A study on the insertion of isocyanides into palladium allyl bond: Effect of their nature on the strategic steps of the process

Article abstract of DOI:10.1016/j.ica.2010.06.055

The coordinative capabilities of tert-butyl isocyanide (TIC) and 2,6-dimethylphenyl isocyanide (DIC) were shown to be perfectly comparable in spite of their different steric and electronic features. As a matter of fact, when equimolar amounts of these two isocyanides are made to compete for the same coordination sites of a Pd-allyl substrate the statistical mixture of the possible products is always observed. On the contrary, the DIC proved to be much more efficient than TIC in promoting the migratory insertion of an allyl fragment. This conclusion was simply based on the analysis of the products resulting from the reaction of an appropriate Pd-allyl complex with both isocyanides simultaneously.

Full text of DOI:10.1016/j.ica.2010.06.055

Inorganica Chimica Acta 363 (2010) 3426–3431
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A study on the insertion of isocyanides into palladium allyl bond: Effect of
their nature on the strategic steps of the process
*
Luciano Canovese, Gavino Chessa, Fabiano Visentin
Dipartimento di Chimica, Università Ca’ Foscari Venezia, Calle Larga S. Marta 2137, 30123 Venice, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The coordinative capabilities of tert-butyl isocyanide (TIC) and 2,6-dimethylphenyl isocyanide (DIC) were
shown to be perfectly comparable in spite of their different steric and electronic features. As a matter of
fact, when equimolar amounts of these two isocyanides are made to compete for the same coordination
sites of a Pd-allyl substrate the statistical mixture of the possible products is always observed.
On the contrary, the DIC proved to be much more efficient than TIC in promoting the migratory inser-
tion of an allyl fragment. This conclusion was simply based on the analysis of the products resulting from
the reaction of an appropriate Pd-allyl complex with both isocyanides simultaneously.
Ó 2010 Elsevier B.V. All rights reserved.
Received 30 March 2010
Received in revised form 22 June 2010
Accepted 28 June 2010
Available online 8 August 2010
Keywords:
Isocyanide insertion
Palladium allyl complexes
Mechanistic study
1. Introduction
in trans position to the chloride, represents the crucial intermedi-
ate for the formation of the final imino-3-butenyl product (see
The insertion of unsaturated molecules into carbon–palladium
bonds represents a fundamental step in many catalytic processes
[1]. In this context isocyanide (CNR) insertion into Pd–C bonds
has been extensively studied [2]. Isocyanides are a class of unsatu-
rated carbon ligands isoelectronic with carbon monoxide but
potentially more versatile since their electronic and steric proper-
ties can be widely varied depending on the nature of the substitu-
ent R. The insertion ability of isocyanides differs in part from that
of carbon monoxide. In particular, consecutive CO insertions are
disfavoured while isocyanides easily undergo the consecutive mul-
tiple process [3]. This behaviour, albeit leading to new types of or-
ganic compounds difficult to synthesize by other methods, renders
the mechanistic investigations of the system very difficult. Also for
this reason the kinetic studies reported in the literature are not so
numerous, and the most exhaustive results were obtained when it
was possible to limit the process to monoinsertion. To attain this
aim the choice of the spectator ligands is of fundamental impor-
tance: aryl- or methyl-palladium complexes with nitrogen biden-
tate ligands [4] and with mixed phosphorus–nitrogen or
sulphur–nitrogen ligands [5] give monoinserted iminoacyl deriva-
tives in the presence of a stoichiometric amount of isocyanide
whereas under the same conditions the exclusive use of phos-
phines as ancillary ligands often induces the polyinsertion [6].
Recently we have proposed a detailed study of the monoinser-
tion of isocyanides into Pd–allyl complexes [7]. According to this
Scheme 1). For this reason it becomes of remarkable importance
to asses the coordinating capability of the employed isocyanide
since it determines the concentration of the ‘‘active” species (I).
At the same time it is fundamental to establish the efficiency of iso-
cyanide in promoting the subsequent insertion, which represents
the rate-determining step of the process.
The main goal of this report is to prove that the electronic and
steric features of the isocyanide influence the rate-determining
insertion step much more than pre-coordination to the metal
centre.
2. Results and discussion
The strategy adopted to attain our aim is based on the direct
comparison between two different isocyanides which are made
to compete for the same palladium substrate in solution. In this
connection we chose the tert-butyl isocyanide (TIC) and the 2,6-
dimethylphenyl isocyanide (DIC) as representative compounds.
These two species are sufficiently different, the former being much
more electron-donating and more sterically encumbered than the
latter. This significant difference should magnify the effect of the
nature of the isocyanide substituent on its coordinating ability
and on its capacity of promoting migratory insertion of the allyl
group.
mechanism, the neutral species (I), bearing the
g
1-allyl fragment
2.1. Coordination efficiency
A simple experiment allowed us to compare the coordination
efficiency of the two isocyanides. It consists in adding equimolar
* Corresponding author.
E-mail address: fvise@unive.it (F. Visentin).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.06.055

L. Canovese et al. / Inorganica Chimica Acta 363 (2010) 3426–3431
3427
Scheme 1.
Fig. 1. 1H NMR spectrum (in CDCl3 at 253 K) related to the products of the reaction of [Pd( 3-allyl)]2 with 2 equiv. of DIC and 2 equiv. of TIC in the presence of an excess
l-Cl)(g
of NaClO4.
amounts of DIC and TIC to the [Pd (
experimentally obtained by adding 2 equiv. of DIC and two equiv-
alents of TIC to a solution of the dimer [Pd( -Cl)(
3-C3H5)]2, dis-
solved in a 3:1 mixture of CH2Cl2:CH3OH, in the presence of an
excess of NaClO4 [8]. These conditions allow the complete removal
of chloride which otherwise would compete for the coordination
sites and basically start up the process of isocyanide insertion [7].
The 1H NMR spectrum of the products obtained by this proce-
dure is reported in Fig. 1.
g
3-allyl)] fragment. This can be
thesized. The residual signals are consistent with those of the
mixed species [Pd(DIC)(TIC)(
g
3-C3H5)]+ (ꢀ50%).
l
g
The significance of this response is that DIC and TIC exhibit
essentially the same coordinating ability toward the palladium(II)
substrate.
2.2. Insertion efficiency
As pointed out in Scheme 1 the presence of chloride is a neces-
sary condition to promote the insertion of isocyanides into the Pd–
allyl bond [7]. Moreover, the intermediate I represents the species
The result is clear and noteworthy since a statistical mixture of
the three possible products was practically obtained. The signals of
the two homoleptic complexes [Pd(DIC)2(
g
3-C3H5)]+ (ꢀ25%) and
in which migration of the allyl group effectively occurs. The g
1-al-
[Pd(TIC)2(
g
3-C3H5)]+ (ꢀ25%) are located in the spectrum by com-
lyl fragment can potentially move toward both the coordinate iso-
cyanides in cis position, but if they are different one of the two sites
parison with the spectra of authentic samples independently syn-

3428
L. Canovese et al. / Inorganica Chimica Acta 363 (2010) 3426–3431
Chart 1.
Fig. 2. Extract of 1H NMR spectrum (in CDCl3 at 298 K) related to the reaction of [Pd(DIC)Cl( 3-C3H3Me2)] with 1 equiv. of DIC and 1 equiv. of TIC.
g
might become preferable. With this assumption, an accurate anal-
ysis of the product mixture obtained by the reaction of an appro-
priate allyl palladium substrate with DIC and TIC in the presence
of chloride may be a useful tool to evaluate the insertion capability
of the two isocyanides. An experimental option to realize this
study consists in the addition of one equivalent of TIC and one of
However, as results from the analysis of the 1H NMR spectrum1
recorded after the mixing of the reagents (Fig. 2), the only observa-
1
For the identification of the compounds present in the final mixture it is
especially profitable to consider the different positions of the signals relative to the
coordinate and to the inserted DIC in the 1H NMR spectrum. In particular the methyl
signals of the inserted DIC (one singlet for every species) lie to higher field than those
of the coordinated DIC (one singlet for every species). Thus it is possible to establish
the number of the species with inserted DIC (1, 2, 3) and then, by integration of the
signals (and in some case by 1H–1H COSY inter-peaks) to determinate the signals of
the coordinated DIC (if existing) and of the allyl protons of each species. Moreover it is
important to remind that the spectra of species 1 and 6 are already known (see Refs.
[6]).
DIC to a CDCl3 solution of the neutral complex [Pd(DIC)Cl(g3
-
C3H3Me2)]. The choice of a dimethyl derivative simplifies the inter-
pretation of the final spectrum, whereas the 3:1 ratio between pal-
ladium and the overall isocyanide species ensures the complete
conversion of the allylic reagent into the insertion products. The
six possible final complexes are represented in Chart 1.

L. Canovese et al. / Inorganica Chimica Acta 363 (2010) 3426–3431
3429
Scheme 2.
ble products are the species 1, 2, and 3, namely only those with DIC
inserted into the Pd–C bond.
(respectively 1 or 2 and 2 or 3), since the allyl migration on the aro-
matic isocyanide is far more favoured than that on the alkyl isocy-
anide. At this point one equivalent of free TIC and one equivalent of
free DIC remain available for occupying the fourth coordination
site. According to the previously proved comparable coordinating
capability, the statistical distribution between mixed species 2
and homoleptic species 1 and 3 will again be obtained.
This high selectivity attests the marked preference of allyl frag-
ment to migrate on the aromatic isocyanide. Interestingly complex
2, which presents the correct stoichiometric ratio between DIC and
TIC (2:1), amounts to the 50% of the mixture, whereas each species
1 and 3 represents 25% of the total. This evidence needs a further
elucidation. It is likely that immediately after the addition of DIC
In a second experiment we have added two equivalents of TIC to
and TIC to the complex [Pd(DIC)Cl(
g
3-C3H3Me2)] three different
a solution of [Pd(DIC)Cl(
of verifying if an increased ratio between TIC and DIC is sufficient
to induce partial insertion of the TIC into the Pd-
1allyl bond.
g
3-C3H3Me2)] in CDCl3, with the purpose
reactive intermediates are formed as a consequence of the intro-
duction of an isocyanide molecule into the coordination sphere
and the reduction of hapticity of the allyl fragment from g3 to
g1 (Scheme 2), the possibility of the fast exchange between coor-
dinated and free isocyanides being also taken into account. Appar-
ently only the intermediates A and B evolve into the final products
g
Actually the resulting mixture contained some amounts of the spe-
cies 5 and 6 (Fig. 3), but curiously no trace of 4.
As depicted in Scheme 2 the only pathway to complex 4 goes
through the intermediate B and involves the subsequent preferen-

3430
L. Canovese et al. / Inorganica Chimica Acta 363 (2010) 3426–3431
Fig. 3. Extract of 1H NMR spectrum (in CDCl3 at 298 K) related to the reaction of [Pd(DIC)Cl( 3-C3H3Me2)] with 2 equiv. of TIC.
g
tial migration of the allyl fragment on the coordinated TIC and
eventually the filling of the resulting vacant site by a molecule of
DIC. The absence of species 4 in the final products excludes this
possibility and indicates once again the absolute preference of allyl
fragment to migrate on DIC over TIC. On the other hand, the obser-
vable species 5 which theoretically might also be obtained from
the intermediate B indeed derives from the alternative precursor
C. This intermediate, for which the insertion of TIC is obviously
the only option, gives rise also to the final complex 6 which ap-
pears in the final spectrum (Scheme 2). Therefore in this second
experiment the favourable ratio TIC/DIC, in addition to the faster
disappearance of the DIC for insertion, renders competitive the
path involving intermediate C, which was virtually negligible in
the first experiment. Moreover, the low amount of DIC available
is also the reason for the absence of species 1 among the products.
Peaks are labelled as singlet (s), doublet (d), triplet (t), quartet
(q), multiplet (m) and broad (br). The proton assignment was per-
formed also by 1H-2D COSY.
[Pd(l-Cl)(g , [Pd(DIC)Cl(g
3-C3H5)]2 3-C3H3Me2)]7, trans-[Pd-
10
(DIC)2(C@N(2,6-Me2C6H3)CH2CHCMe2)Cl]7 (1) and trans-[Pd(TIC)2
(C@N(CMe3)CH2CHCMe2)Cl]7 (6), were synthesized according to
previously reported procedures. The isocyanides tert-butyl isocya-
nide (TIC) and the 2,6-dimethylphenyl isocyanide (DIC) were pur-
chased from Fluka and used without purification. All other
chemicals and solvents were reagent grade and were used without
further purification.
4.1. Synthesis of [Pd(DIC)2(
g
3-C3H5)]ClO4
To 0.12 g (0.328 mmol) of [Pd(
l-Cl)(g
3-C3H5)]2 in 15 mL of
CH2Cl2, 0.172 g (1.312 mmol) of DIC in 5 mL of the same chlori-
nated solvent was added. Addition of an excess of NaClO4ÁH2O
(0.184 g, 1.312 mmol) dissolved in 6 mL of CH3OH to the stirred
mixture yielded the precipitation of NaCl. The reaction mixture
was stirred for 30 min and then the solvent was removed under re-
duced pressure. The whitish solid was extracted with 20 mL of
CH2Cl2, treated with activated charcoal and filtered off through
Celite. Evaporation to small volume yielded the crude precipitate,
which was recrystallized from CH2Cl2,/Et2O as a white solid and
dried in vacuo. Yield 0.302 g (90%). 1H NMR (CDCl3, 298 K) d 2.51
3. Conclusion
In this contribution we have demonstrated that the nature of
the isocyanide scarcely affects its coordinative capability on palla-
dium(II) but heavily influences the efficiency of the migratory
insertion of the allyl fragment. These results are achieved on the
basis of the mere analysis of the mixture of products obtained by
the reaction of appropriate palladium substrates with variable
amounts of two different isocyanides.
In previous works it was observed that the overall insertion rate
increases with increasing electrophilicity and with reducing steric
hindrance on the isocyanide carbon [4,5,9,10]. This tendency is
confirmed in the present paper but in addition we have shown that
pre-coordination of the isocyanide, a preliminary step of the pro-
cess, is practically insensitive to its electronic and steric features.
For this reason the effect of the nature of the isocyanide on the
overall reaction rate can be wholly attributed to its influence on
the migration step.
r
, 12Y, DIC CH3), 3.77 (d, J = 13.4 Hz, 2H, allyl Hant), 5.03 (d,
J = 7.3 Hz, 2H, allyl Hsyn), 5.90 (m, 1H, allyl Hcentral), 7.19 (d, J
= 7.7 Hz, 4H, DIC Hmeta), 7.32(t, J = 7.7 Hz, 2H, Hpara). 13C{1H} NMR
(CDCl3, T = 298 K, ppm): d 18.72 (CH3, DIC CH3), 71.45 (CH2, allyl
C
terminal), 122.98 (CH, allyl Ccentral), 125.44 (C, DIC C–N), 128.24
(CH, DIC Cmeta), 130.61 (CH, DIC Cpara), 135.87 (C, DIC C2, Cortho),
145.26 (C, C–Pd). IR (KBr pellet, cmÀ1
= 2179 (CN), 1094.5 (ClO4).
)
m
Anal. Calc. for C21H23ClN2O4Pd: C, 49.52; H, 4.55; N, 5.50. Found
C, 49.64; H, 4.56; N, 5.39%.
4.2. Synthesis of [Pd(TIC)2(g
3-C3H5)]ClO4
4. Experimental
The synthesis of the title complex is analogous to that of
[Pd(DIC)2(
3-C3H5)]ClO4, starting from 0.12 g (0.328 mmol) of
[Pd( -Cl)(
3-C3H5)]2, 0.109 g (1.312 mmol) of TIC and an excess
of NaClO4 H2O (0.184 g,1.312 mmol). Yield 0.239 g (88%).1H NMR
1D and 2D NMR spectra were recorded using a Bruker DPX300
spectrometer. Chemical shifts (ppm) are given relative to TMS
(1H NMR).
g
l
g
.

L. Canovese et al. / Inorganica Chimica Acta 363 (2010) 3426–3431
3431
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(CDCl3, 298 K) d 1.62
r, 18Y, C(CH3)3), 3.36 (d, J = 13.4 Hz, 2H, allyl
H
anti), 4.78 (d, J = 7.3 Hz, 2H, allyl Hsyn), 5.64 (m, 1H, allyl Hcentral).
13C{1H} NMR (CDCl3, T = 298 K, ppm): d 29.93 (CH3, C(CH3)3),
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tral), 131.14 (C, C–Pd). IR (KBr pellet, cmÀ1
(ClO4).
) m = 2211 (CN), 1093.0
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4.3. Synthesis of the mixture of [Pd(TIC)(DIC)(
g
3-C3H5)]ClO4 (50%),
g
3-C3H5)]ClO4 (25%)
[Pd(DIC)2(
g
3-C3H5)]ClO4 (25%) and [Pd(TIC)2(
(k) K. Onitsuka, M. Yamamoto, S. Suzuki, S. Takahashi, Organometallics 21
(2002) 581;
(l) J. Vicente, J.A. Abad, E. Martinez-Viviente, P.G. Jones, Organometallics 21
(2002) 4454;
This statistical mixture is obtained by the same method used for
the two previous complexes, starting from 0.12 g (0.328 mmol) of
[Pd(l-Cl)(g
3-C3H5)]2, 0.086 g (0.661 mmol) of DIC and 0.055 g
(m) D.P. Curran, W. Du, Org. Lett. 4 (2002) 3215;
(0.661 mmol) of TIC. Yield 0.282 g (93%).
(n) Y.J. Kim, X.H. Chang, J.T. Han, M.S. Lim, S.W. Lee, Dalton Trans. (2004) 3699;
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(p) M. Bortoluzzi, G. Paolucci, B. Pitteri, A. Vavasori, V. Bertolasi,
Organometallics 28 (2009) 3247.
1H NMR (CDCl3, 253 K) of mixed species [Pd(TIC)(DIC)(g3
-
C3H5)]ClO4: d 1.63
r, 9Y, TIC C(CH3)3), 2.48 r, 6Y, DIC CH3), 3.49
(d, J = 13.4 Hz, 1H, allyl Hanti), 3.53 (d, J = 13.4 Hz, 1H, allyl Hanti),
4.90 (d, J = 7.3 Hz, 1H, allyl Hsyn), 4.96 (d, J = 7.3 Hz, 1H, allyl Hsyn),
5.76 (m, 1H, allyl Hcentral), 7.20 (d, J = 7.7 Hz, 2H, DIC Hmeta), 7.34 (t,
J = 7.7 Hz, 1H, Hpara).
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(2007) 5590.
4.4. Study of the products of insertion of TIC and DIC to complex
[Pd(DIC)Cl(g
3-C3H3Me2)]
These studies were carried out by 1H NMR technique. In the first
experiment we added in succession one equivalent of DIC (0.003 g)
[6] (a) K. Onitsuka, H. Ogawa, T. Joh, S. Takahashi, Y. Yamamoto, H. Yamazaki, J.
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and one equivalent of TIC (2.7
l
L) to 0.8 cm3 of a solution (3 Â
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4799.
10À2 mol dmÀ3) of the title complex in CDCl3. The final spectrum,
recorded after 10 min, remained unchanged with time.
In the second experiment we added with a micropipette two
equivalents (5.4
l
L) of TIC to 0.8 cm3 of a solution (3 Â 10À2
mol dmÀ3) of the title complex in CDCl3. The spectrum of the reac-
tion is recorded after 10 min, and it did not change with time.
[7] L. Canovese, F. Visentin, C. Santo, C. Levi, Organometallics 28 (2009) 6762.
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