2 mg, 50% yield,
90% purity).
Spectral Data for Substrates
1
1-Isocyano-2,4,6-trimethylbenzene (1g).15 White solid, 1.06 g, 73%; H NMR (400 MHz, CDCl3): δ
6.89 (s, 2H), 2.37 (s, 6H), 2.29 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 166.8, 138.8, 134.7, 128.5,
124.1 (t, J = 11.9 Hz, C), 21.2, 18.9; MS (EI) [M]+ m/z = 145.
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1-Isocyano-2,6-diisopropylbenzene (1h). 15 Brown liquid, 1.5 g, 80%; 1H NMR (400 MHz, CDCl3): δ
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7.31 (t, J = 7.7 Hz, 1H), 7.16 (d, J = 7.7 Hz, 2H) 3.38 (septet, J = 6.8 Hz, 2H), 1.27 (d, J = 6.8 Hz, 12H);
13C{1H} NMR (100 MHz, CDCl3): δ 168.8, 145.0, 129.4, 124.4 (t, J = 11.4 Hz), 123.4, 29.9, 22.7; MS
(EI) [M]+ m/z = 187.
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1-Isocyano-3,5-dimethylbenzene (1i).16 Colorless liquid, 884.4 mg, 67%; 1H NMR (400 MHz, CDCl3):
δ 6.98 (s, 1H), 6.93 (s, 2H), 2.28 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 163.3, 139.4, 131.2, 126.4
(t, J = 12.9 Hz), 123.9, 21.0; MS (EI) [M]+ m/z = 131.
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1-Isocyano-3-methoxybenzene (1j).17 Green liquid, 763.8 mg, 57%; H NMR (400 MHz, CDCl3): δ
7.23 (t, J = 8.1 Hz, 1H), 7.00-6.86 (m, 2H), 6.81 (s, 1H), 3.75 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3):
δ 164.3, 160.1, 130.3, 127.3 (t, J = 13.5 Hz), 118.6, 115.6, 111.9, 55.5; MS (EI) [M]+ m/z = 133.
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1-Isocyano-4-methylbenzene (1k).16 Yellow liquid, 660.8 mg, 56%; H NMR (400 MHz, CDCl3): δ
7.17 (d, J = 8.6 Hz, 2H), 7.11 (d, J = 8.1, 2H), 2.31 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 163.9,
139.8, 130.0, 126.1, 124.1 (t, J = 13.3 Hz), 21.3; MS (EI) [M]+ m/z = 117.
1-Isocyano-4-methoxybenzene (1l).16 Yellow liquid, 1.20 g, 90%; 1H NMR (400 MHz, CDCl3): δ 7.25
(d, J = 8.6 Hz, 2H), 6.87-6.81 (m, 2H), 3.78 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 163.1, 159.9,
127.7, 119.4 (t, J = 13.8 Hz), 114.6, 56.6; MS (EI) [M]+ m/z = 133.
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1-Chloro-4-isocyanobenzene (1m).16 Yellow solid, 938.4 mg, 68%; H NMR (400 MHz, CDCl3): δ
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7.41-7.36 (m, 2H), 7.33 (d, J = 8.6 Hz, 2H); C{1H} NMR (100 MHz, CDCl3): δ 165.7, 135.5, 129.8,
127.7, 125.1 (t, J = 13.8 Hz); MS (EI) [M]+ m/z = 137.
1-Fluoro-4-isocyanobenzene (1n).16 Green liquid, 536.8 mg, 44%; 1H NMR (400 MHz, CDCl3): δ 7.48-
7.32 (m, 2H), 7.17-7.03 (m, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 164.5, 162.4 (d, JC-F = 252.0 Hz),
128.5 (d, JC-F = 9.6 Hz), 122.9 (dt, J = 3.8 (C-F), 13.8 Hz) 116.7 (d, JC-F = 23.9 Hz); 19F NMR (376 MHz,
CDCl3): δ -108.7; MS (EI) [M]+ m/z = 121.
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1-Isocyano-4-nitrobenzene (1o).6 Yellow solid, 1.11 g, 75%; 1H NMR (400 MHz, CDCl3): δ 8.32 (d, J
= 9.2 Hz, 2H), 7.60 (d, J = 8.7 Hz, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 169.7, 147.6, 131.3 (t, J =
14.1 Hz), 127.7, 125.2; MS (EI) [M]+ m/z = 148.
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1-(4-isocyanophenyl)ethan-1-one (1p).16 Green liquid, 1.23 g, 85%; H NMR (400 MHz, CDCl3): δ
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8.00 (d, J = 8.7 Hz, 2H), 7.48 (d, J = 8.7 Hz, 2H), 2.62 (s, 3H); C{1H} NMR (100 MHz, CDCl3): δ
196.3, 167.4, 137.4, 130.0 (t, J = 14.4 Hz), 129.6, 126.8, 26.8; MS (EI) [M]+ m/z = 145.
2-Biphenylisocyanide (1q).6 Green liquid, 1.64 g, 91%; 1H NMR (400 MHz, CDCl3): δ 7.42 (d, J = 7.8
Hz, 2H), 7.40-7.25 (m, 6H), 7.24-7.13 (m, 1H); 13C{1H} NMR (100 MHz, CDCl3): δ 167.7, 138.9, 137.2,
130.8, 129.8, 129.2, 128.8, 128.6, 128.4, 128.0, 124.7 (t, J = 10.5 Hz); MS (EI) [M]+ m/z = 179.
Tetra-p-tolylplumbane (2b).18 White solid, 1.86 g, 26%, mp 238-240 oC (decompose); H NMR (400
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MHz, CDCl3): δ 7.49 (d, J = 8.1 Hz, 8H, with satellite, JH-207Pb = 79.3 Hz), 7.22 (d, J = 7.7 Hz, 8H, with
satellite, JH-207Pb = 24.0 Hz), 2.38 (s, 12H, with satellite, JH-207Pb = 29.9 Hz); 13C{1H} NMR (100 MHz,
CDCl3): δ 146.5, 138.2, 137.5, (with satellite, JC-207Pb = 69.6 Hz), 130.3 (with satellite, JC-207Pb = 87.7 Hz),
21.5; MS (FAB) m/z = 481 (M+ -PhMe, 208Pb), 480 (M+ -PhMe, 207Pb).
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Tetrakis(4-(tert-butyl)phenyl)plumbane (2c). White solid, 2.47 g, 26%, mp above 300 C; H NMR
(400 MHz, CDCl3): δ 7.53 (d, J = 8.1 Hz, 8H, with satellite, JH-207Pb = 72.3 Hz), 7.39-7.29 (m, 8H), 1.29
(s, 36H); C{1H} NMR (100 MHz, CDCl3): δ 150.9, 149.8, 137.7 (with satellite, JC-207Pb = 71.5 Hz),
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126.5, (with satellite, JC-207Pb = 68.7 Hz), 34.7, 31.4; HRMS (MALDI-TOF) m/z: [M+] Calcd for C40H52Pb
740.3835; Found 740.3816.
Tetrakis(4-methoxyphenyl)plumbane (2d).18 White solid, 2.14 g, 27%, mp 195-197 C (decompose);
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1H NMR (400 MHz, CDCl3): δ 7.47 (d, J = 8.6 Hz, 8H, with satellite, JH-207Pb = 70.0 Hz), 6.89 (d, J = 8.6
Hz, 8H, with satellite, JH-207Pb = 23.6 Hz), 3.78 (s, 12H); C{1H} NMR (100 MHz, CDCl3): δ 159.7,
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143.4, 138.9 (with satellite, JC-207Pb = 81.0 Hz), 115.4 (with satellite, JC-207Pb = 73.4 Hz), 55.2; MS (FAB)
[M-PhOMe]+ m/z = 529.
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o
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Tetrakis(4-chlorophenyl)plumbane (2e). White solid, 178 mg, 5%, mp 197-199 C; H NMR (400
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MHz, CDCl3): δ 7.60-7.32 (m, 16H); C{1H} NMR (100 MHz, CDCl3): δ 146.8, 138.4 (with satellite,
JC-207Pb = 76.3 Hz), 135.6, 130.0 (with satellite, JC-207Pb = 87.7 Hz); MS (FAB) [M+-PhCl] m/z = 541
(207Pb). Anal. Calcd for C24H16Cl4Pb: C, 44.12; H, 2.47. Found: C, 43.77; H, 2.66.
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Tetrakis(4-fluorophenyl)plumbane (2f). White solid, 398 mg, 5%, mp 128-130 oC; 1H NMR (400 MHz,
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CDCl3): δ 7.68-7.37 (m, 8H), 7.23-7.05 (m, 8H); C{1H} NMR (100 MHz, CDCl3): δ 163.6 (d, JC-F
=
248.0 Hz), 143.8 (d, JC-F = 3.8 Hz), 138.8 (d, JC-F = 7.0 Hz, with satellite, JC-207Pb = 81.5 Hz), 117.0 (d,
JC-F = 19.7 Hz, with satellite, JC-207Pb = 92.0 Hz); 19F NMR (376 MHz, CDCl3): δ -111.6; HRMS (MALDI-
TOF) m/z: [M-PhF]+ Calcd for C18H12F3Pb 493.0657; Found 493.0681.
Spectral Data for Products
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N-tert-butyl-1,1-diphenylmethanimine (3aa).19 Colorless oil, 34.1 mg, 72%; H NMR (400 MHz,
CDCl3): δ 7.56-7.51 (m, 2H), 7.41-.7.35 (m, 3H), 7.32-7.23 (m, 3H), 7.22-7.16 (m, 2H), 1.17 (s, 9H);
13C{1H} NMR (100 MHz, CDCl3): δ 163.7, 142.1, 139.9, 129.4, 128.5, 128.2, 128.0, 127.9, 127.88, 57.1,
31.7; IR (NaCl, ν/cm–1): 694, 776, 1212, 1268, 1628, 2358, 2967; MS (EI) [M]+ m/z = 237; HRMS (CI)
m/z: [M+H]+ Calcd for C17H19N 238.1596; Found 238.1605.
N-tert-butyl-1,1-di-p-tolylmethanimine (3ab).19 White solid, 45.6 mg, 86%, mp 106-108 oC; 1H NMR
(400 MHz, CDCl3): δ 7.42 (d, J = 8.2 Hz, 2H), 7.17 (d, J = 7.8 Hz, 2H), 7.06 (dd, J = 8.2,8.2 Hz, 4H),
2.40 (s, 3H), 2.31 (s, 3H), 1.16 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ 163.8, 139.8 , 139.4, 137.5,
137.1, 128.6, 128.5, 128.4, 128.1, 56.9, 31.7, 21.4, 21.3; IR (KBr, ν/cm–1): 817, 942, 957, 1210, 1605,
2264, 2627, 2961; MS (EI) [M]+ m/z = 265; HRMS (EI) m/z: [M]+ Calcd for C19H23N 265.1830; Found
265.1830.
(1E,2E)-N1,N2-di-tert-butyl-1,2-di-p-tolylethane-1,2-diimine (4ab).6 White solid, 4.2mg, 12%, mp 84-
1
86oC; H NMR (400 MHz, CDCl3): δ 7.65 (d, J = 8.2 Hz, 4H), 7.11 (d, J = 8.2 Hz, 4H), 2.34 (s, 6H),
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1.23 (s,18H); C{1H} NMR (100 MHz, CDCl3): δ 159.2, 139.7, 138.0, 129.0, 127.7, 57.8, 30.1, 21.4.
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MS (EI) [M]+ m/z = 348.
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N-tert-butyl-1,1-bis(4-(tert-butyl)phenyl)methanimine (3ac). White solid, 63.6 mg, 91%, mp 77-79 oC;
1H NMR (400 MHz, CDCl3): δ 7.47 (d, J = 8.7 Hz, 2H), 7.37 (d, J = 8.2 Hz, 2H), 7.29 (d, J = 8.7 Hz,
2H), 7.10 (d, J = 8.7 Hz, 2H), 1.36 (s, 9H), 1.28 (s, 9H), 1.16 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3):
δ 164.0, 152.4, 150.8, 139.7, 137.0, 128.1, 127.9, 124.9, 124.6, 56.9, 34.7, 34.7, 31.7, 31.5, 31.3; IR (KBr,
ν/cm–1): 828, 843, 1216, 1270, 1361, 1625, 2963; MS (EI) [M]+ m/z = 349; HRMS (EI) m/z: [M]+ Calcd
for C25H35N 349.2770; Found 349.2776.
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N-tert-butyl-1,1-bis(4-methoxyphenyl)methanimine (3ad).19 White solid, 52.3 mg, 88%, mp 78-80 oC;
1H NMR (400 MHz, CDCl3): δ 7.47 (d, J = 9.2 Hz, 2H), 7.09 (d, J = 8.7 Hz, 2H), 6.91 (d, J = 8.7 Hz,
2H), 6.79 (d, J = 8.7 Hz, 2H), 3.85 (s, 3H), 3.78 (s, 3H), 1.17 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3):
δ 163.4, 160.8, 159.2, 135.3, 132.1, 129.8, 129.5, 113.23, 113.2, 56.8, 55.4, 55.3, 31.9, 31.8; IR (KBr,
ν/cm–1): 758, 837. 1034, 1245, 1508, 1600, 2963; MS (EI) [M]+ m/z = 297; HRMS (EI) m/z: [M]+ Calcd
for C19H23NO2 297.1729; Found 297.1723.
N-tert-butyl-1,1-bis(4-chlorophenyl)methanimine (3ae). Pale yellow solid, 30.5 mg, 50%, mp 106-108
oC; 1H NMR (400 MHz, C6D6): δ 7.47 (d, J = 8.7 Hz, 2H), 7.11 (d, J = 8.7 Hz, 2H), 6.95 (d, J = 8.2 Hz,
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2H), 6.53 (d, J = 8.2 Hz, 2H), 1.12 (s, 9H); C{1H} NMR (100 MHz, CDCl3): δ 161.2, 140.1, 137.8,
135.8, 134.2, 129.8, 129.4, 128.4, 128.2, 57.3, 31.6; IR (KBr, ν/cm–1): 829, 1089, 1483, 1625; MS (EI)
[M]+ m/z = 305; HRMS (CI) m/z: [M+H]+ Calcd for C17H17 Cl2N 306.0816; Found 306.0821.
N-tert-butyl-1,1-bis(4-fluorophenyl)methanimine (3af). White solid, 40.4 mg, 74%, mp 117-119 oC; 1H
NMR (400 MHz, CDCl3): δ 7.54-7.45 (m, 2H), 7.19-7.07 (m, 4H), 7.00-6.92 (m, 2H), 1.15 (s, 9H);
13C{1H} NMR (100 MHz, CDCl3): δ 164.4 (d, JC-F =138.9 Hz), 161.9 (d, JC-F = 138.0 Hz), 161.5, 138.1
(d, JC-F =2.9 Hz), 135.5 (d, JC-F =3.8 Hz), 130.1 (d, JC-F =8.6 Hz), 130.0 (d, JC-F =8.6 Hz), 115.2 (d, JC-F
=22.0 Hz), 114.8 (d, JC-F =21.1 Hz), 57.1, 31.6; 19F NMR (376 MHz, CDCl3): δ -112.4, -113.3; IR (KBr,
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ν/cm–1): 841, 850, 1218, 1505; MS (EI) [M]+ m/z = 273; HRMS (CI) m/z: [M+H]+ Calcd for C17H17 F2N
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274.1407; Found 274.1398.
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(1E,2E)-1,2-diphenyl-N1,N2-bis(2,4,4-trimethylpentan-2-yl)ethane-1,2-diimine (4ba).6 White solid,
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15.5 mg, 36%, mp 107-110 C; H NMR (400 MHz, CDCl3): δ 7.82-7.71 (m, 4H), 7.37-7.27 (m, 6H),
1.50 (d, J = 1.4 Hz, 4H), 1.37 (s, 6H), 1.16 (s, 6H), 1.03 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3): δ
158.7, 140.8, 129.5, 128.2, 128.0, 62.1, 57.1, 30.2, 30.028, 30.035, 29.6; MS (EI) [M]+ m/z = 432.
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(1E,2E)-N1,N2-dipentyl-1,2-diphenylethane-1,2-diimine (4ca).6 Yellow oil, 8.0 mg, 23%; H NMR
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(400 MHz, CDCl3): δ 7.76-7.71 (m, 4H), 7.43-7.32 (m, 6H), 3.32 (t, J = 7.3 Hz, 4H), 1,73-1,64 (m, 4H),
1.38-123 (m, 8H), 0.86 (t, J = 7.3 Hz, 6H); C{1H} NMR (100 MHz, CDCl3): δ 165.2, 136.2, 130.6,
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128.8, 127.3, 54.5, 30.7, 30.0, 22.6, 14.1 ; MS (EI) [M]+ m/z = 348.
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N-cyclohexyl-1,1-diphenylmethanimine (3da). Colorless oil, 24.7 mg, 47%; H NMR (400 MHz,
CDCl3): δ 7.61-7.54 (m, 2H), 7.48-7.38 (m, 3H), 7.36-7.27 (m, 3H), 7.18-7.12 (m, 2H), 3.21 (tt, J = 7.2,
14.5 Hz, 1H), 1.79-1.67 (m, 2H), 1.65-1.53 (m, 5H), 1.32-1.06 (m, 3H); 13C{1H} NMR (100 MHz,
CDCl3): δ 165.7, 140.5, 137.5, 129.64, 129.56, 128.4, 128.1, 128.0, 127.8, 61.5, 34.0, 25.8, 24.5; MS (EI)
[M]+ m/z = 263; HRMS (CI) m/z: [M+H]+ Calcd for C19H21N 264.1752; Found 264.1758.
(1E,2E)-N1,N2-dicyclohexyl-1,2-diphenylethane-1,2-diimine (4da).6 White solid, 12.3 mg, 33%, mp 89-
93oC; 1H NMR (400 MHz, CDCl3): δ 7.81-7.74 (m,4H), 7.40-7.28 (m, 6H), 3.22 (tt, J = 4.1, 9.6 Hz, 2H),
1.87-1.72 (m, 2H), 1.71-1.41 (m, 10H), 1.40-1.16 (m, 6H), 1.13-0.70 (m, 2H); 13C{1H} NMR (100 MHz,
CDCl3): δ 162.2, 137.3, 130.4, 128.5, 127.7, 63.0, 34.0, 33.0, 25.8, 24.3, 24.2; MS (EI) [M]+ m/z = 372.
N-benzyl-1,1-diphenylmethanimine (3ea).20 Colorless solid, 37.9 mg, 70%, mp 54-57oC; 1H NMR (400
MHz, CDCl3): δ 7.72-7.66 (m, 2H), 7.55-7.11 (m, 13H), 4.61 (s, 2H); MS (EI) [M]+ m/z = 271; HRMS
(CI) m/z: [M+H]+ Calcd for C20H18N 272.1439; Found 272.1440.
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(1E,2E)-N1,N2-bis(2,6-dimethylphenyl)-1,2-diphenylethane-1,2-diimine (4ea).6 White solid, 7.8 mg,
20%; 1H NMR (400 MHz, CDCl3): δ 7.88-7.83 (m, 4H), 7.44-7.13 (m, 16H), 4.6 (d, J = 15.9 Hz, 2H),
4.53 (d, J = 15.9 Hz, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 166.0, 139.7, 135.8, 131.2, 129.0, 128.5,
128.0, 127.6, 126.9, 57.9; MS (EI) (M+) m/z = 388.
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N-(2,6-dimethylphenyl)-1,1-diphenylmethanimine (3fa).21 Yellow oil, 13.7 mg, 12%; H NMR (400
MHz, CDCl3): δ 7.79 (d, J = 8.2 Hz, 2H), 7.50-7.40 (m, 3H), 7.28-7.19 (m, 3H), 7.10 (d, J = 8.2 Hz, 2H),
6.88 (d, J = 7.8 Hz, 2H), 6.79 (t, J = 7.8 Hz, 1H), 2.04 (s, 6H); MS (EI) [M]+ m/z = 285; HRMS (EI) m/z:
[M]+ Calcd for C21H19N 285.1517; Found 285.1510.
(1E,2E)-N1,N2-bis(2,6-dimethylphenyl)-1,2-diphenylethane-1,2-diimine (4fa).6,9 Yellow oil, 71.5 mg,
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86%; H NMR (400 MHz, CDCl3) [as a mixture of two geometrical isomers, an isomer ratio of A/B =
5.3/1]: δ 8.27-8.20 (m, 4H), 7.60-6.68 (m, 12H), 2.13-1.74 (m, 12H); MS (EI) [M]+ m/z = 416. It was
difficult to separate the isomers of 4fa.
Data for isomer A: 1H NMR (400 MHz, CDCl3) δ 8.26 (d, J = 5.9 Hz, 2H), 7.61-6.70 (m, 14H), 1.98-
1.74 (m, 12H); C{1H} NMR (100 MHz, CDCl3): δ 165.6, 147.4. 134.9, 131.0, 128.7, 128.1, 127.63,
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125.7, 123.3, 18.7.
Data for isomer B; 1H NMR (400 MHz, CDCl3) δ 7.61-6.70 (m, 16H), 2.06 (s, 12H); 13C{1H} NMR
(100 MHz, CDCl3): δ 165.3, 147.8, 138.0, 129.9, 128.8, 128.2, 127.58, 126.6, 123.5, 19.0.
(1E,2E)-N1,N2-dimesityl-1,2-diphenylethane-1,2-diimine (4ga). Yellow oil, 59.5 mg, 67%; H NMR
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(400 MHz, CDCl3) [as a mixture of two geometrical isomers, an isomer ratio of A/B = 5/1]: δ 8.31-6.58
(m, 14H), 2.25-2.11 (m, 6H), 2.05-1.72 (m, 12H); HRMS (ESI) m/z: [M+Na]+ Calcd for C32H32N2Na
467.2463; Found 467.2453. It was difficult to separate the isomers of 4ga.
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Data for isomer A: 1H NMR (400 MHz, CDCl3) δ 8.31-8.16 (m, 2H), 7-59-6.58 (m, 12H), 2.25-2.11
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(m, 6H), 1.90-1.72(m, 12H); C{1H} NMR (100 MHz, CDCl3): δ 166.1, 145.3, 135.2, 132.4, 130.9,
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128.9, 128.6, 128.2, 126.4, 20.8, 18.5.
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Data for isomer B; H NMR (400 MHz, CDCl3) δ 7-59-6.58 (m, 14H), 2.25-2.11 (m, 6H), 2.01 (s,
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12H); 13C{1H} NMR (100 MHz, CDCl3): δ 165.5, 144.8, 138.2, 132.6, 129.7, 128.7, 128.0, 127.6, 125.6,
20.7, 18.9.
(1E,2E)-N1,N2-bis(2,6-diisopropylphenyl)-1,2-diphenylethane-1,2-diimine (4ha). Yellow oil, 54.9 mg,
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52%; H NMR (400 MHz, CDCl3) [as a mixture of two geometrical isomers, an isomer ratio of A/B =
3.5/1]: δ 8.20-6.84 (m, 16H), 3.29-2.20 (m, 4H), 1.39-0.37 (m, 24H); HRMS (ESI) m/z: [M+Na]+ Calcd
for C38H44N2Na 551.3402; Found 551.3395. It was difficult to separate the isomers of 4ha.
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N-(3,5-dimethylphenyl)-1,1-diphenylmethanimine (3ia). Yellow oil, 17.1 mg, 15%; H NMR (400
MHz, CDCl3): δ 7.76-7.68 (m, 2H), 7.49-7.33 (m, 3H), 7.30-7.21 (m, 3H), 7.16-7.09 (m, 2H), 6.55 (s,
1H), 6.34 (s, 2H), 2.15 (s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 167.7, 151.2, 140.0, 138.0, 136.5,
130.6, 129.5, 129.3, 128.5, 128.2, 127.9, 124.9, 118.8, 21.3; IR (KBr, ν/cm–1): 503, 673, 694, 727, 780,
847, 1441, 1506, 1573; MS (EI) [M]+ m/z = 285; HRMS (EI) m/z: [M]+ Calcd for C21H19N 285.1517;
Found 285.1509.
(1E,2E)-N1,N2-bis(3,5-dimethylphenyl)-1,2-diphenylethane-1,2-diimine (4ia). Yellow solid, 49.9 mg,
60%, mp 157-159 oC; 1H NMR (400 MHz, CDCl3): δ 7.89-7.83 (m, 4H), 7.46-7.35 (m, 6H), 6.64 (s, 2H),
6.14 (s, 4H), 2.09 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3): δ 164.0, 149.6, 138.1, 137.8, 131.0, 128.7,
128.3, 126.3, 117.8, 21.3; IR (KBr, ν/cm–1): 694, 839, 1447, 1586; MS (EI) [M]+ m/z = 416; HRMS (EI)
m/z: [M]+ Calcd for C30H28N2 416.2252; Found 416.2248.
N-(3-methoxyphenyl)-1,1-diphenylmethanimine (3ja). Yellow oil, 21.8 mg, 19%; 1H NMR (400 MHz,
CDCl3): δ 7.77-7.72 (m, 2H), 7.50-7.44 (m, 1H),7.43-7.37 (m, 2H) 7.30-7.23 (m, 3H), 7.16-7.11(m, 2H),
7.02 (t, J = 7.8 Hz, 1H), 6.48 (ddd, J = 0.9, 2.7, 8.2, 1H), 6.33 (t, J = 1.8 Hz, 1H), 6.28 (ddd, J =0.9, 1.8,
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7.8, 1H), 3.66 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 168.4, 159.9, 152.6, 139.7, 136.3, 130.8,
129.5, 129.4, 129.3, 128.7, 128.3, 128.0, 113.5, 109.3, 106.6, 55.2; IR (NaCl, ν/cm–1): 693, 1128, 1262,
1575; MS (EI) [M]+ m/z = 287; HRMS (EI) m/z: [M]+ Calcd for C20H17NO 287.1310; Found 287.1306.
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(1E,2E)-N1,N2-bis(3-methoxyphenyl)-1,2-diphenylethane-1,2-diimine (4ja). Yellow solid, 37.0 mg,
44%, mp 95-98 oC; 1H NMR (400 MHz, CDCl3): δ 7.93-7.88 (m, 4H), 7.47-7.37 (m, 6H), 6.97 (t, J = 8.2
Hz, 2H), 6.57 (dd, J = 1.8, 8.2 Hz, 2H), 6.18 (ddd, J = 0.9, 1.8, 7.8 Hz, 2H), 5.99 (t, J = 2.3 Hz, 2H), 3.49
(s, 6H); 13C{1H} NMR (100 MHz, CDCl3): δ 164.2, 159.7, 150.8, 137.7, 131.3, 129.2, 128.9, 128.5, 111.8,
111.7, 105.3, 54.9; IR (KBr, ν/cm–1): 691, 783, 1149, 1278, 1588; MS (EI) [M]+ m/z = 420; HRMS (EI)
m/z: [M]+ Calcd for C28H24N2O2 420.1838; Found 420.1840.
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1,1-diphenyl-N-(p-tolyl)methanimine (3ka).22 Yellow oil, 5.4 mg, 5%; 1H NMR (400 MHz, CDCl3): δ
7.76-7.70 (m, 2H), 7.49-7.35 (m, 3H), 7.29-7.24 (m, 3H), 7.14-7.09 (m, 2H), 6.93 (d, J = 8.2 Hz, 2H),
6.62 (d, J = 8.2 Hz, 2H), 2.22 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 168.0, 148.6, 140.0, 136.6,
132.7, 130.6, 129.6, 129.3, 129.1, 128.5, 128.2, 128.0, 121.1, 20.9; MS (EI) [M]+ m/z = 271.
(1E,2E)-1,2-diphenyl-N1,N2-di-p-tolylethane-1,2-diimine (4ka).23 Yellow solid, 31.0 mg, 40%, mp
161-163 oC; 1H NMR (400 MHz, CDCl3): δ 7.90-7.83 (m, 4H), 7.44-7.33 (m, 6H), 6.88 (d, J = 8.6 Hz,
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4H), 6.51 (d, J = 8.2 Hz, 4H), 2.23 (s, 6H) ; C{1H} NMR (100 MHz, CDCl3): δ 163.9, 146.8, 137.6,
134.7, 131.0, 129.1, 128.8, 128.3, 120.4, 21.0; MS (EI) [M]+ m/z = 388.
(1E,2E)-N1,N2-bis(4-methoxyphenyl)-1,2-diphenylethane-1,2-diimine (4la).6 Yellow solid, 60.5 mg,
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72%, mp 164-166 C; H NMR (400 MHz, CDCl3): δ 7.89-7.83 (m, 4H), 7.44-7.32 (m, 6H), 6.71-6.61
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(m, 8H), 3.71 (s, 6H); C{1H} NMR (100 MHz, CDCl3): δ 163.4, 157.4, 142.6, 137.5, 130.9, 128.8,
128.2, 122.2, 113.8, 55.4; MS (EI) [M]+ m/z = 420.
N-(4-chlorophenyl)-1,1-diphenylmethanimine (3ma). Yellow oil, 14.0 mg, 12%; 1H NMR (400 MHz,
CDCl3): δ 7.76 (m, 2H), 7.50-7.35 (m, 3H), 7.33-7.22 (m, 3H), 7.12-7.05 (m, 4H), 6.67-6.61 (m, 2H);
13C{1H} NMR (100 MHz, CDCl3): δ 169.1, 149.8, 139.5, 136.0, 131.0, 129.5, 129.4, 128.9, 128.7, 128.5,
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The Journal of Organic Chemistry
128.3, 128.2, 122.4; MS (EI) [M]+ m/z = 291; HRMS (EI) m/z: [M]+ Calcd for C19H14NCl 291.0815;
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Found 291.0818.
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N-(4-fluorophenyl)-1,1-diphenylmethanimine (3na). Yellow oil, 22.0 mg, 20%; H NMR (400 MHz,
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CDCl3): δ 7.79-7.69 (m, 2H), 7.50-7.36 (m, 3H), 7.34-7.22 (m, 3H), 7.12-7.07 (m, 2H), 6.87-6.79 (m,
2H), 6.70-6.63 (m, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 169.0, 159.2 (d, JC-F = 241.3 Hz), 147.3 (d,
JC-F = 2.8 Hz), 140.0, 136.2, 131.0, 129.4 (d, JC-F = 18.1 Hz), 128.8, 128.2 (d, JC-F = 18.1 Hz), 122.5, 122.4,
115.4, 115.2; 19F NMR (376 MHz, CDCl3) δ -120.7; IR (NaCl, ν/cm–1): 694, 779, 836, 1209, 1498, 1617;
MS (EI) [M]+ m/z = 275; HRMS (EI) m/z: [M]+ Calcd for C19H14NF 275.1110; Found 275.1104.
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(1E,2E)-N1,N2-di([1,1'-biphenyl]-2-yl)-1,2-diphenylethane-1,2-diimine (4qa).6 Yellow solid, 69.7 mg,
68%, mp 182-184 oC; 1H NMR (400 MHz, CDCl3): δ 7.58 (d, J = 7.3 Hz, 4H); 7.41-7.33 (m, 2H), 7.28-
7.21 (m, 6H), 7.20-7.10 (m, 6H), 7.11-7.01 (m, 2H), 6.91-6.80 (m, 6H), 6.46 (d, J = 7.3 Hz, 2H); 13C{1H}
NMR (100 MHz, CDCl3): δ 163.0, 145.9, 139.5, 137.7, 134.9, 130.7, 130.4, 129.8, 128.7, 128.4, 127.53,
127.46, 126.4, 125.9, 118.4; MS (EI) [M]+ m/z = 512.
The authors declare no competing financial interest.
SUPPORTING INFORMATION
1H NMR and 13C NMR spectra. This material is available free of charge via the internet at
http://pubs.acs.org.
ACKNOWLEDGEMENTS
Tran C. C. thanks to the Graduate Course for System-inspired Leaders in Material Science
(SiMS) Scholarship Program. This work is also supported by Grant-in-Aid for Scientific Research on
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