Zinc-mediated carbene insertion to C-Cl bonds of chloromethanes and isolable zinc(II) isocyanide adducts

Article abstract of DOI:10.1021/acs.inorgchem.5b00929

The zinc adduct {[HB(3,5-(CF₃)₂Pz)₃]Zn}⁺, which was generated from [HB(3,5-(CF₃)₂Pz)₃]ZnEt and [Ph₃C]{B[3,5-(CF₃)₂C₆H₃]₄}, catalyzes the activation of C-halogen bonds of chloromethanes via carbene insertion. Ethyl diazoacetate serves as the carbene precursor. The presence of {[HB(3,5-(CF₃)₂Pz)₃]Zn}⁺ in the reaction mixture was confirmed by obtaining {[HB(3,5-(CF₃)₂Pz)₃]Zn(CN^tBu)₃}⁺ using CN^tBu as a trapping agent. {[HB(3,5-(CF₃)₂Pz)₃]Zn(CN^tBu)₃}⁺ loses one zinc-bound CN^tBu easily to produce five-coordinate {[HB(3,5-(CF₃)₂Pz)₃]Zn(CN^tBu)₂}⁺.

Full text of DOI:10.1021/acs.inorgchem.5b00929

Communication
pubs.acs.org/IC
Zinc-Mediated Carbene Insertion to C−Cl Bonds of Chloromethanes
and Isolable Zinc(II) Isocyanide Adducts
Naveen V. Kulkarni, Animesh Das, Naleen B. Jayaratna, Muhammed Yousufuddin, and H. V. Rasika Dias*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States
S
* Supporting Information
(CF3)2Pz)3]Ag(THF) (1; THF = tetrahydrofuran) is also an
ABSTRACT: The zinc adduct {[HB(3,5-(CF3)2Pz)3]-
Zn}+, which was generated from [HB(3,5-(CF3)2Pz)3]-
ZnEt and [Ph3C]{B[3,5-(CF3)2C6H3]4}, catalyzes the
activation of C−halogen bonds of chloromethanes via
carbene insertion. Ethyl diazoacetate serves as the carbene
precursor. The presence of {[HB(3,5-(CF3)2Pz)3]Zn}+ in
the reaction mixture was confirmed by obtaining {[HB-
(3,5-(CF3)2Pz)3]Zn(CNtBu)3}+ using CNtBu as a trapping
agent. {[HB(3,5-(CF3)2Pz)3]Zn(CNtBu)3}+ loses one
zinc-bound CNtBu easily to produce five-coordinate
{[HB(3,5-(CF3)2Pz)3]Zn(CNtBu)2}+.
excellent catalyst for the functionalization of unactivated
hydrocarbons.7,10 Here we present an interesting new develop-
ment in this field and describe the use of an earth-abundant 3d
block metal, in particular a zinc catalyst, {[HB(3,5-(CF3)2Pz)3]-
Zn}+ (2), in the functionalization of chloromethanes via carbene
insertion.
The catalyst {[HB(3,5-(CF3)2Pz)3]Zn}+ (containing a {B-
[3,5-(CF3)2C6H3]4}− counterion) used in this work was
generated in situ by treating the catalyst precursor [HB(3,5-
(CF3)2Pz)3]ZnEt11 with triphenylmethyl tetrakis[3,5-bis-
(trifluoromethyl)phenyl]borate [Ph3 C]{B[3, 5-
(CF3)2C6H3]4}12 in a 1:1 molar ratio at 60 °C. In a typical
process, a mixture of 1 equiv of [HB(3,5-(CF3)2Pz)3]ZnEt and
[Ph3C]{B[3,5-(CF3)2C6H3]4} was placed in 1 mL of alkyl
rocesses that facilitate the functionalization of C−Cl bonds
1
Pare of significant interest because they allow the conversion chloride (substrate and also the solvent) and treated with 20
equiv of EDA, which is the carbene precursor. After the mixture
was maintained at 60 °C for 1 h, the resulting products were
analyzed using several methods. To our delight, the reaction
involving chloroform produced ethyl 2,3,3-trichloropropionate
(4), which is the product of the insertion of carbene :CHCO2Et
into a C−Cl bond, in 59% yield (Table 1). A further increase in
the reaction time did not lead to a significant improvement in the
yield. When the catalyst/EDA molar ratio was changed to 1:50, a
decrease in the product yield was observed, while changing the
of relatively cheap and widely available substrates like alkyl
chlorides (especially compared to bromo and iodo analogues) to
more valuable organic compounds and may even provide a route
to deal with environmentally troublesome halogenated products
(like hydrochlorofluorocarbons).2 Activation of unfunctional-
ized C−Cl bonds, however, is a challenging task because of their
high bond strength.3−5
In 2003, we reported a novel silver-catalyzed carbene insertion
route for the functionalization of C−Cl and C−Br bonds of
halomethanes using commercially available ethyl diazoacetate
(EDA) as the carbene source.6,7 This reaction involves the
formation of a new sp3−sp3 C−C bond while retaining the halide
on the product. The silver adduct (e.g., 1 in Figure 1) supported
by highly fluorinated tris(pyrazolyl)borate ligand [HB(3,5-
(CF3)2Pz)3]− serves as the catalyst.8 High Lewis acidity at the
metal site is a key factor for the success of C−halogen bond
activation in these reactions. Others have studied this silver-
mediated process and provided insight into the mechanism
speculated in the original communication.9 [HB(3,5-
a
Table 1. Catalysis Data
b
c
d
catalyst catalyst/EDA
subtrate
product NY %
GY %
IY %
2
2
2
2
2
1
3
1:20
1:50
1:10
1:20
1:20
1:20
1:20
CHCl3
CHCl3
CHCl3
CH2Cl2
CCl4
4
4
4
5
6
4
4
59
23
68
20
64
78
0
67
21
76
22
32
16
53
CHCl3
CHCl3
75
0
a
All of the reactions were carried at 60 °C for 1 h. Yields are based on
b
1
an average of at least two runs. Yield based on the integration of H
c
NMR signals. Yield based on gas chromatography (GC) analysis.
d
Yield based on isolated products. Note: The isolated product yields
are lower than the NMR (or GC) yields because of the loss of volatile
products during the workup.
Figure 1. Complexes used in the present catalysis process: 1, 2 (used as
its {B[3,5-(CF3)2C6H3]4}− salt), and [HB(3,5- (CF3)2Pz)3]Cu(C2H4)
(3).
Received: April 24, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00929
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Inorganic Chemistry
Communication
[catalyst]/[EDA] ratio to 1:10 improves the yield (Table 1).
This can be explained on the basis of the increased number of
catalytically active species per substrate molecules in the latter
case. Similarly, dichloromethane and tetrachloromethane were
also converted into ethyl 2,3-dichloropropionate (5) and ethyl
2,3,3,3-tetrachloropropanoate, respectively, in 20% and 64%
yield (Table 1). The yields of products 4−6 follow the relative
C−Cl bond energies of the starting alkyl halide substrates.4 The
rest of EDA in these reactions remained mostly unreacted (see
Table S1 in the Supporting Information, SI). When n-BuCl was
used as a substrate under similar reaction conditions, no C−Cl
insertion product was observed. Instead, C−H insertion
products (15%) and ethyl 2-chloroacetate (which results from
HCl elimination from n-BuCl and addition to carbene
:CHCO2Et) were found in the reaction mixture. The rest of
EDA remained essentially unreacted, with a small amount ending
up as (∼3%) carbene dimers.
found to be catalytically inactive for C−Cl bond activation,
suggesting that it could indeed be a termination product.
In order to confirm generation of {[HB(3,5-(CF3)2Pz)3]Zn}+,
we performed a trapping reaction using tert-butyl isocyanide
instead of EDA in dichloromethane. We were able to isolate a
zinc(II) isocyanide adduct, {[HB(3,5-(CF3)2Pz)3]Zn-
(CNtBu)3}{B[3,5-(CF3)2C6H3]4}, in good yield. It was charac-
terized using several methods including X-ray crystallography
(Figure 2). Such structurally authenticated zinc(II) isocyanide
Generation of the zinc cation from [HB(3,5-(CF3)2Pz)3]ZnEt
is a key step for which a strong hydride abstracting agent,
[Ph3C]{B[3,5-(CF3)2C6H3]4}, was used.12 It reacts with [HB-
(3,5-(CF3)2Pz)3]ZnEt, producing {[HB(3,5-(CF3)2Pz)3]Zn}+
as its {B[3,5-(CF3)2C6H3]4}− salt with liberation of an ethylene
molecule. When [HB(3,5-(CF3)2Pz)3]ZnEt was used alone, no
catalytic activity was observed under various conditions,
indicating the need for generation of {[HB(3,5-(CF3)2Pz)3]-
Zn}+. The elevated temperature (60 °C) is necessary for this
reaction. When the same reaction was carried at room
temperature, no notable product formation was observed
because of the sluggish generation of catalytically active cationic
zinc species. Other zinc compounds such as Zn(OTf)2, Et2Zn,13
or the combination of Et2Zn and [Ph3C]{B[3,5-(CF3)2C6H3]4}
or just [Ph3C]{B[3,5-(CF3)2C6H3]4} with 20 equiv of EDA in
chloroform at 60 °C did not produce any measurable amounts of
4.
The C−Cl bond activation chemistry of CHCl3 was probed
under comparable conditions using the well-established silver
catalyst 16 for comparison. Results show that the silver adduct 1
(which is a neutral complex and a 4d metal-based catalyst) is still
the better catalyst compared to the zinc catalyst 2 (Table 1).
However, the copper(I) catalyst 3 (Figure 1) produced only
carbene dimers (100% consumption of EDA) under similar
conditions, indicating its inability to mediate the halogen
activation process.14
The zinc(II)-mediated process is likely to proceed via a metal−
carbene intermediate, which results from the reaction between
EDA and {[HB(3,5-(CF3)2Pz)3]Zn}+ with the loss of N2 (see the
SI for the proposed mechanism).9 Chloroalkanes then bind to
the electrophilic metallacarbene carbon center via a chlorine
atom, forming a ylide, which after additional steps generates
products of formal carbene insertion into C−Cl bonds, releasing
the zinc(II) catalyst. These reaction mixtures containing the
{[HB(3,5-(CF3)2Pz)3]Zn}+ catalyst produced [HB(3,5-
(CF3)2Pz)3]ZnCl (from CHCl3 and CH2Cl2 solutions) in
about 85% yield (with respect to the amount of [HB(3,5-
(CF3)2Pz)3]ZnEt used) upon standing. This compound might
have resulted from the migration of a chloride ion to the metal
center (rather than to the metallacarbene carbon)9 or from the
direct reaction of {[HB(3,5-(CF3)2Pz)3]Zn}+ with a chlorinated
solvent and likely represents a termination step of the catalytic
reaction. We have also prepared [HB(3,5-(CF3)2Pz)3]ZnCl from
an independent route (see the SI) and employed it in the same
catalytic reaction with [Ph3C]{B[3,5-(CF3)2C6H3]4}. It was
Figure 2. View of the molecular structure of {[HB(3,5-(CF3)2Pz)3]-
Zn(CNtBu)3}+. Selected bond lengths (Å) and angles (deg): Zn−N1 =
2.259(7), Zn−N5 = 2.325(7), Zn−N3 = 2.287(7), Zn−C16 =
2.149(10), Zn−C21 = 2.137(10), Zn−C26 = 2.178(10), C21−N8 =
1.175(12), C16−N7 = 1.145(12), C26−N9 = 1.114(11); N1−Zn−N3
= 82.5(3), N1−Zn−N5 = 84.7(2), N3−Zn−N5 = 82.2(2), Zn−C21−
N8 = 173.3(9), Zn−C16−N7 = 178.6(9), Zn−C26−N9 = 174.0(8).
adducts are of interest because they could serve as models for
intermediates in zinc-catalyzed processes involving isocya-
nides.15,16 The six-coordinate zinc atom in this adduct adopts
an octahedral geometry, with three isonitriles occupying the
facial configuration. [HB(3,5-(CF3)2Pz)3]− serves as a typical κ3
ligand.
Interestingly, one of the isocyanide ligands in {[HB(3,5-
(CF3)2Pz)3]Zn(CNtBu)3}{B[3,5-(CF3)2C6H3]4} can be re-
moved quite easily under reduced pressure (in fact, one
isocyanide coordinates to zinc with a longer Zn−C bond relative
to the other two), leading to the formation of a five-coordinate
bis(isocyanide) adduct, {[HB(3,5-(CF3)2Pz)3]Zn(CNtBu)2}{B-
[3,5-(CF3)2C6H3]4}. The X-ray crystal structure of this adduct is
illustrated in Figure 3. It has a distorted trigonal-bipyramidal zinc
center. The κ3-bound [HB(3,5-(CF3)2Pz)3]− ligand occupies
two equatorial and one axial sites, as is evident from two long and
one short Zn−N distances. The axial and equatorial Zn−C
distances are also very different, 2.204(6) and 2.013(7) Å,
respectively. Structurally characterized zinc(II) isocyanide
adducts are rare,15,17 and four-coordinate ZnBr2(CNtBu)2 is
one such example in the literature. The Zn−C distances of this
adduct [2.069(5) and 2.072(4) Å] are close to the average Zn−C
B
DOI: 10.1021/acs.inorgchem.5b00929
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
ACKNOWLEDGMENTS
■
H.V.R.D. thanks the National Science Foundation (Grant CHE-
1265807) and the Robert A. Welch Foundation (Grant Y-1289)
for financial support of this research.
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Figure 3. View of the molecular structure of {[HB(3,5-(CF3)2Pz)3]-
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Zn}+ by stabilizing it using tert-butyl isocyanide and obtained two
rare and isolable zinc(II) isocyanide complexes. Catalyst
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ASSOCIATED CONTENT
■
S
* Supporting Information
Details of the synthesis and characterization and catalysis,
proposed mechanism, additional figures and tables, and X-ray
crystallographic data in CIF format. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.inorgchem.5b00929.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dias@uta.edu.
Author Contributions
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.inorgchem.5b00929
Inorg. Chem. XXXX, XXX, XXX−XXX