Migratory insertion of allyl groups across the Pd-C bond in palladium(ii) isocyanide complexes. the fundamental interplay between temperature and allyl hapticity

Article abstract of DOI:10.1021/om900786e

The 2,6-dimethylphenyl- and tert-butylisocyanides (DIC and TIC, respectively) react in chlorinated solvents with the allyl dimers [Pd(μ-Cl)(η3-C₃H₃Me₂)]₂ and [Pd(μ-Cl)(η³-C₃H₅)]2

Full text of DOI:10.1021/om900786e

6762 Organometallics 2009, 28, 6762–6770
DOI: 10.1021/om900786e
Migratory Insertion of Allyl Groups across the Pd-C Bond
in Palladium(II) Isocyanide Complexes. The Fundamental Interplay
between Temperature and Allyl Hapticity
Luciano Canovese,* Fabiano Visentin, Claudio Santo, and Carlo Levi
ꢀ
Dipartimento di Chimica, Universita Ca’ Foscari, Venice, Italy
Received September 9, 2009
The 2,6-dimethylphenyl- and tert-butylisocyanides (DIC and TIC, respectively) react in chlori-
nated solvents with the allyl dimers [Pd(μ-Cl)(η3-C3H3Me2)]2 and [Pd(μ-Cl)(η3-C3H5)]2, giving the
insertion products trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCMe2)Cl], trans-[Pd(TIC)2(Cd
N(CMe3)CH2CHCMe2)Cl], and trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCH2)Cl], respec-
tively. In particular, the reaction between the complex [Pd(μ-Cl)(η3-C3H3Me2)]2 and DIC was
studied in detail, and a number of different species involved in the insertion process were identified.
A mechanistic network taking into account all the involved derivatives was proposed on the basis of
independently measured equilibrium and rate constants.
In this respect, two independent equilibria, both involving the formation of η1-allyl intermediates,
were detected and the related constants determined. The formation of such intermediates bearing the
η1-allyl fragment in cis position to the isocyanide is crucial when the subsequent insertion is
considered. The intermediates, however, do not display the same reactivity owing to the different
thermodynamic parameters governing their formation. In particular the formation of [Pd(η1-
C3H3Me2)(DIC)2Cl] (MD2Cl) represents the privileged path to the product trans-[Pd(DIC)2(Cd
N(2,6-Me2C6H3)CH2CHCMe2)Cl] (P) when the insertion process is carried out at RT. The
alternative intermediate [Pd(η1-C3H3Me2)(DIC)3]þCl- (MD3) hardly contributes to the formation
of the product P since it is disfavored by increasing the temperature.
Introduction
reaction yielding new interesting organic or organometallic
fragments might occur.4 Isocyanides (CNR) represent a
favorable alternative to the isoelectronic carbon monoxide,
owing to their high reactivity, the easy handling, and their
chemical and steric characteristics, which can be widely
modulated by the appropriate choice of the substituent R.5
However, in spite of the great number of reported isocyanide
insertions into the Pd-C bond6 and at variance with carbon
monoxide,7 to the best of our knowledge they were occa-
sionally employed as reactants toward palladium allyl deri-
vatives.8 Moreover, the mechanism involved in this sort of
reactions was never determined. Recently, we have noticed
that the product of insertion of isocyanide CNR across the
Pd-R0 bond was markedly influenced by both the nature
of R and R0 and the molar ratio between the complex and
the isocyanide.6r,9 Thus, the reaction of the complexes
[Pd(L-L0)(R0)X)] (L-L0=pyridyl thioether, diphenyl phos-
phanylquinoline ligands, X = Cl, I; R0 = Me, C6H4-
Me-4) with an equimolecular amount of 2,6-dimethylphenyl
isocyanide (DIC) yielded quantitatively the monoinserted
derivative [Pd(L-L0)(C(Me)dNC6H3Me)2X]. On the other
hand, addition of DIC in a 2:1 ratio to a solution of the
tolyl derivatives [Pd(L-L0)(C6H4-Me-4)X] gave rise to an
Palladium-catalyzed nucleophilic substitution on allyl
substrates represents an important and versatile methodol-
ogy that has attracted a great deal of interest for its potential
applications in organic synthesis.1 Generally, when nucleo-
philes such as amines or carbanions are employed, the
nucleophilic attack occurs at the terminal carbon of the allyl
moiety, although alternative mechanisms were proposed.2
However, the propensity of the allyl moiety to assume the
monohapto configuration under the attack of strong or
chelating nucleophiles is well known,3 and therefore, in the
presence of an adequate unsaturated moiety an insertion
*Corresponding author. E-mail: cano@unive.it.
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Synthesis; Wiley: Chichester, U.K. 1995; p 290. (d) Handbook of Organo-
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r
pubs.acs.org/Organometallics
Published on Web 11/06/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 23, 2009 6763
equimolecular mixture of trans-[Pd(DIC)2(C(C6H4Me-4)d
NC6H3(Me)2)X] and the unreacted starting complexes.6n,r
The imidoyl dimeric complex [Pd(CNC6H3Me2)(C(Me)dNC6-
Scheme 1. Isocyanides and Complexes Involved in the Migratory
Allyl Insertion
H3Me2)Cl] was instead obtained under similar experimental
2
conditions when the methyl complexes [Pd(L-L0)(Me)X)] were
used as starting substrates. In their early studies Tsuji and
Crociani8a,b claimed that dimeric μ-Cl species were obtained
when the complexes [Pd(η3-C3H5)(CNR)Cl] (CNR = cyclo-
hexyl, phenyl, and p-nitrophenyl isocyanide) were reacted with
one further molecule of isocyanide. In an attempt to obtain
similar complexes containing the DIC isocyanide we observed
that a completely different and far more complicated reaction
took place. Therefore, since no further papers have hitherto
appeared, with the aim of investigating in detail the reactivity
toward the palladium allyl complexes of the isocyanide deriva-
tives, we have undertaken an exhaustive study on the com-
pounds and reactions summarized in Scheme 1. The results are
reported in the present paper.
Results and Discussion
When the dimer [Pd(μ-Cl)(η3-C3H3Me2)]2 reacts at RT in
chlorinated solvents with six equivalents of DIC isocyanide,
the almost immediate formation of complex P takes place,
which represents the product of an allyl migratory insertion.
(reaction 1 in Scheme 1). No hints of the dimeric complexes
described by Crociani and Tsuji are detectable under our
experimental conditions. Such a reaction seems to be of
general application since similar insertion products are ob-
served when the dimer [Pd(μ-Cl)(η3-C3H5)]2 reacts with DIC
or when the dimer [Pd(μ-Cl)(η3-C3H3Me2)]2 reacts with the
more hindered TIC (the reaction in the latter case takes,
however, a longer time, i.e. ∼60 min).
between the starting complex and the isocyanide. Thus, the
ratio [DIC]/[[Pd(μ-Cl)(η3-C3H3Me2)]2] = 2:1 yields the iso-
meric mixture of complexes 2a and 2b (reaction 2 in
Scheme 1), which can be easily isolated and unambiguously
characterized. The assignment of the structures of the two
isomers was obtained by phase-sensitive NOESY and stems
from the key interligand cross-peaks between the syn allyl
protons and the CH3 protons of the coordinated DIC in
isomer 2a. The distribution between isomers is 2a/2b = 5:1.
Upon addition at RT of only one equivalent of DIC to a
chlorinated solvent solution of the complexes 2 (2a þ 2b), an
equimolecular mixture of the product P and the unreacted 2
isomers in rapid interconversion was produced. Dechlorina-
tion of this solution by addition of a methanol solution of
NaClO4 yields the easily separable cationic complex [Pd-
(η3-C3H3Me2)(DIC)2]ClO4 as an air-stable single product,
which was completely characterized (see Experimental
Section). Remarkably, removal of the chloride from complex
P causes the elimination of the inserted isocyanide. As will
be discussed later, the presence or the absence of chloride
in solution is of paramount importance in governing the
migratory insertion reactions.
In an attempt at determining the key species involved in
the overall reaction yielding product P, we carried out a
detailed NMR study at low temperature. Thus, we prelimi-
narily added two equivalents of DIC to a solution of the
complexes 2 at 203 K in CD2Cl2. No formation of the
product P was observed in that case. On the contrary, some
different species were detectable, none of which turned into
complex P at that temperature even after a long time.
A subsequent detailed analysis of the system allows the
identification of all the species and their role in the insertion
process (vide post). Scheme 2 anticipates the results of such
an analysis. The species relevant to the calculation pro-
cedures are indicated with the abbreviations used in the
numerical refinement processes.
However, an accurate insight into reaction 1 allows a more
detailed picture of the whole mechanism. A stepwise addition
of DIC to a solution of the dimer [Pd(μ-Cl)(η3-C3H3Me2)]2
yields different species as a function of the molar ratio
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(9) Gambino, G. “Thesis”, Universita Ca’ Foscari, Venice, 2006.

6764 Organometallics, Vol. 28, No. 23, 2009
Canovese et al.
Scheme 2. Mechanism of the Migratory Allyl Insertion
The validation of Scheme 2 was achieved by determining
independently the equilibrium constants KE1 and KE2 and
the rate constant k.
Determination of the Equilibrium Constant KE1. As already
stated, dechlorination of complex 2 and the subsequent
addition of an equimolecular amount of DIC yields the
complex [Pd(η3-C3H3Me2)(DIC)2]ClO4 (the cationic com-
plex [Pd(η3-C3H3Me2)(DIC)2]þ is always indicated as MD2
independently of the counterion). A further addition of two
equivalents of DIC at RT does not induce any insertion
process. On the contrary, only a fast (on the NMR time scale)
η3-η1 selective allyl isomerization process takes place (see
Figure 1).
Figure 1. Fluxional η3-η1 rearrangement induced upon addi-
tion of DIC in excess to a solution of [Pd(η3-C3H3Me2)-
(DIC)2]ClO4 in CD2Cl2 at different temperatures.
As a matter of fact, upon DIC addition the RT 1H NMR
spectrum displays the collapse of the two original doublets at
∼4.6 and ∼3.5 ppm, ascribable to the allyl Hsyn and Hanti
protons into a doublet centered at ∼4.0 ppm, and the original
doublet of doublets centered at ∼5.5 ppm related to the
central allyl proton reverts into a triplet at the same chemical
shift. The Mesyn and Meanti, on the contrary, are hardly
affected by DIC addition. Such a process is clearly a regio-
selective η3-η1 fluxional phenomenon in which the mono-
hapto allyl fragment coordinated to the palladium center via
the unsubstituted carbon terminus represents a less energy
demanding path to the rearrangement than its analogue with
the allyl fragment coordinated via the l,1-bis-substituted
carbon.10 No insertion processes are observable in this case
within some hours (Figure 1a).
Surprisingly, decreasing the temperature of the solution
previously studied to 213 K induces a dramatic effect on the
1H NMR spectrum. The triplet at ∼5.5 ppm undergoes no
marked transformation, whereas the doublet related to the
protons of the unsubstituted allyl terminus (Me2C-CH-CH2)
shifts upfield by ∼3.5 ppm and the signals due to the methyl
protons bound to the other allyl terminus resonate as two
singlets at ∼1.9 and ∼1.6 ppm (Figure 1b). The difference in
the chemical shifts of the methyl substituents is in this case
markedly reduced if compared with that of the starting
complex.
The upfield shift of the Me2C-CH-CH2 protons but
especially the ensuing proximity of the methyl signals repre-
sents clear evidence of the presence in solution of species
bearing an η1-allyl fragment bound to palladium via the
unsubstituted terminal carbon. As a matter of fact, the
independently synthesized and characterized complex P,
which bears a structurally comparable allyl fragment,
displays similar behavior with close methyl group signals
(Δδ = 0.03 ppm).
At fixed temperature these two effects are magnified upon
increasing the concentration of added DIC (see Scheme 2
and Figure 2).
These spectral features suggest a rapid interconversion (on
the NMR time scale) between the two complexes MD2
([Pd(η3-C3H3Me2)(DIC)2]þ) and MD3 ([Pd(η1-C3H3Me2)-
(DIC)3]þ), which bear a differently coordinated allyl frag-
ment. At fixed DIC concentration the ratio between
complexes MD2 and MD3 is also markedly influenced by
temperature changes.
We therefore decided to study the reaction between the
complexes MD2 and MD3 by adding successive aliquots of
DIC to a solution of MD2 in CD2Cl2. The titrations were
repeated at five different temperatures in the interval 193-233 K,
and within this range no decomposition processes nor
reactions yielding byproduct were observed. The reaction
appeared to be reversible and can be described by eq 1.
(10) Canovese, L.; Visentin, F.; Uguagliati, P.; Bandoli, G. Inorg.
Chim. Acta 1988, 277, 247–252.
MD2 þ DIC ¼ MD3
ð1Þ

Article
Organometallics, Vol. 28, No. 23, 2009 6765
Figure 3. Nonlinear regression analysis for the titration of the
complex [Pd(η3-C3H3Me2)(DIC)2]ClO4 (MD2) with DIC in
CD2Cl2 at 213 K.
Table 1. Initial Concentrations of MD2, Interval of the Used DIC
Concentrations, Temperatures, δI, and the Determined Para-
meters δF and KE1 Related to the Study of Equilibrium 1
in CD2Cl2
[MD2]0 ꢁ 102 [DIC]0 ꢁ 101
mol dm-3
mol dm-3
T, K δI, ppm
δF, ppm
KE1
Figure 2. Resulting 1H NMR spectra when increasing amounts
of DIC are added to a CD2Cl2 solution of the complex [Pd(η3-
C3H3Me2)(DIC)2]ClO4 (MD2) at 213 K.
1.86
1.86
1.88
1.86
1.88
0-1.264
0-1.364
0-1.994
0-1.878
0-3.226
193.1
203.1
213.1
223.1
233.1
2.19
2.21
2.22
2.22
2.23
1.61 ( 0.01 440 ( 50
1.63 ( 0.01 163 ( 7
1.64 ( 0.01 84 ( 2
1.66 ( 0.01 72 ( 5
1.67 ( 0.01 38 ( 2
In Figure 2 the effect of the titration by DIC on the
chemical shifts of the methyl groups of the allyl termini at
213 K is reported. As can be seen, the difference between the
chemical shift related to the syn and anti methyl groups of the
trihapto allyl moiety (Δδ = 0.64 ppm) decreases to a lesser
extent when the allyl fragment is forced into an η1 position
by the incoming DIC nucleophile (extrapolated Δδ =
0.04 ppm). Notably, the signal due to the protons of the
methyl group in anti position hardly changes upon DIC
addition; therefore only the chemical shift δ related to the syn
protons can be taken into consideration in the regression
analysis (vide infra).
In Figure 3 the plot of the regression analysis of the
titration carried out at 213 K is reported, whereas the whole
set of regression plots are reported in the Supporting In-
formation.
As already noticed, the KE1 value considerably decreases
with increasing the temperature, thereby suggesting the
secondary importance of this equilibrium when the overall
mechanism is studied at higher temperatures. We carried out
the van’t Hoff linear regression of the data reported in
Table 1, and the values of ΔH0 = -4.95 ( 0.5 kcal mol-1
and ΔS0 = -14 ( 2 cal mol-1 K-1 were determined. In
particular, the negative ΔS0 value is in accord with the
associative equilibrium process, the total charge of both
involved complexes being unchanged. The extrapolated
values at 298.15 and 283.15 K for KE1 are 3.7 ( 5 and
5.8 ( 8, respectively (to the extent that this extrapolation
procedure over such a wide temperature range is warranted,
taking into account the change in polar features of the
solvent involved).
Each equilibrium determination at any temperature ex-
plored was analyzed by a regression analysis based on the
following equations carried out under the SCIENTIST
environment.
KE1 ¼ ½MD3ꢀ=ð½MD2ꢀ0½MD3ꢀÞð½DICꢀ0½MD3ꢀÞ
ð2Þ
δM ¼ δI -½MD3ꢀðδI -δFÞ=½MD2ꢀ0
ð3Þ
where [MD2]0 represents the initial concentration of MD2,
[DIC]0 the total concentration of DIC added at every titra-
tion interval, [MD3] the calculated concentration of MD3
upon every DIC addition, δI the proton chemical shift of the
methyl in anti position of MD2 before DIC addition, δF the
proton chemical shift of the methyl group in η1 position in
MD3 (chemical shift of the final complex), and δM the
ensuing value of the chemical shift at every titration interval.
Notably, δF and δI are slightly temperature dependent.
The optimized KE1 and δF parameters, the almost tem-
perature-independent δI, the [DIC]0 interval, the experi-
mental temperatures T, and [MD2]0 are summarized in
Table 1.
Determination of the Equilibrium Constant KE2. Upon
addition of one equivalent of DIC to the isomeric mixture
of 2 in CD2Cl2 at 193 K, the formation of the couple of
complexes MD2 and MD2Cl is observed (see Scheme 2). In
the presence of coordinating chloride and in the absence of
free DIC, the amount of MD3 in solution is negligible.
The equilibrium between the already described MD2 and
the MD2Cl substrate is slow with respect to the NMR time
scale. Therefore, at variance with the previously discus-
sed case in which only the averaged spectrum of two rapidly
interconverting species was observable, in this case both
the species were distinguishable. MD2Cl is characterized
by a difference between the chemical shifts of the methyl

6766 Organometallics, Vol. 28, No. 23, 2009
Canovese et al.
Figure 4. 1H NMR spectrum displaying the equilibrium be-
tween the complexes [Pd(η3-C3H3Me2)(DIC)2]Cl (MD2) and
[Pd(η1-C3H3Me2)(DIC)2Cl] (MD2Cl) in CD2Cl2 at 213 K.
substituents at the allyl termini (C-(CH3)2), considerably
smaller than that of the starting complex (Δδ = 0.03 vs
0.64 ppm) and similar to that of the already discussed MD3
species. Moreover, the CH2 allyl terminus coordinated to
palladium resonates as a doublet at significantly low chemi-
cal shift (∼2.97 ppm) (see Figure 4).
All these observations suggest the monohapticity of the
allyl fragment via the unsubstituted carbon termini in
MD2Cl11 (see Scheme 2).
At variance with the previously described equilibrium
(eq 1; KE1) the equilibrium summarized in eq 4 cannot be
studied by means of a titration with a soluble organic
chloride (e.g., TEBACl; tetrabutylammonium chloride) of
the CD2Cl2 solution of complexes MD2 and MD2Cl.12 As a
matter of fact, the butyl protons of TEBA obscure a relevant
part of the spectrum, and moreover a general fluxionality of
the allylic signals take place when nucleophiles or adventi-
tious impurities acting as nucleophiles are added to solutions
of palladium allyl derivatives.13
Figure 5. Equilibrium distribution between the complexes
MD2 and MD2Cl at different temperatures.
Table 2. Temperatures, Complex Concentrations, and Related
Equilibrium Constants Determined by Addition of DIC to an
Equimolecular Solution of 2 in CD2Cl2 ([DIC]0 = [2]0 = 2.92
ꢁ 10-2 mol dm-3
)
[MD2Cl]e ꢁ 103
([MD2]e; [Cl-]e)
ꢁ 102 mol dm-3
T, K
mol dm-3
KE2
a
MD2 þ Cl- ¼ MD2Cl
ð4Þ
193.1
203.1
213.1
223.1
233.1
2.72
4.48
6.40
8.73
10.3
2.65
2.47
2.28
2.05
1.89
3.87
7.34
12.3
20.8
28.8
We therefore decided to study equilibrium 4 by determin-
ing the concentrations of the complexes involved by a
deconvolution process of the methyl signals resonating in
the interval 1.45-1.65 ppm recorded at different tempera-
tures in the interval 193-233 K. The equimolecular amount
of DIC added to the isomeric mixture of complex 2 induces
the formation of only the complexes MD2 and MD2Cl. The
equilibrium constants were thereafter calculated by means of
eq 5 on the basis of the concentrations of MD2 and MD2Cl
determined at any imposed temperature.
aKE2 values are reported without uncertainties since they were calcu-
lated on the basis of one single measurement. See text.
ranted in a low-polarity solvent at low temperatures, where
ionic pairs cannot be ruled out. However this is a customary,
albeit semiquantitative approach in this type of deter-
mination.
2
The van’t Hoff linear regression of the ln KE2 vs 1/T data
gives the following result: ΔH0 = þ4.50 ( 0.15 kcal mol-1
and ΔS0 = þ26 ( 1 cal mol-1 K-1. From the determined
parameters within the validity limits pointed to earlier, we
can extrapolate the KE2 values at 298.15 and 283.15 K, which
are 247 ( 112 and 165 ( 98, respectively.
Notably, at variance with the KE1 value, KE2 increases
with increasing temperature. It is therefore possible to push
the direction of the migratory insertion by only acting on the
reaction temperature. In the case of equilibrium 4, despite the
associative nature of the process, a positive value of ΔS0 is
determined. This very fact suggests a massive release of the
initially organized molecules of the polarized solvent around
the charged [Pd(η3-C3H3Me2)(DIC)2]þCl- (MD2) molecule
when the latter reverts into the neutral [Pd(η1-C3H3Me2)-
(DIC)2Cl] (MD2Cl). The positive ΔS0 value along with its
KE2 ¼ ½MD2Clꢀ=½MD2ꢀ
ð5Þ
The recorded spectra are reported in Figure 5, while the
one-shot equilibrium constants and the related temperatures
are summarized in Table 2. At this juncture a caveat is in
order: equilibrium 4 involves the knowledge of free ionic
species concentrations (MD2 and Cl-), which is not war-
(11) In Scheme 2 the complex MD2Cl should display a cis structure
according to the theory of the nucleophilic substitution in square-planar
complexes, which states that any substitution occurs with retention of
the initial configuration. We, however, cannot exclude that in the
presence of free nucleophile such as chloride and/or DIC an extensive
cis-trans isomerization takes place, as the spectra seem to indicate.
(12) The presence of chloride as counterion of the complex MD2
induces the formation of the MD2Cl derivative at any concentration.
(13) Canovese, L.; Visentin, F.; Uguagliati, P.; Chessa, G.; Pesce, A.
J. Organomet. Chem. 1998, 566, 61–71.

Article
Organometallics, Vol. 28, No. 23, 2009 6767
Figure 7. Nonlinear regression analysis of the kinetics of the
insertion of DIC across the Pd-C bond of the complex [Pd-
(η3-C3H3Me2)(DIC)2]Cl (MD2) in CD2Cl2 at 283.1 K.
Figure 6. Linear regression plots of the van’t Hoff equation for
KE1 and KE2. The temperatures of inversion and of the kinetic
measurements are reported.
½DICꢀ0 ¼ 0:0823 mol dm-3
related positive ΔH0 justifies the existence of an inversion
temperature (∼235.2 K, see Figure 6). Beyond that point, at
the kinetic measurement temperature (283.1 K), the KE2
value becomes far more important than that of KE1, which
is modulated by unfavorable thermodynamic parameters.
Determination of the Rate Constant k. Owing to the com-
plexity of Scheme 2, only a stepwise approach to the kinetic
problem allows a solution of the mechanistic network. Thus,
by means of the independent determination of KE1 and KE2
and of the subsequent kinetic measurements, the complete
disentanglement of the problem by direct determination of
the rate constant k was achievable. According to Scheme 2,
the ensuing value of k would be independent of DIC con-
centration. In order to prove it, the reaction was followed at
two different DIC concentrations. Unfortunately, the con-
centration of the reactant cannot be in large excess with
respect to the other species under NMR conditions. Thus,
the whole process was triggered by setting up a solution
obtained by mixing at 283.1 K14 the isomeric mixture of
complex 2 and DIC in the ratios 1:3.6 and 1:2, respectively
([2]0:[DIC]tot=0.0317:0.114 and 0.033:0.066 mol dm-3, res-
pectively). The derivative [Pd(η3-C3H3Me2)(DIC)2]Cl (MD2)
was immediately and quantitatively formed ([MD2]T = [2]0).
In the presence of the isocyanide at concentration [DIC]0 =
[DIC]tot - [2]0 the complex MD2 undergoes the pre-equilibria
previously described (KE1, KE2), giving rise to the equilibrium
mixture formed by the species MD2, MD3, and MD2Cl,
which eventually evolves into the final complex P by the
subsequent migratory insertion. The smooth progression of
the reaction was followed by recording the decrease of the 1H
NMR average signals at 5.52 ppm related to the S mixture and
the increase of those of complex P at 5.74 ppm as a function of
time and was analyzed by an iterative process carried out
under the SCIENTIST environment based on the following
equations:
KE1 ¼ 5:8 ðextrapolated equilibrium constantÞ
KE2 ¼ 165 ðextrapolated equilibrium constantÞ
½DICꢀ0 ¼ ½DICꢀ þ ½MD3ꢀ þ ½Pꢀ ðmass balanceÞ
½MD2ꢀT ¼ ½MD2ꢀ þ ½MD3ꢀ þ ½MD2Clꢀ þ ½Pꢀ
½MD2ꢀT ¼ ½Cl-ꢀ þ ½MD2Clꢀ þ ½Pꢀ
KE1 ¼ ½MD3ꢀ=½MD2ꢀ½DICꢀ
ð6Þ
KE2 ¼ ½MD2Clꢀ=½MD2ꢀ½Cl-ꢀ
½Sꢀ ¼ ½MD2ꢀ þ ½MD2Clꢀ þ ½MD3ꢀ
d½Pꢀ=dt ¼ k½MD2Clꢀ
Under these conditions the ensuing k value was (2.18 (
0.05) ꢁ 10-3 s-1, and the related regression analysis is shown
in Figure 7.
In the second case the concentration of DIC at time t = 0
([DIC]0) is equal to the concentration of [MD2]T. Such a
concentration, however, albeit sufficient for the quantitative
formation of the final product P, induces a strong instability
in the nonlinear regression procedure since the concentration
of complex MD3, which represents 5.6% of the total con-
centration of the starting complexes at time t = 0, suddenly
drops to a very low value upon consumption of free DIC.
The most convenient solution of the problem could be a
simplification of the reaction scheme consisting in ignoring
the equilibrium described by KE1 and treating the whole
system as a simple pre-equilibrium between MD2 and
½MD2ꢀT ¼ 0:0317 mol dm-3 ðtotal metal concentrationÞ
(14) 283 K represents the optimized temperature for the migratory
insertion process, which occurs at a rate compatible with the 1H NMR
technique.

6768 Organometallics, Vol. 28, No. 23, 2009
Canovese et al.
Scheme 3. Mechanism for the Migratory Allyl Insertion Carried
Out under Equimolecular Conditions ([MD2]T = [DIC]0)
out at different temperatures and the determination of the
thermodynamic parameters of the equilibria allowed the
determination of the rate constant k and some general
conclusions. The migration of an allyl fragment occurs when
the latter is η1-coordinated. Therefore, in order to obtain the
subsequent insertion, a strategy to force the monohapticity
of the allyl moiety is necessary. Usually the allyl monohap-
ticity was induced by forcing its coordinating ability by
strong tris-chelating ligands15 or by using tris- or bis-chelat-
ing ligands in the presence of chloride.3a,16 However,
although the coordinating capability of chloride toward
platinum group metals in nonprotic solvents is well known,
no hypothesis on the driving force governing the allyl
coordinative choice was advanced. As a matter of fact, only
the van’t Hoff analysis of the equilibria between the differ-
ently coordinated allyl derivatives can provide the correct
answer. We were able to establish that in polarized nonprotic
solvents the formation of neutral complexes containing
strong nucleophiles and the η1-coordinated allyl fragment
(MD2Cl) is favored by increasing the temperature thanks to
the positive ΔH0 value involved in the equilibrium reaction.
The presence of the chloride ion justifies the positive ΔS0
value of the equilibrium reaction 4, which is probably due to
the disorder associated with the release of solvent molecules
as a consequence of the formation of the neutral derivative
MD2Cl.17 The ionic complex MD3, albeit containing the η1-
coordinated allyl moiety in the favorable cis position with
respect to the isocyanide, does not display similar reactivity
since the extent of its formation is governed by unfavorable
thermodynamic parameters. In this case the negative entropy
value is in accord with the associative equilibrium process
between substrates bearing the same charge. In other words,
complex MD3 does not give insertion since an increase in
temperature (which in principle would induce the increase on
reaction rate) disfavors the formation of the complex itself.
The nature of the solvent is however crucial. As a matter of
fact, in coordinating and polar solvents such as water or
alcohols, soluble ionic allyl complexes are very stable and no
hints of η1 neutral species are evident.18 The migratory
insertion rate constant k, which is independent of the iso-
cyanide concentration, was eventually obtained, and the
conclusion that such an insertion occurs only when the allyl
is η1-coordinated and in cis position to the Pd-C bond can
be safely confirmed.
MD2Cl in the presence of equimolecular Cl- . The complex
MD2Cl in the presence of free isocyanide (which does
not affect the kinetics) gives the product P according
to Scheme 3:
The numerical regression carried out under the SCIEN-
TIST environment was based on the following equations in
which the symbols used and the related meaning are the same
as those previously employed:
½MD2ꢀT ¼ 0:033 mol dm-3 ðtotal metal concentrationÞ
½DICꢀ0 ¼ 0:033 mol dm-3 ðinitial concentration of DICÞ
KE2 ¼ 165 ðextrapolated equilibrium constantÞ
½MD2ꢀT ¼ ½MD2ꢀ þ ½MD2Clꢀ þ ½Pꢀ
½MD2ꢀT ¼ ½Cl-ꢀ þ ½MD2Clꢀ þ ½Pꢀ
KE2 ¼ ½MD2Clꢀ=½MD2ꢀ½Cl-ꢀ
½Sꢀ ¼ ½MD2ꢀ þ ½MD2Clꢀ
d½Pꢀ=dt ¼ k½MD2Clꢀ
ð7Þ
Inthiscasethe ensuing k value was (2.12( 0.06) ꢁ 10-3 s-1
,
which is in very good agreement with the previous determina-
tion. The independence of the rate constant k on DIC con-
centration was therefore demonstrated together with the
proposed mechanistic picture.
Conclusion
We were able to resolve the complex mechanism governing
the allyl migratory insertion across the Pd-C bond in
isocyanide complexes of palladium by means of an accurate
kinetic analysis carried out by a stepwise validation of the
network proposed in Scheme 2. In particular we have first
determined the equilibrium constants related to the forma-
tion of different complexes bearing the coordinated isocya-
nide, chloride, and the tri- or monohapto allyl fragment (eqs
1 and 4). The two independent equilibrium studies carried
Experimental Section
Materials. Unless otherwise stated, all manipulations were
carried out under an argon atmosphere using argon immediately
prior to use. 1D- and 2D-NMR spectra were recorded using a
Bruker DPX300 spectrometer. Chemical shifts (ppm) are given
relative to TMS (1H and 13C NMR).
(16) (a) Braunstein, P.; Naud, F.; Dedieu, A.; Rohmer, M.-M.;
DeCian, A.; Rettig, S. J. Organometallics 2001, 20, 2966–2981.
(b) Kolmar, M.; Helmchen, G. Organometallics 2002, 21, 4771–4775.
(c) Braunstein, P.; Zhang, J.; Welter, R. J. Chem. Soc., Dalton Trans.
2003, 507–509.
(17) It is noteworthy that Braunstein et al. already noticed (see
ref 16a) that at low temperature in the presence of chloride the
trihapticity of the η3-allyl coordination is favored even in the presence
of potentially terdentate ligands. On increasing the temperature, forma-
tion of the neutral η1-allyl complex bearing the chloride ion coordinated
together with the potentially tris-chelating bis(oxazoline) acting as a bis-
chelating ligand was observed.
€
(15) (a) Rulke, R. E.; Kliphuis, D.; Elsevier, C. J.; Fraanje, J.;
Goubitz, K.; van Leeuwen, P. W. N. M.; Vrieze, K. J. Chem. Soc.,
Chem. Commun. 1994, 1817–1819. (b) Wehman, P.; R€ulke, R. E.; Kaasjager,
V. E.; Kamer, P. C. J.; Kooijman, H.; Spek, A. L.; Elsevier, C. J.; Vrieze, K.;
van Leeuwen, P. W. N. M. J. Chem. Soc., Chem. Commun. 1995, 331–332.
(c) R€ulke, R. E; Kaasjager, V. E.; Vehman, P.; Elsevier, C. J.; van Leeuwen, P.
W. N. M.; Vrieze, K.; Fraanje, J.; Goubitz, K.; Spek, A. L. Organometallics
1996, 15, 3022–3031. (d) Byers, P. K.; Canty, A. J. J. Chem. Soc., Chem.
Commun. 1988, 639–641. (e) Ramdeehul, S.; Barloy, L.; Osborn, J. A.; De
Cian, A.; Fisher, J. Organometallics 1996, 15, 5442–5444. (f) Barloy, L.;
Ramdeehul, S.; Osborn, J. A.; Carlotti, C.; Taulelle, F.; De Cian, A.; Fisher, J.
Eur. J. Inorg. Chem. 2000, 2523–2532.
(18) Mesnager, J.; Kuntz, E.; Pinel, C. J. Organomet. Chem. 2009,
694, 2513–2518.

Article
Peaks are labeled as singlet (s), doublet (d), triplet (t), quartet
Organometallics, Vol. 28, No. 23, 2009 6769
(CH, allyl central). IR (KBr), cm-1: 2169 νCN. Anal. Calcd for
C14H18ClNPd: C, 49.14; H, 5.30; N, 4.09. Found: C, 49.28; H,
5.23; N, 4.18.
(q), quintet (qn), multiplet (m), and broad (br). The proton and
carbon assignment was performed by 1H-2D COSY, 1H-2D
NOESY, 1H-13C HMQC, and HMBC experiments.
[Pd(η3-C3H5)(DIC)Cl]). A 0.100 g (0.762 mmol) sample of
2,6-dimethylphenylisocyanide (DIC) was added as a solid to a
solution of [Pd(μ-Cl)(η3- C3H5)]2 (0.139 g, 0.381 mmol) in
dichloromethane (15 mL). The clear solution was stirred at
room temperature for 10 min, after which time the solvent was
removed under vacuum. The light yellow residue was washed
with diethyl ether (3 ꢁ 3 mL) and then with n-pentane (2 ꢁ
3 mL). Finally the resultant solid was dried under vacuum.
Yield: 0.208 g (87%). 1H NMR (CDCl3, 298 K): δ 2.47 (s, 6H,
DIC CH3), 3.00 (d, J = 12.5 Hz, 1H, allyl Hanti trans-Cl), 3.55 (d,
J = 13.3 Hz, 1H, allyl Hanti trans-DIC), 4.28 (d, J = 7.4 Hz, 1H,
allyl Hsyn trans-Cl), 4.58 (d, J = 7.4 Hz, 1H, allyl Hsyn trans-
DIC), 5.57 (m, allyl Hcentral), 7.14 (d, J = 7.5 Hz, 2H, DIC Hc),
7.27 (t, J = 7.5 Hz, 1H, DIC Hd). 13C{1H} NMR (CDCl3, 298
K): δ 18.7 (CH3, DIC CH3), 58.1 (CH2, allyl trans-Cl), 75.0
(CH2, allyl trans-DIC), 117.4 (CH, allyl central), 128.0 (CH,
DIC Cc), 128.0 (C, DIC Ca), 129.8 (CH, DIC Cd), 135.6
(C, DIC Cb), 149.2 (C, DIC CNR). IR (KBr), cm-1: 2180
[Pd(μ-Cl)(η3-C3H5)]2 and [Pd(μ-Cl)(η3-1,1-C3H3Me2)]2
19
20
were prepared following literature procedures. All other chemi-
cals were commercial grade and were used without further
purification.
Equilibrium and Kinetic Measurements. The equilibrium mea-
surements were analyzed by 1H NMR technique. In the case of
equilibrium 1 the complex MD2 was dissolved in 0.8 mL of
CD2Cl2 ([MD2]0 = 1.88 ꢁ 10-2 mol dm-3) and titrated by
addition of weighed amounts of solid DIC in the range 1.02 ꢁ
10-2 to 0.332 mol dm-3. Any addition was carried out upon
freezing the test tube in an acetone/liquid nitrogen mixture. The
resulting solution was then taken to the experimental tempera-
ture into the NMR probe. The titrations were repeated at five
different temperatures in the interval 193-233 K. The equilib-
rium position was evaluated from the difference between the
chemical shifts of the Me substituents of the allyl fragment. In
the case of equilibrium 4 the complex 2 (2a þ 2b) was dissolved in
0.8 mL of CD2Cl2 ([2]0 = 2.92 ꢁ 10-2 mol dm-3) and frozen at
193 K in an acetone/liquid nitrogen mixture. An equimolecular
amount of DIC was then added, and the resulting solution was
taken to the experimental temperatures. The equilibrium posi-
tion was estimated from methyl signal integration carried out at
any temperature by peak deconvolution (vide post).
The reaction kinetics were followed by integration of the
peaks at 5.52 and 5.74 ppm of the solution obtained by dissol-
ving the complex 2 (2a þ 2b) with an appropriate amount of DIC
([2]0:[DIC]tot = 0.0317:0.114 and 0.033:0.066 mol dm-3) in
CD2Cl2 at 298 K. The uncertainty in the temperature measure-
ments is estimated at (1 K in any case.
Line-Fitting Analysis. Line-fitting analysis of NMR peaks was
performed using the line-fitting module available in MestReC (v
4.9.9.6). Fitting was performed in the interval 1.45-1.65 ppm
without any constraint on peak parameters. The default con-
vergence criteria were used.
ν
CN. Anal. Calcd for C14H18ClNPd: C, 45.88; H, 4.49; N,
4.46. Found: C, 45.64; H, 4.39; N, 4.56.
[Pd(η3-1,1-C3H3Me2)(DIC)2]ClO4. To a solution of [Pd-
(μ-Cl)(η3-1,1-C3H3Me2)]2 (0.12 g, 0.284 mmol) in CH2Cl2
(15 mL) was added DIC (0.149 g, 1.14 mmol) dissolved in 5
mL of the same chlorinated solvent. Addition of NaClO4 H2O
3
(0.184 g, 1.14 mmol) in CH3OH to the stirred mixture yielded
the precipitation of NaCl with the concomitant decoloration of
the solution. The reaction mixture was stirred for 30 min, and
the solvent was removed under reduced pressure. The resulting
sticky solid was dissolved in 20 mL of CH2Cl2, treated with
activated charcoal, and filtered through Celite. The resulting
clear solution, concentrated under reduced pressure, yielded the
crude product as an off-white solid upon addition of diethyl
ether. Finally the crude product was recrystallized from
CH2Cl2/diethyl ether. Yield: 0.257 g (84%).
1H NMR (CDCl3, 298 K): δ 1.69 (s, 3H, allyl CH3anti), 2.25 (s,
3H, allyl CH3syn), 2.50 (s, 12H, DIC CH3), 3.58 (dd, J3 = 13.8
Hz, J5 = 1.2 Hz, 2H, allyl Hanti), 4.64 (d,d J3= 7.9 Hz, J5 = 1.2
Hz, 2H, allyl), 5.63 (dd, J = 13.8 Hz, J = 7.9 Hz 1H, allyl
Synthesis of the Complexes. [Pd(η3-1,1-C3H3Me2)(DIC)Cl]).
A 0.100 g (0.762 mmol) amount of 2,6-dimethylphenylisocya-
nide (DIC) was added as a solid to a solution of [Pd(μ-Cl)(η3-
1,1-C3H3Me2)]2 (0.161 g, 0.381 mmol) in dichloromethane
(15 mL). The clear solution was stirred at room temperature
for 10 min, after which time the solvent was removed under
vacuum. The light yellow residue was washed with diethyl ether
(3 ꢁ 3 mL) and then with n-pentane (2 ꢁ 3 mL). Finally the
resulting solid was dried under vacuum. Yield: 0.231 g (89%).
1H NMR (CDCl3, 298 K): Isomer 2a δ 1.44 (s, 3H, allyl
CH3anti), 1.86 (s, 3H, allyl CH3syn), 2.46 (s, 6H, DIC CH3), 3.05
(d, J = 12.8 Hz, 1H, allyl Hanti trans-Cl), 3.96 (d, J = 7.4 Hz, 1H,
allyl Hsyn trans-Cl), 5.13 (dd, J = 12.8 Hz, J = 7.4 Hz 1H, allyl
Hcentral), 7.18 (d, J = 7.8 Hz, 2H, DIC Hc), 7.21 (d, J = 7.8 Hz,
0
2H, DIC Hc ), 7.32 (t, J = 7.8 Hz, 1H, DIC Hd), 7.34 (t, J = 7.8
0
Hz DIC, 1H, Hd ). 13C{1H} NMR (CDCl3, 298 K): δ 18.7 (CH3,
DIC CH3), 22.4 (CH3, allyl CH3syn), 28.5 (CH3, allyl CH3anti),
61.6 (CH2, C3 allyl), 111.0 (C, C1 allyl), 116.0 (CH, allyl central),
0
125.4 (C, DIC Ca), 128.2 (CH, DIC Cc), 128.4 (CH, DIC Cc ),
0
128.4 (C, DIC Ca), 130.6 (CH, DIC Cd), 130.7 (CH, DIC Cd ),
0
135.6 (C, DIC Cb), 135.8 (C, DIC Cb ), 146.0 (C, DIC CNR),
146.0 (C, DIC CNR0). IR (KBr), cm-1: 2166 νCN, 1090 νClO, 624
δClO. Anal. Calcd for C14H18ClNPd: C, 49.14; H, 5.30; N, 4.09.
Found: C, 49.27; H, 5.36; N, 4.12.
H
central), 7.13 (d, J = 7.6 Hz, 2H, DIC Hc), 7.25 (t, J = 7.6 Hz,
1H, DIC Hd). Isomer 2b δ 1.49 (s, 3H, allyl CH3syn), δ 1.92 (s, 3H,
allyl CH3anti), δ 2.46 (s, 6H, DIC CH3), 3.51 (d, J = 13.4 Hz, 1H,
allyl Hanti trans-DIC), 4.23 (d, J = 7.4 Hz, 1H, allyl Hsyn trans-
DIC), 5.36 (dd, J = 13.4 Hz, J = 7.4 Hz 1H, allyl Hcentral), 7.13
(d, J=7.6 Hz, 2H, DIC Hc), 7.25 (t, J = 7.6 Hz, 1H, DIC Hd).
13C{1H} NMR (CDCl3, 298 K): Isomer 2a δ 18.8 (CH3, DIC
CH3), 20.8 (CH3, allyl CH3syn), 26.3 (CH3, allyl CH3anti), 50.3
(CH2, allyl trans-Cl), 110.1 (CH, allyl central), 127.9 (CH, DIC
Cc), 127.9 (C, DIC Ca), 129.6 (CH, DIC Cd), 135.4 (C, DIC Cb),
150.5 (C, DIC CNR) (the quaternary C allyl trans-DIC not
detectable). Isomer 2b For this isomer only A few signals
are detectable and distinguishable: 22.8 (CH3, allyl CH3syn),
29.0 (CH3, allyl CH3anti), 94.0 (CH2, allyl trans-DIC), 113.6
trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCMe2)Cl]. To
0.100 g (2.37 mmol) of [Pd(μ-Cl)(η3-1,1-C3H3Me2)]2 in CH2Cl2
(10 mL) was added as a solid 0.187 g (1.42 mmol) of DIC under
inert atmosphere (Ar) at 0 °C. The resulting solution was stirred
for 15 min and then evaporated under vacuum. The resulting
off-white solid was washed with diethyl ether (3 ꢁ 3 mL) and
with n-pentane (2 ꢁ 3 mL).
Yield: 0.270 g (94%). The product decomposes in several
hours in chlorinated solvent but can be stored as a solid at
1
-18 °C for weeks. H NMR (CDCl3, 298 K): δ 1.79 (d, J5 =
1.4 Hz, 3H, allyl CH3), 1.82 (d, J5 = 1.4 Hz, 3H, allyl CH3), 2.11
(s, 6H, CdN{C6H3(CH3)2}), 2.39 (s, 12H, DIC CH3), 3.63 (d,
J = 7.4 Hz, 2H, allyl H3), 5.75 (dqn, J3= 7.4 Hz, J5 = 1.4 Hz,
0
0
1H, allyl H2), 6.88 (m, 3H, Hc ,Hd ), 7.12 (d, J = 7.7 Hz, 4H, DIC
Hc), 7.27 (t, J = 7.7 Hz, 2H, DIC Hd). 13C{1H} NMR (CDCl3,
298 K): δ 18.5 (CH3, allyl CH3), 18.6 (CH3, DIC CH3), 18.7
(19) Hartley, F. R.; Jones, S. R. J. Organomet. Chem. 1974, 66, 465–
473.
(20) Auburn, P. R.; Mackenzie, P. B.; Bosnich, B. J. Am. Chem. Soc.
1985, 107, 2033–2046.
(CH3, CdN{C6H3(CH3)2}), 25.8 (CH3, allyl CH3), 48.5
0
(CH2, allyl C3), 119.4 (CH, C2 allyl), 123.0 (CH, Cd ), 125.4

6770 Organometallics, Vol. 28, No. 23, 2009
Canovese et al.
0
0
0
(C, DIC Ca), 126.5 (C, Ca ), 128.0 (CH, DIC Cc and Cc ), 130.0
allyl C3), 117.5 (CH2, C1 allyl), 123.1 (CH, Cd ), 127.9 (CH,
0
(CH, DIC Cd), 135.8 (C, DIC Cb), 144.7 (C, DIC CNR0), 149.1
DIC Cc and Cc ), 129.6 (CH, DIC Cd), 135.5 (C, DIC Cb), 144.2
(C, DIC CNR0), 149.2 (C, C ), 176.9 (C, CdN{C6H3(CH3)2}).
b0
0
(C, Cb ), 177.2 (C, CdN{C6H3(CH3)2}). IR (KBr), cm-1: 2174
The partial decomposition of the product does not allow a saf0e
assignment of the signals of the quaternary carbons Ca and Ca .
IR (KBr), cm-1: 2173 νCN. Anal. Calcd for C30H32ClN3Pd: C,
62.50; H, 5.60; N, 7.29. Found: C, 62.64; H, 5.66; N, 7.39.
trans-[Pd(TIC)2(CdN(CMe3)CH2CHC(Me)2)Cl]. An NMR
sample of [Pd(μ-Cl)(η3-1,1-C3H3Me2)]2 (0.0082 g in 0.8 mL of
CDCl3) was treated with 6 equiv of TIC (13.2 μL). The reaction
went to completion in 1 h with formation of one single product.
νCN. Anal. Calcd for C32H36ClN3Pd: C, 63.58; H, 6.00; N, 6.95.
Found: C, 63.64; H, 6.09; N, 6.88.
trans-[Pd(DIC)2(CdN(2,6-Me2C6H3)CH2CHCH2)Cl]. To
0.100 g (2.73 mmol) of [Pd(μ-Cl)(η3-C3H5)]2 in CH2Cl2
(10 mL) was added 0.215 g (1.64 mmol) of DIC as a solid under
inert atmosphere (Ar) at 0 °C. The resulting solution was stirred
for 15 min and then evaporated under vacuum. The resulting
off-white solid was washed with diethyl ether (3 ꢁ 3 mL) and
with n-pentane (2 ꢁ 3 mL).
1
The features of the H NMR spectrum were compatible with
those of the title compound.
Yield: 0.290 g (92%). The product decomposes in several
hours in chlorinated solvent but can be stored as a solid at -18 °C
for weeks. 1H NMR (CDCl3, 298 K): δ 2.11 (s, 6H, CdN-
{C6H3(CH3)2}), 2.40 (s, 12H, DIC CH3), 3.71 (d, J = 7.2 Hz,
2H, allyl H3), 5.26 (d, J = 10.0 Hz, 1H, allyl cis H1), 5.34 (d, J =
17.0 Hz, 1H, allyl trans H1), 6.34 (m, 1H, allyl H2), 6.90 (m, 3H,
1H NMR (CDCl3, 298 K): δ 1.46-147 (bs, 24H, TIC C(CH3)3
and allyl CH3), 1.75 (s, 9H, CdN{CdN(C(CH3)3}), 3.20 (d, J =
7.4 Hz, 2H, allyl H3), 5.38 (dqn, J3 = 7.4 Hz, J5 = 1.4 Hz, 1H,
allyl H2). The product decomposes in a few hours in the NMR
tube.
0
0
Hc ,Hd ), 7.13 (d, J = 7.5 Hz, 4H, DIC Hc), 7.24 (t, J = 7.5 Hz,
2H, DIC Hd). 13C{1H} NMR (CDCl3, 298 K): δ 18.8 (CH3,
CdN{C6H3(CH3)2}), 19.0 (CH3, DIC CH3), 53.7 (CH2,
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.