Structure-kinetic relationship study of organozinc reagents

Article abstract of DOI:10.1039/c4cc01135j

Phenylzinc reagents prepared from various zinc halides show distinct kinetic features in the palladium-catalyzed Negishi-type oxidative coupling reaction, in which the phenylzinc reagent prepared from ZnI₂ gives the highest rate. In situ infrared and X-ray absorption spectroscopy studies show that the higher reaction rate was observed for longer Zn-C bond distances.

Full text of DOI:10.1039/c4cc01135j

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Structure–kinetic relationship study of organozinc
reagents†
Cite this: Chem. Commun., 2014,
50, 8709
Guanghui Zhang,ab Jing Li,a Yi Deng,ab Jeffrey T. Miller,b A. Jeremy Kropf,a
Emilio E. Bunelb and Aiwen Lei*abc
Received 12th February 2014,
Accepted 16th May 2014
DOI: 10.1039/c4cc01135j
www.rsc.org/chemcomm
Phenylzinc reagents prepared from various zinc halides show combination of nuclear magnetic resonance (NMR), electro-
distinct kinetic features in the palladium-catalyzed Negishi-type spray ionization (ESI) mass and conductivity measurements
oxidative coupling reaction, in which the phenylzinc reagent pre- have been used to study the structure of phenylzinc reagents,
pared from ZnI2 gives the highest rate. In situ infrared and X-ray and the presence of lithium zincate complexes has been con-
absorption spectroscopy studies show that the higher reaction rate firmed.14–16 Knowledge of the coordination geometry and bond
was observed for longer Zn–C bond distances.
distances is often necessary for a better understanding of the
related reaction rates, pathways and selectivity. This is of
Organozinc compounds were among the first organometallic particular importance for designing new reactions in which
compounds ever made, among which zinc dialkyls found organozinc reagents or intermediates are involved. In this
extensive use as alkylating reagents in organic synthesis, espe- communication, we combined in situ infrared (IR) and X-ray
cially in the early development of organometallic chemistry.1,2 absorption spectroscopy (XAS) to study the kinetic and struc-
In the past few decades, the discovery of Reformatsky, Simmons– tural relationship of phenylzinc reagents prepared from phenyl-
Smith, Fukuyama and Negishi coupling reactions brought organo- lithium reagents and various zinc halides.
zinc chemistry into its second rapidly developing stage.3–10 The
Three different phenylzinc reagents I, II and III, were pre-
main challenge in the kinetic investigation of organozinc reagent- pared by mixing phenyllithium reagents with different zinc
involved reactions is that the transmetallation step is usually halides, as shown in Fig. 1. When these phenylzinc reagents
faster than the other elementary steps in the catalytic cycle. Using were applied in the Pd-catalyzed Negishi-type oxidative homo-
Ni-catalyzed oxidative coupling as the kinetic model, the trans- coupling reaction, a dramatic difference in the reaction rates
metallation of arylzinc with a nickel catalyst has been revealed. was observed. When phenylzinc reagent I was used, the reac-
This offered an opportunity to quantitatively acquire the kinetic tion was complete within 500 s; while the reaction using
data of the transmetallation of arylzinc reagents.11 A quantitative
kinetic study was also carried out for the transmetallation
between an arylzinc reagent and Pd-R.12 Additionally, in the
Ni-catalyzed Negishi-type oxidative homo-coupling reaction,
arylzinc reagents prepared by different methods possess very
different kinetics due to their varying structural features.13 The
structure of the phenylzinc reagent prepared from the phenyl
Grignard reagent has been determined by single crystal analy-
sis; however, the structure of the phenylzinc reagent made from
the phenyllithium reagent still remains unknown. Recently, a
a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
Hubei 430072, China. E-mail: aiwenlei@whu.edu.cn
b Chemical Sciences and Engineering Division, Argonne National Laboratory,
9700 South Cass Avenue, Argonne, IL 60439, USA
c State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China
Fig. 1 Synthesis of phenylzinc reagents and the kinetic features of the
† Electronic supplementary information (ESI) available: Experimental section
Pd-catalyzed Negishi-type oxidative homo-coupling reaction.
and data analysis. See DOI: 10.1039/c4cc01135j
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phenylzinc reagent III took more than 2000 s. It has previously
been demonstrated that transmetallation is the rate-limiting
step in the phenylzinc reagent III-involved homo-coupling
reaction,12 and further kinetic investigation revealed that trans-
metallation is also rate-limiting when phenylzinc reagent I and
II are applied (see ESI† for details).
XAS was then used to investigate the structural differences
between the three phenylzinc reagents. XAS is a unique and
powerful technique for probing the local geometric and elec-
tronic structure of metal ions in noncrystalline systems invol-
ving solids and liquids, in which the X-ray is able to penetrate
the organic solution.17,18 These advantages make XAS an excel-
lent tool for investigating the structures of organozinc reagents
in solution.19–24
Fig. 3 XAS spectra of phenylzinc reagent II. (a) XANES spectra of solid
ZnBr2, ZnBr2/THF solution and phenylzinc reagent II. The inset shows the
first derivatives of the XANES spectra. (b) Fitting results of the k2-weighted
R-space EXAFS spectrum of the phenylzinc reagent II solution. FT range:
2.66–12.50 ÅÀ1; fitting range: 1.12–2.53 Å.
Zinc iodide adopts a tetrahedral coordination geometry phenyl anion to Zn led to the increase of both the Zn–O and
which gives a sharp edge jump originating from the 1s to 4p Zn–I bond distances due to electron donation from the phenyl
electron transition in the X-ray absorption near-edge structure anion to Zn. Although attempts to obtain the single crystal
(XANES) spectrum.25 As shown in Fig. 2a, the XANES spectrum structure of the phenylzinc reagent prepared from PhLi failed,
of the THF solution of ZnI2 has similar features to solid ZnI2, several crystal structures of substituted and very bulky arylzinc
which indicates the tetrahedral coordination geometry of the iodides have been reported.26,27 The bond distances that we
Zn species in THF. The shift of the edge energy from 9661.9 eV obtained from XAS are very close to the reported crystal
to 9662.1 eV and the change in shape of the XANES spectrum structures. Previous NMR and ESI investigations also suggested
suggests the coordination of the solvent THF to Zn. Fitting of that the lithium organozincate monomer complex was the major
the extended X-ray absorption fine structure (EXAFS) spectrum species under similar conditions,14 therefore the local structure
suggests that the Zn species in the ZnI2/THF solution has two of phenylzinc reagent I was determined as [PhZnI2(THF)]À.
iodide ligands with a bond distance of 2.53 Å and two oxygen
ZnBr2 has the same coordination geometry as ZnI2, i.e., four
ligands at a distance of 2.01 Å, so the structure is assigned as bromides are tetrahedrally coordinated to Zn with an average
ZnI2(THF)2 (Fig. S1, ESI†). ZnI2 has 4 iodides coordinated at an Zn–Br bond distance of 2.42 Å.25 The edge energy of ZnBr2 is
average distance of 2.62 Å. It can be seen that the coordination determined to be 9662.3 eV. As shown in Fig. 3a, coordination
of THF led to a decrease in the Zn–I bond distance. Addition of of THF to Zn does not change the edge energy, but leads to a
1 equivalent of PhLi to the ZnI2/THF solution results in a shift change in the shape of the XANES spectrum above the edge,
of the edge energy to 9660.4 eV. The new species also adopts a which is similar to ZnI2(THF)2. For phenylzinc reagent II, a
tetrahedral coordination geometry, since the shape of the decrease of the ‘‘white line’’ height and a shift in the edge were
XANES spectrum is still similar to ZnI2. The decrease in the also observed. The formation of the Zn–C bond shifts the edge
height of the ‘‘white line’’ (the sharp intense peak in the rising to a lower energy by 1.7 eV which is the same as the shift from
absorption edge of the XANES spectrum) and the change of the ZnI2(THF)2 to [PhZnI2(THF)]À. Two bromide anions, one carbon
edge energy suggests the coordination of a phenyl anion to Zn. anion and one oxygen atom were found to coordinate to Zn
Based on the fitting results of the EXAFS spectrum in Fig. 2b, through fitting the EXAFS spectrum (Fig. 3b). The Zn–Br bond
the major species in phenylzinc reagent I is coordinated to two distance was determined to be 2.45 Å. The Zn–O and Zn–C bond
iodides at 2.62 Å, one oxygen atom at 2.09 Å and one carbon distances were determined to be 2.11 Å and 1.91 Å, respectively.
anion at 1.96 Å. It is reasonable that the coordination of a The local structure of the major species in phenylzinc reagent II
was thus assigned as [PhZnBr2(THF)]À. Phenylzinc reagent III
was also investigated by XAS. As shown in Fig. 4a, the edge
energy of ZnCl2 is 9662.6 eV, which is 0.2 eV higher than the
ZnCl2/THF solution. The edge energy of phenylzinc reagent III is
9660.5 eV, 1.9 eV lower than the ZnCl2/THF solution. To sum-
marize all of the XANES results, coordination of THF to zinc
halides changes the edge energy. For ZnCl2, the shift is À0.2 eV;
for ZnI2, it is 0.2 eV, but for ZnBr2, no shift was observed.
Coordination of the phenyl anion to Zn led to an energy shift
of À1.7 or À1.9 eV compared with the corresponding ZnX2/THF
solution.
Fig. 2 XAS spectra of phenylzinc reagent I. (a) XANES spectra of solid ZnI2,
As shown in Fig. 4b, the EXAFS fitting results suggested a
ZnI2/THF solution and phenylzinc reagent I. The inset shows the first
similar structure of phenylzinc reagent III to those of I and II.
Two chlorides are bonded at a distance of 2.33 Å, one carbon
anion with a bond distance of 1.90 Å and one oxygen atom at
derivatives of the XANES spectra. (b) Fitting results of the k2-weighted
R-space EXAFS spectrum of phenylzinc reagent I. FT range: 2.86–
10.82 ÅÀ1; fitting range: 1.07–2.80 Å.
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to coordinate to Zn. The increase in the Zn–C bond distance
was observed upon changing the halide anion from chloride to
bromide and to iodide. It was also observed that a higher
transmetallation rate occurs with longer Zn–C bond distances.
This work was supported by the 973 Program (2012CB725302),
the National Natural Science Foundation of China (21390400,
21025206, 21272180 and 21302148), the Research Fund for the
Doctoral Program of Higher Education of China (20120141130002)
and the Program for Changjiang Scholars and Innovative Research
Team in University (IRT1030). Use of the Advanced Photon Source
was supported by the U. S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357. MRCAT operations were supported by
the Department of Energy and the MRCAT member institutions.
Fig. 4 XAS spectra of phenylzinc reagent III. (a) XANES spectra of solid
ZnCl2, ZnCl2/THF solution and phenylzinc reagent III. The inset shows the
first derivatives of the XANES spectra. (b) Fitting results of the k2-weighted
R-space EXAFS spectrum of phenylzinc reagent III. FT range: 2.76–
12.60 ÅÀ1; fitting range: 1.02–2.23 Å.
Table 1 Summary of the EXAFS fitting results
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2.05 Å were determined. The EXAFS fitting results of all of
the three phenylzinc reagents are summarized in Table 1. The
Zn–C bond distance in [PhZnI2(THF)]À is longer than that in
[PhZnBr2(THF)]À, while [PhZnCl2(THF)]À has the shortest Zn–C
bond distance. The difference between the Zn–C bond distance
and the reactivity is probably caused by the electronic proper-
ties of the halide ligands. The electronegativity of the halogens
increases up the group as a consequence of the halide electro-
negativity and the availability of their s electrons. In the
absence of p interactions between the halide anions and Zn2+
(d10 electronic configuration), iodide would be expected to
donate electron density to the metal through s interactions.
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in electron density on Zn2+, and therefore the highest trans-
metallation rate is expected (See ESI† for further discussion).
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all of the three phenylzinc reagents form zincate complexes
with distorted tetrahedral geometries in THF. The phenyl
anion, two halide anions and one THF molecule were found
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Chem. Commun., 2014, 50, 8709--8711 | 8711