A mechanistic investigation of carbon-hydrogen bond stannylation: Synthesis and characterization of nickel catalysts

Article abstract of DOI:10.1039/c2dt30310h

The complex (ⁱPr₃P)Ni(η²-Bu ₃SnCHCH₂)₂ (1a) was characterized by NMR spectroscopy and was identified as the active species for catalytic C-H bond stannylation of partially fluorinated aromatics, for example in the reaction between pentafluorobenzene and Bu₃SnCHCH₂, which generates C₆F₅SnBu₃ and ethylene. The crystalline complex (ⁱPr₃P)Ni(η²-Ph ₃SnCHCH₂)₂ (1b) provides a more easily handled analogue, and is also capable of catalytic stannylation with added Ph ₃SnCHCH₂ and C₆F₅H. Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of catalytic stannylation is proportional to [C₆F ₅H] and inversely proportional to [Ph₃SnCHCH₂]. This is consistent with a mechanism where reversible Ph₃SnCHCH ₂ dissociation provides (ⁱPr₃P) Ni(η²-Ph₃SnCHCH₂), followed by a rate-determining reaction with C₆F₅H to generate the stannylation products. Kinetic competition reactions between the fluorinated aromatics pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,5- tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene and 1,3-difluorobenzene all suggest significant Ni-aryl bond formation in the rate-determining step under catalytic conditions. Labelling studies are consistent with an insertion of the hydrogen of the arene into the vinyl group, followed by β-elimination or β-abstraction of the SnPh₃ moiety.

Full text of DOI:10.1039/c2dt30310h

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PAPER
A mechanistic investigation of carbon–hydrogen bond stannylation: synthesis
and characterization of nickel catalysts†
Samuel A. Johnson,* Meghan E. Doster, Jacob Matthews, Manar Shoshani, Michelle Thibodeau,
Amanda Labadie and Jillian A. Hatnean
Received 10th February 2012, Accepted 9th March 2012
DOI: 10.1039/c2dt30310h
The complex (iPr3P)Ni(η2-Bu3SnCHvCH2)2 (1a) was characterized by NMR spectroscopy and was
identified as the active species for catalytic C–H bond stannylation of partially fluorinated aromatics, for
example in the reaction between pentafluorobenzene and Bu3SnCHvCH2, which generates C6F5SnBu3
and ethylene. The crystalline complex (iPr3P)Ni(η2-Ph3SnCHvCH2)2 (1b) provides a more easily
handled analogue, and is also capable of catalytic stannylation with added Ph3SnCHvCH2 and C6F5H.
Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of
catalytic stannylation is proportional to [C6F5H] and inversely proportional to [Ph3SnCHvCH2]. This is
consistent with a mechanism where reversible Ph3SnCHvCH2 dissociation provides (iPr3P)Ni(η2-
Ph3SnCHvCH2), followed by a rate-determining reaction with C6F5H to generate the stannylation
products. Kinetic competition reactions between the fluorinated aromatics pentafluorobenzene, 1,2,4,5-
tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene and 1,3-
difluorobenzene all suggest significant Ni–aryl bond formation in the rate-determining step under catalytic
conditions. Labelling studies are consistent with an insertion of the hydrogen of the arene into the vinyl
group, followed by β-elimination or β-abstraction of the SnPh3 moiety.
This catalytic functionalization is of interest for several
Introduction
reasons. The products provide useful reagents for Stille coupling
reactions.7 Although the catalytic borylation of C–H bonds have
Over the past decades, catalytic C–H bond functionalization has
seen extensive development over the past two decades,8,9 and
undergone significant developments as a practical, economical
and green synthetic approach.1 Our research has targeted
these C–B bond containing compounds are of utility to synthetic
chemists due to their use in Suzuki coupling reactions,10 similar
methods to utilize available and cost effective Ni complexes for
C–H activation in place of more expensive noble metal com-
plexes (e.g. Pt, Ir, Rh, Au) that are commonly used in catalytic
C–H functionalization reactions. Although Ni complexes have
been suggested as better suited for selective C–F activation2 for
thermodynamic reasons,3 Ni complexes are finding increasing
use in the catalytic transformation of C–H bonds.4 We have
found that partially fluorinated arenes and pyridines can undergo
oxidative addition of their C–H bonds to Ni(0) phosphine com-
plexes, which suggests that these complexes should be capable
of catalytic C–H bond functionalization.5 Previously, we com-
municated that the reaction of partially fluorinated arenes with
Bu3SnCHvCH2 resulted in catalytic C–H bond stannylation
with the loss of ethylene gas, as shown in Scheme 1.6 The reac-
tion was catalysed by a combination of Ni(COD)2 and either the
nitrogen donor MeNC5H4NiPr2a or a phosphine donor.
reactions to form other important carbon–heteroatom bonds are
lacking. To the best of our knowledge this is the first example of
catalytic C–H bond stannylation. Equally intriguing is the mech-
anism of the conversion. Typical C–H borylation reactions occur
with reagents that contain B–H or B–B bonds. The transform-
ation of the Sn–C bond in Scheme 1 to form a new Sn–C in the
product with the loss of an ethylene bond provides a unique
mechanism of C–H bond functionalization. Although this reac-
tion is currently limited to activated aromatics, for example fluor-
obenzene has been found to react but not benzene, insight into
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, ON, Canada. E-mail: sjohnson@uwindsor.ca;
Fax: +1 519 973 7098; Tel: +1 519 253 3000
†CCDC 866976. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt30310h
Scheme 1
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Scheme 2
Fig. 1 ORTEP of complex 1b, shown with 50% thermal ellipsoid para-
meters. Hydrogen atoms are omitted, and only the ipso carbons of the
aromatic rings are shown for clarity. Selected bond distances (Å) and
angles (°): Ni(1)–C(2), 1.984(7); Ni(1)–C(4), 1.986(7); Ni(1)–C(1),
1.989(7); Ni(1)–C(3), 1.998(8); Ni(1)–P(1), 2.203(2); C(1)–C(2), 1.387
(11); C(3)–C(4), 1.369(12); C(2)–Ni(1)–C(4), 171.0(4); C(2)–Ni(1)–C
(3), 131.4(4); C(2)–Ni(1)–P(1), 94.7(2); C(4)–Ni(1)–P(1), 94.0(3); C
(1)–Ni(1)–P(1), 135.2(2); C(3)–Ni(1)–P(1), 133.8(3).
the mechanism may allow for the design of catalysts capable of
the stannylation of a broader scope of substrates. Similarly,
knowledge of the reaction mechanism may allow for the design
of catalysts capable of converting C–H bonds to other carbon–
heteroatom bonds, such as C–Si bonds. In our previously pub-
lished work, no catalytically active species were identified. In
this work, we describe isolable species that perform catalytic
C–H bond stannylation, and investigate the mechanism of this
reaction.
Results and discussion
The resting state of nickel C–H bond stannylation catalysts
The reaction of Ni(COD)2 with one equivalent of triisopropyl-
phosphine and two equivalents of Bu3SnCHvCH2 provided the
species (iPr3P)Ni(η2-Bu3SnCHvCH2)2 (1a), as shown in
Scheme 2. The 31P{1H} NMR spectrum displays a resonance at
δ 50.2 with 119Sn/117Sn satellite peaks (3JSnP = 29.7 Hz)
of appropriate intensities for two coordinated Bu3SnCHvCH2
moieties (117Sn, 7.7%, and 119Sn, 8.6% abundant, both I = 1/2).
The different couplings to 117Sn and 119Sn were not resolved
owing to the line-widths and modest difference in gyromagnetic
ratios between these isotopes. The 119Sn{1H} NMR spectrum
displayed the expected doublet from coupling to a single phos-
Scheme 3
displayed a signal at δ 49.8 with satellites separated by 32.3 Hz
due to coupling to two equivalent Sn nuclei, and the 119Sn NMR
spectrum displayed a doublet at δ −122.0 with the same coup-
ling constant. The H NMR spectrum displayed diastereotopic
methyl groups on the Pr3P donor, consistent with the lack of a
mirror plane of symmetry in 1b, and featured coordinated vinyl
moiety environments at δ 3.00, 3.09 and 4.11. The NMR para-
meters are all comparable to those for 1a.
1
i
1
phorus nucleus. The intensities in the H NMR spectrum were
consistent with the proposed formulation, and featured the
chemical shifts for the coordinated vinyl moieties shifted several
ppm upfield relative to free Bu3SnCHvCH2.
Stoichiometric stannylation using 1b
Although 1a was an isolable air-sensitive oil, it proved
impossible to crystallize, so characterization was limited to mul-
tinuclear NMR spectroscopy. We chose to investigate
Ph3SnCHvCH2 as a reagent that could provide a crystalline and
easily handled stannylation catalyst that was more amenable to
mechanistic studies. The reaction of Ni(COD)2 with one equival-
The addition of C6F5H to solutions of 1b in C6D6, shown in
Scheme 3, provided conversion to the C–H activation product
C6F5SnPh3, as monitored by 19F and 119Sn{1H} NMR spectro-
scopy. The reaction proceeds slowly at room temperature under
the conditions used. Two additional nickel-containing products
i
1
ent of Pr3P and two equivalents of Ph3SnCHvCH2 provided
were readily identified from a combination of 31P{1H}, H and
the complex, (iPr3P)Ni(η2-Ph3SnCHvCH2)2 (1b), which was
crystallized from toluene at −40 °C. An ORTEP of the solid-
state molecular structure as determined by X-ray crystallography
is shown in Fig. 1. The structure is as expected, with η2-coordi-
nated Ph3SnCHvCH2 groups. The two Ph3Sn substituents
arrange themselves so that they are far away from the bulky
PiPr3 donor, and on opposite sides of the Ni coordination plane
to best avoid each other, which gives a complex with pseudo-C2
symmetry.
119Sn{1H} NMR spectroscopy. Early in the reaction, a product
assigned as (PiPr3)Ni(η2-Bu3SnCHvCH2)(η2-C2H4) (2) was
observed, with a 31P{1H} shift of δ 50.9 and satellites with a
25.2 Hz separation and intensities consistent with coupling to a
single Sn environment. The 119Sn{1H} NMR spectrum of 2 fea-
tures a doublet at −109.2, with a JPSn of 25.2 Hz, which
confirms a single phosphine is coordinated to the metal centre.
1
The H NMR spectrum features diastereotopic Me groups from
the iPr3P moiety and three distinctive multiplets from the coordi-
nated vinyl moiety at δ 3.22, 3.03 and 2.78 that integrate to 1H
each. A pair of second-order multiplets at δ 2.94 and 2.66
The NMR spectra of 1b in C6D6 displayed resonances consist-
ent with the solid-state structure. The 31P{1H} NMR spectrum
8136 | Dalton Trans., 2012, 41, 8135–8143
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assigned as the coordinated ethylene moiety integrate to 2H
environments each, consistent with rapid rotation about the
Ni–(η2-C2H4) bond at room temperature, which exchanges only
the trans-disposed hydrogen environments. Before 1b is fully
consumed to form 2, the reaction of 1b with C6F5H also gener-
ates (PiPr3)Ni(η2-C2H4)2 (3), presumably either by reaction of 2
with C6F5H or from ligand redistribution between two equiva-
lents of 2. This known11 complex was identified by its distinctive
1H NMR spectrum, which features a single iPr3P methyl
environment, a singlet at δ 2.73 for the coordinated C2H4
moiety, and a singlet in the 31P{1H} NMR spectrum at δ 52.5.
Unfortunately, no further intermediates were observed, to
provide insight into the mechanism of C–Sn bond formation.
The compositions of 2 and 3, which could not be isolated from
these reaction mixtures, were further confirmed by an alternate
synthesis. Ethylene was added to a C6D6 solution of 1b in an
NMR tube, followed by warming the sealed tube to 50 °C in an
NMR probe, as shown on the bottom of Scheme 3. This reaction
provided equilibrium amounts of 2 and 3, as analysed by multi-
nuclear NMR spectroscopy. The reverse reaction, the replace-
ment of coordinated ethylene in 2 and 3 by Ph3SnCHvCH2,
regenerated 1b from 2 and 3, and thus allows the catalytic stan-
nylation of C–H bonds with vinyltin reagents.
Fig. 2 Rate of formation of C6F5SnPh3 versus the concentration of the
catalyst (iPr3P)Ni(η2-Ph3SnCHvCH2)2 (1b) in the catalytic stannylation
of C6F5H with Ph3SnCHvCH2.
Rate law and mechanism of catalytic stannylation
The failure to observe any additional intermediates in the stoi-
chiometric reaction of 1b with C6F5H provides little additional
insight into the mechanism of this unusual C–H functionaliza-
tion reaction. The effect of the concentration of nickel catalyst,
C6F5H, and Ph3SnCHvCH2 was examined in an attempt to
determine the rate law. The determination of exact rate constants
in these systems was complicated by small amounts of Ni metal
precipitate over the course of reactions, and the multiple com-
ponents sometimes observed in solution (e.g. 1b and 2);
however, experimentally it proved possible to generate reprodu-
cible and informative rate data.
Scheme 4
A stock solution in toluene with concentrations of 0.0052 M of
catalyst 1b and 0.212 M Ph3SnCHvCH2 was used to make five
solutions with concentrations of C6F5H ranging from 0.123 to
1.97 M. These were transferred to an NMR probe preheated to
338 K and the initial catalytic reaction rates were monitored. The
reaction rate was found to be linearly proportional to the C6F5H
concentration. A similar experiment where a stock solution with
constant catalyst 1b concentration (0.0052 M) and C6F5H con-
centration (0.476 M) was used to make five solutions with differ-
ent Ph3SnCHvCH2 concentrations (0.053, 0.094, 0.218, 0.507,
and 1.13 M) showed that reaction rate is inversely proportional
to the concentration of Ph3SnCHvCH2. With the lowest concen-
tration of Ph3SnCHvCH2 used (0.053 M) the initial reaction
rate did not remain constant over the course of minutes, but
instead slowed rapidly until the reaction was complete after only
20 min. The precipitation of a visibly large amount of nickel
suggests that the catalyst is not stable with low Ph3SnCHvCH2
concentrations at the temperature used.
The reaction kinetics are suggestive of the mechanism shown
in Scheme 4. The initial step is a reversible dissociation of one
of the two Ph3SnCHvCH2 moieties from 1b to generate the
unobserved species (iPr3P)Ni(η2-Ph3SnCHvCH2) (4) with
forward rate constant k1 and reverse rate constant k−1. This step
is consistent with the reduction in reaction rate upon increased
Ph3SnCHvCH2 concentration. The rate-determining step is a
reaction between 4 and C6F5H, with rate constant k2. Though
Given the fact that Ni complexes sometimes undergo reactions
to form dinuclear complexes that perform transformations invol-
ving both C–C bond formation12 and C–H activation,5f,13 we
chose to initially verify that the active catalyst remains mono-
nuclear during the rate determining steps of the reaction. By
using a stock solution of 0.172 M Ph3SnCHvCH2 and 0.177 M
1
C6F5H with both a 19F and H NMR internal standard, different
masses of the catalyst (iPr3P)Ni(η2-Ph3SnCHvCH2) 1b were
added to 0.6 mL aliquots to give approximate catalyst concen-
trations of 0.005 M, 0.01 M, 0.02 M, 0.04 M and 0.08 M,
respectively. These solutions do not react appreciably at room
temperature, and transferring to an NMR probe preheated to
338 K provided a convenient means to monitor the initial reac-
tion rates, which remained constant for several minutes under
these conditions. The results clearly show a linear correlation
between reaction rate and catalyst concentration, as shown in
Fig. 2. This supports a mononuclear nickel complex as the active
species during the rate determining steps of catalytic stannyla-
tion. This data also suggests that metallic nickel or nickel nano-
particles are not the active species.
Similar experiments were performed to determine the effect
C6F5H and Ph3SnCHvCH2 concentration have on reaction rate.
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Table 1 Relative reaction rates compared to pentafluorobenzene in
competition experiments with stannylation using Ph3SnCHvCH2 and
catalyst 1b
Relative
rate
ΔΔG‡
ΔΔH‡
Substrate
C6F5H
(kcal mol−1
)
Equiv. H (kcal mol−1
)
1.00
0.00
0.05
0.77
2.05
2.18
4.13
1
2
2
1
3
1
0.00
0.51
1.24
2.05
2.92
4.13
1,2,4,5-C6F4H2 0.93
1,2,3,5-C6F4H2 0.32
1,2,4-C6F3H3
1,3,5-C6F3H3
1,3-C6F2H4
0.047
0.039
0.0021
Fig. 3 Rate of C6F5SnPh3 formation versus [1b][C6F5H]/
[Ph3SnCHvCH2] at 338 K for the catalytic stannylation of C6F5H with
Ph3SnCHvCH2, shown for nine different sets of concentrations for the
reagents but identical catalyst concentration, [1b]. The solid line is a
least squares linear fit.
From DFT calculations, it was hypothesized that this is due to a
mechanism where C–H activation and insertion occur in one
step,14 although this analysis does not explain the similarly low
KIE that was reported for the oxidative addition of 1,2,4,5-
tetrafluorobenzene-d1 to an (iPr3P)2Ni synthon at 298 K.5a The
KIE of 2.0 that we report here supports a typical oxidative
addition process, though ligand assistance of this process by a bar-
rierless insertion into the vinyl group step cannot be discounted.
more than one step may be required to reach 5 from 4, little
insight into these steps are provided from the rate data. Complex
5 should readily associate Ph3SnCHvCH2 to generate 2, which
can then lose ethylene to form 4, and is in equilibrium with the
favoured species 1b, but 5 is a speculative intermediate, and
alternate pathways where the ethylene moiety is lost prior to
C–Sn bond formation are viable. An alternate mechanism where
arene coordination precedes vinyl dissociation also cannot be
discounted.
ð3Þ
The rate law given in eqn (1) can be derived using a steady-
state approximation for the concentration of intermediate 4.
Under catalytic conditions, a simpler rate law can be derived pro-
viding that k2[C6F5H] is much less than k−1[Ph3SnCHvCH2],
and is shown in eqn (2). The rate law is consistent with a rapid
pre-equilibrium formation of 4, followed by rate determining
reaction with C6F5H. As expected, a plot of reaction rate versus
[1b][C6F5H]/[Ph3SnCHvCH2] for the catalytic data provided
a linear plot as shown in Fig. 3. The observed rate constant,
(k1/k−1) × k2, can be estimated as 0.0016(2) s–1 at 338 K from
these data.
Further support for significant formation of a metal–carbon
bond in the C–H bond cleaving step was obtained from a com-
parison of reaction rate with different fluorinated arenes contain-
ing two ortho-F substituents. The rates of reaction relative to
pentafluorobenzene for the substrates 1,2,4,5-tetrafluorobenzene,
1,2,3,5-tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluoro-
benzene and 1,3-difluorobenzene are shown in Table 1. The rela-
tive rates were obtained by competition studies between these
substrates at 338 K. The ratio of initial products can be used to
generate a difference in Gibbs free energy of activation, ΔΔG‡,
for these substrates relative to pentafluorobenzene. An estimated
difference in enthalpy of activation, ΔΔH‡, can be obtained by
correcting for the statistical increase in activation that occurs due
to the presence of multiple identical sites of activation. Previous
computational studies allow for an estimate of aryl–H and aryl–
Ni bond dissociation energies.15 The ΔΔH‡ values correlate well
with the difference between aryl–H and aryl–Ni bond strengths,
as shown in Fig. 4. Both the relative enthalpies and relative bond
dissociation energy differences are with respect to pentafluoro-
benzene, the most reactive fluorinated benzene. There are several
interesting trends observed from this plot. The first is that there is
a clear correlation between the ΔΔH‡ and the difference between
predicted C–H and C–Ni bond strengths for these substrates.
Secondly, there is a clear effect of meta-substitution, where the
substrates with more meta-fluorine substituents react faster than
similar substrates with para-fluorine substituents, and this corre-
lates with the greater importance of meta-F substituents towards
Ni–C bond strength than C–H bond strength. Interestingly, the
plot has an initial slope of ∼1 for the substrates 1,2,4,5- and
1,2,3,5-tetrafluorobenzene, suggestive of a transition state where
Ni–C bond formation and C–H bond cleavage is substantial.
Steady-state rate law:
d½C6F5SnPh3ꢀ
dt
k1k2½1bꢀ½C6F5Hꢀ
¼
ð1Þ
k
ꢁ1
½Ph3SnCHvCH2ꢀ þ k2½C6F5Hꢀ
Under catalytic conditions, where
d½C6F5SnPh3ꢀ
dt
k1k2½1bꢀ½C6F5Hꢀ
½Ph3SnCHvCH2ꢀ
¼
ð2Þ
k
ꢁ1
Various studies were performed to gain insight into the nature
of the step that cleaves the arene C–H bond in the catalytic
stannylation reaction. The catalytic reaction of 1b with 1,2,4,5-
tetrafluorobenzene-d1 provided an intramolecular kinetic isotope
effect (KIE) of 2.0 for the catalytic conversion to Ph3Sn–2,3,5,6-
C6F4D and to Ph3Sn–2,3,5,6-C6F4H, as shown in eqn (3). This
is similar to the equilibrium isotope effect we previously
reported for the reversible C–H/D activation of 1,2,4,5-tetrafluoro-
benzene-d1 to a (Et3P)2Ni synthon.5c In contrast, the seemingly
closely related alkenylation of the C–H bond in para-
MeOC6F4H using a 3% loading of a catalyst obtained from Ni
(COD)2 and P(Cyp)3 has been reported to have a KIE of 1.0.4f
8138 | Dalton Trans., 2012, 41, 8135–8143
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Fig. 4 Plot of the estimated difference in relative Ni–aryl vs. H–aryl
bond dissociation energies, ΔDC–H–ΔDNi–C, versus the difference in
enthalpy of activation for catalytic stannylation, ΔΔH‡, determined from
competition reactions between a series of fluorinated substrates. The
relative ΔD value and ΔΔH‡ values are both with respect to
pentafluorobenzene.
Scheme 5
For the less fluorinated substrates the ΔΔH‡ values increase
faster than the difference in dissociation energies between the C–
H and Ni–C bonds, perhaps indicative of an earlier transition
state with less Ni–C bond formation.
trans-propene-d1 is suggestive of an insertion pathway as illus-
trated in Scheme 5. The first step proceeding from the binding of
the pentafluorobenzene may involve distinct oxidative addition
and insertion step, or this may be ligand assisted, with no barrier
to insertion upon C–H bond breaking. Either β-elimination or
β-abstraction could conceivably lead to the product. Future com-
putational studies may provide greater detail to the exact mech-
anism of this reaction during and after the rate-determining step.
The reaction of pentafluorobenzene-d1 and cis-1-propenyl-tri-
butyltin catalysed by 1b produced primarily trans-propene-d1, as
1
analysed by H and 13C{1H} NMR spectroscopy and shown in
eqn (4); this was incorrectly assigned previously6 due to a data-
base error in the assignment of the 1H NMR spectrum of
1
propene.16 The H NMR spectrum of the reaction mixture fea-
tures two resonances for trans-propene-d1. The hydrogen
attached to C-1 that is cis to the methyl group was at δ 4.99,
with a 17.0 Hz coupling to C-2 hydrogen, and a 1.7 Hz quartet
coupling to the methyl group. The C-2 hydrogen geminal to the
methyl group further revealed the location of the deuterium
label, with a 1.5 Hz cis HD coupling providing a small 1 : 1 : 1
triplet splitting of the 17 Hz doublet and 6.4 Hz quartet splitting.
The 13C{1H} NMR spectrum was also definitive regarding the
Conclusions
Experimental evidence shows that monophosphine nickel com-
plexes such as (iPr3P)Ni(η2-Bu3SnCHvCH2)2 (1a) and (iPr3P)
Ni(η2-Ph3SnCHvCH2)2 (1b) are the active catalysts for the cata-
lytic stannylation of partially fluorinated aromatics such as pen-
tafluorobenzene. Complex 1b is a solid which allowed for facile
handling and thus was amenable to mechanistic studies. The
observed kinetic data is consistent with a mononuclear nickel
complex throughout the key steps of the catalytic cycle, and with
a dissociative step with the loss of a Ph3SnCHvCH2 moiety
prior to reaction with C6F5H. The Sn–C bond forming step
appears to occur via β-elimination or β-abstraction after hydro-
gen insertion into the vinyltin moiety. Competition studies
suggest that the rate determining step occurs with significant
metal–aryl bond formation with highly fluorinated aromatics.
Further studies are needed to better understand the importance of
stannyl substituents and ancillary donor choice on both catalyst
thermal stability and reaction rate.
1
carbon to which the deuterium is attached. Only C-1 has a JCD
coupling, which was a 1 : 1 : 1 triplet with a distinctive 24.5 Hz
coupling at δ 115.6. The C-2 carbon was a singlet at δ 133.6.
Reaction with a mixture of pentafluorobenzene-d1 and a mixture
of cis- and trans-1-propenyl-tributyltin catalysed by 1b provided
both cis and trans-propene-d1, which confirmed that the
single isotopomer of propene-d1 obtained using cis-1-propenyl-
tributyltin is not due to initial isomerization of the vinyl reagent.
ð4Þ
Experimental
General procedures
The relative rates of the different fluorinated substrates, as well
as the observed KIE with 1,2,4,5-C6F4HD, are consistent with a
mechanism where the stannylation reaction has a step where
both aryl–H bond breaking and Ni–C bond formation to the
fluoroaryl moiety occurs in a concerted manner, leading to oxi-
dative addition. We had previously reported that labelling studies
suggested a σ-bond metathesis pathway,6 but the reassignment as
Unless otherwise stated, all manipulations were performed under
an inert atmosphere of nitrogen using either standard Schlenk
techniques or an MBraun glovebox. Benzene-d6 was dried by
refluxing with Na/K and was then vacuum transferred and
degassed by three freeze–pump–thaw cycles. All other solvents
were purchased anhydrous from Aldrich and further purified
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3
using a Grubbs’ type column system17 produced by Innovative
Technology. 1H, 13C{1H}, 19F{1H}, 31P{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
1P, JPSn = 30.7 Hz). 119Sn{1H} (toluene, 25 °C, 111.96 MHz):
3
δ −122.0 (d, 1Sn, JSnP = 32.4 Hz). Anal. Calcd for
C49H57NiPSn2: C, 60.48; H 5.90. Found: C, 60.29; H 5.79%.
1
to proton nuclei. H NMR spectra were referenced to residual
Stoichiometric stannylation of C6F5H with 1b
protons (C6D6, δ 7.15) with respect to tetramethylsilane at δ
0.00. 13C{1H} spectra were referenced relative to solvent reson-
ances (C6D6, δ 128.0). 19F{1H} NMR spectra were referenced to
an external sample of 80% CCl3F in CDCl3 at δ 0.0. 31P{1H}
NMR spectra were referenced to an external sample of phospho-
ric acid at δ 0.0. 119Sn{1H} NMR spectra were referenced to an
external sample of SnMe4 at δ 0.0. C6D6 and toluene-d8 was pur-
chased from Aldrich. The compounds pentafluorobenzene, PiPr3,
tributyl(vinyl)tin, and triphenyltin chloride were purchased from
Aldrich. The compounds cis-tributyl(1-propenyl) tin, cis/trans-
tributyl(1-propenyl) tin, bromo-2,3,4,5,6-pentafluorobenzene,
bromo-2,3,5,6-tetrafluorobenzene and Bn3SnCl were purchased
from Alfa Aesar. The compounds Ni(COD)2,18 C6F5D,19
C6F4HD,20 Ph3Sn(vinyl)21 were prepared by literature pro-
cedures. Elemental analyses were conducted at the Centre for
Catalysis and Materials Research at the University of Windsor.
The addition of 40 mg C6F5H to 40 mg 1b in benzene-d6 was
monitored by 1H, 19F and 119Sn NMR spectroscopy. The product
C6F5SnPh3 and complexes 2 and 3 were the only species
observed. The assignment was confirmed by consistency with
the reaction products observed in the reaction of 1b and
ethylene, provided below.
Catalysis and characterization of C6F5SnPh3
A solution of pentafluorobenzene (0.052 g, 0.309 mmol) in
C6D6 was added to a mixture of triphenyl(vinyl)tin (0.039 g,
0.103 mmol) and (iPr3P)Ni[η2-Ph3SnCHvCH2]2 (0.003 g,
0.0031 mmol). The solution was heated at 338 K for 0.5 h to
allow the reaction to go to completion (95% yield by NMR spec-
troscopy). 1H NMR (C6D6, 65 °C, 300.13 MHz): δ 7.28 (m,
2
17H, Ar-H); 7.70 (m with Sn satellites, 8H, Ar-H, JHSn = 55.0
Hz). 19F{1H} NMR (C6D6, 65 °C, 282.40 MHz): δ −118.6 (AA′
MM′N second order with Sn satellites, 2F, 2,6-Ar-F); −151.0 (tt
Synthesis of (iPr3P)Ni[η2-Bu3SnCHvCH2]2 (1a)
3
4
A toluene solution of Ni(COD)2 (500 mg, 1.82 mmol), PiPr3
(291.3 mg, 1.82 mmol), and Bu3SnCHvCH2 (1.15 g,
3.64 mmol, 2 equiv) were reacted immediately at room temper-
ature. The solvent was removed under vacuum leaving an oil.
The resultant oil was identified by multinuclear NMR spec-
troscopy to be (PiPr3)Ni(η2-C2H3SnBu3)2, 1a. 1H NMR
(toluene-d8, 25 °C, 300.13 MHz): δ 0.93 (overlapping m, 30H,
with Sn satellites, 1F, 4-Ar-F, JFF = 19.6 Hz, JFF = 2.7 Hz);
−159.7 (AA′MM′N second order, 2F, 3,5-Ar-F). 119Sn{1H}
NMR (C6D6, 25 °C, 111.96 MHz): δ −137.9 (m).
Reaction of 1b with ethylene
A solution of 1b (40 mg) in benzene-d6 was transferred into an
NMR tube equipped with a Teflon valve. The nitrogen atmos-
phere was removed by two freeze–pump–thaw cycles, and an
atmosphere of ethylene was added. The sample was heated at
50 °C for 30 minutes. The probe was then cooled to 25 °C and
spectra were collected consistent with 2 and 3.
3
SnCH3 and SnCH2CH2CH2, JHH = 7.3 Hz); 0.97 and 0.99 (d,
3
18H, Pr-CH3, JHH = 7 Hz); 1.39 (m with Sn satellites, 12H,
3
3
SnCH2, JHSn = 60.4 Hz, JHH = 7.3 Hz); 1.51 (m, 3H, Pr-CH);
1.62 (m, 12H, SnCH2CH2); 2.80 (dd with Sn satellites, 2H,
vinyl-CH, 3JHH = 6.6 Hz, 3JHH = 2.3 Hz, 3JHSn = 62.1 Hz); 2.86
(m, 2H, vinyl-CH); 3.35 (ddd with Sn satellites, 2H, vinyl-CH,
2JPH = 15.3 Hz, 3JHH = 6.3 Hz, 3JHH = 4.3 Hz, 2JHSn = 67.8 Hz).
31P{1H} NMR (toluene-d8, 25 °C, 121.5 MHz): δ 50.2 (s with
Sn satellites, 1P, 3JPSn = 29.7 Hz). 119Sn{1H} (toluene-d8, 25 °C,
111.96 MHz): δ −35.7 (d, 1Sn, 3JSnP = 30.3 Hz).
Reaction of C6F5D, Bu3Sn(cis,trans-propenyl), and 5% catalyst
loading (iPr3P)Ni(η2-Ph3SnCHvCH2)
A solution of C6F5D (0.046 g, 0.2718 mmol), Bu3Sn(cis,trans-
propenyl) (0.090 g, 0.2718 mmol) in 1 mL of C6D6 was mixed
with (iPr3P)Ni(η2-Ph3SnCHvCH2) (0.013 g, 0.0136 mmol) and
placed in a preheated NMR probe at 338 K. After 20 min the 1H
NMR spectrum was used to confirm that both cis-propene-d1
and trans-propene-d1 are produced in equal amounts.
Synthesis of (iPr3P)Ni[η2-Ph3SnCHvCH2]2, (1b)
To a solution of triphenylvinyl tin (0.719 g, 1.90 mmol) in
15 mL of toluene was added PiPr3 (0.262 g, 0.95 mmol) and Ni
(COD)2 (0.153 g, 0.95 mmol). The solution was stirred for
30 min and the solvent was removed, leaving a yellow solid.
Characterization of (iPr3P)Ni(η2-Ph3SnCHvCH2)(η2-C2H4), (2)
1H NMR (C6D6, 25 °C, 500.13 MHz): δ 0.85 (dd, 9H, CH3,
1
(0.898 g, 97% yield). H NMR (C6D6, 25 °C, 500.13 MHz): δ
3
3
0.64 (dd, 9H, CH3, JHH = 7.1 Hz, JHP = 12.3 Hz); 0.77 (dd,
3
3
3
2
9H, CH3, JHH = 7.1 Hz, JHP = 12.3 Hz); 1.90 (d septet, 3H,
2JHH = 7 Hz, JHP = 12.5 Hz); 0.90 (dd, 9H, CH3, JHH = 7 Hz,
3JHP = 12.5 Hz); 2.00 (m, 3H, CH); 2.70 (2nd order m, 2H,
CHH on η2 ethylene); 2.97 (2nd order m, 2H, CHH on η2
2
3
CH, JHP = 7.2 Hz, JHH = 6.9 Hz); 3.00 (dd with Sn satellites,
2H, vinyl-CH, 2JHH = 11.7 Hz, 3JHH = 4.1 Hz, JHSn = 151 Hz);
2
3
2
3
2
3.09 (dd, 2H, vinyl-CH, JHH = 14.8 Hz, JHH = 9.4 Hz); 4.11
ethylene); 2.79 (dd, 1H, vinyl-CH, JHH = 15.5 Hz, JHH = 6.5
2
3
3
2
(ddd with Sn satellites, 2H, vinyl-CH, JHH = 14.5 Hz, JHH
=
Hz); 3.06 (dd, 1H, vinyl-CH, JHH = 12 Hz, JHH = 6.5 Hz);
12.0 Hz, 3JHH = 3.0 Hz, 3JHSn = 61.6 Hz); 7.15 (m, 12H, Ar-H);
7.6 (m with Sn satellites, 12H, Ar-H, 3JHSn = 45.2 Hz). 31P{1H}
NMR (C6D6, 25 °C, 202.46 MHz): δ 49.8 (s with Sn satellites,
3.25 (ddd, 1H, vinyl-CH, JHH = 12 Hz, JHH = 15.5 Hz, 3JHP
=
3
3
3.5 Hz); 7.19 (m, H, Ar-H); 7.60 (m, 2H, Ar-H); 7.78 (m with
Sn satellites, 2H, Ar-H, 3JHSn = 44.5 Hz). 31P{1H} NMR (C6D6,
8140 | Dalton Trans., 2012, 41, 8135–8143
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3
25 °C, 121.54 MHz): δ 50.89 (s with Sn satellites, 1P, JSnP
=
observed reaction rates were 3.64 × 10–6, 6.76 × 10–6, 1.68 ×
10–5, 2.72 × 10–5, and 7.64 × 10–5 M s−1, respectively.
24.3Hz). 119Sn{1H} (C6D6, 25 °C, 186.48 MHz): δ −109.8 (d,
1Sn, 3JSnP = 24.9 Hz).
Catalytic reaction rate versus [Ph3SnCHvCH2]
Characterization of (iPr3P)Ni(η2-C2H4)2, (3)
A stock solution of the reagent Ph3SnCHvCH2 (100 mg,
0.265 mmol), the reagent C6F5H (400 mg, 2.38 mmol) the cata-
lyst (iPr3P)Ni(η2-Ph3SnCHvCH2)2 25 mg, 0.026 mmol) and
the internal standard C6H5F (100 mg, 1.04 mmol) was dissolved
in 5 mL toluene, to provide a solution that was 0.053 M, 0.476
M 0.0052 M and 0.208 M in these four components, respect-
ively. Five NMR tubes were loaded with 0, 10, 30, 70 and
162 mg of Ph3SnCHvCH2, and the stock solution was added to
dilute the solution to a total volume of 0.64 mL. No reaction was
observed at room temperature. The samples were introduced into
an NMR spectrometer probe preheated to 338 K, and the rate of
production of concentration of C6F5SnPh3 was monitored versus
time by 19F NMR spectroscopy. The slope of this plot was found
to be linear for extended periods, and the slope was recorded as
the reaction rate. The initial concentrations of Ph3SnCHvCH2
for the five separate samples are calculated to be 0.053, 0.094,
0.177, 0.342, and 0.722 M, and the observed reaction rates were
1.18 × 10–4, 4.39 × 10–5, 1.80 × 10–5, 7.45 × 10–6, and 3.12 ×
10–6 M s−1, respectively.
1H NMR (C6D6, 25 °C, 500.13 MHz): δ 0.97 (dd, 18H, CH3,
3
2JHH = 7.5 Hz, JHP = 12.5 Hz); 1.93 (m, 3H, CH); 2.76 (s, 4H,
C2H4). 31P{1H} NMR (C6D6, 25 °C, 121.54 MHz): δ 52.46 (s,
1P).
Catalytic reaction rate versus [catalyst]
A stock solution of the reagent Ph3SnCHvCH2 (232 mg,
0.62 mmol), the reagent C6F5H (107 mg, 0.64 mmol), the
internal standard C6H5F (60 mg, 0.62 mmol) and the internal
standard hexamethyldisiloxane (HMDSO) (100 mg, 0.62 mmol)
were dissolved in toluene and the solution was diluted to
3.6 mL, to provide
a solution that is 0.172 M of
Ph3SnCHvCH2, C6H5F and HMDSO and 0.177 M of C6F5H.
Approximate masses of 3, 6, 12, 24 and 48 mg of the catalyst
(iPr3P)Ni(η2-Ph3SnCHvCH2) were weighed into a vial and
0.6 mL of the stock solution was added to give approximate cat-
alyst concentrations of 0.005 M, 0.01 M, 0.02 M, 0.04 M and
0.08 M, respectively. The resulting solution was transferred to an
NMR tube. The samples did not react appreciably at room temp-
erature, and were introduced into an NMR spectrometer probe
preheated to 338 K. The rate of reaction production was moni-
tored via concentration of C6F5SnPh3 formed versus time. The
concentration of C6F5SnPh3 formed was estimated from inte-
gration of the 19F NMR signals compared to the internal standard
C6H5F. Plotting concentration of product formed versus time, the
slope was found to be linear for extended periods of time and
was recorded as the reaction rate. The observed reactions rates
Reaction of 1,2,4,5-C6F4HD, Ph3Sn(vinyl), and 5% catalyst
loading (iPr3P)Ni(η2-Ph3SnCHvCH2)
A solution of 1,2,4,5-C6F4HD (0.015 g, 0.103 mmol), triphenyl
(vinyl)tin (0.039 g, 0.103 mmol) in 1 mL of C6D6 was mixed
with (iPr3P)Ni(η2-Ph3SnCHvCH2) (0.005 g, 0.005 mmol) and
placed in a preheated NMR probe at 338 K. The 19F NMR spec-
trum of the reaction mixture was recorded after 5 min of
initiation of the reaction in order to determine the initial deuter-
ium isotope effect for C–H vs. C–D activation. Activation of
hydrogen over deuterium can be confirmed by a ∼0.3 ppm shift
of any ortho fluorine adjacent to the remaining deuterium in the
product and the isotope effect can be determined through inte-
gration. Oxidative addition is favoured for C–H over C–D
bonds, and the ratio of integrals for the products Ph3Sn(2,3,5,6-
C6F4D) and Ph3Sn(2,3,5,6-C6F4H) were found to be 2.0 : 1 at
338 K. 19F{1H} NMR (C6D6, 65 °C, 282.40 MHz): −118.7
(AA′BB′ second order, 3F, 2,6-Ar-F); −136.9 (AA′BB′ second
order, 1F, 3,5-Ar(H)-F); −137.2 (AA′BB′ second order, 2F, 3,5-
Ar(D)-F).
were found to be 1.347 × 10−5, 2.162 × 10−5, 3.961 × 10−5
,
8.387 × 10−5 and 1.794 × 10−4 M s−1, respectively. Plotting cat-
alyst concentration versus the respective reaction rates, yields a
linear slope.
Catalytic reaction rate versus [C6F5H]
A stock solution of the reagent Ph3SnCHvCH2 (400 mg,
1.06 mmol), the catalyst (iPr3P)Ni(η2-Ph3SiCHvCH2)2, 1b,
(25 mg, 0.026 mmol) and the internal standard C6H5F (100 mg,
1.04 mmol) were dissolved in 5 mL toluene to provide a solution
that was 0.212 M, 0.0052 M and 0.208 M in these three com-
ponents, respectively. Approximate masses of 12, 25, 50, 100
and 200 mg of C6F5H were weighed directly into five 5 mm
NMR tubes, and the stock solution was added to dilute the sol-
ution to a total volume of 0.64 mL. The samples did not react
appreciably at room temperature. The samples were introduced
into an NMR spectrometer probe preheated to 338 K, and the
concentration of C6F5SnPh3 was monitored versus time by 19F
NMR spectroscopy. The slope of this plot was found to be linear
for extended periods, and the slope was recorded as the reaction
rate. The concentrations of C6F5H for the five separate samples
were estimated from integration compared to the internal stan-
dard to be 0.123, 0.218, 0.480, 0.82 and 1.97 M, and the
Fluorinated aromatic competition reactions
A stock solution of Ph3SnCHvCH2 (400 mg, 1.06 mmol), cata-
lyst 1b (30 mg, 0.031 mmol) and the internal standard monofl-
uorobenzene (100 mg, 1.04 mmol) were dissolved in 5 mL of
toluene. Equimolar amounts of fluorinated aromatics were
added. The following proportions were used with each compe-
tition: (a) C6F5H (100 mg, 0.595 mmol) vs. 1,2,4,5-C6F4H2
(89 mg, 0.595 mmol); (b) 1,2,4,5-C6F4H2 (100 mg,
0.666 mmol) vs. 1,2,3,5-C6F4H2 (100 mg, 0.666 mmol); (c)
1,2,3,5-C6F4H2 (100 mg, 0.666 mmol) vs. 1,2,4-C6F3H3 (88 mg,
0.666 mmol); (d) 1,2,4-C6F3H3 (100 mg, 0.757 mmol) vs. 1,3,5-
This journal is © The Royal Society of Chemistry 2012
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C6F3H3 (100 mg, 0.757 mmol); and (e) 1,2,4-C6F3H3 (100 mg,
0.757 mmol) vs. 1,3-C6F2H4 (86 mg, 0.757 mmol). Soon after
the reactants were mixed 19F NMR spectroscopy was used to
analyze the sample at 338 K. At this point the conversion was
minimal relative to the starting materials, which allowed inte-
gration of the products 19F NMR resonances to be used to deter-
mine relative rates. Using the relative rate constant, the change in
Gibbs free energy of activation and change in enthalpy of acti-
vation versus pentafluorobenzene were also determined.
Notes and references
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Reaction of C6F5D, Bu3Sn(cis,trans-propenyl), and 5% catalyst
loading (iPr3P)Ni(η2-Ph3SnCHvCH2)
A solution of C6F5D (0.046 g, 0.2718 mmol), Bu3Sn(cis,trans-
propenyl) (0.090 g, 0.2718 mmol) in 1 mL of C6D6 was mixed
with 1b (0.013 g, 0.0136 mmol) and placed in a preheated NMR
probe at 338 K. After 20 min the 1H NMR spectrum was used to
confirm that both the ratio of cis-propene-d1 to trans-propene-d1
was equal to the trans : cis ratio of the Bu3Sn-propenyl starting
material.
X-ray crystallography
The X-ray structure of 1b was obtained at −100 °C, with the
crystal covered in Paratone and placed rapidly into the cold N2
stream of the Kryo-Flex low-temperature device. The data was
collected using the SMART22 software on a Bruker APEX CCD
diffractometer using a graphite monochromator with MoKα radi-
ation (λ = 0.71073 Å). A hemisphere of data was collected using
a counting time of 10 s per frame. Data reductions were per-
formed using the SAINT23 software, and the data were corrected
for absorption using SADABS.24 The structures were solved by
direct methods using SIR9725 and refined by full-matrix least-
squares on F2 with anisotropic displacement parameters for the
non-H atoms using SHELX-9726 and the WinGX27 software
package, and thermal ellipsoid plots were produced using
ORTEP32.28 The hydrogen atoms on the coordinated carbon
atoms of the vinyl moiety were located in the electron-density
difference map and their positions were refined. The remaining
hydrogen atoms were placed in idealized locations using the
AFIX command in SHELX.
3 M. Reinhold, J. E. McGrady and R. N. Perutz, J. Am. Chem. Soc., 2004,
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Details of crystal data, data collection, and structure refine-
ment data for 1b: empirical formula, C49H57NiPSn2; formula
weight, 973.01 g mol−1; monoclinic; a = 9.6240(12), b = 44.359
(6), c = 11.7793(11) Å; β = 114.822(8)°; V = 4564.1(9) Å3;
space group P21/c; Z = 4; Dcalc = 1.416 g cm−3; μ(MoKα) =
1.560 mm−1; temperature = 173(2) K; 2θmax 50.0°; total no. of
reflns = 43 373; no. unique reflns = 8037; Rint = 0.0527; trans-
mission factors = 0.93–0.56; no. with I ≥ 2σ(I) = 6953; no. vari-
ables = 508, reflections/parameters = 15.3, wR2 (all data) =
0.121; GOF = 1.274; residual density = 1.14; −1.363 e− Å−3
.
11 (a) N. V. Petrushanskaya, A. I. Kurapova and V. S. Fel’dblyum, Zh. Org.
Khim., 1973, 9, 2620; (b) K. R. Poerschke, J. Am. Chem. Soc., 1989, 111,
5691.
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J. Am. Chem. Soc., 2010, 132, 6296; (c) S. Lin and T. Agapie, Synlett,
2011, 1.
13 A. L. Keen and S. A. Johnson, J. Am. Chem. Soc., 2006, 128, 1806.
14 J. Guihaume, S. Halbert, O. Eisenstein and R. N. Perutz, Organometal-
lics, 2012, 31, 1300.
Acknowledgements
Acknowledgement is made to the Natural Sciences and Engin-
eering Research Council (NSERC) of Canada for a Discovery
Grant for S.A.J. and postgraduate scholarships for M.E.D and
J.A.H.
8142 | Dalton Trans., 2012, 41, 8135–8143
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