Formation of ethane from mono-methyl palladium(II) complexes

Article abstract of DOI:10.1021/ja412338k

This article describes the high-yielding and selective oxidatively induced formation of ethane from mono-methyl palladium complexes. Mechanistic details of this reaction have been explored via both experiment and computation. On the basis of these studies, a mechanism involving methyl group transmetalation between Pd^II and Pd^IV interediates is proposed.

Full text of DOI:10.1021/ja412338k

Article
pubs.acs.org/JACS
Formation of Ethane from Mono-Methyl Palladium(II) Complexes
†
†
‡
§
§
,§
Monica D. Lotz, Matthew S. Remy, David B. Lao, Alireza Ariafard, Brian F. Yates, Allan J. Canty,*
‡
,†
James M. Mayer, and Melanie S. Sanford*
†
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, United States
School of Chemistry, University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia
‡
§
*
S Supporting Information
ABSTRACT: This article describes the high-yielding and selective oxidatively
induced formation of ethane from mono-methyl palladium complexes. Mecha-
nistic details of this reaction have been explored via both experiment and
computation. On the basis of these studies, a mechanism involving methyl group
II
IV
transmetalation between Pd and Pd interediates is proposed.
1
1
INTRODUCTION
thermodynamically unfavorable and (b) the subsequent C−C
■
II
coupling step at dimethyl Pd complexes is slow and low
The catalytic oxidative coupling of methane is an attractive
route to ethane and higher hydrocarbons. While heteroge-
11
1
yielding, even under forcing conditions. We hypothesized that
these challenging steps could be circumvented via the catalytic
cycle proposed in Scheme 2. This would involve: (i) CH4
1
,2
neous catalysts are known for this transformation, their
modest selectivities and high reaction temperatures have moti-
vated the pursuit of more selective and milder homogeneous
3
Scheme 2. Proposed Catalytic Cycle for Pd-Catalyzed
Methane Oxidative Dimerization
alternatives. To date, examples of homogeneous catalytic
4
,5
alkane/alkane coupling remain rare. We hypothesized that
the development of such processes could be informed by
analogy to better-known arene C−H oxidative coupling reac-
6
tions. For instance, the Pd-catalyzed dimerization of dimethyl
phthalate is a key step in the industrial preparation of the
7
monomer precursor to Upilex. The catalytic cycle for this reac-
tion is believed to involve the following steps: (i) arene C−H
II
activation by A to generate mono-aryl Pd complex B (Ar =
Ar
II
aryl), (ii) disproportionation to form bis-aryl Pd adduct C ,
Ar
(
iii) C−C bond-forming reductive elimination to release the
0
0
product biaryl and Pd , and (iv) oxidation of Pd to regenerate
the Pd catalyst (Scheme 1).
II
II
Scheme 1. Pd-Catalyzed Arene Oxidative Dimerization
activation at Pd to generate mono-methyl complex B , (ii)
Me
IV
oxidatively induced transmetalation to form dimethyl Pd
species D, and (iii) C−C coupling from D to regenerate the
II
7
Pd catalyst.
Importantly, CH −CH coupling at Pd and PdIV dimethyl
III
3
3
complexes (step iii) is well-known to occur much more readily
II
3c,10,12,13
than that from analogous Pd species.
Furthermore,
we have previously demonstrated a related oxidatively induced
II
CH −CH reductive elimination starting with the dimethyl Pd
3
3
complex (dtbpy)Pd(CH ) (1, dtbpy = 4,4′-di-tert-butyl-2,2′-
3
2
3a
bipyridine) (eq 1). The treatment of 1 with 1.1 equiv of
+
ferrocenium (Fc ) yielded ethane in 49% yield (maximum
theoretical yield = 50%) via Pd intermediate 2. This reaction
IV
II
Methane activation at Pd to form mono-methyl complexes
5
,8,9
provides proof-of-principle for the feasibility of the proposed
oxidative transmetalation step (Scheme 2, step ii). However,
analogous to BAr has significant precedent.
In contrast, the
generation of ethane from analogues of B (putative steps ii
Ar
10
and iii) has not been reported. The primary challenges asso-
II
ciated with ethane formation from mono-methyl Pd com-
Received: December 10, 2013
II
plexes are that (a) transmetalation at Pd (step ii) is highly
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Article
Importantly, the maximum possible yield of ethane in these
experiments is 50%. Minimal CH D (or CH ) is observed with
3
4
most oxidants (≤1% in all cases except with hypervalent iodine
oxidants, entries 2 and 3). The yield of ethane varies signifi-
cantly as a function of oxidant. In some cases, this is due to
competitive processes. For instance, hypervalent iodine oxi-
dants bearing carboxylate ligands (e.g., PhI(OAc) , PhI(OTFA) ,
there is currently no precedent for accessing Pd dimethyl
2
2
14
entries 2 and 3) afford significant quantities of CH OAc and
complexes like 1 via CH activation. Thus, in order to realize
3
4
CH OTFA as well as CH D. Overall, the highest yields of
the catalytic cycle in Scheme 2, it is critical to achieve CH −CH
3
3
3
3
−
9
ethane (and cleanest reactions) are observed with the 1e
oxidant acetylferrocenium tetrafluoroborate (AcFcBF ) and
the 2e oxidant N-fluoropyridinium triflate (NFTPT) (entries 6
coupling from mono-methyl intermediates like B . Herein, we
Me
17
report oxidatively induced ethane formation from mono-methyl
4
−
II
Pd complexes. Both experimental and computational details
and 7). The reaction of 3 with 1.2 equiv of each of these
oxidants affords ethane in 36 and 31% yield, respectively. With
about the mechanism of this transformation are discussed.
RESULTS AND DISCUSSION
Reactivity of 3. Our initial studies focused on the genera-
tion of ethane from mono-methyl Pd complex (dtbpy)Pd-
1.2 equiv of NFTPT, there was ∼15% of a Pd(CH ) species
3
■
remaining at the end of this reaction. The use of an excess of
oxidant (5 equiv) could be used to push the reaction to full
conversion; under these conditions, ethane was formed in 37%
yield (entry 8).
II
(
CH )(OAc) (3). Acetate and other carboxylate ligands have
3
been widely utilized in Pd-catalyzed methane functionalization
9
,15
The inorganic byproducts of these transformations are (dtbpy)-
reactions.
These are often proposed to promote C−H acti-
II
II
+
Pd (OAc)(BF ) (4) with AcFcBF and (dtbpy)Pd (OAc)(X)
vation via concerted metalation−deprotonation-type mecha-
nisms. Complex 3 was synthesized by the reaction of (dtbpy)-
4
4
1
6
X = F− (5-F); X = 2,4,6-trimethylpyridine (5-TP)] with
NFTPT. The X ligands of 4 and 5 (BF , F , and TP) are in
[
−
−
II
Pd (CH )(Cl) with AgOAc and was characterized by NMR
4
3
rapid exchange with solvent (acetone or water); thus, the com-
plexes were converted to the corresponding iodide salts to
determine yields. This procedure established that the yield of 4
is 43% and the yield of 5-F/TP is 55% under our standard
reaction conditions.
spectroscopy and elemental analysis. This complex is quite
stable in the absence of an oxidant. For instance, it can be
stored in the solid state at −33 °C for several months without
detectable decomposition. It is also stable in acetone at room
temperature under N for at least 24 h. Complex 3 does
2
decompose upon heating to 100 °C for 21 h in acetone-d6
Mechanistic Studies. We next sought to gain insights into
the mechanism of ethane formation in these systems using a
combination of experiment and theory. A first set of experi-
ments was performed to assess whether methyl radical inter-
(
eq 2). This results in CH D as the major organic product (83%
3
yield) along with only small amounts of ethane (7%). Notably,
no CH OAc was observed under these thermolysis conditions.
3
mediates (CH •) are involved in ethane formation from
3
complex 3. The reactions of 3 with AcFcBF and NFTPT were
4
performed in the presence of 40 equiv of the H atom donor
3a
1
,4-cyclohexadiene (CHD). As discussed in our prior work,
CH • intermediates formed under these conditions would be
3
expected to participate in rapid H• abstraction from cyclo-
In contrast, 3 reacts rapidly with various oxidants at 25 °C
hexadiene to afford CH in high yield. As shown in Table 2,
4
in acetone-d to form ethane in 11−37% yield (Table 1).
entries 3 and 4, ≤6% methane was detected under these con-
ditions, with only small decreases in the yield of ethane. These
6
−
−
a
results suggest against CH • formation as a major pathway to
Table 1. Oxidation of 3 with 1e and 2e Oxidants
3
Table 2. Influence of Cyclohexadiene and Light of Oxidative
a
Ethane Formation from 3
yield
yield
yield
3
b
c
entry
oxidant
K S O
C H
CH4
CH OAc
2
6
1
2
3
4
5
6
7
11%
15%
28%
17%
22%
36%
31%
37%
<1%
31%
3%
<1%
37%
2
2
8
PhI(OAc)2
PhI(OTFA)2
CAN
yield
C H
yield
CH4
d
entry
oxidant
conditions
49%
2
6
<1%
1%
<1%
<1%
<1%
<1%
<1%
1
2
AcFcBF
NFTPT
4
none
36%
37%
29%
35%
38%
39%
<1%
1%
AgBF4
none
b
AcFcBF4
NFTPT
NFTPT
<1%
<1%
1%
3
AcFcBF
NFTPT
4
CHD
2%
b
4
CHD
6%
e
c
8
5
AcFcBF
NFTPT
4
no light
no light
<1%
<1%
c
6
a
Conditions: 3 (4 μmol, 1 equiv), oxidant (4.8 μmol, 1.2 equiv), 1,1,2-
trichloroethane (4 μmol, 1 equiv, standard), acetone-d , 25 °C, 24 h.
a
Conditions: 3 (4 μmol, 1 equiv), AcFcBF (4.8 μmol, 1.2 equiv) or
6
4
b
c
The theoretical maximum yield of ethane is 50%. The theoretical
NFTPT (20 μmol, 5 equiv), 1,1,2-trichloroethane (4 μmol, 1 equiv,
standard), acetone-d , 25 °C, 1 h. 40 mmol of 1,4-cyclohexadiene
(CHD) added under the standard conditions. Reaction conducted
d
b
maximum yield of CH and CH OAc is 100%. CH OTFA is formed
4
3
3
6
e
c
in 49% yield in this reaction. Conditions: 3 (4 μmol, 1 equiv),
NFTPT (20 μmol, 5 equiv), 1,1,2-trichloroethane (4 μmol, 1 equiv,
under standard conditions but in a foil-wrapped NMR tube to exclude
standard), acetone-d , 25 °C, 1 h.
ambient light.
6
B
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Journal of the American Chemical Society
Article
ethane. We also compared the reactions of 3 with AcFcBF and
3′ + NFTP (N-fluoropyridinium) to 5′-F/5′-TP and ethane.
These steps are (1) oxidation of 0.5 equiv of 3′ by NFTP to
4
NFTPT carried out under ambient light to those conducted in
foil-wrapped NMR tubes. As shown in entries 5 and 6, nearly
identical yields were obtained in the presence and absence of
light, suggesting against mechanisms involving light-induced
IV
generate Pd isomers 6′ and 7′, (2) methyl group trans-
II
metalation between 1 equiv of 7′ and 1 equiv of Pd starting
1
9,20
material 3′ to form 8′ and 5-F′,
and (3) ethane reductive
cleavage of the Pd−CH bonds.
elimination from 8′ to release the products. The details of each
step are discussed below. The initial oxidation of 3′ with NFTP
3
We next conducted low temperature NMR experiments in an
attempt to observe organometallic intermediates during the
oxidation of complex 3. Importantly, our previous studies of the
+
proceeds via TS_I′ (Figure 1) with transfer of F to the Pd
⧧
center. This activation barrier (ΔG = 17.6 kcal/mol) is the
reaction of Pd dimethyl complex 1 with FcBF showed the
highest for the entire sequence. The initial product 6′ iso-
merizes to 7′ via a transition structure (TS_II′) with a very low
4
IV
3a
formation of trimethyl Pd intermediate 2 (eq 1). Complex 2
was proposed to form via initial oxidation of 1 followed by
subsequent methyl group transmetalation between a dimeth-
IV
barrier, as would be anticipated for a labile five-coordinate Pd
1
2
center. The isomerization is highly thermodynamically
favorable (ΔG = −8.7 kcal/mol).
IV
II
yl Pd intermediate and the dimethyl Pd starting material.
Intermediate 2 is stabilized by the presence of three strongly
electron donating methyl groups; thus, its detection at low
Transmetalation of the methyl ligand between 3′ and 7′
proceeds via an early transition state in which the transferring
1
IV
temperature by H NMR spectroscopy was relatively
straightforward.
methyl group is much closer to the initial Pd than the in-
1
2
II
coming Pd nucleophile, as reflected in the Pd−CH distances
3
We hypothesized that an analogous methyl group trans-
metalation process could be occurring during the oxidation of
(2.183 and 2.756 Å, respectively, in TS_III′). The geometry at
the carbon of the bridging methyl ligand is not quite trigonal
3
(Scheme 3). However, here, the penultimate organometallic
bipyramidal, as reflected in CH angles (117.4−117.9°). The
2
transition structure shows a single imaginary frequency, with an
“umbrella” motion for the methyl group. The transmetalation
reaction is thermodynamically downhill (ΔG = −19.8 kcal/mol)
and is essentially barrierless. The transmetalation process is
facilitated by the excellent orbital match between the HOMO
of 3′ and the LUMO of 7′ (Figure 1).
Scheme 3. Proposed Mechanism for Ethane Formation
from 3
Finally, ethane formation occurs via H C−CH coupling at
3
3
the five-coordinate complex 8′ (containing an additional weak
Pd···O interaction). The barrier for this process is calculated to
be low (10.5 kcal/mol) in the absence of a strongly coordi-
nating ligand to fill the sixth coordination site. The other
possible reductive elimination reaction from 8′ (H C−OAc cou-
3
pling) is much less favorable, with a barrier of 32.4 kcal/mol.
This is consistent with the fact that this product is not observed
with NFTPT as the oxidant.
Model Complex to Detect Intermediates. We reasoned
that replacing the OAc of 3 with a more stabilizing X-type
ligand might enable detection of some of the short-lived inter-
intermediate would be the dimethyl PdIV species 8, which is
expected to be significantly less stable than its trimethyl
mediates in this ethane-forming reaction. Several recent reports
1
2
IV
analogue. The treatment of 3 with NFTPT or AcFcBF at
have shown that CF
tes.
3
ligands stabilize related Pd intermedia-
4
1
8b,c,d,21
1
−
78 °C in CD Cl resulted in H NMR spectra with broad,
uninterpretable signals. The formation of ethane occurred upon
Furthermore, CF ligands are notoriously inert in
3
2
2
reductive elimination reactions, suggesting the possibility that
methyl ligand transmetalation between Pd centers could out-
warming these samples to −60 °C. The addition of 100 equiv
IV
of CH CN (which was anticipated to stabilize cationic Pd
compete reductive elimination to form F
3
C−CH
next targeted (dtbpy)Pd (CH )(CF ) (9) as a model com-
3 3
3
. Thus, we
3
12
II
complexes such as 8) did marginally enhance the stability of
the intermediates formed during this process. However, the
spectra remained broad and showed a complex mixture of H
NMR signals. Furthermore, warming this mixture to −40 °C
resulted in the rapid liberation of ethane.
pound for probing reaction intermediates and mechanisms of
oxidatively induced ethane formation from mono-methyl Pd
1
complexes.
II
Complex 9 was prepared by treating (tmeda)Pd (CH
)(I)
3
We turned to DFT to obtain a mechanistic picture of the
reactivity of 3 with oxidants. We selected the reaction of
NFTPT with 3 for detailed computational study for two rea-
sons. First, it affords the highest yield of ethane of any of the
transformations in Table 1. Second, NFTPT is well-known to
with CsF/TMSCF and then dtbpy. It was characterized by
3
NMR spectroscopy and elemental analysis. Palladium(II) com-
plex 9 is even more stable than 3 in the absence of oxidants. It
can be stored under N in the solid state for >6 months without
2
detectable decomposition and is stable at 25 °C in acetone for
−
II
18
function as a 2e oxidant for Pd centers, thus limiting the
at least 2 days under N . Even at 100 °C in acetone, it only
2
likelihood of odd-electron intermediates, which would be much
decomposes completely after 8 days, yielding CH D as the
3
19
more difficult to treat computationally. Notably, these calcu-
lations were performed using bipyridine (bpy) in place of dtbpy
as the ligand at palladium; as such, the starting material is
termed 3′ and subsequent intermediates are denoted with a
major organic product in 74% yield. Trace ethane (<5%) and
no F C−CH were detected (eq 3).
3
3
Complex 9 reacts rapidly with AcFcBF and NFTPT at
4
25 °C in acetone-d to afford ethane in 34 and 41% yield,
6
20
−
prime. The DFT calculations show a low energy 2e oxi-
respectively (Table 3, entries 1 and 2). As with complex 3, ≤2%
dation path that has three steps in the overall conversion of
yield of methane was detected in these reactions. The inorganic
C
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Journal of the American Chemical Society
Article
Figure 1. Energy profile for the reaction of (bpy)Pd(CH )(CF ) (3′) with the N-fluoropyridinium cation, together with views of the HOMO for 3′
3
3
and LUMO for 7′. TP = 2,4,6-trimethylpyridine. Energies ΔG (ΔH) in kcal/mol.
exclusion of light (entries 5 and 6). As with 3, these results
suggest against CH • intermediates as a major pathway to
3
ethane formation.
DFT calculations show that the lowest energy path for the
reaction of 9′ (bipyridine analogue of 9) with NFTP is di-
rectly analogous to that for 3. It involves three steps: (1)
a
Table 3. Reaction of 9 with AcFcBF and NFTPT
4
IV
oxidation of 0.5 equiv of 9′ by NFTP to generate Pd iso-
mers 11′ and 12′, (2) methyl group transmetalation between 1
II
equiv of 12′ and 1 equiv of Pd starting material 9′ to form
2
3−25
13′,
and (3) ethane reductive elimination from 13′ to
yield
yield
CH4
26
entry
oxidant
conditions
C H
release the products (Scheme 4). This sequence directly par-
allels that found for 3′ (Figure 1); as a result, most of the details
are discussed in the Supporting Information. The barrier for
2
6
1
2
AcFcBF4
NFTPT
AcFcBF4
NFTPT
AcFcBF4
NFTPT
none
34%
41%
29%
42%
39%
42%
2%
none
<1%
2%
b
3
CHD
b
4
CHD
2%
Scheme 4. Proposed Mechanism of Reaction of 9 with
NFTPT
c
5
no light
no light
<1%
<1%
c
6
a
Conditions: 9 (4 μmol, 1 equiv), AcFcBF (4.8 μmol, 1.2 equiv) or
4
NFTPT (20 μmol, 5 equiv), 1,1,2-trichloroethane (4 μmol, 1 equiv,
b
standard), acetone-d , 25 °C, 1 h. 40 mmol of 1,4-cyclohexadiene
6
c
(
CHD) added under otherwise standard conditions. Reaction
conducted under standard conditions but in a foil-wrapped NMR
tube to exclude ambient light.
II
+
byproduct of both of these reactions is (dtbpy)Pd (CF )(X)
3
−
(
10, X = solvent, F , or TP), which is formed in 95 and 72%
yield, respectively. The reactions of 9 with AcFcBF and
22
4
NFTPT were essentially unaffected by the presence of 40 equiv
of 1,4-cyclohexadiene (CHD, entries 3 and 4) and by the
D
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Journal of the American Chemical Society
Article
II
ethane reductive elimination from 13′ is computed to be lower
than that from 8′ (6.0 vs 10.5 kcal/mol, respectively). This
difference can be attributed to the second weak C-O inter-
action in 8′; generally, reductive elimination from six-coordinate
intermediate and 1 equiv of the Pd starting material. The key
IV
Pd dimethyl intermediate was detected and characterized
during the reaction of (dtbpy)Pd(CH )(CF ) with AcFcBF .
Overall, these transformations provide exciting precedent for
the possibility of achieving C−C coupling of methane via a
catalytic cycle like that in Scheme 2. Studies toward this goal
are currently underway.
3
3
4
27
complexes is slower than that from five-coordinate complexes.
The DFT calculations suggest that intermediates 11, 12, and
3 should not be detectable, since the oxidation step has the
1
highest barrier. However, we hypothesized that the addition
of a coordinating solvent might trap 11, 12, and/or 13 as six-
coordinate cationic species that could be detected at low tem-
perature by NMR spectroscopy. Conducting the reaction of 9
with NFTPT at −40 °C in CD Cl in the presence of 100 equiv
ASSOCIATED CONTENT
Supporting Information
Experimental and spectral details for all new compounds and all
reactions reported. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
*
S
2
2
of CD CN resulted in a complex and uninterpretable mix-
3
1
19
28
ture of H and F NMR signals. However, under otherwise
identical conditions, the reaction of 1 equiv of 9 with 1.2 equiv
AUTHOR INFORMATION
Corresponding Authors
mssanfor@umich.edu
■
+
1
19
of AcFc afforded an intermediate with H and F NMR signals
consistent with formulation as complex 13-CD CN (Scheme 5).
3
Allan.Canty@utas.edu.au
Notes
+
Scheme 5. Reaction of 9 with AcFc with Added CD CN
3
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF under the CCI Center for
Enabling New Technologies through Catalysis (CENTC)
Phase II Renewal, CHE-1205189, and the Australian Research
Council.
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■
(
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II
1
(
(
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3
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3
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+
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3
2
3
1
19
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29
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IV
involvement of Pd intermediate 13-CD CN in the oxidatively
3
3
3
(
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(
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(
(
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(
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G. B. Dalton Trans. 2013, 42, 12794.
CONCLUSIONS
In summary, this report describes the oxidatively induced
■
1
II
generation of H C−CH from mono-methyl Pd acetate and
3
3
trifluoromethyl complexes. Experimental studies suggest that
(9) Key examples: (a) Kao, L. C.; Hutson, A. C.; Sen, A. J. Am. Chem.
the major pathway for ethane formation does not involve CH •
3
Soc. 1991, 113, 700. (b) Muehlhofer, M.; Strassner, T.; Herrmann, W.
intermediates. DFT calculations on the reaction of the OAc and
A. Angew. Chem., Int. Ed. 2002, 41, 1745.
(10) For the generation of ethane from a Pd mono-methyl complex,
III
CF systems with NFTP show a low energy pathway involv-
ing methyl group transmetalation between 1 equiv of a Pd
3
IV
see: (a) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M.
E
dx.doi.org/10.1021/ja412338k | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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(
2
(
11) Remy, M. S.; Cundari, T.; Sanford, M. S. Organometallics 2010,
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IV
3
3
IV
II
Skelton, B. W.; White, A. H. J. Chem. Soc., Chem. Commun. 1986, 1722.
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(24) Pd to Pt methyl group transmetalation, including kinetics:
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(
̈
III
IV
Canty, A. J. Organometallics 1994, 13, 2412. (d) Canty, A. J. Dalton
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III
(
13) CH −CH coupling at Pd : (a) Khusnutdinova, J. R.; Qu, F.;
3
3
Zhang, Y.; Rath, N. P.; Mirica, L. M. Organometallics 2012, 31, 4627.
b) Tang, F.; Zhang, Y.; Rath, N. P.; Mirica, L. M. Organometallics
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(
2
(
(26) On the basis of logic analogous to that in ref 19, we believe that
it is reasonable to propose that the oxidation of 9 with NFTPT and
IV
AcFcBF
likely proceed via the same penultimate Pd intermediate 13.
4
(27) (a) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem.
Soc. 2000, 122, 1456. (b) Crumpton, D. M.; Goldberg, K. I. J. Am.
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(28) The complex mixtures of intermediates observed with NFTPT
are likely due to the presence of two potential counterions (F and
OTf) as well as two potential L-type ligands (solvent and 2,4,6-
trimethylpyridine) in this system.
(
6048. (b) An, Z.; Pan, X.; Liu, X.; Han, X.; Bao, X. J. Am. Chem. Soc.
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610.
(29) The expected organic byproduct benzothiophene was formed in
(
16) (a) Siegbahn, P. E. M.; Crabtree, R. H.; Norlund, P. J. Biol. Inorg.
3
9% NMR yield at the end of this reaction.
Chem. 1998, 3, 314. (b) Garcia-Cuadrado, D.; Braga, A. A. C.;
Maseras, F.; Echavarren, A. M. J. Am. Chem. Soc. 2006, 128, 1066.
(
(
c) Lafrance, M.; Fagnou, K. J. Am. Chem. Soc. 2006, 128, 16496.
d) Lafrance, M.; Gorelsky, S. I.; Fagnou, K. J. Am. Chem. Soc. 2007,
1
29, 14570. (e) Gorelsky, S. I.; Lapointe, D.; Fagnou, K. J. Am. Chem.
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9, 1118. (g) Sun, H.-Y.; Gorelsky, S. I.; Stuart, D. R.; Campeau, L.-C.;
3
Fagnou, K. J. Org. Chem. 2010, 75, 8180. (h) Gorelsky, S.; Lapointe,
D.; Fagnou, K. J. Org. Chem. 2012, 77, 658. (i) Maleckis, A.; Kampf, J.
W.; Sanford, M. S. J. Am. Chem. Soc. 2013, 135, 6618.
(
17) AcFcBF provided higher yields than FcBF . The former is well-
4
4
documented to be a stronger oxidant: Connelly, N. G.; Geiger, W. E.
Chem. Rev. 1996, 96, 877.
(
18) (a) Hull, K. L.; Anani, W. Q.; Sanford, M. S. J. Am. Chem. Soc.
2
006, 128, 7134. (b) Ball, N. D.; Kampf, J. W.; Sanford, M. S. J. Am.
Chem. Soc. 2010, 132, 2878. (c) Racowski, J. M.; Ball, N. D.; Sanford,
M. S. J. Am. Chem. Soc. 2011, 133, 18022. (d) Racowski, J. M.; Gary, J.
B.; Sanford, M. S. Angew. Chem., Int. Ed. 2012, 51, 3414. (e) Maleckis,
A.; Sanford, M. S. Organometallics 2011, 30, 6617.
II
(
19) The initial oxidation of Pd with AcFcBF almost certainly
4
−
occurs via an outer sphere 1e pathway, and the resulting open shell
intermediate(s) are very challenging to assess computationally.
+
However, our prior studies of the reactivity of 1 with Fc show that
−
IV
1
e oxidants can ultimately yield Pd intermediates (i.e., 2) via methyl
−
group transmetalation and a second 1e oxidation. (Importantly, these
latter two steps could occur in either order, with CH group
3
III
IV
transmetalation occurring at either Pd or Pd ; see refs 23−25 for
examples of each possibility.) These results suggest that it is reasonable
that ethane formation from 3 with AcFcBF4 and NFTPT could
IV
proceed via an analogous penultimate Pd intermediate (8).
(
20) Gaussian 09 was used at the B3LYP level for geometry
optimization with dichloromethane as solvent utilizing the SDD basis
set on Pd and the 6-31G(d) basis set for other atoms. Single point
energy calculations for all structures at the M06 level employed the
quadrupole-ξ valence def2-QZVP basis set on Pd along with the
corresponding ECP and the 6-311+G(2d,p) basis set on other atoms.
See the Supporting Information for full details.
(
21) (a) Ye, Y.; Ball, N. D.; Kampf, J. W.; Sanford, M. S. J. Am. Chem.
Soc. 2010, 132, 14682. (b) Powers, D. C.; Lee, E.; Ariafard, A.;
Sanford, M. S.; Yates, B. F.; Canty, A. J.; Ritter, T. J. Am. Chem. Soc.
2
(
012, 134, 12002.
22) The X ligands of 10 (BF , F, and TP) are in rapid exchange with
4
solvent (acetone or water); thus, the complexes were converted to the
corresponding iodide salts to determine yields.
IV
II
(
23) Pd to Pd methyl group transmetalation: (a) Markies, B. A.;
Canty, A. J.; Boersma, J.; van Koten, G. Organometallics 1994, 13,
F
dx.doi.org/10.1021/ja412338k | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX