Intermolecular migratory insertion of unactivated olefins into palladium-nitrogen bonds. Steric and electronic effects on the rate of migratory insertion

Article abstract of DOI:10.1021/ja205722f

We report a detailed examination of the effect of the steric and electronic properties of the ancillary ligand and the alkene reactant on the rate of migratory insertion of unactivated alkenes into the palladium-nitrogen bond of isolated palladium amido complexes. A series of THF-bound and THF-free amidopalladium complexes ligated by cyclometalated benzylphosphine ligands possessing varied steric and electronic properties were synthesized. The THF-free complexes react with ethylene at -50 °C to form olefin-amido complexes that were observed directly and that undergo migratory insertion, followed by β-hydride elimination to generate enamine products. The effect of the steric properties of the ancillary ligand on the binding of the alkene and the rate of migratory insertion were evaluated individually. The relative binding affinity of ethylene vs THF is larger for the less sterically hindered complex than for the more hindered complex, but the less hindered complex undergoes the insertion of ethylene more slowly than does the more hindered complex. These two changes in relative equilibrium and rate constants cause the rates of reaction of ethylene with the two THF-ligated species having different steric properties to be similar to each other. Reactions of the complexes containing electronically varied ancillary ligands showed that the more electron-poor complexes underwent the migratory insertion step faster than the more electron-rich complexes. Reactions of a THF-ligated palladium-amide with substituted vinylarenes showed that electron-poor vinylarenes reacted with the amido complex slightly faster than electron-rich vinylarenes. Separation of the energetics of binding and insertion indicate that the complex of an electron-rich vinylarene is more stable in this system than the complex of a more electron-poor vinylarene but that the insertion step of the bound, electron-rich vinylarene is slower than the insertion step with the bound, electron-poor vinylarene.

Full text of DOI:10.1021/ja205722f

ARTICLE
pubs.acs.org/JACS
Intermolecular Migratory Insertion of Unactivated Olefins into
PalladiumꢀNitrogen Bonds. Steric and Electronic Effects on the Rate of
Migratory Insertion
Patrick S. Hanley and John F. Hartwig*
Department of Chemistry, University of Illinois at UrbanaꢀChampaign, 600 South Mathews Avenue, Urbana, Illinois 61801,
United States
S Supporting Information
b
ABSTRACT: We report a detailed examination of the effect of
the steric and electronic properties of the ancillary ligand and
the alkene reactant on the rate of migratory insertion of
unactivated alkenes into the palladiumꢀnitrogen bond of
isolated palladium amido complexes. A series of THF-bound
and THF-free amidopalladium complexes ligated by cyclome-
talated benzylphosphine ligands possessing varied steric and
electronic properties were synthesized. The THF-free com-
plexes react with ethylene at ꢀ50 °C to form olefin-amido complexes that were observed directly and that undergo migratory
insertion, followed by β-hydride elimination to generate enamine products. The effect of the steric properties of the ancillary ligand
on the binding of the alkene and the rate of migratory insertion were evaluated individually. The relative binding affinity of ethylene
vs THF is larger for the less sterically hindered complex than for the more hindered complex, but the less hindered complex
undergoes the insertion of ethylene more slowly than does the more hindered complex. These two changes in relative equilibrium
and rate constants cause the rates of reaction of ethylene with the two THF-ligated species having different steric properties to be
similar to each other. Reactions of the complexes containing electronically varied ancillary ligands showed that the more electron-
poor complexes underwent the migratory insertion step faster than the more electron-rich complexes. Reactions of a THF-ligated
palladium-amide with substituted vinylarenes showed that electron-poor vinylarenes reacted with the amido complex slightly faster
than electron-rich vinylarenes. Separation of the energetics of binding and insertion indicate that the complex of an electron-rich
vinylarene is more stable in this system than the complex of a more electron-poor vinylarene but that the insertion step of the bound,
electron-rich vinylarene is slower than the insertion step with the bound, electron-poor vinylarene.
’ INTRODUCTION
dimethylacetylene dicarboxylate, but alkenes and unactivated
alkynes did not react.21
Migratory insertions of alkenes are among the most docu-
mented organometallic transformations. This class of reaction is
a common step in numerous catalytic processes, including the
polymerization of alkenes,1ꢀ4 the hydroarylation of alkenes,5ꢀ9
the difunctionalization of alkenes,10ꢀ13 and the olefination of aryl
halides, commonly termed the Mizoroki-Heck reaction.14ꢀ17
Many organometallic complexes react to form new CꢀC and
CꢀH bonds by the transfer of a transition metal hydrocarbyl or
hydride group to a coordinated olefin.18 However, the transfer of
an amido group from an isolated transition metal amido complex
to an olefin to generate a new CꢀN bond is much less established.
Many of the prior reactions of alkenes with isolated amido
complexes have required activated alkenes. Milstein, Casalnuovo,
and co-workers reported an iridium(III) arylamido complex that
inserts norbornene, but reactions with less-strained olefins were
not observed.19 Trogler et al. reported the transfer of an amido
ligand from an isolated platinum-amido complex to acrylonitrile,
but this reaction most likely occurs by direct nucleophilic attack
of the nitrogen of the amide on the electrophilic acrylonitrile.20
Boncella et al. reported the reaction of a palladium amide with
Less-activated alkenes undergo intramolecular insertion
into amido complexes characterized in situ, and recent catalytic
hydroaminations of secondary aminoalkenes catalyzed by group
IV transition metal complexes are thought to occur by alkene
insertion into the metalꢀamide bond.22ꢀ24 For example, Marks
et al. reported the intramolecular insertion of an olefin into a
lanthanide-amide generated in situ,25 and Hultzsch has reported
the hydroamination of secondary aminoalkenes catalyzed by
cationic ziroconcene and titanocene complexes.26 More recently,
Wolfe reported the intramolecular insertion of an alkene into a
palladium-amido complex generated and characterized in situ.27,28
The authors’ group has reported the isolation and full char-
acterization of a series of isolated rhodium(I) amido complexes
that react intermolecularly with unactivated alkenes and vinylar-
enes to generate imine products.29 Potential pathways for the
transfer of the amido group were investigated by a series of
Received: June 20, 2011
Published: August 04, 2011
r
2011 American Chemical Society
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Scheme 1. Reactions of THF-Ligated Palladium-Amides
2aꢀe with Ethylene and Octene
ethylene to form a species proposed to be the amido ethylene
precursor to the actual migratory insertion process.
These studies on the reactions of alkenes with discrete
amidopalladium complexes set the stage for a detailed analysis
of the effect of the steric and electronic properties of the ancillary
ligand and of the alkene on the rate of insertion. Such effects
could then be compared to those known for insertions of alkenes
into metalꢀalkyl complexes, and the effects of the differences in
bond polarity, bond strength, and the presence or absence of an
electron pair could be determined.
To this end, we report the synthesis of a series of amidopalla-
dium complexes possessing varied steric and electronic proper-
ties and studies on the reactions of these complexes with olefins
that reveal detailed information on the insertion process. This
series of complexes reacts with ethylene in high yield through a
directly observed ethylene amido complex, and they react with
vinylarenes having systematically varied electronic properties.
This combination of experiments creates a map of the effects of
the steric and electronic properties of the system on this new
class of alkene insertion reaction. Some of the effects on the rates
of these reactions are distinct from those for insertions into
metalꢀcarbon bonds but other effects parallel those observed for
insertions into metalꢀcarbon bonds, despite the significant
differences in the properties of the reactive metalꢀligand bond.
mechanistic experiments. All the mechanistic data were consis-
tent with a pathway initiated by coordination of the alkene to the
metal center, migratory insertion of the olefin into the rhodium-
amido bond, and β-hydrogen elimination from the resulting
aminoalkene complex.
In addition, migratory insertion has been proposed to occur
during several classes of palladium-catalyzed aminations of alkenes,
including carboaminations,30ꢀ34 oxidative aminations,35ꢀ37
aminoacetoxylations,38 diaminations,39 chloro-aminations,40 and
hetero-Heck type transformations.41 Stereochemical evidence for
syn-aminopalladation by migratory insertion has been obtained
during studies of some of these systems. For example, Stahl and
co-workers have shown that a complex generated from Pd-
(OAc)2 and pyridine catalyzes the oxidative cyclization of
deuterium-labeled aminoalkenes to generate products from
syn-aminopalladation,42 and Wolfe and co-workers have shown
that the stereochemical outcome of alkene carboaminations they
developed is consistent with a mechanism involving the migra-
tory insertion of an alkene into a metalꢀamide bond.27 Although
data have been reported that imply that alkenes insert into
palladiumꢀnitrogen bonds during catalytic reactions, discrete
complexes involved in palladium-catalyzed olefin aminations,
other than those studied in the current work, have not been
isolated. Studies aimed at revealing steric and electronic effects
on the insertions of alkenes into the PdꢀN bonds of phosphine-
ligated intermediates in palladium-catalyzed carboamination
reactions were stated to reveal the effect of these properties on
the rate of ligand dissociation, rather than the rate of the CꢀN
bond-forming migratory insertion step.28
We recently reported the intermolecular insertion of ethylene
and octene into the PdꢀN bonds of discrete, isolated palladium
diarylamido complexes to form enamine products.43 Stable,
THF-bound Pd-amido complexes ligated by a cyclometalated
benzylphosphine ligand reacted with ethylene to form N-vinyl
diarylamine products over 2 h at ꢀ10 °C (Scheme 1). The
stereochemical outcome of the reaction of trans-ethylene-d2
implied that the reaction occurred by insertion of the alkene
into the metal-amido bond. Reactions of ethylene with a series of
amido complexes containing different substituents on the diaryl-
amide showed that the palladium-amido complexes containing
more electron-donating amido groups reacted with alkenes faster
than those containing less electron-donating amido groups.
During these studies, a diarylamido complex lacking the THF
ligand also was isolated, and this unsaturated complex reacted
with ethylene at ꢀ50 °C in toluene-d8 to form vinyl diphenyla-
mine in high yield. Moreover, this amido complex lacking
coordinated THF reacted at low temperatures with 13C-labeled
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Palladium-Diarylamido
Complexes. To prepare a series of amidopalladium complexes
that undergo migratory insertion reactions with alkenes, we
previously targeted complexes that are isoelectronic with the
rhodium amides we had shown to undergo intermolecular alkene
insertion reactions. To encourage the isolation of monomeric
complexes and to discourage CꢀN bond-forming reductive
elimination from unsaturated arylpalladium-amido complexes,
cyclometalated species generated from bulky, chelating PꢀC
ligands were synthesized. The synthesis of these complexes was
accomplished by a two-step procedure involving initial formation
of a dimeric complex with bridging chloride ligands from the
reaction of Pd(OAc)2 with di-tert-butylbenzylphosphine and
subsequent addition of LiCl in methanol. Reaction of the dimeric
chloride complexes with potassium diarylamides generated the
diarylamido complexes 2aꢀ2e shown in Scheme 1. The char-
acterization of these complexes was reported previously in
communication form.43
To map the effects of the electronic properties of the ligands
on the rates of these migratory insertions of alkenes into the
PdꢀN bonds of the diarylamido complexes, we sought to
prepare a series of complexes bearing phosphine ligands with
varied steric and electronic properties. Dimeric, cyclometalated
palladium-chloride complexes 1a, 1g, and 1h containing different
substituents on the cyclometalated benzylphosphine ligand were
prepared from the parent benzyl di-tert-butylphosphine and from
benzyl di-tert-butylphosphines containing meta-trifluoromethyl
and meta-methoxy substiutents on the aryl ring. The palladium-
diphenylamido complexes were synthesized from the isolated
chloride dimers by the addition of KNPh2 (Scheme 2).
Attempts to prepare the analogous amido complexes contain-
ing less hindered ligands led to complicated mixtures of products.
Dimeric, cyclometalated palladium chloride complexes contain-
ing isopropyl, phenyl, and cyclohexyl groups were synthesized,
but attempts to prepare monomeric palladium-diarylamido
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Scheme 2. Synthesis of THF Ligated Palladium-Amides 2a, 2fꢀh, and THF-Free Pd Amides 3a, 3fꢀh, and 4a, 4fꢀh
Table 1. Selected Bond Distances (Å) and Angles (degrees)
in Palladium-Amides 2a and 2f
entry
parametera
2a
2f
1
2
3
4
PꢀPd
2.249 (1)
82.4 (2)
2.242 (1)
82.2 (3)
PꢀPdꢀC(17)
PꢀPdꢀN
176.5 (2)
113.0 (3)
175.9 (2)
106.2ꢀ111b
C(24)ꢀPꢀC(27)
a The numbers used to identify the carbon atoms in the angles listed
below correspond to the numbering scheme for 2f in the paper. b The
tert-butyl and isopropyl substituents in complex 2f were disordered over
two positions. The range between the largest and the shortest bond
angles is shown.
Figure 1. ORTEP diagram of 2f with 35% probability ellipsoids.
Hydrogen atoms are omitted for clarity.
room temperature by solution NMR spectroscopy, whereas the
analogous dinuclear species 4a containing di-tert-butylphosphino
groups was observed in an equilibrium with the THF-free
complex 3a only at low temperatures.
complexes from these less hindered chloride precursors led to a
complex mixture of products. However, less dramatic changes to
the steric properties led to isolable species; the monomeric Pd-
diarylamido complex 2f, which is analogous to 2a but containing
a ligand derived from benzyl(tert-butyl)(isopropyl)phosphine,
was synthesized in pure form by the sameroute used to prepare2a.
The structure of the 2-((t-Bu)(i-Pr))PCH2C6H4-ligated
complex 2f was determined by X-ray crystallography (Figure 1),
and this analysis showed that the hydrogen atom in the
isopropyl group is directed toward the opposing tert-butyl group
and that the isopropyl methyl groups face the coordination site
occupied by the THF molecule. Thus, the steric effects imparted
by the di-tert-butyl phosphino and tert-butyl(isopropyl)phosphino
groups on the coordination site that would be occupied by the
alkene ligand are not as large as one might first envision. Selected
bond angles and lengths of the 2-(t-Bu)2PCH2C6H4-ligated 2a
and 2f are listed in Table 1. The structures are similar to each
other, but the C(24)ꢀPꢀC(27) angle between the quaternary
carbon atom of the tert-butyl group and the tertiary carbon of the
isopropyl group in 2f (109.2°) is about 3.8° smaller than the
angle between the two quaternary carbons of the tert-butyl
groups in 2a (113.0°). The smaller size of the CꢀPꢀC angle
in complex 2f causes the alkyl substituents on the phosphorus
atom of 2f to be placed closer to the square plane than they are in
complex 2a.
Three-coordinate amido complexes that lack bound THF
would form alkene adducts more readily than the THF-ligated
species. To generate amido complexes lacking bound solvent,
we conducted the synthesis of the amido complexes in the
absence of THF or other basic solvents. The reaction of the
cyclometalated palladium chloride dimers with excess KNPh2
in toluene formed the solvent-free amido complexes 3a, 3f, 3g,
and 3h. Longer reaction times were needed to prepare 3f, 3g,
and 3h than was needed to prepare cyclometalated benzyl
phosphine amido complex 3a. After one day, all of the chloride
complexes 1f, 1g, and 1h were consumed, but intermediates,
presumed to be complexes containing bridging chloride and
amido groups, were the primary products. After stirring an
additional 1ꢀ6 days, these intermediates reacted with addi-
tional KNPh2 to generate the desired products. Recrystalliza-
tion of the crude material from reactions conducted at room
temperature gave pure samples of the THF-free amido com-
plexes in acceptable yields of 26ꢀ66%. Conducting these
reactions at higher temperatures to increase the reaction rate
led to the formation of two new, unknown species, as deter-
mined by 31P NMR spectroscopy, and reactions at higher
temperatures were not pursued further.
Reaction of Palladium-Diarylamido Complexes with
Alkenes. The THF-ligated diarylamido complexes reacted with
ethylene and R-olefins to generate enamine products (Scheme 3).
Reactions of 2a, 2f, 2g, and 2h with ethylene in toluene-d8
occurred at ꢀ10 °C to generate N-vinyldiphenylamine, Pd[P(t-
Bu)(R)(Bn*)]2 (R = t-Bu or i-Pr; Bn* = ꢀCH2-3-R0-C6H4,
While the steric environment around the palladium center of
2-((t-Bu)(i-Pr))PCH2C6H4-ligated 2f is not dramatically differ-
ent from that of 2a, this difference in steric property did affect
the structure of the ground state. Significant amounts of the
unsymmetrical dinuclear species 4f (Scheme 2) were observed at
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R0 = H, CF3, OCH3) and Pd black. All reactions generated
greater than 75% yield of N-vinyldiphenylamine. The enamine
was characterized by comparison of the 1H NMR spectrum to
that of genuine material prepared independently. The Pd(0)
complex Pd[P(t-Bu)(R)(Bn*)]2 was the only species detected
by 31P NMR spectroscopy, and no palladium black was formed
in the presence of 1 additional equivalent of P(t-Bu)2(Bn).
The THF-free amido complexes 3a, 3f, 3g, and 3h also reacted
with ethylene to generate N-vinyl diphenylamine and Pd[P(t-
Bu)(R)(Bn*)]2 in toluene-d8. These reactions occurred below
room temperature (ꢀ50 °C) to form enamine products in
greater than 75% yield. Again, Pd[P(t-Bu)(R)(Bn*)]2 was the
only palladium product detected by 31P NMR spectroscopy. The
rates of the reactions of 3a, 3f, 3g, and 3h at ꢀ50 °C were similar
to those of the THF-ligated species at the higher temperature
of ꢀ10 °C.
Scheme 3. Reactions of THF Ligated Pd-Amides 2a, 2fꢀh,
and THF-Free Palladium-Amides 3a, 3fꢀh, with Ethylene
Complex 2a also reacted with vinylarenes in high yield, but this
reaction occurred with regioselectivity distinct from the reactions
of alkenes. The reaction of palladium-amide 2a with 10 equiv of
styrene in benzene-d6 at 60 °C generated (E)-N-benzylidene-1,1-
diphenylmethanamine in 78% yield (eq 1). In contrast to the
reaction of 1-octene with 2a that forms enamine products from
1,2-insertion into the metalꢀnitrogen bond, followed by β-
hydrogen elimination (Figure 1),43 the reaction of styrene
formed only the enamine product from initial 2,1 insertion into
the metalꢀnitrogen bond. Presumably, the formation of this
isomer is favored by a combination of the steric interaction of the
aryl group on the alkene with the two aryl substituents at nitrogen
and the electronic preference for formation of an intermediate
with the aryl group located R to the palladium center.18 The
identity of the organic products from reaction with substituted
styrenes was confirmed by independent synthesis through
palladium-catalyzed coupling of the corresponding trans-R-bro-
mostyrene with HNPh2. The regioselectivity of this process
contrasts with that for reactions of [Rh(PEt3)3(NHAr)] and
[Rh(PEt3)3(NAr2)] with styrene, which formed ketimine and
branched enamine products, respectively.29
Figure 2. ORTEP drawing of 5 with 35% probability ellipsoids
(the hydrogen atoms, except for the hydoxyl hydrogen, were omitted
for clarity).
vinylarenes with amido complexes 2a, 2fꢀ2h, 3a, and 3fꢀ3h,
the reactions must be conducted under strictly anhydrous
conditions. Preparation of the amido complex in solvents that
were not carefully dried generated palladium amido hydroxo
complex 5 shown in Figure 2 (identified by NMR spectroscopy
and X-ray diffraction; see the Supporting Information). Reac-
tions of the amido complex with ethylene in the presence of
adventitious water generated the same hydroxo amido complex.
Comparison of Ethylene and THF Binding to Palladium-
Amide 3a. To measure the rate of migratory insertion directly,
the equilibrium constants for dissociation of THF and binding of
ethylene to the unsaturated species are needed to ensure that
kinetic measurements are conducted under conditions in which
the alkene-amido complex is the major species and the observed
rate constant corresponds as close as possible to that for the
migratory insertion step. The THF-bound amido complex 2a
equilibrates with the combination of free THF and the three-
coordinate amido species 3a on the NMR time scale (eq 3). This
equilibration was revealed by the large difference in 31P NMR
chemical shifts of the THF-ligated amide 2a in THF-d8 (δ 88.0
ppm) and benzene-d6 (δ 98.7 ppm). For reference, the 31P NMR
chemical shift of 3a in benzene-d6 was δ 101.7 ppm. The
equilibrium constant for dissociation of THF from 2a at 22 °C
Palladium amido complex 2a containing the cyclometalated
benzylphosphine ligand is much more reactive toward alkene
insertion than the isoelectronic [Rh(PEt3)3(N(p-anisyl)2] com-
plex.29 This rhodium complex reacted with styrene to form only
10ꢀ15% of the enamine product in the presence of 200 equiv of
styrene at 60 °C (eq 2), whereas the palladium complexes formed
the enamine product at room temperature in good yield with less
than 10 equiv of styrene.
Nonproductive Reactions of Amide 2a. To obtain high
yields of the enamine from the reactions of alkenes and
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was determined more precisely by obtaining spectra of samples
of 2a containing different concentrations of THF in toluene-d8.
The equilibrium constant was determined to be 6.8 Mꢀ1 at room
temperature from a fit of the plot (Figure 3) of the 31P chemical
shift vs the concentration of added THF to eq 4.
δ3a þ K1δ2a½THFꢁ
δobs
¼
ð4Þ
1 þ K1½3aꢁ½THFꢁ
The chemical shift of THF-bound 2a also depended on
temperature, due to the change in the equilibrium constant
for dissociation of THF with temperature. The chemical shift
at ꢀ60 °Cwasδ88.0 ppm; the chemical shift at 40 °C(101.1 ppm)
was closer to that of the THF-free species 3a. From the
observed chemical shifts at various temperatures, the equilibrium
constants for the association of THF shown in eq 3 were
determined (see Supporting Information), and from the tem-
perature dependence of the equilibrium constants, the enthalpy
and entropy were found to be ΔH = ꢀ11.3 kcal/mol and
ΔS = ꢀ34.0 cal/mol. The enthalpy corresponds to the PdꢀO
bond strength.
Figure 3. Plot of 31P chemical shift of 3a versus concentration of added
THF. The data were fit to eq 4.
equilibrium process at low temperature. The olefin-amido com-
plex undergoes insertion at ꢀ50 °C. At ꢀ50 °C, two resonances
corresponding to the dinuclear species are observed in the 31P
NMR spectrum, in addition to the resonance observed for 8.
These two complexes decay in concert, implying that the
equilibrium between 3a and 4a is established faster than the
olefin insertion process.
The equilibria involving binding of THF and ethylene to
amido complex 3a described in this paper, along with the
previously reported data concerning the reaction of ethylene
amido species 8, support the mechanism shown in Scheme 4 for
the reaction of palladium-diarylamides 2a and 3a with ethylene.
In this mechanism, THF rapidly and reversibly dissociates from
complex 2a, and the resulting THF-free complex 3a binds
ethylene to generate the olefin-amido adduct 8.44 Complex 8
then undergoes migratory insertion to generate the proposed
alkylpalladium intermediate 9. Intermediate 9 then rapidly
decomposes by β-hydrogen elimination to form a Pd-hydride
complex (10), and complex 10 undergoes CꢀH bond-forming
reductive elimination to generate Pd[P(Bn)(t-Bu)2]2 (11) and
Pd black. In the presence of excess P(Bn)(t-Bu)2, the unsaturated
product of reductive elimination binds the phosphine, and
We previously reported that the THF-free complex 3a equili-
brates with the ethylene-amido complex 8 in the presence of
ethylene and determined that the equilibrium constant for
ethylene binding at ꢀ65 °C is 0.09 (ΔG = 1.0 kcal/mol).43
If we assume the entropy for binding of ethylene is similar to that
for binding of THF, then the Pdꢀethylene bond dissociation
enthalpy is ꢀ6.1 kcal/mol, and the Pdꢀethylene bond is weaker
than the PdꢀTHF bond. These data are consistent with
the lack of observation by 31P NMR spectroscopy of an ethylene
complex during the reaction of THF-ligated amido complex 2a
with 50 equiv of ethylene at ꢀ10 °C.
Mechanism of the Reaction of Ethylene with Palladium-
Amide 2a. As previously reported, the addition of ethylene to the
THF-free amido species 3a at ꢀ65 °C generates a rapidly
equilibrating mixture of 3a and ethylene amido complex 8
(Scheme 4). In addition, the monomeric Pd-amido species 3a
partially converts to the unsymmetrical dimer 4a in an
Scheme 4. Proposed Reaction Mechanism of the Reaction of Pd-Amides 2a and 3a with Ethylene
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Scheme 5. Simulated Reaction Mechanism of Pd-Amide 3a with Ethylene
Figure 4. Approximate observed rate constant45 for the reaction of 3a
Figure 5. Experimental and simulated decay curves of the reaction of 3a
with ethylene.
with ethylene vs the concentration of added ethylene. For a comment on
the approximation of the rate constants, see footnote.45
was observed by 31P NMR spectroscopy, and the resonances
for 4a decay in concert with that observed for the ethylene
adduct 8.
Pd[P(Bn)(t-Bu)2]2 is the only product observed by 31P NMR
spectroscopy. Intermediates 9 and 10 were not observed during
the course of the reaction.
To determine precisely the rate constant for the migratory
insertion step (k4), we simulated the kinetic data of the simplified
process in Scheme 5 with the software KINSIM46 and iteratively
fit the experimental decays of 4a and 8 over time to the simulated
data with the software FITSIM.47 The experimental data fit the
simulated decay curves well (see Figure 5), and this analysis
showed the rate constant for the CꢀN bond-forming migratory
Reactions of Alkene-Amido Complex 8. To gain informa-
tion on the rate constant of the individual insertion step (k4),
the observed rate constant for the reaction of the pure olefin-
amido complex 8 was measured at different concentrations
of ethylene. If the mechanism in Scheme 4 is followed, then
the value of kobs should approach the value of the insertion
step under conditions in which the equilibrium lies far
toward the ethylene amido complex 8. A plot of the approx-
insertion step (k4) to be (8.7 ( 0.5) ꢂ 10ꢀ4 sꢀ1
.
45
Steric Effects of the Ancillary Ligand. To evaluate the steric
effects of the ancillary ligand on the migratory insertion step,
we compared the structure and reactivity of amido complex
2f ligated with a cyclometalated (tert-butyl)(isopropyl)benzyl-
phosphine ligand to those of the complex 2a ligated by the
cyclometalated di(tert-butyl)benzylphosphine ligand. The rate
constants for the reactions of THF-ligated complexes 2a and 2f
with ethylene are shown in eq 5. These observed rate constants
correspond to the product of the equilibrium constant for the
exchange of ethylene for THF and the rate constant for the
migratory insertion step. The reaction of the less hindered 2f was
faster than that of 2a, but by approximately the sum of the
standard deviations. The similarity in the rate constant for
reaction of 2a and 2f could be due to similar equilibrium
constants for binding and insertion of ethylene by the two
complexes or to counterbalancing steric effects on the binding
imate kobs for the reaction of 3a vs the concentration of
ethylene is shown in Figure 4. At low concentrations of
ethylene, the rate is positively dependent on the concentra-
tion of ethylene, but at high concentrations, the rate constant
approaches a constant value.
Under conditions in which 3a is allowed to react with high
concentrations of ethylene (150 equiv), the concentration of
the olefin adduct is much larger than the concentration of the
three-coordinate monomer 3a, and the kinetic expression
corresponding to the mechanism in Scheme 4 can be simpli-
fied. However, fitting of the dependence of kobs on [ethylene] is
complicated by the equilibrium between 3a and the unsymme-
trical dimer 4a. Fortunately, the olefin adduct 8 equilibrates
with the unsymmetrical dimer 4a and free ethylene faster than
it undergoes migratory insertion. During the reaction of 3a
with 150 equiv of ethylene, 15% of the unsymmetrical dimer 4a
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and insertion step. The following experiments and computations
distinguish between these possibilities.
Figure 6. Optimized ground state and transition state structure of the
olefin amido adduct 8.
The rate constants for migratory insertion of ethylene into the
PdꢀN bond of complexes 3a and 3f are shown in eq 6. In
contrast to the reactions of 3a, the reaction of the 2-((t-Bu)-
(i-Pr))PCH2C6H4-ligated 3f with ethylene at ꢀ50 °C did not
contain measurable quantities of the unsymmetrical dimer 4f
after the pre-equilibrium for binding of ethylene had been
established. After addition of ethylene, the amount of the
ethylene adduct was small, but after approximately 20 min (vs
a t1/2 = 2 h for the insertion process), the dinuclear species 4f was
completely converted to the ethylene adduct of 3f. Thus, the
decay of this adduct was measured after the pre-equilibrium had
been established, and this portion of the data were fitted to an
exponential decay to determine the rate constant for migratory
insertion. The rate constant for insertion of ethylene into the less
sterically encumbered amido complex 3f was 0.97 ꢂ 10ꢀ4 sꢀ1
(ΔG‡ = 17.0 kcal/mol). This rate constant is almost an order
of magnitude smaller than the rate constant for insertion
of ethylene into the more sterically encumbered complex 3a
(8.7 ꢂ 10ꢀ4 sꢀ1, ΔG = 16.0 kcal/mol). These results suggest that
steric hindrance at the phosphorus atom decreases the barrier to
the CꢀN bond-forming step of the migratory insertion process.
Computed Free Energy Barriers for Insertion of Ethylene
into the PdꢀN Bonds of Amido Complexes 3a and 3f. To
evaluate further the effect of the steric environment around the
palladium atom on the migratory insertion process, a series of
computations were performed on the reaction of olefin amido
complexes 3a and 3f. These computations were conducted using
density functional theory as implemented by the Gaussian 09
package.48 The B3LYP functional with polarized triple-ζ basis
sets has been shown to generate accurate results for many
palladium systems49ꢀ51 and was, therefore, used for calculations
of the reactivity of 3a, 3f, and an analogous complex ligated by a
truncated 2-(CH3)2PCH2C6H4 ligand.
Figure 7. Optimized ground state and transition state free energies for
ethylene insertion into PꢀC ligated Pd-amido complexes.
the reactions of ethylene with the cis isomers of a series of olefin
amido complexes ligated by 2-((t-Bu)(i-Pr))PCH2C6H4 and
2-(CH3)2PCH2C6H4 were computed.
The optimized ground-state structure of 8, and the structure of
the transition state for the insertion reaction of 8 are shown in
Figure 6. In the ground-state structure of 8, the centroid of the
ethylene ligand lies in the square plane, and the CꢀC double bond
is oriented perpendicular to that of the square plane. The computed
distance of the Pd-ethylene bond is 2.50 Å. In the transition state
structure for insertion, the olefin has rotated approximately 90° and
lies parallel to the PdꢀN bond. The distance between the
palladium and the closest ethylene carbon is 2.27 Å, and the
CꢀN distance is 2.04 Å. The hydrogen atoms on the ethylene are
splayed away from the Pd-center, and the HꢀCꢀH angle is 106°,
which is less than the 109° of an sp3 hybridized carbon.
The immediate product from the insertion process has an
azametallocyclobutane structure in which the nitrogen atom
forms a dative bond to the palladium center. The angles at the
β-carbon of this product indicate that the ethylene carbons are
hybridized in an sp3 fashion. The PdꢀC distance (2.14 Å) is
typical for a palladiumꢀcarbon bond, and the CꢀN distance
(1.52 Å) is typical for a CꢀN single bond.
Because the precise structure of the ethylene amido complex 8
is unknown, reactions of the two isomers of 8 with the alkene cis
or trans to the phosphorus atom were computed. The cis isomer
contains the alkene in the coordination site of the THF in com-
plex 2a. The ground state of the cis isomer is lower than that of
the trans isomer by 2.4 kcal/mol, and the free energy of activation
from the cis ground state to the cis transition state was 0.5 kcal/
mol lower than the free energy of activation from the trans
ground state to the trans transition state. Thus, to determine the
steric effect of the ancillary ligand on the migratory insertion step,
The computed free energies for migratory insertion of ethy-
lene into the amido complexes containing the three cyclometa-
lated ligands, 2-(t-Bu)2PCH2C6H4, 2-((t-Bu)(i-Pr))PCH2C6H4,
and 2-(CH3)2PCH2C6H4, are shown in Figure 7. The free
energy of activation for reaction of complex 8, which is ligated
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by the bulkiest ligand 2-(t-Bu)2PCH2C6H4 was computed to be
the lowest (15.6 kcal/mol). The enthalpy of activation of this
process (14.0 kcal/mol) was computed to be close to that of the
free energy, indicating that the entropy for this process has the
small value expected for an intramolecular reaction. The free
energy barrier for reaction of the 2-((t-Bu)(i-Pr))PCH2C6H4-
ligated complex was computed to be about 1 kcal/mol higher
(16.5 kcal/mol) than that of the 2-(t-Bu)2PCH2C6H4-ligated
complex, and the free energy barrier for reaction of the least bulky
complex ligated by 2-(CH3)2PCH2C6H4 to be the highest (18.0
kcal/mol). These computed free energies are consistent with the
activation barriers measured experimentally. The difference in free
energies determined experimentally for reaction of 3a and 3f was
1.0 kcal/mol. Thus, both the experimental and computational
results indicate that greater steric bulk on the phosphorus atom
leads to a lower barrier for migratory insertion, even though the
coordination number of the complex does not change during
insertion of an alkene into an amido complex.
An analysis of the computed structures of the ground and
transition states provides a rationalization for the steric effect. In
the computed ground-state structure, the olefin is located cis to
the phosphorus atom, and the steric interactions between the
ethylene hydrogens and the alkyl groups on the phosphorus
atom are greater than they would be in other conformations. As
the complex adopts the structure of the transition state in which
the CꢀC bond lies parallel to the PdꢀN bond, the distances
between the alkyl groups on the phosphorus atom and the
termini of the alkene ligand decrease. This decrease in steric
interaction from the ground to transition state is consistent with
the lower barrier for reaction of the more hindered complex.
Although the barrier for the migratory insertion step for the
more sterically congested complex is lower than that for the less
sterically congested complex, the reactions of ethylene with the
THF-ligated amides 2a and 2f are not faster for the more
hindered complex than for the less hindered compound. The
difference between the rates of reaction of the sterically more and
less hindered complexes containing and lacking bound THF
results from a difference in binding energy of ethylene and THF
to the more and less hindered amidopalladium fragments. Details
on the measurement of these equilibrium constants for 3f are
provided in the Supporting Information and the determination
of this value for the binding of ethylene to 3a was included in
reference 43. The Keq at 22 °C for binding of THF to the less
sterically congested 3f is 14 Mꢀ1, whereas the Keq for binding of
THF to the more hindered 3a is 6.8 Mꢀ1 (ratio of Keq values = 2).
tantalumꢀhydride bonds of metal alkene hydride complexes.53
The rate constants measured for insertion of permethylme-
tallocene and unsubstituted metallocene propylene hydride
complexes followed the trend Cp*2Nb(CH2dCHCH3)H>Cp2Nb-
(CH2dCHCH3)H > Cp*2Ta(CH2dCHCH3)H > Cp2Ta-
(CH2dCHCH3)H. The authors attributed this trend to the
greater steric interactions between the methyl group of the
propene and the methyl groups on the Cp* ligands in the ground
states than in the transition states. In the ground state, the olefin
lies in the plane bisecting the two Cp* ligands and the hydrogen
atoms and methyl group on the alkene are pointed toward
the methyl groups on the Cp* ligand. The transition state structure
is less sterically congested because the methyl group becomes
moredistantfrom theCp rings asthe product alkyl ligand emerges.
This change in structure and accompanying decrease in steric
effectsfrom thegroundtotransition state leads tothefasterratefor
insertion of the permethylmetallocene derivatives.
Electronic Effects of the Ancillary Ligand. To examine the
electronic effect of the ancillary ligand on the rate of migratory
insertion, THF-ligated, palladium-diarylamido complexes were
synthesized that contain substituents in the 3-position of the
benzylphosphine aryl ring. Complex 2h, which contains an
electron-withdrawing trifluoromethyl group and complex 2g,
which contains an electron-donating methoxy group in this
position on the benzylic aryl ring, were allowed to react with
ethylene in toluene at ꢀ10 °C. The decay of the palladium-amide
was monitored by 1H NMR spectroscopy, and the rate constants
for reactions of these complexes and the unsubstituted complex
2a are shown in eq 7. Trifluoromethyl-substituted 2h reacted
about 2.5 times faster than the parent complex 2a, and complex
2a reacted slightly faster than methoxy-substituted 2g.
These observed rate constants are a composite of the equilib-
rium constants for exchange of ethylene for THF and the rate
constant for insertion. Thus, to measure the electronic effects of
the ancillary ligand on the individual migratory insertion step, the
reactions of ethylene with the benzylphosphine-ligated amido
complexes 3a, 3g, and 3h lacking a THF were conducted.
Complexes 3a, 3g, and 3h, were allowed to react with a large
excess of ethylene at ꢀ50 °C, and the decay was monitored by
31P NMR spectroscopy. The rate constants for the migratory
insertion step with complexes 3a and 3g were determined by
kinetic simulation as described earlier in this paper. Because the
reaction of 3h with ethylene did not form an observable amount
of dimer 4h, the rate constant for migratory insertion could be
determined directly from the exponential decay of the ethylene
adduct of 3h. The reaction rates and the free energies of
activation are listed in eq 8. The trifluoromethyl-substituted
complex 3h reacted about two times faster than the unsubstituted
complex 3a, and complex 3a reacted about two times faster than
the methoxy-substituted complex 3g. These kinetic data indicate
that the complexes in this series containing a more electron-poor
metal center undergo the elementary migratory insertion step
faster than those containing a more electron-rich metal center.
The Keq at ꢀ65 °C for binding of ethylene to 3f is 149 Mꢀ1
,
whereas the Keq for binding of ethylene to 3a is 11 Mꢀ1 (ratio of
Keq values = 13). These data demonstrate that the binding affinity
of THF to the less sterically congested 3f is two times greater than
the binding affinity of THF to 3a. However, the binding affinity of
ethylene to 3f is about 13 times greater than the binding affinity of
ethylene to 3a. Although the ethylene adduct of the THF-free amido
complex 3f has a 1.0 kcal/mol higher barrier for the migratory
insertion step than the ethylene adduct of 3a, the difference in binding
of ethylene to 3f and 3a is larger than that for binding of THF, and this
difference leads to the slightly faster overall rate for the reaction of 2f
with ethylene than for the reaction of 2a with ethylene.
Steric effects on migratory insertion reactions have rarely been
evaluated.52,53 The binding equilibrium and the migratory inser-
tion step are rarely separated, and the direct precursor to
insertion is rarely observed directly.54ꢀ58 In one case, Bercaw
did report insertions of propene into the niobiumꢀhydride and
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Table 2. Reactivity of Palladium-Amide 2a with
Functionalized
The trend observed for the insertions of ethylene into the
metalꢀnitrogen bond of palladium-amido complexes parallels
the trends observed for insertion of ethylene into metalꢀcarbon
and metalꢀhydrogen bonds. Generally, complexes containing
more electrophilic metal centers undergo migratory insertion
faster than those containing less electrophilic centers.59,60 How-
ever, it is difficult to deduce whether the equilibrium constant for
binding of the olefin or the rate constant for migratory insertion
is responsible for the faster rates typically observed for the inser-
tions of more electron-poor metal centers, because increasing the
electrophilicity of the metal center typically results in stronger
binding of the olefin.
A comparison of the rates of migratory insertion of the
palladiumꢀamido complexes and more electrophilic lanthanide
and group IV amido complexes can be made, although one must
appreciate that there are multiple differences between the struc-
tures and the reactions of the early metal systems and the
palladium complexes. The amido groups in the more electro-
philic amido complexes (alkylamides) are different from those in
the palladium amido complexes reported here (diarylamides),
and the reactions of the lanthanide systems are intramolecular,
whereas the reactions of the palladium amides are intermolecular.
Despite these differences, the two types of complexes undergo
insertion of alkenes into metal-amido linkages with similar rates.
Marks and co-workers reported computational and experi-
mental studies of the mechanism of the cyclization of aminoalk-
enes catalyzed by organolanthanide complexes containing
permethylcyclopentyldienyl ligands.25,61 This work showed that
the free energy of activation (ΔG‡) for the migratory insertion of
an alkene unit within a cyclic lanthanide aminoalkene complex is
roughly 21ꢀ24 kcal/mol.62 Thus, this free energy of activation
for insertion of an alkene into a more electrophilic lanthanide-amido
complex is higher than the 16 kcal/mol free energy barrier for
insertion of ethylene within the amidopalladium olefin complex
8. However, the entropic contribution to the ΔG‡ for insertion
into the lanthanide amides was found to be large. The computed
ΔH‡ for the migratory insertion of ethylene within complex 8 of
14.0 kcal/mol is similar to the experimentally measured ΔH‡
barrier of 12.7 kcal/mol for organolanthanide complexes. Of
these two values, ΔH‡ would be most affected by the electronic
properties of the two systems. Perhaps the effect of the greater
electrophilicity of the lanthanide amide complexes and the
greater bond strength of the lanthanide amides counterbalance
each other.
lanthanideꢀamide and palladiumꢀamide bonds. This higher
barrier for reaction of the ziroconium complex likely results from
an increased strength of the metalꢀnitrogen bond due to
π-donation of the amide lone pair to the d0 metal center.64,65 This
comparison of the barriers to group IV amides is made tentatively
because the entropy and enthalpy of activation for insertions into
the group IV amido complexes have not been reported.
Electronic Effects of the Olefin on the Rate of Insertion.
The reactions of 2a with a series of vinylarenes was conducted to
reveal the effect of the electronic properties of the vinylarene on
the rates of insertion. Amido complex 2a was allowed to react
with a series of vinylarenes containing electron-donating and
electron-withdrawing substituents at the meta and para-posi-
tions, and the rate constants for these reactions were measured by
1H NMR spectroscopy. The yields and rate constants for these
reactions are summarized in Table 2.
In general, reactions of Pd-amide 2a with electron-poor
vinylarenes generated enamine products in higher yields than
reactions of 2a with electron-rich vinylarenes. The reaction of 2a
with p-(trifluoromethyl)styrene gave 95% yield of the (E)-
β-aminoethylarene, whereas the reaction of amide 2a with
p-methoxystyrene gave only 11% yield of the enamine product.
In the latter case, decomposition of the amido complex by protona-
tion of the palladiumꢀnitrogen bond was faster than migratory
insertion.
Consistent with the difference in reaction yields, the reactions
of complex 2a with vinylarenes containing electron-withdrawing
substituents were faster than those containing electron-donating
substituents. For example, the reaction of 2a with p-(trifluoromethyl)-
styrene was more than three times faster than the reaction of 2a
with styrene. Likewise, the reaction of 2a with styrene was
almost two times faster than the reaction of 2a with the more
electron-rich p-methylstyrene. Figure 8 shows a Hammett plot
of log(kx/kh) versus σ derived from the data in Table 2. The F
value is 1.04 (r = 0.988). This positive F value indicates an
accumulation of negative charge or a decrease in the partial
positive charge on the olefin in the transition state for migratory
insertion.
The free energy barriers for insertions of alkenes into group IV
amides are also higher than those for the palladium amides. The
hydroamination of an aminoalkene catalyzed by constrained-
geometry ansa-ligated Zr complexes occurred over days at
100ꢀ120 °C.22,63 This reaction was proposed to occur by turn-
over-limiting insertion of the alkene into the metalꢀamido bond.
The elevated temperatures and long times required for cycliza-
tion indicate that the barrier for insertion of the alkene into the
ZrꢀN bond is much higher than that for insertion into both the
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Figure 8. Hammett plot of the reaction of palladium-amide 2a with
substituted styrenes.
To begin to determine the origin of the observed electronic
effects, we evaluated the relative binding affinities of the different
vinylarenes. We attempted to generate the amido styrene com-
plex from the reaction of amido complex 3a lacking THF with
Figure 9. Energy diagram for the reaction of palladium-amide 2a with
substituted styrenes. The relative energies of the DFT-optimized, ground-
state styrene complexes are shown. All energies have the units of kcal/mol.
1
styrene in benzene-d8, but no change in the 31P or H NMR
signals of complex 2a was observed.66 Thus, we assessed the
relative binding energies of the vinylarenes to the benzylpho-
sphine-ligated palladium-amido fragments using density func-
tional theory. Although the differences in binding energies of the
different vinylarenes are not large, the ground-state energies
should be reliable, and the relative, rather than absolute, binding
energies are the important values for this analysis.
Scheme 6. Reaction of Palladium-Amide 2a with Substituted
Vinylarenes
The energies of the vinylarene-amido complexes containing
4-methylstyrene and 3-methoxystyrene were calculated. The
initial structures were generated by replacing ethylene in the
computed structures of the ethylene amido complex 8 with a
vinylarene. The energies of four isomers with different orienta-
tions of the vinylarenes were minimized, and the same isomer
was found to have the lowest energy for both vinylarenes
complexes. The palladiumꢀstyrene bond is relatively long for a
Pdꢀalkene bond (2.72 Å). The alkene unit is oreinted perpen-
dicular to the square plane, and the aryl group of the styrene is
located cis the phosphorus atom.
The computed relative free energies (ΔG°) of the styrene-
amido complexes are shown in the free energy diagram for
reaction of THF-complex 2a with 4-methylsytrene and 3-meth-
oxystyrene (Figure 9). The complex of the more electron-rich
vinylarene, 4-methylsytrene, was computed to be 1.2 kcal/mol
more stable than the complex of 3-methoxystyrene, whereas the
experimental free energy barrier (ΔG‡) for reaction of the
3-methoxystyrene complex was found to be 0.4 kcal/mol lower
than that of 4-methylstyrene. As depicted in Figure 9, this
combination of experimental and computational data imply that
the free energy barrier for the elementary migratory insertion
step of the 3-methoxystyrene complex is 1.6 kcal/mol smaller
than that for the same reaction of the 4-methylstyrene complex.
Becuase electron-rich vinylarenes bind more strongly to the
palladium center than electron-poor vinylarenes, the coordina-
tion of alkenes to this Pd(II) center involves a net flow of
electrons from the alkene to the metal. Typically, the alkene in
palladium-olefin complexes acts primarily as a σ donor.67,68
The immediate product from insertion of the olefin into the
metalꢀnitrogen bond is an aryl-substituted 2-aminoalkyl ligand
that places a negative charge on the Pd-bound carbon. Therefore,
we propose that the observed electronic effect results from a
positively charged alkene ligand becoming a negatively charged
alkyl ligand through a transition state that reflects this overall
change in the property of the R-carbon of the vinylarene
(Scheme 6).
Comparison of the Electronic Effects of the Alkene to
Those of Insertions of Alkenes into MetalꢀHydride and
MetalꢀAlkyl Bonds. The F value observed for the insertion of
substituted styrenes into the palladiumꢀnitrogen bonds of
palladium-amides (F = 1.04, r = 0.988) was similar to the F
value obtained by Brookhart and co-workers (F = 1.1 ( 0.1,
r = 0.991) during a study of the electronic effects of the migratory
insertion of para-substituted vinylarenes into methylpalladium
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Scheme 7. Reaction of para-Substituted Styrenes with
Methyl Palladium Complexes
and computing the relative binding energies of the vinylarenes,
we also revealed the effect of the olefin electronics on the
individual steps of olefin binding and insertion. Understanding
these effects on this new class of organometallic transformation is
necessary for the rational design of new catalysts for the amina-
tion of olefins. The following conclusions have been drawn from
these studies.
Steric Effects On the Insertion Process
(1) Bulky substituents on the phosphine create stronger steric
interactions in the ground state than in the transition state
for migratory insertion into the palladium amides. In the
transition state, the alkene lies along the PdꢀN bond, and
the steric interactions between the alkene hydrogens and
the alkyl groups on the phosphorus atom in the ancillary
ligand are weaker than they are in the ground state.
Because the steric interactions are greater in the ground
state than in the transition state, the barrier for migratory
insertion is lower for the complexes ligated with the
bulkier phosphine than for the complex ligated by the
smaller phosphine, as deduced by both experimental and
computational work.
(2) The effect of the phosphine steric properties on binding
of the alkene prior to the migratory insertion counter-
balances the effect of these steric properties on the CꢀN
bond-forming step, and this combination of effects causes
the reactions initiated by the THF-ligated species with
larger and smaller ligands to be similar to each other.
Thus, careful balancing of the size of the ancillary ligand is
needed to increase the rate of reaction of amido com-
plexes with alkenes.
complexes (Scheme 7).69 In this study, the binding affinities of
the para-substituted styrenes were measured, and the relative
free energies of activation (ΔΔG‡) were determined. These data
indicate that the effect of the electronic properties of the olefin on
the relative energies of the ground states was larger than the effect
of the electronic properties of the olefin on the relative energies
of the transition states. This result parallels the trend we deduced
from our studies of the rates of migratory insertion of substituted
vinylarenes into PdꢀN bonds.
The electronic effects on the coordination and insertion steps
of the reaction of substituted vinylarenes with RhH2Cl(PPh3)3
measured during early studies by Halpern were different from
those deduced for the insertions into palladium amide and alkyl
bonds.70 Electron-poor styrenes bind to these rhodium com-
plexes more strongly than do electron-rich styrenes, and the
insertion of the styrene into the RhꢀH bond was slower for
electron-poor styrenes than for electron-rich styrenes. Similarly,
Bercaw studied the binding of vinylarenes to a cyclopentadienyl-
ligated niobium hydride complex and the subsequent migratory
insertion of the styrene into the NbꢀH bond. The F values for
binding of the styrene to the Nb center and for the insertion
reaction of endo-Cp2NbH(4-X-C6H4CHdCH2) were +2.2,
and ꢀ1.06, respectively.53,71 These results demonstate that the
more electron-poor, p-substituted vinylarenes form more stable
adducts to these niobium complexes, (most likely due to
increased π-backbonding from the metal to the alkene unit of
the more electron-poor vinyalrene), but the complex of the more
electron-poor vinylarene underwent the migratory insertion step
more slowly than did the complex of the more electron-rich
vinylarene.53 Thus, the electronic effects observed for the
migratory insertion of vinylarenes into the palladium-amides
in the current work are the opposite of those observed for
the insertions of vinylarenes into the metalꢀhydride bonds of
hydridometal vinylarene complexes of niobium(III) and
rhodium(III). Instead, they parallel the electronic effects on
the insertions of vinylarenes into the palladiumꢀcarbon bonds
of cationic Pd(II) vinylarene complexes.
(3) The complex containing the more hindered ancillary
ligand adopts more of the monomeric structure than
the complex containing the less hindred ancillary ligand.
Because insertion involves binding of the olefin cis to a
terminal amide and the dimeric strutures form stable,
unreactive amido species, monomeric amides are neces-
sary for facile formation of the olefin-amido adduct.
Electronic Effects on the Insertion Process
(1) The amido complexes in this study containing the more
weakly donating ancillary ligands, and therefore the more
electrophilic metal centers, inserted alkenes faster than
those containing the more strongly donating ancillary
ligands. This result parallels the general trend in prior
studies showing that alkene insertions into metalꢀ
alkyl bonds of complexes containing more electrophilic
metal centers are faster than those into into metalꢀalkyl
bonds of complexes containing more electron-rich metal
centers.
(2) The rates of insertions of alkenes into the palladium-
amides in this study are similar to those for insertions into
the much more electrophilic lanthanide and Group IV
amido alkene complexes. The similarity of these rates
suggests that insertions of alkenes into metalꢀamide
bonds could be as broad in scope as insertions of alkenes
into metalꢀcarbon bonds.
’ SUMMARY
In this paper, we have reported studies on the migratory
insertions of unactivated olefins into the PdꢀN bond of isolated
palladium-diarylamido complexes that reveal the steric and
electronic effects of the ancillary ligand and olefin on the rate
of the migratory insertion. By varying the electronic and steric
properties of the ancillary benzylphosphine ligand, we revealed
the effects of the structure of the ancillary ligand on the individual
steps of alkene binding and insertion into the PdꢀN bond. In
addition, by conducting the reactions with a series of vinylarenes
(3) The THF-ligated benzylphosphine-ligated palladium-
amides reacted with electron-poor vinylarenes slightly
faster than with electron-rich vinylarenes. The relative
ground-state energies of styrene-bound amidopalladium
complexes were assessed by DFT methods, and the
complex of the more electron-donatining 4-methylstyrene
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was computed to be more stable than the complex of the
less electron-donating 3-methoxystyrene. This combina-
tion of the experimentally measured free energies of
activation for the reaction of THF-ligated palladium amide
with the different vinylarenes and the computed relative
ground-state energies of the vinylarene complexes show
that the elementary step of migratory insertion is faster
for complexes of electron-poor vinylarenes than for com-
plexesofelectron-richvinylarenes and that the difference in
free energies of activation for this step is larger than that
for the overall reaction initiated with the THF-ligated
palladium amide.
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b
characterization of complexes, insertion products, and computa-
tional details. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jhartwig@illinois.edu
’ ACKNOWLEDGMENT
We thank the Department of Energy for support, Johnson-
Matthey for the donation of Pd(OAc)2, Danielle Gray for
collection of X-ray data, Dale Pahls for his assistance with the
computations, and Prof. William D. Jones for assistance with
kinetic simulations.
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