Isolation and structural characterization of geminal di(iodozincio)methane complexes stabilized with nitrogen ligands

Article abstract of DOI:10.1021/ja5114535

Treatment of gem-di(iodozincio)methane with pyridine or diamine derivatives resulted in the isolation of a storable gem-di(iodozincio)methane species. Use of the sterically bulky bipyridine ligand gave a gem-di(iodozincio)methane complex, which allowed the first X-ray structural analysis of such species. This work represents a rare example of the isolation of an organometallic reactive species in Schlenk equilibrium and thus provides new insight into the design of efficient and storable organometallic reagents. The isolated gem-di(iodozincio)methane complexes serve as effective methylene dianion synthons for olefination of carbonyl compounds.

Full text of DOI:10.1021/ja5114535

Communication
pubs.acs.org/JACS
Isolation and Structural Characterization of Geminal
Di(iodozincio)methane Complexes Stabilized with Nitrogen Ligands
Yusuke Nishida,† Naoki Hosokawa,† Masahito Murai,*,† and Kazuhiko Takai*,†,‡
‡
†Division of Chemistry and Biotechnology, Graduate School of Natural Science and Technology and Research Center of New
Functional Materials for Energy Production, Storage, and Transport, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama
700-8530, Japan
S
* Supporting Information
with titanium salts.5 These features make them quite useful
especially for the total synthesis of natural products.7 Recently,
ABSTRACT: Treatment of gem-di(iodozincio)methane
with pyridine or diamine derivatives resulted in the
isolation of a storable gem-di(iodozincio)methane species.
Use of the sterically bulky bipyridine ligand gave a gem-
di(iodozincio)methane complex, which allowed the first X-
ray structural analysis of such species. This work represents
a rare example of the isolation of an organometallic
reactive species in Schlenk equilibrium and thus provides
new insight into the design of efficient and storable
organometallic reagents. The isolated gem-di(iodozincio)-
methane complexes serve as effective methylene dianion
synthons for olefination of carbonyl compounds.
gem-di(iodozincio)methanes have been shown to be useful
reagents for methylenation and ring-closing metathesis cascades
of olefinic esters,8 1,4-addition to enones, nickel-catalyzed
carbozincation of alkynes, and palladium-catalyzed sequential
cross-coupling reactions.9
Despite numerous studies on their synthetic applications,
crystallographic characterization of these useful reagents has
remained elusive, presumably due to their tendency to
decompose in solution via a Schlenk equilibrium and difficulties
associated with producing single crystals.10,11 Matsubara et al.
studied the structure of CH2(ZnI)2 species in THF using
extended X-ray absorption fine structure, small angle neutron
scattering, and anomalous X-ray scattering analysis.12 We have
long suspected that isolation and direct structural character-
ization would shed light on the exact nature of the reactive
species, the origin of their unique selectivity, and their further
synthetic application.13 Moreover, this study was expected to
provide a new insight into the design of more efficient as well as
storable gem-dimetallic reagents for olefination of carbonyl
compounds. This report describes the synthesis and isolation of a
series of storable gem-di(iodozincio)methane complexes and
their first X-ray characterization. The performance of these
complexes in the methylenation of several carbonyl compounds
is also investigated.
To stabilize the generated gem-di(iodozincio)methane species,
we first chose 2,2′-bipyridine as a ligand. The reaction of gem-
di(iodozincio)methane, which can be easily prepared from zinc
and diiodomethane using a catalytic amount of PbCl2,5 with 2
equiv of 2,2′-bipyridine in THF at 25 °C for 30 min gave the
bipy-CH2(ZnI)2 complex 1 as a pale yellow solid in 77% yield
(Scheme 1). The formation of 1 was indirectly confirmed by
NMR spectroscopy through the appearance of a new methylene
ompounds with two metal atoms on the same carbon,
1
2
C
categorized as gem-dimetalloalkanes (R2CM M ), con-
stitute a classic and important class of organometallic reagents in
organic synthesis.1 Among them, gem-dimetallomethanes (R =
H), such as Tebbe reagent (Ti−CH2−Al), Nysted reagent (Zn−
CH2−Zn), and gem-dizinciomethanes (Zn−CH2−Zn) are
frequently employed in methylenation and related reactions.2−4
In contrast to the synthetic use, isolation and structural
characterization of these reagents, apart from Tebbe reagent, are
generally difficult due to their tendency to decompose via the
Schlenk equilibrium, which results from their inherent instability.
The structure of gem-di(iodozincio)methane (CH2(ZnI)2) was
first proposed in the methylenation of a ketone and aldehydes by
Fried and Miyano, in which they considered the reagent as a
dianion derived from further reduction of Simmons−Smith
reagents (I−CH2−ZnI) with zinc.4a,b Oshima, Takai, and Nozaki
confirmed its structure indirectly by trapping with Me3SnCl and
demonstrated its wide synthetic utility.4c Later, it was proved that
these seminal outcomes were strongly dependent on the nature
of the zinc used. After intensive studies, it was disclosed that the
presence of trace amounts of lead was the key factor for the
generation of gem-di(iodozincio)methane compounds.5 The
reduction of a zinc carbenoid (I−CHR−ZnI) into gem-
di(iodozincio)methanes (RCH(ZnI)2) was dramatically accel-
erated by the use of a PbCl2 catalyst, and a proceudure for general
preparation as a robust alkylidenation method for various
carbonyl compounds was established. This alkylidenation
reaction offers some advantages over typical olefination reaction
such as the Wittig reaction,6 including expansion of the scope to
easily enolizable or sterically hindered ketones,3,4c,h high
chemoselectivity,3,4i and alkylidenation of ester carbonyl groups
Scheme 1. Synthesis of bipy-CH2(ZnI)2 Complex 1
Received: November 13, 2014
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
proton at −0.68 ppm in THF-d8. This singlet (Zn−CH2−Zn)
and a multiplet, which was assigned to CHN, with an
integration of 1:2, suggested the presence of two 2,2′-bipyridine
ligands per a gem-di(iodozincio)methane unit. Elemental analysis
also supported this proposed structural formula. Moreover, ESI-
TOF-MS analysis clearly proved the presence of complex 1 even
in the solution state (see Supporting Information). The
corresponding gem-di(iodozincio)methane complex 2, contain-
ing lutidine as a ligand, was synthesized in a similar manner in
76% yield. Complex 3 having a diamine ligand such as
N,N,N′,N′-tetramethylethylenediamine (TMEDA) was also
prepared (Figure 1). The chemical shift of the active methylene
absent after the complexes lutidine-CH2(ZnI)2 2 and TMEDA-
CH2(ZnI)2 3 were kept in CD2Cl2 or CDCl3 at 25 °C for 24 h.16
Diiodozinc complexes with the corresponding nitrogen ligands
were isolated from the reaction mixture, and their presence
confirmed by 1H and 13C NMR spectroscopy as well as elemental
analysis. These analytical data were consistent with the data
obtained from an authentic sample prepared from the reaction of
ZnI2 with the corresponding nitrogen ligands (see Figure S3).
Moreover, when the insoluble white solid was treated with conc.
HCl in CDCl3 at 25 °C, evolution of methane gas was observed
by 1H NMR analysis (Figure S4). These observations
demonstrate the decomposition of gem-di(iodozincio)methane
complexes 1−3 by a Schlenk equilibrium in the solution state.
Since ligand exchange shown in Scheme 2b probably proceeds
through a bimolecular associative process, we postulated that the
bulky ligand 6-mesityl-2,2′-bipyridine (MesBipy) would prevent
the Schlenk equilibrium and generate the more stable gem-
di(iodozincio)methane species.15 When a mixture of gem-
di(iodozincio)methane and 6-mesityl-2,2′-bipyridine (1:2
ratio) was stirred in THF at 25 °C, formation of a yellow
precipitate was observed. Evaporation of the solvent followed by
extraction with CH2Cl2 and cooling of the extract at −25 °C
resulted in the formation of orange prisms. Note that the
recrystallization should be done at a low temperature, as the
Mesbipy-CH2(ZnI)2 complex 4 (Figure 1) was unstable in
solution and decomposed to the corresponding diiodozinc
complex 4′ after only 1 day at 25 °C. X-ray crystallographic
analysis under a cold stream of nitrogen finally gave the
molecular structure of gem-di(iodozincio)methane. Figure 2
Figure 1. Structures of gem-di(iodozincio)methane complexes 2−4 with
nitrogen ligands (Mes = 2,4,6-trimethylphenyl).
carbon in 3 was −0.76 ppm, compared with −0.55 ppm for that
of 1 (both in CDCl3), reflecting electron donation from the
TMEDA ligand. Complexes 1−3 were soluble in dichloro-
methane and THF, but almost insoluble in hexane. Additionally,
they were found to be stable in the solid state. No obvious
decomposition was observed by NMR at 25 °C even after a year,
when they were kept in the solid state under an argon
atmosphere.
Unfortunately, all attempts to obtain suitable crystals for X-ray
analysis by recrystallization led to the isolation of the diiodozinc
complex 1′ (bipy-ZnI2) with the corresponding nitrogen ligands
(Figure S1 and Table S1).14 This can be understood by
considering the change in chemical structure through a Schlenk
equilibrium, which is often proposed for main-group metal-based
organometallic reagents having metal−halogen bonds, such as
Grignard and organozinc reagents (Scheme 2a).12c,15 The
Scheme 2. Schlenk Equilibrium for gem-
Di(iodozincio)methanes
Figure 2. X-ray crystal structure of the gem-di(iodozincio)methane
complex 4. Left: side view; right: top view. Thermal ellipsoids are drawn
at the 50% probability level. Co-crystallizing dichloromethane is omitted
for clarity.
clearly shows the structure of 4, which has two zinc centers
bridged by a methylene ligand. Selected bond distances and
angles are summarized in Table 1. The zinc center displays a
distorted tetrahedral geometry. The Zn−C−Zn bond angle
Table 1. Selected Bond Lengths (Å) and Angles (°) of 4
bond lengths (Å)
Zn1−N1
Zn1−N2
Zn1−C39
Zn1−I1
Zn2−N3
Zn2−N4
Zn2−C39
Zn2−I2
bond angles (°)
Zn1−C39−Zn2
2.091(4)
2.146(4)
1.944(5)
2.7013(9)
2.163(5)
2.127(4)
1.961(4)
2.6533(8)
117.0(2)
77.56(14)
118.21(13)
94.04(12)
76.96(16)
125.23(14)
98.57(10)
N1−Zn1−N2
C39−Zn1−I1
N2−Zn1−I1
N3−Zn2−N4
C39−Zn2−I2
N4−Zn2−I2
Schlenk equilibrium is known to proceed via self-transmetalation
of gem-dizinciomethanes R2C(ZnX)2 to produce dimeric,
trimeric, and oligomeric organozinc species (Scheme 2b). The
methylene proton signal of the bipyridine complex 1 in CD2Cl2
disappeared gradually at room temperature, and the formation of
1
an insoluble white solid was observed. The singlet H NMR
resonances for other methylene protons were also found to be
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Journal of the American Chemical Society
Communication
117.0° was larger than the previously reported Mg−C−Mg bond
angle (112.2°) for (Me3Si)2C(MgBr)2.14a Two mesityl rings
point in the same direction, located as if to “protect” the active
methylene carbon from a Schlenk equilibrium. The two Zn−C
bond distances, 1.944(5) and 1.961(4) Å, are much shorter than
that reported for the gem-dizinciomethanes (CR2Zn2) (longer
than 2.024 Å).10 Note that these previously reported gem-
dizinciomethanes do not have halide atoms attached to the zinc
atom and are more stable due to the absence of a tendency to
decompose via the Schlenk equilibrium. The presence of
substituents on the methylene carbon atoms to further stabilize
these complexes might also be a reason for the longer Zn−C
bond. The current Zn−C bond distances are even shorter than
those of the iodomethylzinc carbenoid species [Zn(CH2I)2]
(typically longer than 1.995 Å)11 but are comparable to the
typical Zn−C bond length (1.9−2.1 Å).
those reported by one of the present authors previously, using
freshly prepared gem-di(iodozincio)methane CH2(ZnI)2.4c,h,5b
Note that one significant drawback of the previously employed
gem-di(iodozincio)methanes was their instability; the reagents
needed to be prepared beforehand and titrated in order to
determine the factor. In contrast, the gem-di(iodozincio)-
methane complexes with nitrogen ligands prepared in this
work were quite stable.21 In the current study, 8 month-old
powders of 2 and 3 were found to remain effective reagents for
methylenation, as shown in Scheme 3.
Further utility was demonstrated by the stereoselective 1,4-
addition reaction of gem-di(iodozincio)methane to enones.9g In
the presence of chlorotrimethylsilane, an addition reaction of the
lutidine-CH2(ZnI)2 complex 2 with chalcone proceeded
efficiently via the formation of a silyl enol ether of β-zinciomethyl
ketone followed by deuteration to furnish the deuterated (Z)-
silyl enol ether 6 in 82% yield (Scheme 4).
While the gem-di(iodozincio)methane complexes under
discussion are stable in the solid state, some of them can be
utilized as methylenation reagent for the carbonyl compounds.
For example, 1-ethenylnaphthalene was obtained in 82% yield
when 2 equiv of the lutidine-CH2(ZnI)2 complex 2 was
employed as a methylenation reagent (Scheme 3a). In sharp
Scheme 4. Stereoselective 1,4-Addition Reaction of a gem-
Di(iodozincio)methane Complex to Chalcone
Scheme 3. Methylenation of Aldehydes, Ketones, and Esters
with gem-Di(iodozincio)methane Complexes
In conclusion, this study shows that the sterically crowded
bipyridine ligand provides a unique environment for isolation of
the first example of structurally characterized gem-di-
(iodozincio)methane species. The resulting complexes serve as
sources of storable methylenation reagents for carbonyl
compounds, including aldehydes, ketones, and esters. This result
also revealed changes in reactivity of the gem-di(iodozincio)-
methane species depending on the ligands used. As zinc atoms
can easily be replaced with other transition metals by
transmetalation, this study will open up new perspectives on
the design and development of synthetically useful gem-
dimetallomethane species that are difficult to access by
conventional protocols.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
contrast, the bipy-CH2(ZnI)2 complex 1 did not show any
activity for this transformation. Although the TMEDA-
CH2(ZnI)2 complex 3 afforded the expected olefin, the yield
was quite low. The same lutidine-CH2(ZnI)2 complex 2
promoted the methylenation of ketones such as 2-dodecanone,
furnishing 2-methyl-1-dodecene in 70% yield when the TMEDA-
titanium complex 517 was used as an additive (Scheme 3b).18
Other additives, including BF3·OEt2, TiCl4, β-TiCl3, MnCl2,
Me3SiOTf, AlCl3, and ZnI2, were totally ineffective, and the
expected 2-methyl-1-dodecene was not obtained in any of these
reactions. While the role of the titanium additive is unclear at the
moment, the formation of a titanium methylidene species and
activation of the ketone as a Lewis acid are considered the most
likely modes of action.5,19 The use of a combination of TMEDA-
CH2(ZnI)2 complex 3 and TMEDA-titanium complex 5
promoted methylenation of esters, affording a mixture of 2-
methoxy-1-decene and 2-dodecanone in 81% and 10% yields,
respectively (Scheme 3c).20 These results are comparable to
AUTHOR INFORMATION
Corresponding Authors
masahito.murai@cc.okayama-u.ac.jp
ktakai@cc.okayama-u.ac.jp
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by a Grant-in-Aid (no.
26248030) from MEXT, Japan, and MEXT program for
promoting the enhancement of research universities. The
authors gratefully thank Prof. Munetaka Akita, Dr. Yuya Tanaka,
and Ms. Kaho Sugimoto (Tokyo Institute of Technology, Japan)
for ESI-TOF-MS measurements.
C
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Communication
(11) For the X-ray crystallographic studies on a monometallic
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(10) X-ray crystallographic studies of gem-dizinciomethanes (CR2Zn2
(R ≠ H)) have been reported. It should be noted that these complexes
do not contain halide atoms on the zinc atom and are thus much more
stable due to the absence of decomposition via the Schlenk equilibrium.
Substituents on the methylene carbon atoms also help to stabilize these
complexes. See: (a) Steiner, M.; Grutzmacher, H.; Pritzkow, H.; Zsolnai,
L. Chem. Commun. 1998, 285. (b) Andrews, P. C.; Raston, C. L.;
Skelton, B. W.; White, A. H. Organometallics 1998, 17, 779.
(c) Westerhausen, M.; Guckel, C.; Mayer, P. Angew. Chem., Int. Ed.
2001, 40, 2666. (d) Wooten, A.; Carroll, P. J.; Maestri, A. G.; Walsh, P. J.
J. Am. Chem. Soc. 2006, 128, 4624. (e) Javed, S.; Hoffman, D. Dalton
Trans. 2010, 39, 11439.
(20) NMR analysis clarified that 2-dodecanone was formed in situ
before the aqueous workup by keto−enol tautomerization of 2-
methoxy-1-decene under the current reaction conditions.
(21) Lutidine-CH2(ZnI)2 complex 2 was relatively stable against air
and heat (∼50 °C) but unstable against water. See Scheme S1 for the
deital.
D
DOI: 10.1021/ja5114535
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX