Inorganica Chimica Acta
Microwave-assisted synthesis of (N-heterocyclic carbene)Ni(Cp)Cl complexes
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Brant Landers, Oscar Navarro
Department of Chemistry, University of Hawaii at Manoa, 2545 McCarthy Mall, Honolulu, HI 96822, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 6 October 2011
The use of microwave heating for the synthesis of a series of (N-heterocyclic carbene)Ni(Cp)Cl complexes
is presented. This protocol allows for a considerable reduction of the reaction times required using con-
ventional heating, while affording comparable or better yields of the desired complexes. Their application
as pre-catalysts in the microwave-assisted anaerobic oxidation of secondary alcohols is also discussed.
Ó 2011 Elsevier B.V. All rights reserved.
Young Investigator Award Special Issue
Keywords:
N-heterocyclic carbene
Nickel
Microwave heating
Alcohol oxidation
1. Introduction
for the oxidation of secondary alcohols under anaerobic conditions
using 2,4-chlorotoluene as oxidant [12–14].
The unique steric and electronic features of N-heterocyclic
carbenes (NHC) have attracted much interest in the last two dec-
ades, and their use as ligands in the synthesis of new organometal-
lic compounds is widely found in the literature [1,2]. Many of these
complexes have been successfully applied as catalysts or pre-cata-
lysts in a variety of C–C and C–heteroatom bond-forming reactions
[1,2]. In many cases, the synthesis of NHC-bearing complexes in-
volves the combination of a metal precursor (a metal salt or an
organometallic compound) and an imidazolium or imidazolinium
salt in a solvent. The mixture is then stirred over extended periods
of time at high temperature [3–7]. As an alternative to this general
protocol, our group recently decided to explore the use of micro-
wave heating for the synthesis of this type of complexes, in order
to provide a ‘‘greener’’ alternative by shortening reaction times,
and therefore decrease the amount of energy required for the pro-
cess, while maintaining or increasing the yields of the desired
products. Although organic synthesis has benefited from this tech-
nology for quite some time [8,9], examples of the use of microwave
heating in the synthesis of organometallic compounds are, inter-
estingly, scarce [10]. We recently reported the successful use of
microwave heating for the synthesis of (NHC)Pd(acac)Cl and
(NHC)Pd(3-Cl-pyridine)Cl2 complexes [11]. This protocol afforded
the desired compounds in yields comparable to those obtained
using conventional heating, but drastically reducing reaction
times. Herein, we wish to report a similar protocol for the synthesis
of (NHC)Ni(Cp)Cl complexes. In addition, we took further advan-
tage of this technology by using these complexes as pre-catalysts
2. Results and discussion
The reported synthesis of (NHC)Ni(Cp)Cl complexes (Fig. 1)
using conventional heating were carried out in refluxing THF for
periods that range from 0.5 h for the synthesis of (IMes)Ni(Cp)Cl
(1) [6] to 2.5 h for (IPr)Ni(Cp)Cl (3) and overnight for the synthesis
of the saturated carbene-bearing complexes (SIPr)-, and (SIMes)-
Ni(Cp)Cl (2 and 4, respectively) [7]. Using the same solvent, we car-
ried out the synthesis of 1–4 using a microwave reactor, increasing
the temperature to 110 °C (Scheme 1). The reaction times were
considerably reduced under these conditions: 1 mmol scale reac-
tions were completed after 5 min in the case of 1 and 3 and
30 min in the case of 2 and 4, with no further conversions observed
in any case if the reaction times were increased. The desired com-
plexes 1–4 were isolated in comparable or higher yields than those
reported using conventional heating, with the exception of 3.
Since Cowley’s early report on the synthesis of (IMes)Ni(Cp)Cl
[6], (NHC)Ni(Cp)Cl complexes have been used as pre-catalysts for
dehalogenation [7], aryl amination [7], styrene polymerization
[15], and hydrothiolation reactions [16]. We decided to add a new
entry in this list by performing catalytic oxidations of secondary
alcohols under anaerobic conditions, using 2,4-dichlorotoluene as
oxidant and solvent [12,13]. A preliminary screening revealed that,
in opposition to previous work by our group using other (NHC)-Ni
catalysts, the oxidation of propylphenyl alcohol could not be per-
formed at mild temperatures (25–40 °C) even after an extended
period of time (Table 1, entries 1, 2). This result suggested that
the Ni(II) was not being reduced to Ni(0), a step necessary to initiate
the catalytic cycle. We thus decided to perform the oxidations un-
der microwave heating, with much better results. In agreement
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