Palladium-catalyzed hydroesterification of alkynes employing aryl formates without the use of external carbon monoxide

Article abstract of DOI:10.1002/adsc.201000750

A highly efficient hydroesterification of alkynes employing aryl formates has been developed without the use of external carbon monoxide and at ambient pressure. The reaction in the presence of a palladium-xantphos catalyst system selectively affords α,β-unsaturated esters in good to high yields. Use of an aryl formate is crucial and alkyl formates did not react at all. The hydroesterification of norbornene and terminal alkenes also readily proceeded under similar reaction conditions. A mechanistic study showed that conversion of aryl formates to carbon monoxide and phenol derivatives occurred in the hydroesterification. Xantphos is highly effective as a ligand both in the conversion of aryl formates and the hydroesterification reactions.

Full text of DOI:10.1002/adsc.201000750

FULL PAPERS
DOI: 10.1002/adsc.201000750
Palladium-Catalyzed Hydroesterification of Alkynes Employing
Aryl Formates without the Use of External Carbon Monoxide
Yuko Katafuchi,a Tetsuaki Fujihara,a Tomohiro Iwai,a Jun Terao,a
and Yasushi Tsujia,*
a
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510,
Japan
Fax : (+81)-75-383-2514; phone: (+81)-75-383-2515; e-mail: ytsuji@scl.kyoto-u.ac.jp
Received: October 5, 2010; Published online: February 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000750.
Abstract: A highly efficient hydroesterification of al- study showed that conversion of aryl formates to
kynes employing aryl formates has been developed carbon monoxide and phenol derivatives occurred in
without the use of external carbon monoxide and at the hydroesterification. Xantphos is highly effective
ambient pressure. The reaction in the presence of a as a ligand both in the conversion of aryl formates
palladium-xantphos catalyst system selectively af- and the hydroesterification reactions.
fords a,b-unsaturated esters in good to high yields.
Use of an aryl formate is crucial and alkyl formates
did not react at all. The hydroesterification of nor- Keywords: alkynes; aryl formates; homogeneous cat-
bornene and terminal alkenes also readily proceeded alysis; hydroesterification; palladium; phosphane li-
under similar reaction conditions. A mechanistic gands
Introduction
plex[5a] and the addition of formamides to alkynes cat-
alyzed by a palladium complex.[5b] These reactions ef-
The hydroesterification of alkynes and alkenes is one ficiently afforded the corresponding syn-addition
of the most straightforward reactions affording one- products. Herein, we now report the formal syn-addi-
carbon elongated esters. Especially, the reaction of al- tion of formate esters to alkynes in the presence of a
kynes affords useful a,b-unsaturated esters. The hy- palladium-xantphos[6] catalyst system. Simply mixing
droesterification is usually carried out under carbon alkynes and readily available aryl formates with the
monoxide pressure with an alcohol in the presence of catalyst in a usual flask (no pressure bottle is re-
a transition metal catalyst.[1] However, a transforma- quired) realizes the highly efficient hydroesterifica-
tion without the use of external carbon monoxide is tion of the alkynes. The reaction can be carried out
highly desirable.[2]
without the use of external carbon monoxide, and no
Formic acid and its esters can be readily derived directing group is required.
from carbon monoxide.[3] Thus, formate esters may be
regarded as liquid condensates and expedient alterna-
tive sources of carbon monoxide and alcohols. Actual- Results and Discussion
ly, much effort has been made to realize the hydroes-
terification reaction employing formate esters[4] with- Optimization
out the use of external carbon monoxide. However,
so far the hydroesterification utilizing formate ester First, the reaction of phenyl formate (1a) with diphe-
has been suffering from low efficiency: often high re- nylacetylene (2a) was carried out in the presence of a
action temperature (>1808C)[4b–f,k] and/or even catalytic amount of [Pd
ACHTUNGTRENNUNG
Recently, we have developed the addition of acid Without an added ligand, no product was obtained at
chlorides to alkynes catalyzed by an iridium com- all (entry 1). With the addition of monodentate phos-
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Table 1. Effect of phosphanes on the palladium-catalyzed
hydroesterification of diphenylacetylene (2a) with phenyl
formate (1a).[a]
Other Pd precursors such as [PdCl
[Pd2A(dba)3] were not so effective.
2ACHTUNGTRENN(UNG PhCN)2] and
CTHUNGTRENNUNG
Under the same reaction conditions as in entry 10,
alkyl formates such as butyl formate (1b), 2-phenyl-
ethyl formate (1c), and benzyl formate (1d) did not
convert at all and the corresponding alkyl ester prod-
ucts (3) could not be obtained. Thus, uniquely phenyl
formate (1a) can be a good substrate in the present
hydroesterification. Notably, the resulting phenyl
ester (3a) is very reactive in the acid-catalyzed trans-
esterification reaction. Indeed, the resulting 3a readily
reacted with various alcohols and the corresponding
alkyl esters 3b–d were isolated in high yields in a one-
pot reaction from 1a and 2a (Scheme 1). This is an-
other benefit in employing phenyl formate as the sub-
strate.
Entry
Ligand
Yield [%][b]
E/Z[c]
1
2
none
PPh3
0
1
–
–
3
4
5
6
7
8
9
10
11
12
P
PCy3
dppf
dppe
dppb
dppbz
rac-binap
xantphos
xantphos[d]
xantphos[f]
(o-Tol)3
18
27
54
75
99
93
99
99
100/0
100/0
100/0
100/0
84/16
84/16
83/17
100/0
100/0
100/0
Scope of Substrates
99 (97)[e]
6
Various internal alkynes 2b–j reacted with 1a under
the standard reaction conditions (Table 2). Bis(4-ace-
tylphenyl)acetylene (2b) and bis(4-methylphenyl)ace-
tylene (2c) afforded the corresponding products 3e
and 3f stereoselectively in high yields (entries 1 and
2). Aliphatic alkynes such as 5-decyne (2d) and 4-
octyne (2e) also gave the corresponding products, 3g
and 3h, stereoselectively in high yields (entries 3 and
4). As for unsymmetrical alkynes, 1-phenyl-1-propyne
(2f) provided two regioisomers 3i and 3i’, but each
[a]
Diphenylacetylene (2a, 0.50 mmol), phenyl formate (1a,
2.0 mmol), [Pd(OAc)2] (0.025 mmol, 5.0 mol%), phos-
phane (0.050 mmol or 0.10 mmol, P/Pd=4), mesitylene
(0.50 mL) at 1008C for 20 h.
Yield based on the GC internal standard technique.
Determined by GC analysis.
0.0375 mmol (P/Pd=3).
ACHTUNGTRENNUNG
[b]
[c]
[d]
[e]
[f]
Isolated yield of 2a.
0.030 mmol (P/Pd=2.4).
[6]
phanes such as PPh3, PACHTUNTRGNEU(GN o-Tol)3 and PCy3 3a was af- product was isolated in pure form in 64% and 29%
forded in only low yields (entries 2–4). In contrast, yields, respectively (entry 5). Alkynes with ester and
the use of bidentate phosphanes such as dppf, dppe, amide functionalities (2g and 2h) successfully gave
dppb, dppbz and rac-binap[6] improved the catalytic the hydroesterification products and each product was
activity (entries 5–9), although yields (entries 5 and 6) isolated in pure form (entries 6 and 7). Although it
or stereoselectivities (entries 7–9) of the product were was difficult to determine the structure of the product
not satisfactory. Among the ligands examined, xant- bearing an amide functionality, the structure of 3k
phos gave the best result affording 3a in 99% yield was confirmed by X-ray crystallography (Supporting
with perfect (E)-stereoselectivity as confirmed by X- Information). With 1-cyclohexyl-1-propyne (2i) the re-
ray
crystallography
(Supporting
Information) gioselectivity was improved and 3l and 3l’ were isolat-
(entry 10). No E-Z isomerization occurred during the ed in 79% and 7% yields, respectively (entry 8). Grat-
reaction. Evidently, the P/Pd ratio affected the cata- ifyingly, 1-phenyl-2-(trimethylsilyl)ethyne (2j) afford-
lytic activity. While the yields at P/Pd=4 and 3 were ed a single regioisomer 3m in 96% yield selectively
99% (entries 10 and 11), the yield decreased dramati- (entry 9). Aryl formates bearing both electron-rich
cally to 6% at P/Pd=2.4 (entry 12).
(entries 10 and 11) and electron-poor (entries 13 and
Scheme 1. One-pot transesterification reaction of 3a.
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Palladium-Catalyzed Hydroesterification of Alkynes Employing Aryl Formates
Table 2. Hydroesterification of internal alkynes 2 with aryl formates 1.[a]
Entry
1
2
Product
Yield [%][b]
(E/Z)[c]
AHCTUNGTRENNUNG
1
1a
83 (100/0)
78 (100/0)
2[d]
1a
3[e]
4[f]
1a
1a
88 (99/1)
78 (100/0)
64 (100/0) and
29 (100/0)
5
1a
1a
1a
6[g]
64 (100/0)
55 (100/0) and
30 (100/0)
7[g]
79 (100/0) and
7 (100/0)
8
1a
1a
9
96 (99/1)
94 (99/1)
98 (99/1)
10[g]
2a
2a
11
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Yuko Katafuchi et al.
Yield [%][b]
FULL PAPERS
Table 2. (Continued)
Entry
1
2
Product
AHCTUNGTRENNUNG
(E/Z)[c]
12
2a
2a
94 (99/1)
92 (100/0)
93 (100/0)
13
14
2a
[a]
Internal alkyne (2: 0.5 mmol), formate (1: 2.0 mmol), [Pd
4), mesitylene (0.50 mL) at 1008C for 20 h.
Yield of the isolated product.
Determined by GC analysis.
At 1308C.
Xantphos (0.0375 mmol, P/Pd=3), at 908C.
At 908C.
At 1208C.
ACHTUNGRTEN(NGNU OAc)2] (0.025 mmol, 5.0 mol%), xantphos (0.050 mmol, P/Pd=
[b]
[c]
[d]
[e]
[f]
[g]
14) phenyl moieties gave the corresponding products Reaction Mechanisms
(3n–r) in high yields.
Terminal alkynes were also successfully utilized in During the course of the catalytic reaction, the facile
the hydroesterification. In Table 3, various terminal conversion of aryl formates to the corresponding phe-
alkynes 2k–r afforded the adducts in high yields with nols and carbon monoxide was observed. Therefore,
an excess 1a in the presence of [Pd2ACTHNUTRGNEU(GN dba)3]-xantphos the hydroesterification could proceed via the very ef-
catalyst system. The reaction of phenylacetylene (2k) ficient formation of carbon monoxide and phenols
with 1a proceeded regioselectively, giving a single re- from aryl formates. Actually, the reaction of 2a with
gioisomer 3s in 93% yield (entry 1). 4-Ethynyltoluene phenol in the presence of the [PdACHTNURTGEN(UNG OAc)2]-xantphos
(2l) also afforded a single adduct 3t regioselectively catalyst system under carbon monoxide atmosphere
(entry 2). 1-Ethynyl-4-methoxybenzene (2m) and 1- (1 atm) afforded the corresponding product 3a in
chloro-4-ethynylbenzene (2n) provided the corre- 99% yield [Scheme 3 (A)]. Furthermore, the reaction
sponding products with high regioselectivity (entries 3 of 1a with 2a in the presence of p-cresol afforded
and 4). Other terminal alkynes 2o–r having alkyls or a products 3a and 3n in 49% and 30% yields, respec-
silyl substituents on the sp carbon also afforded the tively [Scheme 3 (B)]. These results clearly indicate
corresponding products in high yields with high regio- that with xantphos as the ligand the reaction proceeds
selectivities (entries 5–8). It is noteworthy that the via conversion of aryl formates to phenols and carbon
silyl substituent reversed the regioselectivity (entry 8 monoxide, not via direct addition of aryl formates to
vs. entries 1–7).
alkynes. Usually, the reactivity of phenol in the hydro-
Besides alkynes, the reaction could be applied to al- esterification and related Reppe carbonylations is
kenes (Scheme 2). The reaction of 2-norbornene (4a) quite low due to its low nucleophilicity. In these
with 1a proceeded smoothly to afford 5a in 91% yield former reactions, the established general reactivity
exo-selectively [Scheme 2 (A)]. In the reaction of sty- order is as follows: aliphatic alcohol>water>phe-
rene (4b) with 1a, two regioisomers 5b and 5b’ were nol.[1b] Thus, the present catalyst system (Pd-xantphos,
isolated in pure form in 66% and 28% yields, respec- P/Pd>3) is uniquely active to utilize phenols and aryl
tively. 1-Dodecene (4c) also afforded a mixture of 5c formates very efficiently in the hydroesterification.[7]
and 5c’ in 70% total yield with high regioselectivity
[8/92, Scheme 2 (B)].
The conversion of aryl formates to phenols and
carbon monoxide is crucial in the present hydroesteri-
fication. Therefore, this step was examined in detail
with 1a. The conversion showed a zero-order depend-
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Palladium-Catalyzed Hydroesterification of Alkynes Employing Aryl Formates
Table 3. Hydroesterification of terminal alkynes with 1a.[a]
Entry
Alkyne 2k–r
Total yield of 3+3’ [%][b]
Selectivity (3/3’)[c]
3s/3s’=100/0
1
2
3
2k
2l
3s: 93
3t: 83
3u: 74
3t/3t’=100/0
2m
3u/3u’=100/0
4
5
6
2n
2o
2p
3v+3v’: 50
3w+3w’: 94
3x+3x’: 97
3v/3v’=93/7
3w/3w’=90/10
3x/3x’=96/4
7
2q
2r
3y: 77
3z’: 90
3y/3y’=100/0
3z/3z’=0/100
8
[a]
Terminal alkyne (0.50 mmol), 1a (5.0 mmol), [Pd
sitylene (1.5 mL) at 1008C for 20 h.
Yield of the isolated product.
2ACHTUNGTREN(NNUG dba)3] (0.0125 mmol, 5.0 mol%), xantphos (0.050 mmol, P/Pd=4), me-
[b]
[c]
Determined by GC analysis.
Scheme 2. Hydroesterificaion of alkenes with 1a.
ence on the concentration of aryl formates up to 80% provided the activation parameters: DH° =13.4 kcal
conversion. Furthermore, the nature of the ligand molÀ1 and DS° =À38.9 calmolÀ1 KÀ1. Further, the
strongly affected the rate constant with 1a (Table 4). effect of a substituent on the phenyl ring of the for-
Without phosphane ligands, no conversion of 1a was mates (1e, 1f, 1h and 3-methoxyphenyl formate 1j)
observed. Bidentate ligands such as xantphos and was examined. The Hammett plot of these rate con-
dppf showed high efficiency as the ligand (entries 1 stants with the corresponding s values[8] showed a
and 2), while dppf was not a suitable ligand and xant- straight line with the slope 1=3.46 (Figure 1), indicat-
phos was the best one in the hydroesterification reac- ing that more electron-withdrawing substituents accel-
tion (entries 5 and 10 in Table 1). The reaction using erate the conversion.
xantphos as a ligand (entry 1 in Table 4) was carried
Under these reaction conditions, the reactivity of
out at 808C, 1008C, and 1108C, and the Eyring plot alkyl formates such as 1b, 1c, and 1d was quite low
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Yuko Katafuchi et al.
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Scheme 3. Hydroesterification of 2a with phenol derivatives.
Table 4. Effect of phosphane on the conversion of phenyl
formate (1a) to phenol and carbon monoxide.[a]
were estimated by the MP4ACHTUNTGRENNUG(SDQ)/6-31+GACHTUNGTRENNUNG(2d,p)//
B3LYP/6-31G(d) method[9] for the homolytic cleavage
[Eq. (1)].
Entry
Ligand
kobs (molLÀ1 minÀ1)[b]
1
2
3
4
xantphos
dppf
PPh3
1.95ꢁ10À2
3.29ꢁ10À2
3.5ꢁ10À3
7ꢁ10À4
The calculated bond energies of the alkyl formate
(1b: 100.90 kcalmolÀ1, 1c: 100.58 kcalmolÀ1, and 1d:
100.60 kcalmolÀ1) are considerably larger than those
of the aryl formates (1a: 83.91 kcalmolÀ1, 1e:
81.69 kcalmolÀ1, 1f: 80.16 kcalmolÀ1, 1h: 82.93 kcal
molÀ1, and 1j: 84.46 kcalmolÀ1). These stronger bonds
of the alkyl formates could cause the much lower re-
activity in the hydroesterification.
PCy3
[a]
1a (2.0 mmol), [PdACHTNUGTRNEUNG(OAc)2] (0.025 mmol), phosphane
(0.050 mmol or 0.10 mmol, P/Pd=4), mesitylene
(1.0 mL) at 1008C.
[b]
Àd[1a]/dt=kobs[1a]0.
A possible catalytic cycle is shown in Scheme 4.
Aryl formate (1) is converted to phenols and carbon
monoxide in cycle I. The kinetic measurement of this
cycle indicated the rate has a zero-order dependence
on the concentration of 1. Thus, precoordination of 1
to active catalyst species A with a large equilibrium
constant might be involved. The electron-withdrawing
substituents on 1 accelerate the cycle I (Figure 1).
These substituents might stabilize the aryloxy inter-
mediate such as C. The conversion of 1 in cycle I
must be faster than the hydroesterification in cycle II,
since phenols and carbon monoxide were accumulat-
ed in the reaction system. In the present hydroesterifi-
cation, phenols of lower nucleophilicity show the
higher efficiency. Thus, nucleophilic attack[1] of phe-
nols onto the acryloyl palladium species would be un-
likely. The generated phenols might react with the
catalyst species A to afford C (step II-1). Then, inser-
tion of alkyne provides an alkenyl intermediate D
(step II-2) followed by insertion of CO to provide a
Figure 1. The Hammett plot for the palladium-catalyzed
conversion of 1. Reaction conditions: (2.0 mmol),
[Pd(OAc)2] (0.025 mmol), xantphos (0.050 mmol, P/Pd=4),
mesitylene (1.0 mL) at 1008C.
1
ACHTUNGTRENNUNG
and the hydroesterification did not proceed at all carbonyl intermediate E or E’ (step II-3). Finally, re-
(vide supra). To gain some insight into the different ductive elimination could provide the hydroesterifica-
reactivity, the bond dissociation energies between the tion product (3) and the active catalyst species (A) re-
formyl carbons and the alkoxy or aryloxy oxygens generates (step II-4).
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Palladium-Catalyzed Hydroesterification of Alkynes Employing Aryl Formates
Scheme 4. A plausible catalytic cycle.
Conclusions
25 m). Column chromatography was carried out on silica gel
(Kanto N60, spherical, neutral, 63–210 mm). TLC analyses
were performed on commercial glass plates bearing a
0.25 mm layer of Merck Silica gel 60F254.
The hydroesterification of alkynes employing aryl for-
mates occurred at 1008C at ambient pressure in the
presence of the Pd-xantphos catalyst system to afford
a,b-unsaturated esters regio- and stereoselectively.
The reaction can be carried out without the use of ex-
ternal carbon monoxide, and no directing group is re-
quired. During the reaction aryl formates were con-
verted to carbon monoxide and the corresponding
phenols. The reactivity of alkyl formates in the hydro-
esterification reaction was quite low. The hydroesteri-
fication of alkenes also readily proceeded under simi-
lar reaction conditions.
General Procedure for Palladium-Catalyzed
Hydroesterification of Internal Alkynes (2a–2j) with
Aryl Formates (1) (Table 1 and Table 2)
[Pd
(OAc)2]
(5.6 mg,
0.025 mmol)
and
phosphane
(0.050 mmol or 0.10 mmol, P/Pd=4) were added to a 10-mL
flask equipped with a three-way glass-plug stopcock at-
tached by a standard ground glass joint. The flask was evac-
uated and backfilled with argon three times. Then, aryl for-
mate (1a, 1e–i, 2.0 mmol) and mesitylene (0.50 mL) were
added to the flask under flowing Ar, and the resulting solu-
tion was stirred at room temperature for 15 min. Internal
alkyne (2a–j, 0.50 mmol) was added to the flask under Ar
and the mixture was stirred for additional 10 min. The reac-
tion was carried out at 1008C (bath temperature). After
cooling to room temperature, the mixture was evaporated
and the product was isolated by silica gel chromatography
using hexane-AcOEt as an eluent. The GC yields were ob-
tained using tridecane as an internal standard.
Experimental Section
General
All manipulations were performed under an argon atmos-
phere using standard Schlenk-type glassware on a dual-
manifold Schlenk line. All solvents were dried and purified
by usual procedures.[10] Unless otherwise noted, materials
obtained from commercial suppliers were used without fur-
ther purification. IR spectra were determined on a Shimad-
General Procedure for Palladium-Catalyzed
Hydroesterification of Terminal Alkynes (2k–r) with
Phenyl Formate (1a) (Table 3)
zu FTIR-8300 spectrometer. H and 13C NMR spectra were
1
measured with
a JEOL ECX-400P spectrometer. The
1H NMR chemical shifts are reported relative to tetrame-
thylsilane (TMS, 0.00 ppm). The 13C NMR chemical shifts
are reported relative to CDCl3 (77.0 ppm). EI-MS were re-
corded on a Shimadzu GCMS-QP5050 A with a direct inlet.
MALDI-TOF mass spectra were recorded on a Bruker Au-
toflex. Elemental analysis was carried out at Center for Or-
ganic Elemental Microanalysis, Graduate School of Pharma-
ceutical Science, Kyoto University. GC analysis was carried
out using a Shimadzu GC-17A equipped with an integrator
(C-R8A) with a capillary column (CBP-5, 0.25 mm i.d.ꢁ
N
2ACHTUNGTRENUN(NG dba)3] (11.5 mg, 0.0125 mmol) and xantphos (29 mg,
a
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Yuko Katafuchi et al.
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(bath temperature) for 24 h. After cooling to room tempera-
ture, the mixture was evaporated and the product was isolat-
ed by silica gel chromatography using hexane-AcOEt as an
eluent.
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General Procedure for Palladium-Catalyzed
Conversion of Phenyl Formate (1a) (Table 4)
[Pd
(OAc)2] (5.6 mg, 0.025 mmol) and
a
phosphane
(0.050 mmol or 0.10 mmol, P/Pd=4) were added to a 10-mL
flask equipped with a three-way glass-plug stopcock at-
tached by a standard ground glass joint. The flask was evac-
uated and backfilled with argon three times. Then 1a
(226 mL, 2.0 mmol), tridecane (98 mL, 0.40 mmol, internal
standard) and mesitylene (1.0 mL) were added to the flask
under flowing Ar, and the resulting solution was stirred at
room temperature for 15 min. The reaction was carried out
at 1008C (bath temperature). A small aliquot (40 mL) was
taken out from the reaction mixture at a suitable interval
and a conversion of 1a was analyzed by GC. A time-course
profile for the conversion of 1a using dppf as a ligand is
shown in the Supporting Information, Figure S1. From the
slope of the linear plot, a zero-order rate constant kobs in
Àd[1a]/dt=kobs[1a]0 was determined.
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[6] Abbreviations: xantphos: 4,5-bis(diphenylphosphino)-
9,9-dimethylxanthene, o-Tol=2-methylphenyl, Cy=cy-
Acknowledgements
clohexyl,
dppe=1,2-bis(diphenylphosphino)ethane,
dppb=1,2-bis(diphenylphosphino)butane, dppbz=1,2-
This work was supported by Grant-in-Aid for Scientific Re-
search on Priority Areas (“Synergy of Elements” and
“Chemistry of Concerto Catalysis“) from Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan.
bis(diphenylphosphino)benzene,
phenylphosphino)-1,1’-binaphthyl,
rac-BINAP=bis(di-
dppf=1,2-bis(di-
AHCTUNGTERGpNNUN henylphosphino)ferrocene.
[7] Miura et al. reported the palladium-catalyzed hydroes-
terification of terminal alkynes with phenol under
carbon monoxide. However, their catalyst system was
not so active to allow use of less reactive internal al-
kynes: K. Itoh, M. Miura, M. Nomura, Tetrahedron
Lett. 1992, 33, 5369–5372. Very recently, Kuniyasu and
Kambe et al. reported the palladium-catalyzed hydroes-
terification of internal alkynes with phenol under
carbon monoxide in the presence of Zn: H. Kuniyasu,
T. Yoshizawa, N. Kambe, Tetrahedron Lett. 2010, 51,
6818–6821.
[8] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91,
165–195.
[9] Gaussian 03 program: Revision C.02, Gaussian, Inc.,
Pittsburg, PA, 2004.
[10] W. L. F. Armarego, C. L. Chai, Purification of Labora-
tory Chemicals, 5th edn., Butterworth-Heinemann,
Oxford, 2003.
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Adv. Synth. Catal. 2011, 353, 475 – 482