Oxidatively induced reductive elimination of dioxygen from an η2-peroxopalladium(II) complex promoted by electron-deficient alkenes

Article abstract of DOI:10.1021/ja057753b

The first example of associative displacement of dioxygen from a peroxopalladium(II) complex is reported. Electron-deficient alkenes, p-X-trans-β-nitrostyrene (X = OCH₃, CH₃, H, F, Br, CF₃, NO₂), react quantitatively with (bc)Pd(η²-O²) (bc = bathocuproine) in dichloromethane at room temperature to form the corresponding palladium(0)-alkene complexes. Mechanistic studies indicate that ligand substitution proceeds through an associative mechanism, and the electronic characteristics of the reactions are consistent with an oxidatively induced reductive elimination pathway. Copyright

Full text of DOI:10.1021/ja057753b

Published on Web 02/09/2006
“
Oxidatively Induced” Reductive Elimination of Dioxygen from an
2-
η Peroxopalladium(II) Complex Promoted by Electron-Deficient Alkenes
Brian V. Popp and Shannon S. Stahl*
Department of Chemistry, UniVersity of WisconsinsMadison, 1101 UniVersity AVenue, Madison, Wisconsin 53706
Received November 14, 2005; E-mail: stahl@chem.wisc.edu
Palladium-catalyzed oxidation reactions employ a variety of
stoichiometric oxidants, most commonly, Cu(II) salts and benzo-
Scheme 1. Dioxygen- and Benzoquinone-Mediated Oxidation of
Pd(0)
1
quinone. In some cases, efficient catalytic turnover can be achieved
2
with dioxygen as the sole oxidant. The origin of oxidant specificity
for individual reactions is poorly understood. The relationship
between benzoquinone and dioxygen is particularly intriguing
because these molecules possess different electronic structures
2
(
singlet vs triplet), but both react with Pd(0) to produce η -adducts
that subsequently react with acid to generate catalytically active
Pd(II) (Scheme 1).3,4 In recent mechanistic studies of alkene self-
Scheme 2. Quantitative Dioxygen Trapping Experiment
exchange at Pd(0), we speculated about the relationship between
reactions of dioxygen and alkenes with well-defined palladium(0)
5
complexes. Here, we report the discovery that electron-deficient
2
alkenes (nitrostyrenes) can displace dioxygen from an η -peroxo-
palladium(II) complex. Mechanistic studies indicate this reaction
proceeds by an “oxidatively induced” reductive elimination pathway
in which the incoming alkene withdraws electron density from the
Pd(II) center in the transition state. These observations amplify the
similar reactivity of alkenes and dioxygen with palladium and have
direct implications for the use of benzoquinone and dioxygen as
oxidants in Pd-catalyzed oxidation reactions.
this generalization.4,8,9 The only reported exception is the dissocia-
tion of O
0-70 °C over an extended time period (38 h).
The unexpected facility of eq 1 prompted us to investigate the
mechanism of this reaction. The kinetics of dioxygen displacement
2 2 2 2
from (t-Bu PhP) Pd(O ), which occurs under vacuum at
10
6
We recently reported the synthesis and characterization of a
peroxopalladium(II) complex, 1, bearing the aromatic diimine ligand
bathocuproine (bc).4 Shortly thereafter, we began probing whether
CF3
1
by ns were studied initially by H NMR spectroscopy. Under
a
CF3
2 2
pseudo-first-order conditions (12 equiv of ns ) in CD Cl at 25
1
could effect oxygen-atom transfer to electron-deficient alkenes,
CF3
°
C, 1 converted cleanly into 2 in ∼20 min with an exponential
6
such as nitrostyrene. Although no oxygenation products were
observed, we obtained a different, unexpected result: nitrostyrene
dependence on [1] (Figures S2 and S3). No intermediates were
observed during the reaction. UV-visible spectroscopy proved to
be a more convenient method to acquire kinetics data. A representa-
displaces dioxygen from the Pd center. For example, addition of
CF
1
0 equiv of p-CF
methane-d
characterized alkene adduct, (bc)Pd(ns
3
-trans-â-nitrostyrene (ns
3
) to 1 in dichloro-
NO2
tive time-course for the reaction of 1 with ns is shown in Figure
A, and similar data were acquired for five different ns derivatives
results in nearly complete conversion to the previously
X
2
1
CF
CF
) (eq 1).5 The
3
) (2
3
3 2
(X ) H, F, Br, CF , NO ). Exponential fits to the time-course data
CF
was confirmed by comparison of H and 19F NMR
1
identity of 2
3
enabled us to obtain kobs values for each alkene over a range of
concentrations (Figure 1B). In each case, the kobs values exhibit a
spectral data with those from an authentic sample.
X
linear dependence on [ns ], and the fit intersects the origin.
Activation parameters, obtained by monitoring the temperature
dependence of the displacement reaction for four different nsX
3 2
derivatives (X ) F, Br, CF , NO ), reveal large negative entropies
q
of activation, ∆S ) -30 to -36 eu (Figure S4, Table S2).
Together, these data indicate that dioxygen displacement by nsX
proceeds exclusively via an associatiVe mechanism.
A Hammett plot, derived from the bimolecular rate constants
for displacement of dioxygen by the different ns derivatives, yields
X
X
Other nitrostyrene derivatives (ns , X ) OCH
3
, CH
3
, H, F, Br,
NO
sponding alkene complex 2 . With the more electron-rich alkenes,
2
) also react with 1, displacing dioxygen to form the corre-
p
a linear correlation with σ parameters (Figure 2). As Figures 1B
X
and 2 reveal, the more electron-deficient alkenes react faster, and
the substantial positive slope in the Hammett plot (F ) 1.53)
indicates that significant Pd f alkene charge transfer occurs in
the transition state of the reaction.
OCH
CH
3
ns
3
and ns
, a large excess (>100 equiv) is required to achieve
quantitative conversion in a sealed reaction vessel. It was possible
to characterize and quantify the evolved dioxygen gas by trapping
it with (bc)Pd(dba) (dba ) dibenzylideneacetone) in a separate flask
The mechanistic picture that emerges from these data is closely
related to that of alkene exchange at Pd(0), which has been
7
attached to the reaction vessel (Scheme 2).
2
5
The formation of Pd(η -O
2
) adducts via Pd(0) oxygenation is
characterized experimentally and computationally. The latter
generally irreversible, and prior observations in our lab supported
reaction proceeds via charge transfer from a palladium-centered
2804
9
J. AM. CHEM. SOC. 2006, 128, 2804-2805
10.1021/ja057753b CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
questions concerning the relationship between dioxygen and ben-
zoquinone as oxidants in palladium-catalyzed oxidation reactions.
Benzoquinone is an electron-deficient alkene, and in preliminary
studies, we find that benzoquinone can displace dioxygen from
1
5,16
2
(bc)Pd(O ) to form the corresponding alkene adduct 3 (eq 2).
In future studies, we hope to elucidate the kinetic and thermody-
namic factors that differentiate the reactivity of dioxygen and
electron-deficient alkenes (including benzoquinone) with palladium
centers.
Acknowledgment. This work was supported by the Dreyfus
Foundation (New Faculty and Teacher-Scholar Award), the Sloan
Foundation (Research Fellowship), and the NSF (CAREER Award,
CHE-0094344). NMR instrumentation is supported by NIH (1 S10
RR04981 and 1 S10 RR08389) and the NSF (CHE-8813550, CHE-
9629688, CHE-9208463, and CHE-9709065).
Figure 1. (A) Representative single wavelength (425 nm) UV-visible
NO
spectroscopic time-course for the reaction of 1 with ns
2
. (B) Olefin
concentration dependence on the rate of dioxygen displacement from 1 by
1
Supporting Information Available: Experimental details, H NMR
X
X
X
ns to form 2 . Reaction conditions: [Pd] ) 0.5 mM, [ns ] ) 5-39 mM,
mL of CH2Cl2, 298 K.
spectral time-course and concentration traces, Eyring data. This material
is available free of charge via the Internet at http://pubs.acs.org.
3
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Figure 3. Mechanistic model for associative displacement of alkenes (A)
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4
2
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(
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(
5b,11
3A), thus reflecting an “oxidative” trajectory.
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2
-displacement reaction (Figure 3B). By
(
(
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2
from Pd(II).1
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2
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1
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8
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(
(15) Popp, B. V.; Stahl, S. S. Unpublished results.
(
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JA057753B
J. AM. CHEM. SOC.
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