Reductive coupling of aliphatic cyclic imides with benzophenones by low-valent titanium

Article abstract of DOI:10.1016/j.tetlet.2013.10.053

The reductive coupling of aliphatic cyclic imides with benzophenones by Zn–TiCl₄ in THF gave two- and four-electron reduced products selectively by controlling the reaction conditions. Although cyclic and acyclic products were formed as mixtures in most cases, cyclic dehydrated products could be selectively obtained by heating the product mixtures in the presence of cat. p-TsOH.

Full text of DOI:10.1016/j.tetlet.2013.10.053

Tetrahedron Letters 54 (2013) 6944–6948
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reductive coupling of aliphatic cyclic imides with benzophenones
by low-valent titanium
⇑
Naoki Kise , Syn Kinameri, Toshihiko Sakurai
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101, Koyama-cho Minami, Tottori 680-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The reductive coupling of aliphatic cyclic imides with benzophenones by Zn–TiCl4 in THF gave two- and
four-electron reduced products selectively by controlling the reaction conditions. Although cyclic and
acyclic products were formed as mixtures in most cases, cyclic dehydrated products could be selectively
obtained by heating the product mixtures in the presence of cat. p-TsOH.
Received 11 September 2013
Revised 8 October 2013
Accepted 10 October 2013
Available online 17 October 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Reductive coupling
Low-valent titanium
McMurry coupling
Aliphatic cyclic imides
Benzophenones
Cross McMurry coupling is a powerful tool for the reductive
coupling of two different carbonyl compounds because of its
versatility, convenience, and economical efficiency.1,2 Recently,
we reported the reductive coupling of uracils3a and N-methoxycar-
bonyl lactams3b with benzophenones by low-valent titanium. In
this context, we report herein the reductive coupling of aliphatic
cyclic imides, such as succinimides and glutarimides, with benz-
ophenones by low-valent titanium generated from Zn–TiCl4
(Scheme 1). We found that two- and four-electron reduced adducts
could be selectively obtained by controlling the reaction condi-
tions. In each case, cyclic and acyclic products were formed
depending on the substrates and the conditions of workup after
the reductive coupling. In most cases, the products converged on
cyclic dehydrated products by reflux of the product mixtures in
benzene or toluene in the presence of cat. p-TsOH. Therefore, this
method provides a synthetic route to a new class of five- and
six-membered nitrogen heterocycles, 5-(diarylmethylene)-1H-pyr-
rol-2(5H)-ones, 6-diarylmethylpyridin-2(1H)-ones, and their
hydrogenated analogs.4 The reaction mechanisms of the reductive
coupling and following reactions are also discussed.
n
1) Zn-TiCl4
Ar
Ar
Ar
Ar
+
O
O
O
N
N
R
N
R
O
2) cat. p-TsOH
O
R
Ar
Ar
Δ
n = 1
n = 2
n = 1, 2
R = Me, H
two-electron reduced products
n
Ar
O
N
R
n = 1, 2
Ar
four-electron reduced products
Scheme 1. Reductive coupling of aliphatic imides with benzophenones by Zn–TiCl4.
⇑
Corresponding author.
E-mail address: kise@bio.tottori-u.ac.jp (N. Kise).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.053

N. Kise et al. / Tetrahedron Letters 54 (2013) 6944–6948
6945
Table 1
Reductive coupling of 1a,b with 2a by Zn–TiCl4
1) Zn-TiCl4
Ph
Ph
Ph
Ph
THF
Ph
Ph
Ph
Ph
+
O
O
O
O
O
N
R
N
R
N
R
N
R
OH
2) workup
O
OH
OH
1a: R = Me
1b: R = H
(1 mmol)
2a
3a: R = Me
3b: R = H
4a: R = Me
4b: R = H
5a: R = Me
5b: R = H
(2 mmol)
Ph
Ph
Ph
RHNOC
RHNOC
O
Ph
Ph
N
OH
O
R
Ph
O
6a: R = Me
7a: R = Me
8a: R = Me
6b: R = H
7b: R = H
8b: R = H
Workupb
Yieldc (%)
a
Run
1
TiCl4 (mmol)
Temp (°C)
3
4
5
6
7
8
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
2
2
2
2
4
4
2
2
2
4
4
0
0
0
a
a
a
a
a
a
a
b
b
b
b
b
18
—
—
18
—
—
10
—
5
13
—
—
—
—
—
9
—
10
—
—
20
73
80
—
14
12
7
53
—
7
32
10
—
61
—
—
30
—
—
—
—
—
31
54
—
—
—
—
—
—
—
28
—
—
—
—
13
—
ad
b
c
0
30
30
0
0
0
a
b
a
b
c
a
42
—
—
10
11
30
30
—
—
36
47
b
5
a
Zn/TiCl4 = 2/1.
b
(a) 1 M HCl, 25 °C, 15 min; (b) crude product mixture obtained by workup with 1 M HCl (workup a) was refluxed in benzene in the presence of cat. p-TsOH for 30 min; (c)
satd NaHCO3 aq, 25 °C, 3 h.
c
Isolated yields.
1 M HCl, 25 °C, 3 h.
d
The reaction conditions of the reductive coupling were sur-
veyed with N-methylsuccinimide (1a) and benzophenone (2a) as
the substrates and the results are summarized in Table 1.5 The mo-
lar ratio of Zn/TiCl4 was fixed to 2/1. First, the reduction was car-
ried out with the ratio of 1a/2a/TiCl4 as 1/2/2 in THF at 0 °C for
12 h (runs 1–4). After usual workup with 1 M HCl at 25 °C for
15 min, four cross-coupled products 3a–6a were formed as
two-electron reduced products (run 1). When the workup time
L-selectride/THF
-78 °C, 1 h
85-90%
Ph
Ph
O
N
O
N
R
R
Ph
Ph
DDQ/benzene
reflux, 1 h
90-95%
5a,b
7a,b
Scheme 2. Mutual transformation between 5a,b and 7a,b.
Table 2
Reductive coupling of 1a,b with 2b,c,d by Zn–TiCl4
1) Zn-TiCl4
THF
Ar
Ar
Ar
Ar
Ar
Ar
+
O
RHNOC
O
O
O
O
N
R
N
R
N
R
Ar
OH
2) workup
OH
O
Ar
OH
O
1a: R = Me
1b: R = H
(1 mmol)
2b: Ar = p-FC6H4
3
5
6
2c: Ar = p-MeOC6H4
2d: dibenzosuberone
(2 mmol)
3-7c: R = Me; Ar = p-FC6H4
3-7d: R = Me; Ar = p-MeOC6H4
3-7e: R = H; Ar = p-FC6H4
Ar
Ar
N
R
3-7f: R = H; Ar = p-MeOC6H4
3-7g: R = H; Ar2
=
7
Workupb
Yieldc (%)
a
Run
1
2
TiCl4 (mmol)
Temp (°C)
3
5
6
7
1
2
3
4
5
6
1a
1a
1a
1a
1a
1a
2b
2b
2b
2c
2c
2c
2
2
4
2
2
4
0
0
30
ꢀ10
ꢀ10
30
b
c
b
b
c
c
c
c
d
d
d
—
81
—
10
73d
—
—
71
—
—
57
—
—
—
61
—
—
—
—
—
22
—
b
17
58
(continued on next page)

6946
N. Kise et al. / Tetrahedron Letters 54 (2013) 6944–6948
Table 2 (continued)
a
Run
1
2
TiCl4 (mmol)
Temp (°C)
Workupb
Yieldc (%)
3
5
6
7
7
8
9
10
11
12
1b
1b
1b
1b
1b
1b
2b
2b
2c
2c
2d
2d
2
4
2
4
2
4
0
30
0
b
b
b
e
e
f
f
g
g
—
—
—
—
—
—
53
9
49
4
48
Trace
—
—
—
—
—
—
4
44
8
43
6
52
30
b
ꢀ10
be
be
30
a
b
c
Zn/TiCl4 = 2/1.
(b) Crude product mixture obtained by workup with 1 M HCl was refluxed in benzene in the presence of cat. p-TsOH for 30 min; (c) satd NaHCO3 aq, 25 °C, 3 h.
Isolated yields.
Obtained with 9 (8%).
Crude product mixture was refluxed in toluene for 2 h.
d
e
for stirring with 1 M HCl was prolonged to 3 h, the cyclic product
5a was obtained as the major product (73%) with a small amount
of the acyclic product 6a (run 2). Therefore, the crude product mix-
ture obtained from run 1 was refluxed in the presence of catalytic
amount of p-TsOH in benzene for 30 min to give 5a (80%) as the
sole product (run 3). In contrast, 6a was formed mainly (61%) after
workup with satd NaHCO3 aq at 25 °C for 3 h (run 4). Next, the
reduction was performed with the ratio of 1a/2a/TiCl4 as 1/2/4 in
THF at 30 °C for 12 h (runs 5 and 6). After workup with 1 M HCl
at 25 °C for 15 min, cyclic product 7a and acyclic product 8a were
formed as four-electron reduced products (run 5). As expected, 7a
was obtained as the major product (54%) after reflux of the product
mixture in benzene similarly to run 3 (run 6). Instead of 1a, succin-
imide (1b) was employed as the substrate and afforded similar re-
p-TsOH
benzene
reflux, 1 h
Ar
O
MeHNOC
Ar
Ar
Ar
N
O
OH
82 %
Me
O
Ar = p-MeOC6H4
6d
9
+H+
-H2O
-H+
Ar
MeHNOC
Ar
O
Scheme 3. Transformation of 6d to 9.
Table 3
Reductive coupling of 10a,b with 2a–d by Zn–TiCl4
1) Zn-TiCl4
THF
Ar
Ar
Ar
+
Ar
Ar
Ar
O
N
R
O
O
N
R
O
N
R
RHNOC
RHNOC
Ar
Ar
2) workup
O
OH
Ar
OH
O
O
10a: R = Me
10b: R = H
(1 mmol)
2a-d
11
12
13
15
(2 mmol)
Ar
11-15a: R = Me; Ar = Ph
11-15b: R = H; Ar = Ph
Ar
11-15c: R = H; Ar = p-FC6H4
O
N
R
11-15d: R = H; Ar = p-MeOC6H4
11-15e: R = H; Ar2
Ar
14
=
a
Run
10
2
TiCl4 (mmol)
Temp (°C)
Workupb
Yieldc (%)
13
11
12
14
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
10a
10a
10b
10b
10b
10b
10b
10b
10b
10b
10b
10b
10b
10b
10b
10b
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2d
2d
2d
2
4
2
2
2
4
2
2
4
4
2
4
4
2
4
4
0
30
0
a
a
a
b
a
b
a
b
a
b
a
a
b
a
a
b
a
a
b
b
b
b
c
c
c
c
d
d
d
e
e
e
—
—
33
—
—
—
54
—
—
—
—
—
—
70
—
—
—
—
—
78
—
—
—
78
—
—
—
—
—
—
—
—
76
—
51
—
—
—
30
—
—
—
77
—
—
—
—
—
12
72
—
—
64
—
—
60
—
—
—
55
—
—
56
—
—
64
—
0
30
30
0
0
—
30
30
0
30
30
0
16
68
6
15
70
—
—
11
—
30
30
13
75
—
a
b
c
Zn/TiCl4 = 2/1.
(a) 1 M HCl, 25 °C, 15 min; (b) Crude product mixture obtained by workup with 1 M HCl (workup a) was refluxed in toluene in the presence of cat. p-TsOH for 2 h.
Isolated yields.

N. Kise et al. / Tetrahedron Letters 54 (2013) 6944–6948
6947
5). In the reaction of 1b with dibenzosuberone (2d), the reaction
mixtures were refluxed in toluene for 2 h to give 5g and 7g, since
the cyclization of acyclic products was slow in refluxing benzene
(runs 11 and 12).
p-TsOH
toluene
Ph
reflux, 24 h
Ph
Ph
MeHNOC
MeHNOC
Ph
Ph
N
O
O
OH
Me
65%
O
O
Ph
Ph
This reductive coupling was also effective for glutarimides
10a,b in place of 1a,b (Table 3). The reductive coupling of N-meth-
ylglutarimide (10a) with 2a was carried out under the same condi-
tions as above (runs 1 and 2). After workup with 1 M HCl, acyclic
product 13a (76%) and its deoxygenated product 15a (64%) were
formed exclusively. Since cyclization of these acyclic products
was slow, a prolonged reaction time was needed even in refluxing
toluene (Scheme 4). In the reaction of glutarimide (10b) with 2a
(runs 3–6), mixtures of cyclic and acyclic products were formed
after workup with 1 M HCl (runs 3 and 5). Reflux of the reaction
mixtures in toluene for 2 h gave cyclic dehydrated products 12b
(78%) and 14b (72%), respectively (runs 4 and 6). The reactions of
10b with other benzophenones 2b–d also selectively afforded cyc-
lic products 12c and 14c–e in moderate to good yields (runs 8, 10,
13, and 16). Acyclic product 13d (77%) and cyclic product 11e (70%)
were formed from 2c and 2d, respectively, after workup with 1 M
HCl (runs 11 and 14). Unfortunately, the corresponding cyclic
dehydrated products 12d and 12e could not be obtained, since
the heating of 13d and 11e in cat. p-TsOH/toluene resulted in com-
plex mixtures.
The presumed mechanism of the reductive coupling of 1a with
2a is illustrated in Scheme 5. Initially, dianion intermediate A is
generated by two-electron transfer from low-valent titanium to
2a. The major by-products are the homo-coupled pinacol and its
further reduced McMurry-type alkene, 1,1,2,2-tetraphenylethene.
For the cross coupling, the nucleophilic attack of A on 1a forms ad-
duct B. The adduct B is stable at 0 °C and, therefore, the workup of
B with water affords diol 3a. During workup, the dehydration of 3a
is catalyzed under acidic conditions to give 4a and 5a, while the
ring-opening of 3a is promoted under basic conditions to lead 6a.
The ring-closing of 6a to 3a and subsequent dehydration of 3a to
5a through 4a are effected by reflux in benzene in the presence
13a
15a
12a
p-TsOH
toluene
Ph
reflux, 6 h
N
Me
80%
14a
Scheme 4. Transformation of 13a and 15a to 12a and 14a.
sults (runs 7–11). Although the yields were moderate, cyclic dehy-
drated products 5b (53%) and 7b (47%) could selectively be ob-
tained (runs 8 and 11). While the two-electron reduced products
5a,b were formed exclusively after reflux in benzene (runs 3 and
8), the four-electron reduced products 7a,b were obtained as mix-
tures with small amounts of 5a,b (runs 6 and 11). Incidentally,
these two cyclic products could readily be transformed to each
other by reduction with L-Selectride and oxidation with DDQ
(Scheme 2). All of the products were determined by spectroscopic6
and X-ray crystallographic analyses.7
The reductive coupling of 1a,b with benzophenone derivatives
2b–d was carried out under the same conditions as those in Table 1
(Table 2). The reaction mixtures obtained from the reductive cou-
pling were refluxed in benzene as described above and cyclic dehy-
drated products 5 and 7 were obtained selectively in all cases. In
the reaction of 1a with 4,40-dimethoxybenzophenone (2c), a small
amount of six-membered cyclized product 9 (8%) was obtained
with major five-membered cyclized product 5d (73%) (run 4). It
was confirmed that 9 was selectively produced by reflux in ben-
zene from acyclic product 6d (Scheme 3); 6d was prepared by
workup with satd NaHCO3 aq after the reductive coupling (run
Ph
O
Ph
O
2a
+2e
+2e
Ph
Ph
Ti(+X)
+2e
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
O
O
A
Ti
2a
Ph
O
Ph
Ph
O
O
N
A
+
N
O
O
O
Me
N
Me
Me
Ti
7a
+H2O
workup
1a
B
Ph
ring-
ring-
opening
-H2O
Ph
OH
closing
O
N
OH
Me
-H2O
Ph
Ph
3a
MeHNOC
MeTiNOC
Ph
Ph
Ph
Ph
ring-
opening
OTi
O
O
O
N
Me
ring-
OH
8a
C
closing
+H2O
4a
+2e
-H2O
workup
Ph
Ph
Ph
MeTiNOC
MeHNOC
O
Ph
N
Ph
OH
Me
5a
Ph
O
O
D
6a
Scheme 5. Presumed reaction mechanism of reductive coupling of 1a with 2a by low-valent titanium.

6948
N. Kise et al. / Tetrahedron Letters 54 (2013) 6944–6948
1 M HCl (20 mL) at 0 °C and the mixture was stirred for 15 min at 25 °C. The
mixture was extracted with ethyl acetate three times. The organic layer was
washed with aqueous NaCl and dried over MgSO4. After the solvent was
removed, the residue was purified by column chromatography on silica gel to
give 3a–6a.
of cat. amount of p-TsOH to give 5a as the sole product eventually.
This result suggests that 5a is thermodynamically the most stable
of the four products 3a–6a. The DFT calculations8 at the B3LYP/6-
311+G(2d,p) level of the four products also support this result;
the relative energies in benzene (PCM) at 353 K are 3a: 11.6 kcal/
mol; 4a+H2O: 1.6 kcal/mol; 5a+2H2O: ꢀ20.1 kcal/mol; 6a: 0 kcal/
mol. On the contrary, the reductive b-elimination of the adduct B
by low valent titanium is promoted at 30 °C to give 7a. Alterna-
tively at the elevated temperature, acyclic product 8a is formed
by the ring-opening of B, subsequent reduction of resulting C to
D, and then work up of D with water. The ring-closing of 8a and
following dehydration to 7a are also effected by reflux in cat. p-
TsOH/benzene. DFT calculations8 exhibit that 7a is more stable
than 8a; the relative energies in benzene (PCM) at 353 K are
7a+H2O: ꢀ3.5 kcal/mol; 8a: 0 kcal/mol.
6. See Supplementary data.
7. All measurements of X-ray crystallographic analysis were made on a Rigaku
RAXIS imaging plate area detector with graphite monochromated Mo
Ka
radiation. The structure was solved by direct methods with SIR-97 and refined
with SHELXL-97. The non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were refined isotropically. All calculations were performed
using the Yadokari-XG software package. Crystal data for 3a, 4a, 5a, 5b, 6a, 7a,
7b, 11b, 12b, and 14b are as follows: CCDC 959918–959927 contain the
supplementary crystallographic data. These data can be obtained free of charge
from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif. Compound 3a (CCDC 959918): C18H19NO3, FW = 297.34, mp
171–172 °C, triclinic, P-1 (no 2), colorless block, a = 9.206(4) Å, b = 12.204(5) Å,
c = 15.212(4) Å,
T = 298 K, Z = 4, Dcalcd = 1.293 g/cm3,
(CCDC 959919): C18H17NO2, FW = 279.33, mp 204–205 °C, triclinic, P-1 (no 2),
colorless block, a = 7.897(3) Å, b = 9.993(5) Å, c = 10.888(5) Å, = 105.66(4),
b = 105.03(3),
= 107.33(3), V = 733.9(6) Å3, T = 298 K, Z = 2, Dcalcd = 1.264 g/
cm3, = 0.82 cmꢀ1
GOF = 1.089. Compound 5a (CCDC 959920): C18H15NO,
a
= 69.732(19), b = 85.263(18),
c
= 72.399(10),V = 1527.8(9) Å3,
l
= 0.88 cmꢀ1, GOF = 0.941. Compound 4a
In summary, the reductive coupling of succinimides 1a,b and
glutarimides 10a,b with benzophenones 2a–d by Zn–TiCl4 gave
two- and four-electron reduced products as cyclic and acyclic
products. The two- and four-electron reduced products could be
prepared selectively by controlling the reaction conditions. The
product selectivity of cyclic and acyclic products depends on the
employed substrates and the conditions of workup. In most cases,
all products were transformed to the corresponding cyclic dehy-
drated products by heating in the presence of cat. p-TsOH. Conse-
quently, five-(5 and 7) and six-membered cyclized products (12
and 14) were synthesized selectively by this method. The applica-
tion of these compounds is an issue in the future.
a
c
l
,
FW = 261.31, mp 121–122 °C, monoclinic, P21/c (no 14), yellow block,
a = 10.54(2) Å, b = 17.82(4) Å, c = 15.22(2) Å, b = 87.88(16), V = 2857(9) Å3,
T = 298 K, Z = 8, Dcalcd = 1.215 g/cm3,
l
= 0.75 cmꢀ1, GOF = 0.876. Compound 5b
(CCDC 959921): C17H13NO, FW = 247.28, mp 189–191 °C, monoclinic, P21/a (no
14), yellow block, a = 9.969(3) Å, b = 10.2341(18) Å, c = 12.943(4) Å, b = 93.333(6),
V = 1318.2(6) Å3, T = 298 K, Z = 4, Dcalcd = 1.246 g/cm3,
l
= 0.78 cmꢀ1, GOF = 1.027.
Compound 6a (CCDC 959922):
C18H19NO3, FW = 297.34, mp 168–169 °C,
monoclinic, P21/a (no 14), colorless block, a = 15.152(5) Å, b = 10.429(6) Å,
c = 14.911(8) Å,
b = 42.328(15),
l
V = 1586.6(13) Å3,
T = 298 K,
Z = 4,
Dcalcd = 1.245 g/cm3,
= 0.85 cmꢀ1, GOF = 1.077. Compound 7a (CCDC 959923):
C
18H17NO, FW = 263.33, mp 133 °C, monoclinic, P21/c (no 14), colorless block,
a = 10.364(4) Å, b = 17.706(7) Å, c = 15.497(7) Å, b = 93.08(4), V = 2840(2) Å3,
T = 298 K, Z = 8, Dcalcd = 1.232 g/cm3, = 0.76 cmꢀ1, GOF = 1.029. Compound 7b
(CCDC 959924): C17H15NO, FW = 249.30, mp 187–188 °C, triclinic, P-1 (no 2),
colorless block, a = 7.136(3) Å, b = 7.914(3) Å, c = 12.230(4) Å, = 90.35(2),
b = 104.28(2),
= 94.090(5), V = 667.4(5) Å3, T = 298 K, Z = 2, Dcalcd = 1.240 g/cm3,
= 0.77 cmꢀ1
GOF = 1.067. Compound 11b (CCDC 959925): C18H17NO2,
FW = 279.33, mp 178–179 °C, triclinic, P-1 (no 2), colorless block, a = 5.982(3) Å,
l
a
Supplementary data
c
l
,
Supplementary data (experimental procedures, characteriza-
tion data for products, 1H and 13C NMR spectra of products, X-
ray crystallographic structures, and results of DFT calculations)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.10.053.
b = 14.544(6) Å,
c = 18.251(8) Å,
T = 298 K,
a
Z = 4,
= 73.67(4),
b = 82.65(4),
c
= 78.89(3),
V = 1490.5(11) Å3,
Dcalcd = 1.245 g/cm3,
l
= 0.81 cmꢀ1
,
GOF = 0.893. Compound 12b (CCDC 959926): C18H15NO, FW = 261.31, mp 215–
217 °C, triclinic, P-1 (no 2), colorless block, a = 7.973(16) Å, b = 9.220(20) Å,
c = 10.492(20) Å,
Z = 2, Dcalcd = 1.299 g/cm3,
959927): 18H17NO, FW = 263.33, mp 153–154 °C, monoclinic, P21 (no 4),
colorless block, a = 8.708(2) Å, b = 9.836(2) Å, c = 8.9882(15) Å, b = 113.838(17),
a
= 65.07(18), b = 75.19(17),
c
= 75.8(2), V = 668(2) Å3, T = 298 K,
l
= 0.80 cmꢀ1
,
GOF = 1.097. Compound 14b (CCDC
C
References and notes
V = 704.2(3) Å3, T = 298 K, Z = 2, Dcalcd = 1242 g/cm3,
l
= 0.77 cmꢀ1, GOF = 1.044.
8. All DFT calculations were carried out with the GAUSSIAN 099 program. Geometry
optimization was performed at the B3LYP/6-311+G(2d,p) level using the IEFPCM
models for benzene solvent to take the solvent effect into consideration. The
optimized geometries were verified by the vibrational analysis and their
energies were thermally corrected to 353 K based on the frequencies. For
optimized structures of 3a–8a, see Supplementary data.
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5. Typical procedure for the reductive coupling of 1a with 2a (Table 1, run 1) is as
follows. To a solution of 1a (1 mmol), 2a (2 mmol), and zinc powder (4 mmol) in
THF (10 mL) was added TiCl4 (2 mmol) dropwise at 0 °C and then the dark blue
suspension was stirred for 12 h at this temperature. To the mixture was added