Selective formylation or methylation of amines using carbon dioxide catalysed by a rhodium perimidine-based NHC complex

Article abstract of DOI:10.1039/c8gc03094d

Carbon dioxide can play a vital role as a sustainable feedstock for chemical synthesis. To be viable, the employed protocol should be as mild as possible. Herein we report a methodology to incorporate CO₂ into primary, secondary, aromatic or alkyl amines catalysed by a Rh(i) complex bearing a perimidine-based NHC/phosphine pincer ligand. The periminide-based ligand belongs to a class of 6-membered NHC ligand accessed through chelate-assisted double C-H activation. N-Formylation and -methylation of amines were performed using a balloon of CO₂, and phenylsilane as the reducing agent. Product selectivity between formylated and methylated products was tuned by changing the solvent, reaction temperature and the quantity of phenylsilane used. Medium to excellent conversions, as well as tolerance to a range of functional groups, were achieved. Stoichiometric reactions with reactants employed in catalysis and time course studies suggested that formylation and methylation reactions of interest begin with hydrosilylation of CO₂ followed by reaction with amine substrates.

Full text of DOI:10.1039/c8gc03094d

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Selective formylation or methylation of amines
using carbon dioxide catalysed by a rhodium
perimidine-based NHC complex†
a
Cite this: DOI: 10.1039/c8gc03094d
Raphael H. Lam, a Caitlin M. A. McQueen,b Indrek Pernik,
*
Roy T. McBurney, a Anthony F. Hill*b and Barbara A. Messerle
*
a
Carbon dioxide can play a vital role as a sustainable feedstock for chemical synthesis. To be viable, the
employed protocol should be as mild as possible. Herein we report a methodology to incorporate CO2
into primary, secondary, aromatic or alkyl amines catalysed by a Rh(I) complex bearing a perimidine-based
NHC/phosphine pincer ligand. The periminide-based ligand belongs to a class of 6-membered NHC
ligand accessed through chelate-assisted double C–H activation. N-Formylation and -methylation of
amines were performed using a balloon of CO2, and phenylsilane as the reducing agent. Product selecti-
vity between formylated and methylated products was tuned by changing the solvent, reaction tempera-
ture and the quantity of phenylsilane used. Medium to excellent conversions, as well as tolerance to a
range of functional groups, were achieved. Stoichiometric reactions with reactants employed in catalysis
and time course studies suggested that formylation and methylation reactions of interest begin with
hydrosilylation of CO2 followed by reaction with amine substrates.
Received 3rd October 2018,
Accepted 8th December 2018
DOI: 10.1039/c8gc03094d
rsc.li/greenchem
methylsulfate amongst others.26–29 Thus, CO2 would represent
a safer and more benign substitute. The first example of amine
Introduction
Carbon dioxide is potentially a sustainable feedstock for the formylation with CO2 was reported by Cantat and co-workers
chemical industry due to its non-toxic nature, low cost and in 2012, using organic bases as catalysts.30,31 The methylation
increasing abundance in the atmosphere.1–3 The key challenge of amines with CO2 was later achieved independently by the
of using CO2 as a building block in the chemical industry is to Cantat and Beller groups in 2013 using Zn32 and Ru10 com-
overcome the inertness of CO2, such that mild reaction con- plexes as catalysts. Several organometallic catalysts that
ditions could be employed.1 Industrial examples of using CO2 promote the methylation of amines have been reported since
as a feedstock are comparatively limited, but include the syn- then by other groups.32–48 Among these, only two reports fea-
thesis of poly-/cyclic carbonates,1,4 urea2,3,5 and salicylic tured a rhodium catalyst, and all NHC-based catalysts featured
acid.2,5–7
The formylation or methylation of amines using CO2 are
the traditional 5-membered NHC motif (Fig. 1a (left) and b).39,40
N-Heterocyclic carbenes (NHCs) and pincer-type ligands are
potentially important processes.3,8–11 Formamides are com- popular because of their capacity to stabilise metal complexes
monly used in the production of drug molecules and against decomposition during catalysis. Typically, NHCs are
agrochemicals.12–15 Methylated amines can improve based on five-membered heterocyclic rings, such as the
lipophilicity16–20 and potency18–21 of drug molecules, or some- 5-membered imidazole-based structure (Fig. 1a). There are
times control the conformations of molecules.20,22–25 Current only a handful of examples of 6-membered NHCs, using a peri-
methylation protocols rely heavily on the stoichiometric use of midine-based scaffold (Fig. 1a, right).49–59 We are particularly
toxic reagents such as formaldehyde, methyl iodide and di- interested in using 6-membered NHC ligands, as they place
the carbene centre in an electron-rich, annulated 14 π-electron
aromatic system, as in a perimidine molecule.60 Furthermore,
aDepartment of Molecular Sciences, Macquarie University, NSW 2109, Australia.
E-mail: barbara.messerle@mq.edu.au, indrek.pernik@mq.edu.au
bResearch School of Chemistry, The Australian National University, Canberra,
ACT 0200, Australia. E-mail: a.hill@anu.edu.au
expanding the NHC ring from five- to six-membered influences
the steric impact of the ligand, decreasing the angle α (Fig. 1a)
and directing the amine substituents closer to the coordinated
metal.61 Studies by Richeson and co-workers on a range of free
and metallated perimidinylidene-based NHCs (per-NHCs)58,59
have highlighted 6-membered NHC systems. Their reports
†Electronic supplementary information (ESI) available. CCDC 898438. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8gc03094d
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Scheme 1 Preparation of 2,3-dihydroperimidine pro-ligands and their
reactions with [RhCl(PPh3)3] to form per-NHC pincer complexes.
spectra was triphenylphosphine (δP ∼ −5), and in the latter
rapid reaction the evolution of dihydrogen was also evident in
1
the H NMR spectrum (δH = 4.47) and the observation of gas
evolution. Thus, this direct preparation with rhodium54 and
also ruthenium55 complexes have provided rare preliminary
reports of atom-efficient instances of NHC ligand installation
Fig. 1 (a) N-Heterocyclic carbene ligands with imidazole-based (left)
and perimidine-based (right) structures. (b) Previous work on the
addition of CO2 to amines by Kobayashi.39,40 (c) This work – catalysed via double geminal C–H activation.62–64 This preparation of an
addition of CO2 to amines. Respective pre-catalysts in insets.
NHC complex avoids the typical use of either base or silver
transmetallation to activate cationic azolium precursors.65
Both rhodium complexes could be precipitated from the
included experimental data indicating enhanced σ-basicity for
the per-NHC ligands relative to their five-membered ring ana-
logues, as did
reaction mixtures to afford high isolated yields (2a, 88%; 2b,
92%). The formulations of 2a and 2b as NHC pincer complexes
are based on the spectroscopic data, which each comprise a
rhodium-103-coupled doublet resonance in the 31P{1H} NMR
spectra (2a, δP = 22.9; 2b, δP = 38.3), and one 1H NMR reso-
nance for the PCH2 protons (2a, δH = 4.14; 2b, δH = 3.58) with
no additional CH2 protons evident. Virtual triplet coupling of
PCH2 protons to the phosphorus nuclei is observed for 2a,
with a small JH–P coupling value of 2 Hz, though it could not
a
later report from Herrmann et al.50
Furthermore, studies by the Özdemir49 and Dötz51 groups have
found palladium per-NHC complexes to be effective catalysts,
and the latter of these reports noted that their per-NHC
system, which incorporates the per-NHCs as the axial donors
in a pincer framework, was more efficient in Heck coupling
reactions than its imidazole and benzimidazole analogues.
We wanted to determine the catalytic activity of rhodium
complexes featuring the novel 6-membered perimidine-based
NHC ligand in CO2 incorporation reactions (Fig. 1c). Herein,
we report the selective formylation and methylation of primary
and secondary amines with CO2, catalysed by per-NHC
rhodium complexes [RhCl{C(NCH2PPh2)2C10H6}] (2a) and
[RhCl{C(NCH2PCy2)2C10H6}] (2b), using mild reaction tempera-
tures (25–90 °C) and low pressures of CO2 (balloon).
Switchable product selectivity (formylation vs. methylation)
was achieved by controlling the quantity of hydrosilane and by
changing the reaction solvent and temperature.
1
be discerned in the H NMR spectrum of 2b. Though the low
solubilities of the complexes precluded acquisition of ade-
quate 1D 13C NMR spectra, the carbene carbon resonance of
1
2b was detected in a H–13C HMBC experiment at δC = 206.7
within the range observed for previously reported per-NHC
rhodium(I) complexes.56,59
Molecular structure of 2b. Previously only the molecular
structure of 2a had been determined; in this work the X-ray
structure of 2b is also reported which allows comparisons to
be made. The space-filling representations of both 2a and 2b
are shown (Fig. 2), illustrating the enhanced steric crowding of
the metal centre by the cyclohexyl groups in 2b relative to the
phenyl groups in 2a. The structure of 2b straddles a crystallo-
graphic C2 axis that passes through C1 and Rh1, with the ring
system of the pincer ligand lying within the coordination
plane. However, broadness of the PCH2 resonance in the 1H
Results and discussion
Catalyst preparation
Synthesis of rhodium pincer per-NHC complexes. As NMR spectrum is consistent with twisting of the ring system
described in an earlier communication,54 double chelate- occurring in solution. The Rh1–C1 and Rh1–Cl1 distances are
assisted aminal C–H activation of 1a and 1b occurred upon comparable to those observed in 2a.
direct reaction with [RhCl(PPh3)3] to give the per-NHC pincer
Investigation on the formation of 2a. Though the complete
complexes [RhCl{C(NCH2PR2)2C10H6}] (R = Ph 2a, R = Cy 2b, formation of 2a required many hours, the starting materials
Scheme 1).54 Pro-ligands 1a and 1b were prepared by combin- appeared to have been completely consumed within one hour
ing 1,8-diaminonaphthalene, paraformaldehyde and a second- to give a major intermediate species that gradually converted
ary phosphine HPR2 in a remarkably convenient one-pot syn- to the final product 2a. The 31P{1H} NMR spectrum of this
thesis (Scheme 1).
intermediate species showed a doublet of doublet resonance at
2
The reactions proceeded within 21 hours and 10 minutes δP = 61.3 (1JRh–P = 133 Hz, JP–P = 24 Hz) and a doublet of
2
for 2a and 2b, respectively, and were monitored by 31P NMR triplet resonance at δP = 17.9 (1JRh–P = 83 Hz, JP–P = 24 Hz),
spectroscopy. The only side product observed in the 31P NMR suggesting that both the pincer ligand and a triphenyl-
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sequently react with the free HCl and lose H2 and PPh3 to
afford 2a. A number of observations support 4a as the likely
formulation of the intermediate species. Firstly, the formation
1
of a pentacoordinate intermediate is supported by the JRh–P
coupling of the diphenylphosphino 31P resonance at δP = 61,
which shows a large value of 133 Hz. This is slightly reduced
relative to that in the four-coordinate 2a (1JRh–P = 153 Hz),
though significantly larger than those observed for a related
hexacoordinate rhodium(III) complex (vide infra), which gives a
value of around 100 Hz. Secondly, the mass spectra of the
intermediate showed strong peaks consistent with complex 4a.
Furthermore, a later experiment was performed in which 1a
was treated with n-butyllithium, in order to deprotonate the
central methylene group, prior to addition of [RhCl(PPh3)3].
Fig. 2 Molecular structure of 2b (aryl and cylohexyl hydrogen atoms
omitted, 50% displacement ellipsoids, asterisks denote symmetry-gen-
erated atoms (crystallographic C2 axis through C1 and Rh1), and space-
filling representations of 2a and 2b viewed along the coordination
planes. Selected bond lengths (Å) and angles (°) for 2b: Rh1–C1 =
1
The 31P{1H} and H NMR spectra of the resulting product are
1.930(6), C1–N1 = 1.390(5), Rh1–P1 = 2.2338(12), Rh1–Cl1 = 2.3998(16), very similar to those of the observed intermediate in the prepa-
C1–Rh1–P1 = 84.90(3), C1–Rh1–Cl1 = 180, P1–Rh1–P1* = 169.79(6).
ration of 2a, but again attempts to isolate a pure sample of
this product were plagued by the inevitable formation of 2a.
Nonetheless, this result suggests that 4a is the product of this
reaction, as well as the tenuous intermediate.
1
phosphine ligand were coordinated to rhodium. The H NMR
spectrum of this species showed a multiplet hydride resonance
at δH = −17.6 (C6D6), and two mutually coupled resonances at
δH = 3.95 and 4.83 (2JH–H = 13 Hz) that could be attributed to
PCH2 protons, indicating diastereotopic protons on these
methylene groups due to a low symmetry molecule. When
attempting to cleanly isolate this intermediate, small quan-
tities of complex 2a were always present, despite not being
detected in the reaction mixture, precluding complete charac-
terization of this species. All crystallisation attempts yielded
only crystals of 2a.
As noted, the formation of 2b (10 min) is much more rapid
than that of 2a (21 h), and no intermediate was observed by
NMR spectroscopy. The persistence of intermediate species 4a in
the latter reaction is presumably a consequence of both the
reduced steric profile and σ-basicity of the -PPh2 groups relative
to their cyclohexyl analogues, which decreases the steric and elec-
tronic incentive for expulsion of a σ-donor ligand such as PPh3.
CO2 incorporation into amines
Two possible formulations for this observed intermediate
were considered, and are shown in Scheme 2. Oxidative
addition of 1a to [RhCl(PPh3)3] may yield a six-coordinate σ-2-
perimidinyl rhodium(III) species (3a), which subsequently
eliminates dihydrogen and triphenylphosphine to form 2a.
There are certainly examples of double C–H activation via such
a two-step process.66 Furthermore, single C–H activation to
form σ-2-perimidinyl complex [IrHCl(CO){CH(NCH2PCy2)2
C10H6}] was observed in the reaction between 1b and [IrCl(CO)
(PPh3)2].54
Catalyst screening for formylation reactions. The per-NHC
rhodium complexes 2a and 2b, together with a small selection
of locally available rhodium and iridium complexes—some
possessing strong σ-donor ligands—were screened for
their catalytic activity in the formylation of aniline using CO2
with phenylsilane as the reducing agent in THF at 50 °C
(Chart 1). Wilkinson’s catalyst (7) achieved 15% conversion to
Alternatively, the reaction between 1a and [RhCl(PPh3)3]
may release HCl to form the five-coordinate carbene complex
[RhH(PPh3){C(NCH2PPh2)2C10H6}] 4a, which could sub-
Chart 1 Catalyst screen for the formylation of aniline (5a) with CO2.
Conversions were determined with 1H NMR spectroscopy using 1,1,2,2-
tetrachloroethane as internal standard.
Scheme 2 Reactions of H2C(NCH2PPh2)2C10H6 with [RhCl(PPh3)3],
showing proposed intermediates in blue.
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the formanilide product. No desired product was found
when using Vaska’s complex (8) as catalyst. Rhodium com-
plexes bearing NHC-triazole bidentate ligand (9a and 9b),67 or
N,N-bidentate ligands (1068 and 1169–71 were also found to be
inactive as catalysts. Interestingly, very different conversions
were achieved when testing the per-NHC complexes 2a and 2b.
Complex 2a was inactive as a catalyst; whereas an excellent
93% conversion was achieved using complex 2b as the catalyst.
Scheme 3 Oxidative addition of dichloromethane to 2b.
Similar ligand effect for the formylation reaction was recently evident from the development of a doublet resonance in the
reported by He.72 An isolated yield of 86% was obtained after 31P{1H} NMR spectrum at δP = 22.1, with the coupling constant
1
purification by column chromatography. X-ray structures of 2a of JRh–P = 99 Hz, cf. 148 Hz for 2b, reflecting the octahedral
and 2b displayed similar bond lengths and bite angles about geometry at rhodium.
1
the Rh centres in these two complexes. The markedly different
The H NMR spectrum of 12b in CDCl3 showed one multi-
conversions are therefore most likely a result of different σ- plet peak at δH = 4.45, which was attributed to both the PCH2
donating ability of the phosphine ligand motifs (-PPh2 vs. groups and the CH2Cl protons based on relative integrals. In
-PCy2), given that the shape of the cavity through which reac- C6D6, however, these resonances separated to give two doub-
tants approach the Rh centre during catalysis would be lets at δH = 4.24 and 4.31 for the diastereotopic PCH2 protons,
expected to be more size-exclusive for 2b than for 2a. When no indicating cis coordination of the two chlorides, and a triplet
3
catalyst was added to the reaction mixture, no product for- of doublets at δH = 4.93 corresponding to CH2Cl, with JP–H
2
mation was observed (Scheme 1).
and JRh–H coupling constants of 7 and 2 Hz, respectively (cf.
3
2
Conditions screening for formylation reactions. With a suit- δH = 3.61, JP–H = 8 Hz, JRh–H = 3 Hz in CDCl3 for the reported
able catalyst identified, we proceeded to optimise the reaction complex 12a). The CH2Cl moiety appears in the 13C{1H} NMR
conditions for the formylation of amines. Aniline was used as spectrum as a doublet of triplets at δC = 37.2 (1JRh–C = 30 Hz,
a model amine substrate. Highest conversion (93% conv.; 86% 2JP–C = 8 Hz), while the carbene resonance, also a doublet of
isolated yield) was obtained using THF as solvent, 1 equiv. of triplets, is located at δC = 205.8 (1JRh–C = 50 Hz, JP–C = 5 Hz),
2
PhSiH3 as reductant, and the reaction temperature of 50 °C. close to that of the precursor 2b.
Decreasing the reaction temperature to 25 °C resulted in a
Formylation reaction scope. Having established that aniline
slight drop in conversion to the desired product to 83%. (5a) undergoes formylation under the optimised reaction
Replacing THF (50 °C) with toluene (90 °C) maintained high conditions, a range of substrates was tested to gauge the
conversion (91%). Lowering the reaction temperature of impact of different groups (Chart 2). Halogens (F to I) were
toluene to 50 °C, however, led to a significant drop in conver- tolerated giving good to excellent yields (6b–e). Extremely elec-
sion to 56%. Acetonitrile was also tested as solvent for the reac- tron withdrawing 4-trifluoromethyl aniline (5f) gave a moder-
tion but did not yield any desired product. The tests above are ate conversion of 66%. 4-Nitroaniline (5g) resulted in no reac-
summarised in the ESI, Tables S1 and 2.† Chlorinated solvents tion under the reducing condition. Benzamide gave 20%
were obviated as complex 2b was found to lead to side reac- conversion to the desired product (6h), and no side-reactions
tions (vide infra).
were observed. High or near complete conversions were
Different hydrosilanes were also tested in THF at 50 °C. obtained using substrates with electron donating groups (5i–j).
Replacing phenylsilane with diphenylsilane decreased the con- Primary aliphatic and benzyl amine substrates (5k–n) led to
version from 93% to 36%. Polymethylhydrosiloxane (PMHS)— low to medium conversions. Heterocycles (5o & 5p) were
a by-product in the chemical industry—was an attractive candi- unreactive.
date which unfortunately did not react.
Secondary amine substrates were considered next. High
A catalyst loading evaluation demonstrated that 5 mol% conversion was obtained using N-methylaniline (13a) as sub-
was the optimal catalyst loading, as lower loadings led to high strate, and medium conversions were observed for piperidine
silane to rhodium ratios and thus reduced reaction rates (see and morpholine substrates (5r–s). N,N-Diphenylamine, N,N-
the stoichiometric studies section, and ESI†).
diisopropylamine and 2,2,6,6-tetramethylpiperidine (5q, 5t–u)
The effect of using chlorinated solvent. Given that the were unreactive, likely due to the steric congestion about
rhodium(III) complex [RhCl2(CH2Cl){C(NCH2CH2PPh2)2C2H2}] the nitrogen atom. N-Benzylethylamine and N,N-dibenzyla-
(12a) has previously been reported to result from oxidative mine (13n and 5v) afforded the desirable products with
addition of dichloromethane to an (inferred) 16-electron similar conversions to those observed using benzylamine
rhodium(I) species,73 we anticipated that our 16-electron (5n).
rhodium(I) complexes might similarly activate a C–Cl bond.
Conditions screening for methylation reactions. With the
Though complex 2b appeared to be stable in dichloromethane success in formylation reactions established, extension of the
for several hours at ambient temperature, at reflux 2b cleaves a scope of catalysed reactions to include methylation of amines
dichloromethane C–Cl bond to afford the Rh(III) complex was explored (Scheme 4), with the expectation that this would
[RhCl2(CH2Cl){C(NCH2PCy2)2C10H6}] (12b, Scheme 3). The require stronger reducing conditions to convert the formamide
reaction took place over 20 hours, product formation being group into a methylamino group. Starting with the optimal
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Chart 2 N-Formylation of amines with CO2. Amine substrates (0.5 mmol), PhSiH3 (0.5 mmol, 1 equiv.) and catalyst 2b (25 μmol, 5 mol%) were
heated with stirring in THF (2 mL, 50 °C) under CO2 atmosphere (balloon). Reactions were quenched by cooling to r.t. and adding deionised water
(40 μl, 2.2 mmol). 1,1,2,2-Tetrachloroethane (53 μL, 0.5 mmol) was then added as internal standard and the reaction mixtures were analysed using 1H
NMR spectroscopy. Isolated yield in parenthesis.
improvement in the formation of methylanilines (13a and 15a)
vs. formanilides (6a and 14a). At the same time, this increase
in phenylsilane resulted in a significant reduction in the
overall conversion to the products from 93% (1 equiv. PhSiH3)
to 31% (4 equiv. PhSiH3). When 4 molar equivalents of
Ph2SiH2 was used instead of 4 equiv. of PhSiH3, no substantial
change in the formation of methylanilines was observed.
Scheme 4 Methylation of amines.
To improve the overall conversion, toluene was tested as the
conditions for formylation of aniline (THF, 50 °C), the amount reaction solvent in order to raise the reaction temperature to
of phenylsilane was increased to 4 equivalents with respect to 90 °C. Phenylsilane was added in three separate experiments
the aniline substrate (Table 1). This resulted in a substantial (2, 4 and 8 equiv.) and the product distribution was found to
Table 1 Optimisation of reaction conditions for N-methylation of amines, using aniline as test substrate (cf. Scheme 4). The benchmark reaction
condition for subsequent amine methylation is highlighted in green
Entry
Solvent
THF
Hydrosilane
PhSiH3
Equiv.
Temp. (°C)
Conv. (%) (6a : 13a : 14a : 15a)
Total Conv. (%)
1
2
3
4
5
6
7
8
1
4
1
4
1
2
4
8
4
50
50
50
50
90
90
90
90
90
90
93 : <2 : <2 : <2
<2 : 19 : <2 : 12
36 : <2 : 0 : 0
65 : 4 : 0 : 0
93
31
36
69
98
70
89
73
80
35
Ph2SiH2
Tol
PhSiH3
91 : 0 : 7 : 0
55 : 2 : 10 : 3
28 : 14 : < 2: 47
0 : 36 : 0 : 37
12 : 29 : 3 : 36
32 : 0 : <2 : 3
10
11
Ph2SiH2
Ph3SiH
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be affected by the amount of PhSiH3 added. Highest conver- methylation products could be achieved. Indeed the reaction
sion with formation of the largest amount of N,N-dimethyl- with 4-fluoroaniline (5b) and 4-methoxyaniline (5j) led to sub-
aniline product (15a) was obtained when using 4 equivalents stantial improvements in the selectivity for the corresponding
of PhSiH3 (Table 1, entry 7, used as benchmark conditions for methylated products (13b, 13j, 15b and 15j). Benzylamine (5n)
subsequent methylation reactions of amines). The highest resulted selectively in the monomethylated product 13n. Using
selectivity for the methylation products 13a and 15a was piperidine (5r) and N-benzylmethylamine (13n) as substrates
achieved when using 8 equivalents of PhSiH3, but this came at led to improvement in selectivity towards the methylated pro-
the cost of overall conversion, which was significantly reduced ducts (13r and 15n) but still some formylation products
(Table 1, entry 8).
formed. The dibenzyl substrate, N,N-dibenzylamine 5v resulted
The use of 4 equiv. of Ph2SiH2 (instead of 4 equiv. of in clean conversion to the methylated product 13v. As
PhSiH3) resulted in a similar product distribution to that expected, some reagents led to lower conversions when higher
observed for using PhSiH3, but with a lower conversion amounts of PhSiH3 were used, but interestingly in the case of
(Table 1, entry 10); triphenylsilane afforded 32% of formani- 13n and 5v no substantial decrease was observed and in the
lide and the reaction only formed methylated products to a case of 5r a significant increase in the overall conversion was
minimal extent.
obtained.
Methylation reaction scope. The scope of the methylation
protocol was investigated for a range of amine substrates using
4 equiv. of PhSiH3, toluene solvent (90 °C) and CO2 atmo-
Stochiometric studies
Reactions of 2b with CO2, CS2 and CO. To gain insight to
sphere (balloon). Methylamine products were favoured over what processes may be in operation during catalysis, 2b was
formamide products in most cases (Chart 3). Anilines with exposed to carbon dioxide. In addition, CS2 and CO were inves-
halogen substituents at the para-position (5b–f) resulted in tigated to better understand the interactions of small mole-
lower yields. Apart from 4-fluoroaniline (5b) which afforded cules with the rhodium centre of 2b. In all cases, quantitative
formamide and methylanilines (6b, 13b and 15b), these sub- conversion to new products was observed as shown by 31P
strates gave methylated products only in low to medium con- NMR spectroscopy. Removal of solvent under reduced
versions. The nitro group on 4-nitroaniline (5g) was subjected pressure, however, resulted in reversion to the starting
to reduction, and further reacted under the methylation reac- complex 2b.
1
tion conditions as observed in the H NMR spectra. The main
Exposure of a benzene solution of 2b to CO2 atmosphere
products for this entry could not be identified. Interestingly, did not lead to noticeable colour change. The 31P{1H} NMR
benzamide (5h) did not react to give the desired product or spectrum, however, showed a new sharp doublet at δP = 30.0
1
1
decompose when used as a substrate under the same reaction with JRh–P = 96 Hz; whereas the H NMR spectrum (500 MHz)
conditions. displayed three new distinct doublet peaks at δH = 3.83, 4.14,
Aromatic substrates with electron donating substituents and 6.10, and two broad signals at δH = 2.53–2.62, and
were then explored. Using 2,4,6-trimethylaniline (5i) as sub- 2.72–2.88 which could indicate dynamic exchange of co-
strate afforded the dimethylated product cleanly with high ordinated and free CO2 in the solution. Poor solubility of 2b
conversion. 4-Methoxyaniline (5j), however, led to a mixture of precluded the acquisition of informative 13C{1H} NMR data.
formamide and methylaniline products. Hexylamine (5k)
afforded N,N-dimethylhexylamine in high conversion.
To better understand how CO2 might interact with 2b
during catalysis, the analogous CS2 molecule was used to gain
A
mixture of products (13–15n) was obtained on using benzyla- insights. In contrast to reaction with CO2, the formation of the
mine (5n) as substrate. Indole (5p) was inactive as a substrate, CS2 adduct [RhCl(η2-CS2){C(NCH2PCy2)2C10H6}] (16b) from 2b
as in the catalysed formylation reaction.
was well supported by NMR spectroscopic and X-ray crystallo-
Secondary amine substrates were also investigated. graphic data. When a suspension of 2b in C6D6 was treated
N-Methylaniline (13a) led to N,N-dimethylaniline (15a) in high with carbon disulfide, a rapid colour change from bright
conversion. Diphenylamine (5q), however resulted in a signifi- orange to pale orange was observed. The 31P{1H} NMR spec-
cant drop in product conversion. Similar observations were trum showed a sharp doublet at δP = 31.3 with 1JRh–P = 103 Hz,
1
also made on comparing the results of using piperidine (5r) while the H spectrum displayed two distinct doublet peaks at
and 2,2,6,6-tetramethylpiperidine (5u) substrates. This is, as δH = 4.32 and 4.40. The 13C{1H} NMR spectrum confirmed CS2
before, likely due to steric hindrance about the nitrogen atom coordination, displaying a rhodium-coupled doublet peak at
of the amine substrates impeding catalysis. Morpholine (5s) δC = 260.3 (1JRh–C = 12 Hz), close to those of literature examples
gave 33% of the target product. N-Benzylmethylamine (13n) of 18-electron Rh(I)(η2-CS2) complexes which give resonances
and dibenzylamine (5v) led to a mixture to formylated and at δC = 251.1 74 and 256.9.75 The small Rh–C coupling (cf.
methylated products in medium to high conversions.
24.1–38.8 Hz in reported complexes) may be a result of the
For the substrates that led to the methylation products, as weak binding of the CS2 ligand to rhodium, as evidenced by
well as the formylation products, the reactions were repeated its ready dissociation. A doublet carbene resonance was also
using 8 equiv. of PhSiH3. Although the benchmark optimi- observed at δC = 212.8 (1JRh–C = 60 Hz). The anticipated
sation reactions (Table 1) showed that this leads to lower con- carbene 13C–31P coupling was too small to be resolved.
versions, it also showed that higher selectivity towards the Infrared data for 16b in solution were obtained using neat
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Chart 3 N-Methylation of amines with CO2. Amine substrates (0.5 mmol), PhSiH3 (2 mmol, 4 equiv.) and catalyst 2b (25 μmol, 5 mol%) were heated
with stirring in toluene (2 mL, 90 °C) under CO2 atmosphere (balloon). Reactions were quenched by cooling to r.t. and adding deionised water
(40 μl, 2.2 mmol). 1,1,2,2-Tetrachloroethane (53 μL, 0.5 mmol) was then added as internal standard and the reaction mixtures were analysed using 1H
NMR spectroscopy (CDCl3). Isolated yield in parenthesis. Results marked with asterisks (*) were obtained using 8 equiv. of PhSiH3 (4 mmol). The pro-
ducts 6, 13 and 14 are only added to the chart if they formed. The unaccounted material was either starting material (major) or unidentified side-
products.
carbon disulfide as a solvent. However, the region in which the the IR spectrum of the precursor 2b and the free ligand 1b,
ν(CvS) and ν(RhCS) modes of an η2-CS2 complex should be such that no unambiguous assignments could be made.
observed (ca. 990–1280 cm−1 and 630–650 cm−1, respectively76)
Although 16b was highly air sensitive and readily lost CS2,
includes numerous bands of the pincer ligand, as observed in crystals of the complex that formed slowly in the reaction
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ent decomposition.80 Solution IR spectroscopy of the reaction
mixture showed a broad, weak band in the carbonyl region at
1988 cm−1 with a shoulder at 2000 cm−1. The weakness of the
bands and the fact that there were two and not one may
suggest that the sample was decomposing under the nitrogen
atmosphere in the infrared cell. Given the poor quality of these
spectroscopic data, the proposed formation of 17b remains
speculative. Another possible formulation is that CO had dis-
placed the chloride anion to give a dicarbonyl cationic species,
which may account for the second infrared CO stretching
Fig. 3 Molecular structure of 16b in a crystal of 16b·2(C6H6) (aryl and
cyclohexyl hydrogen atoms omitted, 50% displacement ellipsoids). band, though such a species would be expected to exhibit very
Positional disorder was observed between the CS2 and Cl ligands, and
this was modelled as two distinct positions for both ligands with partial
occupancies of 0.9 and 0.1. The CS2 and Cl ligands with 0.9 occupancies
low solubility in the nonpolar benzene solvent.
are shown. Selected bond lengths (Å) and angles (°): Rh1–C1 = 1.999(5),
C1–N1 = 1.359(6), C1–N2 = 1.353(6), Rh1–P1 = 2.3074(13), Rh1–P2 =
Studies of catalytic reaction routes
In addition to stoichiometric studies of 2b with CO2, and the
analogous CS2 and CO, the reactions of 2b with the other sub-
strates present during the formylation and methylation reac-
tions were investigated. We undertook this to obtain an under-
standing of mechanism and product selectivity during the cat-
alysed reactions.
A commonly held reaction mechanism for the N-formyla-
tion and N-methylation of amines with CO2 starts with metal
catalyst promoted hydrosilylation of CO2 to silyl formate.
There are then two possible routes leading to the final for-
mation of the N-methylamines where the amine substrate
reacts either with the silyl formate intermediate (Pathway 2) or
the reduced silyl ether intermediate (Pathway 1, Fig. 4).32,81–83
On treating complex 2b with 2 molar equivalents of aniline
in toluene-d8, no new products were formed – with no change
observed in either the 1H or 31P{1H} NMR spectra at room
temperature or after heating at 90 °C. The same observations
were also noted when aniline was added under the CO2 atmo-
sphere. On treating complex 2b with PhSiH3 in deuterated
toluene a mixture of products was formed, including multiple
2.3233(13), Rh1–Cl1
2.4437(15), S1–C70 = 1.704(6), S2–C70 = 1.569 (7), C1–Rh1–P1 =
81.94(14), C1–Rh1–P2 = 82.43(14), P1–Rh1–P2 = 164.32(5).
= 2.588(2), Rh1–C70 = 1.998(7), Rh1–S1 =
mixture proved stable enough for X-ray structural characteris-
ation. The crystal structure (Fig. 3) confirms the η2-coordi-
nation of the CS2 ligand through one of the CvS bonds. The
adopted stereochemistry places the π-acidic C70 pseudo-trans
to the sole π-donor (Cl). The ring system of the pincer ligand
exhibits a twist angle of 24.8° relative to the coordination
plane, larger than observed in the structures of the square-
planar complexes 2a (19.8°)54 and 2b (essentially no twist) pre-
sumably to accommodate the additional ligand. The Rh–C,
Rh–Cl and Rh–P bond distances are significantly larger than
those in 2a and 2b, as expected due to the increased coordi-
nation number. The Rh–C70 bond length is within the range
observed for the few structurally characterised examples of Rh
(η2-CS2) complexes,74,77,78 though the Rh–S1 bond length is
significantly longer than the range of 2.3662(11)–2.387(5) Å
observed in these cases, perhaps reflecting the aforementioned
weak binding.
Placing a suspension of 2b in benzene under an atmo-
sphere of carbon monoxide resulted in an immediate colour
change from orange to yellow,79 accompanied by replacement
of the 31P{1H} NMR resonance of 2b with a broad doublet at
δP = 52.5 (1JRh–P = 130 Hz). The methylene protons appeared as
a broad singlet peak in the 1H NMR spectrum at δH = 5.25. The
broad peaks suggest a dynamic process in which the CO
ligand is coordinating to the metal centre to form [RhCl(CO){C
(NCH2PCy2)2C10H6}] (17b) and dissociating to reform 2b
(Scheme 5). This exchange precluded the acquisition of infor-
mative 13C NMR spectral data. Furthermore, attempts to
isolate the product resulted in either reversion to 2b, or appar-
Fig. 4 Proposed reaction pathways for N-formylation and -methylation
of amines with CO2 and phenylsilane. Pathway 1 (green): reduction of
silyl formate to silyl ether, then reaction with amine substrate to form
the final methylamine product; Pathway 2 (pink): formation of forma-
mide by reacting amine substrate with silyl formate, then reduction of
formamide to methylamine. In this work, the transition metal catalyst
[M] is complex 2b.
Scheme 5 Reversible reactions of complex 2b with CS2 and CO.
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rhodium species containing metal bound hydrides, as con-
firmed by 1H and 31P NMR spectroscopy (ESI†).
The treatment of 2b with 2 molar equivalents of PhSiH3
under a CO2 atmosphere was then undertaken in order to
further evaluate the plausibility of the general mechanism pro-
posed in Fig. 4. A number of products formed, and based on
1H NMR spectra of the products at room temperature, the new
species included silyl formate in small quantities and the silyl
ether intermediate (Fig. 4 and Fig. S7†). The formation of silyl
formate was confirmed on observation of formic acid following
1
hydrolysis of the products using H NMR spectroscopy. By 31P
NMR spectroscopy, the formation of Rh bound reaction inter-
mediates at room temperature was confirmed, however, these
could not be isolated (Fig. S8†). Upon raising the reaction
temperature to 70 °C, all the new peaks that were seen in the
31P{1H} NMR spectrum at room temperature disappeared and
the only peak present in the 31P{1H} NMR spectrum at that
temperature was the doublet corresponding to the starting
complex 2b. This suggested that the reaction of 2b with
PhSiH3 and CO2 was rapid, and that the integrity of 2b was re-
Fig. 5 Reaction profile of N-methylation of aniline in toluene at 90 °C;
ca. 5 mol% of 2b, PhSiH3 (4 equiv.) and a CO2 balloon was used. 1,3,5-
trimethoxybenene (0.2 equiv.) was added as internal standard. The reac-
tion was monitored by 1H NMR spectroscopy.
established after the reaction, indicating the potential for 2b N-methylaniline product (13a, Scheme 6), indicating that
as a recyclable catalyst.
Pathway 2 of the mechanism is also plausible as a route for
Having established that 2b can readily promote the for- the methylation reaction.
mation of silyl formate on reaction with CO2 and PhSiH3, and
Time course studies. The mechanistic pathways for the
subsequently silyl ether, thus paving the way to the methylated methylation reaction (Scheme 4) were further investigated
product, we proceeded to explore whether the final using a time course study of the reaction under catalytic con-
N-methylamine product can form through Pathway 1 as part of ditions (toluene at 90 °C; 5 mol% of 2b, PhSiH3 (4 equiv.) and
the mechanism (Fig. 4). As a first step, PhSiH3 was added to a CO2 atmosphere) (Fig. 5). In the initial stages of the reaction
complex 2b under a CO2 atmosphere, and the mixture was (up to 5 h), where the concentration of PhSiH3 will be highest,
allowed to react for 16 h at 90 °C under analogous conditions only the methylation products N-methylaniline (13a) and N,N-
1
to those used for the methylation catalysis. The H NMR spec- dimethylaniline (15a) were observed. Significant amounts of
trum of the reaction mixture showed the presence of silyl the formylation products 6a and 14a formed in the final part
ethers, free PhSiH3 and no silyl formate (Fig. S9†). The CO2 of the time course (5–7 h), where the PhSiH3 concentration
atmosphere was removed from the system with 3 freeze– was lower. The only product that decreased in concentration
pump–thaw cycles. Aniline (5a) was then added and allowed to between 5 h and 7 h (22% vs. 3% respectively) was the methyl-
react for another 16 h, affording N-methylaniline (13a) and ation product 13a which was consumed as a precursor to 14a.
N,N-dimethylaniline (15a), with some unreacted aniline
These data match well with the mechanistic proposal
present (Fig. S10–12†). This suggests that the methyl group (Fig. 4), where both pathways require PhSiH3 for the formation
could be transferred from the intermediate silyl ethers onto of the N-methylation products. When the relative concen-
the nitrogen atom of amine substrates as shown in proposed tration of PhSiH3 drops, the reduction of the N-formyl pro-
Pathway 1 (Fig. 4). No formylation products were observed in ducts is slowed down, and build-up of the relative concen-
the reaction mixture, showing that Pathway 1 is not reversible.
tration of 6a and 14a occurs. This also links to the data seen
In order to investigate the viability of Pathway 2 (Fig. 4) the when comparing the effect of different molar equivalences of
N-formyl reaction product, formanilide (6a) was used as a sub- PhSiH3 used for the methylation reaction (Table 1, entries
strate (Scheme 6). Under a nitrogen atmosphere, with no 5–8). Lower relative ratios of PhSiH3 promoted the formylation
additional CO2 present, 3 molar equivalents of PhSiH3 were products, while higher molar equivalences of PhSiH3 resulted
added to 6a in the presence of the catalyst 2b. This led to a in the methylation products as the mayor or exclusive species
clean reduction reaction that resulted in the corresponding formed.
Observations gathered from the mechanistic investigations,
scope studies and the screening of reaction conditions were in
line with reaction pathways proposed herein and in the
literature.32,81–84 Both proposed pathways in Fig. 4 were shown
to be viable under the reaction conditions of catalysed methyl-
ation of amines; it is also clear that product conversions and
ratios are highly dependent on the identity of the substrates
but also the silane reagent and the relative quantity of it.
Scheme 6 Reduction of formanilide (6a) to N-methylaniline (13a) in
the presence of PhSiH3 and complex 2b.
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Under the catalysis conditions, catalyst 2b remained intact
(resting state) as a four-coordinate, chloride bound complex
after catalysis as shown in spectroscopic studies.
6 H. Kolbe and E. Lautemann, Annalen, 1859, 113, 125–127.
7 R. Schmitt and E. Burkard, Ber. Dtsch. Chem. Ges., 1877,
2699.
8 E. Blondiaux, J. Pouessel and T. Cantat, Angew. Chem., Int.
Ed., 2014, 53, 12186–12190.
9 O. Jacquet, C. Das Neves Gomes, M. Ephritikhine and
T. Cantat, ChemCatChem, 2013, 5, 117–120.
10 Y. Li, X. Fang, K. Junge and M. Beller, Angew. Chem., Int.
Ed., 2013, 52, 9568–9571.
11 F. D. Bobbink, S. Das and P. J. Dyson, Nat. Protocols, 2017,
12, 417–428.
12 H. G. Grant and L. A. Summers, Aust. J. Chem., 1980, 33,
613–617.
13 B. C. Chen, M. S. Bednarz, R. Zhao, J. E. Sundeen, P. Chen,
Z. Shen, A. P. Skoumbourdis and J. C. Barrish, Tetrahedron
Lett., 2000, 41, 5453–5456.
14 K. Kobayashi, S. Nagato, M. Kawakita, O. Morikawa and
H. Konishi, Chem. Lett., 1995, 24, 575–576.
15 C. Fang, C. Lu, M. Liu, Y. Zhu, Y. Fu and B.-L. Lin, ACS
Catal., 2016, 6, 7876–7881.
16 B. Testa, P.-A. Carrupt, P. Gaillard, F. Billois and P. Weber,
Pharm. Res., 1996, 13, 335–343.
17 E. S. Istvan and J. Deisenhofer, Science, 2001, 292, 1160–
1164.
18 U. Fagerholm, Pharm. Res., 2008, 25, 625–638.
19 A. Malkia, L. Murtomaki, A. Urtti and K. Kontturi,
Eur. J. Pharm. Sci., 2004, 23, 13–47.
Conclusions
In conclusion, the formation of the per-NHC rhodium complex
2b is described as well as an in-depth structural characteris-
ation of 2b, following up on its’ initial report in a preliminary
communication.54 The complex 2b was found to be an active
catalyst for the N-formylation and -methylation of a wide range
of amine substrates using CO2 as the carbon source. Selectivity
between the formylation and methylation reactions was
achieved by varying the solvent, temperatures and quantity of
hydrosilane added, and has the potential to lead to control of
product formation.
The reactivity of 2b was probed using a range of small mole-
cules as well as reagents for methylation of aniline. Complex
2b was found to reversibly coordinate CS2, CO2 and CO on the
basis of crystallographic and spectroscopic data; 2b can also
activate C–Cl and Si–H bonds via oxidative addition, making it
a potential catalyst for other reactions. In the context of
N-methylation of aniline, 2b reacted with PhSiH3 and CO2, but
not aniline. Rhodium-mediated hydrosilylation of CO2 was
likely the first step of the N-methylation reaction catalysed by
2b and precedes attack by aniline (or other amine substrates).
Plausible pathways for the 2b catalysed formylation and
methylation of amines have been proposed by examining the
results of stoichiometric studies and the reaction profile as
well as results from reaction condition optimisation.
20 E. J. Barreiro, A. E. Kummerle and C. A. M. Fraga, Chem.
Rev., 2011, 111, 5215–5246.
21 H. Schoenherr and T. Cernak, Angew. Chem., Int. Ed., 2013,
52, 12256–12267.
22 E. Buchdunger, J. Zimmermann, H. Mett, T. Meyer,
M. Mueller, U. Regenass and N. B. Lydon, Proc. Natl. Acad.
Sci. U. S. A., 1995, 92, 2558–2562.
Conflicts of interest
23 J.
Zimmermann,
E.
Buchdunger,
H.
Mett,
There are no conflicts to declare.
T. Meyer and N. B. Lydon, Bioorg. Med. Chem. Lett., 1997, 7,
187–192.
24 J. Zimmermann, E. Buchdunger, H. Mett, T. Meyer,
N. B. Lydon and P. Traxler, Bioorg. Med. Chem. Lett., 1996,
6, 1221–1226.
25 J. Zimmermann, P. Furet and E. Buchdunger, ACS Symp.
Ser., 2001, 796, 245–259, DOI: 10.1021/bk-2001-0796.ch015.
26 F. Fache, L. Jacquot and M. Lemaire, Tetrahedron Lett.,
1994, 35, 3313–3314.
Acknowledgements
We gratefully acknowledge the Australian Research Council
(DP110101611) for funding. R. H. L. acknowledges Macquarie
University Research Training Pathway Scholarship.
27 R. A. da Silva, I. H. S. Estevam and L. W. Bieber,
Tetrahedron Lett., 2007, 48, 7680–7682.
Notes and references
1 Q. Liu, L. Wu, R. Jackstell and M. Beller, Nat. Commun., 28 J. R. Harding, J. R. Jones, S.-Y. Lu and R. Wood, Tetrahedron
2015, 6, 5933–5947. Lett., 2002, 43, 9487–9488.
2 P. Braunstein, D. Matt and D. Nobel, Chem. Rev., 1988, 88, 29 T. Maarten and R. Govind, EP 375333A2, 1990.
747–764.
30 O. Jacquet, C. Das Neves Gomes, M. Ephritikhine and
T. Cantat, J. Am. Chem. Soc., 2012, 134, 2934–2937.
31 C. Das Neves Gomes, O. Jacquet, C. Villiers, P. Thuery,
M. Ephritikhine and T. Cantat, Angew. Chem., Int. Ed.,
2012, 51, 187–190.
3 Y. Li, X. Cui, K. Dong, K. Junge and M. Beller, ACS Catal.,
2017, 7, 1077–1086.
4 T. Sakakura and K. Kohno, Chem. Commun., 2009, 1312–
1330.
5 M. Aresta, A. Dibenedetto and A. Angelini, Chem. Rev., 32 O. Jacquet, X. Frogneux, C. Das Neves Gomes and
2014, 114, 1709–1742. T. Cantat, Chem. Sci., 2013, 4, 2127–2131.
Green Chem.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Green Chemistry
Paper
33 Y. Li, I. Sorribes, T. Yan, K. Junge and M. Beller, Angew. 59 P. Bazinet, T.-G. Ong, J. S. O’Brien, N. Lavoie, E. Bell,
Chem., Int. Ed., 2013, 52, 12156–12160.
G. P. A. Yap, I. Korobkov and D. S. Richeson,
Organometallics, 2007, 26, 2885–2895.
60 A. F. Pozharskii and V. V. Dal’nikovskaya, Usp. Khim., 1981,
50, 1559–1560.
61 For a regular pentagon and hexagon, the corresponding
angles α are 72 and 60, respectively.
62 V. M. Ho, L. A. Watson, J. C. Huffman and K. G. Caulton,
New J. Chem., 2003, 27, 1446–1450.
34 K. Beydoun, G. Ghattas, K. Thenert, J. Klankermayer and
W. Leitner, Angew. Chem., Int. Ed., 2014, 53, 11010–11014.
35 K. Beydoun, T. vom Stein, J. Klankermayer and W. Leitner,
Angew. Chem., Int. Ed., 2013, 52, 9554–9557.
36 L. Zhang, Z. Han, X. Zhao, Z. Wang and K. Ding, Angew.
Chem., Int. Ed., 2015, 54, 6186–6189.
37 K. Beydoun, K. Thenert, E. S. Streng, S. Brosinski,
W. Leitner and J. Klankermayer, ChemCatChem, 2016, 8, 63 A. Prades, M. Poyatos, J. A. Mata and E. Peris, Angew.
135–138. Chem., Int. Ed., 2011, 50, 7666–7669.
38 Z. Yang, B. Yu, H. Zhang, Y. Zhao, Y. Chen, Z. Ma, G. Ji, 64 H. Valdes, M. Poyatos and E. Peris, Organometallics, 2013,
X. Gao, B. Han and Z. Liu, ACS Catal., 2016, 6, 1268–
1273.
39 T. V. Q. Nguyen, W.-J. Yoo and S. Kobayashi, Adv. Synth.
Catal., 2016, 358, 452–458.
32, 6445–6451.
65 For a review of these methods see: E. Peris, Top. Organomet.
Chem., 2007, 21, 83.
66 H. Werner, Angew. Chem., Int. Ed., 2010, 49, 4714–4728.
40 T. V. Q. Nguyen, W.-J. Yoo and S. Kobayashi, Angew. Chem., 67 K. Q. Vuong, M. G. Timerbulatova, M. B. Peterson,
Int. Ed., 2015, 54, 9209–9212.
41 X. Frogneux, O. Jacquet and T. Cantat, Catal. Sci. Technol.,
2014, 4, 1529–1533.
42 P. Daw, S. Chakraborty, G. Leitus, Y. Diskin-Posner, Y. Ben-
David and D. Milstein, ACS Catal., 2017, 7, 2500–2504.
M. Bhadbhade and B. A. Messerle, Dalton Trans., 2013, 42,
14298–14308.
68 R. H. Lam, D. B. Walker, M. H. Tucker, M. R. D. Gatus,
M. Bhadbhade and B. A. Messerle, Organometallics, 2015,
34, 4312–4317.
43 O. Santoro, F. Lazreg, Y. Minenkov, L. Cavallo and 69 L. D. Field, B. A. Messerle, M. Rehr, L. P. Soler and
C. S. J. Cazin, Dalton Trans., 2015, 44, 18138–18144. T. W. Hambley, Organometallics, 2003, 22, 2387.
44 H. Liu, Q. Mei, Q. Xu, J. Song, H. Liu and B. Han, Green 70 S. Burling, L. D. Field, B. A. Messerle and P. Turner,
Chem., 2017, 19, 196–201. Organometallics, 2004, 23, 1714–1721.
45 S. Zhang, Q. Mei, H. Liu, H. Liu, Z. Zhang and B. Han, RSC 71 L. P. Soler, PhD thesis, University of Sydney, 1999.
Adv., 2016, 6, 32370–32373.
72 X.-D. Li, S.-M. Xia, K.-H. Chen, X.-F. Liu, H.-R. Li and
46 K. Motokura, N. Takahashi, D. Kashiwame, S. Yamaguchi,
L.-N. He, Green Chem., 2018, 20, 4853–4868.
A. Miyaji and T. Baba, Catal. Sci. Technol., 2013, 3, 2392– 73 J. Y. Zeng, M.-H. Hsieh and H. M. Lee, J. Organomet. Chem.,
2396. 2005, 690, 5662–5671.
47 L. Gonzalez-Sebastian, M. Flores-Alamo and J. J. Garcia, 74 M. Doux, N. Mezailles, L. Ricard and P. Le Floch,
Organometallics, 2015, 34, 763–769. Organometallics, 2003, 22, 4624–4626.
48 Z.-Z. Yang, B. Yu, H. Zhang, Y. Zhao, G. Ji and Z. Liu, RSC 75 A. F. Hill, A. J. P. White, D. J. Williams and
Adv., 2015, 5, 19613–19619.
49 I. Özdemir, B. Alici, N. Gurbuz, E. Cetinkaya and
B. Cetinkaya, J. Mol. Catal. A: Chem., 2004, 217, 37–40.
J. D. E. T. Wilton-Ely, Organometallics, 1998, 17, 3152–
3154.
76 K. K. Pandey, Coord. Chem. Rev., 1995, 140, 37–114.
50 W. A. Herrmann, J. Schuetz, G. D. Frey and E. Herdtweck, 77 C. Bianchini, D. Masi, C. Mealli, A. Meli and M. Sabat,
Organometallics, 2006, 25, 2437–2448. Organometallics, 1985, 4, 1014–1019.
51 T. Tu, J. Malineni, X. Bao and K. H. Dötz, Adv. Synth. Catal., 78 E. Lindner, B. Keppeler, H. A. Mayer, K. Gierling, R. Fawzi
2009, 351, 1029–1034.
and M. Steinmann, J. Organomet. Chem., 1996, 526, 175–
52 H. Tsurugi, S. Fujita, G. Choi, T. Yamagata, S. Ito,
183.
H. Miyasaka and K. Mashima, Organometallics, 2010, 29, 79 Though by no means definitive, in many cases decolouriza-
4120–4129.
tion of d6 or d8 complexes often accompanies an increase
in coordinative saturation.
53 G. Choi, H. Tsurugi and K. Mashima, J. Am. Chem. Soc.,
2013, 135, 13149–13161.
54 A. F. Hill and C. M. A. McQueen, Organometallics, 2012, 31,
8051–8054.
55 A. F. Hill and C. M. A. McQueen, Organometallics, 2014, 33,
1909–1912.
56 K. Verlinden and C. Ganter, J. Organomet. Chem., 2014,
750, 23–29.
80 (16) Decomposition was suggested by peaks in the NMR
spectra that matched those observed when 2b was exposed
to air, despite all manipulations being carried out under
nitrogen using standard Schlenk techniques. This
decomposition product could not be well characterised,
despite several attempts, but gave a distinctive 31P NMR
doublet resonance at P = 30 (1JRh–P = 102 Hz) and most
57 W. P. Fehlhammer and W. Finck, J. Organomet. Chem.,
1991, 414, 261–270.
likely corresponds to
{C(NCH2PCy2)2C10H6}].
a dioxygen adduct [RhCl(2-O2)
58 P. Bazinet, G. P. A. Yap and D. S. Richeson, J. Am. Chem. 81 W. Sattler and G. Parkin, J. Am. Chem. Soc., 2012, 134,
Soc., 2003, 125, 13314–13315.
17462–17465.
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82 R. Lalrempuia, M. Iglesias, V. Polo, P. J. Sanz Miguel, 83 K. Motokura, D. Kashiwame, A. Miyaji and T. Baba, Org.
F. J. Fernandez-Alvarez, J. J. Perez-Torrente and Lett., 2012, 14, 2642–2645.
L. A. Oro, Angew. Chem., Int. Ed., 2012, 51, 12824– 84 M. Hulla, G. Laurenczy and P. J. Dyson, ACS Catal., 2018, 8,
12827.
10619–10630.
Green Chem.
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