Structure and Reaction Chemistry of Magnesium Organocuprates Derived from Magnesiacyclopentadienes and Copper(I) Salts

Article abstract of DOI:10.1002/anie.201607355

The chemistry of magnesium organocuprates, including their synthesis, structures, and reactions, remains underexplored. In this work, by taking advantage of the high reactivity and ready availability of magnesiacyclopentadienes, a series of magnesiacyclopentadiene-based organocuprates were synthesized and structurally characterized. A variety of CuX salts (X=Cl, Br, I, or alkynyl) were successfully applied to react with magnesiacyclopentadienes. Besides CuX salts, AgX salts (X=Cl, alkynyl) also undergwent the above reaction to afford the corresponding magnesium organoargentates. Single-crystal X-ray structural analysis and DFT calculations of these butadienyl magnesium organocuprates revealed unique structural characteristics and bonding modes. These results are also very useful to understand the transmetalation process, since the product can be viewed as the resting-state intermediate of a transmetalation reaction between organomagnesium compounds and coinage-metal salts. Preliminary information on the reaction chemistry of these magnesium organocuprates is provided by their reactions with allyl bromide, benzoyl chloride, and CO₂.

Full text of DOI:10.1002/anie.201607355

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International Edition: DOI: 10.1002/anie.201607355
German Edition: DOI: 10.1002/ange.201607355
Organometallic Compounds
Structure and Reaction Chemistry of Magnesium Organocuprates
Derived from Magnesiacyclopentadienes and Copper(I) Salts
Liang Liu, Junnian Wei, Yue Chi, Wen-Xiong Zhang,* and Zhenfeng Xi*
Abstract: The chemistry of magnesium organocuprates,
including their synthesis, structures, and reactions, remains
underexplored. In this work, by taking advantage of the high
reactivity and ready availability of magnesiacyclopentadienes,
a series of magnesiacyclopentadiene-based organocuprates
were synthesized and structurally characterized. A variety of
CuX salts (X = Cl, Br, I, or alkynyl) were successfully applied
to react with magnesiacyclopentadienes. Besides CuX salts,
AgX salts (X = Cl, alkynyl) also undergwent the above
reaction to afford the corresponding magnesium organoargen-
tates. Single-crystal X-ray structural analysis and DFT calcu-
lations of these butadienyl magnesium organocuprates
revealed unique structural characteristics and bonding modes.
These results are also very useful to understand the trans-
metalation process, since the product can be viewed as the
resting-state intermediate of a transmetalation reaction between
organomagnesium compounds and coinage-metal salts. Pre-
liminary information on the reaction chemistry of these
magnesium organocuprates is provided by their reactions
with allyl bromide, benzoyl chloride, and CO2.
We recently reported a facile synthesis of multisubstituted
magnesiacyclopentadienes (1) as the first example of alkaline-
earth metallocyclopentadienes.[4] Single-crystal X-ray struc-
tural analysis of these magnesiacyclopentadienes revealed
a nearly coplanar ring skeleton. Although the lengths of the
2
À
chemical bonds of the ring, including two Mg C(sp ) bonds,
are comparable to those reported, the bite angle of C1-Mg-C4
(91.548) is much smaller than the classic C-Mg-C angles in the
usual tetracoordinate magnesium compounds (115–1308).[5]
This structural information, including the nearly coplanar ring
skeleton and the small bite angle, indicates that these
magnesiacyclopentadienes should possess a high degree of
ring strain and thus should be highly reactive. As part of our
continued interest in butadienyl-based organometallic
chemistry, we initiated research on the reaction chemistry of
these magnesiacyclopentadienes with metal salts. We envi-
sioned that the high reactivity of these magnesiacyclopenta-
dienes would lead them to undergo facile transmetalation
reactions with CuX to generate the corresponding magnesium
organocuprates, in which the butadienyl skeleton should
stabilize well-defined structures. Herein, we report the
isolation and structural characterization of a series of novel
monomeric (Cu/Mg = 1:2) and linearly linked dimeric (Cu/
Mg = 2:3) magnesium organocuprates. These results also shed
light on the transmetalation process,[6] since the product can
be viewed as the resting-state intermediate of a transmetala-
tion reaction between organomagnesium compounds and
coinage-metal salts.
O
rganocopper(I) compounds are ubiquitously involved in
Cu-based reactions as synthetic reagents or key intermedi-
ates.[1] Among these compounds, lithium organocuprates have
been widely applied in synthetic chemistry. Many lithium
organocuprates have been structurally characterized.[2] How-
ever, in sharp contrast, magnesium organocuprates remain
almost unexplored, particularly in terms of their well-defined
structures, although different structural characteristics and
useful reaction chemistry can be expected. The limited
examples of structurally characterized magnesium organo-
cuprates are all based on aryl skeletons, for example, aryl
magnesium organocuprates.[3] To the best of our knowledge,
there have been no alkenyl magnesium organocuprates
isolated to date.
Treatment of magnesiacyclopentadiene 1a with 0.5 equiv
of copper(I) salt in hexane at room temperature for 1 h
afforded a yellow precipitate. The solid was filtered and
recrystallized in Et2O and THF to give the magnesium
organocuprates 2a, 2b, and 2c in 71%, 70% and 79% yields
of isolated product, respectively (Scheme 1). When 0.75 and
1.0 equiv of copper(I) salts were reacted with 1a, products 2
were always obtained as the major products. Similarly, when
1a was reacted with 0.5 equiv of AgCl·PMe3, the correspond-
ing magnesium organoargentate 2d was isolated in 68%
[*] L. Liu, Dr. J. Wei, Y. Chi, Prof. Dr. W.-X. Zhang, Prof. Dr. Z. Xi
Beijing National Laboratory for Molecular Sciences (BNLMS), Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University
Beijing 100871 (China)
yield.
In
addition,
(phenylethynyl)copper(I)
and
(phenylethynyl)silver(I) were both compatible under similar
reaction conditions (hexane/THF = 2:1), with more than 75%
conversions of 1a, as determined by 1H NMR spectroscopy in
C6D6 with hexamethylbenzene as an internal standard.
Finally, alkynyl magnesium organocuprate 3a and alkynyl
magnesium organoargentate 3b were obtained in 41% and
24% yields of isolated product, respectively. When 1b was
treated with 0.5 equiv of copper(I) salt under the above-
mentioned reaction conditions, different magnesium organo-
cuprates (4a and 4b) with a Cu/Mg ratio 2:3 were obtained as
major products. After optimization of the reaction conditions
E-mail: wx_zhang@pku.edu.cn
zfxi@pku.edu.cn
Prof. Dr. Z. Xi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry, Shanghai 200032 (China)
Supporting information for this article (including experimental
details, X-ray crystallographic data, and scanned NMR spectra of all
new products) can be found under:
http://dx.doi.org/10.1002/anie.201607355.
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Figure 1. a) ORTEP drawing of 2b with thermal ellipsoids drawn at
30% probability. H atoms are omitted for clarity. b) Selected bond
lengths [ꢀ] and bond angles (8) of 2b.
increases noticeably in 2b. Although the C1-Mg-C4 bite
angle [88.7(3)8] became much smaller, the strained magne-
À
siacyclopentadiene skeleton can be stabilized by Mg Br bond
formation [2.549(2) ꢀ] and the resulting obtuse C-Mg-Br
Scheme 1. Synthesis of magnesiacyclopentadiene-based magnesium
organocuprates and organoargentates.
À
angles [(117.13(19)8, 128.92(19)8]. The Cu C bond length
2
À
[1.945(6) ꢀ] is in the range of Cu C(sp ) bond lengths in
diaryl organocuprates (1.91–2.35 ꢀ).[2a] However, the C-Cu-C
bond angle [156.8(4)8] is significantly less obtuse than that of
diaryl lithium organocuprates (163–1788), which indicates
(2/3 equiv CuX; Et2O, RT, 10 min), magnesium organocup-
rates 4a and 4b were generated in 58% and 53% yields of
isolated product, respectively. Products 2–4 were all obtained
as air- and moisture-sensitive yellow crystals and subjected to
single-crystal X-ray structural analysis. The results show that
both inorganic and organic coinage-metal salts can be
effectively fused with magnesiacyclopentadienes 1 through
À
that a higher degree of covalency in the Mg C bond induces
À À
a corresponding increase in the Cu C Mg three-center-two-
electron bonding character.[3f] The Cu Mg distance [2.709-
(2) ꢀ] is shorter than the sum of van der Waals radii of Cu
À
À
(2.0 ꢀ) and Mg (2.2 ꢀ) atoms, which indicates a weak Cu Mg
À
the formal cleavage of M X bonds. Furthermore, the
interaction.[7] Overall, in analogy to lithium diorganocuprate–
lithium halide aggregates,[8] the six central atoms (C4, C4’,
Cu1, Mg1, Mg1’, and Br1) in 2b form a monomeric magne-
sium organocuprate following an incomplete transmetalation
event between magnesiacyclopentadienes and coinage-metal
substituents of the butadienyl skeletons significantly influ-
ence the composition and configuration of the products.
Compounds 2a and 2b both crystallized in the ortho-
rhombic Fdd2 space group, while 2c crystallized in the
monoclinic P21/n space group. Compounds 2a, 2b, and 2c
have almost identical configurations. Figure 1 shows the
single-crystal X-ray structure of 2b (for the structures of 2a
and 2c, see the Supporting Information). Compound 2b is
symmetrical, with the line through Cu1 and Br1 as the C2 axis.
Compound 2b consists of two magnesiacyclopentadiene units
linked by one discrete CuBr unit. The non-planar magnesia-
cyclopentadiene moieties in 2 are distorted, with C4, C4’
atoms bending away from the original five-membered plane
toward the Cu atom. The Mg atoms are tetracoordinated by
two alkenyl carbon atoms, one bromide atom, and one oxygen
atom. Compared to that in the magnesiacyclopentadiene 1a-
À
salts. In this process, the Br atom and one Mg C bond are
retained in the product skeleton, whereas they would be
released as MgBr2 salt in other general transmetalation
reactions. Compound 2d crystallized in the orthorhombic
Pbca space group. The structure of 2d is similar to that of 2b
À
except for a more distorted configuration and longer Ag C
bond lengths (for structural details of 2d, see the Supporting
Information). It is noteworthy that 2d is the first example of
a magnesium organoargentate.[9] Compounds 3a and 3b have
analogous configurations to 2a and 2d, respectively, except
for the bridging phenylethynyl ligands on two Mg atoms (for
structures of 3a and 3b, see the Supporting Information).
To gain further insight into the bonding situations of the
magnesium organocuprates, DFT calculations were carried
TMEDA,[4] the Mg C1 bond length [2.113(7) ꢀ] almost does
À
À
not change, while the Mg C4 bond length [2.341(7) ꢀ]
2
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out by using natural bond orbital (NBO) analysis[10] and
atoms in molecules (AIM) analysis.[11] Compound 2b was
structurally optimized at the B3LYP/LANL2DZ[12] (for Cu,
Br) or 6-31 + G*[13] (for the other elements) level in the gas
phase by Gaussian09.[14] In the optimized structure, the
configuration and all bond lengths were consistent with the X-
À
ray structure. As shown in Table 1, NBO analysis of the Mg
Table 1: NBO calculations for 2b.
Entry
Bond
Length [ꢀ]
Wiberg Bond Index
À
1
2
3
4
5
6
Mg1 C1
2.1219
2.3396
1.9810
2.7673
2.6232
3.7188
0.3823
0.2150
0.3375
0.1543
0.3470
0.0482
À
Mg1 C4
À
Cu1 C4
À
Cu1 Mg1
À
Br1 Mg1
À
Br1 Cu1
À
À
C1 and Mg C4 bonds clearly suggested one classical Mg
2
C(sp2) bond[5] and another drastically weakened Mg C(sp )
À
Figure 2. a) ORTEP drawing of 4a with thermal ellipsoids drawn at
bond, respectively (Table 1, entry 1–2). The Wiberg bond
30% probability for non-H atoms. H atoms are omitted for clarity.
b) Topological structure of 4a. Me and SiMe3 substitutes are omitted
for clarity.
À
À
index for Cu1 Mg1 indicates a weak Cu Mg interaction,
which is in accordance with the short distance in the single-
crystal structure of 2b (Table 1, entry 4). The Wiberg bond
À
index of Cu1 Br1 was nearly zero, which indicates complete
À
cleavage of the Cu Br bond by two magnesiacyclopenta-
Supporting Information). Compared to that in the corre-
dienes (Table 1, entry 6). AIM methods located a ring critical
point (CP) in the six-membered ring (C4, Cu1, C4’, Mg1,
Mg1’, Br1), while no bond path (or bond CP) was found
between Cu and Br atoms, which supports the non-existence
of a Cu Br interaction (for details, see the Supporting
Information). Furthermore, AIM analysis located no bond
path (or BCP) between the Cu and Mg atoms, whereas the
sponding magnesiacyclopentadiene 1b-TMEDA,[4] the Mg
À
À
C5 bond is longer [2.2872(17) ꢀ], while the Mg C1 [2.0913-
À
(19) ꢀ] and Mg C4 [2.3086(18) ꢀ] bonds become slightly
À
shortened and dramatically elongated, respectively. The Cu
À
C bond lengths [1.9438(17), 1.9506(17) ꢀ] and C-Cu-C bond
angles [164.11(7)8] are both in the range expected for diaryl
organocuprates.[2a] As the topological structure shows,
dimeric 4a can be regarded as a combination of two
[(MeTMS-vinyl)2Cu]À cuprate units and two [(MeTMS-
vinyl) Mg1.5Cl]+ countercation units.
À
Mg C bond paths are slightly bent toward the Cu atom. This
result suggests that the Cu Mg interaction is likely to be very
À
weak.
7
1
As shown in Scheme 2, Li and H NMR spectroscopy
demonstrate that when treated with an excess of LiCl in C6D6
at room temperature for 24 h, 2c can be transformed into 2a
in 92% yield through a halide-exchange process. Moreover,
2c reacted with 1.0 equiv of (phenylethynyl)potassium[15] in
THFat room temperature over 3 h to afford 3a quantitatively,
As an alternative perspective, magnesiacyclopentadienes
in the above reactions act in the same way as amphiphilic
ligands, with the more exposed terminal alkenyl carbon atoms
as Lewis bases and the unsaturated magnesium atoms as
Lewis acids.[16] The synergistic coordination of terminal C(sp2)
atoms and Mg atoms on MX salts leads to heterolytic
1
À
as determined by H NMR spectroscopy in C6D6.
cleavage of the M X bonds through the transfer of anionic
Both 4a and 4b crystallized in the monoclinic C2/c space
group. Compound 4a is symmetrical, with the line through
Mg2 and the center of C6-C6’ as the C2 axis (Figure 2). It
comprises three nonplanar magnesiacyclopentadiene units
and two separated CuCl units (for the structure of 4b, see the
X units from M centers to Mg centers.[17] Accordingly, the
magnesiacyclopentadienes could be considered as examples
of ambiphilic ligands, with incorporation of Mg-based Lewis
acids and C-based Lewis bases, which were previously limited
to Group 13 (B, Al) and Group 15 (N, P) atoms.
The preliminary reactivity of magnesium organocuprate
2c, which has four reactive sites (C1, C1’, C4, C4’), was
investigated (Scheme 3). Treatment of 2c with allyl bromide
afforded bis-allylated product 5 in 93% yield of isolated
product. Reaction of 2c with benzoyl chloride (even with
3.0 equiv) gave the cyclization product 6. 2,5-Bis(trimethyl-
silyl)-substituted cyclopentadienone 7 was obtained in 97%
yield of isolated product from the reaction of 2c with CO2.
In summary, we have synthesized and structurally char-
acterized a series of rigid alkenyl magnesium organocuprates
Scheme 2. Structural transformation of 2c into 2a and 3a.
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Scheme 3. Preliminary reactivities of 2c.
or organoargentates based on s-cis 1,3-butadiene skeletons
through synergistic activation of the Cu X bonds by magne-
À
siacyclopentadienes. The resulting magnesium ate complexes
can be considered to be the resting-state intermediate of
a transmetalation reaction between magnesiacyclopenta-
dienes and copper(I)/silver(I) compounds. The novel finding
is fundamentally significant for understanding the mechanism
of transmetalation processes and the bonding characters of
magnesium cuprates/argentates.
Acknowledgements
This work was supported by the 973 Program
(2012CB821600) and Natural Science Foundation of China
[12] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270.
[13] W. J. Hehre, R. Ditchfield, J. A. Pople, J. Chem. Phys. 1972, 56,
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[14] Gaussian09 (Revision C.01), M. J. Frisch, et al., Gaussian, Inc.,
Wallingford CT, 2010. For full reference, see the Supporting
Information.
À
Keywords: Cu X bond cleavage · magnesiacyclopentadienes ·
magnesium organocuprates · organometallic compounds ·
transmetalation
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À
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Received: July 29, 2016
Published online: && &&, &&&&
4
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Communications
Organometallic Compounds
L. Liu, J. Wei, Y. Chi, W.-X. Zhang,*
Cut and Paste: A series of unprecedented
alkenyl magnesium organocuprates and
organoargentates were synthesized and
structurally characterized. Unique struc-
tural characteristics and bonding situa-
tions were revealed. The ligand-exchange
reaction was realized through the syner-
Z. Xi*
&&&& — &&&&
Structure and Reaction Chemistry of
Magnesium Organocuprates Derived
from Magnesiacyclopentadienes and
Copper(I) Salts
À
gistic activation of M X bonds by reactive
magnesiacyclopentadienes.
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These are not the final page numbers!