[(RCN)2PdCl2]-catalyzed E/Z isomerization of alkenes: A non-hydride binuclear addition-elimination pathway

Article abstract of DOI:10.1002/anie.201103947

A crepuscular catalyst: Alkene migration catalyzed by [(RCN) ₂PdCl₂] complexes proceeds through an in situ generated Pd-H species. Addition of a 1,5-diene inhibits migration and allows the slower background catalysis of E/Z interconversion to be studied. Experimental and computational results suggest this interconversion proceeds through a conformational equilibrium in dipalladacycles (see picture). Copyright

Full text of DOI:10.1002/anie.201103947

Communications
DOI: 10.1002/anie.201103947
Reaction Mechanisms
[(RCN)2PdCl2]-Catalyzed E/Z Isomerization of Alkenes:
A Non-Hydride Binuclear Addition–Elimination Pathway**
Emily H. P. Tan, Guy C. Lloyd-Jones,* Jeremy N. Harvey,* Alastair J. J. Lennox, and
Benjamin M. Mills
It has been known since the 1960s that [(RCN)2PdCl2]
complexes efficiently catalyze double-bond migration and
geometric E/Z interconversion in alkenes.[1] Herein we
propose a new addition–elimination mechanism with a
binuclear Pd species that selectively effects E/Z interconver-
sion;[2] before describing this, we consider the mechanisms
previously proposed for this process.[1,3,4] A key aspect of the
discussion herein, is that although there has been consider-
able debate as to whether isomerization involves, inter
alia,[1f,g,3h,4] [Pd-H] species and h1-alkyl intermediates,[1a,i] or
h3-Pd(H) intermediates,[1b,d,e,g,h,2a] when concurrent with
alkene migration,[1a–e,g–i] the process of E/Z interconversion
is assumed to arise through the same mechanism (pathway A,
Scheme 1).
Figure 1. Isomerization of 1-(Z)-[2H]-[1] (0.15m) to a) [2H]-[2] and
b) 1-(E)-[2H]-[1] catalyzed by [(tBuCN)2PdCl2] and diene 3 (0 mol%
Scheme 1. Alkene isomerization catalyzed by [(RCN)2PdCl2]. Pathway A
effects both migration and E/Z interconversion through addition–
(crosses), 1 mol% (open circles), 5 mol% (filled circles)). a) Dashed
lines are solely a guide to the eye. b) kobs/[Pd]=2.7ꢀ10ꢀ3 mꢀ1 sꢀ1
DG° =22 kcalmolꢀ1 [12]
;
ꢀ
elimination of in situ generated [Pd-H] species, or allylic C H
.
insertion. Pathway B effects E/Z interconversion without migration.
detailed analysis of the kinetics was precluded by extensive
Unpublished observations[5] made during mechanistic H/D exchange.[9] Repeating the isomerization in the presence
studies[6] of Pd-catalyzed 1,6-diene cycloisomerization[7] led of 1 to 5 mol% 1,5-diene 3,[6e,f] an efficient [Pd-H] trap,
us to suspect that there is a mechanistic dichotomy in induced substantial suppression of both the alkene migration
[(RCN)2PdCl2]-catalyzed alkene isomerization,[1] and thus to and the isotopic scrambling. Simple pseudo first-order
investigate selective decoupling of migration from E/Z equilibrium kinetics were then observed for E/Z interconver-
isomerization using [Pd-H] traps.[6e,f,8] We began with the sion, see solid line in Figure 1b.[10]
isomerization of deuterium labeled methyl allyl malonate
The sigmoidal kinetic profile for migration and its
(Z)-1-[2H]-1 (Figure 1). Alkene migration displayed sigmoi- inhibition by substoichiometric [Pd-H] trap 3, adds to the
dal kinetic profiles, whereas, E/Z interconversion proceeded body of evidence[1a,i] for [(RCN)2PdCl2]-catalyzed alkene
directly, that is, without an induction period. However, migration involving low concentrations of in situ generated
[Pd-H] species. The selectivity of the inhibition[11] is the first
experimental demonstration that concurrent E/Z intercon-
version can follow a pathway (B) that is mechanistically
independent from the pathway for migration (A).
[*] Dr. E. H. P. Tan, Prof. Dr. G. C. Lloyd-Jones, Prof. Dr. J. N. Harvey,
A. J. J. Lennox, B. M. Mills
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
E-mail: guy.lloyd-jones@bris.ac.uk
The isomerization of Z-1-phenyl-buten-2-ene (Z-4) was
studied under analogous conditions (Scheme 2).[12] For this
alkene, migration is thermodynamically more favorable[13]
[**] G.C.L.-J. is a Royal Society Wolfson Research Merit Award Holder.
than E/Z interconversion, resulting ultimately in > 95%
E-5. Nonetheless, both isomerization processes displayed
approximate pseudo first-order equilibrium kinetics, allowing
We thank AstraZeneca for generous funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103947.
9602
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9602 –9606

nation results in 31% “softening” of the double bond
character.[16] However, examination of models shows that
=
torsion of the C C bond alone is not enough to effect E/Z
isomerization, and we were unable to find an isomerization
transition state of this type in DFT calculations.[18] From an
experimental study of [(PhCN)2PdCl2]-catalyzed E/Z isomer-
ization in 1,2-[2H2]-ethylene, which cannot proceed through
ꢀ
allylic C H insertion, and does not involve intermolecular
Scheme 2. Isomerization of Z-4 and Z-[2H2]-4 (0.5m) catalyzed by
10 mol% [(MeCN)2PdCl2][15] at 218C in solvent polarities ranging from
e=0.9 to 21.0.[12] Linear correlations of empirical rate constants for
pseudo first-order equilibration: kM (sꢀ1) = 2.7ꢀ10ꢀ6 e, r2 =0.94;
kZ/E (sꢀ1) = 0.8ꢀ10ꢀ6 e, r2 =0.67.
H/D migration, Wells et al.[1f] proposed that alkene coordi-
nation to PdII results in an incipient b-carbocation,[4,19]
facilitating C C rotation, see upper pathway in Scheme 3.
ꢀ
direct analysis of the relative rates (kZ/E/kM) under a variety of
conditions.[12] Isomerization was favored by polar solvents,
with a linear correlation of kobs versus solvent dielectric (e) for
both processes.[12] However, the effect was relatively small;[14]
ruling out substantial charge generation in the catalytic
intermediates in either process. With Z-1,1-[2H2]-4, migration
was attenuated by a primary kinetic isotope effect (kH/kD =
4.5)[12] and accompanied by extensive intermolecular deute-
rium migration, just as would be expected for a [Pd-H]-based
mechanism for pathway A. Addition of [Pd-H] traps such as
Scheme 3. Generic mechanisms for geometric interconversion of an
alkene through incipient b-carbocation and nucleophilic addition
routes.
diene
3
or TEMPO ((2,2,6,6-tetramethylpiperidin-1-
Indeed, we found the more electrophilic complexes
[(MeCN)4Pd][OTf]2 and [(MeCN)2Pd(OTs)2] induced
> 100-fold faster isomerization of Z-4 than [(RCN)2PdCl2].[12]
However, on addition of the [Pd-H] trap 3, there was
substantial suppression (up to 3000-fold) in both isomer-
ization processes (kZ/E/kM ꢁ 1), indicative of predominant
catalysis by [Pd-H] species (pathway A) with these sulfonate
catalysts.
yl)oxyl),[8] led to substantial (up to 600-fold) increases in
kZ/E/kM,[12] through selective inhibition of migration (kM).
The kinetics of E/Z interconversion of Z-b-methyl styrene
6 (Figure 2a),[2a,b] for which alkene migration is endergonic,[13]
were not suppressed by a [Pd-H] trap, and corresponded to a
simple pseudo first-order conversion of Z-6 to E-6 (K = 32),
through pathway B. Alternative mechanisms to the h1-
alkyl[1a,i] and h3-Pd(H)[1b,d–h,2a] pathways (A) have been
previously suggested[1f,16] for PdII-catalyzed E/Z isomeriza-
tion.[17] Rericha et al. have proposed that PdII–alkene coordi-
The low sensitivity of pathway B to solvent polarity,[12] as
well as a negligible secondary KIE (kH/kD = 0.95) obtained
with (Z)-1,1-[2H2]-4[12] weigh against the incipient b-carbo-
cation mechanism.[1f,4,19] This conclusion was firmly rein-
forced by DFT studies[18] that indicated a high DG° (53.3 kcal
ꢀ1
ꢀ
mol ) for C C rotation in [(ethene)2PdCl2], through a b-
carbocation, inconsistent with the values observed for alkenes
1, 4 and 6 (20–24 kcalmolꢀ1). Varying the computational
method or the ancillary ligands (e.g. [(ethene)-
(CH3CN)PdCl2]) did not materially change the calculated
barrier; adding an alkene substituent ([(propene)-
(ethene)PdCl2]) lowers the barrier, but only to 46.6 kcal
molꢀ1.
An addition–elimination process, see lower pathway in
Scheme 3, could bypass the generation of a b-carbocation,
provided that it can proceed through a syn-then-anti, or an
anti-then-syn, sequence.[20] Alkene chloropalladation with
[Li2Pd2Cl6] in AcOH has been shown by Henry to be non-
stereospecific.[21]
An
analogous
mechanism
with
[(RCN)2PdCl2] (Scheme 3; Nu = Cl) would effect E/Z inter-
conversion. However, this would necessitate addition–elimi-
nation of chloride ion,[21] inconsistent with the low sensitivity
of the reaction to solvent polarity.[12] Nonetheless, the concept
of a reversible chloropalladation process was further consid-
ered when a more detailed analysis of the kinetics of E/Z
interconversion of Z-6 (Figure 2b) revealed alkene inhibition,
and a palladium dependency that was greater than first-
Figure 2. a) Isomerization of Z-6 to E-6 catalyzed by [(tBuCN)2PdCl2] in
CD2Cl2 at 258C with [6]TOT =0.144–0.205m and [Pd]=8.2–30.6 mm.
Conditions a–f (mol% Pd): a, 6.0; b, 9.8; c, 13.7; d, 17.0; e, 9.6; f,
10.8.[12] Lines through data are kinetic simulations employing the
model in Scheme 4. b) Emꢀp1irical pseudo first-order rate constant:
k
obs =1.3ꢀ10ꢀ3 [Pd]1.6[6]TOT
.
Angew. Chem. Int. Ed. 2011, 50, 9602 –9606
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9603

Communications
order.[12] A simple model in which there is an alkene-
dependent equilibrium between mononuclear (7) and binu-
clear (8) alkene complexes,[22] with E/Z interconversion
catalyzed only by the binuclear complex 8 (Scheme 4)
satisfactorily simulated all of the kinetic data (see solid lines
through data in Figure 2) using a single set of equilibrium and
rate constants (K, kZ!E and kE!Z).[12]
Scheme 5. E/Z interconversion of a generic 1,2-disubstituted (a,b)
alkene (in Newman projection when Pd coordinated) through syn and
anti binuclear 1,2-chloropalladation. See Figure 3 for structures and
energies, according to DFT calculations, where the alkene is propene.
Scheme 4. Model used for simulation of the kinetics of E/Z intercon-
version of b-methyl styrene (6), starting from Z-6 (Figure 2).[12]
binuclear 1,2-chloropalladation (!10/11), binuclear elimina-
tion (!9), and bridge-closure (!8) effects catalysis of E/Z
isomerization in the bulk alkene (Scheme 5). The turnover-
limiting step for pathway B with propene (Figure 3) is
predicted to be the anti addition (or anti elimination in the
reverse direction), with a calculated DG° of 22.9 kcalmolꢀ1
above propene + 8 (26.8 kcalmolꢀ1 in the case of ethene),
consistent with the measured rates of E/Z isomerization for
alkenes 1-[2H]-1, 4 and 6 (DG° = 20–24 kcalmolꢀ1).
In summary, based on prior observations in diene cyclo-
isomerization,[6] we have reinvestigated the isomerization of
alkenes catalyzed by [(RCN)2PdCl2].[1,2] Using [Pd-H]
traps[6e,f,8] to inhibit migration, we reveal a background
Using model complexes where L = h2-ethene, DFT pre-
dicts DG =+ 3.9 kcalmolꢀ1 for 7 + 7!8 + 2L,[18] consistent
within computational error with the dimerization equilibrium
employed in the kinetic model (DG =+ 0.7 kcalmolꢀ1), where
L = h2-b-methyl styrene 6 (Scheme 4). The same binuclear
species 8 [{(alkene)PdCl2}2] was suggested by Sparke et al.[1c]
in 1965 to be the active catalyst for alkene isomerization
(migration and E/Z interconversion) by [(PhCN)2PdCl2],
albeit without kinetic evidence or a specific mechanistic
proposal. The involvement of binuclear complexes was also
noted by Henry for anti-chloropalladation of alkenes by
[Li2Pd2Cl6].[20] However, the reaction was proposed to pro-
ceed in a pseudo mononuclear manner, with the second Pd
center simply serving to reduce the electrostatic repulsion of
an external chloride nucleophile. Since these anionic inter-
actions do not apply in the current system, we sought to find a
mechanism that specifically requires the involvement of both
Pd centers. In other words, a binuclear non-ionic chloropalla-
dation process, that is able to proceed in both a syn and an anti
manner, thereby generating a pathway for E/Z isomerization.
DFT calculations[18] show that an alkene (ethene or
propene) can add to bis-chloro bridged complex 8 (L = h2-
ethene) to form various metastable species lying just a few
kcalmolꢀ1 higher in free energy than 8 + alkene, including a
monobridged species 9 (L = h2-ethene, Scheme 5).[3g] The long
ꢀ
Pd Cl bonds (2.38–2.45 ꢀ) and acute Cl-Pd-Cl bond angles
(908) in the key monobridged species 9 facilitate a reversible
binuclear 1,2-chloropalladation of the alkene, involving the
coordinated Pd unit and a chloride on the “other” Pd center.
There are two modes of binuclear 1,2-chloropalladation:
an anti-addition and a syn-addition, generating six-membered
ring adducts 10 and 11, respectively, with boatlike geometries.
These adducts can undergo facile interconversion through
processes akin to conformational isomerization in a cyclo-
hexane ring. The net process of bridge-cleavage (!9),
Figure 3. DG0(298 K) profile (kcalmolꢀ1; DFT[18]) for degenerate
geometric isomerization (permutation) of propene catalyzed by the
binuclear complex 8, where L=h2-ethene.
9604 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9602 –9606

557 – 604; p) C. Dugave, L. Demange, Chem. Rev. 2003, 103,
2475 – 2532.
interconversion of E/Z isomers that does not involve [Pd-H]
[1b,d–h,2a]
species,[1a,i] or allylic C H insertion,
or incipient b-
ꢀ
carbocations.[1f,4,19] Kinetic simulations, isotopic labeling and
DFT studies support E/Z interconversion proceeding through
reversible syn and anti binuclear chloropalladation of the
alkene (Scheme 5 and Figure 3).
[3] For related mechanistic discussion of isomerization by Pd-based
systems, mostly PdCl4 and PdCl2 in AcOH or MeOH, see:
a) N. R. Davies, Nature 1964, 201, 490 – 491; b) J. F. Harrod, A. J.
Chalk, Nature 1965, 205, 280 – 281; c) N. R. Davies, Aust. J.
Chem. 1964, 17, 212 – 218; d) B. I. Cruikshank, N. R. Davies,
Aust. J. Chem. 1966, 19, 815 – 825; e) R. Cramer, R. V. Lind-
sey, Jr., J. Am. Chem. Soc. 1966, 88, 3534 – 3544; f) B. I.
Cruikshank, N. R. Davies, Aust. J. Chem. 1973, 26, 1935 – 1947;
g) F. Conti, L. Raimondi, G. F. Pregaglia, R. Ugo, J. Organomet.
Chem. 1974, 70, 107 – 120; h) A. M. Zawisza, S. Bouquillon, J.
Muzart, Eur. J. Org. Chem. 2007, 3901 – 3904.
This binuclear addition–elimination mechanism may also
explain why double-bond[23] and H-migration[1f] is not
observed in other Pd-catalyzed E/Z isomerization processes.
The geometries of the proposed intermediates should also
facilitate the design of selective isomerization catalysts based
on variation of the spectator alkenes (L) in complexes 8–11.
The selective inhibition of Pd-catalyzed alkene migration,
using [Pd-H] traps such as 1,5-diene 3[6e,f] or TEMPO,[8] may
also find application in other catalytic processes, for example
in Mizoroki–Heck reactions.[24] It is also evident from
Figure 3, that 9!10!11!9 can effect intramolecular enan-
tiofacial interconversion of a coordinated prochiral alkene,
that is without dissociation from the metal or involvement of
an external ligand; a feature that may be of importance in
asymmetric catalysis.
[4] For a detailed discussion of the incipient carbocation mecha-
nism, see: a) A. Sen, T.-W. Lai, Inorg. Chem. 1981, 20, 4036 –
4038; b) A. Sen, T.-W. Lai, J. Am. Chem. Soc. 1981, 103, 4627 –
4629; c) A. Sen, T.-W. Lai, Inorg. Chem. 1984, 23, 3257 – 3258.
[5] Cycloisomerization proceeded with sigmoidal kinetics[6d,e]
1
whereas E/Z interconversion, detected by H NMR spectrosco-
py in D-labeled substrate, proceeded with pseudo first-order
equilibrium kinetics and no induction period. See the Supporting
Information for full details.
[6] a) K. L. Bray, I. J. S. Fairlamb, G. C. Lloyd-Jones, J. Chem. Soc.
Chem. Commun. 2001, 187 – 188; b) R. A. Widenhoefer, Acc.
Chem. Res. 2002, 35, 905 – 913; c) G. C. Lloyd-Jones, Org.
Biomol. Chem. 2003, 1, 215 – 236; d) K. L. Bray, I. J. S. Fairlamb,
J.-P. Kaiser, G. C. Lloyd-Jones, P. A. Slatford, Top. Catal. 2002,
19, 49 – 59; e) K. L. Bray, G. C. Lloyd-Jones, M. P. MuÇoz, P. A.
Slatford, E. H. P. Tan, A. R. Tyler-Mahon, P. A. Worthington,
Chem. Eur. J. 2006, 12, 8650 – 8663; f) L. A. Evans, N. Fey, G. C.
Lloyd-Jones, M. P. MuÇoz, P. A. Slatford, Angew. Chem. 2009,
121, 6380 – 6383; Angew. Chem. Int. Ed. 2009, 48, 6262 – 6265.
[7] The Pd-H based mechanism below has been proposed for
Received: June 9, 2011
Revised: July 18, 2011
Keywords: alkenes · homogeneous catalysis · isomerization ·
.
palladium · reaction mechanisms
cycloisomerization
of
dimethyl
diallyl
malonate.[6b–e]
[1] a) J. F. Harrod, A. J. Chalk, J. Am. Chem. Soc. 1964, 86, 1776 –
1779; b) G. C. Bond, M. Hellier, J. Catal. 1965, 4, 1 – 5; c) M. B.
Sparke, L. Turner, A. J. M. Wenham, J. Catal. 1965, 4, 332 – 340;
d) J. F. Harrod, A. J. Chalk, J. Am. Chem. Soc. 1966, 88, 3491 –
3497; e) N. R. Davies, A. D. DiMichiel, V. A. Pickles, Aust. J.
Chem. 1968, 21, 385 – 395; f) B. Hudson, D. E. Webster, P. B.
Wells, J. Chem. Soc. Dalton Trans. 1972, 1204 – 1207; g) P.
Golborn, F. Scheinmann, J. Chem. Soc. Perkin Trans. 1 1973,
2870 – 2875; h) D. Bingham, B. Hudson, D. E. Webster, P. B.
Wells, J. Chem. Soc. Dalton Trans. 1974, 1521 – 1524; i) S. Sakaki,
H. Kanai, K. Tarama, Can. J. Chem. 1974, 52, 2857 – 2860.
[2] Development and application of [(RCN)2PdCl2]-catalyzed
alkene E/Z interconversion in synthesis: a) R. G. F. Giles, V. R.
Lee Son, M. V. Sargent, Aust. J. Chem. 1990, 43, 777 – 781; b) J.
Yu, M. J. Gaunt, J. B. Spencer, J. Org. Chem. 2002, 67, 4627 –
4629; c) J. E. Moses, J. E. Baldwin, R. Marquez, R. M. Adling-
ton, A. R. Cowley, Org. Lett. 2002, 4, 3731 – 3734; d) P. Lemaire,
G. Balme, P. Desbordes, J.-P. Vors, Org. Biomol. Chem. 2003, 1,
4209 – 4219; e) J. E. Moses, J. E. Baldwin, S. Brꢁckner, S. J. Eade,
R. M. Adlington, Org. Biomol. Chem. 2003, 1, 3670 – 3684; f) S.
Brꢁckner, J. E. Baldwin, R. M. Adlington, T. D. W. Claridge, B.
Odell, Tetrahedron 2004, 60, 2785 – 2788; g) F.-D. Wang, J.-M
Yue, Synlett 2005, 2077 – 2079; h) M. F. Jacobsen, J. E. Moses,
R. M. Adlington, J. E. Baldwin, Org. Lett. 2005, 7, 2473 – 2476;
i) S. Chandrasekhar, S. S. Sultana, Tetrahedron Lett. 2006, 47,
7255 – 7258; j) S.-Y. Seo, J.-W. Jung, J.-K. Jung, N.-J. Kim, Y.-W.
Chin, J. Kim, Y.-G. Suh, J. Org. Chem. 2007, 72, 666 – 668; k) S.
Kim, Y. M. Lee, J. Lee, T. Lee, Y. Fu, Y. Song, J. Cho, D. Kim, J.
Org. Chem. 2007, 72, 4886 – 4891; l) D. L. J. Clive, E. J. L.
Stoffman, Org. Biomol. Chem. 2008, 6, 1831 – 1842; m) J.-W.
Jung, J. Jang, S.-Y. Seo, J.-K. Jung, Y.-G. Suh, Tetrahedron 2010,
66, 6826 – 6831; n) M. Rçssle, D. J. Del Valle, M. J. Krische, Org.
Lett. 2011, 13, 1482 – 1485; for selected reviews on E/Z
interconversion see: o) P. E. Sonnet, Tetrahedron 1980, 36,
[8] DPPH and TEMPO are efficient [Pd-H] traps, through PdII to
PdI reduction, see: A. C. Albꢂniz, P. Espinet, R. Lꢃpez-
Fernꢄndez, A. Sen, J. Am. Chem. Soc. 2002, 124, 11278 – 11279.
[9] GCMS/NMR analysis showed deuterium transfer to and from
C(1) in 1 to C(2)/C(3) in 1 and 2. Intermolecular transfer was
faster than migration, consistent with nonproductive hydro-
=
palladation/b-hydride elimination at C(1) C(2), in both orien-
tations.
[10] Linear regression of 0.5 ꢅ ln(xZꢀ0.5) = ꢀkobs t; xZ = mol fraction
Z-1.
[11] Other [Pd-H] traps were also tested, including norbornadiene,
DPPH,[8] TEMPO,[8] C(1)-Ph 3, N(iPr)2Et), Proton Sponge, and
CCl4.[12]
[12] See the Supporting Information for full details.
[13] Equilibrium mol fractions for 4/5 at 408C in THF were estimated
as Z-4 (0.002), E-4 (0.032), Z-5 (0.011), and E-5 (0.955) by
isomerization of 5 (E/Z 50/50; Wittig olefination) for 4 days.
Equilibrium mol fractions at 218C in CH2Cl2 for 6 were
estimated analogously, as Z-6 (0.03), E-6 (0.97), and 1-phenyl-
prop-2-ene (< 0.001).
[14] For example, there is only a 12-fold increase in kM and a 7-fold
increase in kZ/E, on changing from CH2Cl2 (e = 0.9) to acetone
(e = 21); see the Supporting Information for full details.
[15] [(tBuCN)2PdCl2], [(MeCN)2PdCl2], and [(PhCN)2PdCl2] all
catalyzed isomerization at similar rates.[12] No isomerization
was detected over a period of 15 days using 10 mol% [Pd(CN)2],
Angew. Chem. Int. Ed. 2011, 50, 9602 –9606
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 9605

Communications
[(PPh3)2PdCl2], [(MeCN)2Pd(O2CCF3)2], [Pd(O2CCF3)2], or
[(DMSO)2PdCl2].
[22] For early examples of crystal structures of binuclear m-chlor-
oalkene Pd complexes analogous to 8, see: a) J. N. Dempsey,
N. C. Baenziger, J. Am. Chem. Soc. 1955, 77, 4984 – 4987; b) J. R.
Holden, N. C. Baenziger, J. Am. Chem. Soc. 1955, 77, 4987 –
4990; c) M. Zettlitzer, H. tom Dieck, L. Stamp, Z. Naturforsch. B
1986, 41, 1230 – 1238; for a recent review of binuclear Pd
complexes and their reactions, see: d) V. K. Jain, L. Jain, Coord.
Chem. Rev. 2005, 249, 3075 – 3197.
[23] See for example: I. S. Kim, G. R. Dong, Y. H. Jung, J. Org.
Chem. 2007, 72, 5424 – 5426; and specifically entries 5, 6 and 8 in
Table 2.
[24] See for example: a) F. Ozawa, A. Kubo, Y. Matsumoto, T.
Hayashi, E. Nishioka, K. Yanagi, K. Moriguchi, Organometallics
1993, 12, 4188 – 4196; b) G. C. Lloyd-Jones, P. A. Slatford, J. Am.
Chem. Soc. 2004, 126, 2690 – 2691; c) B. M. M. Wheatley, B. A.
Keay, J. Org. Chem. 2007, 72, 7253 – 7259.
[16] R. Ponec, R. Rericha, J. Organomet. Chem. 1988, 341, 549 – 557.
[17] The mechanism of Pd0-catalyzed interconversion of E/Z alkenes
has been studied in detail: L. Canovese, C. Santo, F. Visentin,
Organometallics 2008, 27, 3577 – 3581; there is no evidence for
reduction of PdII to Pd0 in the systems reported herein.
[18] Calculations were performed using the B3LYP functional, a
relativistic effective core potential (ECP) on Pd with an
associated triple-zeta valence basis, and the 6-31G(d) basis on
other atoms. The ꢀD3 dispersion correction to energies was
included. See the Supporting Information for full details.
[19] P. M. Henry, J. Am. Chem. Soc. 1972, 94, 7316 – 7322.
[20] Addition of neutral nucleophiles, for example, RCN or H2O, to
Pd-catalyzed E/Z interconversion of Z-6 caused no significant
rate enhancement.[12]
[21] P. M. Henry, J. Org. Chem. 1972, 37, 2443 – 2447.
9606 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9602 –9606