Carbon-hydrogen bond stannylation and alkylation catalyzed by nitrogen-donor-supported nickel complexes: Intermediates with Ni-Sn bonds and catalytic carbostannylation of ethylene with organostannanes

Article abstract of DOI:10.1021/om4003889

The reaction of H₂C=CHSnR₃ with C₆F ₅H, where R = Bu, Bn, Ph, was catalyzed by Ni(COD)₂ and the nitrogen donor ancillary ligand MeNC₅H₄N ⁱPr. These reactions produced the stannylation products C ₆F₅SnR₃ (1^R) and C-H alkylation products C₆F₅CH₂CH₂SnR₃ (3^R). The Bu substituent provided the best selectivity for stannylation, whereas the Ph substituent provided primarily the alkylation product. The catalytic intermediate (MeNC₅H₄N ⁱPr)Ni(η²-H₂C=CHSnR₃) ₂ (2^R) was observed by NMR spectroscopy and isolated in the case of R = Ph. A second catalytic intermediate, cis-(MeNC₅H ₄NⁱPr)₂Ni(C₆F₅)(SnR ₃) (4^R), was observed by NMR spectroscopy and isolated for R = Bn, Ph by the reaction of C₆F₅SnR₃ with MeNC₅H₄NⁱPr and Ni(COD)₂. The reaction of C₆F₅SnR₃ with ethylene in the presence of catalytic MeNC₅H₄NⁱPr and Ni(COD)₂ provided the carbostannylation product 3^R. Mechanistic studies of the C-H stannylation/alkylation mechanism were performed to propose a mechanistic manifold for these transformations.

Full text of DOI:10.1021/om4003889

Article
pubs.acs.org/Organometallics
Carbon−Hydrogen Bond Stannylation and Alkylation Catalyzed by
Nitrogen-Donor-Supported Nickel Complexes: Intermediates with
Ni−Sn Bonds and Catalytic Carbostannylation of Ethylene with
Organostannanes
Meghan E. Doster and Samuel A. Johnson*
Department of Chemistry & Biochemistry, University of Windsor, Windsor, Ontario, Canada N9B 3P4
S
* Supporting Information
ABSTRACT: The reaction of H2CCHSnR3 with C6F5H,
where R = Bu, Bn, Ph, was catalyzed by Ni(COD)2 and the
nitrogen donor ancillary ligand MeNC5H4NiPr. These
reactions produced the stannylation products C6F5SnR3 (1R)
and C−H alkylation products C6F5CH2CH2SnR3 (3R). The Bu
substituent provided the best selectivity for stannylation,
whereas the Ph substituent provided primarily the alkylation
product. The catalytic intermediate (MeNC5H4NiPr)Ni(η2-
H2CCHSnR3)2 (2R) was observed by NMR spectroscopy
and isolated in the case of R = Ph. A second catalytic
intermediate, cis-(MeNC5H4NiPr)2Ni(C6F5)(SnR3) (4R), was
observed by NMR spectroscopy and isolated for R = Bn, Ph by
the reaction of C6F5SnR3 with MeNC5H4NiPr and Ni(COD)2.
The reaction of C6F5SnR3 with ethylene in the presence of catalytic MeNC5H4NiPr and Ni(COD)2 provided the
carbostannylation product 3R. Mechanistic studies of the C−H stannylation/alkylation mechanism were performed to propose a
mechanistic manifold for these transformations.
i
or Pr3P catalyze the reaction of H2CCHSnBu3 and
INTRODUCTION
■
fluorinated aromatics via selective C−H bond stannylation to
give C6FnH5−nSnBu3 (1Bu) and ethylene, as shown for the
substrate C6F5H in the top reaction in Scheme 1, where R
=Bu.13
Reactions that convert simple hydrocarbons into molecules that
bear versatile functional groups are of great importance in
organic and organometallic chemistry.1 For example, carbon−
boron bonds can be accessed directly from hydrocarbons via
either C−H bond borylation or the hydroboration2 of
unsaturated hydrocarbons. Similarly, hydrosilylation2,3 is the
best-developed route to C−Si bonds from hydrocarbons,
though C−H bond silylation catalysts are of considerable
interest.4 These heteroatom bond forming reactions are
important because these products are important substrates for
further transformations such as Miyaura−Suzuki5 or Hiyama6
cross-coupling reactions.
Scheme 1
Efficient catalytic routes to C−Sn bonds have been less
developed,7 despite the utility of these reagents in the
functional group tolerant Stille cross-coupling reaction.8
Typical hydrostannylation2,9 reactions are limited in scope,
with alkynes as the primary substrates, and often proceed by
radical mechanisms. Hydrostannylation of alkenes is more
difficult and usually requires a strained alkene10 or the addition
of a Lewis acid11 or radical initiator12 to overcome side
reactions which produce H2 and distannanes. New routes to
these bonds could expand the scope and utility of Stille
coupling.
We recently reported that Ni(COD)2 (COD = 1,5-
Received: May 3, 2013
Published: July 22, 2013
cyclooctadiene) and an ancillary ligand such as MeNC5H4NiPr
© 2013 American Chemical Society
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With iPr3P as the ancillary ligand, the resting state of the C−
H bond stannylation reaction is (iPr3P)Ni(η2-H2CCHSnR3)2
(R = Bu, Ph). The proposed mechanism involves the reversible
dissociation of one of the H2CCHSnR3 moieties from the
catalyst to produce the unobserved intermediate (iPr3P)Ni(η2-
H2CCHSnR3), which reacts with C6F5H to produce 1Bu or
1Ph. Little is known about the C−Sn bond-cleavage or C−Sn
bond-forming steps in this system, because they occur after the
rate-determining step.14
Crystals suitable for analysis by X-ray crystallography were
obtained by cooling a saturated toluene solution of 2Ph to −40
°C. An ORTEP diagram of the solid-state molecular structure
of 2Ph is shown in Figure 1. The structure features η2-H2C
This paper details a mechanistic study of the reactions shown
in Scheme 1 when the ancillary ligand iPr3P is replaced with the
neutral quasi-amido donor MeNC5H4NiPr, abbreviated herein
as {NQA}.15a Though the bond lengths of the solid-state
structure of {NQA} suggest that the conjugated imine
resonance structure best describes the ground state, as shown
in Scheme 2, this nitrogen donor is a strong base in aqueous
Figure 1. Solid-state molecular structure of 2Ph as determined by X-ray
crystallography. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ni(1)−N(1), 1.975(3); Ni(1)−C(3),
1.975(3); Ni(1)C(1), 1.984(3); Ni(1)−C(4), 1.986(4); Ni(1)−
C(2), 1.986(4); C(1)−C(2), 1.392(5); C(3)−C(4), 1.399(5); C(3)−
Ni(1)−N(1), 133.86(13); C(3)−Ni(1)−C(1), 92.69(15); N(1)−
Ni(1)−C(1), 133.38(13); N(1)−Ni(1)−C(4), 92.77(14); C(1)−
Ni(1)−C(4), 133.11(16); C(3)−Ni(1)−C(2), 133.66(15); N(1)−
Ni(1)−C(2), 92.49(14); C(4)−Ni(1)−C(2), 172.55(18).
Scheme 2. The Neutral Quasi-Amido Donor {NQA}
solution, which can be attributed to its aromatic resonance
structure shown on the right of Scheme 2. This donor has been
shown to support reactivity similar to that of carbene donors16
in Ni(0) complex C−F bond activation.13b Combinations of
MeNC5H4NiPr with Ni(COD)2 have also been shown to
catalyze the C−H bond stannylation of pentafluorobenzene
CHSnPh3 groups and {NQA} σ-bound to the nickel metal
center. The two SnPh3 substituents arrange themselves so that
they are as far away from the {NQA} ligand as possible, on
opposite sides of the Ni coordination plane, to best avoid each
other. This renders the complex C1 symmetric. The Ni(1)−
C(1), Ni(1)−C(2), Ni(1)−C(3), Ni(1)−C(4), C(1)−C(2),
and C(3)−C(4) bond lengths are all within 3 standard
i
and H2CCHSnBu3 at room temperature, whereas the Pr3P-
supported complexes are exceedingly slow under these
conditions. This paper not only provides additional insight
into the C−H bond stannylation mechanism but also
investigates C−H bond alkylation and ethylene carbostannyla-
tion reactions that provide new routes to Stille coupling
reagents, as illustrated in Scheme 1.
i
deviations of the previously published structure with Pr3P as
the ancillary ligand.14 The Ni(1)−N(1) bond length of
1.975(3) Å for this Ni(0) complex is slightly longer than that
for previously characterized square-planar Ni(II) complexes
with {NQA}, which had a bond length of 1.9256(15) Å.15a This
complex is the first example of a hard {NQA} donor supporting
Ni in a formally zero oxidation state. A CH3 group on the
ancillary ligand iPr substituent has a hydrogen atom that is only
2.6543(5) Å from the Ni center, and the closest ancillary ligand
hydrogen on the heterocyclic ring has a hydrogen that is only
2.5279(5) Å from the nickel center. Low-temperature NMR
spectroscopy provided no evidence as to whether these are
nonbonding close contacts or agostic interactions.
RESULTS AND DISCUSSION
Similar to the approach used to isolate intermediates in the
■
i
stannylation reactions catalyzed by Pr3P-supported Ni(0)
complexes,14 we attempted to generate Ni(0) catalysts
supported by {NQA} using the crystalline vinylstannane
H2CCHSnPh3 in lieu of H2CCHSnBu3. The reaction of
Ni(COD)2 with 1 equiv of {NQA} and 2 equiv of H2C
CHSnPh3 in pentane provided the complex {NQA}Ni(η2-
H2CCHSnPh3)2 (2Ph) as a yellow precipitate in 69% isolated
yield, as shown in Scheme 3.
The 1H NMR spectrum of 2Ph in C6D6 displayed resonances
consistent with the symmetry observed in the solid-state
structure. Complex 2Ph has six proton environments for the two
coordinated vinyl moieties between δ 2.72 and 3.59, which are
shifted several ppm upfield relative to free H2CCHSnPh3.
The four proton environments for the nitrogen-containing ring
of {NQA} are shifted several ppm upfield relative to free
{NQA}. A pair of resonances was observed for the
diastereotopic methyl environments of the isopropyl group,
which integrated to 3H each. The IR spectrum of 2Ph displays a
CN stretching frequency at 1654.6 cm−1 which is
comparable to the CN stretching frequency of free {NQA}
at 1661.6 cm−1, which supports the N-bound nature of the
{NQA} donor in solution. The retention of C1 symmetry in
solution suggests that rotation about the Ni−N bond does not
Scheme 3. Synthesis of 2Ph
1
occur on the H NMR time scale.
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Similar reactions with H2CCHSnBn3 (where Bn =
CH2Ph) in pentane did not yield the desired product
{NQA}Ni(η2-H2CCHSnBn3)2 (2Bn) as an isolable solid.
When the reaction was performed in benzene-d6 and monitored
immediately by 1H NMR spectroscopy, peaks assignable as 2Bn
were observed; however, the solution turned from a bright
yellow to a dark green within 10 min, and the sample
decomposed, making isolation and further characterization
impossible. A similar reaction with H2CCHSnBu3 in toluene
complete conversion of 1Ph to 3Ph was observed by 19F NMR
spectroscopy. The alkylation product 3Ph has three 19F
environments at δ −144.7, −158.4, and −162.7. The 1H
NMR spectra of 3Ph has two distinct CH2 environments that
are multiplets with Sn satellites at δ 1.66 and 2.96 that integrate
to 2H each. When the reaction was complete, the 119Sn{1H}
NMR spectrum had a single environment for 3Ph at δ −103.7,
which is shifted downfield from H2CCHSnPh3 at δ −134.4.
Vinyl Stannane Substituent Effects. The relative
reactivities of H2CCHSnPh3, H2CCHSnBn3, and H2C
CHSnBu3 were next examined under identical reaction
conditions to determine the influence of different substituents
at Sn on the kinetics and thermodynamics of the C−H bond
stannylation and C−H bond alkylation reactions. Catalytic
amounts of Ni(COD)2 (20 mol %) and {NQA} (40 mol %)
were reacted with C6F5H and H2CCHSnR3 (R = Bu, Bn,
Ph), as shown in Scheme 5. The progress of the reaction was
tracked by 19F NMR spectroscopy every 30 min for 120 min to
get the initial product ratios. Note that the large catalyst
loadings used are not necessary but made monitoring by NMR
spectroscopy on this time scale convenient. The results showed
a dramatic influence of the R substituent on the initial product
mixtures, with the Bu substituents providing almost exclusively
1Bu, with the majority of the C6F5H consumed within 2 h. Both
R = Bn and R = Ph consumed C6F5H at a similar rate, with
∼50% remaining after 2 h, but gave significantly different ratios
of 1R to 3R, with the Ph substituent producing primarily 3R.
Similar to the case for the Bu substituents, the Bn group
favored the formation of 1R.
1
produced resonances between δ 2.8 and 3.5 in the H NMR
and a pair of closely spaced resonances at δ −39.6 and −39.9 in
the 119Sn{1H} NMR spectrum, which is consistent with the
desired C1-symmetric product {NQA}Ni(η2-H2C
CHSnBu3)2 (2Bu). The persistence of Ni(COD)2, H2C
CHSnBu3, and {NQA} in this solution suggest the formation of
only an equilibrium amount of 2Bu. Due to both this
equilibrium and the oily nature of the product mixture, imbued
by the Bu substituents, 2Bu could not be isolated, and further
characterization was not possible.
C−H Bond Alkylation. The reaction of H2CCHSnPh
and C6F5H in a sealed tube with catalytic amounts of the
isolated complex 2Ph or Ni(COD)2 and {NQA} for 4−5 days
provided the C−H bond alkylation1 7 product
C6F5CH2CH2SnPh (3Ph), as shown in Scheme 4. This C−H
Scheme 4
Synthesis of cis-{NQA}2Ni(C6F5)(SnR3) Complexes 4Ph
and 4Bn. To better understand the mechanism of C−H bond
stannylation and its relation to C−H bond alkylation, attempts
were made to isolate intermediates 4Ph and 4Bn. The reaction of
Ni(COD)2 with 2 equiv of {NQA} and either Ph3SnC6F5 (1Ph)
or Bn3SnC6F5 (lBn) provided an alternate route to the
complexes cis-{NQA}2Ni(C6F5)(SnR3), where R = Ph (4Ph),
Bn (4Bn), as shown in Scheme 6. Oxidative addition of the C−
Sn bond of 1R to form 4R, where R = Ph, Bn, is formally the
microscopic reverse of the C−Sn bond-forming step in catalytic
C−H bond stannylation.
The solid-state structures of 4Bn and 4Ph were both
determined by X-ray crystallography, and an ORTEP depiction
of 4Ph is shown in Figure 2. The quality of the structure of 4Bn
is poor compared to that of 4Ph, due to the platelike habit of the
crystals, but the connectivities are identical and confirm that
products 4Bn and 4Ph are both square planar with cis-disposed
{NQA} donors. The rings of the ancillary ligands and
pentafluorophenyl moiety lie approximately perpendicular to
the coordination plane, and the isopropyl substituents of the
ancillary ligands are situated on opposite faces of the square
plane. The structures are slightly distorted from square planar,
with a compressed C(1)−Ni(1)−Sn(1) angle in 4Ph of
82.3(3)°.
bond alkylation can be carried out at room temperature with 5
mol % of catalyst. The rate of the reaction can be increased by
heating to 315 K, but temperatures exceeding 315 K cause
catalyst decomposition and decreased yields.
Monitoring the reaction by 19F{1H} NMR spectroscopy
shortly after the reaction was started showed that the initial
product distribution was sensitive to reaction conditions, but
reproducibility provided the same three initial products. After 4
h the anticipated C−H bond stannylation product 1Ph was a
minor product of the product mixture and was typically less
than 7% of the product mixture. The major product was the
C−H bond alkylation product C6F5CH2CH2SnPh3 (3Ph). The
complex cis-{NQA}2Ni(C6F5)(SnPh3) (4Ph) was a resting state
of the nickel catalyst, though catalyst 2Ph was also observed in
solution by 1H NMR spectroscopy after 4 h. Over 2 weeks the
1
Complexes 4Ph and 4Bn were characterized by H, 19F{1H},
13C{1H}, and 119Sn{1H} NMR spectroscopy. Several of the
resonances observed in both the 1H and 19F{1H} NMR spectra
were broad at room temperature. Variable-temperature NMR
spectroscopy was used to probe the fluxionality in these
complexes, which appears to arise from two separate processes
and two isomers. The low-temperature spectra allowed the
observation of a second species, assigned as the minor isomer
4R-m. The data are consistent with the intermediacy of the T-
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Scheme 5
Scheme 6. Synthesis of 4Ph and 4Bn and Fluxionality in
Solution
Figure 2. Solid-state molecular structure of 4Ph as determined by X-ray
crystallography. Hydrogen atoms and phenyl rings are omitted for
clarity. Selected bond lengths (Å) and angles (deg): C−Ni(1),
1.931(9); N(1)−Ni(1), 2.005(7); N(3)−Ni(1), 1.956(7); Ni(1)−
Sn(1), 2.4760(12); N(1)−Ni(1)−C(1), 93.0(3); N(1)−Ni(1)−N(3),
93.4(3); C(1)−Ni(1)−N(3), 171.1(4); N(1)−Ni(1)−Sn(1),
174.8(2); C(1)−Ni(1)−Sn(1), 82.3(3); N(3)−Ni(1)−Sn(1), 91.0(2).
temperature with CD2Cl2 to form a complex that was
tentatively assigned as trans-{NQA}2NiCl(C6F5) (5) on the
basis of the similarity of its NMR spectra to that of the
analogous fluoride trans-{NQA}2NiF(C6F5), but without the
Ni−F 19F resonance.15a Below the coalescence temperature for
complexes 4Bn and 4Ph the ortho fluorines at δ −110 begin to
separate and sharpen and the meta-fluorines at δ −165 sharpen
into two complex multiplets. The 19F{1H} variable-temperature
NMR spectra of 4Ph and 4Bn were modeled using WinDNMR18
to estimate the rate constant for the fluxional process; the
Arrhenius equation was then used to estimate the activation
energy to be 25 and 26 kcal mol−1 for 4Ph and 4Bn, respectively.
The low-temperature limit for this process was reached at 273
K.
Below 273 K there was still some broadening of the ortho-
and meta-fluorines associated with a lower-energy fluxional
process that occurs via the rapid equilibrium process shown in
Scheme 6. Figure 3 displays the 19F{1H} NMR experimental
and modeled spectra for the ortho-fluorine region of 4Bn
between 223 and 273 K. The 273 K 19F{1H} NMR spectrum
has two ortho resonances for the fluorinated aromatic at δ
shaped three-coordinate intermediate 4R-int, which exchanges
4R and 4R-m, as shown on the right-hand side of Scheme 6.
The 19F{1H} NMR spectra of 4Ph and 4Bn near room
temperature indicate that there is a process that exchanges the
ortho-F as well as meta-F environments, which could occur via
rotation around the Ni−C6F5 bond in 4R, 4R-m, or 4R-int but
could also occur via rotation around the Ni−N bond in 4R-int,
1
the latter of which is supported by H NMR data. At 293 K
both the ortho and meta 19F environments of 4Bn are near
coalescence, with broad peaks at δ −110 and −165,
respectively. At 323 K the ortho fluorine resonance for complex
4Bn sharpens into a doublet with a coupling constant of 34 Hz,
and the meta fluorine resonance is a second-order multiplet.
Complex 4Ph has poor solubility in d8-toluene, and thus the
variable-temperature NMR was obtained in CD2Cl2. The fast-
exchange limit 19F NMR spectrum of 4Ph could not be
obtained, due to a reaction that occurred over 30 min at room
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Figure 3. (a) Simulated and (b) experimental variable-temperature 282.4 MHz 19F{1H} NMR spectra showing the decoalescence of peaks for the
ortho-fluorines of 4Bn and 4Bn-m. Coupling was not included in the simulation.
−109.2 and −110.9, but with significantly different line widths.
4Ph and at δ 4.91 and 3.31 for 4Bn, respectively. The protons
associated with C(1) and C(14) of the nitrogen-containing
rings were shifted downfield, due to their close proximity to the
nickel center, and were observed as a broad multiplet and as a
doublet at δ 8.65 and 9.69 for 4Ph and at δ 8.70 and 9.71 for 4Bn
at 298 K. Attempts to resolve the set of broad ligand resonances
were unsuccessful even upon cooling the samples to 223 K.
Below 273 K the two resonances sharpen into two doublets
3
with JFF values of approximately 35 Hz, and a second set of
smaller peaks, approximately 14% in intensity, begin to
separate. The model displayed in Figure 3 was used to confirm
that the second set of smaller peaks was in exchange with the
larger peaks by simulating the observed line widths. The peak
of 4Bn at δ −110.9 is in exchange exclusively with the nearly
coincident 4Bn-m peak at δ −110.6, which leads to the rapid
broadening at low temperatures. Similarly, the δ −109.2
resonance of 4Bn is in exchange with the 4Bn-m peak at δ
−107.0. The variable-temperature NMR spectra were modeled
using WinDNMR,18 and the Arrhenius equation was used to
estimate an activation energy of 5.5 kcal mol−1 for the exchange
between the isomers 4Bn and 4Bn-m. Similar variable-temper-
ature 19F NMR spectra were observed for 4Ph between 223 and
273 K.
1
The H NMR spectra of 4Bn when 0.5, 1, and 2 equiv of
{NQA} were added at 293 K were compared to confirm that
ligand dissociation was the source of line broadening for the
one set of ligand peaks. The broad set of ligand resonances and
the free ligand peaks in the 1H NMR spectra were observed as a
single set of broad and shifted resonances. The results suggest
that the broadening of the one set of {NQA} resonances is due
to a dissociative process where only one of the {NQA} ligands
exchanged with the free ligand. The relatively larger trans effect
of the SnR3 (R = Ph, Bn) ligand versus the pentafluorophenyl
ring versus the moiety presumably determines which ancillary
ligand dissociates. The results also suggest that, if this process is
dissociative, then a T-shaped structure must be adopted by the
three-coordinate intermediate, because the two {NQA}
environments of 4Bn were not in exchange with each other,
and one set of {NQA} resonances remained sharp and were not
exchanged with free ligand. In support of the involvement of
4Bn-int in the process that exchanges 19F NMR ortho-F
environments near room temperature, the addition of
increasing concentrations of {NQA} to solutions of 4Bn causes
the room-temperature 19F NMR line shapes to change to
resemble slower exchange spectra.
Carbostannylation of Ethylene. To determine if ethylene
can be inserted into the Ni−Sn bond of 4Bn and, if the
intermediate with ethylene inserted into the Ni−Sn bond,
({NQA})Ni(CH2CH2SnR3)(C6F5) (6R) can be isolated or
observed, 4Bn was reacted with an atmosphere of ethylene and
the reaction was tracked by 19F{1H} NMR. Conversion to 3Bn
was observed by 19F NMR spectroscopy at 303 K; however, the
desired intermediate where ethylene was inserted into the Ni−
Sn bond was not observed. There were only very small
unidentified 19F{1H} NMR resonances at δ −115, −164, and
−165 throughout the reaction.
The most likely identity of the minor species observed in the
223 K 19F NMR is the isomer 4Bn-m, where the isopropyl
groups of {NQA} lie on the same side of the coordination
plane, rather than on opposite faces, and this hypothesis was
1
investigated by variable-temperature H NMR. At 298 K, the
1H NMR spectra for the SnR3 moiety of 4Ph and 4Bn displayed
three and four resonances, respectively, as expected. There were
14 resolved resonances for the two {NQA} ancillary ligands in
4
Ph and 4Bn, with two broad resonances integrating to 6H each,
which indicated that two diastereotopic pairs of chemically
inequivalent isopropyl methyl environments were in exchange,
but the two ligand environments were not in exchange on the
NMR time scale. At 253 K, the isopropyl methyl group
resonances at δ 0.82 and 0.84 for 4Ph and δ 1.23 and 1.27 for
4Bn were both resolved into two sets of doublets. These are
assigned as the diastereotopic methyl groups on the {NQA}
moiety that remains coordinated in 4Bn-int. Rotation around
the Ni−N bond in 4Bn-int accounts for the observed
coalescence of the diastereotopic methyl groups for this
group near room temperature. The iPr methyl group
resonances for the second {NQA} ligand remains as a single
broad peak that integrates to 6 H at δ 1.08 in 4Ph and δ 1.60 in
4Bn, consistent with a facile dissociation and association of this
ligand. This exchanges the diastereotopic methyl groups for the
dissociating ligand in 4Ph by virtue of the fact that the free
{NQA} can associate with either enantiomer of 4R-int.
To confirm that catalytic carbostannylation of ethylene is
viable, independently synthesized 1Ph was reacted with ethylene
using catalytic amounts of {NQA} and Ni(COD)2 and the
reaction mixture was monitored by 19F NMR spectroscopy.
Under these conditions 1Ph was converted to 3Ph via
carbostannylation, as shown in Scheme 7. A similarly catalyzed
reaction of 1Bn with ethylene produced 3Bn, whereas the
reaction of 1Bu and ethylene yielded a complex mixture of
products.
Consistent with the proposed fluxional process, the
1
remaining H NMR resonances associated with the ancillary
ligands featured a set of sharp resonances for the ligand that
remains associated and a broad set for the ligand that
dissociates. The isopropyl CH protons at 298 K were observed
as an unresolved multiplet and a septet at δ 1.98 and 3.29 for
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slows. The derived rate law for a simplified reaction mechanism
where an equilibrium reaction of 8 with 1Ph to form 4-int is
followed by a rate-determining reaction of 4-int with ethylene
to provide 3Ph has an inverse dependence on ethylene
concentration.
Scheme 7
Mechanism. A plausible mechanistic manifold that includes
intermediates 2R and 4R and is capable of catalytic C−H
alkylation, C−H bond stannylation and ethylene carbostanny-
lation to form 1R and 3R is shown in Scheme 8. The C−H bond
stannylation and alkylation mechanisms both involve the initial
formation of the complexes 2R from the reaction of Ni(COD)2
with 2 equiv of H2CCHSnR3 (R = Bu, Ph, Bn) and 1 equiv
of L (where L ={NQA}) to form 2R. We have previously
proposed for L = PiPr3 that this step is followed by reversible
dissociation of one of the two H2CCHSnR3 moieties to
generate the unobserved species (L)Ni(η2-H2CCHSnR3),
which then reacts with C6F5H either by oxidative addition of
the C−H bond and insertion of H2CCHSnR3 or by a one-
step H transfer to give the unobserved intermediate 6R.14 The
C−H bond stannylation product 1R may be formed from 6R by
β-elimination of the SnR3 group to form intermediate 7,
followed by ligand substitution with the loss of ethylene to
form 4R and then reductive elimination. Product 3R can be
formed by C−H bond alkylation via reductive elimination from
6R or carbostannylation via C−Sn bond activation of 1R to the
The formation of 3Ph via carbostannylation of ethylene is
significantly slower than the C−H alkylation reaction between
H2CCHSnPh3 and C6F5H, which yields the same final
product. Increased ethylene concentration was examined as a
method to increase the rate of carbostannylation. Instead, the
addition of increased ethylene concentrations to catalytic
{NQA} and Ni(COD)2 in the presence of the substrate 1Ph
was found to slow carbostannylation. This can be explained by
the formation of an equilibrium amount of the ethylene
1
complex {NQA}Ni(η2-C2H4)2 (8), which was observed by H
NMR spectroscopy but could not be isolated. Complex 8 was
present throughout the catalytic carbostannylation reaction but
was formed reversibly and was only present in equilibrium
amounts. Increased ethylene concentrations decreased the
concentration of 4Ph due to increased equilibrium concen-
trations of 8, and thus the catalytic conversion to 3Ph actually
Scheme 8
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Organometallics
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nickel metal center followed by coordination of ethylene to the
metal center and insertion into the Ni−Sn bond and reductive
elimination.
Scheme 9
Kinetics of Stannylation and Alkylation. These reactions
feature multiple resting states and inhibition by ethylene, slow
decomposition of the catalyst to Ni over time, and many rate
constants. It proved impossible to reproducibly model the
reaction kinetics to fit each rate constant; however, data
supportive of the proposed mechanism were obtained by
monitoring product and intermediate concentrations versus
time under different reaction conditions. An example is shown
in Figure 4, a plot of concentrations of the catalytic
are not known, as well as four-coordinate intermediates such as
7R where there are multiple geometric isomers whose
interconversion may be necessary and rate limiting; due to
the lack of experimental evidence, these are omitted from the
reaction schemes. These labeling studies also demonstrate that
the initial C−H bond activation step shown in Scheme 9 can be
treated as irreversible under these reaction conditions, as there
is no formation of C6F5H after several hours of reaction, as
monitored by 19F NMR spectroscopy.
Figure 4. Concentrations of the catalytic intermediate (4Bn) and
catalytic stannylation (1Bn) and alkylation (3Bn) products early in the
reaction of C6F5H with H2CCHSnBn3 using the catalyst precursor
{NQA} and Ni(COD)2.
The effect of H2CCHSnBn3 concentration on the rates of
formation of the products and intermediates was also
determined. Early in the reaction, prior to the buildup of 4Bn,
increased [H2CCHSnBn3] inhibits the C−H bond activation
step, which requires dissociation of H2CCHSnBn3 from 2Bn.
This results in a decreased sum of all C−H activation products
1Bn, 3Bn, and 4Bn early in the reaction for increased [H2C
CHSnBn3]. The fraction of Ni catalyst in the 4Bn resting state
versus time for different initial [H2CCHSnBn3] as monitored
by integration of 19F NMR spectra versus an internal standard
is shown in Figure 5 and is also representative of the relative
intermediate (4Bn) and catalytic stannylation (1Bn) and
alkylation (3Bn) products versus time early in the slow reaction
of C6F5H with H2CCHSnBn3 using the catalyst precursor
{NQA} and Ni(COD)2 at 298 K.
This graph shows that the initial formation of 1Bn is slow
until 4Bn accumulates to an approximately steady-state
concentration near 3600 s, as expected if the reductive
elimination step associated with rate constant k3 in Scheme 8
is a slow equilibrium. In contrast, the initial rate of production
of 3Bn is highest early in the reaction, when the highest
concentration of 2Bn is present. This shows that the initial
formation of 3Bn is primarily from 6Bn, prior to its conversion to
4Bn, and that the reverse reaction where 4Bn is converted back
to 6Bn by reaction with ethylene is negligible relative to the rate
of the C−H bond activation step. Thus, the relative ratio of
alkylation (3Bn) to stannylation (1Bn) products is dictated by
the relative rates of the reductive elimination associated with k4
versus the β-elimination and ethylene dissociation associated
with k1 and k2.
Deuterium-labeling studies suggest that the step associated
with k1 is fast compared to the steps associated with k4 and k2
and that the step associated with k2 is rate-limiting for the
conversion of 6Bn to 4Bn. The alkylation of C6F5D with H2C
CHSnR3 (R = Ph, Bn) produced initial mixtures where both
the H and D NMR spectra had near 1:1 peak intensities for
the two different methylene groups, suggestive of an equal
mixture of C6F5CH2CHDSnR3 and C6F5CHDCH2SnR3. If the
rate of reductive elimination was much faster than β-
elimination from 6R (k4 ≫ k1), the product 3R-d1 would be
expected to have a single isotopomer with the deuterium
located in the CH2 group adjacent to the SnPh3 group, due to
the preference to insert H2CCHSnPh3 into the Ni−D bond
with the Ph3Sn moiety away from the nickel center, as shown in
Scheme 9. It should be noted that these mechanisms feature
three-coordinate intermediates such as 6R where the geometries
Figure 5. Fraction of Ni catalyst present as 4Bn vs time with different
initial [H2CCHSnBn3] in the presence of 0.21 M C6F5H and 0.04
M Ni(COD)2 and 0.08 M {NQA}. Lines are shown as a guide only.
1
2
rate of the C−H activation step early in the reaction. The only
other significant catalytic nickel species identified under these
conditions was 2Bn. The concentration of 4Bn reaches an
approximate steady-state value after 3600 s, with larger amounts
of the catalytic nickel species present as 4Bn for lower [H2C
14
i
CHSnBn3]. Unlike with nickel catalysts supported by Pr3P,
which showed an inverse relationship between the rate of
stannylation and [H2CCHSnBn3], after 1 h the rates of
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Organometallics
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formation of 1Bn and 3Bn are approximately identical with those
shown in Figure 4 under all the [H2CCHSnBn3] studied.
This can be rationalized by the fact that, although increased
[H2CCHSnBn3] inhibits the C−H activation step, it
increases the amount of catalytic Ni species present as 2Bn
rather than 4Bn.
The influence of the concentration of {NQA} on the rate of
reaction between C6F5H and H2CCHSnBn3 with catalytic
Ni(COD)2 was also examined. Early in the reaction, the
equilibrium between 4Bn and 1Bn in solution was notably
influenced by [{NQA}], as expected on the basis of Le
Chatelier’s principle. However, the rate of C−H activation and
the selectivity or rate of the reaction toward the production of
1Bn or 3Bn was not significantly influenced by increased
[{NQA}].
analogous manner by refluxing over Na and CaH2, respectively. All
other solvents were purchased anhydrous from Aldrich and further
purified using a Grubbs type column system,28 produced by Innovative
Technology. 1H, 13C{1H}, 19F{1H}, and 119Sn{1H} NMR spectra were
recorded on a Bruker AMX spectrometer operating at 300 MHz or,
where stated, at 500 MHz with respect to proton nuclei. All chemical
shifts are reported in parts per million (ppm), and all coupling
constants are in hertz (Hz). 1H NMR spectra were referenced to
residual protons (C6D5H, δ 7.15; CDHCl2, δ 5.32; C7D7H, δ 2.09;
CHCl3, δ 7.26) with respect to tetramethylsilane at δ 0.00. 13C{1H}
spectra were referenced relative to solvent resonances (C6D6, δ 128.0;
CDCl3, δ 77.0; C7D8, δ 20.4). 19F{1H} NMR spectra were referenced
to an external sample of 80% CCl3F in CDCl3 at δ 0.0. 119Sn{1H}
NMR spectra were referenced to an external sample of SnMe4 at δ 0.0.
C6D6, C7D8, CDCl3, and CH2Cl2 were purchased from Aldrich. The
compounds pentafluorobenzene, H2CCHSnBu3, ClSnPh3, and
ClSnBn3 were purchased from Aldrich. The reagent iPr3P was
purchased from Strem Chemicals. Ethylene was purchased from
BOC gases. The compounds MeNC5H4NiPr,15a Ni(COD)2,29
CONCLUSIONS
■
32
C6F5D,30 H2CCHSnPh3,31 and C6F5SnBn3 were prepared by
Experimental evidence shows that nickel complexes such as
{NQA}Ni(η2-H2CCHSnPh3)2 (2Ph) are catalysts for C−H
bond alkylation of fluorinated aromatics and H2CCHSnR3
(R = Ph, Bn) to form products of the type C6F5CH2CH2SnR3
(3R) (R = Ph, Bn). Isolated intermediates of the type cis-
{NQA}2Ni(C6F5)(SnR3), where R = Ph (4Ph), Bn (4Bn), show
that the C−Sn bond of the organostannane reagents 1R (R =
Ph, Bn) can be activated by Ni(0) and {NQA} and that they
can react with ethylene to provide C6F5CH2CH2SnR3 (3R)
where R = Ph, Bn. The reaction of 1R (R = Ph, Bn) with an
atmosphere of ethylene and catalytic amounts of
MeNC5H4NiPr and Ni(COD)2 provides a rare example of
carbostannylation with an aryl organostannane and an
unactivated alkene.7c Literature examples of related reactivities
have modest scope; carbostannylation7b,19 of alkynes or 1,2-
dienes for the synthesis of alkenylstannanes is largely limited to
organostannane reagents containing alkynyl,20 alkenyl,21 allyl,22
or acyl20c,23 groups. There is an example of carbostannylation
with an aryl organostannane, which undergoes a double-
insertion reaction with alkynes but requires a methoxy group in
the para position of the aryl group.24 The carbostannylation of
alkenes is more difficult and, to the best of our knowledge, has
only been observed in systems that involve strained alkenes25
or radical chain mechanisms.22a,26 Although the scope of
carbostannylation by the catalyst {NQA}/Ni(COD)2 is
currently limited, this reactivity provides an alternative method
for preparing new organostannane reagents for the Stille
coupling reaction directly from the C−Sn bond of organo-
stannane reagents. In the future, alternate ligands will be
screened to generate more selective and robust catalysts.27
This study has also determined the nature of the
intermediates involved in C−Sn bond-breaking and -making
steps in catalytic C−H bond stannylation, as well as the
reversibility of some of these steps. This insight aids the design
of better catalysts, both for this transformation and for other
transformations of C−H bonds into carbon−heteroatom
bonds.
literature procedures. The compounds H2CCHSnBn3 and
C6F5SnPh3 were prepared by procedures analogous to those for
31
32
H2CCHSnPh3 and C6F5SnBn3, respectively. Elemental analysis
and mass spectrometry were performed at the University of Windsor,
Windsor, Ontario, Canada by Dr. Janeen Auld, Instrument Technician.
Synthesis of {NQA}Ni(η2-H2CCHSnPh3)2 (2Ph). To a solution
of H2CCHSnPh3 (0.100 g, 0.265 mmol) in 10 mL of pentane were
added {NQA} (0.020 g, 0.133 mmol) and Ni(COD)2 (0.036 g, 0.133
mmol). The solution was left undisturbed for 2 h, which yielded a
yellow solid. The solid was filtered, rinsed with pentane, and then
1
dried (0.128 g, 69% yield). H NMR (C6D6, 25 °C, 500.13 MHz): δ
3
0.57 (d, 3H, CH(CH3)2, 3JHH = 6.4 Hz); 0.75 (d, 3H, CH(CH3)2, JHH
= 6.3 Hz); 1.53 (s, 3H, NCH3); 2.72 (d with Sn satellites, 1H, vinyl-
2
2
2
CH, JHH = 15.1 Hz, JHSn = 91 Hz); 2.81 (d, 1H, vinyl-CH, JHH
=
15.1 Hz); 2.97 (overlapping m, 3H, vinyl-CH, vinyl−CH and
3
CH(CH3)2); 3.45 (dd with Sn satellites, 1H, vinyl-CH, JHH = 15.2
2
3
Hz, JHH = 11.4 Hz, JHSn = 60 Hz); 3.59 (dd with Sn satellites, 1H,
3
2
3
vinyl-CH, JHH = 15.1 Hz, JHH = 11.3 Hz JHS3n = 63.0 Hz); 4.88 (d
2
with Sn satellites, 1H, C5H4N, JHH = 7.5 Hz, JHSn = 89.5 Hz); 5.02
(d, 1H, C5H4N, 2JHH = 7.5 Hz); 5.19 (d, 1H, C5H4N, 2JHH = 7.5 Hz);
2
5.77 (d, 1H, C5H4N, JHH = 7.5 Hz); 6.75 (overlapping m, 18H, Ph-
H); 7.35 (m, 12H, Ph-H). 119Sn{1H}NMR (C6D6/CD2Cl2, 25 °C,
111.96 MHz): δ −136.3 (br, coincident Sn environments) (s, 1Sn).
Anal. Calcd: C, 61.11; H, 5.23; N, 2.91. Found: C, 60.46; H, 5.33; N,
2.83.
Observation of {NQA}Ni(η2-H2CCHSnBn3)2 (2Bn). To a
solution of H2CCHSnBn3 (0.020 g, 0.048 mmol) in 1 mL of
C6D6 were added {NQA} (0.004 g, 0.024 mmol) and Ni(COD)2
1
(0.006 g, 0.024 mmol). The solution was characterized by H NMR
1
spectroscopy at room temperature after 20 min. H NMR (C6D6, 25
°C, 500.13 MHz): δ 1.24 (d, 3H, CH(CH3)2, 3JHH = 6.5 Hz); 1.27 (d,
3
3H, CH(CH3)2, JHH = 6.5 Hz); 2.00 (s, 3H, NCH3); 2.20 (s, 12H,
CH2Ph); 2.6 − 3.4 (m, 6H, vinyl-CH); 5.5 − 6.7 (m, 4H, C5H4N); 6.7
−7.2 (m, 30H, Ph-H). 119Sn{1H} NMR (C6D6, 25 °C, 111.96 MHz):
δ −54.8 (s, coincident Sn environments).
Synthesis of C6F5CH2CH2SnPh3 (3Ph). A solution of pentafluor-
obenzene (0.225 g, 1.32 mmol) and H2CCHSnPh3 (0.500 g, 1.32
mmol) in 10 mL of toluene was added to {NQA} (0.020 g, 0.132
mmol) and Ni(COD)2 (0.018 g, 0.067 mmol). The solution was
stirred at room temperature for 5 days. The reaction mixture was
filtered through silica, and the solvent was removed, leaving a colorless
powder (95% yield by 19F NMR spectroscopy, 74% isolated yield). 1H
NMR (CDCl3, 25 °C, 300.13 MHz): δ 1.66 (m with Sn satellites, 2H,
EXPERIMENTAL SECTION
■
2
SnCH2, JHSn = 53.3 Hz); 2.96 (m with Sn satellites, 2H, SnCH2CH2,
General Procedures. All reactions were performed under an
atmosphere of dry oxygen-free dinitrogen by means of standard
Schlenk or glovebox techniques. The organotin compounds prepared
can be handled in air, but the nickel catalysts and intermediates are
extremely air-sensitive. Benzene-d6 was dried by refluxing with Na/K
and was then vacuum-transferred and degassed by three freeze−
pump−thaw cycles. Toluene-d8 and CD2Cl2 were dried in an
3JHSn = 45.6 Hz); 7.25−7.32 (m, 9H, m-Ar-H and p-Ar-H); 7.44 (m
3
with Sn satellites, 6H, o-Ar-H, JHSn = 46.4 Hz). 19F{1H} NMR
(CDCl3, 25 °C, 282.40 MHz): δ −144.7 (AA′MM′ second-order m,
3
2F, 2,6-Ar-F); −158.4 (t, 1F, 4-Ar-F, JFF = 20.9 Hz); −162.7
(AA′MM′X second-order m, 2F, 3,5-Ar-F). 13C{1H} NMR (CDCl3,
25 °C, 75.47 MHz): δ 10.3 (s with Sn satellites, SnCH2, 1JC Sn = 367
119
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1
Hz, JC
= 349 Hz); 19.6 (s, SnCH2CH2); 117.5 (tm, ipso-ArF-C,
6H, Ph-H); 7.02 (m, 3H, Ph-p-H); 7.15 (m, 6H, Ph-H); 8.70 (br, 1H,
117Sn
3
4
C5H4N); 9.71 (dd, 1H, C5H4N, JHH = 7.6 Hz, JHH = 2.5 Hz). 19F
2JCF = 18.9 Hz); 128.6 (s, Ph-C); 136.8 (s, Ph-C); 137.3 (dm, ArF-C,
1JCF = 249.1 Hz); 137.7 (s, Ph-C); 139.1 (s, ipso-Ph-C); 139.2 (dm,
NMR (C6D6, 25 °C, 282.40 MHz): −109.5 (br, 2F, 2,6-Ar-F); −164.3
3
(t, 1F, 4-Ar-F, JFF = 19.3 Hz); −165.1 (br, 2F, 3,5-Ar-F). 13C{1H}
ArF-C, JCF = 267.9 Hz); 144.7 (dm, ArF-C, JCF = 244.3 Hz).
119Sn{1H} NMR (C6D6, 25 °C, 111.96 MHz): δ −103.7 (s, Sn). MS:
calcd for C26H19F5Sn, [M+ − C6H5], 469.0038; found, m/z 469.0076.
Synthesis of C6F5CH2CH2SnBn3 (3Bn). A solution of pentafluor-
obenzene (0.200 g, 1.19 mmol) and H2CCHSnBn3 (0.500 g, 1.19
mmol) in 10 mL of toluene was added to {NQA} (0.036 g, 0.24
mmol) and Ni(COD)2 (0.033 g, 0.012 mmol). The solution was
stirred at room temperature for 2 weeks. The reaction mixture was
filtered through silica, and the solvent was removed, which left a
colorless oil (81% yield by NMR spectroscopy). 1H NMR (CDCl3, 25
°C, 300.13 MHz): δ 0.89 (m with Sn satellites, 2H, SnCH2CH2, 2JHSn
= 46.8 Hz); 2.35 (s with Sn satellites, 6H, SnCH2Ph, 2JHSn = 58.5 Hz);
1
1
NMR (C6D6, 25 °C, 125.77 MHz): δ 21.1 (s, SnCH2); 24.2 (s,
CH(CH3)2); 28.1 (s, CH(CH3)2); 40.4 (s, NCH3); 41.0 (s, NCH3);
50.9 (s, CH(CH3)2); 106.9 (s, C5H4N); 117.0 (s, C5H4N); 122.2 (s, (s,
Ph-C); 135.1 (s, C5H4N); 136.0 (s, C5H4N); 146.3 (s, Ph-C); 156.7 (s,
C5H4N). 119Sn{1H} NMR (C6D6, 25 °C, 186.47 MHz): δ −236 (br,
1Sn, 1−Sn). Anal. Calcd: C, 58.86; H, 5.38; N, 6.10. Found: C, 58.97;
H, 5.55; N, 5.83.
Synthesis of trans-{NQA}2NiCl(C6F5) (5). A solution of 4Ph
(0.030 g, 0.034 mol) in 0.6 mL of CD2Cl2 decomposes after 30−40
min at room temperature or 15 min at 313 K to form complex 5 and a
1
tin byproduct. H NMR (CD2Cl2, 25 °C, 500.13 MHz): δ 0.24 (3H,
3
NCH(CH3)2); 2.56 (3H, NCH(CH3)2); 3.13 (1H, NCH); 3.50 (s,
3H, NCH3); 5.92 (1H, C5H4N); 6.78 (1H, C5H4N); 7.79 (1H,
C5H4N); 9.78 (br, 1H, C5H4N). 19F{1H} NMR (CD2Cl2, 25 °C,
2.47 (m, 2H, SnCH2CH2); 6.88 (d, 6H, o-Ar-H, JHH = 7.4 Hz); 7.04
(t, 3H, p-Ar-H, 3JHH = 7.2 Hz); 7.19 (m, 6H, m-Ar-H). 19F{1H} NMR
(CDCl3, 25 °C, 282.40 MHz): δ −145.1 (AA′MM′ second-order m,
3
3
470.54 MHz): −115.7 (AA′BB′ apparent d, 2H, o-F, JFF = 28.1 Hz);
2F, 2,6-Ar-F); −158.3 (t, 1F, 4-Ar-F, JFF = 20.8 Hz); −162.8
−163.5 (t, 1H, p-F, J = 19.6 Hz); −166.6 (AA’BB’C apparent t, 2H, m-
F, J = 23.5 Hz). 13C{1H} NMR (CD2Cl2, 25 °C, 125.77 MHz): δ 19.8
and 24.9 (s, (NCHCH3)2); 43.0 and 51.1 (s, NCH3 and CH); 107.4 (s,
C5H4N); 118.6 (s, C5H4N); 134.8 (s, C5H4N); 137.4 (s, C5H4N);
158.1 (s, C5H4N). The spectra are nearly superimposable with those of
trans-{NQA}2NiFl(C6F5),15a but without a fluoride resonance in the
19F NMR.
(AA′MM′X second-order m, 2F, 3,5-Ar-F). 13C{1H} NMR (CDCl3,
1
25 °C, 75.47 MHz): δ 10.2 (s with Sn satellites, SnCH2CH2, JCSn
=
1
279.6); 18.8 (s with Sn satellites, SnCH2Ph, JCSn(119) = 265.1 Hz,
1JCSn(117) = 253.3 Hz); 19.2 (s with Sn satellites, SnCH2CH2, JCSn
=
2
36.5 Hz); 118.2 (t, ipso-ArF-C, JCF = 20.5 Hz); 123.9 (s with Sn
satellites, Ph-C, JCSn = 15.4 Hz); 127.4 (s with Sn satellites, Ph-C, JCSn
= 23.5 Hz); 128.8 (s with Sn satellites, Ph-C, JCSn = 12.6 Hz); 137.7
2
Carbostannylation of Ethylene with C6F5SnR3 (R = Bu, Ph,
Bn). A solution of C6F5SnBu3 (0.015 g, 0.033 mmol), C6F5SnPh3
(0.017 g, 0.033 mmol), or C6F5SnBn3 (0.019 g, 0.033 mmol), {NQA}
(0.010 g, 0.067 mmol), and Ni(COD)2 (0.009 g, 0.033 mmol) in 0.6
mL of C6D6 was transferred to an NMR tube equipped with a Teflon
valve. The reaction mixture was allowed to react for 1 h to form
complexes 4Ph and 4Bn in situ. The nitrogen atmosphere was then
removed by two freeze−pump−thaw cycles, and an atmosphere of
ethylene was added. The sample was then placed in the NMR probe,
and the reaction progress was monitored by 19F{1H} NMR
spectroscopy. Slow conversion to 3Ph or 3Bn was observed for the
reactions with H2CCHSnPh3 and H2CCHSnBn3, respectively;
however, clean conversion was not observed for H2CCHSnBu3.
Reaction of Ethylene with 4Bn. A solution of 4Bn (0.020 g, 0.022
mmol) in 0.6 mL of d8-toluene was transferred to an NMR tube
equipped with a Teflon valve. The nitrogen atmosphere was then
removed by two freeze−pump−thaw cycles, and an atmosphere of
ethylene was added. The sample was then placed in the NMR probe,
and the reaction progress was monitored by 19F{1H} NMR
spectroscopy as the temperature was increased from 273 to 313 K.
Slow conversion to 3Bn was observed once the temperature reached
298 K; upon heating further to 303 K an increase in the rate of
conversion was observed while temperatures at or above 313 K caused
the catalyst to decompose.
(dm, ArF-C, JCF = 250.8 Hz); 141.2 (dm, ArF-C, JCF = 280.6 Hz);
1
1
141.6 (s, ipso-Ph-C); 144.8 (dm, ArF-C, JCF = 247.5 Hz). 119Sn{1H}
1
NMR (C6D6, 25 °C, 111.96 MHz): δ −43.0 (s, Sn). MS: calcd for
C29H25F5Sn, [M+ − CH2C6H5], 497.04; found, m/z 497.10.
Synthesis of cis-{NQA}2Ni(SnPh3)(C6F5) (4Ph). To a solution of
{NQA} (0.290 g, 1.94 mmol) and Ni(COD)2 (0.266 g, 0.97 mmol) in
20 mL of toluene was added C6F5SnPh3 (0.500 g, 0.97 mmol). The
solution was stirred for 2 h, the solvent was removed, and the residue
was washed with pentane and dried to give an orange solid (0.690 g,
79% yield). 1H NMR (CD2Cl2, −20 °C, 500.13 MHz): δ 0.83 (d, 3H,
3
3
CH(CH3)2, JHH = 7.4 Hz); 0.86 (d, 3H, CH(CH3)2, JHH = 6.6 Hz);
1.08 (br, 6H, CH(CH3)2); 1.98 (br, 1H, CH(CH3)2); 2.31 (s, 3H,
NCH3); 3.29 (m, 1H, CH(CH3)2); 3.38 (s, 3H, NCH3); 5.81 (d, 1H,
C5H4N, 3JHH = 7.3 Hz); 6.04 (d, 1H, C5H4N, 3JHH = 7.3 Hz); 6.60 (d,
1H, C5H4N, 3JHH = 7.6 Hz); 6.64 (d, 1H, C5H4N, 3JHH = 6.7 Hz); 6.88
(d, 1H, C5H4N, 3JHH = 7.0 Hz); 7.01 (m, 6H, Ph-H); 7.18 (m, 3H, Ph-
p-H); 7.23 (m, 6H, Ph-H); 7.58 (d, 1H, C5H4N, 3JHH = 6.8 Hz); 8.65
(br, 1H, C5H4N); 9.69 (br, 1H, C5H4N). 19F NMR (CD2Cl2, −20 °C,
282.40 MHz): −109.9 (br d, 1F, o-F, 3JFF = 36 Hz); −112.6 (br, 1F, o-
F); −166.2 (br, 1F, m-F); −166.6 (br t, 1F, p-F, 3JFF = 34 Hz); −167.5
(br, 1F, m-F). 13C{1H} NMR (CD2Cl2, −20 °C, 125.77 MHz): δ 14.1
(s, CH(CH3)2); 21.3 (s, CH(CH3)2); 22.9 (s, CH(CH3)2); 28.2 (s,
CH(CH3)2); 34.4 (s, CH(CH3)2 or NCH3); 43.7 (s, CH(CH3)2 or
NCH3); 43.8 (s, CH(CH3)2 or NCH3); 50.9 (s, CH(CH3)2 or NCH3);
125.4 (s, C5H4N); 126.2 (s, Ph-C); 126.9 (s with Sn satellites, Ph-C,
3JCSn = 27.6 Hz); 128.4 (s, C5H4N); 129.2 (s, C5H4N); 137.2 (s with
Product Distributions from the Reaction of H2CCHSnR3 (R
= Bu, Ph, Bn) and C6F5H. The reagents {NQA} (0.010 g, 0.067
mmol), Ni(COD)2 (0.009 g, 0.033 mmol), and H2CCHSnPh3
(0.063 g, 0.167 mmol), H2CCHSnBn3 (0.070 g, 0.167 mmol), or
H2CCHSnBu3 (0.053 g, 0.167 mmol) were weighed into a vial, 0.6
mL of a 0.278 M stock solution of C6F5H diluted with C6D6 was
added, and the components were placed into an NMR tube. The
reaction was tracked by 19F{1H} NMR spectroscopy every 30 min for
120 min at room temperature to determine the initial product ratios.
The reaction with H2CCHSnBu3 best promoted the formation of
3
Sn satellites, Ph-C, JCSn = 30.3 Hz); 147.2 (s, Ph-C); 157.7 (s,
C5H4N). 119Sn{1H} NMR (CD2Cl2, −20 °C, 186.50 MHz): δ −334
(br, 1Sn, 1−Sn). Anal. Calcd: C, 57.57; H, 4.95; N, 6.39. Found: C,
56.82;33 H, 5.35; N, 6.14.
Synthesis of cis-{NQA}2Ni(SnBn3)(C6F5) (4Bn). To a solution of
{NQA} (0.268 g, 1.79 mmol) and Ni(COD)2 (0.246 g, 0.89 mmol) in
20 mL of toluene was added C6F5SnBn3 (0.500 g, 0.89 mmol). The
solution was stirred for 2 h, the solvent was removed, and the residue
was washed with pentane and dried to give an orange solid (0.735 g,
80% yield). The solid was recrystallized in toluene at −40 °C to give
1
Bu with only trace amounts of 3Bu, approximately 22:1 by integration,
H2CCHSnBn3 had a slight preference for 1Bn over 3Bn
,
approximately 6:1 by integration, and H2CCHSnPh3 best promoted
the formation of 3Ph with only minor amounts of 1Ph, approximately
0.05:1 by integration.
1
X-ray-quality crystals. H NMR (C6D6, 25 °C, 300.13 MHz): δ 1.25
(br, 6H, CHCH3); 1.60 (br, 6H, CHCH3); 1.98 (s, 3H, CH3); 2.17 (s,
3H, CH3); 2.36 (s with Sn satellites, 6H, CH2, JHSn = 33.0 Hz); 3.31
(septet, 1H, CH, JHH = 6.6 Hz); 4.91 (br, 1H, CH); 5.43 (dd, 1H,
2
Effect of Increasing Concentration of {NQA} on Stannylation
vs Alkylation. Various amounts of {NQA} were weighed into vials:
0.005 g, 0.033 mmol; 0.010 g, 0.067 mmol; 0.020 g, 0.133 mmol; 0.080
g, 0.533 mmol. In each vial was added 0.45 mL of a 0.37 M stock
solution containing equal molar amounts of H2CCHSnBn3 and
3
3
4
C5H4N, JHH = 8.1 Hz, JHH = 2.4 Hz); 5.62 (virtual s, 2H, C5H4N);
3
4
5.69 (dd, 1H, C5H4N, JHH = 8.2 Hz, JHH = 3.1 Hz); 6.03 (d, 1H,
C5H4N, 3JHH = 7.5 Hz); 6.41 (d, 1H, C5H4N, 3JHH = 7.8 Hz); 6.91 (m,
4182
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Organometallics
Article
C6F5H, 0.2 mL of a second stock solution containing 0.167 M of
Ni(COD)2 and 0.333 M of C6H5F as an internal standard. Each
mixture was placed into an NMR tube, and the concentrations of 1Bn,
3Bn, and 4Bn, determined by integration compared to the internal
standard C6H5F, were monitored every 20 min for 80 min by 19F
NMR spectroscopy. After 80 min the equilibrium amount of 1Bn in
solution was found to be 0.071, 0.041, 0.0069, and 0 M after the
addition of 1, 2, 4, and 16 equiv of {NQA}, respectively. The rate of
formation of 4Bn was increased with increasing amounts of {NQA}
with initial relative rates of 1, 9.9, 19, and 31 M s−1 for the addition of
1, 2, 4, and 16 equiv of {NQA}, respectively. The rate of formation of
3Bn was unaffected by the addition of excess {NQA} with an average
rate of formation of 1.09 × 10−6 M s−1.
Effect of Increasing Concentration of {NQA} on Stannylation
vs Alkylation. Solid H2CCHSnBn3 was weighed into vials: 0.035 g,
0.083 mmol; 0.070 g, 0.167 mmol; 0.140 g, 0.333 mmol; 0.279 g, 0.666
mmol. To each vial was added 0.6 mL of a stock solution containing
0.11 M of {NQA} and 0.28 M C6F5H, and 0.2 mL of a second stock
solution containing 0.167 M of Ni(COD)2 and 0.333 M of C6H5F as
an internal standard, for a total volume of 0.8 mL. Each mixture was
placed into an NMR tube, and the concentrations of 1Bn, 3Bn, and 4Bn
were monitored every 20 min for 80 min by 19F NMR.
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Ph were located in the electron density map, and their locations were
refined. All other hydrogen atoms were placed in idealized positions.
ASSOCIATED CONTENT
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We acknowledge the National Science and Engineering Council
(NSERC) of Canada for a Discovery Grant for S.A.J. and a
postgraduate scholarship for M.E.D.
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