Copper(I) chelate complexes with novel ζ-conjugated 1,2-bis(2-pyridylethynyl)benzene ligands: Synthesis, structure, and reactivity

Article abstract of DOI:10.1246/cl.2001.1230

Novel π-conjugated nitrogen ligands, 1,2-bis(2-pyridylethynyl)benzenes, and their Cu(I) complexes have been synthesized and structurally characterized: the reactivity of the complexes for radical cyclization has also been examined.

Full text of DOI:10.1246/cl.2001.1230

1230
Chemistry Letters 2001
Copper(I) Chelate Complexes with Novel π-Conjugated 1,2-Bis(2-pyridylethynyl)benzene
Ligands: Synthesis, Structure, and Reactivity
Tomikazu Kawano, Jun Kuwana, Tatsuji Shinomaru, Chong-Xu Du, and Ikuo Ueda*
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received September 13, 2001; CL-010905)
Novel π-conjugated nitrogen ligands, 1,2-bis(2-
cessful when CuCl was used as the Cu(I) salt. During the reac-
tion of 4b with CuCl in chloroform, a yellow solution of 4bC
was initially obtained, which gradually turned into a blue-green
solution. Under anaerobic conditions, however, the yellow
color of 4bC remained unchanged. This indicates that 4bC is
remarkably air-sensitive and highly reactive in solution. By
contrast to 4bC, both 4bA and 4bB were found to be very air-
stable even if oxygen gas was bubbled into the reaction solu-
tion.
pyridylethynyl)benzenes, and their Cu(I) complexes have been
synthesized and structurally characterized: the reactivity of the
complexes for radical cyclization has also been examined.
Cu(I) complexes with polydentate nitrogen ligands have
received considerable attention within the fields of supramolec-
ular chemistry1 and bioinorganic chemistry.2 Among these
complexes, some two-coordinate Cu(I) complexes have been
shown to be effective in transition-metal-catalyzed reactions
due to their enhanced reactivity.3 In these catalytic reactions,
C2-symmetric nitrogen ligands, which act in a chelating mode,
play an important role in the catalytic activity of the complex.
However, few extensive structural studies of the complexes
derived from the chelating nitrogen ligands have been
reported.4 In this study, 1,2-bis(2-pyridylethynyl)benzene lig-
ands bearing an extended π-conjugated system were designed
as a new class of nitrogen ligands. The ligands and their com-
plexes might be expected to have potential as catalysts, antitu-
mor agents, electrochemical and photochemical devices. We
herein describe the syntheses and structural characterizations of
the novel ligands and their Cu(I) complexes, along with the
reactivity of the complexes in radical cyclization.
The new ligands (3a–d) were synthesized according to the
procedure outlined in Scheme 1. Reaction of the sodium salt of
2-bromo-6-(hydroxymethyl)pyridine (1)5 with methyl iodide
afforded the 6-(methoxymethyl) derivative in 97% yield.
Reaction of sodium salts of phenol or i-PrOH with the 6-
(iodomethyl) derivative, which was obtained by mesylation of
1, followed by treatment of the mesylated derivative with sodi-
um iodide, gave the 6-(phenoxy)methyl and 6-
(isopropoxy)methyl derivatives in 93% and 95% yields, respec-
tively. The latter synthetic protocol allows the preparation of
bromopyridine derivatives with a variety of substituents at the
6-position. The cross-coupling reaction of 1,2-bis(ethynyl)ben-
zene6 with the corresponding 2-bromopyridine derivatives
(2a–d) in the presence of a catalytic amount of Pd(PPh3)4 in
diethylamine at 80 °C afforded the desired ligands in good
yields. All new ligands obtained were characterized by spec-
troscopy (NMR, IR, and FABMS) and elemental analysis.
The reaction of the ligand (3b) with an equimolar quantity
of [Cu(CH3CN)4]X (X = PF6 or ClO4) in dichloromethane at
room temperature proceeded smoothly to afford yellow
mononuclear Cu(I) complexes [Cu(3b)](PF6), 4bA, and
[Cu(3b)](ClO4), 4bB, in 90% and 67% yields, respectively. The
molecular formulas of 4bA and 4bB were determined on the
basis of FAB mass spectroscopy and elemental analysis.
Complexes 4aA, 4cA, 4cB and 4dA were obtained in a similar
manner in 87%, 100%, 61% and 71% yields, respectively. On
the other hand, the isolation of [Cu(3b)]Cl, 4bC, was unsuc-
The structures of the complexes thus obtained were unam-
biguously assigned on the basis of 1H NMR, UV and X-ray crys-
tallography. The 1H NMR spectra of 4bA in dichloromethane-d2
solution consisted of a simple pattern of peaks corresponding to
3b, indicative of the formation of a symmetric species. However,
1H NMR measurement of 4bA at higher concentration (about 10
mM) resulted in the appearance of new peaks, which implies the
formation of a different Cu(I) complex from 4bA. Concentration-
dependent behavior was also observed in the case of 4cA and
4dA, where, in particular, a new complex was generated as the
main product at concentrations above 16 mM solution of 4cA.
This finding will be reported in detail elsewhere. The UV elec-
tronic spectra of 4bA displayed a new absorption at 360 nm,
which was assigned to a metal-to-ligand charge-transfer band.
The NMR spectra and electronic spectra of other complexes
mentioned above indicated a similar behavior. Further structural
information came from the single crystal X-ray analysis of 4bB
and 4cA. The X-ray quality crystals were obtained from 4bB
and 4cA by recrystallization from dichloromethane–hexane and
acetone, respectively. Figure 1 shows the crystal structure of
4bB.7 The complex 4bB consists of one ligand (3b), one two-
coordinate Cu(I) ion, and one perchlorate anion. The ligand
coordinates Cu(I) ions in a trans-chelating mode. The average
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1231
dihedral angle between the aromatic rings in 4bB is 3.63°,
indicative of the formation of a planar complex. The bond
length of Cu–N is 1.935 Å and the bite angle of N–Cu–N is
–
170.28(7)°. The ClO4 anion adopts position nearly perpendi-
cular to the N–Cu–N axis. The distance between the Cu(I) ion
–
and the adjacent oxygen atom of the ClO4 anion is 2.43 Å,
indicative of the presence of a relatively strong interaction. The
crystal structure of the cation of 4cA also had a planar structure
similar to that of 4bB.
complexes such as 4bA and 4bB occurred. Table 1 summarizes
the yields and diastereomer ratios of the reaction products.
In conclusion, we have demonstrated that the reaction of a
new class of π-conjugated nitrogen ligand, 1,2-bis(2-
pyridylethynyl)benzenes, with Cu(I) salts proceeds smoothly to
give novel planar two-coordinate Cu(I) complexes in good
yields, and that radical cyclization of N-allyltrichloroacetamides
using these complexes yields trans-lactam derivatives with good
stereoselectivity.
We thank the Material Analysis Center of our institute for
assisting with elemental analysis.
References and Notes
Complex 4bC might be expected to be useful for some
organic reactions due to its high reactivity. Nagashima et al.
established that the cyclization of N-allyltrichloroacetamides
can be successfully achieved in the presence of a 1:1 mixture of
CuCl and bipyridine under mild conditions.8 The CuCl-bipyri-
dine complex, which was prepared in the reaction mixture,
plays an important role in this reaction. Therefore, radical
cyclization of N-methyl-N-(1-methyl-2-propen)trichloroac-
etamide was examined using the complexes synthesized in this
study (Scheme 2).
1
J. -M. Lehn, “Supramolecular Chemistry: Concepts and
Perspectives,” VCH, Weinheim (1995); M. Fujita, K. Umemoto,
M. Yoshizawa, N. Fujita, T. Kusukawa, and K. Biradha, Chem.
Commun., 2001, 509.
W. Kaim and J. Rall, Angew. Chem., Int. Ed. Engl., 35, 43 (1996);
W. B. Tolman, Acc. Chem. Res., 30, 227 (1997).
M. M. -C. Lo and G. C. Fu, J. Am. Chem. Soc., 120, 10270 (1998);
D. A. Evans, M. M. Faul, M. T. Bilodeau, B. A. Anderson, and D.
M. Barnes, J. Am. Chem. Soc., 115, 5328 (1993).
2
3
4
5
6
7
Cambridge Structural Database, Version 5.20, October 2000,
CCDC, Cambridge, U.K.
D. Cai, D. L. Hughes, and T. R. Verhoven, Tetrahedron Lett., 37,
2537 (1996).
S. Takahashi, Y. Kuroyama, K. Sonogashira, and N. Hagihara,
Synthesis, 1980, 627.
Crystal data f–or 4bA: C24H20ClN2O6Cu, Mr = 531.43, triclinic,
space group P1 (no. 2), a = 10.276(2), b = 12.226(2), c = 9.455(1)
Å, α = 96.52(1), β = 91.81(1), γ = 71.93(1)°, V = 1121.9(3) Å3, Z
= 2, Dcalc = 1.573 g cm–3, µ(Mo Kα) = 11.37 cm–1, T = 223 K,
5462 reflections measured, 5160 unique reflections, Rint = 0.011,
4250 observed reflections with I > 3σ(I), Final R = 0.029 and Rw =
0.032. CAUTION. Perchlorate salts of metal complexes with
organic ligands are potentially explosive. Only small amounts of
materials should be prepared and handled with care.
H. Nagashima, N. Ozaki, M. Ishii, K. Seki, M. Washiyama, and K.
Itoh, J. Org. Chem., 58, 464 (1993).
The acetamide was subjected to the complex 4dC, generat-
ed in situ from CuCl and 3d, in dichloromethane at room tem-
perature to give trans-lactam as the major product (trans:cis =
93:7). Obviously, no radical cyclization using stable Cu(I)
8