Oxidatively resistant ligands for palladium-catalyzed aerobic alcohol oxidation

Article abstract of DOI:10.1021/om101037k

The complex [(neocuproine)Pd(OAc)]₂[OTf]₂ (1) catalyzes the aerobic oxidation of 2-heptanol at room temperature, but competitive ligand oxidation leads to low catalyst lifetimes. In an effort to mitigate the oxidative degradation of the ligand, a variety of Pd complexes ligated by fluorinated phenanthrolines (bis-2,9-(trifluoromethyl)-1,10- phenanthroline (btfm-phen), 4-methyl-2-(trifluoromethyl)-1,10-phenanthroline (tfmm-phen), and 2-(o-difluorophenyl)-1,10-phenanthroline (odfp-phen)) were prepared and tested. Active catalyst precursors were generated by in situ conproportionation of (N-N)Pd(OAc)₂ and [(N-N)Pd(NCCH ₃)₂[OTf]₂. Pd complexes derived from tfmm-phen catalyzed the aerobic oxidation of 2-heptanol at room temperature with up to 22 turnovers, nearly double that of 1.

Full text of DOI:10.1021/om101037k

ARTICLE
pubs.acs.org/Organometallics
Oxidatively Resistant Ligands for Palladium-Catalyzed Aerobic
Alcohol Oxidation
David M. Pearson, Nicholas R. Conley, and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States
S Supporting Information
b
ABSTRACT: The complex [(neocuproine)Pd(OAc)]2[OTf]2
(1) catalyzes the aerobic oxidation of 2-heptanol at room
temperature, but competitive ligand oxidation leads to low
catalyst lifetimes. In an effort to mitigate the oxidative degrada-
tion of the ligand, a variety of Pd complexes ligated by fluorinated
phenanthrolines (bis-2,9-(trifluoromethyl)-1,10-phenanthroline
(btfm-phen), 4-methyl-2-(trifluoromethyl)-1,10-phenanthroline (tfmm-phen), and 2-(o-difluorophenyl)-1,10-phenanthroline
(odfp-phen)) were prepared and tested. Active catalyst precursors were generated by in situ conproportionation of (N-
N)Pd(OAc)2 and [(N-N)Pd(NCCH3)2[OTf]2. Pd complexes derived from tfmm-phen catalyzed the aerobic oxidation of
2-heptanol at room temperature with up to 22 turnovers, nearly double that of 1.
’ INTRODUCTION
phenanthroline (phen), bis-2,9-(trifluoromethyl)-1,10-phenan-
throline (btfm-phen), 4-methyl-2-(trifluoromethyl)-1,10-phe-
nanthroline (tfmm-phen), and 2-(o-difluorophenyl)-1,10-
phenanthroline (odfp-phen). Of these, phen is commercially
available and btfm-phen can be synthesized by radical chlorina-
tion of neocuproine29 followed by halogen exchange with
SbF3/SbF5.30
Several 4,5-disubstituted 2-(trifluoromethyl)-1,10-phenan-
throlines are known,31,32 but 2-(trifluoromethyl)-1,10-phenan-
throline (tfm-phen) has not been reported. We initially
attempted the synthesis of tfm-phen by a Skraup-Doebner-
Palladium-catalyzed aerobic alcohol oxidation1-6 has ad-
vanced considerably since the initial report in 1977.7 More than
a decade later, it was discovered that coordinating solvents such
as DMSO or pyridine could effectively stabilize Pd(0) inter-
mediates and facilitate their reoxidation by oxygen before Pd
black formation occurs.8-11 Subsequently, other ligands were
used to similar effect, and well-defined complexes of palladium
with sparteine,12-14 phenanthroline,15-19 and N-heterocyclic
carbene (NHC)2,20-23 scaffolds have been reported for aerobic
alcohol oxidation. Frequently these systems rely on acetate
counterions due to their ability to facilitate deprotonation of
coordinated alcohols; while other coordinating anions are toler-
ated, they typically require additional base for catalysis.2,24,25
We previously reported that the 2,9-dimethyl-1,10-phenan-
throline (neocuproine) complex [(neocuproine)Pd(OAc)]2
[OTf]2 (1) is active for the aerobic oxidation of 2-heptanol at
room temperature.26 Complex 1 exhibits a high initial turnover
rate (∼78 Pd-1 h-1), but degrades rapidly due to competitive
oxidation of the ligand by partially reduced oxygen species
(Scheme 1). Stahl and co-workers27 reported similar ligand
oxidation after partial reduction of oxygen by Pd, yielding
a reactive Pd(II)-hydroperoxo species. The use of other oxi-
dants, such as benzoquinone, can mitigate this destructive side
reaction, but the synthetic utility of oxygen as a terminal oxidant
remains attractive.28 Herein we document efforts to develop
more oxidatively resistant phenanthroline ligands to mitigate
competitive ligand oxidation pathways in the aerobic oxidation
of alcohols.
Von Miller reaction using R,β-unsaturated carbonyl
4
(Scheme 2) and 8-aminoquinoline.33,34 However, isolation of
the aldehyde 4 from Et2O proved difficult due its high volatility.35
The less volatile ketone derivative 536 was easier to purify, and
condensation of 5 with 8-aminoquinoline yielded the desired
trifluoromethyl-substituted phenanthroline 6.33
Fluorinated aryl phenanthrolines have been proposed as
oxidatively resistant ligands as reported by Sadighi et al.37 in
Cu-mediated transfer of nitrenes. Related 2-polyfluoroaryl-phe-
nanthrolines have also been reported by Deacon et al.38,39 We
adapted a synthetic route to 2-phenyl-1,10-phenanthroline re-
ported by Thummel and co-workers (Scheme 3).40 Substitution
of 20,60-difluoroacetophenone for acetophenone in the final step
yielded 7 in 58% yield from 8-amino-7-quinolinecarbaldehyde.
Despite being five steps, this strategy provides an expeditious
route to 7, requiring no purification steps until the penultimate
step. Attempts to synthesize this compound through Sauvage-
type conditions41 using 2,6-difluorophenyl lithium42 and phe-
nanthroline provided low yields of 7, which could not readily
be purified.
’ RESULTS AND DISCUSSION
Synthesis of Oxidatively Resistant Ligands. Four phenan-
throline derivatives were targeted as potential ligands: 1,10-
Received: November 3, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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Scheme 1. Ligand Oxidation Resulting from Oxygen Reduction in Aerobic Alcohol Oxidation
Figure 1. Substituted phenanthroline ligands.
CH3CN and Et2O generated [(phen)Pd(CH3CN)2][OTf]2 as a
Scheme 2. Synthesis of 4-Methyl-2-(trifluoromethyl)-1,10-
phenanthroline (6)a
1
single species. The observed H NMR chemical shifts were
consistent with that previously reported.44
Addition of [(phen)Pd(CH3CN)21][OTf]2 to (phen)Pd
(OAc)2 in CD3CN yielded a complex H NMR spectrum that
was consistent with the formation of several new species.
Removal of the solvent under vacuum yielded an orange
solid, characterized as the dimeric [(phen)Pd(OAc)]2
[OTf]2, 8. X-ray quality crystals of 8 were obtained by slow
diffusion of diethyl ether into a saturated acetonitrile solution
of a 1:1 mixture of [(phen)Pd(CH3CN)2][OTf]2 and (phen)Pd
(OAc)2.
The structure of [(phen)Pd(OAc)]2[OTf]2 (8, Figure 2) is
similar to that previously reported for its neocuproine analogue
126 and other palladium phen dimers.45-47 A comparison of
selected bond lengths and angles of 8 with the neocuproine
analogue 1 is provided in the Supporting Information. Due to the
absence of flanking methyl groups, the Pd-N bond lengths of
the dimeric phen complex 8 are shorter than those for the
neocuproine dimer 1. The O-Pd-O bond angles in 8
(91.26(15) and 90.67(18) Å) are closer to the ideal 90° for a
square-planar geometry than those found in 1 (84.17(9) and
81.85(8) Å), and the Pd-Pd distance in 8 (2.8548(9) Å) is
shorter than that of 1 by 0.1 Å and in the range of that proposed
for weak Pd-Pd interactions.47-50
a Conditions: (a) Na2S2O4, NaHCO3, CH3CN, H2O, 10 °C, 1 h.; (b)
Et2O, polyphosphoric acid, rt, 2 h (63%); and (c) 8-aminoquinoline, NaI
(1 mol %), H2SO4, 110 °C, 5 h (21%).
Synthesis of Phenanthroline Palladium Complexes. The
synthesis of cationic (N-N)Pd acetate complexes was carried
out by conproportionation of a 1:1 mixture of the diacetate
(N-N)Pd(OAc)2 (A) and ditriflate [(N-N)Pd(NCCH3)2]-
[OTf]2 (B) complexes by a procedure previously described
(Scheme 4).26
For the phenanthroline complexes, the diacetate complex
(phen)Pd(OAc)2 was synthesized as previously reported.43
Our attempts to generate the ditriflate complex [(phen)Pd
(CH3CN)2][OTf]2 by reaction of (phen)PdCl2 with AgOTf
in acetonitrile44 were unsuccessful and yielded at least two
species by 1H NMR. Attempted ligation of phenanthroline with
(CH3CN)4Pd(OTf)2 also yielded multiple unidentified species,
Despite a previous report describing (btfm-phen)Pd(OAc)2
as an effective alcohol oxidation catalyst at 80 °C in DMSO/
water,18 our attempts to prepare (btfm-phen)Pd(OAc)2 were
unsuccessful; we were unable to observe complexation of this
ligand to Pd(OAc)2 in either CH3CN or toluene/CH2Cl2 (see
Supporting Information).
1
as determined by H NMR. However, treatment of (phen)Pd
(OAc)2 with triflic acid followed by two recrystallizations from
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Scheme 3. Abridged Synthetic Route to odfp-phen 7
Figure 3. X-ray crystal structure of [(tfmm-phen)Pd(CH3CN)]2-
[OTf]2 (10) with ellipsoids drawn at 50% probability. Hydrogen atoms
and triflate counterions are omitted for clarity. Select bond lengths (Å)
and angles (deg): Pd(1)-N(1), 2.073(3); Pd(1)-N(2), 1.984(3);
Pd(1)-N(3), 2.000(3); Pd(1)-N(4), 1.992(3); N(2)-Pd(1)-
N(4), 93.48(11); N(2)-Pd(1)-N(3), 175.77(11); N(4)-Pd(1)-
N(3), 82.58(11); N(2)-Pd(1)-N(1), 81.70(10); C(1)-N(1)-Pd-
(1), 133.6(2); and C(14)-N(2)-Pd(1), 127.2(2).
Scheme 4. General Procedure for the Formation of Cationic
Palladium Complexes in situ
Figure 4. X-ray crystal structure of 12 with ellipsoids drawn at 50%
probability. Hydrogen atoms have been omitted for clarity. Select bond
lengths (Å) and angles (deg): Pd(1)-N(2), 1.987(2); Pd(1)-N(1),
1.998(2); Pd(1)-O(1), 1.9521(19); Pd(1)-O(2), 2.0340(19); N(2)-
Pd(1)-N(1), 84.05(9); O(1)-Pd(1)-N(2), 92.99(8); O(1)-Pd(1)-
O(2), 91.36(8); N(1)-Pd(1)-O(2), 91.48(9); O(1)-Pd(1)-N(1),
176.45(9); N(2)-Pd(1)-O(2), 174.49(8); O(1)-C(18)-C(13),
127.0(2); O(1)-C(18)-C(17), 113.7(2); and Pd(1)-O(1)-C(18)-
C(13), 4.1(4).
to palladium by stirring an equimolar solution of 6 and Pd(OAc)2
in toluene/CH2Cl2 to yield (tfmm-phen)Pd(OAc)2 (9) as a red
solid. Complex 9 is only sparingly soluble in chloroform,
dichloromethane, and acetonitrile, but exhibits higher solubility
in 1,1,2,2-tetrachloroethane (TCE). H NMR spectroscopy in
d2-TCE showed a single species with two inequivalent
Figure 2. X-ray crystal structure of [(phen)Pd(OAc)]2[OTf]2 (8) with
ellipsoids drawn at 50% probability. The unit cell contains two dimeric
complexes. An interdimer Pd-Pd distance of 3.0996(11) Å was observed.
The second dimeric unit, hydrogen atoms, triflate counterions, and
solvent molecules have been omitted for clarity. Select bond lengths
(Å) and angles (deg): Pd1-Pd2, 2.8548(9); Pd1-O1, 2.007(3); Pd1-
O3, 2.021(4); Pd1-N1, 2.014(4); Pd1-N2, 2.000(4); Pd2-O2,
2.013(4); Pd2-O4, 2.016(4); Pd2-N3, 2.034(4); Pd2-N4, 2.032(4);
O3-C27, 1.286(7); C27-O4, 1.299(7); O1-C25, 1.282(6); C25-O2,
1.308(7); O1-Pd1-O3, 91.26(15); O2-Pd2-O4, 90.67(18); O1-
Pd1-N1, 93.38(16); and N1-Pd1-N2, 82.21(16).
1
acetate ions.
The dicationic analogue [(tfmm-phen)Pd(NCCH3)2][OTf]2
(10) was prepared by addition of [(CH3CN)4Pd][OTf]2 to 6;
preparation by addition of triflic acid to 9 did not yield 10 cleanly.
Slow diffusion of Et2O into a solution of 10 in acetonitrile yielded
crystals suitable for X-ray diffraction (Figure 3). The Pd-
N1 bond (2.073(3) Å) is elongated compared to Pd-N2
(1.984(3) Å), and it is also slightly longer than previously
observed Pd-N bond lengths in analogous dicationic phenan-
throline-based systems.26,44
The less sterically demanding 2-(trifluoromethyl)-4-methyl-
1,10-phenanthroline (tfmm-phen) 6 could be readily complexed
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Scheme 5. Proposed Mechanism for Palladium-Mediated Fluoride Substitution by Adventitious Water
Conproportionation of 9 and 10 in a 1:1 molar ratio produced
a complex 1H NMR spectrum consistent with the generation of
several new cationic species; no residual 9 or 10 was detectable
after mixing. The complexity in the 1H NMR spectrum is likely a
consequence of monomer-dimer equilibria26 of a variety of
stereoisomeric dimers of the asymmetric phenanthroline Pd
complexes.
The Pd(OAc)2 complex of o-difluorophenyl-phenanthroline
(odfp-phen) was prepared by mixing Pd(OAc)2 with odfp-phen
in dry toluene/CH2Cl2 to yield (odfp-phen)Pd(OAc)2 (11) as a
yellow solid. A half-equivalent of CH2Cl2 remained even after
long periods under high vacuum, but the identity of 11 was
established by 1H and 13C NMR spectroscopy and combustion
analysis (as the CH2Cl2 solvate).
The diacetate complex 11 was crystallized over several days
from a saturated acetonitrile solution, and analysis of the X-ray
structure revealed that the new phenoxide complex 12 had
Figure 5. X-ray crystal structure of 15 with ellipsoids drawn at 50%
probability. Hydrogen atoms and a triflate counterion have been omitted
for clarity.
1
formed (Figure 4). Complex 12 was fully characterized by H
and 13C NMR spectroscopy, both of which were consistent with
substitution of one of the fluorine atoms with oxygen. Solutions
of 11 in rigorously dried acetonitrile did not convert to 12,
suggesting that the substituted oxygen in 12 arises from
adventitious water.
In contrast to 11, the monocationic complex 15 did not
hydrolyze in wet CD3CN and showed no sign of fluoride
displacement, even after heating to 70 °C for 7 days. The higher
hydrolytic stability of 15 relative to 11 suggests that the presence
of the ancillary acetate ligand in 11 facilitates the displacement of
the fluoride to give 12.
Treatment of 15 with excess AgOTf in refluxing acetonitrile
for several days afforded the dicationic bis-acetonitrile complex
14 (Scheme 6). The dicationic 14 was less hydrolytically stable
than 15, and over the course of several days in wet acetonitrile,
the complex degraded into several unidentifiable species.
Combination of 11 and 14 in a 1:1 molar ratio in dry CD3CN
yielded several new species. Analysis of the 1H NMR spectrum
was complicated by the increased number of possible stereo-
isomers combined with the known solution equilibria;26 analo-
gous to the mixing of 9 and 10, no residual peaks attributable to
11 or 14 were observed after mixing.
Catalytic Aerobic Alcohol Oxidation. To assess the influ-
ence of the ligands on the activity of the Pd complexes for aerobic
alcohol oxidation, we investigated the oxidation of 2-heptanol
in acetonitrile in air at room temperature, as previously
described (Figure 6 and Table 1).26 For these experiments,
the cationic Pd complexes were either generated in situ (from
9/10 and 11/14) or introduced as the isolated Pd dimers
(complexes 1 and 8) at a Pd concentration of 1.5 mM (3 mol %
relative to 2-heptanol).
The formation of 12 by hydrolysis of 11 was confirmed by the
addition of water to a dry acetonitrile solution of 11. After three
days, crystals of 12 could be isolated in 36% yield. We propose
that adventitious water displaces one of the acetate ligands to
generate the Pd-hydroxo complex 13, which undergoes intra-
molecular nucleophilic aromatic substitution of fluoride, yielding
12 (Scheme 5). Alternatively, reversible dissociation of one of the
acetates could generate a cationic Pd complex, which could
activate one of the ortho-fluorines toward intermolecular SNAr
by hydroxide. We currently disfavor the second hypothesis, as
similar substitution by methoxide was not observed when
methanol was used as a solvent. The transformation of 11 to
12 may have implications for carbon-fluorine bond activation,
such as those reported for Pt(II)51-53 and Ni(0)54-58 com-
plexes, and is similar to substitution reactions observed by
Usꢀon et al.59
Initial attempts to prepare the palladium ditriflate complex of
odfp-phen (14) from a mixture of (CH3CN)2PdCl2 and 2 equiv
of AgOTf in acetonitrile led to the cationic Pd chloride complex
[(odfp-phen)Pd(NCCH3)Cl][OTf] (15) in 10-20% yield
(Scheme 6). Crystallization of 15 from acetonitrile afforded
crystals suitable for X-ray analysis (Figure 5). The asymmetrically
substituted complex 15 exists as a single isomer in the solid state.
The coordinated chloride is trans to the nitrogen bearing the o-
difluorophenyl substituent. The o-difluorophenyl substituent lies
out of plane of the phenanthroline ligand by 58.8(5)°, as
evidenced by the N2-C12-C13-C18 torsion angle.
As previously reported, the dimeric complex 1 exhibits a fast
initial rate (Figure 6, Table 1), but the activity drops off rapidly
and the maximum conversion under these conditions is only
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Scheme 6. Synthesis of 14 and 15
Figure 6. Reaction progress for the palladium-catalyzed aerobic oxidation of 2-heptanol to 2-heptanone using 1 (b), 8 (9), 9 (2), 10 (`), 9/10 (1),
and 11/14 (() at room temperature in acetonitrile; in all cases, 3 mol % of Pd was employed. Dashed lines are included only to serve as guides for the eye.
Table 1. Efficiency of Various Cationic Palladium Complexes
for the Aerobic Oxidation of 2-Heptanol at Room
Temperature
Addition of water to dimer 8 leads to the formation of 16
(identified by 1H NMR and independent synthesis60) along with
several other unidentified species. Complexes analogous to 16
have been proposed as resting states by Sheldon and co-
workers16,18 in aerobic alcohol oxidations carried out at higher
temperatures. It has been previously observed that increased
steric bulk or higher temperatures are required with
(phen)Pd(OAc)2 to destabilize the μ-hydroxo intermediates
and achieve greater alcohol-oxidation activity.18 This hypothesis
provides a rationale for the poor activity of 8; if the μ-hydroxo-
bridged dimer 16 is formed in the course of the catalytic cycle,
this dimer would likely be inactive at the low temperatures
investigated here. These results imply that substitution at the
2- and/or 9-positions of phenanthroline is important to retain
activity at low temperature, but these substituents must also resist
competitive oxidation.26
c
entry
complex
yield (%)a
TONb
12
TOFi (h-1
78
)
1
2
3
4
1
36
8
trace
67
9/10
11/14
22
21
60
8
2.5
a After 24 h reaction time. b TON = moles ketone/mol Pd. c Initial
turnover frequency (mol ketone/(mol Pd h)).
3
The introduction of a single trifluoromethyl group to the
2-position of phenanthroline was expected to destabilize dimers
analogous to 16 while preventing the harmful degradation path-
ways caused during oxygen reduction. A 1:1 mixture of com-
plexes 9 and 10 catalyzed the oxidation of 2-heptanol at room
temperature to afford 2-heptanone in 67% yield after 24 h (entry
3). In accordance with our previous observations,26 the diacetate
complex 9 or ditriflate complex 10 under identical conditions
provided only 6% and 3% conversion, respectively, after 24 h.
36%.26 As previously discussed, this behavior was attributed to
competitive ligand oxidation, generating the inactive carboxylate
3 (Scheme 1).
The cationic phenanthroline dimer 8, lacking oxidizable
methyl groups, was completely inactive under these conditions
(Table 1, entry 2). We attribute the lack of activity to the
formation of the stable of μ-hydroxo-bridged dimer 16.26,60
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Scheme 7. Formation of μ-Hydroxo-Bridged Dimers
Catalysts derived from a mixture of 9/10 reached a turnover
number of 22 after 24 h, which is nearly twice that previously
observed with 1. Full conversion of 2-heptanol at 3 mol % catalyst
loading was not obtained even at extended reaction times.
Though a substoichiometric amount of palladium black was
observed after 24 h of reaction, a sharp decrease in the rate of
alcohol oxidation preceded Pd precipitation by several hours,
indicating that Pd-black formation in itself may not be respon-
sible for the observed loss in activity. Analysis of the reaction
mixture after 24 h by ESI-MS yielded the expected mass of the
ligand with no evidence of ligand oxidation. Attempts to prolong
catalyst lifetime by introducing additional ligand into the reaction
mixture (0.5 equiv relative to Pd) resulted in slower TOFs but
similar levels of conversion (61% yield at 72 h). Thus, while the
activity and longevity of catalysts derived from the tfmm-phen
ligand 6 are superior to that of catalyst 1 derived from neocu-
proine, the modest turnover numbers observed implicate addi-
tional catalyst deactivation pathways and highlight the need for a
better understanding of catalyst inactivation processes to enable
further optimization.
’ CONCLUSION
Cationic Pd-acetate complexes ligated by neocuproine or
2-substituted phenanthroline ligands are active for the aerobic
oxidation of secondary alcohols under mild conditions (room
temperature, 1 atm air). The dimeric complex [(neocuproine)
Pd(OAc)]2[OTf]2 (1) exhibits high initial turnover frequency
for the aerobic oxidation of 2-heptanol; however, oxidative
degradation of the ligand leads to rapid deactivation and low
turnover numbers.26 The synthesis of two new phenanthroline
ligands, bearing CF3 or 1,5-difluorophenyl substituents, was
carried out in an effort to generate more oxidatively robust Pd
catalysts. Pd complexes derived from these ligands were structu-
rally characterized, and their activity in the aerobic oxidation of
2-heptanol was compared to that of complex 1. The inactivity of
the [(phen)Pd(OAc)]2[OTf]2 (8) implies that substituents at
the 2- and/or 9- positions of the phen ligand are important for
catalytic activity. The higher catalyst lifetimes of Pd complexes
bearing the 2-CF3-substituted ligand 4-methyl-2-(trifluoro-
methyl)-1,10-phenanthroline (tfmm-phen) reveal that inhibiting
ligand oxidation can lead to more robust catalysts for aerobic
alcohol oxidation. Nevertheless, the modest turnover numbers
observed (22 mol ketone/mol Pd) are less than those observed
with Pd carbene complexes (approximately 100 TO).21 While we
observe no evidence for ligand oxidation, other decomposition
pathways may limit the lifetimes of catalysts derived from tfmm-
phen. Pd complexes derived from the 2-(o-difluorophenyl)-1,10-
phenanthroline (odfp-phen) ligand exhibit poor catalyst life-
times as a consequence of facile substitution of the ortho-fluoro
substituent by water (a byproduct of the reaction). These results
provide insights into the structure/property relationships of Pd
oxidation catalysts that may guide future design of oxidatively
robust catalysts for the aerobic oxidation of alcohols.
Investigation of a 1:1 mixture of 11 and 14 for the aerobic
oxidation of 2-heptanol in dry acetonitrile (entry 4) revealed
initial rates of alcohol oxidation comparable to 1, but the
reaction rate quickly dropped off (Figure 6, diamonds), and
only 2.5 turnovers were achieved after 24 h. Precipitation of the
resulting palladium complex was accomplished by addition of
1
diethyl ether, and the H NMR spectrum of the precipitate
exhibited splitting consistent with the formation of a chelated
phenoxide complex, analogous to 12. Two resonances were
observed by 19F NMR, consistent with the presence of a triflate
counterion and a single fluoride on the ligand, suggesting the
catalyst degradation product is the cationic complex 17. It is
worth noting that fluorine substitution by 2-heptanol was not
observed. The formation of tridentate complex 17 inactivates
the Pd in a manner that is reminiscent of the inactivation of 1 by
intramolecular ligand oxidation to tridentate complex 3,26
which exhibited negligible activity compared to its bidentate
precursor. Addition of molecular sieves to the 2-heptanol-
oxidation reaction mixture catalyzed by 11/14 increased the
TON of the catalyst to 4.5, suggesting that water produced
during oxygen reduction can be scavenged competitively with
fluoride displacement.
’ EXPERIMENTAL SECTION
Materials. Unless otherwise stated, solvents were purchased from
Sigma-Aldrich or Fisher Chemical and used as received. Deuterated
solvents were purchased from Cambridge Isotopes and used as received.
7-Methylquinoline was purchased from TCI America and used without
further purification. 8-Aminoquinoline was purchased from Sigma-
Aldrich and used without further purification. (1,10-Phenanthroline)Pd-
(OAc)2,431,26 4,35 and 536 were prepared as previously described. [(CH3
61
CN)4Pd][OTf]2 was prepared in analogy to [(CH3CN)4Pd][BF4]2
and was used in situ. 8-Amino-7-quinolinecarbaldehyde was prepared by
the route established by Thummel and co-workers.40 All NMR spectra
were acquired on Varian Inova 300, Mercury 400, or Inova 500 MHz
spectrometers. 1H and 13C NMR spectra are referenced to the solvent
residual peaks. 19F NMR spectra are referenced to CF3CO2H (-76.55
ppm) or free triflate anion (-77.6 ppm). For splitting patterns, “s”
refers to singlet, “d” refers to doublet, “t” refers to triplet, and “q” refers
to quartet.
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2-(Trifluoromethyl)-4-methyl-1,10-phenanthroline (6). Enone
536 (6.86 g, 49 mmol) was added over 5 h to a stirred solution of
8-aminoquinoline (4.24 g, 29 mmol) and sodium iodide (0.043 g, 0.28
mmol) in 70% sulfuric acid (10.6 mL) at 110 °C. The reaction was
allowed to continue for an additional hour and was then cooled to room
temperature. The dark orange-red mixture was poured into 1 M Na2CO3
(123 mL) and extracted with CH2Cl2 (3 ꢀ 100 mL). The combined
organics were extracted with 12 M HCl (5 ꢀ 50 mL). The aqueous,
acidic layer was neutralized using 3 M NaOH/1 M Na2CO3 and
extracted with CH2Cl2 (3 ꢀ 100 mL). The compound was purified
by silica gel column chromatography (CH2Cl2/MeOH; polarity was
slowly increased until product eluted), yielding 6 (1.6 g, 21%) in
reasonable purity as a brown solid. Further purification can be achieved
by crystallization (CHCl3/hexanes) or by column chromatography
(acetone/hexanes, 1:4), affording 6 as a white solid. 1H NMR (CDCl3)
δ: 9.29 (dd, J = 1.7, 4.6 Hz, 1H), 8.30 (dd, J = 1.7, 8.0 Hz, 1H), 8.07 (d, J =
9.2Hz, 1H), 7.95 (d, J = 9.2 Hz, 1H), 7.84 (s, 1H), 7.69(dd, J = 4.6, 8.0 Hz,
1H), 2.90 (s, 3H). 13C NMR (CDCl3) δ: 151.3, 147.4 (q, 2JCF = 34.4 Hz),
146.9, 146.2, 145.4, 136.2, 129.5 (q, 4JCF = 1.0 Hz), 128.8, 128.5, 123.6,
solution was stirred for 15 min. Removal of solvent under vacuum
afforded 8 quantitatively. Anal. Calcd for C30H22F6N4O10Pd2S2: C,
36.42; H, 2.24; N, 5.66. Found: C, 35.87; H, 1.56; N, 6.09. X-ray quality
crystals of 8 were obtained by slow diffusion of diethyl ether into a
saturated acetonitrile solution of a 1:1 mixture of [(phen)Pd(CH3
CN)2][OTf]2 and (phen)Pd(OAc)2.
(4-Methyl-2-(trifluoromethyl)-1,10-phenanthroline)Pd-
(OAc)2 (9). Pd(OAc)2 (0.0856 g, 0.38 mmol) was added to a stirred
solution of 6 (0.100 g, 0.38 mmol) in acetone (10 mL). The solution was
allowed to stir overnight, during which time a fine, dark red precipitate
formed. The red solid was isolated by vacuum filtration, rinsed repeat-
edly with acetone and Et2O, and finally dried under high vacuum to give
9 (0.138 g, 74%). 1H NMR (Cl2CDCDCl2) δ: 8.62 (d, J = 6.8 Hz, 2H,
H7 þ H9), 8.16 (d, J = 9.1 Hz, 1H, H5), 8.11 (d, J = 9.1 Hz, 1H, H6), 8.07
(s, 1H, H3), 7.90 (m, 1H, H8), 2.98 (s, 3H, Me), 2.17 (s, 3H, OAc), 1.99
(s, 3H, OAc). 13C NMR (Cl2CDCDCl2) δ: 178.8, 178.0, 152.4, 150.2,
147.2, 146.82, 146.78, 138.8, 130.7, 130.3, 128.7, 125.5, 124.2, 123.5,
23.2, 23.1, 19.7. 19F NMR (Cl2CDCDCl2) δ: -61.2, -61.4, -62.2, -
65.5. Anal. Calcd for C18H15F3N2O4Pd (H2O): C, 42.83; H, 3.39; N,
3
122.0, 121.8 (obsd d, 1JCF = 275.5 Hz), 120.19 (q, 3JCF = 2.2 Hz), 19.6. 19
F
5.55. Found: C, 42.54; H, 2.88; N, 5.88. HRMS (ESþ): calcd for
C16H12N2O2F3Pd [M - OAc] 426.9886, found 426.9887.
NMR (CDCl3) δ: -65.5 (s) Anal. Calcd for C14H9F3N2: C, 64.12; H,
3.46; N, 10.68. Found: C, 62.88; H, 3.10; N, 10.36. HRMS-(ESþ): calcd
for C14H10F3N2 [M þ H] 263.0796, found 263.0795.
[(4-Methyl-2-(trifluoromethyl)-1,10-phenanthroline)Pd-
(NCCH3)2][OTf]2 (10). Trifluoromethyl ligand 6 (0.109 g, 0.42
mmol) was added to a stirred solution of [CH3CN)4Pd][OTf]2
(0.580 g, 1.02 mmol) in acetonitrile (4 mL), during which time the
solution became orange. The solution was filtered, and Et2O was
carefully layered onto the filtrate (i.e., without mixing) to induce
crystallization. The resulting needlelike crystals were recrystallized two
times from acetonitrile/Et2O, yielding pure 10 (0.11 g, 35%). The
mother liquor from the two previous recrystallizations was concentrated
and treated with Et2O to yield an additional 0.045 g (49% combined
yield) of the title compound. 1H NMR (CD3CN) δ: 9.48 (dd, J = 1.2, 5.6
Hz, 1H), 8.86 (dd, J = 1.2, 8.4 Hz, 1H), 8.37 (d J = 9.0 Hz, 1H), 8.36 (m,
1H), 8.28 (d, J = 9.0 Hz, 1H), 8.01 (dd, J = 5.6, 8.4 Hz, 1H), 3.04 (s, 3H).
13C NMR (CD3CN) δ: 156.9, 153.6, 142.7, 133.0, 132.8, 130.3, 127.1,
125.8, 125.4, 123.2, 121.9, 120.6, 119.7, 19.9. 19F NMR (CD3CN) δ: -
59.83, -77.47. ESI-MS (þ): 665 [M - 2CH3CN], 517 [M - 2CH3CN
- OTf]. Anal. Calcd for C20H15F9N4O6PdS2: C, 32.08; H, 2.02; N, 7.48.
Found: C, 31.95; H, 1.76; N, 6.51. HRMS (ESþ): calcd for
C14H12N2F3Pd [LPdHþ] 368.9831, found 368.9838.
Conproportionation of 9 and 10. Equal molar amounts of the
diacetate 9 and the ditriflate 10 complexes were combined in acetonitrile
to generate a mixture of stereoisomers (Figure S17). A similar spectrum
is obtained upon addition of one equivalent of triflic acid to the diacetate
complex 9 in acetonitrile. The resulting solution exhibits similar catalytic
activity to that of an equal molar solution prepared by the conpropor-
tionation method; the initial rate of oxidation using 9/triflic acid is
slightly retarded, but the final conversion is equal to that obtained by
conproportionation within the limits of experimental uncertainty. The
complexity associated with the solution speciation upon mixing 9 and 10
is highlighted in Figure S17, which shows the aromatic regions of the 1H
NMR spectra of 9, 10, and their mixture.
2-(20,60-Difluorophenyl)-1,10-phenanthroline
(7). This
compound was prepared by adaptation of the procedure reported by
Thummel and co-workers40 for the synthesis of 2-phenyl-1,10-phenan-
throline. A saturated solution of ethanolic potassium hydroxide (0.5 mL)
was added dropwise to a solution of 8-amino-7-quinolinecarbaldehyde
(0.231 g, 1.34 mmol) and 20,60-difluoroacetophenone (0.209 g, 1.34
mmol) in absolute ethanol (15 mL) under an atmosphere of N2, and the
mixture was refluxed for 15 h. The solution was diluted with water (30 mL)
and extracted with CH2Cl2 (3 ꢀ 30 mL). The combined organic extracts
were washed with H2O (2 ꢀ 20 mL) and dried (MgSO4). Flash
chromatography on alumina using CH2Cl2 as the eluent and subsequent
recrystallization with EtOAc/hexane provided the title compound as an
off-white solid (0.228 g, 58%). 1H NMR (CDCl3) δ: 9.22 (dd, J = 1.3,
3.8 Hz, 1H, H9), 8.34 (d,, J = 7.6 Hz, 1H, H4), 8.25(dd, J = 1.3, 7.2 Hz,
1H, H7), 7.84 (s, 2H, H5-6), 7.79 (d, J = 7.6 Hz, 1H, H3), 7.63 (dd, J =
3.8, 7.2 Hz, 1H, H8), 7.38 (m, J = 1.8, 6.7, 6.3 Hz, 1H, p-Ph), 7.04 (m, J =
4.0, 6.5 Hz, 2H, m-Ph). 13C NMR (CDCl3) δ: 161.0 (dd, 1JCF = 250.5
Hz, 3JCF = 7.1 Hz), 150.8, 150.1, 146.4, 146.4, 136.3, 136.1, 130.3 (t, 3JCF
=
10.3 Hz), 129.0, 127.9, 127.2, 126.3, 125.4, 123.1, 119.0 (t, 2JCF =19.2
Hz), 111.8 (m, 2JCF = 24.2 Hz). 19F NMR (CDCl3) δ: -113.0 (t, J = 6.5
Hz). ESI-MS (þ): 293 [M þ H]. Anal. Calcd for C18H10F2N2: C, 73.97;
H, 3.45; N, 9.58. Found: C, 73.71; H, 3.38; N 9.39.
[(1,10-Phenanthroline)Pd(NCCH3)2][OTf]2. Triflic acid (1.5 mL,
0.33 M, 2 equiv) was added dropwise to a stirred solution of
(phen)Pd(OAc)2 (100 mg, 0.247 mmol) in dry acetonitrile (10 mL).
Dry diethyl ether was added to induce precipitation of the resulting
complex, which was recrystallized three times from CH3CN/Et2O to
achieve the desired compound in high purity (22 mg, 0.033 mmol,
1
13%). While this compound has been previously reported,44 the H
(2-(20,60-o-Difluorophenyl)-1,10-phenanthroline)Pd-
(OAc)2 (11). A solution of palladium diacetate (0.076 g, 0.342 mmol)
in CH2Cl2 (1.2 mL) was added to a stirred solution of ligand 7 (0.100 g,
0.342 mmol) in toluene (6.2 mL). The solution was allowed to stir for
24 h at room temperature, during which time a yellow solid formed. The
mixture was poured into petroleum ether (5 mL) to induce further
precipitation, and the yellow precipitate was isolated by vacuum filtra-
tion and dried under high vacuum (0.168 g, 82%). When complex 12 is
prepared by this method, it contains half of an equivalent of CH2Cl2
even after extensive drying, as observed by 1H NMR spectroscopy. Due
to its low solubility in CDCl3, chemical shifts are also reported in
anhydrous CD3CN. 1H NMR (CDCl3) δ: 8.67 (dd, J = 1.2, 5.3 Hz, 1H),
NMR shifts reported were suggestive of two species in a 1:1 mixture.
We have isolated a single species that matches one set of signals from
the previously reported spectrum. 1H NMR (CD3CN) δ: 8.96 (dd, J =
1.2, 8.4 Hz, 2H), 8.85 (dd, J = 1.2, 5.5 Hz, 2H), 8.24 (s, 2H), 8.04 (dd,
J = 5.5, 8.4 Hz, 2H). (signal due to bound CH3CN could not be
resolved from residual water and solvent resonances). 13C NMR
(CD3CN) δ: 153.2, 148.3, 143.5, 132.5, 129.1, 127.4, 122.0 (q,
1JCF = 320.7 Hz). Anal. Calcd for C18H14F6N4O6PdS2: C, 32.42; H,
2.12; N, 8.40. Found: C, 32.31; H, 1.88; N, 8.22.
[(1,10-Phenanthroline)Pd(OAc)]2[OTf]2 (8). To a solution of
(phen)Pd(OAc)2 (10.7 mg, 0.026 mmol) in acetonitrile (2 mL) was
added [(phen)Pd(NCCH3)2][OTf]2 (17.7 mg, 0.026 mmol), and the
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ARTICLE
8.66 (d, J = 9.2 Hz, 1H), 8.57 (dd, J = 1.2, 8.2 Hz, 1H), 8.48 (d, J = 9.2 Hz,
1H), 8.01 (d, J = 8.8 Hz, 1H), 7.91 (d, J = 8.8 Hz, 1H), 7.85 (dd, J = 5.3,
8.2 Hz, 1H), 7.25-7.11 (m, 2H), 6.48 (ddd, J = 1.5, 7.6, 14.4 Hz, 1H).
1H NMR (CD3CN) δ: 8.68 (dd, J = 1.3, 8.3 Hz, 1H), 8.66 (d, J = 8.3 Hz,
1H), 8.28 (dd, J = 1.3, 5.3 Hz, 1H), 8.08 (d, J = 8.8 Hz, 1H), 8.05 (d, J =
8.8 Hz, 1H), 7.76 (dt, J = 1.0, 8.5 Hz, 1H), 7.73 (dd, J = 5.3, 8.2 Hz, 1H),
7.75 (m, 1H), 7.15 (t, J = 8.2 Hz, 2H), 1.99 (2, 3H), 1.28 (s, 3H). 13C
NMR (CD3CN) δ: 177.5 (OAc), 177.4 (OAc), 161.0 (dd, 1JCF = 249.5
Hz, 3JCF = 6.23 Hz), 154.4, 150.7, 148.3, 147.3, 140.6, 140.5, 133.7 (t,
were isolated by cannula filtration and dried under high vacuum, yielding
14 (44 mg, 19%). 1H NMR (CD3CN) δ: 9.03 (d, J = 8.4 Hz, 1H, H4),
8.99(dd, J = 1.1, 8.3 Hz, 1H, H9), 8.85 (dd, J = 1.1, 5.5 Hz, 1H, H7), 8.28
(s, 2H, H5-6), 8.10 (d, J = 8.4 Hz, 1H, H3), 8.04 (dd, J = 5.5, 8.3 Hz, 1H,
H8), 7.78 (m, 1H, Arp), 7.35 (m, 2H, Arm). 13C NMR (CD3CN) δ:
160.8 (dd, 1JCF = 251.4 Hz, 3JCF = 5.6 Hz), 154.4, 154.1, 149.6, 148.8,
143.8, 143.7, 136.3 (t, 3JCF = 10.8 Hz),132.8, 132.1, 130.8, 129.5, 129.0,
127.3, 122.0 (q, 1JCF = 321.2 Hz), 116.2 (t, 2JCF = 18.6 Hz), 113.5 (m,
2JCF = 20.5 Hz). 19F NMR (CD3CN) δ: -77.6, -110.7 (t, J = 7.5 Hz).
Exposure to high vacuum for prolonged periods of time resulted in
partial loss of coordinated acetonitrile. Anal. Calcd for C24H16F8N4
O6PdS2: C, 37.01; H, 2.07; N, 7.19. Found: C, 36.60; H, 1.83; N, 6.68.
Anal. Calcd for C22H13F8N3O6PdS2: C, 35.81; H, 1.78; N, 5.69. Found:
C, 35.56; H, 1.82; N, 6.10.
3JCF = 10.7 Hz), 131.3, 130.57, 130.5, 128.75, 128.3, 126.2, 115 (t, 2JCF
=
19.2 Hz), 112.5 (m, 2JCF = 22.7 Hz), 23.2 (OAc), 22.1 (OAc). 19F NMR
(CD3CN): -111.6 (t, J = 7.1 Hz). ESI-MS(þ): 398.9 [M - 2OAc þ
H], 442.9 [M - 2OAc þ O2CH], 456.9 [M - OAc]. Anal. Calcd for
C22H16F2N2O4Pd 1/2(CH2Cl2): C, 48.32; H, 3.06; N, 5.01. Found: C,
3
48.06; H, 3.33; N, 4.80.
[(2-(20,60-o-Difluorophenyl)-1,10-phenanthroline)Pd(CH3-
CN)Cl][OTf] (15). Cationic-palladium precursor (CH3CN)4Pd
(OTf)2 (320 mg, 0.564 mmol) was prepared in situ by addition of
(CH3CN)2PdCl2 (0.1 g, 0.564 mmol) to AgOTf (0.289 g, 1.128 mmol)
by analogy to previous reports.61 After removing AgCl by filtration, the
precursor solution was added to a stirred solution of odfp-phen (148 mg,
0.508 mmol, 0.9 equiv) in acetonitrile and allowed to stir 2 h, after which
time the reaction mixture was concentrated and precipitated with Et2O.
The resulting solid (0.161 g) contained an acetonitrile-insoluble portion
(we attribute this to additional AgCl), which was removed by redissolv-
ing the precipitate in acetonitrile and filtering. The filtrate was evapo-
rated to dryness, and the complex was recrystallized twice by slow
addition of Et2O to acetonitrile, producing complex 15 in 10% yield
(0.043 g). 1H NMR (CD3CN) δ: 9.43 (dd, J = 1.3 5.6 Hz, 1H, H9), 8.95
(d, J = 8.4 Hz, 1H, H4), 8.86 (dd, J = 1.3, 8.2 Hz, 1H, H7), 8.23 (d, J = 1.5
Hz, 2H, H5-6), 8.08 (d, J = 8.4 Hz, 1H, H3), 7.99 (dd, J = 5.6, 8.2 Hz, 1H,
H8), 7.73 (m, 1H, Arp), 7.32 (m, 2H, Arm). 13C NMR (CD3CN) δ:
160.9 (dd, 1JCF = 250.6 Hz, 3JCF = 5.8 Hz), 153.1, 149.4, 148.5, 143.8,
143.6,142.4, 135.6 (t, 3JCF = 10.4 Hz) 132.4, 131.5, 130.5, 129.3, 128.6,
126.6, 122.0 (q, 1JCF = 320.9 Hz), 116.6 (t, 2JCF = 18.6 Hz), 113.41 (m,
2JCF = 23.1 Hz). 19F NMR (CD3CN) δ: -77.6, -111.2 (t, J = 7.2 Hz).
Anal. Calcd for C21H13ClF5N3O3PdS: C, 40.40; H, 2.10; Cl, 5.68; N,
6.73. Found: C, 40.29; H, 1.71; Cl, 6.07; N, 6.53.
Synthesis of Phenoxide Complex 12. Diacetate 11 (20 mg,
0.038 mmol) was dissolved in acetonitrile (5 mL). The solution was
filtered into a 20 mL vial through a plug of glass fiber to remove any
undissolved material. Two drops of water were added to the solution, and
the vial was capped, shaken, and allowed to sit at room temperature for 3
days. Crystals became visible after 24 h but continued to grow when
allowed to sit for additional time. X-ray-quality, needle-shaped crystals
were isolated by removal of the mother liquor with a pipet and repeated
washing of the crystals with Et2O. Upon drying under high vacuum,
complex 12 was obtained in 36% yield (6.3 mg). Additional crystals were
obtained by concentration of the mother liquor. Complex 12 is nearly
insoluble in acetonitrile and DMSO but is sparingly soluble in chloro-
form. 1H NMR (CDCl3) δ: 8.74 (d, J = 5.4 Hz, 1H, H9), 8.61 (d, J = 9.1
Hz, 1H, H4), 8.55 (d, J = 8.2 Hz, 1H, H7), 8.49 (d, J = 9.1 Hz, 1H, H3),
8.00 (d, J = 8.7 Hz, 1H, H5), 7.88 (d, J = 8.7 Hz, 1H, H6), 7.84 (dd, J = 5.4,
8.2 Hz, 1H, H8), 7.20 (m, 1H, HAr), 7.11(d, J = 8.7 Hz, 1H, HAr), 6.46
(dd, J = 7.8, 14.3 Hz, 1H, HAr), 2.30 (s, 3H, OAc). 1H NMR (d6-DMSO)
δ: 8.99 (dd, J = 1.4, 8.3 Hz, 1H), 8.91 (d, J = 9.2 Hz, 1H), 8.66 (dd, J = 1.8,
9.2 Hz, 1H), 8.62 (dd, J = 1.4, 5.2 Hz, 1H), 8.28 (d, J = 8.6 Hz, 1H), 8.24
(d, J = 8.6 Hz, 1H), 8.11 (dd, J = 5.2, 8.3 Hz, 1H), 7.29 (dd, J = 8.0, 15.5
Hz, 1H), 6.90 (d, J = 8.7 Hz, 1H), 6.60 (dd, J = 8.0, 14.2 Hz, 1H), 2.04 (s,
3H). 13C NMR (CDCl3) δ: 179.1 (OAc), 166.6, 162.5 (d, 1JCF = 251.5
Hz), 150.9, 149.6, 146.3, 145.7, 138.4, 136.0, 132.3 (d, 2JCF =14.8 Hz),
129.3, 127.2, 126.7, 126.5, 126.0, 124.7, 119.4, 102.7 (d, 2JCF = 26.0 Hz),
24.08 (OAc). 19F NMR (CDCl3) δ: -107.9 (dd, JHF = 14.3, 6.5 Hz).
ESI-MS(þ): 394.8 [M - OAc], 412.8 [M - OAc þ H2O], 792.8 [2M -
2OAc þ H], 836.8 [2M - OAc]. Anal. Calcd for C20H13FN2O3Pd: C,
52.82; H, 2.88; N, 6.16. Found: C, 52.47; H, 2.55; N, 5.90.
Conproportionation of 11 and 14. An equal molar quantity of
the ditriflate complex 14 (20 mg, 0.0257 mmol) and the diacetate
complex 11 (13 mg, 0.0257 mmol) were combined in acetonitrile until
all solids were dissolved. Figure S36 shows the aromatic regions of the
1H NMR spectra of 11, 14, and their mixture.
Phenoxide Complex 17. The diacetate complex 11 (8.07 mg,
0.015 mmol) was added to a flame-dried flask with anhydrous CH3CN
(1 mL), and the flask was capped with a rubber septum. To this was
added a stock solution of ditriflate complex 14 (11.68 mg, 0.015 mmol)
in CH3CN (0.5 mL), and the reaction mixture was stirred. Upon
complete dissolution of all solids, a stock solution of 2-heptanol
(0.5 mL, 0.2 M) was added through the septum and the solution was
stirred at room temperature under a balloon full of air for 24 h. The
reaction mixture was filtered, and Et2O was added to effect precipitation
of 17. The solid was collected by centrifugation, washed with Et2O, and
dried briefly under high vacuum. 1H NMR (CD3CN) δ: 8.76 (d, J = 8.2
Hz, 1H), 8.64 (d, J = 5.2 Hz, 1H), 8.51 (dd, J = 15.7, 9.2 Hz, 1H), 8.01
(dd, J = 11.6, 8.7 Hz, 2H), 7.89 (dd, J = 5.2, 8.2 Hz, 1H), 7.18 (dd, J =
14.9, 8.1 Hz, 1H), 6.66 (d, J = 8.6 Hz, 1H), 6.50 (dd, J = 7.8, 14.5 Hz,
1H). 19F NMR (CD3CN) δ: -77.6, -105.9 (dd, J = 14.4, 6.2 Hz)
General Method for Aerobic Oxidation of 2-Heptanol
Using Cationic Palladium Complexes. A solution of 2-heptanol
(146 μL, 1.00 mmol) and decane (100 μL, 0.538 mmol) in acetonitrile
(1.5 mL) was stirred under a balloon of air for 5 min. At t = 0, the
palladium catalyst (3 mol %) was added either as a solid or as a solution
in acetonitrile; in either case, the reaction mixture was adjusted such that
it contained a total acetonitrile volume of 2 mL. The mixture was stirred
[(2-(20,60-o-Difluorophenyl)-1,10-phenanthroline)Pd-
(CH3CN)2][OTf]2 (14). In a flame-dried flask equipped with a reflux
condenser was added PdCl2 (57 mg, 0.32 mmol, 1.1 equiv) and odfp-
phen 7 (87 mg, 0.30 mmol, 1.0 equiv). Dry acetonitrile (5 mL) was
added to the solids, and the solution was refluxed overnight under N2
atmosphere, during which time a yellow solid was formed. The suspen-
sion containing the yellow solid was treated with AgOTf (206 mg, 0.80
mmol, 2.7 equiv) and was refluxed in the dark for 2 days. Additional
AgOTf (103 mg, 0.40 mmol) was added to the reaction mixture, and
refluxing was continued in the dark for 1 day. At this time, the reaction
flask contained a mixture of 15 and 14 in a 2:1 ratio as observed by 1H
NMR spectroscopy. The solvent was evaporated to dryness, dissolved in
dry acetonitrile (5 mL), and filtered to remove residual AgCl, and
AgOTf (106 mg, 0.41 mmol, 1.4 equiv) was added to the solution. The
reaction mixture was heated an additional 4 days in the dark at 60 °C.
The solvent-evaporation/dissolution/filtration process was repeated
two more times, and in this manner, the ratio of 15 to 14 changed to
1:10. Finally, the solvent was evaporated to dryness, dry acetonitrile was
added, and the solution was filtered through Celite. Diethyl ether was
added to the solution until it became slightly turbid, and then the
solution was cooled to -25 °C. Yellow crystals formed overnight, which
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Organometallics
ARTICLE
at room temperature, and aliquots were taken periodically and subjected
to analysis by GC. Initial turnover frequencies (TOFi) were calculated
on the basis of the conversion of 2-heptanol to 2-heptanone after 1 min
of reaction time. 2-Heptanone was confirmed by co-injection with an
authentic sample.
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’ ASSOCIATED CONTENT
Supporting Information. 1H, 13C, and 19F NMR spectra
S
b
of new compounds, experimental procedures for complexation
studies with btfm-phen, CIF files giving crystal data for com-
pounds 8, 10, 12, and 15, and a comparison of the bond lengths
of 8 and 1.This material is available free of charge via the Internet
at http://pubs.acs.org.
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’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: waymouth@stanford.edu.
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’ ACKNOWLEDGMENT
This work was supported by the Global Climate and Energy
Project (No. 33454) at Stanford University. D.M.P. gratefully
acknowledges the Burt and Deedee McMurtry Stanford Gradu-
ate Fellowship Fund.
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