in the ILs explains the former result, however it is less clear
why reactions involving [Bun N][Ph3SiF2] were unsuccessful.
in the two systems, for [1b][PF6] and [1c][PF6], can be
compared to the relative rates of reaction for the two doped
ILs and suggests that smaller nanoparticles lead to increased
rates. A nanoparticle size of ca. 2 nm is optimal for related
coupling processes.9
4
The relative ‘‘nakedness’’ of FÀ could be an important factor,
as FÀ is already coordinated to a hypervalent Si center in
[Bun N][Ph3SiF2]. In contrast, FÀ may be relatively poorly
4
solvated, and thus more nucleophilic, in [Bun N]F dissolved in
Aryl siloxane substituent effects are also seen; alkene-doped
ILs (with 1a) used in this study were tested against para-H, Me
and Cl substituted siloxanes (as shown in Scheme 2). The
relative rates of reaction in a competition experiment involving
all three siloxanes were measured using HRGC (1, 0.5 and 3.7,
respectively). As in DeShong’s work,6 para electron-withdrawing
substituents in aryl siloxanes increase the rate of reaction,
suggesting that the results obtained from the coupling in
alkene-doped ILs exhibit similar trends to comparable
reactions in conventional solvents.
4
these ILs.
In order to probe any substituent effects in the alkene-doped
ILs we have evaluated ionic chalcones [1a][PF6], [1b][PF6] and
[1c][PF6], which contain para-H, OMe and NO2 groups,
respectively. The data in Table 1 show that all three ligands
give doped ILs with [C5MPyrr][Tf2N] that are active for the
coupling reactions studied. However, the isolated yields from
ligands [1b][PF6] and [1c][PF6] (80% and 60%, respectively)
suggest that electron-donating or electron-withdrawing
substituents on the ligand may significantly influence the
reaction. In neutral alkenes, this has been linked to the relative
s-donor, p-acceptor properties of the alkenes when bound to
Pd0/II species.1 In order to better understand this effect in the
alkene-doped ILs the relative rates of reaction for the OMe
and NO2 substituted chalcones were measured by GC and
found to be 1 and 0.12, respectively. These data suggest that
the electron-releasing methoxy group (ligand [1b][PF6])
promotes a significantly faster reaction than the electron-
withdrawing NO2 group (ligand [1c][PF6]). Initial recycling
experiments suggest that the IL–catalyst system is not
catalytically active following extraction of the products. A
build up of salt byproducts or Pd nanoparticle deactivation
could explain this outcome. Work is in progress to address
this issue.
In summary, alkene-doped, functional ILs are excellent
reaction media for Pd-catalysed allyl–aryl couplings, simplify-
ing product recovery. The presence of alkene functionality in
the IL is essential for the formation of the cross-coupled
product and alkene-ligand substituent effects, similar to those
seen using neutral alkene ligands, are operative in these
systems. These effects can be related to the Pd nanoparticle
size formed during the reaction, although a more subtle effect
at PdII cannot be ruled out. Investigations to determine
whether these ligands can be used as probes to help with the
elucidation of catalytic mechanisms using mass spectrometric
methods are in progress.
The authors thank the University of York (J.M.S), the
Royal Society and Astra Zeneca (I.J.S.F), the NRSCC and
ERASMUS (P.S.B.) and the Netherlands Organization for
Scientific Research (NWO) (C.M.) for financial support. We
thank C. Biewer for starting material syntheses.
TEM measurements were performed on the nanoparticulate
Pd formed in situ in these reactions (see ESIw). Micrographs
from the reactions involving ligands [1b][PF6] and [1c][PF6] are
illustrated in Fig. 3. These micrographs, and others that we
have collected during this study, show that the nanoparticulate
Pd formed during the reactions in alkene-doped ILs have
narrow size distributions. Of particular note is the relative size
of the Pd nanoparticles. The nanoparticles formed from the IL
doped with ligand [1b][PF6] have sizes of 1–3 nm, whereas
those from the [1c][PF6]-doped IL have sizes of 10–15 nm. This
suggests that the chalcone ligand could be involved in catalyst
stability, nanoparticle formation or dissolution of mono-
nuclear Pd0 species, and that substituent effects are important
in this process, akin to polyfluorinated-dba stabilised Pd
nanoparticles (4–5 nm).8 The difference in nanoparticle size
Notes and references
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Fig. 3 TEM images of Pd nanoparticles formed during coupling
reactions using conditions in Table 1 entry 4 (left) and entry 5 (right).
ꢀc
This journal is The Royal Society of Chemistry 2009
5736 | Chem. Commun., 2009, 5734–5736