η3-furfuryl and η3-thienyl complexes of palladium and platinum of relevance to the functionalization of biomass-derived furans

Article abstract of DOI:10.1021/om300568z

A number of cationic and neutral η³-furfuryl complexes of palladium and platinum were prepared, including one with a ligand derived from fructose, 5-chloromethylfurfural. The complexes were studied by variable-temperature and heteronuclear correlation NMR spectroscopy, as well as single-crystal X-ray crystallography. Analogous η³-thienyl complexes could be prepared and detected in solution by NMR, but could not be isolated due to facile decomposition.

Full text of DOI:10.1021/om300568z

Article
pubs.acs.org/Organometallics
η3‑Furfuryl and η3‑Thienyl Complexes of Palladium and Platinum of
Relevance to the Functionalization of Biomass-Derived Furans
Yang Shi, Peter Brenner, Stefanie Bertsch, Krzysztof Radacki, and Rian D. Dewhurst*
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97070 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: A number of cationic and neutral η3-furfuryl
complexes of palladium and platinum were prepared, including
one with a ligand derived from fructose, 5-chloromethylfurfu-
ral. The complexes were studied by variable-temperature and
heteronuclear correlation NMR spectroscopy, as well as single-
crystal X-ray crystallography. Analogous η3-thienyl complexes
could be prepared and detected in solution by NMR, but could not be isolated due to facile decomposition.
for a range of other processes such as the catalytic
INTRODUCTION
■
polymerization,9 hydroboration,10 hydrosilylation,11 and hydro-
amination12 of vinyl arenes and the C−H functionalization of
alkylbenzenes.13 In contrast, very little is known about
analogous heterocyclic complexes (e.g., η3-furfuryl and η3-
thienyl). A report from 1969 disclosed the synthesis of η3-
thienyl (V, Scheme 1) complexes of group 6 metals by CO loss
from η1-thienyl metal−carbonyl complexes,14a while a paper
from 1981 reported η3-furfuryl (IV) complexes.14b A η3-furfuryl
complex was later suggested as an intermediate in a palladium-
catalyzed allylic silylation of furfuryl acetate,15a and the
decarboxylative furfurylation of a nitrile.15b An unsuccessful
attempt at Suzuki−Miyaura cross coupling of a furfuryl
carbonate has been reported (traces of product were
detected).15c
One of the 12 chemicals recently identified by the U.S.
Department of Energy as key biomass-derived chemicals is 2,5-
furan dicarboxylic acid, potentially a link between cellulose
carbohydrates and high-volume monomers for condensation
polymerization.1 Since this report, the interest in biomass-
derived furans has grown substantially, inspired in part by the
simplicity of the single-step synthesis of furans, such as furfural
and 5-hydroxymethylfurfural, from cellulose.2,3 A particularly
promising carbohydrate-derived furan is 5-chloromethylfurfural
(I, Scheme 1), long known to be available from sugars by
Scheme 1. Synthesis of 5-Chloromethylfurfural (I) and
Relevant η3 Ligands (II−V)
This project was designed to eliminate the paucity of η3-
heterocyclic (IV, V, Scheme 1) complexes in the literature and
draw parallels between these complexes and related η3-benzyl
(III) complexes, already well known to be intermediates in
catalytic processes. Additionally, we sought to make a
connection between biomass-derived furans and transition
metal catalysis. Herein we present the synthesis of new η3-
furfuryl (IV) complexes, including one derived from the
carbohydrate-derived furan compound I, and the generation of
unstable η3-thienyl (V) complexes. As a proof of principle we
also present the stoichiometric amination of a η3-furfuryl ligand
with benzylamine. A portion of this work has been
communicated previously.16
dehydration with excess concentrated hydrochloric acid.4
Recently, reports from Mascal and co-workers have focused
on I, particularly its synthesis from cellulose, and synthetic
possibilities including hydrogenation, alkoxylation, amination,
ring-opening, and an elegant procedure for the conversion of I
to the natural herbicide δ-aminolevulinic acid.5
From an organometallic perspective, the (chloromethyl)-
furan functionality of I invites comparison to benzyl halides and
their derived η3-benzyl complexes.6 η3-Benzyl ligands (III,
Scheme 1), like their parent ligands, η3-allyls (II),7 play a crucial
role as intermediates in catalytic nucleophilic allylic (or
benzylic) substitution reactions.8 The participation of η3-benzyl
complexes as catalytic intermediates has also been confirmed
RESULTS AND DISCUSSION
■
Synthesis of η3-Furfuryl Complexes of Palladium and
Platinum. Furans have a reputation as poor π-ligands. Harman
and co-workers have performed compelling studies on π-furan
complexes of mid-transition metals and their reactivity; yet
even now only a handful of stable η2- and η4-furan complexes
Received: June 21, 2012
Published: July 30, 2012
© 2012 American Chemical Society
5599
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
are known.17 Our initial forays into the synthesis of η3-furfuryl
complexes led us to treat the zerovalent palladium complex
[Pd(PCy3)2] with a slight excess of methyl 5-(chloromethyl)-2-
furancarboxylate and silver triflate, resulting in a mixture of two
phosphorus-containing complexes as indicated by 31P NMR
spectroscopy. A singlet signal at δ 52.6 (2) and two doublet
signals at δ 42.5 and 36.5 (1; both with JPP = 34.8) were found
in an approximate 5:1 molar ratio. Given the 31P NMR data, the
structure of 1 was assumed to be the cationic bis(phosphine)
η3-furfuryl complex indicated in Scheme 2. A small amount of
Scheme 2. Syntheses of η3-Furfuryl and -Thienyl Complexes
Figure 1. Molecular structure of 2 and the cations of 4b and 4c.
Thermal ellipsoids are set at 50% probability. The minor conformation
of the triflato ligand (2) and hydrogen atoms have been omitted for
clarity. Selected bond lengths [Å] and angles [deg] for 2: Pd1−C6
2.060(3), Pd1−C2 2.212(3), Pd1−C3 2.351(3), C2−C3 1.396(5),
C4−C5 1.338(4); C3−C2−C6 126.7(3). Selected bond lengths [Å]
and angles [deg] for 4b: Pt1−C6 2.132(1), Pt1−C2 2.223(1), Pt1−C3
2.316(1), C2−C3 1.405(7), C4−C5 1.335(6); C3−C2−C6 126.2(4).
Selected bond lengths [Å] and angles [deg] for 4c: Pd1−C6 2.155(3),
Pd1−C2 2.225(3), Pd1−C3 2.306(3), C2−C3 1.397(4), C4−C5
1.346(4); C3−C2−C6 126.6(3).
crystals suitable for single-crystal X-ray diffraction was obtained
from the mixture, indicating the connectivity of the other
component in the mixture, the neutral monophosphine η3-
furfuryl complex 2. The molecular structure of 2 (Figure 1)
suggests that the PCy3 ligand trans to the furfuryl C6 position
has been labilized and replaced by the triflate anion. This
surprising finding provides some measure of the stability of the
η3 coordination of the furfuryl ligand, as the triflate anion
preferentially replaces a phosphine rather than the furan C2−
C3 double bond. The d(Pd−C3)/d(Pd−C6) symmetry ratio in
complex 2 is 1.14 (Table 1), somewhat higher than that
observed in the previously published η3-furfuryl complex 4a
(ratio: 1.06).16 In light of the unselective halide abstraction
process in the [Pd(PCy3)2] system, presumably due to the
reluctance of the large PCy3 ligands to adopt a mutually-cis
geometry and the relatively strong ligating ability of the triflate
anion, we turned our focus to the comparatively sterically
modest ligand PPh3.
5600
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
Table 1. Bond Distances and Angles for 2 and 4a−c
2
4a16
4b
4c
Distances (Å)
M−C6
M−C2
M−C3
2.060(3)
2.163(3)
2.236(2)
2.289(3)
1.06
2.132(1)
2.223(1)
2.316(1)
1.09
2.155(3)
2.225(3)
2.306(3)
1.07
2.212(3)
2.351(3)
1.14
ratio d(M−C3)/d(M−
C6)
C2−C3
C4−C5
1.396(5)
1.338(4)
1.043
1.402(4)
1.342(4)
1.044
1.405(7)
1.335(6)
1.052
1.397(4)
1.346(4)
1.038
ratio d(C2−C3)/d(C4−
C5)
C2−O1
C5−O1
1.369(4)
1.373(4)
1.384(3)
1.382(3)
1.382(5)
1.385(5)
1.374(3)
1.396(3)
Angles (deg)
C3−C2−C6
M−C2−O1
126.7(3)
116.2
126.8(3)
119.9
126.2(4)
118.1
126.6(3)
118.2
M−{midpoint:C2,C3}−
113.5
118.2
117.1
116.9
{furan centroid}
torsion O1−C2−M−
L(trans to C3)
76.8
74.1
89.8
70.7
104.2
63.5
89.8
64.6
torsion O1−C2−M−
L(trans to C6)
C6−C2−{furan
163.2
167.7
167.8
168.2
centroid}
The oxidative addition reactions of 5-(chloromethyl)-2-
furancarboxylate with [Pd(PPh3)4] or [Pt(C2H4)(PPh3)2]
provide different regioisomers of the divalent Pd/Pt complexes
3a and 3b. In the case of Pd, the trans isomer (trans-3a) is
obtained, while the cis isomer (cis-3b) is obtained in the case of
Pt. This is presumably a result of the preorganization of the
phosphine ligands of [Pt(C2H4)(PPh3)2] into a cis orientation,
leading to the product cis-3b. In neither of the above cases was
the alternate isomer detected. Treatment of either trans-3a or
cis-3b with 1.1 equiv of silver tetrafluoroborate provided the
previously published 4a and the new Pt(II) η3-furfuryl complex
1
4b. The H NMR spectrum of 4b showed a doublet signal for
the furan C4H proton at δ 5.61 (JHH = 2.5, 1H) and unresolved
multiplet signals at δ 5.58 (C3H), 3.39 (CH2), and 2.66 (CH2)
characteristic of the η3-furfuryl ligand. Interestingly, variable-
1
temperature H NMR experiments revealed very little depend-
ence of the CH2 resonances on the temperature upon cooling,
even as low as −80 °C. The 31P NMR spectrum of 4b showed
two near-coincident doublet signals (δP 16.17, 14.79; JPP
=
12.2) but with significantly different 195Pt−31P coupling
constants (JPtP = 4558, 3550). A 1H,31P COSY spectrum
(Figure 2) showed cross-peaks allowing the assignment of the
low-field 31P NMR signal to the phosphine ligand trans to the
furan C3H (with a weak correlation to the C4H signal), while
the higher field signal corresponded to the phosphine ligand
trans to the furfuryl methylene group. Single-crystal X-ray
diffraction analysis (Figure 1) showed 4b to be isostructural
with 4a, with only a marginally higher symmetry ratio of 1.09
(cf. 1.06 for 4a) (Table 1).
In order to establish a direct connection between the possible
catalytic relevance of η3-furfuryl ligands and carbohydrate-
derived furans, the aforementioned aldehyde 5-chloromethyl-
furfural (I, Scheme 1) was prepared via dehydration of fructose
as previously described in the literature.4 This compound
slowly darkens when kept at room temperature, presumably
due to polymerization, and is best stored as a solid at −20 °C.
Addition of [Pd(PPh3)4] to the furan provided trans-3c, in
accordance with the synthesis of trans-3a, and subsequent
Figure 2. Variable-temperature (VT, above) and 31P,1H COSY
(below) NMR spectra for 4b and 4c. Only the M−CH2 regions of
the VT NMR spectra are depicted.
addition of silver tetrafluoroborate provided the cationic
1
complex 4c. The H NMR spectrum of 4c showed one very
broad signal centered at δH 3.42, which split into apparent
triplet and overlapping doublet-of-doublet signals when
measured at −80 °C, the appearance of these signals being
very similar to those of Pt(II) complex 4b (Figure 2). The 31P
and 31P−1H COSY NMR spectra of 4c (Figure 2) showed
doublet signals at δP 30.23 (trans to furan C3H) and 20.85
(trans to furfuryl CH2) with a P−P coupling constant of 46.2
Hz, wholly consistent with those of 4a. The molecular structure
of 4c (Figure 1) reveals a symmetry ratio of 1.07 (Table 1),
very similar to that of 4a (1.06) and 4b (1.09). This ratio over
the η3-furfuryl complexes 4a−c consistently matches those of
5601
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
the most symmetrical η3-benzyl complexes, which range from
1.05 to 1.29, further supporting our claim that η3-furfuryl
complexes may be stronger ligands in some cases.
presence of the η3-thienyl complex 4e in the sample.
Unfortunately, the decomposition of the starting material and
all of the derived complexes, presumably due to ring-opening
and/or polymerization of the thiophene ring, is a serious
impediment to the synthesis of stable η3-thienyl complexes.
Reaction of 4a with Benzylamine. Hartwig et al. has
reported a stoichiometric reaction of a cationic η3-naphthyl
complex of palladium with aniline, resulting in amination of the
exocyclic “allylic” carbon atom.12b In order to probe the
reactivity of the η3-furfuryl ligand toward nucleophiles, 4a was
heated with aniline in d8-THF, giving a range of products of
which none could be isolated or identified. Changing the amine
component from aniline to benzylamine produced a mixture,
which separated into two bands after multiple column
chromatography attempts. The first band was identified as
Some key structural parameters determined for complexes 2
and 4a−c are very similar despite their differing connectivity
and composition, such as the O1−C2 and O1−C5 distances,
which are very close across all four complexes, indicating that
the ligation of the furan ring to the metal has little effect on the
oxygen atom (Table 1). Since both σ-donation from and π-
back-bonding to the C2−C3 bond are expected to lengthen this
bond, the ratio of the C2−C3 and C4−C5 distances is
effectively a measure of the participation of the C2−C3 double
bond in bonding with the metal center. This parameter is
somewhat static over the four complexes, from 1.038 (4c) to
1.052 (4b). The C3−C2−C6 angle varies even less: from
126.2° (4b) to 126.8° (4a). The tilt of η3-allyl and η3-benzyl
ligands away from the metal center is an established
phenomenon and can be quantified roughly by the M−C2−
O1 angle, which varies slightly from 116.2° (2) to 119.9° (4a).
Similarly the tilt of the furan ring away from the metal,
quantifiable by the angle made by the metal, the midpoint of
the C2−C3 bond, and the furan ring centroid, varies from
113.5° (2) to 118.2° (4a). By far the most clear distinction
between the complexes is how the η3-furfuryl ligand is oriented
around the imaginary axis connecting the C3−C2−C6 centroid
and the metal. As shown by the analysis of the O1−C2−M−L
torsion angles, Pt complex 4b (angles: 104.2°/63.5°) is notably
distorted from a pseudo-square-planar arrangement around the
metal center. In all four complexes, a noticeable bend of C6
away from the furan ring plane toward the metal was observed.
This bend is greatest for triflato complex 2 (163.2°), while the
three bis(phosphine) complexes 4a−c show more moderate
bends of 167.7−168.2°.
1
pure benzaldehyde by H NMR, while the second band could
be resolved only with GC/MS, showing unambiguous signals
for PPh3, OPPh3, dibenzylamine, and a small signal possibly
indicating the presence of the desired amination product 5
(Scheme 3). The metal-mediated conversion of amines to
Scheme 3. Stoichiometric Reactivity of 4a
Attempted Synthesis of η3-Thienyl Complexes of
Palladium and Platinum. The unstable 2-chloromethyl-
thiophene was prepared by addition of thionyl chloride to 2-
thiophenemethanol as a brown, viscous oil. Addition of the
prepared 2-chloromethylthiophene to [Pd(PPh3)4] resulted in
two phosphorus-containing products with similar 31P NMR
singlet resonances (δP 25.0, 24.0); however, attempts to isolate
these products failed. Consequently, silver tetrafluoroborate
was added to the mixture in situ, resulting in two doublet 31P
NMR signals at δP 27.9 and 23.4 (JPP = 43). The similarity of
the chemical shifts and P−P coupling constants to those of η3-
furfuryl complexes 4a,c indicated the likely formation of the η3-
thienyl product 4d (Scheme 2). This observation also suggests
that the two singlet resonances in the previous reaction may
arise from different isomers of trans-3d, which both converge to
4d upon halide abstraction. Unfortunately, 4d could not be
isolated as a pure substance due to rapid decomposition.
The reaction of 2-chloromethylthiophene with [Pt(C2H4)-
(PPh3)2] provided, in contrast to the furfuryl chemistry, the
trans oxidative addition product trans-4e, as indicated by a
singlet in the 31P NMR spectrum of the reaction mixture (δ
27.5, JPtP = 3217). To avoid the common decomposition of
thienyl complexes seen above, the mixture was treated in situ
with silver tetrafluoroborate, providing a black solid. This solid,
while impure and unstable, appeared to contain the desired
Pt(II) η3-thienyl product 4e, as indicated by two doublet
aldehydes,18 and specifically benzylamine to benzaldehyde,19
has been previously reported in microwave-promoted reactions
using palladium on carbon as catalyst. In these cases water is
used as the oxygen source; therefore the observation of
benzaldehyde in the reaction of 4a is presumably the result of
reaction of water with the excess amine after the furfuryl
amination had taken place.
CONCLUSION
■
Cationic and neutral η3-furfuryl complexes of palladium and
platinum were prepared with furfuryl halides. Notably, the η3-
furfuryl coordination mode is also stable in the presence of a
pendent aldehyde functional group, as demonstrated by the
synthesis of an η3-furfuryl complex incorporating the
carbohydrate-derived furan 5-chloromethylfurfural. Variable-
temperature NMR experiments verify the dynamic behavior of
the η3-furfuryl ligand when bound to Pd, while a related Pt η3-
furfuryl complex shows much more restricted dynamic
behavior. The synthesis of analogous η3-thienyl complexes
was attempted, providing promising NMR data; however the
products could not be isolated due to decomposition,
presumably of the thienyl ring system. A preliminary reaction
of the η3-furfuryl complex 4a with benzylamine showed that
amination of the furfuryl position of the η3-furfuryl ligand
seems possible. We hope that the herein reported results will
increase the understanding of heterocyclic pseudoallyl com-
plexes and lead to the future exploitation of the η3-furfuryl
resonances in its 31P NMR spectrum (δP 19.3, JPP = 8.9, JPtP
=
3509; δP 17.2, JPP = 8.9, JPtP = 4797). The similarity of these
data to Pt(II) η3-furfuryl complex 4b (δP 16.17, JPP = 12.2, JPtP
= 4558; δP 14.79, JPP = 12.2, JPtP = 3550) also indicated the
5602
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
C3H), 5.84 (unresolved dd, 1H, collapses to a singlet upon 31P
decoupling, furan C4H), 3.84 (s, 3H, OCH3), 3.57 (br s, 1H, sharpens
upon 31P decoupling, PdCH2), 3.19 (br s, 1H, sharpens upon 31P
decoupling, PdCH2). 1H NMR (500 MHz, −80 °C, CD2Cl2): δ 7.47−
6.96 (m, 30H, PC6H5), 6.00 (dd, JHH = 3.1, JPH = 8.5, 1H, furan C3H),
5.75 (br s, 1H, furan C4H), 3.81 (s, 3H, OCH3), 3.56 (unresolved dd,
1H, PdCH2), 3.07 (dd, JHH = 6.4, JPH = 10.7, 1H, PdCH2). 13C{1H}
NMR (126 MHz, 25 °C, CD2Cl2): δ 158.4 (unresolved dd, CO2Me),
150.8 (unresolved dd, furan C5), 146.1 (unresolved dd, furan C2),
134.1−133.7 (overlapping 2 × d, JCP = approximately 35.0 and 35.3,
C2,6H(C6H5)), 133.6 (overlaid 2 × d, JCP = 41.8, C3,5H(C6H5)), 130.5
(overlapping 2 × d, JCP = approximately 44.0 and 40.4, C1(C6H5)),
129.4 (overlaid 2 × d, JCP = 8.4, C4H(C6H5)), 120.8 (unresolved dd,
furan C4H), 91.2 (d, JCP = 25.2, furan C3H), 52.8 (s, OCH3), 49.7 (d,
JCP = 42.8, PdCH2). Assignments of 13C signals were assisted by
DEPT-135 and DEPT-90 NMR experiments. 31P{1H} NMR (202
MHz, 25 °C, CD2Cl2): δ 30.6 (d, JPP = 44.5, P trans to furan C3H as
ligand in the catalytic conversion of biomass-derived com-
pounds such as 5-chloromethylfurfural.
EXPERIMENTAL SECTION
■
General Techniques. Unless otherwise stated, all reactions were
carried out under water- and oxygen-free conditions in an atmosphere
of dry argon by means of Schlenk or glovebox techniques. All solvents
were dried over molten alkali metal or alloy, distilled, and stored over 4
Å molecular sieves. NMR spectra were acquired on Bruker Avance
1
200, 400, or 500 NMR instruments. H and 13C{1H} NMR chemical
shifts are reported relative to the (residual) solvent peaks. 31P{1H}
NMR chemical shifts are reported relative to an external standard
(80% H3PO4). Mass spectrometry was performed using a HP G1800A
GCD system. Crystal structure determinations were carried out on a
Bruker Smart Apex I or Bruker FR591 Apex II X-ray diffraction
apparatus.
1
confirmed by H,31P-COSY), 20.6 (d, JPP = 44.5, P trans to furfuryl
Preparation of a Mixture of 1 and 2. In a glovebox,
[Pd(PCy3)2] (40 mg, 60 μmol), methyl 5-(chloromethyl)-2-
furancarboxylate (11 mg, 63 μmol), and toluene were added to a
Schlenk tube, stirred, and left for 1 h. Silver triflate (15 mg, 60 μmol)
was then added to the solution, and the suspension was stirred, then
filtered through Celite. Reduction of the solvent volume resulted in
orange crystals, from which the solvent was removed via cannula, then
dried under vacuum. The NMR data was obtained from this mixture of
crystals. 31P{1H} NMR (81 MHz, 25 °C, CD2Cl2): δ 52.6 (s, complex
2), 42.5 (d, JPP = 34.8, complex 1), 36.5 (d, JPP = 34.8, complex 1).
Integration of the 31P{1H} NMR spectrum showed 2 and 1 to be in an
approximate 5:1 molar ratio.
CH2 as confirmed by 1H,31P-COSY). 1H,1H-COSY (500 MHz, 25 °C,
CD2Cl2): cross-peaks found between the signals at δ 6.12 (furan
1
C3H)/5.84 (furan C4H) and δ 3.57 (PdCH2)/3.19 (PdCH2). H,31P-
COSY (121 MHz, 25 °C, CD2Cl2): cross-peaks found between δP
30.6/δH 6.12 (furan C3H), 5.84 (furan C4H) and δP 20.6/δH 6.12
(furan C3H). Both 31P signals also correlate with elements of the
1
phenyl region of the H NMR spectrum. High-resolution ESI-MS:
1
calcd for [12C43 H3716O331P2102Pd]+ 765.12685, found 765.12699.
Preparation of Platinum(II) η3-Furfuryl Compound 4b via cis-
3b. A toluene solution of methyl 5-(chloromethyl)-2-furancarboxylate
(93 mg, 0.53 mmol) was added dropwise into a Schlenk tube
containing an equimolar amount of bis(triphenylphosphine)(η2-
ethene)platinum(0) (400 mg, 0.535 mmol) in 40 mL of toluene,
and the mixture was stirred at room temperature for 1 h until all solids
dissolved. This dark orange mixture was examined by 31P{1H} NMR
spectroscopy, revealing the formation of the cis-bis(phosphine) η1-
furfuryl intermediate cis-3b. 31P{1H} NMR (202 MHz, 25 °C): δ 23.3
(d, JPP = 16.2, JPtP = 1810), 20.6 (d, JPP = 16.2, JPtP = 4458).
Preparation of Palladium(II) η3-Furfuryl Compound 4a via
trans-3a. A benzene solution of methyl 5-(chloromethyl)-2-
furancarboxylate (30 mg, 0.17 mmol) was added dropwise into a
Schlenk tube containing an equimolar amount of tetrakis-
(triphenylphosphine)palladium(0) (200 mg, 0.173 mmol) in 40 mL
of benzene and stirred at room temperature overnight. The solvent
was reduced under vacuum to 15 mL so that the dark yellow η1-
furfuryl intermediate trans-3a precipitated. The mother liquor
containing free triphenylphosphine was removed with a filter cannula,
and solid product trans-3a was dried under vacuum (82 mg, 59%). 1H
NMR (400 MHz, 25 °C, CD2Cl2): δ 7.77−7.42 (m, 30H, PC6H5),
6.79 (d, JHH = 3.2, 1H, furan C3H), 4.82 (d, JHH = 3.2, 1H, furan C4H),
3.69 (s, 3H, OCH3), 2.33 (s, 2H, PdCH2). 13C{1H} NMR (126 MHz,
25 °C, CD2Cl2): δ 164.9 (s, CO2Me), 159.3 (s, furan C5), 142.0 (furan
C2), 135.6 and 128.5 (s, C2,3,5,6H(C6H5)), 131.5 (br s, C1H(C6H5)),
130.6 (s, C4H(C6H5)), 120.2 (s, furan C4H), 106.4 (s, furan C3H),
51.5 (s, OCH3), 17.2 (s, PtCH2). Assignments of 13C signals were
assisted by DEPT-135 NMR experiments. 31P{1H} NMR (162 MHz,
25 °C, CD2Cl2): δ 28.2 (s).
Silver tetrafluoroborate (114 mg, 0.586 mmol) was added directly
into this mixture, causing a color change to milky white. This
suspension was stirred for 8 h, at which point it was filtered via filter
cannula. The filtered solid was then extracted with DCM, and the
suspension left to settle and filtered once more. The methylene
chloride was removed completely from the orange-brown filtrate
under vacuum to afford off-white solid 4b (280 mg, 56%). Colorless
single crystals suitable for X-ray analysis were obtained by
recrystallization from DCM/hexane at −30 °C. 1H NMR (500
MHz, 25 °C, CD2Cl2): δ 7.43−7.08 (m, 30H, PC6H5), 5.72 (dd, JHH
=
1.5, JPH = 3.0, 1H, furan C4H), 5.58 (m, 1H, furan C3H), 3.82 (s, 3H,
1
OCH3), 3.39 (m, 1H, PtCH2), 2.66 (m, 1H, PtCH2). H NMR (500
Silver tetrafluoroborate (22 mg, 0.11 mmol) was added to the
yellow solution of intermediate trans-3a (82 mg, 0.10 mmol) in
toluene, causing a color change to orange-brown. This suspension was
stirred overnight, at which point it was filtered via filter cannula. The
filtered solid was then extracted with DCM, and the suspension left to
settle and filtered once more. The methylene chloride was removed
completely from the light orange filtrate under vacuum to afford light
orange solid 4a (40 mg, 46%).
MHz, −80 °C, CD2Cl2): δ 7.41−6.98 (m, 30H, PC6H5), 5.61 (d, JHH
=
2.5, 1H, furan C4H), 5.48 (m, 1H, furan C3H), 3.74 (s, 3H, OCH3),
3.33 (m, 1H, PtCH2), 2.56 (m, 1H, PtCH2). 13C{1H} NMR (126
MHz, 25 °C, CD2Cl2): δ 158.1 (d, CO2Me), 150.2 (d, furan C5),
145.0 (unresolved dd, furan C2), 133.9 (dd, JCP = 11.6 and 18.4,
C2,6H(C6H5)), 131.9 (dd, JCP = 2.6 and 35.6, C3,5H(C6H5)), 130.0
(overlapping dd, JCP = 8.5, C1(C6H5)), 129.2 (dd, JCP = 11.0,
C4H(C6H5)), 121.9 (unresolved dd, furan C4H), 81.7 (d, JCP = 26.5,
furan C3H), 52.8 (s, OCH3), 43.9 (d, JCP = 40.3, PtCH2). Assignments
of 13C signals were assisted by DEPT-135 NMR experiments. 31P{1H}
In Situ Synthesis of Palladium(II) η3-Furfuryl Compound 4a.
To a Schlenk tube containing [Pd(PPh3)4] (100 mg, 87 μmol) was
added a solution of methyl 5-(chloromethyl)-2-furancarboxylate (15
mg, 87 μmol) in toluene (20 mL), and the resulting mixture was
stirred for 2 h. In a glovebox, solid silver tetrafluoroborate (20 mg, 100
μmol) was added to the yellow solution, causing a color change to
orange-brown. This suspension was stirred for 1 h, at which point it
was filtered via filter cannula. The filtered solid was then extracted with
dichloromethane, and the suspension left to settle and decanted. The
dichloromethane was removed from the orange decantate under
vacuum, and the orange solid was recrystallized from dichloro-
NMR (202 MHz, 25 °C, CD2Cl2): δ 16.17 (d, JPP = 12.2, JPtP = 4558,
1
P trans to furan C3H as confirmed by H,31P-COSY), 14.79 (d, JPP
=
1
12.2, JPtP = 3550, P trans to furfuryl CH2 as confirmed by H,31P-
COSY). 1H,31P-COSY (121 MHz, 25 °C, CD2Cl2): cross-peaks found
between δP 16.17/δH 5.72 (furan C4H), 5.58 (furan C3H) and δP
14.79/δH 3.39, 2.66 (furfuryl CH2). Both 31P signals also correlate with
1
elements of the phenyl region of the H NMR spectrum.
Preparation of Palladium(II) η3-Furfuryl Compound 4c via
trans-3c. A benzene solution of 5-(chloromethyl)furfural (37.5 mg,
0.260 mmol) was added dropwise into a Schlenk tube containing an
equimolar amount of tetrakis(triphenylphosphine)palladium(0) (300
mg, 0.262 mmol) in 40 mL of benzene, and the mixture was stirred at
1
1
methane/benzene. Yield: 32 mg (43%). H and H{31P} NMR (500
MHz, 25 °C, CD2Cl2): δ 7.42−7.05 (m, 30H, PC6H5), 6.12 (dd, JHH
3.2, JPH = 8.5, 1H, collapses to a singlet upon 31P decoupling, furan
=
5603
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
room temperature overnight. The solvent was reduced under high
vacuum to 15 mL, so that orange η1-furfuryl intermediate trans-3c
precipitated. The mother liquor containing free triphenylphosphine
was removed with a filter cannula, and solid product trans-3c was dried
under vacuum (82 mg, 41%). 1H NMR (200 MHz, 25 °C, CD2Cl2): δ
9.22 (s, 1H, CHO), 7.91−7.04 (m, 30H, PC6H5), 6.62 (unresolved d,
1H, furan C3H), 4.83 (d, JHH = 3.5, 1H, furan C4H), 2.62 (s, 2H,
PdCH2). 31P{1H} NMR (81 MHz, 25 °C, C6D6): δ 28.4 (s).
extracted with DCM, and the suspension left to settle and filtered once
more, providing an impure dark substance. 31P{1H} NMR spectral
data indicated the substance to be the expected η3-thienyl product 4e.
Due to rapid decomposition, no pure sample could be obtained and
1H NMR spectral data are unavailable. 31P{1H} NMR (81 MHz, 25
°C, CD2Cl2): δ 19.3 (d, JPP = 8.9, JPtP = 3509), 17.2 (d, JPP = 8.9, JPtP
=
4797).
Reaction of 4a with Benzyl Amine. η3-Furfuryl palladium
compound 4a (190 mg, 0.222 mmol) and an excess of benzyl amine
(300 mg, 2.80 mmol) were mixed in THF and then heated at 70 °C
for 3 h. The resulting brown mixture was subjected to silica gel column
chromatography (twice, eluting with DCM/hexane, 3:1 and 5:1) to
Silver tetrafluoroborate (22 mg, 0.11 mmol) was added to the
yellow solution of intermediate trans-3c (82 mg, 0.10 mmol) in
toluene, causing a brown precipitate. This suspension was stirred
overnight, at which point it was filtered via filter cannula. The filtered
solid was then extracted with DCM, and the suspension left to settle
and filtered once more. The methylene chloride was removed
completely from the light yellow filtrate under vacuum to afford
yellow solid 4c (25 mg, 31%). 1H NMR (500 MHz, 25 °C, CD2Cl2): δ
9.22 (s, 1H, CHO), 7.41−7.05 (m, 30H, PC6H5), 6.20 (dd, JHH = 3.5,
JPH = 9.0, 1H, furan C3H), 5.86 (m, 1H, furan C4H), 3.42 (br, 2H,
PdCH2). 1H NMR (500 MHz, −80 °C, CD2Cl2): δ 8.94 (s, 1H,
CHO), 7.39−6.93 (m, 30H, PC6H5), 6.10 (dd, JHH = 3.0, JPH = 9.0,
1H, furan C3H), 5.80 (s, 1H, furan C4H), 3.48 (m, 1H, PdCH2), 3.16
(dd, JPH = 10.5 and 4.0, 1H, PdCH2). 31P{1H} NMR (202 MHz, 25
°C, CD2Cl2): δ 30.23 (d, JPP = 46.2, P trans to furan C3H as confirmed
1
yield benzaldehyde. H NMR (200 MHz, 25 °C, C6D6): δ 9.64 (s,
aldehyde 1H), 7.51 (d, JHH = 8, 2H), 7.03 (d, JHH = 6.4, 1H), 6.96
(unresolved dd, JHH = 8, 2H). A second band was also found to be a
mixture and could not be further purified by column chromatography.
Analysis by GC/MS showed four major fractions with the following
major positive ion signals: GC/MS m/z 195 (retention time 9.25
min), benzaldehyde; 243 (10.68, possibly amination product 5); 262
(11.43, PPh3); 277 (14.07, OPPh3).
Crystallographic Details. The crystal data of 2, 4b, and 4c were
collected on a Bruker APEX diffractometer with a CCD area detector
and graphite-monochromated Mo Kα radiation. The structure was
solved using direct methods, refined with the Shelx software package
(G. Sheldrick, Acta Crystallogr. 2008, A64, 112−122), and expanded
using Fourier techniques. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were assigned to idealized positions
and were included in structure factor calculations. In the disordered
parts of the triflate group of 2, all atoms were restrained,
corresponding bond distances were restrained, and equal anisotropic
displacement parameters were used for F1, F2, and O2, respectively.
Crystal data for 2: C26H40F3O6PPdS, Mr = 675.01, yellow block,
1
by H,31P-COSY), 20.85 (d, JPP = 46.2, P trans to furfuryl CH2 as
confirmed by 1H,31P-COSY). 1H,31P-COSY (121 MHz, −80 °C,
CD2Cl2): cross-peaks found between δP 30.87/δH 6.10 (furan C3H)
and δP 20.65/δH 3.48, 3.16 (furfuryl CH2). Both 31P signals also
correlate with elements of the phenyl region of the 1H NMR spectrum.
Preparation of Palladium(II) η3-Thienyl Compound 4d via
trans-3d. A 0.500 mL portion of thionyl chloride (1.63 g·mL−1, 6.85
mmol) was added via syringe into a solution of 2-thiophenemethanol
(0.54 g, 4.75 mmol) in 30 mL of toluene. The brown mixture was
stirred at 50 °C for 2 h; then the solvent and residual thionyl chloride
were removed completely under high vacuum. The remaining dark,
0.21 × 0.17 × 0.08 mm3, triclinic space group P1, a = 9.286(5) Å, b =
̅
9.748(5) Å, c = 16.543(8) Å, α = 101.782(7)°, β = 96.075(8)°, γ =
90.978(8)°, V = 1456.6(13) Å3, Z = 2, ρcalcd = 1.539 g·cm−3, μ = 0.820
mm−1, F(000) = 696, T = 174(2) K, R1 = 0.0453, wR2 = 0.1027, 7286
independent reflections [2θ ≤ 5680°] and 390 parameters.
1
oily, thermally unstable substance was 2-chloromethylthiophene. H
NMR (200 MHz, 25 °C, C6D6): δ 6.75 (m, 1H, thiophene), 6.57 (m,
2H, thiophene), 4.20 (s, 2H, CH2Cl).
Crystal data for 4b: C44H39BCl2F4O3P2Pt, Mr = 1030.49, colorless
A benzene solution of 2-chloromethylthiophene (58 mg, 0.44
mmol) was added dropwise to a Schlenk tube containing an equimolar
amount of tetrakis(triphenylphosphine)palladium(0) (505 mg, 0.437
mmol) in 40 mL of benzene, and the mixture was stirred at room
temperature overnight. The expected η1-thienyl intermediate trans-3d
could not be precipitated by means of reducing the volume of
benzene; instead a brown mixture containing what is believed to be
phosphonium salt [Ph3PCH2(C4H3S)]Cl was obtained. 31P{1H} NMR
(81 MHz, 25 °C, C6D6): δ 25.0 (s), 24.0 (s).
Silver tetrafluoroborate (1.2 equiv, 103 mg, 0.529 mmol) was added
into the above Schlenk flask filled with 30 mL of toluene. This
suspension was stirred overnight, at which point it was filtered via filter
cannula. The filtered solid was extracted with DCM, and the
suspension left to settle and filtered once more. An impure dark
substance was obtained. 31P{1H} NMR spectral data indicated the
substance to be the expected η3-thienyl product 4d. Due to rapid
decomposition, no pure sample could be obtained and 1H NMR
spectral data could not be obtained. 31P{1H} NMR (81 MHz, 25 °C,
C6D6): δ 27.9 (d, JPP = 43), 23.4 (d, JPP = 43).
plates, 0.11 × 0.08 × 0.06 mm3, triclinic space group P1, a = 10.667(8)
̅
Å, b = 12.393(9) Å, c = 17.272(13) Å, α = 74.868(11)°, β =
76.251(11)°, γ = 77.722(11)°, V = 2113(3) Å3, Z = 2, ρcalcd = 1.620
g·cm−3, μ = 3.579 mm−1, F(000) = 1020, T = 174(2) K, R1 = 0.0383,
wR2 = 0.0810, 8377 independent reflections [2θ ≤ 52.24°] and 526
parameters.
Crystal data for 4c: C43H37BCl2F4O2P2Pd, Mr = 911.78, colorless
needle, 0.23 × 0.05 × 0.01 mm3, monoclinic space group P21/c, a =
9.8960(11) Å, b = 22.181(2) Å, c = 18.154(2) Å, β = 96.908(3)°, V =
3955.9(8) Å3, Z = 4, ρcalcd = 1.531 g·cm−3, μ = 0.741 mm−1, F(000) =
1848, T = 100(2) K, R1 = 0.0762, wR2 = 0.0913, 9400 independent
reflections [2θ ≤ 56.58°] and 496 parameters.
Crystallographic data for 2, 4b, and 4c have been deposited with the
Cambridge Crystallographic Data Center as supplementary publication
nos. CCDC-880708, CCDC-880709, and CCDC-880707, respec-
tively. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Preparation of Platinum(II) η3-Thienyl Compound 4e via
trans-3e. A toluene solution of 2-chloromethylthiophene prepared as
above (26.5 mg, 0.20 mmol) was added dropwise into a Schlenk tube
containing an equimolar amount of bis(triphenylphosphine)(η2-
ethene)platinum(0) (150 mg, 0.201 mmol) in 40 mL of toluene,
and the mixture was stirred at room temperature overnight. The
resulting clear orange solution was examined by multinuclear NMR
spectroscopy, which suggested the formation of trans-bis(phosphine)
η1-thienyl intermediate trans-3e. 31P{1H} NMR (81 MHz, 25 °C,
toluene): δ 27.5 (s, JPtP = 3217).
ASSOCIATED CONTENT
■
S
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: r.dewhurst@uni-wuerzburg.de.
In situ addition of 1.1 equiv of silver tetrafluoroborate (44 mg, 0.23
mmol) caused an immediate color change to dark brown and
precipitation of a black solid. This suspension was stirred overnight, at
which point it was filtered via filter cannula. The filtered solid was
Notes
The authors declare no competing financial interest.
5604
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605

Organometallics
Article
Hazari, N.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2008, 130, 2418.
ACKNOWLEDGMENTS
■
Financial support was provided by the Deutsche Forschungs-
gemeinschaft (DFG). We thank Prof. H. Braunschweig for
continued support.
(14) (a) King, R. B.; Kapoor, R. N. Inorg. Chem. 1969, 8, 2535.
(b) Onishi, M.; Ito, T.; Hiraki, K. J. Organomet. Chem. 1981, 209, 123.
(15) (a) Urata, H.; Suzuki, H.; Moro-oka, Y.; Ikawa, T. Bull. Chem.
Soc. Jpn. 1984, 57, 607. (b) Recio, A., III; Heinzman, J. D.; Tunge, J. A.
Chem. Commun. 2012, 48, 142. (c) Molander, G. A.; Elia, M. D. J. Org.
Chem. 2006, 71, 9198.
REFERENCES
■
(1) Top Value Added Chemicals From Biomass, Vol. 1, Technical
Report No. DOE/GO-102004−1992; National Renewable Energy
Lab: Golden, CO, 2004 (available at www.osti.gov/bridge).
(2) Prominent review articles regarding biomass-derived furans:
(a) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.;
Cairney, J.; Eckert, C. A.; Frederick, W. J., Jr.; Hallett, J. P.; Leak, D. J.;
Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T.
Science 2006, 311, 484. (b) Chheda, J. N.; Huber, G. W.; Dumesic, J.
A. Angew. Chem., Int. Ed. 2007, 46, 7164.
(16) Dewhurst, R. D.; Muller, R.; Kaupp, M.; Radacki, K.; Gotz, K.
̈
̈
Organometallics 2010, 29, 4431.
(17) (a) Chen, H.; Hodges, L. M.; Liu, R.; Stevens, W. C., Jr.; Sabat,
M.; Harman, W. D. J. Am. Chem. Soc. 1994, 116, 5499. (b) Brooks, B.
C.; Chin, R. M.; Harman, W. D. Organometallics 1994, 17, 4716.
(c) Liu, R.; Chen, H.; Harman, W. D. Organometallics 1995, 14, 2861.
(d) Chen, H.; Liu, R.; Myers, W. H.; Harman, W. D. J. Am. Chem. Soc.
1998, 120, 509. (e) Brooks, B. C.; Chin, R. M. Organometallics 1998,
17, 4716. (f) Chen, H.; Caughey, R.; Liu, R.; McMills, M.; Rupp, M.;
Myers, W. H.; Harman, W. D. Tetrahedron 2000, 56, 2313.
(g) Friedman, L. A.; Harman, W. D. J. Am. Chem. Soc. 2001, 123,
8967. (h) Friedman, L. A.; Sabat, M.; Harman, W. D. J. Am. Chem. Soc.
2002, 124, 7395. (i) Friedman, L. A.; You, F.; Sabat, M.; Harman, W.
D. J. Am. Chem. Soc. 2003, 125, 14980. (j) Mocella, C. J.; Delafuente,
D. A.; Keane, J. M.; Warner, G. R.; Friedman, L. A.; Sabat, M.;
Harman, W. D. Organometallics 2004, 23, 3772. (k) You, F.; Friedman,
L. A.; Bassett, K. C.; Lin, Y.; Sabat, M.; Harman, W. D. Organometallics
2005, 24, 2903. (l) Keane, J. M.; Harman, W. D. Organometallics 2005,
24, 1786. (m) Bassett, K. C.; You, F.; Graham, P. M.; Myers, W. H.;
Sabat, M.; Harman, W. D. Organometallics 2006, 25, 435.
(3) Prominent communications regarding biomass-derived furans:
(a) Huber, G. W.; Chheda, J. N.; Barrett, C. J.; Dumesic, J. A. Science
2005, 308, 1446. (b) Roman
A. Science 2006, 312, 1933. (c) Roman
́
-Leshkov, Y.; Chheda, J. N.; Dumesic, J.
-Leshkov, Y.; Barrett, C. J.; Liu,
́
Z. Y.; Dumesic, J. A. Nature 2007, 447, 982. (d) Zhao, H.; Holladay, J.
E.; Brown, H.; Zhang, Z. C. Science 2007, 316, 1597. (e) Kunkes, E. L.;
Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; Gartner, C. A.;
̈
Dumesic, J. A. Science 2008, 332, 417.
(4) Hamada, K.; Yoshihara, H.; Suzukamo, G. Chem. Lett. 1982, 617.
(5) (a) Mascal, M.; Nitikin, E. B. Angew. Chem., Int. Ed. 2008, 47,
7924. (b) Mascal, M.; Nitikin, E. B. ChemSusChem 2009, 2, 423.
(c) Mascal, M.; Dutta, S. Green Chem. 2011, 13, 40.
(6) (a) King, R. B.; Fronzaglia, A. J. Am. Chem. Soc. 1966, 88, 709.
(b) Cotton, F. A.; LaPrade, M. D. J. Am. Chem. Soc. 1968, 90, 5418.
(c) Hartwig, J. F. Organotransition Metal Chemistry; From Bonding to
Catalysis; University Science Books: Sausalito, CA, 2010.
(18) Miyazawa, A.; Tanaka, K.; Sakakura, T.; Tashiro, M.; Tashiro,
H.; Surya Prakash, G. K.; Olah, G. A. Chem. Commun. 2005, 2104.
(19) Chandrasekhar, S.; Pavan Kumar Reddy, G.; Nagesh, Ch.; Raji
Reddy, Ch. Tetrahedron Lett. 2006, 48, 1269.
(7) Reviews regarding η3-allyl complexes and their use in synthesis:
(a) Green, M. L. H.; Nagy, P. L. I. Adv. Organomet. Chem. 1965, 2,
325. (b) Clarke, H. L. J. Organomet. Chem. 1974, 80, 155. (c) Faller, J.
W. Adv. Organomet. Chem. 1977, 16, 211. (d) Tsutsui, M.; Courtney,
A. Adv. Organomet. Chem. 1977, 16, 241. (e) Tsuji, J. J. Organomet.
Chem. 1986, 300, 281. (f) Consiglio, G.; Waymouth, R. M. Chem. Rev.
1989, 89, 275. (g) Trost, B. M.; Van Vranken, D. L. Chem. Rev. 1996,
96, 395. (h) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103, 2921.
(i) Patil, N. T.; Yamamoto, Y. Top. Organomet. Chem. 2006, 19, 91.
(8) (a) Kuwano, R.; Kondo, Y.; Matsuyama, Y. J. Am. Chem. Soc.
2003, 125, 12014. (b) Kuwano, R. Synthesis 2009, 1049.
(9) Catalytic polymerization of vinylarenes involving η3-benzyl
intermediates: (a) Rix, F. C.; Brookhart, M.; White, P. S. J. Am.
Chem. Soc. 1996, 118, 2436. (b) Nozaki, K.; Komaki, H.; Kawashima,
Y.; Hiyama, T.; Matsubara, T. J. Am. Chem. Soc. 2001, 123, 534.
(10) Catalytic hydroboration of vinylarenes involving η3-benzyl
intermediates: Hayashi, T.; Matsumoto, Y.; Ito, Y. Tetrahedron:
Asymmetry 1991, 2, 601.
(11) Catalytic hydrosilylation of vinylarenes involving η3-benzyl
intermediates: LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem.
Soc. 1997, 119, 906.
(12) Catalytic hydroamination of vinylarenes involving η3-benzyl
intermediates: (a) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc.
2000, 122, 9546. (b) Nettekoven, U.; Hartwig, J. F. J. Am. Chem. Soc.
2002, 124, 1166. (c) Johns, A. M.; Utsunomiya, M.; Incarvito, C. D.;
Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 1828. (d) Johns, A. M.; Tye,
J. W.; Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 16010.
(13) C−H functionalization of alkylbenzenes involving η3-benzyl
intermediates: (a) Shimada, S.; Batsanov, A. S.; Howard, J. A. K.;
Marder, T. B. Angew. Chem., Int. Ed. 2001, 40, 2168. (b) Lam, W. H.;
Lam, K. C.; Lin, Z.; Shimada, S.; Perutz, R. N.; Marder, T. B. Dalton
Trans. 2004, 1556. (c) Heyduk, A. F.; Driver, T. G.; Labinger, J. A.;
Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 15034. (d) Driver, T. G.;
Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organometallics 2005, 24,
3644. (e) Driver, T. G.; Williams, T. J.; Labinger, J. A.; Bercaw, J. E.
Organometallics 2007, 26, 294. (f) Williams, T. J.; Caffyn, A. J. M.;
5605
dx.doi.org/10.1021/om300568z | Organometallics 2012, 31, 5599−5605