Solvent effect on palladium-catalyzed cross-coupling reactions and implications on the active catalytic species

Article abstract of DOI:10.1002/anie.201101746

Suzuki coupling of the bifunctional substrate 1 using [Pd ₂(dba)₃]/PtBu₃ gives selectivity for C-Cl in nonpolar solvents but for C-OTf in polar solvents. The results of computational and experimental studies suggest that the catalytically active species in polar solvents under conditions employing coordinating additives is inconsistent with monoligated [Pd(PtBu₃)]. Instead, the data are consistent with an anionic palladium complex as the active species.

Full text of DOI:10.1002/anie.201101746

Communications
DOI: 10.1002/anie.201101746
Cross-Coupling
Solvent Effect on Palladium-Catalyzed Cross-Coupling Reactions and
Implications on the Active Catalytic Species**
Fabien Proutiere and Franziska Schoenebeck*
One of the milestones in pushing the limits of the oxidative
addition step in palladium-catalyzed cross-coupling reactions
was the discovery that bulky phosphine ligands trigger
unprecedented high reactivity in palladium catalysis. A
variety of monodentate ligand systems were developed, for
bisligated palladium (which is the active species with PCy3)
[14]
À
favors C OTf insertion. Notably, calculations with differ-
ent ligands, for example, PMe3, showed that only the ligation
state of Pd is crucial for the selectivity, and a rationalization of
that behavior was also deduced.[14] Regioselectivity is con-
trolled by the distortion energy (and thus by the bond
example
PAdtBu2
(Ad = 1-adamantyl),[1]
((Ph5C5)-
(C5H4)Fe)PtBu2 ferrocenyl = (C5H5)(C5H4)Fe (Q-phos),[2]
PAd2nBu,[3] and Buchwaldꢀs biaryl phosphine ligands.[4] The
groups of Nishiyama[5] and Fu[6] demonstrated the power of
the bulky tri-tert-butylphosphine ligand, PtBu3. Fu and co-
workers showed that a combination of [Pd2(dba)3] (dba =
dibenzylideneacetone) and PtBu3 allows cross-coupling of
aryl chlorides at room temperature.[6,7] Experimental and
theoretical mechanistic studies indicated that a monoligated
palladium species would be the active catalyst in reactions of
Pd and PtBu3.[8–12]
dissociation energy of the C X bond) for monoligated Pd,
À
À
which therefore reacts with the weakest bond, that is, C Cl.
With bisligated Pd, in contrast, selectivity is controlled by the
À
greatest interaction in the transition state (TS; reaction at C
OTf).[14] These investigations have thus revealed a reactivity
system to differentiate mono- from bisligated catalytic
species.
Herein, we provide indirect evidence for a change of the
catalytically active species in polar solvents. The results
suggest that the active species under conditions employing
coordinating additives in polar solvents is inconsistent with
monoligated [Pd(PtBu3)] and suggests an anionic palladium
complex as the reactive species.
Fu and co-workers demonstrated the regioselectivity of
the reaction shown in Scheme 1 in THF.[7] We were interested
in studying the solvent effect on the selectivity. We performed
experiments on chloroaryl triflate 1 under the conditions
determined by Fu and co-workers[7] using [Pd2(dba)3]/PtBu3 in
a range of solvents. Table 1 reports the results of the
experiments.[17] We found that in analogy to Fuꢀs original
Fu and co-workers further demonstrated a remarkable
ligand effect in Pd-catalyzed coupling of chloroaryl triflate 1
(Scheme 1).[7,13] Whereas PCy3 gave coupling at the C OTf
À
À
bond, PtBu3 led to a reversal, with exclusive reaction at the C
Cl bond.[7] DFT calculations were applied to explore the
origins of this chemoselectivity.[14]
It was found that monoligated palladium (which is favored
À
with PtBu3 under these conditions) favors C Cl insertion and
À
study in THF, exclusive C Cl insertion takes place in the
nonpolar solvent toluene (see entry 1, Table 1). In polar
solvents such as DMFand MeCN, in contrast, the selectivity is
À
reversed, and there is remarkably high selectivity for C OTf
insertion (entries 2, 3, Table 1).[15,16]
Intrigued by this discovery, we set out to study the origin
of the selectivity reversal in polar solvents. Previous computa-
tional studies concluded that [Pd(PtBu3)] would react selec-
Scheme 1. Ligand-dependant chemoselectivity.[7]
Table 1: Variation of solvent and base in the reaction shown in
Scheme 1.[17][a]
Solvent
Base
t [h]
Yield [%]
[*] F. Proutiere, Prof. Dr. F. Schoenebeck
2
3
1
Laboratorium fꢀr Organische Chemie, ETH Zꢀrich
Wolfgang-Pauli-Strasse 10, 8093 Zꢀrich (Switzerland)
E-mail: schoenebeck@org.chem.ethz.ch
1[b,c]
2[c]
3[c]
4[d]
5[d]
6[e]
toluene
MeCN
DMF
THF
MeCN
MeCN
KF
KF
KF
DIPA
DIPA
lutidine
48
48
48
24
48
48
70[b]
3
3
14
2
–
74
22
7
27
25
20[b]
–
37
41
35
56
Homepage: http://www.schoenebeck.ethz.ch/
[**] We thank ETH Zꢀrich for financial support and the Laboratory of
Organic Chemistry for instrumental support, in particular the
groups of Profs. Bode, Chen, and Diederich for access to their GC-
MS and HPLC facilities. We thank Franziska Lissel for early work on
the project. Calculations were performed on the ETH High-
Performance Cluster Brutus.
<1
[a] With 1.5% [Pd2(dba)3] and PtBu3 as ligand.[b] Heating (at 708C for
48 h) gives 83% 2 and 6% recovery of 1. [c] Conditions: 3.0 equiv KF,
1.01 equiv RB(OH)2, 1.0 equiv 1, RT. [d] Conditions as in [c], but with
3.0 equiv diisopropylamine rather than KF. [e] Conditions as in [c], but
with 3.0 equiv lutidine rather than KF.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101746.
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[14]
À
tively with C Cl. However, these conclusions were based
on studies and calculations in THF and in the gas phase. A
suggests that the experimentally observed selectivity reversal
is due neither to solvent coordination nor to electrostatic
stabilization by the polar solvents. Thus, monoligated [Pd-
(PtBu3)] as active species is inconsistent with the reactivity
observed in polar solvents and suggests that a different
catalytic species is active.
À
polar solvent might stabilize the C OTf insertion TS more
À
strongly than the C Cl insertion TS and could thus cause a
selectivity change. To test this hypothesis, we applied
computational studies.[18] We optimized the transition states
À
À
for C Cl and C OTf insertion by [Pd(PtBu3)] in MeCN using
a CPCM solvation model and several methods. Table 2
What is the active species in polar solvents? Several
previous studies have suggested that anions can coordinate to
the Pd0 catalyst prior to oxidative insertion.[22,23] Roy and
Hartwig performed kinetic investigations on the oxidative
insertion of [Pd{P(o-tolyl)3}2] to ArOTf.[24] They found that
À
reports the results. All calculations gave a preference for C
Cl insertion, although to a varying extent. Thus, the exper-
À
imentally observed preference for C OTf insertion is not due
to electrostatic stabilization by the more polar solvent.[19]
added anions would accelerate oxidative insertion into the C
À
OTf bond and suggested that initial exchange of one ligand
would take place to form [Pd{P(o-tolyl)3}X]À (with X = Br,
Cl), which would then undergo oxidative insertion.[24,25] To
investigate whether the presence of anionic Pd species is
consistent with the reactivity observed in polar solvents, we
undertook DFT calculations[26] of the corresponding anionic
transition states involving [Pd(PtBu3)F]À. These calculations
revealed indeed clear preference for triflate insertion
(DDG° = 2.3 kcalmolÀ1 in MeCN and 5.4 kcalmolÀ1 in tolu-
ene). Thus, if [Pd(PtBu3)F]À were to be formed,[27] it would
preferentially give rise to triflate insertion. To test for the
importance of KF, we did experiments in the absence of KF
and instead used the sterically demanding organic bases
diisopropylamine (DIPA, entry 4, 5, Table 1) or lutidine
(entry 6, Table 1). Despite the absence of fluoride, the
Table 2: Calculation of DDG° for the insertion of C OTf and C Cl by
[Pd(PtBu3)] with different methods.[a]
À
À
Method
DDG° =DG°C-OTfÀDG°
C-Cl
B3LYP/6-31+G(d)[b]
B3PW91/6-31+G(d,p)[c]
BLYP/6-31+G(d,p)[c]
M06L/6-31+G(d,p)[c]
M052X/6-31+G(d,p)[c]
4.1
5.8
1.6
0.8
1.1
[a] Optimized in MeCN, energies in kcalmolÀ1. [b] ECP for Pd is
LANL2DZ. [c] ECP for Pd is SDD.
Polar solvents usually have a greater basicity and nucle-
ophilicity than nonpolar solvents. Thus, there might be
coordination of the polar donor solvent to the palladium
species in the transition state.[20] Our calculations show that
solvent-coordinated transition states indeed favor triflate
insertion (by DDG° = 4.9 kcalmolÀ1 with [Pd(PtBu3)-
(MeCN)]), which would be in accord with the results of the
experiments. However, we calculated the reaction free-
energy paths under solvent coordination and compared
those to the insertion pathways for catalyst without solvent
coordination, that is, [Pd(PtBu3)Sol] versus [Pd(PtBu3)] as
active species. Figure 1 shows the results. The free-energy
reaction profile shows that solvent coordination in the TS is
disfavored with energy barriers much higher (ca. 44 kcal
À
À
selectivity preferences (for C OTf in MeCN and C Cl in
THF) remained the same.[28] In these cases (i.e. entries 4–6,
Table 1), the reactivity in polar solvents would be consistent
with the coordination of deprotonated boronic acid to Pd and
oxidative insertion by [Pd(PtBu3)(ArBO2H)]À.[29] Our calcu-
lations of the TSs of oxidative insertion by [Pd(PtBu3)-
(ArBO2H)]À predict a clear preference for triflate insertion
(DDG° = 3.9 kcalmolÀ1 in MeCN).[30] Figure 2 illustrates the
corresponding anionic TSs. Thus, the applied computational
and experimental studies suggest that in the presence of
coordinating species, such as salt or boronic acid, [Pd-
(PtBu3)X]À (with X = F or ArBO2H) is active in polar
solvents and [Pd(PtBu3)] in nonpolar solvents. Changes in
the polarity of the reaction medium thus have a dramatic
effect on the activity of one species in competition with
another, resulting in a complete selectivity
molÀ1 for C OTf insertion) than those for the monoligated,
À
uncoordinated pathways (27.9 kcalmolÀ1 for C Cl insertion
À
[21]
and 33.7 kcalmolÀ1 for C OTf insertion).
This finding
À
reversal.
If these mechanistic conclusions were
À
correct, predominant C Cl insertion should
be observed in polar solvents in the absence of
coordinating additives such as KF or
ArB(OH)2. To test this hypothesis, we decided
to perform Stille cross-coupling reactions on
substrate 1 [Eq. (1)], as those can be done
additive-free, and the stannane coupling part-
ner is non-coordinating.[31,32] Table 3 gives the
results of the Stille test reactions. We now
À
indeed see high selectivity for C Cl insertion
in DMF if no coordinating anions or coupling
partner are present (see Table 3, entries 1 and
2).[33,34] Addition of KF or CsF (Table 3,
entries 3 and 4) once again results in predom-
Figure 1. Free-energy profile for oxidative insertion to 1.[21] Energies in kcalmolÀ1
Calculated with B3LYP/6-31+G(d), LANL2DZ (Pd).
.
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8193

Communications
In conclusion, using computational studies combined with
experiments, we provide strong support of a change of the
catalytically active species in polar solvents in the presence of
coordinating cross-coupling partner or additives, leading to a
reversal of regioselectivity. The results suggest that the active
species under such conditions in polar solvents is inconsistent
with monoligated [Pd(PtBu3)] and reinforce the proposals of
Amatore, Jutand et al. of anionic palladium as active catalytic
species,[22] in line with recent conclusions by Hartwig.[24]
Changes in solvent polarities can thus have a dramatic
effect on the activity of one catalytic species in competition
with another, resulting in a complete selectivity reversal.
Received: March 10, 2011
Revised: April 30, 2011
Published online: July 12, 2011
Keywords: palladium · regioselectivity · solvent effects ·
.
Suzuki coupling
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[8] a) Experimental studies by Fu and Littke showed sluggish
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J. F. Hartwig, J. Am. Chem. Soc. 2004, 126, 1184.
À
À
Figure 2. Tranistion states for C OTf (left) and C Cl (right) insertion
using [Pd(PtBu3)X]À with X=F (top), ArB(OH)O (bottom), optimized
in MeCN. DDG° =2.3 kcalmolÀ1 (for X=F) and 3.9 kcalmolÀ1 (for
À
X=ArBO2H) in favor of C OTf insertion, calculated with CPCM
(MeCN) B3LYP/6-31+G(d), LANL2DZ (Pd).
Table 3: Stille cross-couplings with 1 [Eq. (1)].
Additive[a]
KPF6
T
t
Coupling
partner
Ratio of compounds[b]
[9] F. Barrios-Landeros, B. P. Carrow, J. F. Hartwig, J. Am. Chem.
Soc. 2009, 131, 8141.
1
4
5
1
RT
38 h Bu3SnPh 45
Bu3SnPh
38 h Bu3SnPh 18
38 h Me3SnPh
47
88
30
21
8
9
51
79
[10] L. Xue, Z. Lin, Chem. Soc. Rev. 2010, 39, 1692.
[11] On the ligand effect in Suzuki–Miyaura coupling: a) J. Jover, N.
Frey, G. C. Lloyd-Jones, J. N. Harvey, J. Mol. Catal. A 2010, 324,
39; b) S. Kozuch, J. M. L. Martin, ACS Catal. 2011, 1, 246.
[12] Computational studies concluded that TSs with [Pd(PtBu3)2]
cannot be found: a) Z. Li, Y. Fu, Q.-X. Guo, L. Liu, Organo-
metallics 2008, 27, 4043; b) K. C. Lam, T. B. Marder, Z. Lin,
Organometallics 2007, 26, 758; c) M. Ahlquist, P.-O. Norrby,
Organometallics 2007, 26, 550.
2[c] KPF6
1008C 7 d
RT
RT
3
3
4
KF
CsF
–
[a] 3.0 equiv. [b] Ratio determined by calibrated GC-MS analysis. [c] 1.5%
[Pd(PtBu3)2]/0.75% [Pd2(dba)3].[34]
[13] For studies on similar systems: a) T. Kamikawa, T. Hayashi,
Tetrahedron Lett. 1997, 38, 7087; b) G. Espino, A. Kurbanga-
lieva, J. M. Brown, Chem. Commun. 2007, 1742; c) For a ligand-
inant triflate insertion.[35] These findings are strong evidence
of the validity of our computational and experimental
mechanistic conclusions.
À
À
dependant selectivity of Csp3 Br versus Csp2 Br bond activation,
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see: C. Mollar, M. Besora, F. Maseras, G. Asensio, M. Medio-
Simon, Chem. Eur. J. 2010, 16, 13390.
[25] Ligandless, anionic Pd complexes were isolated by B. P. Carrow,
J. F. Hartwig, J. Am. Chem. Soc. 2010, 132, 79.
[14] F. Schoenebeck, K. N. Houk, J. Am. Chem. Soc. 2010, 132, 2496.
[15] To test the generality with other aryl boronic acids, we
performed reactions of 1 with ArB(OH)2 (Ar= 4-MeOC6H4)
[26] Calculated with B3LYP/6-31 + G(d) and LANL2DZ (ECP for
Pd). See also: C. J. Cramer, D. G. Truhlar, Phys. Chem. Chem.
Phys. 2009, 11, 10757.
in toluene and MeCN. Exclusive reaction with C Cl is seen in
[27] The formation of [Pd(PtBu3)F]À is favored by 7.6 kcalmolÀ1
relative to [Pd(PtBu3)] and FÀ. Calculation of the formation of
[Pd(PtBu3)F]À relative to KF, favors [Pd(PtBu3)F]À by 2.8 kcal
molÀ1 relative to [Pd(PtBu3)] and KF. [Optimized with CPCM
(MeCN)and B3LYP/6-31 + G(d) and LANL2DZ (for Pd)].
[28] Use of NEt3 was previously found to give rise to lower
conversions, see: A. F. Littke, G. C. Fu, Angew. Chem. 1998,
110, 3586; Angew. Chem. Int. Ed. 1998, 37, 3387.
À
À
toluene and with C OTf in MeCN. Intermolecular competition
experiments of 4-acetylphenyl triflate and 4-acetylphenyl chlo-
ride with o-tolylboronic acid also showed complete selectivity
À
À
for C Cl in toluene and C OTf in MeCN. See the Supporting
Information for further information.
À
[16] A 50:50 mixture of MeCN/toluene results in mixture of C Cl
À
and C OTf insertion. The ratio of compounds 2/3/1 is 25:64:11
(applying conditions [b] in Table 1). See the Supporting Infor-
mation for further information.
[29] Brown and co-workers have previously suggested that the cross-
coupling partner might coordinate in the transition state; see
Ref. [13b]. See also: K. Matos, J. A. Soderquist, J. Org. Chem.
1998, 63, 461.
[17] Table 1 reports the yields after isolation and purification. The
crude mixture prior to purification was analyzed by GC-MS in
each case, confirming that the yields of isolated product are a
true reflection of selectivity. Reaction times of up to 48 h were
chosen to ensure high conversion.
[30] DDG° (preference C OTf insertion): 5.9 (toluene) and 9.1 kcal
À
molÀ1 (gas phase). The formation of [Pd(PtBu3)(ArBO2H)]À is
favored by 7.3 kcalmolÀ1 in MeCN relative to [Pd(PtBu3)] and
ArB(OH)OÀ. [Optimization with CPCM (MeCN) and B3LYP/
6-31 + G(d) and LANL2DZ (for Pd)].
[18] Gaussian09, RevisionA.01; M. J. Frisch et al.; see the Support-
ing Information.
[19] a) P. C. J. Kamer, P. W. N. M. van Leeuwen, J. N. H. Reek, Acc.
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Freixa, P. W. N. M. Van Leeuwen, Chem. Soc. Rev. 2009, 38,
1099.
[20] To gain insights on the active catalytic species in polar solvents,
we performed experiments under phosphine-free but otherwise
identical conditions (cf. the Supporting Information). In THF,
DMF, and MeCN, if at all, only trace conversions were observed.
This result suggests that a phosphine-containing Pd species is
responsible for the observed reactivity.
[31] Echavarren and Stille previously demonstrated Stille reactions
on 4-bromophenyl triflate using [Pd(PPh3)4] and [PdCl2(PPh3)2].
They showed that cross-coupling is also possible in the absence
of any additive, LiCl in their case. They further observed
dependence of selectivity on the additive; a rationale for
behavior, however, was not given: A. M. Echavarren, J. K.
Stille, J. Am. Chem. Soc. 1987, 109, 5478.
[32] On the role of LiCl in Stille reactions: A. L. Casado, P. Espinet,
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[33] Owing to the slow transmetalation step in the absence of
coordinating anions, the reaction requires elevated temperature
for full conversion in the case of KPF6; compare: a) W. J. Scott,
G. T. Crisp, J. K. Stille, J. Am. Chem. Soc. 1984, 106, 4630;
b) W. J. Scott, J. K. Stille, J. Am. Chem. Soc. 1986, 108, 3033.
[34] Control experiments showed that the [Pd(PtBu3)2]/[Pd2(dba)3]
system gives rise to identical selectivity as [Pd2(dba)3]/PtBu3 in
the Suzuki cross-coupling in polar and nonpolar solvents. High
temperature has an effect on selectivity, but the preference
remains the same. Suzuki reactions in the presence of KF at
elevated temperature in DMF gave a 63:37 ratio of 3 and 2 (at
1008C), and a 77:23 ratio of products 3 and 2 in MeCN at 808C.
[35] These conditions were applied by Fu and co-workers in Stille
reactions: A. F. Littke, L. Schwarz, G. C. Fu, J. Am. Chem. Soc.
2002, 124, 6343. CsF was found to be the most efficient additive,
consistent with the high conversion in entry 4 of Table 3.
Me3SnPh was used to show the independence of selectivity
from the tin reagent. Experiments with KFand Me3SnPh showed
a similar result to entry 3 in Table 3.
[21] Corrections to the free energies due owing to different concen-
trations of [Pd(PtBu3)] and MeCN were not applied, as values
for DDG° exceed concentration effects. Gas-phase energies are
given.
[22] a) C. Amatore, M. Azzabi, A. Jutand, J. Am. Chem. Soc. 1991,
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