Isolable and well-defined butadienyl organocopper(I) aggregates: Facile synthesis, structural characterization, and reaction chemistry

Article abstract of DOI:10.1021/ja4114243

Four types of alkenyl organocopper(I) aggregates linked by 1,3-butadienyl and/or 1,3,5,7-octatetraenyl moieties were selectively realized in good isolated yields. All these organocopper(I) aggregates were structurally characterized by single-crystal X-ray

Full text of DOI:10.1021/ja4114243

Communication
pubs.acs.org/JACS
Isolable and Well-Defined Butadienyl Organocopper(I) Aggregates:
Facile Synthesis, Structural Characterization, and Reaction Chemistry
Weizhi Geng,† Junnian Wei,† Wen-Xiong Zhang,† and Zhenfeng Xi*,†,‡
†Beijing National Laboratory for Molecular Sciences (BNLMS), and Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry (SIOC), Shanghai 200032, China
S
* Supporting Information
between dilithio reagents 1 and CuCl. Controlled structural
ABSTRACT: Four types of alkenyl organocopper(I)
aggregates linked by 1,3-butadienyl and/or 1,3,5,7-
octatetraenyl moieties were selectively realized in good
isolated yields. All these organocopper(I) aggregates were
structurally characterized by single-crystal X-ray structural
analysis. These unprecedented aggregates, stabilized by
multiple Cu−Cu interactions and the conjugated 1,3-
butadienyl or 1,3,5,7-octatetraenyl bridges, could undergo
controlled structural transformations. The 1,4-dicopper
1,3-butadienyl aggregate 3 could be efficiently transformed
to aggregate 2, while LiI could disaggregate the 1,3-
butadienyl-1,3,5,7-octatetraenyl aggregate 4 to 1,3,5,7-
octatetraenyl aggregate 5 and 1,3-butadienyl aggregate 2.
Preliminary reaction chemistry and synthetic applications
of these organocopper(I) aggregates were also inves-
tigated.
transformations among these organocopper(I) aggregates were
observed. Preliminary reaction chemistry and synthetic
applications of these aggregates were investigated.
1,2,3,4-Tetrasubstituted 1,4-dilithio-1,3-butadienes 1 could
be readily synthesized and isolated in high yields from the
reaction between their corresponding diiodides and t-BuLi.9a,10
Treatment of the tetrapropyl-substituted dilithio reagent 1a (R
= Pr) with 2.0 equiv of CuCl in Et2O at −78 °C for 2 h
afforded the 1,4-dicopper-1,3-butadiene tetrameric compound
2a in 75% isolated yield (Scheme 1). This compound 2a was
obtained as air- and moisture-sensitive red crystals and
characterized by X-ray single-crystal structural analysis (Figure
1). For this tetrameric organo-dicopper aggregate, the eight
Scheme 1. Synthesis of 1,3-Butadienyl Dicopper(I)
Aggregates 2 and 3, and Their Structural Transformation
rganocopper compounds,1−5 including diorganocuprates
2
O
LiCuR2, hetero-organocuprates MCuRX (M = Li, Cu,
Ag; X = monoanionic group),3 and organocopper(I) oligomers
(CuR)n,4 are structurally interesting and synthetically useful.1−5
Among them, organocuprates have been well investigated and
synthetically widely applied.1−3 However, the study and
characterization of organocopper compounds (CuR)n remains
a great challenge, due to their thermal instability and low
solubility in common organic solvents.4 Although remarkable
achievements have been realized in isolation and structural
characterization of aryl,4 alkynyl,5j and alkylorganocopper(I)4l
compounds (CuR)n, isolable and structurally well-defined
alkenyl organocopper(I) compounds are basically unknown.6
Since alkenylcopper compounds are often proposed as key
intermediates in Cu-mediated or -catalyzed synthetic chem-
istry,7 their isolation and structural elucidation are fundamen-
tally important and synthetically attractive.
We have been studying reactivity, structural features, and
synthetic application of 1,4-dilithio-1,3-butadienes (dilithio
reagents).8,9 Our results have demonstrated that the butadienyl
skeleton shows remarkable cooperative stabilizing ability for
reactive organometallic intermediates.8 Hence, we envisioned
that the butadienyl bridge would construct a favorable structure
for achieving Cu−Cu interactions to stabilize the alkenyl-
dicopper compounds. Herein we report the isolation and
structural characterization of four different types of unprece-
dented alkenylcopper(I) compounds from the reaction
Received: November 9, 2013
Published: December 24, 2013
© 2013 American Chemical Society
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Communication
each terminal carbon atom linked to the same type of Cu atom,
while the C17−C20, C33−C36 butadienyl ligands are unsym-
metrical, with two terminal carbons linked to different types of
Cu atoms. These differences were also observed by their NMR
spectra (see SI for detail). The distances of C−Cu bonds range
from 1.959(6) Å to 2.016(5) Å which are similar to the
reported values (1.95−2.15 Å),4,11 while the Cu−Cl bonds
range from 2.1545(15) Å to 2.1750(17) Å. The angles of 3c-2e
Cu−C−Cu bonds range from 74.4° to 78.6°, which are similar
to those for compounds 2a and 2b. The C−Cu bond distances
and Cu−C−Cu angles for compound 3a tend to be
homogeneous because of the loose arrangement of the
substitutes.
Figure 1. Diamond drawing of 2a with 30% thermal ellipsoids. Propyl
substituents are omitted for clarity.
copper atoms form a slightly distorted hexahedral, and the four
butadienyl moieties are linked to the copper cluster via two
three-center-two-electron (3c-2e) bonds. The bridged 1,3-
butadienyl ligands linked to four Cu atoms formed an
approximate parallelogram with Cu1−Cu4 2.4708(10), Cu3−
Cu4 2.4863(10), Cu1−Cu2 2.4966(11), and Cu2−Cu3
2.6694(10) Å. The Cu−Cu distances indicated d10 metal−
metal interactions (2.45−3.10 Å).11 The Cu−Cu stacking
interaction stabilized the organo-dicopper compounds. Termi-
nal carbons are each linked to two copper atoms in a 3c-2e
bonding mode with Cipso−Cu distances of 1.955(5), 2.081(5)
and 1.961(5), 2.047(5) Å, respectively. The angles of Cu−C−
Cu are 74.5−83.5°, and those C−Cu−C angles are 153.5−
173.3°, indicating a nearly linear geometry. When the tetraethyl
substituted dilithio reagent 1b (R = Et) was used, compound
2b was obtained in 68% isolated yield as light-red crystals.
Although 2b is also a tetramer, the configuration of copper
clusters tended to be more distorted hexahedrally than 2a (see
Supporting Information [SI] for detail). However, the bond
lengths and angles of 2b are almost the same as those of 2a.
These results indicate that the substituents can influence the
Cu−Cu stacking geometry but do not remarkably change the
C−Cu bond distances.4h,i,k
The aggregate 3a could be transformed to 2a in 82% yield
when treated with 1.0 equiv of 1a in Et2O (Scheme 1).
Interestingly, solvents and additives were found to influence
the formation and structure of alkenyl dicopper(I) aggregates
remarkably. As given in Scheme 2, when the dilithio reagent 1a
Scheme 2. Synthesis of 1,3,5,7-Octatetraenyl Dicopper(I)
Aggregates 4 and 5, and Their Structural Transformation/
Disaggregation
A different type of organocopper(I) aggregate 3 was obtained
when the dilithio reagent 1 was treated with an excess amount
of CuCl (Scheme 1). When 2.5 equiv of CuCl was added into
the Et2O solvent of 1a at −78 °C, precipitation was obtained.
The solid was filtered and recrystallized in Et2O with 2.0 equiv
of TMEDA to give yellow crystals 3a in 63% isolated yield.
Single-crystal X-ray structural analysis shows that 3a comprises
eight copper atoms in a hexagonal close-packed arrangement
which forms five tetrahedrons (Figure 2). There are two types
of copper atoms; half of the copper atoms formed Cl−Cu−C
bonds (Cu1, Cu2, Cu5, Cu8), and the others formed C−Cu−C
bonds. Three 1,3-butadienyl ligands have different environ-
ments; the C1−C4 butadienyl ligand is a symmetrical one with
was treated with 2.5 equiv of CuCl in THF in place of Et2O, the
aggregate 4 linked by 1,3-butadienyl and 1,3,5,7-octatetraenyl
moieties was obtained in 43% isolated yield. Apparently,
dimerization of 1,3-butadienyl 1,4-dicopper(I) moieties took
place. When 1a was treated with 2.5 equiv of CuCl and 4.0
equiv of LiI in THF, 1,8-dicopper-1,3,5,7-octatetraene deriva-
tive 5 was obtained in 63% isolated yield.12 The LiI salt could
disaggregate the dicopper complexes and increase their
reactivity to give the dimerization product. When 4 was treated
with 4.0 equiv of LiI in THF, disaggregation took place,
affording 5 in 82% yield and a small amount of 2a (Scheme 2).
The structures of 4 and 5 were both characterized by X-ray
single-crystal structural analysis (Figures 3 and 4). Figure 3
shows that compound 4 contains six copper atoms, which form
three tetrahedrons. There are two types of copper atoms, Cu3
Figure 2. Diamond drawing of 3a with 30% thermal ellipsoids. Propyl
substituents and TMEDA are omitted for clarity.
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Communication
Scheme 3. Demonstration of Reaction Chemistry of 1,4-
Dicopper-1,3-butadiene Tetrameric Aggregates 2
Figure 3. Diamond drawing of 4 with 30% thermal ellipsoids. Propyl
substituents are omitted for clarity.
iminocyclopentadiene derivative 7 in 85% isolated yield via 1,1-
insertion reaction followed by intramolecular coupling.
When the aggregate 5 was dissolved in THF, and the
solution was heated to 50 °C (Scheme 4), an intramolecular
Scheme 4. Thermal Transformation of 1,8-Dicopper-1,3,5,7-
octatetraene Aggregate 5
Figure 4. Diamond drawing of 5 with 30% thermal ellipsoids.
Hydrogen atoms are omitted for clarity.
C−C coupling reaction proceeded, affording its corresponding
cyclooctatetraene derivative 8 in 80% isolated yield with copper
mirror.7k,14 However, when the solvent was changed to toluene,
the semibullvalene derivative 9 was obtained in 56% isolated
yield along with a small amount of cyclooctatetraene 8.7k The
intermediacy of compound 5 was proposed for CuCl-mediated
reactions of dilithio reagents in our early work.7k,13,14
In summary, we have synthesized and structurally charac-
terized the first series of alkenylcopper(I) aggregates which
contain two 3c-2e Cu−C−Cu bonds. The favorable distance of
1,3-butadienyl bridges is considered essential for achieving Cu−
Cu interactions to stabilize the alkenylcopper compounds.
Preliminary reaction chemistry and synthetic applications of
these organocopper(I) aggregates were investigated.
and Cu4 atoms forming C−Cu−C bonds and the others
forming C−Cu−Cl bonds. The terminal carbon atoms of
1,3,5,7-octatetraenyl and 1,3-butadienyl ligands are linked to
the same type of copper atoms, respectively.
As shown in Figure 4, the compound 5 contains three copper
atoms, which form a linear Cu−Cu−Cu coordination
geometry. Each terminal carbon atom is linked with two
copper atoms, forming three-center-two-electron (3c-2e) bonds
with Cipso−Cu distances in the range 1.974(5)−2.005(5) Å.
The angles of Cu−C−Cu are 72.79(15)°and 75.23(17)°,
respectively, which are similar to those for 2a, 2b, and 3a. The
favorable distance of octatetraenyl linker made 5 form a nine-
membered copper-heterocycle and could also achieve the linear
geometry of the C−Cu−C bond (C−Cu−C angle is
171.71(18)°). The distances of Cu−Cu bonds (2.3749(8) Å,
2.4192(8) Å) show stronger Cu−Cu interaction.11c
The reactivities of 1,4-dicopper 2 and 1,8-dicopper 5
aggregates were investigated. All the aggregated organocopper
compounds 2−5 were thermally stable at solid states at room
temperature under dry N2. Intramolecular homocoupling of 2
and subsequent [4 + 2] cycloaddition took place (Scheme 3),
giving tricyclo[4.2.0.02,5]octa-3,7-dienes 6 in high isolated
yields.12,13 Because LiI salt could disaggregate the copper
complexes and increase their reactivity, in the presence of LiI,
2b thus could react with 2-methylphenyl isocyanide to give the
ASSOCIATED CONTENT
* Supporting Information
Experimental details, X-ray data for 2a, 2b, 3a, 4, 5 and scanned
NMR spectra of all new products. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
zfxi@pku.edu.cn
■
Notes
The authors declare no competing financial interest.
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Journal of the American Chemical Society
Communication
Buchholz, J.; Grushin, V. V. J. Am. Chem. Soc. 2011, 133, 20901.
(m) Stollenz, M.; Meyer, F. Organometallics 2012, 31, 7708. (n) van
Koten, G. Organometallics 2012, 31, 7634.
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program
(2012CB821600) and the Natural Science Foundation of
China.
(5) For other organocopper(I) compounds, see: (a) Ford, P. C.;
Cariati, E.; Bourassa, J. Chem. Rev. 1999, 99, 3625. (b) Sharpless, K. B.;
Finn, M. G. Angew. Chem., Int. Ed. 2004, 40, 2004. (c) Hodson, B. E.;
McGrath, T. D.; Stone, F. G. A. Organometallics 2005, 24, 3386.
(d) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Organometallics 2006, 25,
2405. (e) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. J. Am. Chem. Soc. 2006,
128, 11036. (f) Nolte, C.; Mayer, P.; Straub, B. F. Angew. Chem., Int.
Ed. 2007, 46, 2101. (g) Dubinia, G. G.; Furutachi, H.; Vicic, D. A. J.
Am. Chem. Soc. 2008, 130, 8600. (h) Ren, S.; Xie, Z. Organometallics
2008, 27, 5167. (i) Davenport, T. C.; Tilley, T. D. Angew. Chem., Int.
Ed. 2011, 50, 12205. (j) Lang, H.; Jakob, A.; Milde, B. Organometallics
2012, 31, 7661.
(6) Organocopper(I) compounds containing both aryl and alkenyl
groups were reported. see: (a) Noltes, J. G.; ten Hoedt, R. W. M.; van
Koten, G.; Spek, A. L.; Schoone, J. C. J. Organomet. Chem. 1982, 225,
365. (b) Schulte, P.; Behrens, U. Z. Anorg. Allg. Chem. 2000, 626, 1692.
(7) 1,4-Dicopper-1,3-butadiene intermediates were proposed by
Takahashi and others. For selected reviews, see: (a) Takahashi, T.;
Kotora, M.; Hara, R.; Xi, Z. Bull. Chem. Soc. Jpn. 1999, 72, 2591.
(b) Zhou, L.; Yamanaka, M.; Kanno, K.-i.; Takahashi, T. Heterocycles
2008, 76, 923. (c) Zhou, L.; Li, S.; Kanno, K.-i.; Takahashi, T.
Heterocycles 2010, 80, 725. see also: (d) Takahashi, T.; Kotora, M.; Xi,
Z. J. Chem. Soc., Chem. Commun. 1995, 361. (e) Takahashi, T.; Li, Y.;
Stepnicka, P.; Kitamura, M.; Liu, Y.; Nakajima, K.; Kotora, M. J. Am.
Chem. Soc. 2002, 124, 576. (f) Yamamoto, Y.; Ohno, T.; Itoh, K.
REFERENCES
■
(1) For reviews, see: (a) Krause, N. Modern Organocopper Chemistry;
Wiley-VCH: Weinheim, Germany, 2002. (b) Per
́
ez, P. J.; Dea
́
z-
Requejo, M. M. In Comprehensive Organometallic Chemistry III;
Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K.,
2007; Vol. 2, Chapter 3, p 153. (c) Lipshutz, B. H.; Yamamoto, Y.
Chem. Rev. 2008, 108, 2793. (d) Pappoport, Z.; Marek, I. The
Chemistry of Organocopper Compounds; Wiley-VCH: Weinheim,
Germany, 2009. (e) van Koten, G.; Per
Organometallics 2012, 31, 7631.
́
ez, P. J.; Liebeskind, L.
(2) For reviews on organocuprates MCuR2, see: (a) Gschwind, R. M.
Chem. Rev. 2008, 108, 3029. (b) Davies, R. P. Coord. Chem. Rev. 2011,
255, 1226. (c) Yoshikai, N.; Nakamura, E. Chem. Rev. 2012, 112, 2339.
See also: (d) Gilman, H.; Jones, R. G.; Woods, L. A. J. Org. Chem.
1952, 17, 1630. (e) Whitesides, G. M.; Fisher, W. F.; San Filippo, J.,
Jr.; Bashe, R. W.; House, H. O. J. Am. Chem. Soc. 1969, 91, 4871.
(f) van Koten, G.; Jastrzebski, J. T. B. H.; Muller, F.; Stam, C. H. J. Am.
Chem. Soc. 1985, 107, 697. (g) Lorenzen, N. P.; Weiss, E. Angew.
Chem., Int. Ed. Engl. 1990, 29, 300. (h) Olmstead, M. M.; Power, P. P.
J. Am. Chem. Soc. 1990, 112, 8008. (i) Nakamura, E.; Mori, S. Angew.
Chem., Int. Ed. 2000, 39, 3750. (j) Amijs, C. H. M.; Jastrzebski, J. T. B.
H.; Lutz, M.; Spek, A. L.; van Koten, G. Organometallics 2002, 21,
4662. (k) Kronenburg, C. M. P.; Jastrzebski, J. T. B. H.; Lutz, M.;
Spek, A. L.; van Koten, G. Organometallics 2003, 22, 2312.
(l) Woodward, S. Angew. Chem., Int. Ed. 2005, 44, 5560. (m) Davies,
R. P.; Hornauer, S.; White, A. J. P. Chem. Commun. 2007, 304.
(n) Yoshikai, N.; Ida, R.; Nakamura, E. Adv. Synth. Catal. 2008, 350,
1063. (o) Bomparola, R.; Davies, R. P.; Gray, T.; White, A. J. P.
Organometallics 2012, 28, 4632. (p) Haywood, J.; Wheatley, A. E. H.
Organomet. Chem. 2011, 37, 79. (q) Bomparola, R.; Davies, R. P.; Lal,
S.; White, A. J. P. Organometallics 2012, 31, 7877. (r) Bertz, S. H.;
Hardin, R. A.; Heavey, T. J.; Ogle, C. A. Angew. Chem., Int. Ed. 2013,
52, 10250.
Chem.Eur. J. 2002, 8, 4734. (g) Dufkova,
́
L.; Matsumura, H.; Neca
a, P.; Uhlík, F.; Kotora, M. Collect. Czech. Chem. Commun.
2004, 69, 351. (h) Dufkova, L.; Kotora, M.; Císarova, I. Eur. J. Org.
̌
s,
̌
̌ ̌
D.; Stepnick
́
̌
́
Chem. 2005, 2491. (i) Turek, P.; Kotora, M.; Hocek, M.; Votruba, I.
Collect. Czech. Chem. Commun. 2005, 70, 339. (j) Chen, C.; Xi, C.;
Jiang, Y.; Hong, X. J. Am. Chem. Soc. 2005, 127, 8024. (k) Wang, C.;
Yuan, J.; Li, G.; Wang, Z.; Zhang, S.; Xi, Z. J. Am. Chem. Soc. 2006,
128, 4564. (l) Tsubouchi, A.; Onishi, K.; Takeda, T. J. Am. Chem. Soc.
2006, 128, 14268. (m) Luo, Q.; Wang, C.; Zhang, W.-X.; Xi, Z. Chem.
Commun. 2008, 13, 1593. (n) Chen, C.; Xi, C. Chin. Sci. Bull. 2010, 55,
3235.
(8) Xi, Z. Acc. Chem. Res. 2010, 43, 1342.
(3) For structurally characterized heteroleptic organocuprates, see for
cyanocuprates: (a) Hwang, C.-S.; Power, P. P. J. Am. Chem. Soc. 1998,
120, 6409. (b) Boche, G.; Bosold, F.; Marsch, M.; Harms, K. Angew.
Chem., Int. Ed. 1998, 37, 1684. (c) Kronenburg, C. M. P.; Jastrzebski, J.
T. B. H.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 1998, 120, 9688.
(d) Hwang, C.-S.; Power, P. P. Organometallics 1999, 18, 697.
(e) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D.
Organometallics 2000, 19, 5780. (f) Kronenburg, C. M. P.;
Jastrzebski, J. T. B. H.; Boersma, J.; Lutz, M.; Spek, A. L.; van
Koten, G. J. Am. Chem. Soc. 2002, 124, 11675. for amidocuprates:
(g) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 4967.
(h) Davies, R. P.; Hornauer, S.; Hitchcock, P. B. Angew. Chem., Int. Ed.
2007, 46, 5191. (i) Haywood, J.; Morey, J. V.; Wheatley, A. E. H.; Liu,
C.-Y.; Yasuike, S.; Kurita, J.; Uchiyama, M.; Raithby, P. R.
Organometallics 2009, 28, 38. (j) Ito, M.; Hashizume, D.; Fukunaga,
T.; Matsuo, Y.; Tamao, K. J. Am. Chem. Soc. 2009, 131, 18024.
(4) For organocopper(I) aggregates (CuR)n, see: (a) Jarvis, J. A. J.;
Pearce, R.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 1977, 999.
(9) (a) Liu, L.; Zhang, W.-X.; Luo, Q.; Li, H.; Xi, Z. Organometallics
2010, 29, 278. (b) Zhou, Y.; Zhang, W.-X.; Xi, Z. Organometallics
2012, 31, 5546. (c) Zhang, S.; Zhan, M.; Zhang, W.-X.; Xi, Z.
Organometallics 2013, 32, 4020.
(10) (a) Fang, H.; Zhao, C.; Li, G.; Xi, Z. Tetrahedron 2003, 59,
3779. (b) Wang, Z.; Wang, C.; Xi, Z. Tetrahedron Lett. 2006, 47, 4157.
(11) For reviews, see: (a) Ford, P. C.; Cariati, E.; Bourassa, J. Chem.
Rev. 1999, 99, 3625. (b) Yam, V. W.-W.; Lo, K. K.-W. Chem. Soc. Rev.
1999, 28, 323. See also: (c) Yam, V. W.-W.; Lee, W.-K.; Cheung, K.
K.; Lee, H.-K.; Leung, W.-P. J. Chem. Soc., Dalton Trans. 1996, 2889.
(12) Ubayama, H.; Sun, W.-H.; Xi, Z.; Takahashi, T. Chem. Commun.
1998, 1931.
(13) Li, G.; Fang, H.; Xi, Z. Tetrahedron Lett. 2003, 44, 8705.
(14) Wei, J.; Wang, Z.; Zhang, W.-X.; Xi, Z. Org. Lett. 2013, 15, 1222.
(b) Lingnau, R.; Strahle, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 436.
̈
(c) Nobel, D.; van Koten, G.; Spek, A. L. Angew. Chem., Int. Ed. Engl.
1989, 28, 208. (d) Janssen, M. D.; Corsten, M. A.; Spek, A. L.; Grove,
D. M.; van Koten, G. Organometallics 1996, 15, 2810. (e) Eriksson, H.;
̈
Ortendahl, M.; Hakansson, M. Organometallics 1996, 15, 4823.
̊
(f) Eriksson, H.; Hakansson, M. Organometallics 1997, 16, 4243.
(g) Niemeyer, M. Organometallics 1998, 17, 4649. (h) Hakansson, M.;
Eriksson, H.; Jagner, S. Inorg. Chim. Acta 2006, 359, 2519. (i) Boere, R.
T.; Masuda, J. D.; Tran, P. J. Organomet. Chem. 2006, 691, 5585.
(j) Venkatasubbaiah, K.; DiPasquale, A. G.; Bolte, M.; Rheingold, A.
̊
̊
L.; Jakle, F. Angew. Chem., Int. Ed. 2006, 45, 6838. (k) Jakle, F. Dalton
̈
̈
Trans. 2007, 2851. (l) Zanardi, A.; Novikov, M. A.; Martin, E.; Benet-
613
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