Catalytic Conversion of Ethanol to n-Butanol Using Ruthenium P-N Ligand Complexes

Article abstract of DOI:10.1021/acscatal.5b01327

We report several ruthenium catalysts incorporating mixed donor phosphine-amine ligands for the upgrade of ethanol to the advanced biofuel n-butanol, which show high selectivity (≥90%) at good (up to 31%) conversion. In situ formation of catalysts from mixtures of [RuCl₂(η⁶-p-cymene)]₂ and 2-(diphenylphosphino)ethylamine (1) shows enhanced activity at initial water concentrations higher than those of our previously reported diphosphine systems. Preliminary mechanistic studies (electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy) suggest the possibility of ligand-assisted proton transfer in some derivatives.

Full text of DOI:10.1021/acscatal.5b01327

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Article
Catalytic Conversion of Ethanol to n-Butanol
using Ruthenium P-N ligand complexes
Richard L Wingad, Paul J. Gates, Steven T. G. Street, and Duncan F Wass
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01327 • Publication Date (Web): 19 Aug 2015
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Catalytic Conversion of Ethanol to n-Butanol using Ruthenium P-N
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Richard L. Wingad, Paul J. Gates, Steven T. G. Street and Duncan F. Wass*.
School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK. Email: duncan.wass@bristol.ac.uk;
www.wassresearchgroup.com
ABSTRACT: We report several ruthenium catalysts incorporating mixed donor phosphine-amine ligands for the upgrade
of ethanol to the advanced biofuel n-butanol, which show high selectivity (90%+) at good (up to 31%) conversion. In-situ
formation of catalysts from mixtures of [RuCl (η -p-cymene)] and 2-(diphenylphosphino)ethylamine (1) show enhanced
activity at higher initial water concentrations than our previously reported diphosphine systems. Preliminary mechanistic
studies (ESI-MS and NMR spectroscopy) suggest the possibility of ligand assisted proton transfer in some derivatives.
6
2
2
KEYWORDS: Bio-fuels, Ethanol upgrading, Butanol, Ruthenium, Guerbet Catalysis.
INTRODUCTION
Our approach has been to seek ‘Guerbet’ type catalysts for
the conversion of readily available ethanol into this more
Obtaining liquid fuels from renewable biomass sources re-
mains an important component of future energy provision.
Bio ethanol has long been used as a sustainable replacement
for conventional gasoline, often in the form of a blend of the
two. However, ethanol has a number of significant draw-
backs compared to gasoline: it has a lower energy density
5
1
advanced biofuel. This methodology is sometimes termed
6
‘
Borrowed Hydrogen’ chemistry. In a typical catalytic reac-
tion scheme, an alcohol is dehydrogenated to form an alde-
hyde which then undergoes aldol coupling before the result-
ant product is re-hydrogenated to give a longer chain alcohol
(Scheme 1).
(
70% that of gasoline), it can be corrosive to current engine
technology and fuel infrastructure, and it readily absorbs
water leading to separation and dilution problems in storage
tanks. Higher alcohols such as butanol have fuel properties
that more closely resemble those of gasoline and can allevi-
Unfortunately, ethanol is a specifically difficult substrate for
7
this chemistry, primarily because base-catalyzed aldol con-
densation of acetaldehyde is notoriously difficult to control,
8
leading to mixtures of oligomeric and polymeric products.
2
ate many of these problems associated with ethanol. For
These factors combine to make achieving good selectivity in
ethanol-to-butanol conversion extremely challenging. Some
progress has been reported in work by Ishii using a homoge-
neous iridium catalyst, [Ir(COD)(acac)], in the presence of a
phosphine ligand, the additive 1,7-octadiene, and an alkoxide
base to achieve n-butanol selectivity of up to 67% at 12%
example n-butanol is essentially non-corrosive and immisci-
ble with water; the energy density of butanol is also closer to
that of gasoline (90%). This improved performance has led to
butanol being termed an ‘advanced biofuel’ and the commer-
cial availability of this material as a green ‘drop-in’ alterna-
3
9
tive to gasoline gathers pace. However, the bulk synthesis of
conversion. As expected, longer chain alcohols are formed
butanol from biosustainable feedstocks remains challenged
by low conversion and variable selectivity.
as side products of uncontrolled aldol reactions. More re-
cently, heterogeneous catalysts have been reported which
4
10
achieve selectivity of up to 80% at 25% ethanol conversion.
In addition iridium catalysts have been reported which can
11
be used in water and air. We reported a new family of ho-
mogeneous ruthenium-based catalysts which demonstrated a
step change in performance, achieving the upgrading of eth-
anol to n-butanol in over 94% selectivity at good conver-
1
2
sion. Key to this high selectivity was the use of small bite
angle 1,1-bis(diphenylphosphino)methane (dppm) ligands,
diphosphines with larger bite angles being less effective. As
part of our investigation into a wider variety of ligands, we
have discovered that mixed donor P-N ligands are also highly
selective.
Scheme 1. The Guerbet reaction.
1
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Table 1. Ruthenium catalyzed conversion of ethanol to n-butanol.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a
b
c
Run
1
Catalyst
Conversion (Yield)%
25.1 (21.9)
20.4 (17.5)
9.8 (9.8)
Selectivity %
91.1
TON
251
204
98
6
1 + [RuCl (η -p-cymene)]
2
2
d
2
6
dppm + [RuCl (η -p-cymene)]
90.0
>99
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
e
3
6
1 + [RuCl (η -p-cymene)]
2
2
2
f
6
4
1 + [RuCl (η -p-cymene)]
4.9 (4.9)
>99
49
2
e
6
5
dppm + [RuCl (η -p-cymene)]
5.7 (5.3)
95.1
57
2
2
2
f
6
6
dppm + [RuCl (η -p-cymene)]
4.3 (4.0)
95.2
43
2
6
7
8
9
2 eq. 1 + [RuCl (η -p-cymene)]
22.7 (20.8)
23.8 (18.8)
16.7 (12.1)
19.7 (17.5)
31.4 (28.1)
23.6 (20.5)
18.8 (17.1)
20.5 (18.0)
13.3 (1.6)
94.3
227
238
167
197
314
236
188
205
133
211
2
2
6
2 + [RuCl (η -p-cymene)]
84.9
79.6
92.4
92.7
2
2
6
3 + [RuCl (η -p-cymene)]
2
2
6
1
0
1
4 + [RuCl (η -p-cymene)]
2
2
6
1
5 + [RuCl (η -p-cymene)]
2
2
12
6
7
90.8
93.5
13
14
15
g
6
1 + [RuCl (η -p-cymene)]
91.7
2
2
h
8
12.4
16
a
10
21.1 (19.5)
95.1
b
Total conversion of ethanol to Guerbet products (see supporting information), yield of n-butanol in parenthesis. Total selectiv-
ity to n-butanol in the liquid fraction determined by GC. Turnover number (TON) based on mmol of substrate converted to
products per mmol of Ru. Data taken from reference 12, dppm = 1,1-bis(diphenylphosphino)methane. 1.84 ml water added (5%
c
d
e
f
g
h
by volume). 3.89 ml water added (10% by volume). 10 g 3
ing information).
Å
molecular sieves added. Major product is ethyl acetate (see support-
RESULTS AND DISSCUSSION
Our initial catalyst screening utilized the same procedure we
reported for diphosphine ligands: [RuCl (η -p-cymene)] as a
6
2
2
catalyst precursor with a range of P-N donor ligands (Chart
), sodium ethoxide base and a run time of 4 hours at 150 °C.
1
Results are presented in Table 1. Liquid phase selectivities to
n-butanol are reported in line with previous reports for direct
comparison.
9,11-12
Comparison of ligand 1 with dppm (compare runs 1 and 2)
reveals that 1 has very similar performance under these con-
ditions. Further investigation under a more varied set of
conditions reveals that 1 offers certain advantages in terms of
maintaining good conversion and selectivity compared to
dppm at higher water concentrations (compare runs 3 and 5);
this offers distinct technological advantages in industrial
application. However when the concentration of water is
increased further both systems give similar results (compare
runs 4 and 6). 1
Chart 1. Ligands and complexes used in this study.
Some catalyst decomposition is observed through all runs, so
that the reaction solution after 4 hours has a black turbid
appearance. In our previous paper we reported that the use
of bis-chelate ruthenium complexes such as trans-
RuCl (dppm) ] provided more stable catalysts, albeit at the
2 2
cost of rate. When 2 equivalents of 1 were used in conjunc-
tion with [RuCl (η -p-cymene)] we were pleased to find that
the reaction still proceeded with very high selectivity (94.3
) to give an n-butanol yield of 20.8 % after 4 hours (run 7).
This compares favorably to our dppm system which gives
1
2
[
6
2
2
%
12
considerably lower n-butanol yields over 4 hours. In all runs
with two equivalents of ligand the final reaction mixture is
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46
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48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. In situ monitoring of ruthenium complex 6 by ESI mass spectrometry.
clear yellow, with little to no evidence of ruthenium metal
deposition, suggesting a homogeneous system.
Ligand 1 clearly has the potential to be deprotonated under
these reaction conditions, and operate via an ‘outer sphere’
mechanism which is common for catalysts of this type in
1
3
hydrogen transfer reactions. This is believed to be the case
here as will be discussed later, and suggested ligands 2 and 3
with partial and full methylation of the amine may provide
insights. The performance of 2 (run 8) is very similar to 1,
whereas 3 (run 9) gives reduced conversion and selectivity,
albeit not the dramatic switch in performance that may have
been expected if an outer sphere mechanism is essential.
Given the good performance of dppm, the fact that ligand
deprotonation is not critical but may facilitate catalysis
seems reasonable. The related ligands 4 and 5 also perform
well; 5 (run 11) with a conversion of 31.4% and 92.7% selectivi-
ty out-performs even the dppm catalyst. Unfortunately, this
particular ligand decomposes during the run to yield traces
of foul-smelling 3-methylindole, making it an unattractive
derivative for further development. The ruthenium com
6
14
15
plexes [RuCl(η -p-cymene)(1)]Cl 6 and [RuCl (1) ] 7 (Chart
1
in 1.16 g, 14.1 mmol sodium acetate in addition to liquid
products. This result is consistent with the hydrogen also
2
2
1
7
) may be pre-formed by known methods before executing
18
the catalysis. Under our standard catalytic conditions, identi-
cal results within error compared to in situ formed catalysts
are obtained (compare runs 1 and 12, and 7 and 13).
produced during the reaction, and implies parallel Canniz-
zaro or Tishchenko-type pathways (Scheme 2) in addition
to Guerbet chemistry.
19
20
Perhaps the most surprising feature of these catalysts is that
they yield n-butanol as the major product and yet are similar
to reported ruthenium catalysts based on tridentate P-N-P
ligands which produce only ethyl acetate in excellent selec-
16
tivity. A closer examination of the full mass balance for our
catalysts reveals that some acetate products are formed as
solid sodium acetate by-products; for example, run 1 results
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active organometallic species are present. The pre-catalyst 6
3
1
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
6
is observed before addition of base with a distinct P{ H}
NMR spectroscopy resonance at 61.4 ppm; mass spectrome-
6
try reveals a parent ion peak at m/z 500.08 due to [RuCl(η -
+
p-cymene)(1)] . Upon addition of NaOEt at room tempera-
6
+
ture, the ethoxide complex [Ru(η -p-cymene)(1)(OEt)] is not
observed but rather the only species visible in the NMR spec-
trum is a hydride (δ = 74.0 ppm, δ = -9.01 (d, J = 45 Hz),
the mass spectrum of which corresponds to [Ru(η -p-
cymene)(1)(H)] (9: m/z 466.12). After 15 minutes of heating
other species are also observed in the mass spectrum, specifi-
cally a signal at m/z 492.14. Initially we proposed the for-
mation of an amido carbonyl complex, [Ru(η -p-
cymene)(Ph P(CH ) NH)(CO)] (exact mass 492.10),
2
p
H
PH
6
+
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
+
in
2
2 2
which the phosphinoamine ligand is deprotonated at nitro-
gen; the decarbonylation of alcohols by related ruthenium
species is known and we presumed a similar process was
21
operating here. However on isolation of this species and
1
3
1
further analysis (IR, C{ H} and 2D NMR spectroscopy, see
supporting information) the balance of evidence suggests
this species may be a metallated alkyl amine complex
(Scheme 3) in which a CHCH group has inserted into the
3
6
+
Ru-N bond, [Ru(η -p-cymene)(Ph P(CH ) NHCHCH )] 10
2
2
2
3
which is also consistent with the MS data (exact mass
92.14). Complex 10 is catalytically active giving similar yield
and selectivity to complex 6 (run 16) and our working hy-
pothesis is that 10 is an off-cycle catalyst resting state. After
4
4
0 minutes virtually all of the hydride is converted to 10,
Scheme 2. Tishchenko (a) and Cannizzaro (b) type
pathways for the formation of sodium acetate.
which is persistent throughout the reaction time (22 hours).
After 22 hours, a multitude of species is observed: species 10,
as well as compounds consistent with bis-chelate complexes
(m/z 617.16, 645.17 and 673.20) formed via ligand re-
distribution reactions. As shown previously bis-chelate com-
plexes are catalytically active for this process (Table 1, run 13).
Of particular interest is the observation of mass spectrum
data consistent with a bis-chelate complex containing a
As expected, the generation of free water facilitates these
side reactions, and conducting a standard run in the pres-
ence of 3Å molecular sieves considerably reduces acetate
make (0.53 g, 6.4 mmol) whilst retaining excellent conver-
sion and selectivity within the liquid fraction to n-butanol
(run 14). We remained surprised that no n-butanol product
was reported with tridentate ligand complexes given our
observations, and further speculated that the different reac-
tion conditions employed in this ethanol to ethyl acetate
catalysis (typically lower temperature and an open vessel to
allow hydrogen to escape) might be influential. We therefore
tested the tridentate ligand complex 8 (Chart 1) under our
conditions (run 15). Even under these conditions, ethyl ace-
tate is produced in higher yield than n-butanol (5.4% yield,
bound C dioxygenated fragment (m/z 645.17); this opens the
4
tantalizing possibility of a metal-templated aldol condensa-
tion, something we have previously speculated might be a
reason for the excellent selectivity of these systems. Attempts
to isolate and further study such species are underway.
4
0.6% selective) in stark contrast to runs with our catalysts.
This implies a genuine ligand effect on selectivity, and defin-
ing the origin of this effect is an ongoing objective.
In Situ Monitoring of Catalyst: In order to gain mechanis-
tic insight, particularly into the active form of the catalyst
and the origin of the excellent selectivity observed, a series of
ongoing catalytic runs were studied using a combination of
Scheme 3. Evolution of the ruthenium pre-catalyst.
CONCLUSION
1
31
1
H and P{ H} NMR spectroscopy and ESI mass spectrometry
Figure 1). Reaction conditions were changed slightly in order
We have explored the use of mixed donor phosphine-amine
ligands in the ruthenium catalyzed upgrading of ethanol to
the advanced biofuel n-butanol; these systems achieve per-
formance which matches our previously reported ruthenium
-1,1-bis(diphenylphosphino)methane catalysts, as well as giv-
ing certain advantages in terms of water tolerance. Prelimi-
nary mechanistic studies suggest the possibility of ligand
assisted proton transfer in some derivatives. The reasons for
the excellent selectivity of such catalysts on the production
(
to simplify this study and allow the use of low pressure ves-
sels, so that a reaction temperature of 78 °C was used. Under
these conditions, the catalyst still converts ethanol to n-
butanol with good selectivity (96.7%) albeit at much lower
conversion after 4 hours (see supporting information); never-
theless, the products obtained, being very similar to those
from a standard run, lead us to believe the same catalytically
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of n-butanol as opposed to ethyl acetate, observed in very
(7) Carlini, C.; Di Girolamo, M.; Macinai, A.; Marchionna, M.;
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similar systems are an area of continued study.
Noviello, M.; Maria, A.; Galletti, R.; Sbrana, G. J. Mol. Catal.
2003, 200, 137-146.
(8) (a) Furukawa, J.; Saegusa, T.; Tsuruta, T.; Fujii, H.; Tatano,
ASSOCIATED CONTENT
T.; J. Polym. Sci. 1959, 36, 546. (b) Degering, E. F.; Stoudt, T. J.
Polym. Sci. 1951, 7, 653-656.
Supporting Information.
Experimental details – catalysis, kinetic plots, monitoring
experiments, spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(
9) Koda, K.; Matsu-ura, T.; Obora, Y.; Ishii, Y. Chem. Lett.
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(
R.; Kordas, K.; Szabo, M.; Sapi, A.; Arve, K.; Warna, J.; Mikkola,
J.-P. Catalysts 2012, 2, 68-84; for a recent review, see: Galadima,
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AUTHOR INFORMATION
Corresponding Author
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(11) Xu, G.; Lammens, T.; Liu, Q.; Wang, X.; Dong, L.; Caiazzo,
A.; Ashrat, N.; Guan, J.; Mu, X. Green Chem. 2014, 16, 3971-3977.
(12) Dowson, G. R. M.; Haddow, M. F.; Lee, J.; Wingad, R. L.;
Wass, D. F. Angew. Chem. Int. Ed. 2013, 52, 9005-9008; Angew.
Chem. 2013, 125, 9175-9178.
(13) (a) Bacchi, A.; Balordi, M.; Cammi, R.; Elviri, L.; Pelizzi, C.;
Picchioni, F.; Verdolino, V.; Goubitz, K.; Peschar, R.; Pelagatti, P.
Eur. J. Inorg. Chem. 2008, 4462-4473. (b) Hounjet, L. J.; Bieren-
stiel, M.; Ferguson, M. J.; McDonald, R.; Cowie, M. Inorg. Chem.
*
E-mail: Duncan.Wass@bristol.ac.uk
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
2
010, 49, 4288-4300. (c) Hounjet, L. J.; Ferguson, M. J.; Cowie, M.
Organometallics 2011, 30, 4108-4114.
14) Abdur-Rashid, K.; Guo, R.; Chen, X.; Jia, W. Can. Pat.
Appl. 2009, CA2636947 A1 20090106.
15) Saudan, L.; Dupau, P.; Riedhauser, J. J.; Wyss, P. US. Pat.
Appl. Publ. 2010, US20100280273 A1 20101104.
16) (a) Nielsen, M.; Kammer, A.; Cozzula, D.; Junge, H.;
ACKNOWLEDGMENT
BP Biofuels are thanked for funding. The Bristol Chemical
Synthesis Centre for Doctoral Training is thanked for fund-
ing a research sabbatical (EP/G036764/1) to S.T.G.S. Glenn
Sunley, Russell Taylor, Renan Cariou, Aidan Hurley and Hel-
en Mason (all BP) are thanked for useful discussions.
(
(
(
Gladiali, S.; Beller, M. Angew. Chem. Int. Ed. 2011, 50, 9593–9597.
(b) Nielsen, M.; Junge, H.; Kammer, A.; Beller, M. Angew. Chem.
Int. Ed. 2012, 51, 5711–5713. For related ethanol to ethyl acetate
catalysts, see: (c) Spasyuk, D.; Smith, S.; Gusev, D. G. Angew.
Chem. Int. Ed. 2013, 52, 2538-2542. (d) Kossoy, E.; Diskin-Posner,
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(
17) 2.04 g of solid is obtained at the end of run 1. H NMR
(2) (a) Many examples including: Atsumi, S.; Cann, A. F.;
spectroscopy of a weighed amount of this solid with a known
amount of DMSO added as a standard in methanol-d (see
Connor, M. R.; Shen, C. R.; Smith, K. M.; Brynildsen, M. P.;
Chou, K. J. Y.; Hanai, T.; Liao, J. C. Metabolic Engineering 2008,
4
supporting info) shows the solid to contain sodium acetate (1.16
g, 14.1 mmol), sodium formate (0.01 g, 0.1 mmol) and sodium
butanoate (0.08 g, 0.7 mmol). The remaining solid is presumably
sodium hydroxide, formed from the hydrolysis of sodium
ethoxide either during the reaction or during work up of the post
reaction mixture. The maximum amount of sodium hydroxide
should therefore be 0.60 g, assuming the remaining sodium
equivalents are in this form. 0.19 g of solid remains unidentified.
In line with previous reports and to allow comparisons to be
made with other literature catalysts, we have reported selectivity
in the liquid fraction in Table 1; including solid products in the
selectivity calculation gives an overall selectivity to n-butanol of
1
0, 305-311; (b) Harvey, B. G.; Meylemans, H. A. J. Chem. Technol.
Biotechnol. 2011, 86, 2−9
(3) (a) Ramey, D. E. US National Agricultural Biotechnology
Council (NABC) Reports, Agricultural Biofuels: Technology,
Sustainability and Profitability, Part III: Technology: Biomass,
Fuels and Co-Products: Butanol: The Other Alternative Fuel
2007, 19, 136-147. (b) Szulczyk, K. R. Int. J. Energy Environ. 2010,
1
, 501-512. (c) Verma, R. P. 2007, International Symposium on
Biofuels, Butanol – A Possible Alternative Energy Source, Sep-
tember 25-26, New Delhi, India.
(4) (a) Green, E. M. Curr. Opin. Biotech. 2011, 22, 337-343. (b)
Jin, C.; Yao, M.; Liu, H.; Lee, C.-F.; Ji, J. Renew. Sust. Energ. Rev.
2011, 15, 4080-4106.
7
5.6%
18) During the reactions a pressure build up is noted, for
example, for run the internal reactor pressure reaches
(
(5) (a) Guerbet, M. C. R. Acad. Sci. Paris 1899, 128, 1002-1004.
1
(b) Guerbet, M. M. C. R. Acad. Sci. Paris 1909, 149, 129–132. (c)
approximately 10 bar. After cooling approximately 4 bar pressure
remains. GC analysis shows the gas to be a combination of
hydrogen and methane (ratio 1250:1).
(19) (a) Cannizzaro, S. Justus Liebigs Ann. Chem. 1853, 88, 129-
130. (b) Cook, J.; Hamlin, J. E.; Nutton, A.; Maitlis, P. M. J. Chem.
Soc., Dalton Trans. 1981, 2342-2352.
Veibel, S.; Nielsen, J. I. Tetrahedron 1967, 23, 1723–1733. (d)
O'Lenick Jr., A. J. J. Surfactants Deterg. 2001, 4, 311-315. (e)
Gabriëls, D.; Hernández, W. Y.; Sels, B.; Van Der Voort, P.; Ver-
berckmoes, A. Catal. Sci. Technol. 2015, 5, 3876-3902.
(6) Review of borrowed hydrogen activation of alcohols: (a)
Haniti, M.; Hamid, S. A.; Slatford, P. A.; Williams, J. M. J. Adv.
Synth. Catal. 2007, 349, 1555-1575. For related ruthenium dehy-
drogenation-based chemistry, see: (b) Morton, D.; Cole-
Hamilton, D. J.; Utuk, I. D.; Paneque-Sosa, M.; Lopez-Poveda, M.
J. Chem. Soc., Dalton Trans. 1989, 489-495. (c) Edwards, M. G.;
Jazzar, R. F. R.; Paine, B. M.; Shermer, D. J.; Whittlesey, M. K.;
Williams, J. M. J.; Edney, D. D. Chem. Commun. 2004, 1, 90-91.
(20) (a) Claisen, L. Ber. 1887, 20, 646-650. (b) Tischtshenko,
W. J. Russ. Phys. Chem. 1906, 38, 355-418. (c) Tischtshenko, W.
Chem. Zentralbl. 1906, 77, 1309. (d) Seki, T.; Nakajo, T.; Onaka,
M. Chem. Lett. 2006, 35, 824-829.
(21) (a) Sieffert, N.; Réocreux, R.; Lourusso, P.; Cole-Hamilton,
D. J.; Bühl, M. Chem. Eur. J. 2014, 20, 4141-4155. (b) Lau, C. P.; Ng,
S. M.; Jia, G.; Lin, Z. Coord. Chem. Rev. 2007, 251, 2223-2237. (c)
Chen, Y.-Z.; Chan, W.; Lau, C.; Chu, H.; Lee, H.; Jia, G.
Organometallics 1997, 16, 1241-1246.
(d) Black, P. J.; Edwards, M. G.; Williams, J. M. J. Eur. J. Org.
Chem. 2006, 19, 4367-4378.
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