1,3-Butadienylzinc trimer formed via transmetalation from 1,4-dilithio-1,3-butadienes: Synthesis, structural characterization, and application in Negishi cross-coupling

Article abstract of DOI:10.1021/om3005004

The first well-defined 1,3-butadienylzinc trimers have been synthesized by transmetalation of 1,4-dilithio-1,3-butadienes with 1 equiv of ZnBr₂. Their structures have been determined by single-crystal X-ray structural analysis. Their reaction c

Full text of DOI:10.1021/om3005004

Article
pubs.acs.org/Organometallics
1,3-Butadienylzinc Trimer Formed via Transmetalation from 1,4-
Dilithio-1,3-butadienes: Synthesis, Structural Characterization, and
Application in Negishi Cross-Coupling
Yi Zhou,† Wen-Xiong Zhang,† and Zhenfeng Xi*,†,‡
†Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry
of Education, College of Chemistry, Peking University, Beijing 100871, People's Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC), CAS, Shanghai 200032,
People's Republic of China
S
* Supporting Information
ABSTRACT: The first well-defined 1,3-butadienylzinc
trimers have been synthesized by transmetalation of 1,4-
dilithio-1,3-butadienes with 1 equiv of ZnBr2. Their structures
have been determined by single-crystal X-ray structural
analysis. Their reaction chemistry has been demonstrated by
Pd-catalyzed Negishi cross-coupling with iodobenzenes.
rganozinc compounds are useful synthetic reagents and
readily available reactive organometallic intermediates.1
provide 2f in 95% isolated yield (Table 1, entry 6). The
reaction of 2,2′-dilithiobiphenyl 1g with ZnBr2 gave rise to the
o-biphenylenezinc trimer 2g (Table 1, entry 7).8 It should be
noted that the same complexes 2 were obtained as sole
products even with an excess amount of ZnBr2. These 1,3-
butadienylzinc trimers 2a−g are all stable under an N2
atmosphere even at 100 °C and are soluble in common
solvents, such as hexane, benzene, and THF.
O
There are many reports on their reactivities and synthetic
applications. In contrast, much less investigation has been
carried out on their structures.2−4 Walsh and co-workers
reported the structure of the divinylzinc complex Zn[C(R)
CH2]2, which features four-coordinated zinc centers with two
σ-bonded vinyl groups and two π interactions with the
neighboring divinylzinc units.4
We have been interested in the reaction chemistry and
transmetalation of 1,4-dilithio-1,3-butadienes (1).5,6 Our
previous results demonstrated that those organo-dimetallic
compounds formed via this transmetalation show unique
structures and reactivities.6 Herein we report the synthesis,
structural characterization, and reaction chemistry of the first
well-defined 1,3-butadienylzinc trimers (2) formed from 1,4-
dilithio-1,3-butadienes (1) and ZnBr2. Although there are many
metalloles,7 to our knowledge, these are the first well-defined
trimetallic organozinc macrocycles.8
1,3-Butadienylzinc trimers 2a−c were structurally charac-
terized by X-ray single-crystal structural analyses (Figure 1,
2a,c; see the Supporting Information for details), in the triclinic
P1, hexagonal P6 2 , and monoclinic C2/c space groups,
̅
5
2
respectively. They all exhibit a novel trimeric 1,3-butadienezinc
pattern in their solid state, independent of the substituents on
the 1,3-butadienyl skeleton. These results are in marked
contrast with their precursors 1,4-dilithio-1,3-butadienes, the
structures of which are dependent on their substituents.5,9
Three 1,3-butadienylzinc “CCCCZn” units comprise a 15-
membered metallacycle. Each zinc atom is supported by two σ-
bonded 1,3-butadiene units. The distances between two zinc
centers ranged from 2.8440(6) to 2.9255(6) Å in 2a and 3.203
to 3.233 Å in 2c, which are much longer than the Zn−Zn metal
bond in the reported Cp*Zn−ZnCp*,10 indicating the absence
of Zn−Zn bonds. The Zn−C bond lengths in 2a,c (1.939(3)−
1.976(5) Å) were both comparable with the reported C(sp2)−
Zn−C(sp2) bonds in Zn[C(H)CMe2]2 (1.987(9)−2.006(7)
Å).4 The C−Zn−C bond angles between the vinyl σ bonds in
2a,c fall between 172.66(13)−177.52(13) and 163.8(2)−
167.0(2)°, exhibiting a small difference due to the steric effects
RESULTS AND DISCUSSION
■
1,2,3,4-Tetramethyl-substituted 1,4-dilithio-1,3-butadiene (1a)
in Et2O was generated in situ from its corresponding 1,4-
5
t
diiodo-1,3-butadiene and 4 equiv of BuLi. After 1 equiv of
ZnBr2 in Et2O was added at 0 °C, the resulting solution was
warmed to room temperature for 30 min. The pure salt-free
product 2a was obtained in 84% yield via recrystallization in
hexane at −20 °C (Table 1, entry 1). Similar to the case for 2a,
when 1b−e were treated with ZnBr2, their corresponding 1,3-
butadienylzinctrimers 2b−e were produced in excellent isolated
yields (Table 1, entries 2−5). The unsymmetric 1,4-dilithio-1,3-
butadiene 1f could be also applied to the present conditions to
Received: June 8, 2012
Published: July 16, 2012
© 2012 American Chemical Society
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Article
monophenylated product 3a in toluene at 90 °C in 88%
isolated yield upon hydrolysis. The expected double cross-
coupling product was not observed, even with a large excess of
iodobenzene and at higher temperatures. When the reaction
mixture was quenched with D2O, the D-incorporated product
3a-D was isolated in 91% yield (>95% D). These results
indicate that one Zn−C bond remains but is inert toward a
second cross-coupling with iodobenzene, probably due to the
butadienyl conjugation. In contrast, when 1,2-diiodobenzene
was used, the aromatization product 1,2,3,4-tetraethylnaph-
thalene (5) was obtained in 89% isolated yield,12 probably
owing to the aromatization driving force that makes the second
intramolecular Negishi cross-coupling take place.
Table 1. Formation of 1,3-Butadienylzinc Trimers 2 via
Transmetalation of 1,4-Dilithio-1,3-butadienes 1 with ZnBr2
In summary, the first well-defined 1,3-butadienylzinc trimer
compounds 2 having a 15-membered metallacyclic ring with 3
zinc centers have been achieved by transmetalation of 1,4-
dilithio-1,3-butadienes (1) with 1 equiv of ZnBr2. Preliminary
results indicate that these macrocyclic organozinc compounds
should have interesting and useful reactivity and synthetic
applications.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under a
slightly positive pressure of dry and oxygen-free nitrogen by using
standard Schlenk line techniques or under a nitrogen atmosphere in a
glovebox. The nitrogen in the glovebox was constantly circulated
through a copper/molecular sieves catalyst unit. The oxygen and
moisture concentrations in the glovebox atmosphere were monitored
to ensure both were always below 1 ppm. Unless otherwise noted, all
starting materials were commercially available and were used without
further purification. Solvent was distilled from sodium/benzophenone
1
under a nitrogen atmosphere. H NMR and 13C NMR spectra were
1
recorded on a JEOL-AL300 spectrometer (FT, 300 MHz for H; 75
MHz for 13C), a Bruker-400 spectrometer (FT, 400 MHz for 1H; 100
MHz for 13C), and a Bruker-500 spectrometer (FT, 500 MHz for H;
1
125 MHz for 13C) at room temperature. Organometallic samples for
NMR spectroscopic measurements were prepared in the glovebox by
use of J. Young valve NMR tubes (Wilmad 528-JY).
Synthesis of 1,3-Butadienylzinc Trimer. A solution of
substituted 1,4-diiodo-1,3-butadiene (1 mmol) in Et2O (10 mL) was
t
cooled to −78 °C, and BuLi (4 mmol, 1.6 M in pentane) was added
dropwise. The reaction mixture was then stirred at −78 °C for 1 h, and
1,4-dilithio-1,3-butadiene 1 (1 mmol) was generated in situ. The
reaction mixture was warmed to 0 °C, anhydrous ZnBr2 (1 mmol)
dissolved in Et2O was added, and then the resulting solution was
stirred at room temperature for 30 min. The pure salt-free product 2
was obtained by extraction with hexane and recrystallization in hexane
at −20 °C.
1
2a: colorless solid, isolated yield 84%; H NMR (400 MHz, C6D6,
Me4Si) δ 1.82 (s, 18 H, 6 CH3), 1.90 (s, 18 H, 6 CH3); 13C NMR (100
MHz, C6D6, Me4Si) δ 15.45 (s, 6 CH3), 21.01 (s, 6 CH3), 147.93 (s, 6
quat C), 159.52 (s, 6 quat C). Anal. Calcd for C24H36Zn3: C, 55.35; H,
6.97. Found: C, 55.52; H, 7.02. Recrystallization of 2a from hexane at
−20 °C gave crystals suitable for X-ray analysis.
1
2b: colorless solid, isolated yield 92%; H NMR (500 MHz, C6D6)
a
b
δ 1.01−1.05 (m, 36 H, 12 CH3), 2.02−2.18 (m, 12 H, 6 CH2), 2.41−
2.51 (m, 12 H, 6 CH2); 13C NMR (75 MHz, C6D6) δ 13.34 (s, 6
CH3), 17.24 (s, 6 CH3), 22.04 (s, 6 CH2), 27.76 (s, 6 CH2), 157.73 (s,
6 quat C), 161.96 (s, 6 quat C). Anal. Calcd for C36H60Zn3: C, 62.75;
H, 8.78. Found: C, 62.71; H, 8.64. Recrystallization of 2b from hexane
at −20 °C gave crystals suitable for X-ray analysis.
Isolated yields. Conditions: −78 °C for 15 min then room
temperature for 30 min.
of bulky substituents. However, these angles are much larger
than those found in Zn[C(Me)CH2]2 (132.4(3)−
139.4(3)°).4
The reaction chemistry of such macrocyclic organozinc
compounds was demonstrated by the Pd-catalyzed Negishi
cross-coupling with iodobenzenes.11 As shown in Scheme 1, the
reaction between 2b and iodobenzene gave exclusively the
2c: colorless solid, isolated yield 95%; 1H NMR (300 MHz, C6D6) δ
0.18 (s, 54 H, 18 CH3), 1.97 (s, 18 H, 6 CH3); 13C NMR (75 MHz,
C6D6) δ 1.84 (s, 18 CH3), 24.51 (s, 6 CH3), 148.55 (s, 6 quat C),
174.33 (s, 6 quat C). Anal. Calcd for C36H72Si6Zn3: C, 49.72; H, 8.34.
Found: C, 49.77; H, 8.37. Recrystallization of 2c from hexane at −20
°C gave crystals suitable for X-ray analysis.
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Figure 1. ORTEP drawings of 2a (left) and 2c (right) with 30% thermal ellipsoids. Selected bond lengths (Å) and angles (deg) for 2a: Zn(1)−
C(20) = 1.936(3), Zn(1)−C(1) = 1.939(3), Zn(2)−C(4) = 1.944(3), Zn(2)−C(9) = 1.945(3), Zn(3)−C(12) = 1.933(3), Zn(3)−C(17) =
1.936(3), Zn(1)−Zn(2) = 2.9255(6), Zn(1)−Zn(3) = 2.8440(6), Zn(2)−Zn(3) = 2.9101(7); C(20)−Zn(1)−C(1) = 172.66(13), C(4)−Zn(2)−
C(9) = 177.52(13), C(12)−Zn(3)−C(17) = 172.08(13). Selected bond lengths (Å) and angles (deg) for 2c: Zn(1)−C(1) = 1.948(6), Zn(1)−
C(12) = 1.951(6), Zn(2)−C(8) = 1.961(5), Zn(2)−C(9) = 1.976(5), Zn(3)−C(4) = 1.933(5), Zn(3)−C(5) = 1.948(5), Zn(1)−Zn(2) = 3.225,
Zn(1)−Zn(3) = 3.203, Zn(2)−Zn(3) = 3.233; C(1)−Zn(1)−C(12) = 164.6(2), C(8)−Zn(2)−C(9) = 167.0(2), C(4)−Zn(3)−C(5) = 163.8(2).
1
2g: colorless solid, isolated yield 65%; H NMR (500 MHz, C6D6,
Me4Si) δ 6.81−6.85 (m, 6 H, 6 CH), 6.89−6.92 (m, 6 H, 6 CH),
7.14−7.16 (m, 12 H, 12 CH); 13C NMR (75 MHz, C6D6) δ 127.42 (s,
6 CH), 127.45 (s, 6 CH), 128.46 (s, 6 CH), 129.00 (s, 6 CH), 129.01
(s, 6 quat C), 141.67 (s, 6 quat C). Anal. Calcd for C36H24Zn3: C,
66.23; H, 3.71. Found: C, 66.21; H, 3.67.
Scheme 1. Pd-Catalyzed Negishi Cross-Coupling of 1,3-
Butadienylzinc Trimer 2b with Iodobenzenes
Pd-Catalyzed Negishi Cross-Coupling Reaction of 2b with
Iodobenzene or 1,2-Diiodobenzene. Under nitrogen, a mixture of
1,3-butadienylzinc trimer 2b (0.5 mmol), iodobenzene or 1,2-
diiodobenzene (0.5 mmol), and Pd(PPh3)4 (5 mol %) in 2 mL of
toluene was stirred at 90 °C (for iodobenzene) or 100 °C (for 1,2-
diiodobenzene) for 1 h. The reaction mixture was quenched with
water and extracted with Et2O. The extraction was washed with brine
and dried over MgSO4. The solvent was then evaporated under
vacuum, and the residue was purified by column chromatography to
give products 3a and 4.
3a: colorless liquid, isolated yield 88%; 1H NMR (500 MHz,
CDCl3, SiMe4) δ 0.62 (t, J = 9.4 Hz, 3 H, 1 CH3), 0.86 (t, J = 9.5 Hz, 3
H, 1 CH3), 0.91 (t, J = 9.4 Hz, 3 H, 1 CH3), 1.00 (t, J = 9.4 Hz, 3 H, 1
CH3), 1.75−1.90 (m, 4 H, 2 CH2), 2.25 (q, J = 9.4 Hz, 2 H, 1 CH2),
2.40 (q, J = 9.3 Hz, 2 H, 1 CH2), 4.91 (t, J = 9.2 Hz, 1 H, 1 CH),
7.06−7.19 (m, 5 H, 5 CH); 13C NMR (75 MHz, CDCl3, SiMe4) δ
13.34 (s, 1 CH3), 13.37 (s, 1 CH3), 13.45 (s, 1 CH3), 13.92 (s, 1
CH3), 20.99 (s, 1 CH2), 22.81 (s, 1 CH2), 24.49 (s, 1 CH2), 27.30 (s,
1 CH2), 125.23 (s, 1 CH), 127.16 (s, 2 CH), 129.53 (s, 2 CH), 132.24
(s, 1 CH), 137.30 (s, 1 quat C), 139.71 (s, 1 quat C), 141.16 (s, 1 quat
C), 144.40 (s, 1 quat C).13
1
2d: colorless solid, isolated yield 92%; H NMR (300 MHz, C6D6,
Me4Si) δ 0.32 (s, 54 H, 18 CH3), 7.61−7.86 (m, 30 H, 30 CH); 13C
NMR (75 MHz, C6D6) δ −0.19 (s, 18 CH3), 127.43 (s, 6 CH), 128.07
(s, 12 CH), 130.23 (s, 12 CH), 134.45 (s, 6 quat C), 142.57 (s, 6 quat
C), 160.69 (s, 6 quat C). Anal. Calcd for C66H84Si6Zn3: C, 63.82; H,
6.82. Found: C, 63.69; H, 6.74.
1
1
3a-D: colorless liquid, isolated yield 91%; H NMR (400 MHz,
2e: colorless solid, isolated yield 72%; H NMR (500 MHz, THF-
D8) δ −0.13 (s, 36 H, 12 CH3), 7.26−7.37 (m, 60 H, 60 CH); 13C
NMR (75 MHz, THF-D8) δ −0.23 (s, 12 CH3), 93.87 (s, 6 quat C),
106.31 (s, 6 quat C), 124.25 (s, 6 quat C), 128.36 (s, 6 CH), 128.69 (s,
12 CH), 129.00 (s, 12 CH), 129.29 (s, 6 CH), 130.88 (s, 12 CH),
131.82 (s, 6 quat C), 132.57 (s, 12 CH), 142.19 (s, 6 quat C), 162.26
(s, 6 quat C). Anal. Calcd for C108H96Si6Zn3: C, 73.76; H, 5.50. Found:
C, 73.83; H, 5.55.
CDCl3, SiMe4) δ 0.62 (t, J = 9.4 Hz, 3 H, 1 CH3), 0.86 (t, J = 9.5 Hz, 3
H, 1 CH3), 0.91 (t, J = 9.4 Hz, 3 H, 1 CH3), 1.00 (t, J = 9.4 Hz, 3 H, 1
CH3), 1.80−1.88 (m, 4 H, 2 CH2), 2.25 (q, J = 9.4 Hz, 2 H, 1 CH2),
2.40 (q, J = 9.3 Hz, 2 H, 1 CH2), 7.06−7.19 (m, 5 H, 5 CH); 13C
NMR (100 MHz, CDCl3, SiMe4) δ 13.31 (s, 1 CH3), 13.35 (s, 1
CH3), 13.45 (s, 1 CH3), 13.89 (s, 1 CH3), 20.91 (s, 1 CH2), 22.82 (s,
1 CH2), 24.53 (s, 1 CH2), 27.30 (s, 1 CH2), 125.26 (s, 1 CH), 127.18
(s, 2 CH), 129.54 (s, 2 CH), 131.88 (t, JC‑D = 22.9 Hz, 1 CD), 137.37
(s, 1 quat C), 139.67 (s, 1 quat C), 141.19 (s, 1 quat C), 144.43 (s, 1
quat C); HRMS (ESI) calcd for C18H25D [M + H]+ 244.2171, found
244.2170.
1
2f: colorless liquid, isolated yield 95%; H NMR (500 MHz, C6D6,
SiMe4) δ 0.10 (s, 27 H, 9 CH3), 0.80 (t, J = 7.4 Hz, 9 H, 3 CH3),
1.22−1.29 (m, 6 H, 3 CH2), 1.49−1.55 (m, 6 H, 3 CH2), 2.36 (t, J =
8.0 Hz, 6 H, 3 CH2), 6.75−6.88 (m, 15 H, 15 CH), 6.96−7.04 (m, 15
H, 15 CH); 13C NMR (100 MHz, THF-D8) δ 0.54 (s, 9 CH3), 14.46
(s, 3 CH3), 23.79 (s, 3 CH2), 32.96 (s, 3 CH2), 34.43 (s, 3 CH2),
127.29 (s, 3 CH), 127.87 (s, 3 CH), 128.01 (s, 3 quat C), 128.49 (s, 6
CH), 129.20 (s, 6 CH), 130.41 (s, 6 CH), 130.55 (s, 3 quat C), 130.77
(s, 6 CH), 138.37 (s, 3 quat C), 140.91 (s, 3 quat C), 146.64 (s, 3 quat
C), 161.28 (s, 3 quat C). Anal. Calcd for C69H84Si3Zn3: C, 69.41; H,
7.09. Found: C, 69.43; H, 7.11.
4: colorless liquid, isolated yield 89%; 1H NMR (400 MHz, CDCl3,
SiMe4) δ 1.25 (t, J = 7.4 Hz, 6 H, 2 CH3), 1.31 (t, J = 7.5 Hz, 6 H, 2
CH3), 2.85 (q, J = 7.4 Hz, 4 H, 2 CH2), 3.11 (q, J = 7.4 Hz, 4 H, 2
CH2), 7.40−7.42 (m, 2 H, 2 CH), 8.02−8.04 (m, 2 H, 2 CH); 13C
NMR (100 MHz, CDCl3, SiMe4) δ 15.53 (s, 2 CH3), 15.86 (s, 2
CH3), 21.71 (s, 2 CH2), 22.78 (s, 2 CH2), 124.49, 124.53 (s, 2 quat
C), 131.01 (s, 2 quat C), 135.38 (s, 2 quat C), 137.75 (s, 2 quat C).12a
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X-ray Crystallographic Studies. Single crystals of 2a−c suitable
for X-ray analysis were grown in hexane at −20 °C for 24 h. The
crystals of 2a−c were manipulated under a nitrogen atmosphere and
were sealed in a thin-walled glass capillary. Data collections for 2a,b
were performed at −100 °C on a Rigaku CCD Saturn 724
diffractometer, using graphite-monochromated Mo Kα radiation (λ
= 0.710 73 Å). Data collections for 2c were performed at 20 °C on a
Rigaku RAXIS RAPID IP diffractometer, using graphite-monochro-
mated Mo Kα radiation (λ = 0.710 73 Å). The determination of crystal
class and unit cell parameters was carried out by the CrystalClear
program (Rigaku Inc., 2007) for 2a,b or Rapid-AUTO program
package (Rigaku Inc., 2000) for 2c. The raw frame data were
processed using Crystal Clear for 2a,b or Crystal Structure (Rigaku/
MSC, 2000) for 2c to yield the reflection data file. The structures of
2a−c were solved by use of the SHELXTL program. Refinement was
performed on F2 anisotropically for all the non-hydrogen atoms by the
full-matrix least-squares method. The hydrogen atoms were placed at
calculated positions and were included in the structure calculation
without further refinement of the parameters. Crystallographic data
(excluding structure factors) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC-884573 (2a), CCDC-884574 (2b), and CCDC-884575 (2c).
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation of
China and the Major State Basic Research Development
Program (2011CB808700).
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Crystallographic data for 2a: C H Zn , triclinic, space group P1,
̅
24 36
3
a = 9.7112(16) Å, b = 9.9467(14) Å, c = 13.852(2) Å, α = 70.142(8)°,
β = 77.515(9)°, γ = 75.018(8)°, U = 1203.5(3) Å3, Dc = 1.437 Mg
m−3, Z = 2, T = 173(2) K, colorless block, 0.34 × 0.28 × 0.23 mm3.
Data collection was carried out at −100 °C on a diffractometer; 8459
reflections were collected, with 4189 independent reflections (R(int) =
0.0337), giving R1 = 0.0356 for observed unique reflections (F2 >
2σ(F2)) and wR2 = 0.0850 for all data. The maximum and minimum
residual electron densities on the final difference Fourier map were
0.484 and −0.381 e Å−3.
́
Garcιa-Alvarez, P.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Russo, L.
Dalton Trans. 2010, 39, 62−65.
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G. R. J. Org. Chem. 1963, 28, 3592−3593. (c) Evans, D. F.; Wharf, I. J.
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Crystallographic data for 2b: C36H60Zn3, hexagonal, space group
P6522, a = 11.557(4) Å, b = 11.557(4) Å, c = 48.530(19) Å, α = 90°, β
= 90°, γ = 120°, U = 5614(4) Å3, Dc = 1.223 Mg m−3, Z = 6, T =
173(2) K, colorless block, 0.35 × 0.30 × 0.28 mm3. Data collection
was carried out at −100 °C on a diffractometer; 14 890 reflections
were collected, with 3376 independent reflections (R(int) = 0.0464),
giving R1 = 0.0461 for observed unique reflections (F2 > 2σ(F2)) and
wR2 = 0.0951 for all data. The maximum and minimum residual
electron densities on the final difference Fourier map were 0.295 and
−0.262 e Å−3.
Crystallographic data for 2c: C36H72Si6Zn3·1/2C6H14, monoclinic,
space group C2/c, a = 20.106(4) Å, b = 13.786(3) Å, c = 38.576(8) Å,
α = 90°, β = 95.04(3)°, γ = 90°, U = 10651(4) Å3, Dc = 1.138 Mg m−3,
Z = 8, T = 293(2) K, colorless block, 0.20 × 0.20 × 0.10 mm3. Data
collection was carried out at room temperature on a diffractometer; 22
920 reflections were collected, with 12 197 independent reflections
(R(int) = 0.0846), giving R1 = 0.0415 for observed unique reflections
(F2 > 2σ(F2)) and wR2 = 0.0612 for all data. The maximum and
minimum residual electron densities on the final difference Fourier
map were 0.493 and −0.515 e Å−3.
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files, figures, and tables giving crystallographic data for 2a−
c and figures giving NMR spectra for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zfxi@pku.edu.cn. Fax: 86-10-62751708. Tel: 86-10-
62759728.
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Organometallics
Article
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