Counterintuitive kinetics in Tsuji-Trost allylation: Ion-pair partitioning and implications for asymmetric catalysis

Article abstract of DOI:10.1021/ja806278e

The kinetics of Pd-catalyzed Tsuji-Trost allylation employing simple phosphine ligands (L = Ar₃P, etc.) are consistent with turnover-limiting nucleophilic attack of an electrophilic [L₂Pd(allyl)]⁺ catalytic intermediate. Counter-intuitively, when L is made more electron donating, which renders [L₂Pd(allyl)]⁺ less electrophilic (by up to an order of magnitude), higher rates of turnover are observed. In the presence of catalytic NaBArF, large rate differentials arise by attenuation of ion-pair return (via generation of [L₂Pd(allyl)]⁺ [BArF]^-) a process that also increases the asymmetric induction from 28 to 78% ee in an archetypal asymmetric allylation employing BINAP (L*) as ligand. There is substantial potential for analogous application of [M]ⁿ⁺([BArF]^-)_n cocatalysis in other transition metal catalyzed processes involving an ionic reactant or reagent and an ionogenic catalytic cycle. Copyright

Full text of DOI:10.1021/ja806278e

Published on Web 10/08/2008
Counterintuitive Kinetics in Tsuji-Trost Allylation: Ion-Pair Partitioning and
Implications for Asymmetric Catalysis
Louise A. Evans,† Natalie Fey,† Jeremy N. Harvey,† David Hose,‡ Guy C. Lloyd-Jones,*,†
Paul Murray,‡ A. Guy Orpen,† Robert Osborne,‡ Gareth J. J. Owen-Smith,† and Mark Purdie§
UniVersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K., AstraZeneca, AVlon Works, SeVern Road, Hallen,
Bristol BS10 7ZE, and AstraZeneca, Bakewell Road, Loughborough, LE11 5RH, U.K.
Received August 8, 2008; E-mail: Guy.Lloyd-Jones@bris.ac.uk
Table 1. Relative Rates (kNuREL) for 2 + [4(L)2][OTf]f3, Empirical
The Tsuji-Trost allylation of stabilized carbanions (1 + 2 f 3,
Rate Constants (kL) for 1 + 2f3 in THF Employing [4(L)2][OTf] as
Scheme 1) has evolved from a stoichiometric process1 into one
Precatalyst, and Normalized Relative Rates in THF, DMF, and
that is catalytic,2 versatile, and extraordinarily selective.3 The
CH2Cl2
generic catalytic cycle outlined in Scheme 1, involving oxidative
addition (kOA f [4]+), followed by nucleophilic attack (kNu) has
become firmly established in the literature.2b,3 The apparent
simplicity of the mechanism and general robustness of the process
accounts for its ubiquity as a “benchmarking reaction” in ligand
design for asymmetric catalysis.3 Herein we report on the kinetics
c
kL (M-1 s-1 b
1b
1.9
)
kL
REL
of the catalytic reaction in the absence of interfering coligands such
as halide or dba.4h-l We demonstrate that a variation of electron-
demand in the phosphine ligand (L ) i-xiii, Table 1)5 leads to a
counterintuitive spectrum of ligand-dependent turnover rates (kL).
This phenomenon has led us to use catalytic NaBAr′F (NaB[(3,5-
(CF3)2)C6H3)4])6 to attenuate ion-pair return at the stage of the
palladium allyl intermediate [4]+|[X]-; a process analogous to the
“special salt effect” for carbocations.7 Ion-pair partitioning by
catalytic NaBAr′F provides new opportunities to accelerate ally-
lation reactions, to reduce catalyst loadings, and to improve
enantioselectivity, in this well-known reaction.
Despite extensive exploration of influence of ligands on the
selectivity of allylation,3 little has been documented regarding their
influence on the overall reaction rate.4 Nonetheless, there is a
consensus that under catalytic conditions, rate ) kobs[Nu]x.4 This
has given rise to the general assumption that the Pd-allyl intermedi-
ate ([4]+), not L2Pd, is the “resting-state” of the catalytic cycle.3
The corollary of this assumption is that the turnover rate should
depend primarily on the electrophilicity (kNu) of [4]+, as modulated
by ligands “L” (Scheme 1). The effect of a range of simple aryl
and alkyl phosphine ligands (i-xiii) on the stoichiometric reactivity
REL a
entry
L
kNu
1a
1c
THF DMF CH2Cl2
d
1
2
3
4
5
6
7
8
9
(i)
P(p-C6H4CF3)3
2.64
0.73
4.68
8.32
53.1 0.03 0.02
5.58
(ii) P(p-C6H4Cl)3
(iii) P(p-C6H4F)3
(iv) PPh3
2.64
2.15
1.00
0.41
d
b
26.9
1.00 1.00
1.00
0.37
(v) P(p-C6H4Me)3
71.8 397 1980 2.67 1.34
(vi) P(p-C6H4OMe)3 0.36 241
(vii) PPh2Cy
(viii) PPhCy2
(ix) PCy3
0.49
0.13
0.20
3.01
0.19 225
3.95
71.8
99.7
36.9
10 (x) P(OPh)3
0.91
e
11 (xi)
12 (xii)
13 (xiii)
dppf
e
1.44
dppf-(p-CF3)4
dppf-(p-CH3)4
e
0.75 239
a Stoichiometric allylation: by first-order compensated fit of MS
analysis of n in [13Cn]-3 obtained by competition of [4(ii-x)][OTf] and
[4(x-xiii)][OTf] with
[
13C1-4(i)2][OTf] for limiting 2. b Linear
regression of kobs ) kL[Pd]tot; kobs from In[2]0/[2]t ) kobst. c kL for each
solvent is normalized to kL ) 1.00 for L ) PPh3 (iv). d Using BSA/
CH(Me)E2 + cat. NaOAc. e Bidentate ligand.
Scheme 1. The Generic Catalytic Cycle for Tsuji-Trost
Allylation.1-3 a
(kNu) of [4][OTf] toward 2 was readily assessed, Table 1. The data
REL
(normalized to kNu
) 1.00 for L ) PPh3, entry 4) shows the
reactivity of [4]+ spans about 1 order of magnitude and in the
direction expected: electron withdrawing ligands (L) increase the
electrophilicity (kNu).4bc,f
Under catalytic conditions the rate of reaction 1a + 2f3 was
found to be independent of the allyl acetate (1a) concentration:
d[3]/dt ) kL[Pdtot]1[2]1. While fully consistent with the generic
catalytic cycle, Scheme 1, the trend in kL is completely opposed to
that of kNu, across the whole range of ligand systems tested. For
example, the p-tolyl dppf complex [4(xiii)][OTf] generates a catalyst
for the reaction of 1a with 2 that is 166-fold more efficient (kL)
than its p-trifluoromethyl analogue [4(xii)][OTf] (Table 1, entries
12 and 13), yet the complex itself is 5-fold less reactive (kNu) toward
2sa discrepancy of almost 3 orders of magnitude in the ratio of
a kL is the empirical rate constant for turnover as a function of ligand.
Replacing allyl acetate (1a) with allyl benzoate (1b) or allyl
methyl carbonate (1c) led to substantially faster turnover, Table 1
entries 1 and 5, with 1c increasing kL values by just under 2 orders
of magnitude over 1a. However, as with 1a, the more electron rich
ligand P(p-tolyl)3 (v) induced about 2 orders of magnitude faster
turnover than the electron withdrawing ligand P(p-trifluorotolyl)3
(i).8
kL/kNu
.
† University of Bristol.
‡ AstraZeneca, Avlon Works.
§ AstraZeneca, Bakewell Road.
9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 14471–14473 14471

C O M M U N I C A T I O N S
Scheme 2. Modified Standard Cycle (A), with Ion-Pair Partitioning
(kIR/kNu) to Account for the Kinetics, Leaving Group (X) and Ligand
Dependencies in the Pd-Catalyzed Reaction of 1a-c with 2, and
Ion-Pair Metathesis Cycle (B)a
a kL ≈ (kOA/kIR)(kNu+ KBAr′Fk′Nu[NaBAr′F]).
Figure 1. Ligand-selective acceleration of 1a + 2f3 by catalytic
[NaBAr′F]. Linear regression7,13 is of kL ) mx + kL0; a ) (m/kL0); kL
)
kobs/[Pd]tot. The acceleration factor a and predicted kL values at 0 and 1
Since the trend in the electrophilicity (kNu) of [4]+ is opposite
to the trend in turnover rate (kL), the resting state of the catalytic
system can not be [4]+[X]-. Nonetheless, nucleophilic attack by 2
is turnover-limiting, suggesting that [4]+[X]- is generated reversibly
in a low, steady-state, concentration. The catalytic cycle “A”
outlined in Scheme 2, involving reversible generation of [4]+[X]-
from L2Pd(η2-1), is consistent with these observations.9,10
When 1a and [13C1]-1b were coreacted there was no cross-over
of leaving-group (X) detected in unreacted substrates. There is also
no common-ion rate suppression by the NaX, which grows in
parallel with 3. Both of these results suggest that catalytic flux
proceeds via a solvent-separated tight ion-pair [4]+|[X]-.4h-l,11
This ion-pairing accounts for the effects of the ligand, and the
identity of X, on kL. Thus, electron-withdrawing ligands, such as
P(p-trifluorotolyl)3 (i), increase the electrophilicity of [4]+|[X]-
toward ion-pair return of X- (kIR) to a greater degree than they
increase nucleophilic attack by 2 (kNu).9 Better leaving groups (e.g.,
BzO, 1b) reduce the rate of internal return, increasing the rate of
catalytic turnover via nucleophilic attack by 2.
mM NaBAr′F (kL and kL1 mM) are given for comparison.
0
(1.0 equiv) cleanly generated the π-allyl complex [4(v)2][BAr′F]
+ NaOAc, when L ) P(p-tolyl)3 (v). However, the same reaction
conducted with the electron-withdrawing ligand P(p-trifluorotolyl)3
(i) did not generate any significant quantity of [4(i)2][BAr′F],
although both π-allyl complexes ([4(L)2][BAr′F]) were smoothly
generated by NaBAr′F addition to [4(L)2][OTf] with cogeneration
of NaOTf.14b This contrasting behavior clearly demonstrates the
dependence of the energetics of formation of ([4]+[BAr′F]-) on
both the electron-donating ability of the ligand and the nature of X
in [4]+|[X]-.
The acceleration by NaBAr′F, Figure 1, is apparently a balance
between having sufficient electron-donation available from the
ligands (L) to make abstraction of X energetically accessible,14b
but not having so much electron-donation available that it makes
little impact on ion-pair return (kIR) versus nucleophilic attack (kNu).
For the range of simple aryl phosphines tested, PPh3 appears to be
about optimum in this regard.
Solvent obviously plays a key role on the ion-pairing in [4][X]4j
and was briefly explored. In the more ion-pair stabilizing solvent
DMF the reactions proceeded about 3-fold faster than in THF, but
there was no significant change in the effect of the ligand on the
trend in turnover rates (kLREL, Table 1, entries 1, 4, 5). However,
consistent with the generation of a more-dissociated ion-pair, the
susceptibility to NaBAr′F acceleration was negligible. In stark
contrast, the effect of the ligand on the trend in turnover rates was
reversed in the less ion-pair stabilizing solvent CH2Cl2 (kLREL, Table
1, entries 1, 4, 5) using BSA/2-H for in situ nucleophile genera-
tion.15 We are currently investigating the ligand influence on the
energetics of ion-paired and neutral pathways, using computational
methods, and will report on this in full in due course. Nonetheless,
as with reactions in THF, NaBAr′F induces substantial acceleration
in CH2Cl2 when the more electron rich ligands PPh3 (iv) and P(p-
tolyl)3 (v) are employed (a ≈ 1f3 × 103 M-1), while reactions
using P(p-trifluorotolyl)3 (i) are unaffected.
Since the effect of NaBAr′F is to attenuate ion-pair return (kIR)
by X, the acceleration by NaBAr′F should be X-dependent, with
poorer leaving groups such as OAc (1a) accelerated more than better
ones, for example, OBz (1b). Competition between 1a and 1b for
limiting 2 in THF yields their relative reactivity16 (k1a/k1b, Figure
2). In all cases, acetate 1a is substantially less reactive than benzoate
1b (k1a/k1b , 1).16 As predicted by Figure 1, with P(p-trifluorotolyl)3
(i) as ligand (aa ≈ 0 M-1) the selectivity is [NaBAr′F]-independent
(Figure 2). However, with the more electron rich ligands PPh3 (iv)
As the partitioning of the ion-pair (kNu[2] versus kIR, Scheme 2)
appears to be a key parameter in controlling the turnover rate, we
explored the use of NaBAr′F as a source of “naked” Na+.7
Generation of [4]+|[BAr′F]- from [4]+|[X]- will have the effect
of attenuating12 ion-pair return (kIR) of X, analogous to Winstein’s
“special salt effect”.7 A key feature of this process is that the
organic-soluble NaBAr′F is regenerated by 2 (k′Nu) and can be
employed in catalytic quantities, Scheme 2, cycle B.
The kinetics of Pd-catalyzed reactions of 1a with 2 in the
presence of catalytic NaBAr′F (0.6f2.2 mol%) were informative,
Figure 1. A factor a has been employed to quantify the susceptibility
of the catalyst system to acceleration through NaBAr′F mediated
ion-pair partitioning, as compared to reactions with no added
NaBAr′F (kL , y-axis intercept).13 The analysis clearly distinguishes
0
that catalysts bearing moderately electron-rich ligands are the most
substantially accelerated. For example, the catalyst bearing PPh3
(iv, a ≈ 6 × 103 M-1) undergoes 7-fold faster turnover with 1
mM NaBAr′F, while for the more electron rich ligand P(p-anisyl)3
(vi) turnover is only increased 3-fold (a ≈ 2 × 103 M-1). Catalysts
bearing electron-poor ligands, which are already slow in the absence
of NaBAr′F, for example, P(p-trifluorotolyl)3 (i) are hardly acceler-
ated at all (a ≈ -0.02 × 103 M-1), Figure 1.
The ligand-dependency of the effect of NaBAr′F on attenuating
ion-pair return was confirmed by 1H NMR study of a stoichiometric
mixture of LnPd14 and 1a in d8-THF, which does not generate any
detectable π-allyl complex, [4(L)2][OAc]). Addition of NaBAr′F
9
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C O M M U N I C A T I O N S
M. ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: Heidelberg, Germany, 1999; pp 833-886.
(4) Studies reporting on the kinetics of catalytic reaction: (a) Mackenzie, P. B.;
Whelan, J.; Bosnich, B. J. Am. Chem. Soc. 1985, 107, 2046. (b) Åkermark,
B.; Zetterberg, K.; Hansson, S.; Krakenberger, B.; Vitagliano, A. J.
Organomet. Chem. 1987, 335, 133. (c) Ramdeehul, S.; Dierkes, P.; Aguado,
R.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Osborn, J. A. Angew.
Chem., Int. Ed. 1998, 37, 3118. (d) Gais, H.-J.; Jagusch, T.; Spalthoff, N.;
Gerhards, F.; Frank, M.; Raabe, G. Chem.sEur. J. 2003, 9, 4202. Studies
reporting on the kinetics of stoichiometric amination of π-allyl Pd
complexes: (e) Vitagliano, A.; Åkermark, B. J. Organomet. Chem. 1988,
349, C22. (f) Kuhne, O.; Mayr, H. Angew Chem. Int. Ed. 1999, 38, 343.
(g) Amatore, C.; Ge´nin, E.; Jutand, A.; Mensah, L. Organometallics 2007,
26, 1875. Studies reporting on the kinetics of stoichiometric oxidative
addition of allyl esters to generate π-allyl Pd complexes: (h) Amatore, C.;
Jutand, A.; Meyer, G.; Mottier, L. Chem.sEur. J. 1999, 5, 466. (i) Amatore,
C.; Gamez, S.; Jutand, A. Chem.sEur. J. 2001, 7, 1273. (j) Amatore, C.;
Gamez, S.; Jutand, A.; Meyer, G.; Mottier, L. Electrochim. Acta 2001, 46,
3237. (k) Agenet, N.; Amatore, C.; Gamez, S.; Ge´rardin, H.; Jutand, A.;
Meyer, G.; Orthwein, C. ArkiVoc 2002, V, 92. (l) Amatore, C.; Bahsoun,
A. A.; Jutand, A.; Mensah, L.; Meyer, G.; Ricard, L. Organometallics 2005,
24, 1569.
(5) For a similar (but qualitative) trend, ascribed to an “ionic liquid effect”,
see: (a) Ross, J.; Chen, W.; Xu, L.; Xiao, J. Organometallics 2001, 20,
138. For a related acetate sequestering effect through H-bonding, that
suppresses rate, see: (b) Ross, J.; Xiao, J. Chem.sEur. J. 2003, 9, 4900.
(6) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11, 3920.
(7) (a) Winstein, S.; Klinedinst, P. E.; Robinson, G. C. J. Am. Chem. Soc.
1954, 76, 2597. (b) At present it is unclear whether the approximately linear
plots in Figure 1 are the early phases of a special salt effect (which should
progressively inflect to a normal salt effect) or the linear portion of a, rather
powerful, normal salt effect. Scheme 3 supports the former.
Figure 2. Selective acceleration of acetate (1a) over benzoate (1b) by
[NaBAr′F]. k1a/k1b estimated by competition of 1a/[13C]-1b for limiting 2.
Nonlinear regression for (iv)/(v) is of k1a/k1b ) ({k1a/k1b}0)((1 + a1ax)/(1
+ a1bx)). Note16 that k1a/k1b * kL(1a)/kL(1b)
.
Scheme 3. Impact of Cocatalytic NaBAr′F on Enantioselectivity21
(8) Crotyl acetate behaved analogously. See Supporting Information.
(9) An analogous conclusion was made by Jutand and co-workers, for Pd
catalysed allylic amination, but based on the kinetics of separate stoichio-
metric reactions (KOA and kNu), see reference 4g.
(10) Ligand association/dissociation equilibria are not implicated: the bidentate
dppf ligands (entries 11-13) give identical trends. Addition of 1 equiv
ligand v (entry 5) resulted in severe attenuation of turnover rate.
(11) Akermark, B.; Hansson, S.; Zetterberg, K.; Krakenberger, B.; Vitagliano,
A. Organometallics 1984, 3, 679.
(12) Pregosin has demonstrated (from 19F PGSE diffusion) that BAr′F engages
in “very modest but not zero ion pairing”, see: (a) Nama, D.; Butti, P.;
Pregosin, P. S. Organometallics 2007, 26, 4942. (b) Pregosin, P. S. Prog.
Nucl. Magn. Reson. Spectrosc. 2006, 49, 261.
and P(p-tolyl)3 (v), acetate 1a can be differentially accelerated (a1a/
a1b > 1) over benzoate 1b, by an order of magnitude.
(13) The approximation d[3]/dt ≈ kL(1 + a[NaBAr′F])[Pdtot][2] may only hold
for low [NaBAr′F] concentrations. See also ref 7b.
(14) (a) A mixture of 1a and (nominally) (L)2Pd, was generated by reaction of
[Pd2(allyl)2(OAc)2] with i and v (L/Pd ) 2;-78f21 °C). For the analogous
reaction with Cy3P, see: Yamamoto, T.; Saito, O.; Yamamoto, A. J. Am.
Chem. Soc., 1981, 103, 5600. The precatalyst mixture gave an identical kL
value for 1af3 to that obtained with [4(L)2][OTf]. (b)1H-NMR isotherm-
derived equilibrium constants for NaBAr′F + [4(L)2][OTf] ) NaOAc +
[4(L)2][BAr′F] are K ) 0.43 ( 0.11 (i), and K ) 193 ( 147 (v), see
Supporting Information.
Catalytic NaBAr′F is proposed to facilitate accelerated flux via
generation of [4][BAr′F],7,17 and thus a preliminary study18 was
conducted to investigate the impact of the degree of ion-pairing19
at the point of nucleophilic attack (kNu versus k′Nu, Scheme 2), on
the selectivity of an archetypal asymmetric allylation, Scheme 3.20
In summary, counterintuitive ligand influences on the rate of
Tsuji-Trost allylation5 (1 + 2 f 3) can arise through reversible
generation of a tight ion-pair ([4]+|[X]-) from a precomplexed
resting state, L2Pd(η2-1),22 Scheme 2. With appropriately electron-
donating ligands, accelerated flux can be achieved by modulating
the ion-pair partitioning with catalytic quantities of NaBAr′F
(Scheme 2, cycle B).14b,23 This process offers the potential to use
lower catalyst loadings and to improve slow allylation reactions. It
will also be an important consideration in chiral ligand design and
the optimization of enantioselectivity.19,20
(15) Reactions were substantially slower than in THF, see Supporting Informa-
tion, and proceeded with very approximately pseudo-zero-order kinetics,
as might be expected from a steady-state concentration of [2].
(16) For other examples of pseudo-zero-order competitive networks see: (a)
Ferretti, A. C.; Mathew, J. S.; Ashworth, I.; Purdy, M.; Brennan, C.;
Blackmond, D. G. AdV. Synth. Catal. 2008, 350, 1007. (b) Blackmond,
D. G.; Hodnett, N. S.; Lloyd-Jones, G. C. J. Am. Chem. Soc. 2006, 128,
7450. (c) Dominguez, B.; Hodnett, N. S.; Lloyd-Jones, G. C. Angew. Chem.,
Int. Ed. 2001, 40, 4289.
(17) Further evidence for a change in ion-pairing in 4+ at the point of
nucleophilic attack comes from competition between 2 (NaC(Me)E2) and
the less sterically hindered NaC(H)E2 for limiting 1a. Using more electron
rich ligands, the selectivity for 2 could be approximately doubled by addition
of 3.0 mM [NaBAr′F], see Supporting Information.
(18) NaBAr′F (3.0 mM) increased k1a/k1b (0.15 f 0.50) with [4(BINAP)][OTf].
(19) The generation of a less-intimate ion-pair (see ref 12) may also modulate
“memory effects”. For leading references: (a) Trost, B. M.; Bunt, R. C.
J. Am. Chem. Soc. 1996, 118, 235. (b) Lloyd-Jones, G. C.; Stephen, S. C.;
Fairlamb, I. J. S.; Martorell, A.; Dominguez, B.; Tomlin, P. M.; Murray,
M.; Fernandez, J. M.; Jeffery, J. C.; Riis-Johannessen, T.; Guerziz, T. Pure
Appl. Chem. 2004, 76, 589. (c) Svensen, N.; Fristrup, P.; Tanner, D.; Norrby,
P.-O. AdV. Synth. Catal. 2007, 349, 2631.
Acknowledgment. We thank AstraZeneca Global PR&D for
generous financial support of this project and John M. Brown FRS,
Oxford, for open exchange of information about ion-pairing.
Supporting Information Available: Preparation and characteriza-
tion of catalysts; kinetic data and analysis. This material is available
free of charge via the Internet at http://pubs.acs.org.
(20) Detrimental effects of halide on the selectivity of asymmetric allylation
are known, see for example: Clark, T. P.; Landis, C. R. J. Am. Chem. Soc.,
2003, 125, 11792.
(21) Previously reported to proceed in 30% ee with X ) Cl: Yamaguchi, M.;
Shima, T.; Yamagishi, T. Tetrahedron: Asymmetry 1991, 2, 663.
(22) Cation-stabilizing allylic substituents (for example, 1,1,3-Ar3) may decrease
kIR and/or kNu leading to a Pd-allyl species as resting state, see reference
4a.
(23) BAr′F, or analogous “non-interactive” (ref 12) anions are often employed
in systems propagating via contiguous cationic catalytic intermediates. The
results herein show that BAr′F can also be applied to catalytic cycles with
neutral and cationic intermediates.
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