Allylic C-H acetoxylation with a 4,5-diazafluorenone-ligated palladium catalyst: A ligand-based strategy to achieve aerobic catalytic turnover

Article abstract of DOI:10.1021/ja105829t

Pd-catalyzed C-H oxidation reactions often require the use of oxidants other than O₂. Here we demonstrate a ligand-based strategy to replace benzoquinone with O₂ as the stoichiometric oxidant in Pd-catalyzed allylic C-H acetoxylation. Use of 4,5-diazafluorenone (1) as an ancillary ligand for Pd(OAc)₂ enables terminal alkenes to be converted to linear allylic acetoxylation products in good yields and selectivity under 1 atm O ₂. Mechanistic studies have revealed that 1 facilitates C-O reductive elimination from a π-allyl-Pd^II intermediate, thereby eliminating the requirement for benzoquinone in this key catalytic step.

Full text of DOI:10.1021/ja105829t

Published on Web 10/07/2010
Allylic C-H Acetoxylation with a 4,5-Diazafluorenone-Ligated Palladium
Catalyst: A Ligand-Based Strategy To Achieve Aerobic Catalytic Turnover
Alison N. Campbell, Paul B. White, Ilia A. Guzei, and Shannon S. Stahl*
Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison,
Wisconsin 53706, United States
Received July 1, 2010; E-mail: stahl@chem.wisc.edu
Scheme 1. Proposed Mechanism for Palladium-Catalyzed Allylic
Acetoxylation
Abstract: Pd-catalyzed C-H oxidation reactions often require the
use of oxidants other than O2. Here we demonstrate a ligand-
based strategy to replace benzoquinone with O2 as the stoichio-
metric oxidant in Pd-catalyzed allylic C-H acetoxylation. Use of
4,5-diazafluorenone (1) as an ancillary ligand for Pd(OAc)2
enables terminal alkenes to be converted to linear allylic acetoxyl-
ation products in good yields and selectivity under 1 atm O2.
Mechanistic studies have revealed that 1 facilitates C-O reductive
elimination from a π-allyl-PdII intermediate, thereby eliminating
the requirement for benzoquinone in this key catalytic step.
The selective oxidation of C-H bonds in organic molecules is
an important and growing field.1 Many of the emerging methods
utilize palladium catalysts in combination with stoichiometric
organic or transition-metal oxidants, such as PhI(OAc)2, benzo-
quinone, CuII, or AgI.2 Mechanistic studies have suggested that these
oxidizing agents often react with organopalladium(II) intermediates
and promote reductive elimination of carbon-carbon or carbon-
heteroatom bonds.3 Replacement of these oxidants with molecular
oxygen represents a significant fundamental challenge that has
important implications for environmentally benign, large-scale
applications of these methods.4,5 Here we present a ligand-based
strategy to replace benzoquinone (BQ) with O2 as an oxidant in
Pd-catalyzed allylic C-H acetoxylation reactions. Preliminary
mechanistic studies have revealed that the use of 4,5-diazafluo-
renone (1) as an ancillary ligand facilitates C-O bond formation
from π-allyl-PdII species and thereby permits O2 to be used as the
oxidant in a PdII/Pd0 catalytic cycle, eliminating the requirement
for BQ.6
Table 1. Identification of a Ligand for Palladium-Catalyzed Aerobic
Allylic Acetoxylationa
Pd-catalyzed allylic acetoxylation reactions represent a prominent
class of C-H oxidations that have been the subject of extensive
historical7 and contemporary8,9 interest. The vast majority of these
reactions utilize BQ as a stoichiometric or cocatalytic oxidant.
Fundamental studies have implicated at least three possible roles
for BQ in the catalytic mechanism (Scheme 1): (1) promotion of
nucleophilic attack by acetate on a π-allyl-PdII species (step II);3a,d
(2) displacement of the allylic acetate product from Pd0 following
reversible C-O bond formation (step III);8d and (3) oxidation of
Pd0 to PdII (step IV).10 Because molecular oxygen is also capable
of oxidizing Pd0 to PdII,6 we reasoned that an aerobic allylic C-H
oxidation process could be achieved by identifying a ligand for Pd
that could eliminate the requirement for BQ in steps II and III.
Pyridine, phenanthroline (phen), and related nitrogenous ligands
have been widely used in Pd-catalyzed aerobic oxidation reactions.11
a Conditions: 5% Pd(OAc)2 (3 mg, 0.0134 mmol), 5% ligand (0.0134
mmol), allylbenzene (35 µL, 0.268 mmol), AcOH (1 mL), 1 atm O2, 80
°C, 18 h. b GC yields (internal standard ) nitrobenzene). c Here 10%
ligand was used.
Such ligands were tested recently by Lin, Labinger, and Bercaw in
allylic acetoxylation reactions,8d and a bipyrimidine-Pd(OAc)2
catalyst was shown to be effective with 2 equiv of BQ as the
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C O M M U N I C A T I O N S
Table 2. Aerobic Allylic Acetoxylation of Terminal Olefinsa
oxidant. We reevaluated ligands of this type under 1 atm O2 to test
their ability to support aerobic Pd-catalyzed acetoxylation of
allylbenzene (Table 1). Pyridine (entry 2), bipyridine (bpy, entry
3), phen (entry 8), bipyrimidine (bpm, entry 11), and a number of
bpy and phen derivatives (entries 4-7, 9, and 10) were almost
completely ineffective; mostly unreacted allylbenzene was obtained
at the end of the reaction. The lack of Pd black in these reactions
suggested that the problem is a lack of reactivity rather than catalyst
decomposition. A noteworthy exception to these poor results was
obtained with 4,5-diazafluorenone (1), which led to an 81% yield
of the linear acetoxylation product (entry 12). This ligand was
included in the screen on the basis of the hypothesis that the
carbonyl group could promote back-bonding from the PdII center.
We reasoned that the contribution from the PdIV resonance struc-
ture (eq 1) might facilitate C-O reductive elimination from a
π-allyl-PdII intermediate and enable BQ-free catalytic turnover (cf.
Scheme 1, step II). Subsequent control experiments failed to validate
this hypothesis, however. For example, use of di-2-pyridyl ketone
(2), another ligand capable of back-bonding, afforded the allylic
acetoxylation product in only 5% yield, whereas use of another
diazafluorene ligand, 3, in which the carbonyl group in 1 was
replaced with two methyl substituents, resulted in a moderate yield
of the acetoxylation product (50%). The latter result was not as
good as the acetoxylation yield obtained with 1 but was substantially
better than the yield with 2 or the other ligands in Table 1. These
results suggest that the geometric properties of 1 (e.g., the bite angle)
rather than back-bonding underlie the effectiveness of this ligand.
a Reaction conditions: 5 mol % Pd(OAc)2 (0.05 mmol), 5 mol % 1
(0.05 mmol), 20 mol % NaOAc (0.20 mmol), substrate (1.0 mmol),
dioxane (2.8 mL), AcOH (0.9 mL, 16 mmol), 1 atm O2, 60 °C. b Based
on GC analysis of the crude reaction mixture. c Isolated yields. GC
yields are given in parentheses. d A 3:1 mixture of allylic and
homoallylic acetates was obtained. e Using 4 atm O2. A 5:1 mixture of
linear and branched isomers was obtained.
Successful aerobic acetoxylation of allylbenzene provided the
basis for further optimization of the reaction conditions and
evaluation of the substrate scope. It was possible to decrease the
temperature to 60 °C and replace AcOH with dioxane as the reaction
solvent; the optimized conditions featured 5 mol % Pd(OAc)2, 16
equiv of AcOH, and a catalytic amount of NaOAc (20 mol %)
under 1 atm O2 (see the Supporting Information for additional
optimization data). These Pd-catalyzed allylic acetoxylation condi-
tions were then tested with a number of other alkenes (Table 2).
Successful reactivity was observed with the naturally occurring
allylbenzene derivatives estragole and methyl eugenol (entries 2
and 3) as well as simple hydrocarbon-based R-olefins (entries 4
and 5). Silyl ethers were well-tolerated, as were esters, acetals,
amides, and carbamates (entries 6-11). Nearly all of the substrates
examined produced the linear acetoxylation product exclusively in
good yield and exhibited a strong preference for formation of the
E isomer of the alkene. The reactivity appeared to be selective for
terminal olefins, as ꢀ-methylstyrene, cyclohexene, methyl crotonate,
and methylenecyclohexane gave little to no desired product.
These allylic oxidation reactions enable anti-Markovnikov hydra-
tion of R-olefins to be achieved via tandem O2/H2-coupled redox
steps. This net transformation represents a long-standing challenge
in the field of catalysis,12 and it can be achieved in a straightforward
one-pot process consisting of Pd-catalyzed allylic acetoxylation,
Scheme 2. Net Anti-Markovnikov Hydration of Terminal Alkenes
via a One-Pot, Three-Step Sequence
removal of the acetate under basic conditions, and hydrogenation
of the alkene (Scheme 2A). No additional catalyst is required for
the hydrogenation step; addition of activated carbon to the crude
reaction mixture, which still contains Pd from the acetoxylation
reaction, and stirring under 1 atm H2 results in efficient hydrogena-
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C O M M U N I C A T I O N S
tion of the alkene. Good yields of terminal alcohols were obtained
for three representative substrates utilizing this sequence (Scheme
2B). The isolated yield of the alcohol essentially matched that of
the initial acetoxylation step.
Mechanistic studies have begun to provide insights into the ability
of the diazafluorenone ligand 1 to support aerobic BQ-free catalytic
turnover. Two similar [(L)PdII(η3-allyl)]OAc complexes, 4 (L )
1) and 5 (L ) tBu2bpy), were prepared in order to compare the
effects of O2 and BQ on the reactivity of the π-allyl-PdII complexes
under catalytically relevant conditions (Figures 1 and 2). In the
first set of experiments, the ability of 4 and 5 to undergo
1
stoichiometric acetoxylation of the allyl ligand was probed by H
NMR spectroscopy under three separate conditions: in the absence
of an oxidant (i.e., under 1 atm N2), under 3 atm O2,13 and in the
presence of 2 equiv of BQ (Figure 1). The diazafluorenone complex
reacted to form allyl acetate in good yield under all these conditions
(70-90%); however, the reaction time varied from 24 h (N2) to
3 h (O2) to 1 h (BQ).14 Complex 5, with the catalytically
incompetent tBu2bpy ligand, was completely unreactive in the
presence of N2 and O2; however, it underwent acetoxylation in the
presence of 2 equiv of BQ (88%).
Figure 1. Ligand effects on stoichiometric acetoxylation of well-defined
π-allyl-Pd complexes in the absence of an oxidant (under 1 atm N2)
and in the presence of O2 (3 atm) and benzoquinone (2 equiv). The
times given in parentheses are the reaction times needed for >95%
conversion of 4 or 5.
A second set of experiments probed the possibility of reversible
C-O bond formation by monitoring the ability of π-allyl-PdII
complexes 4 and 5 to mediate acetate-d3 incorporation into cinnamyl
acetate (Figure 2A). The proposed Pd-mediated mechanism for
acetate exchange is shown in Figure 2B, and the data are
summarized in Figure 2C. Diazafluorenone complex 4 mediated
extensive acetate exchange (76% in 3 h) in the absence of an
oxidant; however, this reactivity was eliminated by the presence
of O2 or BQ.15 The extent of acetate exchange decreased systemati-
cally as the O2 pressure was increased (Figure 2D), presumably
reflecting the ability of O2 to trap the Pd0-alkene complex (kox in
Figure 2B) and inhibit exchange. Negligible acetate exchange was
observed with tBu2bpy complex 5, even in the absence of an
oxidant.
These results provide insights into the catalytic steps that have
been proposed to involve BQ (Scheme 1, steps II and III). The
reactivity of 5 in the stoichiometric acetoxylation study (Figure 1)
reveals that BQ can induce acetoxylation of an otherwise unreactive
π-allyl-PdII complex. O2 is not an effective BQ surrogate in this
reaction. The stoichiometric reactivity of 4 reveals that BQ is not
required for the C-O bond-forming step when diazafluorenone is
the ancillary ligand. We speculate that the distorted bite angle of
the diazafluorenone ligand16 destabilizes PdII and allows the π-allyl
ligand to undergo nucleophilic attack in the absence of BQ.
Computational studies to probe this hypothesis will be the focus
of future studies. The qualitative rate differences for the acetoxyl-
ation of 4 in the presence of N2, O2, and BQ (Figure 1) can be
rationalized in two ways. One possibility is that O2 and BQ can
enhance the rate by trapping the putative Pd0-alkene species and
preventing reversion to the π-allyl-PdII complex.17 Alternatively,
the higher rate with BQ than with O2 could reflect the ability of
BQ to promote acetoxylation of the π-allyl-PdII complex in
addition to trapping the Pd0 intermediate.
Further work will be needed to probe the effect of ancillary
ligands on other steps of the catalytic mechanism, such as C-H
activation, and their influence on the identity of the turnover-limiting
step and the catalyst resting state. Nevertheless, the results presented
here provide clues into the ability of an ancillary ligand to enable
catalytic turnover with O2 instead of BQ as the stoichiometric
oxidant, and a plausible catalytic cycle is shown in Scheme 3.
This study points to a potentially versatile ligand-based strategy
to achieve aerobic Pd-catalyzed C-H oxidation. For example, BQ
Figure 2. Experiments designed to probe the reversibility of C-O bond
formation from (L)Pd(η3-allyl) complexex. (A) Acetate exchange into
cinnamyl acetate in the presence of a (L)Pd(η3-allyl) complex; (B)
Mechanistic basis for (L)Pd(η3-allyl)-catalyzed acetate exchange in cinnamyl
acetate and the influence of an oxidant on the extent of exchange; (C) Extent
of acetate exchange with different (L)Pd(η3-allyl) complexes in the absence
and presence of an oxidant; (D) O2 pressure effects on the extent of acetate
exchange for the reaction in Figure 2A [(L)Pd(η3-allyl) ) 4].
and other oxidants have been proposed to promote C-C reductive
elimination in oxidative cross-coupling reactions.3g,j,k On the basis
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C O M M U N I C A T I O N S
Scheme 3. Proposed Mechanism for Palladium-Catalyzed Aerobic
Allylic Acetoxylation
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Acknowledgment. We are grateful to the NIH (R01-GM67163
to S.S.S.; F32-GM087890 to A.N.C.) and the Camille and Henry
Dreyfus Postdoctoral Program in Environmental Chemistry for
financial support of this work. High-pressure instrumentation was
supported by the NSF (CHE-0946901).
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Supporting Information Available: Experimental procedures,
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crystallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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