A remote lewis acid trigger dramatically accelerates biaryl reductive elimination from a platinum complex

Article abstract of DOI:10.1021/ja404339u

A strategy for the control of electron density at a metal center is reported, which uses a remote chemical switch involving second-sphere Lewis acid binding that modulates electron density in the first coordination sphere. Binding of the Lewis acid B(C₆F₅)₃ at remote nitrogen positions of a bipyrazine-diarylplatinum(II) complex accelerates biaryl reductive elimination by a factor of 64,000.

Full text of DOI:10.1021/ja404339u

Communication
pubs.acs.org/JACS
A Remote Lewis Acid Trigger Dramatically Accelerates Biaryl
Reductive Elimination from a Platinum Complex
Allegra L. Liberman-Martin, Robert G. Bergman,* and T. Don Tilley*
Department of Chemistry, University of California - Berkeley, Berkeley, California 94720, United States
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 United States
S
* Supporting Information
Scheme 1
ABSTRACT: A strategy for the control of electron
density at a metal center is reported, which uses a remote
chemical switch involving second-sphere Lewis acid
binding that modulates electron density in the first
coordination sphere. Binding of the Lewis acid B(C6F5)3
at remote nitrogen positions of a bipyrazine−
diarylplatinum(II) complex accelerates biaryl reductive
elimination by a factor of 64,000.
benzene to generate mono- or bis-borane adducts 2 and 3
iological catalysts utilize steric and electronic modifications
to attain a remarkable degree of substrate specificity. One
(Scheme 2). Borane adduct formation resulted in dramatic
color changes from red (1), to green (2), to brown (3). In the
presence of 1 equiv of B(C6F5)3, roughly equal amounts of 1, 2,
and 3 were observed (34%, 32%, and 34%, respectively),
whereas with 2 equiv of B(C6F5)3, the product distribution
changed to 3% 1, 4% 2, and 93% 3. To test for the reversibility
of borane coordination, isolated 3 was treated with an excess of
4-dimethylaminopyridine (DMAP), resulting in regeneration of
B
mechanism by which enzymes finely tune reactivity is through
allosteric regulation, in which the enzyme responds to the
presence of a distant activator that induces conformational
changes to alter enzyme activity.1 The operation of a chemical
switching mechanism enables the active site to be protected in a
stable resting state, which is converted to a more reactive form
during catalysis. This allows many biological systems to be
active only when a triggering signal is present, thus protecting
them from decomposition.
1
1 and formation of the DMAP−B(C6F5)3 adduct (by H and
19F NMR spectroscopy).
Although tuning reactivity via chemical triggers is common in
biological systems, there are currently few analogues for
synthetic catalysts.2 In particular, examples involving electronic
triggers are rare. One reported strategy involves coordination of
tris(pentafluorophenyl)borane, B(C6F5)3, to carbonyl- and
imine-containing nickel(II) complexes to activate these species
toward ethylene polymerization catalysis.3−5 Placement of the
Lewis acid binding site remote from the reactive metal center
allowed for electronic modifications without direct alteration of
the coordination environment. Addition of the Lewis acid led
to a 3-fold increase in polymerization activity for these systems.
Herein, we report the synthesis and borane binding studies of
platinum−bipyrazine (bpyz) complexes, as well as mechanistic
studies of their biaryl reductive elimination. The para-
arrangement of nitrogens featured in bpyz was envisioned to
allow borane binding with formation of a zwitterionic complex
having a significant resonance contributor that may be
described as an electrophilic platinum(IV) species (Scheme
1). Because Lewis acid binding occurs at a remote ligand site,
differences in the activity observed between borane-free and
-bound complexes should enable the role of electrophilicity on
reactivity to be evaluated.
Scheme 2
Comparisons of 1H and 195Pt NMR data for 1 and 3 suggest
a substantial impact of Lewis acid binding on electron density at
the platinum center. The H NMR resonance for H-3 of the
pyrazine ring shifts from 8.01 ppm for 1 to 8.68 ppm for 3
(Figure 1). This downfield shift is consistent with coordinated
B(C6F5)3 groups withdrawing electron density from the
pyrazine rings. A gradual change in 195Pt chemical shift was
also observed between 1 (δ = −3197 ppm), 2 (−2654 ppm),
and 3 (−2070 ppm), suggesting an increase in electron
deficiency upon B(C6F5)3 coordination.6 The 19F NMR
separation of para and meta resonances of the B(C6F5)3
1
A bipyrazine−diarylplatinum complex (bpyz)Pt(4-CF3-Ph)2
(1) was prepared by treatment of Pt(4-CF3−Ph)2(SEt2)2 with
bpyz. Complex 1 reacted with variable amounts of B(C6F5)3 in
Received: May 1, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja404339u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
with other neutral donors, such as internal alkenes or
phosphines, were unsuccessful.18
To investigate the mechanism of reductive elimination, the
reaction order in each reagent was determined by monitoring
19F NMR spectra in situ. At 45 °C in p-xylene, reaction of 1
with 2 equiv of B(C6F5)3 formed 3 as the exclusive platinum
species. The disappearance of 3 and concomitant formation of
4,4′-bis(trifluoromethyl)-1,1′-biphenyl proceeded with clean
first-order kinetics, and no intermediates were detected by 19F
NMR spectroscopy (Figure S2 in Supporting Information
[SI]).
The slope of the plot of ln(kobs) vs ln[BTMSA] measured
between 0.14 and 0.59 M BTMSA demonstrates that the
reaction is first-order in BTMSA (Figure 2).19 Previously,
1
Figure 1. H NMR spectra of 1 with added B(C6F5)3, recorded in
toluene-d8 at 23 °C with a 1,3,5-tris(trifluoromethyl)benzene internal
standard (δ = 7.61 ppm).
fragments in 3 is 8.5 ppm, consistent with an anionic four-
coordinate geometry at boron.7
To probe the influence of borane binding on reactivity,
reductive elimination studies were targeted, since group 10
metals have been shown to be highly useful catalysts for
carbon−carbon coupling reactions.8−10 Reductive elimination
rates are dependent both on the nature of the transition metal,
and the steric and electronic properties of the ancillary and
reacting ligands.11 Although the activity of many complexes
toward reductive elimination has been attributed to electron
deficiency at the metal center, few detailed studies have directly
probed the role of this property on reactivity. For example, it
has been shown that fluorinated phosphine ligands promote
reductive elimination at platinum.12,13 However, in such
investigations, the steric environment of the platinum center
is inevitably altered upon ligand modification.
Figure 2. Plot of ln(kobs) vs ln[BTMSA], indicating first-order
behavior.
diphosphine−nickel complexes have been observed to undergo
sp2−sp3 reductive elimination via an associative mechanism in
the presence of free phosphine.20 In contrast, biphenyl
reductive elimination from diphosphine−platinum complexes
in the presence of diphenylacetylene was shown to proceed
without prior alkyne association.13
To probe the influence of the Lewis acid on reductive
elimination, the concentration of B(C6F5)3 was varied. In the
presence of 2 equiv of B(C6F5)3, a rate constant of kobs = 1.74
0.04 × 10−4 M·s−1 was determined. Doubling the concentration
of B(C6F5)3 had, within error, no effect on the observed rate
constant (kobs = 1.78 × 10−4 M·s−1), suggesting that reductive
elimination occurs with both equivalents of B(C6F5)3 still
bound to the bpyz ligand. A decrease in rate with added
B(C6F5)3 present would be expected if borane dissociation
occurred prior to C−C bond formation.
In the presence of 2 or more equiv of bis(trimethylsilyl)-
acetylene (BTMSA), the bis-borane adduct 3 was found to
undergo quantitative biaryl reductive elimination at 45 °C (eq
1). This is a remarkably low temperature for platinum biaryl
To verify that borane binding does not induce bpyz ligand
dissociation prior to reductive elimination, experiments were
performed in the presence of excess free bpyz. Identical
reaction rates were observed in the presence or absence of 2
equiv of free bpyz relative to platinum (kobs = 1.78 × 10−4
M·s−1), suggesting that bpyz dissociation does not occur before
the rate-determining step.
The mechanism presented in Scheme 3 is consistent with the
observed kinetic data. Coordination of alkyne to 3 generates a
five-coordinate platinum complex. Next, biaryl coupling expels
a bpyzPt−(η2-alkyne) adduct, from which displacement of bpyz
by another equivalent of alkyne leads ultimately to 4.
To determine the rate acceleration due to borane
coordination, control experiments were performed in the
absence of borane (eq 2). Biaryl reductive elimination was
substantially slower under these conditions, requiring heating
above 100 °C to observe any biaryl formation. In the absence of
borane, the reaction of 1 exhibited first-order dependence on
reductive elimination, which requires temperatures above 90 °C
for bis-phosphine systems.12−14 In contrast, previous studies of
diaryl(2,2′-bipyridine)platinum complexes demonstrated two
consecutive roll-over metalations of the bipyridine ligand at 150
°C, along with generation of 2 equiv of Ar−H, rather than
biphenyl formation.15 In the current work, after reductive
elimination from 3, the bpyz ligand dissociates from platinum
as a bis-borane adduct, yielding a platinum alkyne complex,
[Pt2(μ-Me3SiCCSiMe3)(Me3SiCCSiMe3)2] (4).16,17Analogous
reactivity is observed with other internal alkynes, such as
diphenylacetylene. Attempts to induce reductive elimination
B
dx.doi.org/10.1021/ja404339u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
°C. To our knowledge, this is the fastest reported biaryl
reductive elimination from a platinum center, and it represents
the first example using a diimine ancillary ligand system. Kinetic
analysis supports a mechanism involving association of an
alkyne to a bis-borane adduct and allowed calculation of a
64,000-fold rate acceleration due to borane binding. Future
efforts will explore the generality of this Lewis acid binding
strategy as a means for studying the role of electrophilicity in
other metal-mediated reactions.
Scheme 3
ASSOCIATED CONTENT
■
S
* Supporting Information
Text and figures giving further experimental and spectroscopic
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
rbergman@berkeley.edu; tdtilley@berkeley.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Science Foundation under Grants CHE-0957106 and CHE-
0841786 and the Gerald E. K. Branch Distinguished Professor-
ship. This work was supported by the Director of the Office of
Energy Research, Office of Basic Energy Sciences, Chemical
Sciences Division, of the U.S. Department of Energy under
Contract DE-AC02-05CH11231.
[BTMSA] (Figure S5 in SI), suggesting that the reductive
elimination still proceeds via a five-coordinate intermediate in
this case.
An Eyring plot was constructed for the background reaction
of 1 with excess BTMSA from 110−150 °C without borane
present (Figure 3). From the temperature dependence of the
observed rate constant in the absence of borane, activation
parameters of ΔH⧧ = 29.0 1.3 kcal/mol and ΔS⧧ = −6.60
3.2 eu were calculated. The small, negative value for ΔS⧧ is
consistent with contributions from both the alkyne coordina-
tion and reductive elimination steps.
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experiments with 3 were performed, a rate constant of kobs
=
2.70 × 10−9 M·s−1 for the background reaction of 1 with
BTMSA was estimated. Comparing the rates for the reaction of
3 or 1 with BTMSA gives a rate acceleration of ∼64,000 due to
borane binding.
In conclusion, binding of Lewis acidic B(C6F5)3 at remote
nitrogen sites for a bipyrazine−diarylplatinum complex was
observed. Reactions in the presence of BTMSA demonstrated
facile biaryl reductive elimination, occurring in 95% yield at 45
C
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Journal of the American Chemical Society
Communication
(19) In the absence of alkyne, reductive elimination from 1 is
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high reaction temperatures (>80 °C) to observe reductive elimination,
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decomposition, with minimal biaryl formation (18%).
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D
dx.doi.org/10.1021/ja404339u | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX