Regioselectivity and memory effects in palladium catalyzed allylic alkylations with bidentate P^PS donor ligands

Article abstract of DOI:10.1016/j.jorganchem.2010.10.022

A series of complexes, [Pd(η³-C₃H ₅)(P^PS)][SbF₆], where P^PS are bidentate bisphosphine monosulfide ligands, were found to catalyze allylic alkylation reactions with high branched:linear selectivity with some ligands. Some of these catalysts also display a regiochemical memory effect, in which the hemilability and rigidity of the P^PS ligands affect the reaction rate and the degree to which a memory effect is observed.

Full text of DOI:10.1016/j.jorganchem.2010.10.022

Journal of Organometallic Chemistry 696 (2011) 879e886
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Regioselectivity and memory effects in palladium catalyzed allylic alkylations
with bidentate P^P]S donor ligands
1
*
Suzanna C. Milheiro , J.W. Faller
Yale University, P.O. Box 208107, New Haven, CT 06520-8107, United States
a r t i c l e i n f o
a b s t r a c t
A series of complexes, [Pd(h
3-C3H5)(P^P]S)][SbF6], where P^P]S are bidentate bisphosphine mono-
Article history:
Received 10 August 2010
Received in revised form
6 October 2010
Accepted 8 October 2010
Available online 20 October 2010
sulfide ligands, were found to catalyze allylic alkylation reactions with high branched:linear selectivity
with some ligands. Some of these catalysts also display a regiochemical memory effect, in which the
hemilability and rigidity of the P^P]S ligands affect the reaction rate and the degree to which a memory
effect is observed.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Allylic alkylation
Memory effect
Catalysis
Rearrangements
Isomerization
Hemilabile
Regioselectivity
Conformational equilibria
Heterobidentate ligands
1. Introduction
The donor atoms of ligands also have a significant influence on
transition metal catalyzed reactions and the use of ligands containing
The design of ligands for asymmetric transition metal catalyzed
reactions continues to grow in importance in synthetic chemistry
[1]. Although great strides have been made in preparing ligands
with the appropriate electronic and steric properties to impart high
activity and selectivity to catalysts, it is often difficult to predict the
properties that a metal complex of a given ligand will exhibit. Some
parameters, however, can offer some predictive capabilities. The
bite angle of bisphosphine ligands is known to influence the
selectivity of transition metal catalyzed reactions [2,3]. This is
usually attributed to a steric effect where a larger bite angle ligand
has increased interactions with a substrate bound to the same metal
center allowing more efficient transfer of chiral or other steric
information [4]. This is particularly useful in palladium-catalyzed
allylic addition reactions [5,6] where the regio- or enantio-deter-
mining steps occur on the side of the substrate remote from the
asymmetric ligand. The rigidity of a ligand backbone further
influences the rate and selectivity of asymmetric reactions [2].
differentdonorshasprovensuccessfulforsomeasymmetricreactions
[7,8] including allylic substitution [9e11]. Heterobidentate ligands
containing two different donors, often one hard and one soft donor,
offer several advantages over traditional symmetrical bisphosphine
ligands by creating steric and electronic asymmetry at the metal
center [12], as well as opening the possibility for fluxional processes
that may enhance the rate and selectivity of catalytic reactions.
Another potential consequence of having donors with different
properties is hemilability [13,14] where one donor is substitutionally
inert and remains attached to the metal center while the other donor
is labile. During a catalytic cycle, dissociation of the more labile donor
can create an open site for substrate binding, while re-coordination of
that donor can temporarily stabilize a potentially coordinatively
unsaturated metal center at another step in the catalytic process.
Hemilability has further applications in asymmetric catalysis
where hemilabile ligands may allow interconversion of diastereo-
meric isomers to form a preferred isomer that could then serve as
an asymmetric catalyst. Previously [15], we studied hemilability
and nonrigidity in a series of rhodium and palladium complexes of
P^P]S donor ligands using variable temperature 1H, 13C, and 31P
NMR. We determined the barrier to hemilability was lowest for
P^P]S donor ligands forming larger chelate rings. The rigidity of
* Corresponding author. Fax: þ1 203 432 6144.
E-mail address: jack.faller@yale.edu (J.W. Faller).
1
Present address: Western New England College, 1215 Wilbraham Road,
Springfield, MA 01119, United States.
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.022

880
S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
the ligand backbones and
h
3-allyl fluxional processes were also
examined. The present study aims to increase understanding of the
regioselectivity and memory effects observed in Pd-catalyzed
allylic alkylations with respect to the donor properties, hemil-
ability, and nonrigidity of the P^P]S ligands.
PPh2
PPh2
Ph2P
Ph2P
PPh2
Ph2P
S
S
S
When butenyl substrates are used for allylic nucleophilic addition
reactions, addition can occur at the terminal or internal position
resulting in linear or branched products respectively. Palladium
complexes of chiral P^P]S ligands are known to be selective for
branched products in enantioselective nucleophilic addition reac-
tions to allylic substrates [10,11]. It is proposed that this is the result of
the steric and electronic asymmetry of P^P]S ligands. The allyl
substrate prefers to bind in an h3 fashion to palladium with its
substituted terminus trans to the bulky PPh2 group and cis to the
smaller Ph2P]S group. The nucleophile then attacks the allyl
a
b
c
PPh2
Ph2P
S
PPh2
S
Ph2P
PPh2
Ph2P
S
e
d
f
substrate trans to the more
p-accepting P(III) donor resulting in
a preference for the branched product. This selectivity is in contrast to
palladium bisphosphine catalysts which generally lead to addition at
the less-hindered allyl terminus and are considered complementary
to tungsten [16] and molybdenum [17,18] systems which lead to
addition at the more hindered terminus [19]. Rhodium [20e23] and
iridium [24e28] systems have also been shown to catalyze nucleo-
philic addition to the more substituted allylic terminus.
S
PPh2
O
Fe
O
PPh2
PPh2
Ph2P
PPh2
Ph2P
S
S
g
h
i
The branched product may result in the formation of a stereo-
center and a number of chiral catalysts that are selective for the
branched product are enantioselective. Although a number of studies
have beenreported on the regioselectivityofallylic addition reactions
based on bite angle [4] and donor effects [29] for palladium systems
giving primarily linear product, high branched selectivity from
palladium catalysts isfarlesscommon [10,11,30]and few studies have
addressed this type of regioselectivity. Formation of branched prod-
ucts is potentially more important because a chiral center could be
generated enantioselectively when addition occurs to the substituted
terminus of an unsymmetrically substituted allyl fragment. The
relationship between bidentate ligand bite angle and regioselectivity
has been explored for bisphosphine ligands [4] and analogous ligands
with different donors [31] including heterobidentate ligands. These
complexes generally give a preference for linear product despite
differences in branched:linear ratios that may be tuned through
ligand bite angle and choice of donor. A few catalysts have been
shownto reverse the normalselectivityof palladium to give primarily
branched product, yet these studies have been mainly with chiral
catalysts and focus on enantioselectivity [11,32e34]. There are fewer
studies focusing on regioselectivity with achiral ligands [31].
Fig. 1. Bisphosphine monosulfide ligands in 1.
resulting from differences in ligand bite angle and backbone
rigidity have been reported [15] for some of these complexes. The
P^P]S ligands were all bisdiphenylphosphine monosulfides as
shown in Fig. 1.
The P^P]S complexes of palladium (1) were tested as catalysts
for the alkylation of allylic carbonates (Scheme 1) in order to assess
the effect of bite angle, hemilability and ligand rigidity on the
regioselectivity. The reactions of sodium dimethyl malonate and
either the crotyl carbonate or the 3-buten-2-yl carbonate were
catalyzed with the appropriate complex at 2.5% loading and
allowed to proceed at 20 ꢀC or ꢁ20 ꢀC. The ratio of branched to
linear products was determined by 1H NMR in each case. A
comparison of the results for the linear and the branched substrate
is presented in Table 1.
Three notable features were observed in comparison of results
for the regioselectivity of the alkylation of crotyl (Table 1) and
3-buten-2-yl (Table 1) carbonates with sodium dimethyl malonate
catalyzed by 1. (1) Relative to palladium complexes incorporating
bisphosphine ligands, the use of P^P]S donor ligands generally
reversed the selectivity from a preference for linear product to
a preference for branched product, particularly when the branched
substrate was used. It was only when the linear substrate was used
that a significant preference for the linear product was observed.
(2) An increase in the chelate ring size of the P^P]S ligand
increased the preference for branched product, with the only
exceptions to this trend being those ligands displaying a memory
effect. (3) A memory effect was observed with several of the P^P]S
ligands so that with a given ligand the branched product was
produced in relatively higher amount when the branched substrate
was used than when the linear substrate was used.
We have shown that achiral P^P]S ligands are effective for palla-
dium-catalyzed regioselective nucleophilic addition to allylic carbon-
ates and have studied thisselectivityinthe absence ofchiral modifiers.
Unlike their P^P donor analogues, P^P]S donor ligands favor
branched product formation. Yet, similarly to the bisphosphine
ligands, larger bite angle P^P]S ligands further increase selectivity for
the branched product. The relative rates of allyl rearrangement,
hemilabilityand nucleophilicattack alsoinfluence the branched:linear
product ratio. The [Pd(
h
3-C3H5)(P^P]S)][SbF6] (1) complexes studied
werefoundtobeactivecatalystsforallylicalkylationreactions,andthe
regioselectivity can be tuned by varying the bite angle of the P^P]S
ligand.Thesecatalysts alsodisplaya regiochemicalmemoryeffect, and
the hemilability and rigidity of the P^P]S ligands affect the reaction
rate and the degree to which a memory effect is observed.
OCO2Et
2. Results and discussion
C(H)(CO2Me)2
[Pd(η3-C3H5)(P^P=S)][SbF6]
+
2.1. [Pd(
h
3-C3H5)(P^P]S)][SbF6] (1aei) catalyzed allylic alkylation
3-C3H5)(P^P]S)][SbF6] (1aei) were
C(H)(CO2Me)2
NaH, H2(CO2Me)2, thf
OCO2Et
A series of complexes [Pd(
h
prepared. Observations of changes in the fluxional processes
Scheme 1. Allylic alkylation of butenyl carbonates.

S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
881
Table 1
Allylic alkylation catalyzed by 1aei.
Substrate
Crotyl carbonate
20 ꢀC
3-Buten-2-yl carbonate
20 ꢀC
(P^P]S)
ꢁ20 ꢀC
Conversion %b
ꢁ20 ꢀC
Conversion %a
B:L
B:L
Conversion %a
B:L
Conversion %b
B:L
Ph2P(S)CH2PPh2 (a)
Ph2P(S)(CH2)2PPh2 (c)
Ph2P(S)(CH2)3PPh2 (d)
Ph2P(S)(o-C6H4)PPh2 (b)
(S)-BINAP(S) (e)
Biphep(S) (f)
Dppf(S) (g)
DPEphos(S) (h)
Xantphos(S) (i)
100
100
85
82
97
100
100
98
50:50
65:35
63:37
53:47
50:50
39:61
63:37
75:25
76:24
100c
100
100
21
100
100
100
100
100
50:50
70:30
57:43
56:44
47:53
31:69
67:33
85:15
80:20
100
100
100
61
100
100
100
100
100
50:50
66:34
64:36
58:42
73:27
81:19
78:22
75:25
76:24
100c
100
100
92
100
100
100
100
100
50:50
70:30
62:38
70:30
79:21
77:23
83:17
85:15
80:20
100
a
After 1 h.
After 20 h.
Reaction was complete after 1 h.
b
c
2.2. Effect of P^P]S donor
h
3-allyl complex induced by heterodonor phosphinooxazoline
ligands. With these complexes, nucleophilic attack was shown to
occur trans to the phosphorus donor [9]. In the same way, the
weaker back donation to the sulfide will decrease the electrophi-
licity of the unsubstituted carbon trans to sulfur, and the stronger
back donation to phosphorus will increase the electrophilicity of
the substituted allyl carbon trans to phosphorus. This will activate
the substituted carbon toward nucleophilic attack.
We have observed a strong preference for branched product
with most P^P]S donor ligands and no ligand giving more than
69% linear product (1f) with the linear substrate or 50% linear
product (1a) with the branched substrate. The nature of the P^P]S
donor ligands is significantly different from that of the P^P donor
ligands and can be considered to be responsible for the reversal of
the normal selectivity. The rate of addition to the substituted
terminus relative to the unsubstituted terminus of an allyl fragment
is a balance between steric and electronic properties of the system
[19,29,35,36]. In the case of the P^P]S donor ligands, the steric
asymmetry primarily dictates the orientation of the substrate,
while the electronic asymmetry directs nucleophilic attack. It is
known [36] that for the cationic complexes [Pd(1,1-dimethylallyl)
(P^P)]þ there is a distortion of the allyl ligand bonding to palladium
(Fig. 2). Interaction of the phenyl groups with the allyl ligand causes
The distortion of an allylic substrate can be probed with 13C
NMR. It has been shown that acceptor ligands increase the prefer-
ence for nucleophilic attack at the more substituted terminus
[29,31,35]. It has also been shown that the rate and regioselectivity
of alkylation reactions correlate well to 13C NMR shifts of allylic
termini [29]. It is therefore thought that these shifts can be used to
compare acceptor properties of ligands. In the case of the P^P]S
donor ligands, the more accepting phosphine may induce a greater
positive charge on the trans terminus relative to the cis terminus.
This can be observed by 13C NMR where the substituted allylic
terminus is shifted further downfield than the terminus cis to
phosphorus. There is little variation in these chemical shifts for
different P^P]S ligands. The chemical shifts for the allylic terminus
a shift from an h3 toward an h1
,
h2 bonding mode where the h1
bound carbon is the unsubstituted carbon. This causes a shortening
of the
in the
h
1 PdeC bond and a lengthening of the PdeC bonds involved
h
2 bond relative to the 3 bond of a symmetrical allyl ligand.
h
Further it has been observed that the PdeP bond trans to the h1
bond will be lengthened while that trans to the h2 bond will be
shortened. This places the methyl substituents on the allyl in the
less sterically hindered position. In the case of the P^P]S ligands
(Fig. 3), the sulfide acts as a spacer distancing the phenyl groups of
one phosphorus from the allyl ligand. Therefore, we can expect the
crotyl ligand to coordinate with a strong thermodynamic prefer-
ence for the methyl group disposed cis to the sulfide.
trans to P(III) ranges from
terminus chemical shifts range from
d
80.3e80.0 and the cis to P(III) allylic
66.2e64.7 in 1c, 1d, and 1i.
d
These 13C NMR data support the premise that the P^P]S donors
help to create the desired electronic asymmetry in the allyl frag-
ment, although the small range of chemical shifts does not allow us
to predict which P^P]S ligands will be most effective for inducing
the distortion.
We can further compare the P^P]S ligands to the P^P case
electronically. In the P^P complexes, there is greater donation and
back donation in the shorter PdeP bond than in the longer PdeP
2.3. Effect of bite angle
We have discussed the difficulty in predicting the bite angles of
complexes of P^P]S donor ligands in a previous report [15].
Although, average bite angles for P^P ligands have been reported
[37], P^P]S ligands do not necessarily follow the same trend as P^P
ligands. In some cases the sulfide donor causes an increase in bite
bond. There will be less back donation to the h
2 bond than to the h1
bond making the substituted terminal allyl position more electro-
philic than the unsubstituted terminal allyl position [29]. Similarly,
the symmetrically substituted 1,3-diphenyl allyl forms a distorted
Nu- attack
P
S
P
S
P
Pd
Nu- attack
Pd
Pd
P
P
P
Fig. 2. Allyl distortion in [Pd(1,1-dimethylallyl)(P^P)]þ
.
Fig. 3. Allyl distortion with P^P]S donor ligands.

882
S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
Table 2
isomerism relative to nucleophilic attack also influences selectivity.
Reported bite angles for P^P and P^P]S ligands.
As suggested above for the orientation of the substituted terminus,
an increased bite angle ligand will increase the relative preference
for the anti-crotyl isomer. In the case of P^P ligands, this effect has
been proposed to explain both increased linear [45] and increased
branched [4] selectivity with increased bite angle depending on the
steric demands of the substrate. Since the analogous crotyl
complexes of the P^P]S allyl complexes studied here were not
isolated, it is not clear if this rationalization would apply.
a
P^P
bave
Pd/P^P]S
Rh/P^P]S
M/P^P]S
b
b
b
Ph2PCH2PPh2
71.53
95.74b
91.21c
89.3d
Ph2P(o-C6H4)PPh2
Ph2P(CH2)2PPh2
Ph2P(CH2)3PPh2
Biphep
81.95
82.55
91.56
91.63g
92.77
90.17e
86.4f
92.85h
92.83i
89.11j
99.01k
100.45l
(S)-BINAP
2.4. Memory effect
Dppf
DPEphos
Xantphos
98.74
101.46
104.64
85.75m
A regiochemical memory effect was observed for some of the
P^P]S donor ligands utilized. In previously observed allylic addi-
tion reactions this effect was found to be increased for certain
monodentate phosphine ligands [30,32]. The memory effect refers
to the observation that nucleophilic attack is more apt to occur at
the position of the leaving group. In other words, for a given
catalyst, the branched substrate yields a greater branched: linear
ratio than the linear substrate. A common explanation of the
memory effect phenomenon is that the equilibration rates of
isomeric allylic palladium intermediates are slow relative to the
rate of nucleophilic attack. Matched and mismatched pairs of
palladium complexes derived from enantiotopic substituted allylic
substrates with enantiopure ligands have been proposed to
proceed through different mechanisms of allylic substitution
resulting in a stereochemical memory effect [46].
99.2n
102.61o
93.0p
a
Average bite angles from the Cambridge Crystallographic Database [37].
b
c
d
e
f
[Pd(h
3-C3H5)-(dppm(S))][Otf] [38].
[Pd(II)(dppm(S))2] [39].
[RhCOD(dppm(S))]SbF6X [15].
[Pd(diphos(S))Cl2] [40].
[Ru(h6 1-NMe2-C6H4eC6H4PCy2)(diphos(S))][SbF6] [41].
:h
g
[Rh(2-(4-tert-butyl-phenyl)-8-methoxy-1,8-dimethyl-bicyclo-[2.2.2]octa-2,5-
diene)(biphep)][SbF6] [42].
h
[Pd(
[Pd(
h
h
3-C3H5)(biphep(S))][SbF6] [11].
i
j
3-C3H5)((S)-BINAP(S))][BF4] [43].
[Pd((S)-BINAP(S))Cl2] [43].
[Pd(
[Pd-(
k
l
h
h
3-cinnamylallyl)((S)BINAP(S))]-[SbF6] [11].
3-1,1-dimethylallyl)((S)-BINAP(S))][SbF6] [11].
m
n
o
p
[Pd(Me8-dppf(S))Cl2] [44].
[Pd(
[Pd(
h
h
3-C3H5)(DPEphos(S))][SbF6].
The 3-buten-2-yl and crotyl carbonates are expected to add to
palladium to give the same intermediates, but in some cases the
memory of the position of the leaving group is maintained. Hayashi
observed the formation of a neutral complex upon oxidative
addition of an allylic acetate to a palladium complex of the large
cone angle ligand MeOeMOP (Fig. 4) [32]. The leaving group leaves
from the position trans to phosphorus so nucleophilic attack at the
site trans to phosphorus results in retention of regiochemistry if
allyl rearrangement is slow. Rhodium and iridium catalyzed allylic
substitution reactions often display a regiochemical memory effect,
3-C3H5)(xantphos(S))][SbF6].
[RhCOD(xantphos(S))][SbF6] [15].
angle while other ligands show contracted bite angles with the
sulfide donor relative to the parent P^P donor ligand. The bite
angles of known P^P]S complexes are presented in Table 2 in order
to illustrate the large variation in bite angle that is possible for
a given P^P]S ligand. This variation is illustrated in the bite angles
observed for (S)-BINAP(S) found in various complexes ranging from
89.11 ꢀ to 100.45 ꢀ while the parent bisphosphine has a bite angle of
92.77 ꢀ. It is, therefore, difficult to draw conclusions on the effect of
the ligand bite angle on branched selectivity, but when the
branched substrate is used, the trend for branched selectivity
generally increases with the size of the P^P]S-metal ring with rare
exceptions according to the following trend: five-membered
ring < six-membered ring z seven-membered ring < eight-
but this has been attributed to the formation of an
h
1-allyl [23,24].
A regiochemical memory effect has also been observed for the
alkylation of allylic benzoates with sodium dimethyl malonate
catalyzed by [Pd(h
3-C3H5)((S)-BINAP(S))][SbF6] [10]. We propose
a similar effect for our ligands, where nucleophilic attack will occur
at the site originally occupied by the leaving group if allyl rear-
rangement or cis-trans isomerization is slow. We have previously
reported studies of the relative rates of these processes occurring in
complexes of several P^P]S ligands [15].
This effect may also be related to the reaction rates observed. By
microscopic reversibility, if nucleophilic attack occurs trans to
phosphorus, then one expects the site of the leaving group to be
oriented trans to phosphorus upon initial ionization of the allylic
substrate. If no rearrangement occurs, one would expect branched
substrate to yield branched product and linear substrate to yield
linear product. It also follows that if orientation with the more
substituted terminus trans to phosphorus is favored, then the linear
substrate may react more slowly owing to the sterically unfavorable
orientation of the bulky allylic terminus cis to phosphorus. The only
ligands displaying no memory effect are xantphos(S) (i), DPEphos
membered ring
< nine-membered ring z dppf(S). We can
conclude that the 9-membered rings and dppf(S) approach the
optimum bite angle for branched selectivity. For these ligands that
induce high branched selectivity, decreased temperature further
increased selectivity for the branched product.
The most straightforward explanation for increased branched
selectivity with larger rings is that larger ligands allow better
“embracing” of the substrate by the phenyl groups on the donor
atoms. This is expected to increase the preference of the allyl
substituent for orientation of its more substituted terminus cis to S
which places the electronically favored position for nucleophilic
attack trans to P. The distortion of the h
3-allyl is also expected to
increase with increased bite angle [36].
The origin of branched regioselectivity has also been rational-
ized in terms of the relative amounts and stabilities of syn and anti
isomers. It has been suggested that attack at a syn substituted
position is disfavored while attack on an anti substituted allyl is
favored at the more substituted position [4]. Generally, in a stoi-
chiometric reaction, syn isomers have been shown to give (E)-linear
product, while anti isomers give mainly branched and some
(Z)-linear product. In catalytic reactions, the rate of syn-anti
MeO
PPh2
Fig. 4. Hayashi’s (R)eMeOeMOP ligand.

S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
883
Nu attack leads
to linear product
Nu attack leads to
branched product
κ2 κ1
Pd
P
κ1 κ2
Pd
T
Y
T
P
Pd
S
Pd
P
S
P
S
S
Scheme 2. A mechanism for cis/trans interconversion via ligand hemilability.
(S) (h) and diphos(S) (c). The former two ligands are among the
most selective and form nine-membereds ring with palladium
which appears to be the optimum ring size for branched selectivity
and may encourage fast allyl exchange relative to nucleophilic
attack, thereby decreasing the contribution of memory effect with
these ligands. Bite angle differences have been shown previously to
affect the rate of allyl isomerization relative to nucleophilic attack
[4]. Relative rates of hemilability may also contribute to the
memory effect since hemilability can exchange the allylic positions
cis and trans to the phosphorus donor. In fact, we reported [15] that
xantphos(S) (i), which displays no memory effect, has a decreased
barrier to hemilability relative to ligands a, c, e, and g.
ligands studied [15]. This ligand allowed for high branched selec-
tivity regardless of the substrate used, which is consistent with fast
cis/trans exchange that eliminates the memory effect.
Another factor that may influence the selectivity of palladium-
catalyzed allylic alkylations is ligand backbone rearrangement, but
this does not directly effect which allylic terminus is trans to
phosphorus, the position of nucleophilic attack. The complexes
1gei contain ligands with flexible backbones and are among the
more active catalysts for palladium-catalyzed allylic alkylation. A
less active catalyst is 1b which contains a P^P]S ligand that has
a fixed backbone geometry. An increase in catalyst activity with
increased backbone flexibility has been observed previously for
palladium catalyzed allylic alkylations [45].
One must recognize that cis/trans interconversion may also
occur via a TeYeT mechanism involving an
h
1-crotyl but retaining
Complex 1d also shows four 31P NMR resonances at low
temperature (<ꢁ60 ꢀC). At 45 ꢀC the 31P NMR spectrum remains
broad. It is probable that this complex forms two different
isomers based on the position of the propyl backbone. An alter-
native is that the formation of the P^P]S chelate is unfavorable
the
k
2-P^P]S, as shown in Scheme 3. It is rather difficult to
delineate the relative rates of the interconversions via the paths in
Schemes 2 and 3. Furthermore, although less likely, there is also the
possibility of cis/trans interconversion via a bimolecular process
involving pseudorotation of a five-coordinate intermediate formed
by addition of another ligand.
Xantphos(S) (i) appears to approach the optimal bite angle that
will allow for high regioselectivity for branched product coupled
with high catalyst activity. A nine-membered ring appears to be
optimal for branched selectivity since DPEphos(S) (h) is similar to
xantphos(S) (i) in its selectivity despite its slightly smaller bite
angle and increased flexibility when compared to other bisphos-
phine monosulfides. Dppf has a similar bite angle to DPEphos and
its monosulfide results in similar regioselectivity when the
branched substrate is used.
with this ligand and
a bimetallic species has formed. The
exchange observed may be due to dissociation of the P]S moiety.
This behavior has been observed previously with dppp(S) (d)
[47,48]. Therefore, the fluxional processes that are available to
each individual ligand may further influence branched selectivity,
reaction rate and memory effect. One might note that flexible
four-atom bridged bidentate ligands such as dppb, sometimes
produce dimers; hence even greater number of species may need
to be considered.
2.6. Structural aspects of xantphos(S) and DPEphos(S) complexes
2.5. Hemilability and nonrigidity
As xantphos and DPEphos are particularly large bite angle
ligands, X-ray crystallographic determinations of the monosulfide
complexes were undertaken to assess the relative bite angles in the
complexes with the increased chelate ring size. These metrical
aspects were also of interest since these complexes were implicated
in rapid crotyl isomeric intermediate interconversions. The crys-
tallographic data are summarized in Table 3 and ORTEP views are
shown in Figs. 5 and 6.
The flexibility of the chelate ring provided by the monosulfides
decreases the bite angles relative to the parent large bite angle
bisphosphines (Table 2). The preference for sulfur in P]S ligands to
produce bonds to metals with angles w90ꢀ yields rings with
substantially different geometries relative to the bisphosphines. A
The barrier to hemilability has been shown to decrease for
ligands forming larger rings. Since those ligands also allow for
increased branched selectivity, there may be
a relationship
between the ability of the ligands to rearrange via an h1 interme-
diate and branched selectivity. Since this mechanism exchanges the
cis and trans allylic positions, it may facilitate the reorientation of
the substrate to the more favorable position with the substituted
end trans to P. This may overcome the memory effect if hemilability
occurs more rapidly than nucleophilic attack. We have shown
previously that the large bite angle ligand xantphos(S) (i) has a low
barrier to hemilability relative to the smaller bite angle P^P]S
Nu attack leads
to linear product
Nu attack leads to
branched product
η2 η1
η1 η2
T
Y
T
Pd
Pd
Pd
S
P
Pd
S
P
S
P
S
P
Scheme 3. A mechanism for cis/trans interconversion via an
h
1-crotyl.

884
S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
Table 3
Crystallographic data for 1h and 1i.
1h
1i
Color, shape
Colorless block
C39H33OF6P2PdSSb
953.84
Mo K
Colorless plate
C43H39Cl2F6OP2PdSSb
1078.83
Empirical formula
Formula weight
Radiation/Å
a
(monochr.)
Mo Ka (monochr.)
0.71073
173
0.71073
173
T/K
Crystal system
Space group
Monoclinic
P21/n (No. 14)
Monoclinic
Cc (No. 9)
Unit cell dimensions
a/Å
b/Å
c/Å
13.5020(4)
16.0393(4)
17.3279(4)
96.859(2)
3725.7(2)
4
9.2421(2)
25.0472(7)
18.4197(5)
91.567(2)
4262.4(2)
4
b
/deg
V/Å3
Z
Dcalc/g cmꢁ3
1.700
14.10
1.681
13.64
m
/cmꢁ1 (Mo K
a)
Crystal size/mm
Total reflections, unique
reflections
0.12 ꢂ 0.12 ꢂ 0.17
0.02 ꢂ 0.12 ꢂ 0.12
15792, 8843
8513, 6502
Rint
0.031
0.025
No. Obs (I > 3
s
(I)
5697
5299
Parameters, constraints
Ra, Rwb, GOF
464, 0
0.030, 0.032, 1.08
ꢁ0.53 < 0.51
516, 0
0.031, 0.032, 1.15
ꢁ0.51 < 0.46
Fig. 6. An ORTEP view of the cation in 1i with 50% probability ellipsoids. The P1ePdeS
angle is 102.59(5)ꢀ and the PdeSeP2 angle is 107.86(6)ꢀ.
resid. density/e Åꢁ3
a
R ¼ SkFojejFck/SjFoj, for all I > 3
s(I).
.
b
Rw ¼ [S[w(jFojejFcj)2]/S[w(Fo)2]]1/2
striking difference offered by the backbone flexibility in the DPE-
phos(S) is that the backbone phenyls are nearly orthogonal;
whereas they are constrained to be nearly parallel in the xantphos
(S) complex.
The xantphos(S) ring system allows several conformational
isomers and this provides the opportunity to vary the bite angle
substantially. This is particularly notable in Table 2 when comparing
the allylpalladium and COD rhodium complexes where the bite
angles differ by w10ꢀ. An ORTEP view of the previously published
rhodium complex is provided in Fig. 7 to illustrate the conforma-
tional differences [15]. Note that in the rhodium complex the ligand
tends to fold so that the backbone is near the metal; whereas in the
palladium complex the backbone is remote from the metal.
Fig. 7. An ORTEP view of the cation in [CODRh(Xantphos(S))]SbF6 (DOBPUG) [15] with
50% probability ellipsoids. The P1ePdeS angle is 92.99(4)ꢀ and the PdeSeP2 angle is
108.47(5)ꢀ.
3. Conclusions
[Pd(h
3-C3H5)(P^P]S)][SbF6] complexes have been shown to
catalyze the allylic alkylation of carbonates with sodium dimethyl
malonate with unusually high selectivity for addition to the
substituted allylic terminus. P^P]S ligands forming larger rings
gave increased branched selectivity and furthermore, ligand back-
bone flexibility increased the rate of the reaction. There was also
a significant memory effect observed with a number of P^P]S
ligands. Low temperatures increased the regioselectivity and the
memory effect for the reaction. While the effect of bite angle is
most likely the result of better embracing of the substrate, the
Fig. 5. An ORTEP view of the cation in 1h with 50% probability ellipsoids. The
P1ePdeS angle is 99.23(3)ꢀ and the PdeSeP2 angle is 105.12(4)ꢀ.

S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
885
3
increased rate of hemilability may increase the rate of allyl equili-
bration, and decrease contributions from the memory effect by
allowing more rapid cisetrans exchange. A difference in linear and
branched substrate reactivity was also observed. Considering the
increased interest in P^S ligands for asymmetric allylic alkylation
[43,49e56], the ability to tune ligands for high branched selectivity
is a crucial aspect of developing highly selective catalysts for
enantiopure branched products.
cis to P, anti, d, JHH ¼ 12.5 Hz); 1.70 (1H, allyl-H, trans to P, anti,
dd, 3JHH ¼ 13.5 Hz, JPH ¼ 10.8 Hz). Minor isomer
d: 4.28 (1H, allyl-
4
3
H, trans to P, syn, ddd, JHH ¼ 2.2 Hz, JHH ¼ 7.5 Hz, JPH ¼ 7.3 Hz);
3
4.10 (1H, dddd, allyl-H, central, JHH ¼ 7.0 Hz, 7.5 Hz, 12.6 Hz,
4
13.8 Hz); 3.94 (1H, allyl-H, cis to P, syn, dd, JHH ¼ 2.2 Hz,
3JHH ¼ 7.0 Hz); 3.53 (1H, allyl-H, trans to P, anti, dd, 3JHH ¼ 13.8 Hz,
JPH ¼ 9.3 Hz); 2.65 (1H, allyl-H, cis to P, anti, d, 3JHH ¼ 12.6 Hz). 31
P
NMR (162 MHz, CD2Cl2, -20 ꢀC) Major isomer
d: 39.8, 11.9. Minor
isomer d: 40.2, 14.7. Anal. Calcd for C39H33O1F6P2Pd1S1Sb1$H2O:
4. Experimental
C, 48.20; H, 3.63. Found: C, 48.38; H, 3.55.
4.1. General methods
4.3. Preparation of a new P^P]S Ligand, DPEphos(S) (h)
All synthetic manipulations were performed under a nitrogen
atmosphere using standard Schlenk techniques. CH2Cl2 was dried
by distillation over CaH2 and THF was dried by distillation over
This preparation followed the general procedure used for all of
the monosulfides. The bisphosphine (2.5 mmol) was dissolved in
THF, and 0.8 equivalent of sulfur (64.1 mg, 2.00 mmol) was added.
The mixture was stirred until all of the sulfur dissolved and then the
THF was removed by rotary evaporation. The resulting mixture of
the bisphosphine, the bisphosphine monosulfide, and the bisphos-
phine disulfide was separated bycolumn chromatography (silica gel,
CH2Cl2/hexane mixture). DPEphos(S) (h) 1H NMR (500 MHz, CDCl3)
sodium and benzophenone. [Pd(h
3-C3H5)Cl]2 [56], complexes 1a
[38], 1b [11], 1c [15], 1e [10], 1f [11], 1g [15], and 1i [15], were
prepared by published methods. The previously known P^P]S
ligands a [57], b [11], c [58], d [58], e [10], f [11], g [59], and i [15],
were prepared in analogy to the published method [41] and
characterization data were in accord with those previously
reported. Pentane and diethyl ether were used as received. NMR
spectra were recorded on Bruker 400 or 500 MHz instruments and
the chemical shifts reported in ppm relative to TMS via reference
to solvent resonances for 1H and 13C spectra. The 31P spectra were
indirectly referenced to phosphoric acid via the lock frequency
d: 8.27 (1H, m, aromatic-H); 7.87e7.01 (22H, m, aromatic-H); 6.98
(1H, m, aromatic-H); 6.89 (1H, m, aromatic-H); 6.75 (1H, m,
aromatic-H); 6.55 (1H, m, aromatic-H); 6.01 (1H, m, aromatic-H). 31P
NMR (162 MHz, CDCl3)
C36H28O1P2S1: C, 75.78; H, 4.95. Found: C, 75.51; H, 4.95.
d
: 41.9 (P]S); ꢁ17.4 (P(III)). Anal. Calcd for
and
X values.
4.4. Catalysis studies
4.2. General procedure for preparation of [Pd(h
3-C3H5)(P^P]S)]
[SbF6] (1)
The data for Table 1 were obtained using the following general
procedure. A 2.5% catalyst loading using (0.00525) mmol of
complex 1aeh was added to a flame-dried flask under nitrogen and
then dissolved in 2 mL THF. The crotyl or butenyl substrate, 30.3 mg
(0.210 mmol), was added and the temperature adjusted. A 50%
excess of a stock THF solution of sodium dimethyl malonate that
had been prepared from sodium hydride and a 3% excess of
dimethyl malonate was added to the mixture (0.315 mmol, 1 mL).
After the required time (1 h or 20 h), pentane was added to the
reaction mixture and then the resulting mixture filtered through
Celite. The products were isolated after evaporation of the solvent
and the mixture analyzed by 1H NMR.
[Pd(h
3-C3H5)Cl]2 (36.6 mg, 0.1 mmol) was dissolved in CH2Cl2
and one equivalent (0.2 mmol) of the appropriate P^P]S ligand
was added to the solution. NaSbF6 (51.7 mg, 0.2 mmol) was then
added and the mixture was stirred for 6 h at room temperature. The
solution was filtered through Celite and the solvent removed by
rotary evaporation. The compounds were crystallized from
concentrated CH2Cl2 solutions layered with ether or pentane.
4.2.1. [Pd(h
3-C3H5)(dppp(S))][SbF6] (1d)
Yield: 62%. Exists as two isomers in a 5:1 ratio at ꢁ80 ꢀC 1H NMR
(500 MHz, CD2Cl2, 0 ꢀC)
d: 7.81 (4H, m, Ph-H); 7.65 (2H, m, Ph-H); 7.41
3
(14H, m, Ph-H); 5.52 (1H, m, allyl-H, central, JHH ¼ 7.6, 7.7, 12.6,
13.9 Hz); 4.54 (1H, dd, allyl-H, trans to P, syn, 3JHH ¼ 7.6 Hz, JPH ¼ 6.7 Hz);
3.42 (1H, dd, allyl-H, trans to P, anti, 3JHH ¼ 12.8 Hz, JPH ¼ 11.5 Hz); 3.83
(1H, br, allyl-H, cis to P, syn); 3.00 (2H, m, CH2); 2.89 (2H, m, CH2); 2.73
(1H, br, allyl-H, cis to P, anti); 1.95 (2H, m, CH2). 31P NMR (162 MHz,
4.5. Structure determination and refinement
Data were collected on a Nonius KappaCCD (Mo Ka radiation)
diffractometer at ꢁ100 ꢀC and were not specifically corrected for
absorption other than the inherent corrections provided by Scale-
pack [60]. The structures were solved by direct methods (SIR92)
[61] and refined on F for all reflections [62]. Non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atoms were included at calculated positions. Relevant crystal and
data parameters are presented in Table 3. Atomic coordinates were
deposited with the Cambridge Crystallographic Data Centre.
CD2Cl2, 0 ꢀC)
Major Isomer
d
: 42.8, br; 14.5 br. 31P NMR (162 MHz, CD2Cl2, e80 ꢀC)
: 42.8; 13.2. Minor Isomer
: 41.2 20.7. 13C NMR (126 MHz,
d
d
CD2Cl2)
d
: 133.4 (d, Ph-C, JPC ¼ 2.8 Hz); 133.2e132.6 (br, Ph-C); 131.5 (d,
Ph-C, JPC ¼ 10.4 Hz); 129.8 (d, Ph-C, JPC ¼ 12.5 Hz); 129.6 (d, Ph-C,
JPC ¼ 10.0 Hz); 120.1 (1C, br, allyl-CH); 80.0 (1C, d, allyl-CH2, trans to P,
JPC ¼ 30.5 Hz); 64.7 (1C, br, allyl-CH2, cis to P); 31.3 (1C, d, PCH2,
1JPC ¼ 52.0 Hz); 26.6 (1C, d, PCH2, 1JPC ¼ 26.0 Hz); 17.6 (1C, d, CH2,
2JPC ¼ 4.1 Hz). Anal. Calcd. for C30H31F6P2Pd1S1Sb1$0.5C5H12:C,45.19;H,
4.32. Found: C, 45.26; H, 4.06.
4.5.1. [Pd(h
3-C3H5)(DPEphos(S))][SbF6] (1h)
Crystals were obtained by slow diffusion of pentane into
a methylene chloride solution of 1h. The complex crystallized in the
monoclinic space group P21/n. The cation showed a 2:1 disorder in
the orientation of the allyl as indicated by common positions for the
terminal carbons of the allyl and the central carbon distributed
above and below the SePdeP plane. Owing to the disorder the
C1eC2 and C2eC3 bond lengths are not reliable; however the
PdeC1 and PdeC3 bond lengths of 2.184(4) and 2.133(3) show
the expected relative lengthening trans to P.
4.2.2. [Pd(h
3-C3H5)(DPEphos(S))][SbF6] (1h)
Yield: 82%. Exists as two isomers in 3:2 ratio at 0 ꢀC 1H NMR
(400 MHz, CD2Cl2, ꢁ20 ꢀC) Both isomers
d: 7.76e6.81 (56H, m,
d: 5.52 (1H, dddd, allyl-H, central,
aromatic-H). Major isomer
3JHH ¼ 6.5 Hz, 7.1 Hz, 12.5 Hz, 13.5 Hz); 4.56 (1H, allyl-H, trans to P,
syn, ddd, 4JHH ¼ 1.7 Hz, 3JHH ¼ 7.1 Hz, JPH ¼ 7.3 Hz); 3.77 (1H, allyl-
4
3
H, cis to P, syn, dd, JHH ¼ 1.7 Hz, JHH ¼ 6.5 Hz); 2.58 (1H, allyl-H,

886
S.C. Milheiro, J.W. Faller / Journal of Organometallic Chemistry 696 (2011) 879e886
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Crystals were obtained by slow diffusion of pentane into
a methylene chloride solution of 1i. The complex crystallized in the
monoclinic space group Cc with one molecule of methylene chloride
in the asymmetric unit. The cation showed an 85:15 disorder in the
orientation of the allyl as indicated by common positions for the
terminal carbons of the allyl and the central carbon distributed
above and below the SePdeP plane. Owing to the disorder the
C1eC2and C2eC3 bond lengthsare notreliable; however the PdeC1
and PdeC3 bond lengths of 2.199(6) Å and 2.113(6) Å show the
expected relative lengthening trans to P.
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Acknowledgements
We thank the donors of the Petroleum Research Fund admin-
istered by the American Chemical Society for support of our work
(PRF#43212). We thank Jonathan Parr for helpful comments.
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Appendix A. Supplementary data
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CCDC No. 788058 and 788059 contain the supplementary
crystallographic data for compounds 1h and 1i of this paper. These
data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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