Alkylation of amines with alcohols catalyzed by palladium nanoparticles

Article abstract of DOI:10.1016/j.ica.2012.03.041

Palladium nanoparticles (PdNPs) generated in situ from the complex [(dippe)PdMe₂] were used as efficient catalyst for the alkylation of amines by aliphatic alcohols using neat conditions, under hydrogen atmosphere. On using short chain alcohols

Full text of DOI:10.1016/j.ica.2012.03.041

Inorganica Chimica Acta 392 (2012) 317–321
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Alkylation of amines with alcohols catalyzed by palladium nanoparticles
⇑
Grisell Reyes-Rios, Juventino J. García
Facultad de Química, Universidad Nacional Autónoma de México, México, DF 04510, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticles (PdNPs) generated in situ from the complex [(dippe)PdMe2] were used as effi-
cient catalyst for the alkylation of amines by aliphatic alcohols using neat conditions, under hydrogen
atmosphere. On using short chain alcohols, the reaction resulted in the generation of dialkylated and tri-
alkylated amines in high yields (>80%). The use of bulky alcohols allowed the production of mono-alkyl-
ated products, also in high yield.
Received 15 February 2012
Received in revised form 21 March 2012
Accepted 21 March 2012
Available online 30 March 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Nanoparticles
Alkylation
Alcohols
Catalysis
1. Introduction
corresponding halides or carbonyl compounds used in other meth-
odologies. This reaction involves the conversion of alcohols into
The chemistry of amines, amides and other nitrogen-containing
compounds plays a central role in modern organic synthesis [1,2].
The relevance of these compounds takes its foundations from sev-
eral important facts, for instance, the interaction between nitro-
genated compounds with living organisms is a basic issue in the
pharmaceutical [2] and agrochemical industries [1,2,4]. Therefore,
the development of new and efficient methods to prepare N-con-
taining derivatives is of particular relevance in these arenas.
The N-alkylation with alkyl halides is a very well-known meth-
od for the synthesis of amines [2]. This typically involves conver-
sion of the alcohol into the corresponding alkyl halide or other
alkylating agent, followed by a nucleophilic substitution with
appropriate C- or N-nucleophiles. However, the production of large
amounts of salts as by-products is not a desirable aspect from the
point of view of the green chemistry and the atom economy.
The reductive amination of aldehydes and ketones is another
classical method, which has been widely employed as a useful tool
in the synthesis of amines. Nevertheless, this method requires the
use of strong reducing agents and it is difficult to selectively yield
mono-alkylated products [3].
amines according to Scheme 1. The catalyst reacts with alcohol
i removing hydrogen to yield the corresponding aldehyde ii. In
the presence of an amine, such aldehyde is converted into the
imine iii by a condensation reaction, and the latter reacts with
the metal-hydride produced in the first step to yield the amine
iv. Moreover, the alkylation of amines is a thermodynamically
favored process where the breaking of a C–O bond to yield a C–N
bond, is largely compensated by the formation of H2O.
Several heterogeneous and homogeneous catalysts have been
developed for the alkylation of amines. To name few: silica [5], alu-
minum oxide [6], Raney-nickel [7], copper (simple or oxide)
[8,13b] have been used as heterogeneous catalysts. The first homo-
geneous catalyst for that kind of reaction was introduced in 1981
by Grigg et al. [9] and Watanabe et al. [10]; since then, complexes
of iridium [12,11], rhodium [10], ruthenium [13] and platinum [14]
have been used as homogeneous catalysts. However, relatively few
reports have been disclosed for the alkylation of amines using alco-
hols employing palladium catalysts.
In this context, the use of palladium nanoparticles (PdNPs) as
efficient catalysts in organic reactions has attracted considerable
interest, particularly if they are rapidly and systematically gener-
ated from homogeneous molecular Pd precursors to yield active
species for a variety of catalytic processes [15]. Most of the exam-
ples where palladium nanoparticles are used as catalysts are
mainly focused in C–C coupling reactions [16]. To the best of our
knowledge, only a handful of examples for the use of PdNPs have
been devoted to C–N coupling reactions. Worthy to mention is
the one by Ranu and co-workers [17], where PdNPs were used in
An interesting alternative method for the formation of a C–N
bond is the N-alkylation of amines with alcohols [4]. This is an effi-
cient procedure, since it does not generate any harmful and/or
wasteful co-products (only H2O as by-product). In addition,
alcohols are more readily available starting materials than the
⇑
Corresponding author. Tel.: +52 55 56223514; fax: +52 55 56162010.
E-mail address: juvent@servidor.unam.mx (J.J. García).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.041

318
G. Reyes-Rios, J.J. García / Inorganica Chimica Acta 392 (2012) 317–321
allylic amination reactions. Recently Sabater and co-workers [18]
reported a bifunctional metal Pd/base catalyst for the selective
mono-alkylation of amines with alcohols.
cat
-H2O
´
+
R-NHR´
R-OH
H2NR
i
iv
In recent years, our group reported the synthesis and use of pal-
ladium nanoparticles (PdNPs) in cross coupling reactions leading to
the desulfurization of dibenzothiophene [19]. These PdNPs were
synthesized by thermal reduction of precursors such as [(L)PdMe2]
(L = PEt3, PPh3, dippe = 1,2-bis(di-isopropyl)phosphino-ethane)
and dppe = 1,2-bis(diphenyl)phosphino-ethane), Scheme 2 shows
the basic reactions leading to PdNPs. The use of such PdNPs in
desulfurization along with stabilizing agents like hexadecylamine
resulted in a good catalytic performance of such nanoparticles,
with a typical diameter size of 3–6 nm and low agglomeration in
the reaction media.
LnM
R
OH
R
O
ii
i
LnM
LnMH2 H2NR´
Herein, we report the use of such PdNPs in the catalytic alkyl-
ation of amines by alcohols under mild conditions (Scheme 3).
The current disclosed results illustrate one of the few reports deal-
ing with the use of PdNPs for this kind of reaction.
R
NHR´
R
cata
NR´
iv
iii
Scheme 1. Catalytic N-alkylation of amines with alcohols.
2. Results and discussion
In order to asses the general reactivity of PdNPs in the amine
alkylation reaction, one equivalent of the complex [(dippe)PdMe2]
(1) [dippe = 1,2-bis(di-isopropyl)phosphino-ethane)], used as PdNPs
precursor [19], along with five equivalents of hexadecylamine were
reacted with methanol as alkylating agent and solvent, at 100 °C
during 72 h. The results for these experiments are summarized in
Table 1.
As found in Table 1, the alkylation reaction of hexadecylamine
using methanol without hydrogen showed the lowest activity
(Table 1, entry 1). In contrast, the highest yield was obtained at
250 psi to give the dialkylated amine as the only product in 93%
(Table 1, entry 3). Noteworthy, a decrease in the activity was ob-
served using a higher hydrogen pressure (Table 1, entry 4), proba-
bly due to the deactivation of the reactive PdNPs by the hydrogen
excess leading to the formation of inactive palladium hydrides.
In order to establish the best PdNPs precursor for the amine-
alkylation reaction, the best reaction conditions found on using
(1) (Table 1, entry 3) were applied to the use of the analog complex
[(PEt3)2PdMe2] (2) [19] as shown at Scheme 4.
metathesis
(L)PdMe2 + 2 MgBrCl
(L)PdCl2 + 2MeMgBr
thermolysis
growth
(L)PdMe2
Pd (0) + H3C-CH3 + L
PdNPs
Pd (0)
Scheme 2. Methodology for the synthesis of PdNPs.
R1
R1
PdNPs, H2
-n
n
R2
+
N
R1
NH2
R2 OH
+
N
H
R2
H2O
R2
Scheme 3. Alkylation of primary amines with alcohols.
In comparison, the use of complex 1 as PdNPs precursor gave a
better performance (93% yield) than the use of complex 2 (70%
yield), under the same reaction conditions. Differences in activity
between these two precursors has been observed previously in
cross coupling reactions leading to desulfurization [19]. This fact
may be adduced to differences in the degree of stabilization of
the nanoparticles and/or nanoparticle-related intermediates. Par-
ticularly, the chelate effect and basicity of the dippe present in
the PdNPs precursor, probably inhibited agglomeration and pre-
vented their precipitation from the reaction medium.
With the aim of extending the scope of this reaction, a variety of
alcohols were further tested under the same reaction conditions,
also using the same alkylamine and nanoparticle precursor. Rele-
vant results are summarized in Table 2.
As can be seen on Table 2, high conversions are generally ob-
tained for alkyl alcohols (>80%). To note, now the selectivity of
the process is prone to the mono-alkylation when bulky alcohols
Table 1
Alkylation of hexadecylamine by methanol mediated by PdNPs from 1.
Entry
H2 (psi)
Yielda (%)
1
2
3
4
0
7
150
250
500
86
93
64
All reactions were carried out in a stainless steel reactor (Parr) at the indicated
pressure (psi) of H2. The reaction mixtures were charged in a dry-box using 10 mL
of methanol at 100 °C over 72 h, typically using 0.0686 mmol of (1) as palladium
nanoparticle precursor.
a
Yields were determined by GC–MS.
T=100 oC, t=72 h, H2
Complex , MeOH
NH2
N
7
+ H2O
2
7
70%
Scheme 4. Alkylation of hexadecylamine by methanol mediated by PdNPs from 2.

G. Reyes-Rios, J.J. García / Inorganica Chimica Acta 392 (2012) 317–321
319
Table 2
are used (Table 2, entries 1 and 2), with a lower yield on increasing
the bulkiness of the corresponding alcohol from isopropanol to
tert-butanol (Table 2, entry 2). In sharp contrast, the use of aro-
matic alcohols such as phenol did not resulted in any activity at
all, at first glance this could be attributed to the non-availability
of a b-hydrogen, which may allow the conversion of the alcohol
into the corresponding aldehyde [4]. However, a tertiary alcohol
such as tert-butanol showed very good reactivity (Table 2, entry
2). Therefore, an alternative explanation to the mechanism out-
lined in Scheme 1 is needed. We speculate in an explanation invok-
ing the elementary steps involved in the dehydration of alcohols, to
generate a carbocation needed to react with the primary amine to
yield the new substituted amine. Since this reaction regularly re-
quires an acid reaction media, it is proposed that Pd(II) species
may be catalyzing such reaction; this proposal is represented on
Scheme 5 [20,21].
Alkylation of hexadecylamine with alcohols.
Entry
R2-OH
Product
Yielda (%)
100
1
2
82
0
3b
–
All reactions were carried out in a stainless steel reactor vessel (Parr) using 10 mL of
alcohol and 5 equivalents of hexadecylamine at 100 °C under 250 psi of H2 over
72 h, typically using 0.08 mmol of (1) as palladium nanoparticle precursor.
a
Yields were determined by GC–MS and characterized by spectroscopic data (1H,
13C{1H} NMR).
b
Ratio Phenol:1 (10.5:1).
R3C-O
R'NH2
HO
R3C-NHR'
[PdNP]
[PdNP]
R C-OH
3
[PdNP]
+
+
H
H
H2O
Scheme 5. Alternative dehydration mechanism for the amine alkylation.
Table 3
Alkylation of amines with alcohols by PdNPs under catalytic conditions.
R1
R1
PdNPs, 200 psi H2
-n H2O
R2
N
R2
R1
NH2
+
n R2
OH
R2
N
H
+
Entry
1b
R1-NH2
R2-OH
MeOH
Product
Yielda (%)
70
N
NH2
7
7
7
2b
3b
60
30
NH2
N
H
7
HO
HO
NH2
NH
4c
5c
40
25
NH2
N
H
7
7
HO
HO
NH2
NH
General reaction conditions: a stainless steel autoclave pressurized at 100 °C under 250 psi of H2 over 72 h. the indicated pressure (psi) of H2. Reaction mixtures were charged
in a dry-box using 15 mL (368 mmol) of alcohol. Catalyst loading: 2% or 1% of (1) relative to the amine.
a
Yields were determined by GC–MS.
2 mol% of (1) relative to the amine.
1 mol% of (1) relative to the amine.
b
c

320
G. Reyes-Rios, J.J. García / Inorganica Chimica Acta 392 (2012) 317–321
Considering the above-discussed results made under stoichiom-
The methylated compounds were prepared in situ by reaction of
etricamountsofPdNPs, atthispointisworthytonotethatjustamin-
or amount of metallic palladium atoms would be in the surface to be
active in catalysis. Thus, catalytic amounts of PdNPs were assessed
for this process, using alkyl amines and alcohols as substrates. Ta-
ble 3 summarizestherelevant resultsfor such catalyticexperiments.
In general, the use of catalytic amounts of PdNPs results in good
to moderate yields. Particularly, the use of 2 mol% of (1) for the
alkylation of hexadecylamine by methanol and isopropanol al-
lowed the highest conversions, 70% and 60%, respectively (Table 3,
entries 1 and 2). The reaction with isopropanol is selective towards
the mono-alkylation. Meanwhile, the dialkylation of the amine
with methanol is observed. Lower yields were obtained on using
a load of 1 mol% of catalyst (Table 3, entry 4). Also, lower conver-
sions on using cyclohexylamine and isopropanol were obtained
(Table 3, entries 3 and 5), probably due to significant steric hin-
drance in both substrates. A complementary mechanistic proposal
was recently formulated by Zhang et al. for the alkylation of
amines with alcohols under catalytic conditions [22]; from these
studies, the occurrence of a hydrogen-borrowing process, started
with the oxidation of the alcohol to form the corresponding car-
bonyl compound, is postulated similarly as illustrated in Scheme 1.
chlorinated compounds with excess of MeMgBr [25].
4.2. Typical procedure for the alkylation of hexadecylamine by
methanol and related alcohols
A typical experiment is described as follows: methylated cata-
lytic precursor (1) or (2) (0.03 g, 0.08 mmol), hexadecylamine
(0.075 g, 0.313 mmol for stoichiometric ratio experiments) and
(0.483 g, 2.5 mmol for catalytic ratio experiments) were mixed
with methanol (15 mL, 368 mmol) in the reactor vessel. This was
closed, taken out from the glovebox. Then, in a well-vented
fume-hood, the autoclave was pressurized with H2 to the corre-
sponding pressure as indicated in Table 1 and heated to the desired
temperature, under constant stirring. After 72 h, heating was
stopped and the reactor was allowed to cool down to room tem-
perature, then, the remaining H2 was released to the fume hood.
The vessel was opened to air and the reaction mixture was filtered
in a Celite column, an aliquot of the mixture was immediately in-
jected into the GC–MS. A second aliquot of the same mixture was
also analyzed by 1H and 13C{1H} NMR.
Acknowledgments
3. Conclusion
We thank CONACyT and PAPIIT-DGAPA-UNAM (IN-201010) for
their financial support to this work. G.R.R. thanks CONACyT for a
Grant. Also we thank Dr. Alma Arévalo for technical support.
In conclusion, we developed a method where the use of PdNPs
afforded the N-alkylation of amines employing alcohols as alkylat-
ing agents in good yields (>80%) under neat conditions and low
hydrogen pressure. The reaction allowed the production of di-
alkylated or mono-alkylated amines, depending on the nature of
the alcohol. Such selectivity is sensitive to the size of the alcohol,
yielding mono-alkylation products when bulky alcohols are used
and dialkylation when small alcohols are used. Likewise, good
yields were obtained using catalytic amounts of PdNPs.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.03.041.
References
4. Experimental
[1] (a) O.J. Plante, S.L. Buchwald, P.H. Seeberger, J. Am. Chem. Soc. 122 (2000)
7148;
(b) B. Zou, H.-F. Jiang, Z.-Y. Wang, Eur. J. Org. Chem. (2007) 4600;
(c) C. Torborg, M. Beller, Adv. Synth. Catal. 351 (2009) 3027;
(d) G. Guillena, D.J. Ramón, M. Yus, Chem. Rev. 110 (2010) 1611;
(e) F. Alonso, P. Riente, M. Yus, Acc. Chem. Res. 44 (2011) 379;
(f) C. Fischer, B. Koenig, Beilstein J. Org. Chem. 7 (2011) 59;
(g) G. Cami-Kobeci, P.A. Slatford, M.K. Whittlesey, M.J.J. Williams, Bioorg. Med.
Chem. Lett. 15 (2005) 535;
(h) D.J.C.L. Constable, P.J. Dunn, J.D. Hayler, G.R. Humphrey, J.L. Leazer Jr., R.J.
Linderman, K. Lorenz, J. Monley, B.A. Pearlman, A. Wells, A. Zaks, T. Zhang,
Green Chem. 9 (2007) 411;
All manipulations were carried out using standard Schlenk
techniques and a MBraun glovebox (H2O and O2 < 1 ppm) under
high-purity argon (Praxair 99.998%). Stainless steel Parr pressure
vessels 4750 (125 mL) were used for catalysis experiments. Meth-
anol and isopropanol (J.T. Baker) were dried and distilled from the
corresponding Grignard reagent. Tert-Butanol (>99%; Aldrich) were
distilled and degassed by the freeze–pump–thaw method prior to
use. MeMgBr (3.0 M in THF solution) was purchased from Aldrich
and was used as received. (COD)PdCl2 and hexadecylamine were
purchased from Aldrich, dried in vacuo and used without further
purification. PEt3 was purchased from Aldrich and was also used
without further purification. 1,2-Bis(diisopropylphosphino)ethane
(dippe) was prepared as reported [23].
Deuterated solvents were purchased from Cambridge Isotope
Laboratories and were stored over 3 Å molecular sieves in the
glovebox. 1H and 13C{1H} NMR spectra were recorded at room tem-
perature on a 300 MHz Varian Unity spectrometer in CDCl3. 1H
chemical shifts (d) are reported relative to the residual proton res-
onances in the deuterated solvent. 31P{1H} NMR spectra were re-
corded relative to external 85% H3PO4. All NMR spectra were
carried out using thin wall (0.38 cm) WILMAD NMR tubes with J.
Young valves. GC–MS determinations were performed using an
Agilent 5975C GC–MS equipped with a 30 m DB-5MS capillary
(0.32 mm ID) column, using triethylamine as internal standard.
(i) S.L. Buchwald, C. Mauger, G. Mignani, U. Scholz, Adv. Synth. Catal. 348
(2006) 23.
[2] (a) A.S. Guram, R.A. Rennels, S.L. Buchwald, Angew. Chem., Int. Ed. 34 (1995)
1348;
(b) J.F. Hartwig, Angew. Chem., Int. Ed. 37 (1998) 2046;
(c) B. Schlummer, U. Scholz, Adv. Synth. Catal. 346 (2004) 1599;
(d) J.F. Hartwig, Acc. Chem. Res. 41 (2008) 1534;
(e) Y. Aubin, C. Fischmeister, C.M. Thomas, J.L. Renaud, Chem. Soc. Rev. 39
(2010) 4130.
[3] (a) A. Levi, G. Modena, G. Scorrano, Chem. Commun. (1975) 6;
(b) M.J. Burk, J.E. Feaster, R.L. Harlow, J. Am. Chem. Soc. 115 (1993) 10125;
(c) R.P. Tripathi, S.S. Verma, J. Pandey, V.K. Tiwari, Curr. Org. Chem. 12 (2008)
1093;
(d) M.E. Dominea, M.C. Hernández-Sotoa, Y. Pérez, Catal. Today 159 (2011) 2.
[4] (a) R.H. Crabtree, Organometallics 30 (2011) 17;
(b) T.D. Nixon, M.K. Whittlesey, J.M.J. Williams, Dalton Trans. (2009) 753;
(c) M.H.S.A. Hamid, C.L. Allen, G.W. Lamb, A.C. Maxwell, H.C. Maytum, A.J.A.
Watson, M.J. Williams, J. Am. Chem. Soc. 131 (2009) 1766;
(d) M.H.S.A. Hamid, A. Slatford, M.J. Williams, Adv. Synth. Catal. 349 (2007)
1555;
(e) J. Pasek, P. Kondelik, P. Richter, Ind. Eng. Chem. Prod. Res. Dev. 11 (1972)
333;
(f) A.J.A. Watson, A.C. Maxwell, J.M.J. Williams, J. Org. Chem. 76 (2011) 2328.
[5] A.B. Brown, E.E. Reid, J. Am. Chem. Soc. 46 (1924) 1836.
[6] F. Valot, F. Fache, R. Jacquot, M. Spagnol, M. Lemaire, Tetrahedron Lett. 40
(1999) 3592.
[7] (a) R.G. Rice, E.J. Kohn, L.W. Daash, J. Org. Chem. 23 (1958) 1352;
(b) R.G. Rice, E.J. Kohn, J. Am. Chem. Soc. 77 (1955) 4052.
4.1. Nanoparticle precursors
Complexes [(dippe)PdCl2] (1) and [(Et3P)2PdCl2] (2) were pre-
pared following the procedures reported in the literature [24].

G. Reyes-Rios, J.J. García / Inorganica Chimica Acta 392 (2012) 317–321
321
[8] A.N. Shuikin, L.S. Glebov, G.A. Kliger, V.G. Zaikin, Petrol. Chem. (1993) 307.
(e) D. Astruc, Inorg. Chem. 46 (2007) 1884;
[9] R. Grigg, T.R.B. Mitchell, S. Sutthivaiyakitt, N. Tongpenayi, Chem. Commun.
(1981) 611.
(f) Y. Lunxiang, J. Liebscher, Chem. Rev. 107 (2007) 133;
(g) S. Ogasawara, S. Kato, J. Am. Chem. Soc. 132 (2010) 4608.
[10] Y. Watanabe, T. Tsuji, Y. Ohsugi, Tetrahedron Lett. 22 (1981) 2667.
[11] (a) K.I. Fujita, R. Yamaguchi, Synlett 4 (2005) 560;
[17] A. Laksmikanta, K. Chattopadhyay, B.C. Ranu, J. Org. Chem. 74 (2009) 3982.
[18] A. Corma, T. Ródenas, M. Sabater, Chem. Eur. J. 16 (2010) 254.
[19] J. Torres-Nieto, A. Arévalo, J.J. García, Inorg. Chem. 47 (2008) 11429.
[20] J. March, M.B. Smith, March’s Advanced Organic Chemistry, John Wiley and
Sons, Inc., USA, 2001.
[21] (a) J. Muzart, J. Molec, Catal. A: Chem. 308 (2009) 15;
(b) S.I. Murahashi, T. Shimamura, I. Moritani, J. Am. Chem. Soc. (1974) 931.
[22] (a) Y. Zhang, X. Qi, X. Cui, F. Shi, Y. Deng, Tetrahedron Lett. 52 (2011) 1334;
(b) M.S. Kwon, S. Kim, S. Park, W. Bosco, R.K. Chidrala, J. Park, Org. Chem. 74
(2009) 2877;
(b) K.I. Fujita, Y. Enoki, R. Yamaguchi, Tetrahedron (2008) 1943.
[12] D. Hollman, A. Tillack, R. Jackstell, M. Beller, Chem. Asian J. 2 (2007) 403.
[13] (a) M. Kosugi, K. Masayuki, M. Toshihiko, Chem. Lett. (1983) 927;
(b) R. Cano, D.J. Ramón, M. Yus, J. Org. Chem. 76 (2011) 5547.
[14] Y. Tsuji, R. Takeuchi, H. Ogawa, Y. Watanabe, Chem. Lett. (1986) 293.
[15] (a) A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757;
(b) R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res. 3
(2001) 181;
(c) D. Astruc, F. Lu, J. Ruiz-Aranzaes, Angew. Chem., Int. Ed. 44 (2005) 7852;
(d) H.P. Choo, K.Y. Liew, H. Liu, J. Mater. Chem. 12 (2002) 934.
[16] (a) M. Moreno-Mañas, R. Pleixats, Acc. Chem. Res. 36 (2003) 638;
(b) S. Jansat, M. Gómez, K. Philippot, G. Muller, G. Guiu, C. Claver, S. Castillon, B.
Chaudret, J. Am. Chem. Soc. 126 (2004) 1592;
(c) K. Otto, L.P. Haack, J.E. deVries, Appl. Catal. B 1 (1992) 1;
(d) D.H. Kim, S.I. Woo, J.M. Lee, O.B. Yang, Catal. Lett. 70 (2000) 35.
[23] F.G.N. Cloke, V.C. Gibson, M.L.H. Green, V.S.B. Mtetwa, K. Prout, J. Chem. Soc.,
Dalton Trans. (1988) 2227.
[24] Z.T. Cygan, J.E. Bender IV, K.E. Litz, J.W. Kampf, M.M.B. Holl, Organometallics 21
(2002) 5373.
(c) M.T. Reetz, J.G. De Vries, Chem. Commun. (2004) 1559;
(d) M.S. Kwon, N. Kim, S.H. Seo, I.S. Park, R.K. Cheedrala, J. Park, Angew. Chem.,
Int. Ed. 44 (2005) 6913;
[25] G. Calvin, G.E. Coates, J. Chem. Soc. (1960) 2008.