An unsymmetrical iron catalyst for the asymmetric transfer hydrogenation of ketones

Article abstract of DOI:10.1055/s-0034-1380147

A new iron(II)(Ph₂P-NH-N-PCy₂) complex with a dicyclohexylphosphino group trans to the NH group was found to catalyze the asymmetric transfer hydrogenation of a variety of ketones with high enantioselectivity.

Full text of DOI:10.1055/s-0034-1380147

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2015, 47, 1775–1779
paper
1775
S. A. M. Smith, R. H. Morris
Paper
Synthesis
An Unsymmetrical Iron Catalyst for the Asymmetric Transfer
Hydrogenation of Ketones
Samantha A. M. Smith
Robert H. Morris*
+
Ph
N
Ph
Br
–
BPh4
H
N
P
Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON, M5S 3H6,
Canada
Fe
C
P
Ph2
Cy2
robert.morris@utoronto.ca
O
OH
*
[Fe] (0.02–0.2 mol%)
KOt-Bu (0.1–0.4 mol%)
O
R2
H
R1
R1
R2
i-PrOH, 28 °C
40–99% conversion
ee up to 99%
Received: 12.12.2014
Accepted: 14.01.2015
Published online: 20.02.2015
DOI: 10.1055/s-0034-1380147; Art ID: ss-2014-c0753-st
ATH of ketones with enzyme-like activity. Turnover fre-
quencies (TOF) up to 200 s–1 and enantiomeric excess (ee)
values of up to 90% were observed for the reduction of 3,4-
bis(trifluoromethyl)acetophenone (K8 in Figure 2). Al-
though complex 1 has unprecedented activity, there is
room for improvement with respect to enantioselectivity.
Complex 2 has lower activity, but provides higher enantio-
meric excess values (up to 98%) for certain substrates. In
some cases there is racemization of the product near the
end of the reaction. Complex 3 has higher activity, but leads
to lower enantioselectivity.
Abstract A new iron(II)(Ph2P–NH–N–PCy2) complex with a dicyclohex-
ylphosphino group trans to the NH group was found to catalyze the
asymmetric transfer hydrogenation of a variety of ketones with high en-
antioselectivity.
Key words hydrogenation, iron catalysis, chirality, ketones, alcohols
The reduction of ketones to give enantiopure alcohols is
a very important transformation in organic synthesis as it
provides valuable building blocks for the flavor, fragrance,
fine chemical, and pharmaceutical industries.1 Precious
metals with expensive chiral ligands are generally required
for the catalytic hydrogenation of ketones, but these metals
are costly and potentially toxic.2 Thus, there is a current re-
search effort to find highly efficient and enantioselective
catalysts based on earth-abundant metals for more eco-
nomical and safer processes, without sacrificing activity
and selectivity. Recently, iron has received significant atten-
tion as a viable catalyst for both asymmetric transfer hydro-
genation (ATH)3 and direct hydrogenation (DH)4 of polar
double bonds. Asymmetric transfer hydrogenation takes
place in the presence of a sacrificial reductant as the H+/H–
source, as shown in Scheme 1, with isopropanol as the re-
ductant. This transformation will be the focus of this paper.
1+
–
Ph
N
Ph
BF4
Cl
H
N
P
Fe
C
P
R1
R2
2
2
O
1: R1 = R2 = Ph
2: R1 = R2 = Xyl
3: R1 = Ph, R2 = p-Tol
Figure 1 Third generation iron(II) complexes previously reported by
our group
For the preparation of the catalyst, the incorporation of
an N–H moiety in the tetradentate ligand required an inten-
sive synthetic approach, where first an enantiomerically
pure (S,S)-R2PCH2CH2NHCH(Ph)CH(Ph)NH2 [(S,S)-P-NH-
NH2] ligand was synthesized and isolated, which then un-
derwent an iron-templated Schiff base condensation with
an α-diarylphosphinoacetaldehyde to produce the P–NH–
N–P ligand on iron. Complexes 1–3 differ only by the aryl
phosphine groups, and complex 3 is so far the only example
with unsymmetrical phosphine donors. We propose that
the use of a more sterically hindered phosphine, such as a
dicyclohexylphosphino group on the ligand, may improve
O
[Fe] (0.02–0.2 mol%)
KOt-Bu (0.1–0.4 mol%)
OH
*
R1
R2
R1
R2
i-PrOH, 28 °C
Scheme 1 General reaction scheme for the ATH of ketones
Three iron(II) complexes (1–3 in Figure 1) of our iron
catalysts for ATH have been synthesized and reported re-
cently.3m,4f Complex 1 was found to be highly active in the
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1775–1779

1776
S. A. M. Smith, R. H. Morris
Paper
Synthesis
Ph
OH
Ph
[Br]2
[BPh4]
Br
[BF4]2
Ph
NaOMe (2 equiv), MeOH
[Fe(H2O)6][BF4]2 (2.5 equiv), MeCN
Ph
N
1) KBr (4 equiv), CO, acetone
3 h, 28 °C
N
H
H
N
P
N
P
PCy2
Fe
N
P
Fe
N
Cy2P
Ph Ph
2) NaBPh4 (1.1 equiv), MeOH
Ph2
Cy2
P
Ph2
Cy2
C
O
(2 equiv), MeOH
NH NH2
PPh2
OH
4
28 °C, 16 h
Scheme 2 Synthesis of the new iron(II) Ph2P–NH–N–PCy2 complex 4
the enantioselectivity of this system, and this phosphine
can be introduced in the final step of the templated ligand
synthesis.
The present study describes the catalytic reduction of
ketones using the new unsymmetrically substituted cata-
lyst 4, which provides alcohols in excellent enantiomeric
enrichment without racemization.
tive for the reduction of acetophenone with a 90% ee, but 4
proved to be superior. Substrates K2 and K4 were reduced
with very high enantiomeric excess values of 94% and >99%,
respectively, however, when a more bulky substrate such as
K3 was tested, no conversion was observed. The effective
reduction of K4 is noteworthy as Noyori-type catalysts do
not hydrogenate this substrate.5 When the substituents on
the phenyl ring were varied as for substrates K5–K8, differ-
ent effects on the catalysis were observed. Complex 4 gave
very high conversions of chloro-substituted ketones K5 and
K6, but afforded lower conversions of methyl- and trifluo-
The new complex 4 was synthesized using a method
similar to that employed for the preparation of 1 and 3 as
described briefly above.3m Starting with the air- and mois-
ture-stable dicyclohexylphosphonium dimer and the enan-
tiomerically pure (S,S)-Ph2P–NH–NH2 ligand, the Schiff base
condensation occurred via a metal-mediated template syn-
thesis as shown in Scheme 2. Substitution of the two aceto-
nitrile moieties with bromide and carbonyl, followed by salt
exchange with sodium tetraphenylborate (NaBPh4), afford-
ed complex 4 as a yellow powder in moderate yield (40%) as
a mixture of two diastereomers. This complex was charac-
terized by NMR and IR spectroscopy and mass spectrome-
try. In the 31P{1H} NMR spectrum, characteristic doublets at
78.38 ppm and 47.34 ppm, as well as 76.05 ppm and 75.48
ppm corresponded to two structural isomers of the com-
plex, the first being the major isomer (75%:25%). Presum-
ably, one isomer would have its C=O syn to the N–H and the
other would be anti. A carbonyl stretch (νCO) was found at
1951 cm–1, which is typical for such a complex.
The catalytic activity of complex 4 was tested for the
ATH of ketones K1–K16 and one aldehyde, A1 (Figure 2).
For all substrates, the (R)-alcohol was produced when using
our iron complexes, which have enantiopure (S,S)-ligands.
The results are summarized in Table 1. Overall, complex 4
has a lower activity than 1, however the enantiomeric ex-
cess values of a few substrates have been increased by using
the bulky dicyclohexylphosphine. Entries 1–3 (Table 1)
show the reduction of acetophenone under different condi-
tions. An enantiomeric excess of 81% was observed when
the catalyst/base/substrate (C/B/S) ratio was 1:2:500, but
when the catalyst loading was reduced, the enantiomeric
excess increased significantly to 93% and 98%, with two and
eight equivalents of base, respectively. We found that com-
plex 4 was slightly less active than 1 in the reduction of K1,
giving a conversion of 11% lower, however the enantiomeric
excess was increased significantly by 20%. In our previous
work,3m we found that complex 2 was more enantioselec-
Table 1 Catalytic Results for the ATH of Substrates K1–K16 and A1
Using Complex 4a
Entry
Substrate
Conv. (%)b
Time (min) ee (%)b
TONc
415
1
2d
3e
4
K1
83
36
71
80
0
20
50
81
93
98
94
–
K1
3550
4346
401
0
K1
120
60
K2
5
K3
120
2
6
K4
98
>99
92
60
38
>99
0
>99
94
52
65
95
34
–
~500
~500
463
300
190
~500
0
7
K5
10
8
K6
10
9
K7
20
10
11
12
13f
14
15
16
17
18
19f
K8
40
K9
20
K10
K11
K12
K13
K14
K15
K16
A1
120
50
44
0
43
–
221
0
120
120
30
0
–
0
92
88
40
>99
–
462
440
200
~500
20
80
5
20
10
–
a Catalyst/base/substrate (C/B/S) ratio = 1:2:500 unless otherwise stated.
b Conversion and ee determined by chiral GC.
c TON = turnover number.
d C/B/S = 1:2:6121.
e C/B/S = 1:8:6121.
f C/B/S = 1:8:500.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1775–1779

1777
S. A. M. Smith, R. H. Morris
Paper
Synthesis
O
Cl
O
O
O
O
O
Cl
Cl
K1
K2
K3
K4
K5
K6
O
O
O
O
O
O
F3C
N
O
K7
K8
K9
K10
K11
K12
CF3
O
O
O
O
O
H
A1
K13
K14
K15
K16
Figure 2 Substrate scope for ATH using complex 4
romethyl-substituted ketones K7 and K8. The enantiomeric
excess values of the more sterically hindered ortho- and
meta-substituted ketones K5 and K8 were higher (94% and
95%, respectively) than the para-substituted ketones K6
and K7. Complex 4 gave a higher enantiomeric excess (95%)
for substrate K8 than 1 (90%), however, the activity was
much lower with only 38% conversion. The resulting alco-
hol serves as an intermediate in the synthesis of Aprepitant
to combat nausea associated with cancer chemotherapy.6
Complex 4 was found to tolerate an imine functional group
with the high conversion of acetylpyridine (K9) to (R)-α-
methyl-2-pyridinemethanol, however, catalysis stopped in
the presence of the oxygen-functionalized acetylfuran
(K10).
Alkyl ketones K11 and K12 are deactivated toward re-
duction by complex 4 with conversions of 44% and 0%, re-
spectively, as is the bulky cyclohexylphenyl ketone K13.
High conversions of benzophenone (K14) and 1-(naphtha-
len-2-yl)ethanone (K15) were observed, with an enantio-
meric excess of 80%, matching complex 1, for the produc-
tion of (R)-α-methyl-2-naphthalenemethanol. The ATH of
the olefinic aryl ketone K16 resulted in a non-selective re-
duction of both the C=O and C=C bonds, with a 40% conver-
O
OH
H
Ph
Ph
O
N
N
Fe
1+
Ph
Ph
H
N
Ph
P
P
Ph
H
Br
Ph2
5a
Cy2
δ−
δ+
H
N
C
O
N
N
KOt-Bu (2–8 equiv)
i-PrOH, 28 °C
Fe
R'
H
δ+
δ+
δ−
Fe
+
δ−
P
P
P
P
H
Ph
Ph
N
Cy2
Ph2
Ph2
Cy2
O
Fe
N
O
N
6
4
Fe
P
P
Ph2
Cy2
O
5b
O
R
OH
H
R
Scheme 3 Proposed catalytic cycle for the ATH of ketones
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1775–1779

1778
S. A. M. Smith, R. H. Morris
Paper
Synthesis
sion to reduced products and only a 5% ee of the olefin (R)-
alcohol. This is different compared to complex 1, which re-
duced only the C=O moiety, albeit with a low yield (55%).
Lastly, the hydrogenation of benzaldehyde (A1) was very ef-
ficient with quantitative conversion in 10 minutes. The
products of the reduction of K5, K8, and K9 were isolated
via extraction from brine with diethyl ether, and filtration
through a pad of Celite. The alcohols 1-(2-chlorophe-
nyl)ethanol and 1-(2-pyridyl)ethanol were isolated in 75%
and 58% yields and 95% and 34% ee values, respectively,
without any starting ketone being present. The reduction of
K8 did not go to completion, however, and the alcohol was
distilled with a resulting yield of only 30% and an enantio-
meric excess of 94%.
The proposed mechanism of the catalytic reaction is
shown in Scheme 3 and is assumed to follow that previous-
ly reported.3m The precatalyst 4 is activated with two or
eight equivalents of base to form the neutral iron ene–ami-
do species 5a and 5b. It has been found that the proton and
hydride of the active catalyst must be on the same side of
the ligand plane, and the orientations of the amide and va-
cant site of 5a do not allow the initial H+/H– transfer to oc-
cur in the correct way. However, complex 5b can dehydro-
genate isopropyl alcohol (i-PrOH) to give the hydride com-
plex 6. The ketone interacts preferentially in the re
configuration with 5b via a six-membered transition state.
The H+/H– transfer then occurs to release the product alco-
hol and the neutral species 5b, thus closing the catalytic cy-
cle.
In conclusion, a newly developed unsymmetrical tetra-
dentate P–NH–N–P′ ligand has been synthesized on iron.
The new precatalyst 4 was tested in the ATH of C=O bonds
and gave a turnover number (TON) of up to 4300 and an en-
antiomeric excess of up to 99%. In comparing the dicyclo-
hexylphosphino-substituted complex 4 with the previously
reported diphenylphosphino complex 1 in the ATH of ke-
tones, we found that the enantioselectivity of this system
was increased at the expense of catalytic activity. For exam-
ple, acetophenone, usually a challenging substrate for enan-
tioselective reduction,1e was hydrogenated with a resulting
enantiomeric excess of 98%. The flexible synthesis of our
third generation catalysts offers the possibility of tuning
their structure for the optimum reduction of a given sub-
strate. This is currently under investigation.
31P NMR spectra were referenced to 85% H3PO4 (0 ppm). The electro-
spray ionization mass spectrometry (ESI–MS) data were collected on
an AB/Sciex QStar mass spectrometer with an ESI source.
trans-(S,S)-[Fe(Br)(CO)(Ph2PCH2CH2NHCHPhCH-
PhN=CHCH2PCy2)][BPh4] (4)
In a glovebox, NaOMe (64 mg, 1.176 mmol) was added to a 100 mL
Schlenk flask charged with a stir bar and dicyclohexylphosphonium
bromide (378 mg, 5.88 × 10–1 mmol), and MeOH (25 mL) was added
with stirring. This solution was stirred for no more than 2 min. A
solution of [Fe(H2O)6][BF4]2 (496 mg, 1.47 mmol) dissolved in MeCN
(20 mL) was added to the Schlenk flask, followed by a solution of (S,S)-
Ph2PCH2CH2NHCH(Ph)CH(Ph)NH2 in MeOH (10 mL). This resulted in a
purple solution which was left to stir at r.t. for 16 h, during which
time a color change from purple to pink was observed. The solvent
was removed in vacuo. KBr (140 mg, 4.70 mmol) was added to the
flask which was sealed and removed from the glovebox. The flask was
purged and placed under a CO atmosphere using Schlenk techniques.
Acetone (20 mL), in a syringe, was removed from the glovebox and in-
jected into the flask with stirring for 1.5 h. The solvent was removed
under vacuum and the residual orange solid was redissolved in ace-
tone (20 mL) while under a CO atmosphere. The solution was stirred
for 1 h, after which the solvent was removed and the flask transferred
into the glovebox, where the remaining steps occurred. The solid resi-
due was dissolved in CH2Cl2 (20 mL) and filtered through a pad of
Celite, then through a 25 mm Syringe Filter PTFE membrane (pore
size 0.45 μm). The clear orange solution was dried in vacuo and then
redissolved in a minimum amount of MeOH. This solution was added
to a vial charged with a stir bar and NaBPh4 (402 mg, 1.294 mmol)
dissolved in a minimum volume of MeOH, from which a yellow solid
precipitated. This solid was filtered off, washed with Et2O (3 × 15 mL)
and dried overnight. If the purity by NMR was not sufficient, the solid
was redissolved in a minimum volume of CH2Cl2 and precipitated out
by addition of Et2O.
Yield: 530 mg (40%); yellow powder.
IR (KBr): 1951 (CO) cm–1
.
1H NMR (600 MHz, CD2Cl2): δ = 1.19–1.94 [m, 22 H, P(C6H11)2], 2.58
(m, 2 H, CH2), 2.74 (m, 2 H, CH2), 4.38 (t, JHH = 11.88 Hz, 2 H, PCH2),
4.76 [t, JHH = 12.14 Hz, 1 H, N(H)(Ph)], 4.83 [m, 1 H, N(H)(Ph)], 6.83–
7.56 (m, 40 H, ArH), 7.79 (m, 1 H, N=CH).
31P{1H} NMR (242 MHz, CD2Cl2): δ = 78.38 and 47.34 (d, JPP = 33.5 Hz,
isomer 1), (d, JPP = 33.48 Hz), 76.05 and 75.48 (d, JPP = 38.7 Hz, isomer
2).
HRMS (ESI–TOF, CH2Cl2): m/z [M+] calcd for C43H53BrFeN2OP2:
809.2084; found: 809.2070.
Asymmetric Transfer Hydrogenation; General Procedure (Table 1,
Entries 1 and 4–19)
A 20 mL vial was charged with a stir bar and complex 4 (10 mg,
8.9 × 10–3 mmol). The substrate (4.43 × 10 mmol) was added and the
mixture stirred. If the substrate was a liquid, the mixture was stirred
until complex 4 had completely dissolved. i-PrOH (3.61 g) was added
and the solution was stirred for 5 min. A stock solution of t-BuOK (20
mg, 0.18 mmol) in i-PrOH (0.98 g) was stirred until all the base had
dissolved. This stock solution (0.1 g or 0.4 g, 2 and 8 equiv, respective-
ly) was diluted with i-PrOH (1.0 g or 0.7 g, respectively) and added to
the reaction vial to activate the precatalyst and start catalysis. Sam-
ples (0.1 mL) were taken via syringe and injected into Teflon-sealed
GC vials prepared with wet, aerated i-PrOH to quench catalysis.
All manipulations were performed under an inert atmosphere of ei-
ther N2 or Ar using Schlenk techniques or a glovebox, unless other-
wise stated. Solvents were dried and degassed under standard proce-
dures prior to use. IR spectra were obtained on samples prepared as
KBr discs on a Paragon 500 spectrophotometer. NMR spectra were re-
corded at ambient temperature and pressure using an Agilent DD2
600 MHz spectrometer [1H (600 MHz) and 31P{1H} (242 MHz)]. The
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1775–1779

1779
S. A. M. Smith, R. H. Morris
Paper
Synthesis
Asymmetric Transfer Hydrogenation; General Procedure (Table 1,
Entries 2 and 3)
(2) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282.
(3) (a) Chen, J. S.; Chen, L. L.; Xing, Y.; Chen, G.; Shen, W. Y.; Dong, Z.
R.; Li, Y. Y.; Gao, J. X. Acta Chim. Sin. 2004, 62, 1745. (b) Sui-Seng,
C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem. Int. Ed.
2008, 47, 940. (c) Mikhailine, A. A.; Lough, A. J.; Morris, R. H.
J. Am. Chem. Soc. 2009, 131, 1394. (d) Meyer, N.; Lough, A. J.;
Morris, R. H. Chem. Eur. J. 2009, 15, 5605. (e) Zhou, S.; Fleischer,
S.; Junge, K.; Das, S.; Addis, D.; Beller, M. Angew. Chem. Int. Ed.
2010, 49, 8121. (f) Lagaditis, P. O.; Lough, A. J.; Morris, R. H.
J. Am. Chem. Soc. 2011, 133, 9662. (g) Sues, P. E.; Lough, A. J.;
Morris, R. H. Organometallics 2011, 30, 4418. (h) Yu, S.; Shen,
W.; Li, Y.; Dong, Z.; Xu, Y.; Li, Q.; Zhang, J.; Gao, J. Adv. Synth.
Catal. 2012, 354, 818. (i) Mikhailine, A. A.; Maishan, M. I.; Lough,
A. J.; Morris, R. H. J. Am. Chem. Soc. 2012, 134, 12266.
(j) Mikhailine, A. A.; Maishan, M. I.; Morris, R. H. Org. Lett. 2012,
14, 4638. (k) Prokopchuk, D. E.; Morris, R. H. Organometallics
2012, 31, 7375. (l) Sonnenberg, J. F.; Coombs, N.; Dube, P. A.;
Morris, R. H. J. Am. Chem. Soc. 2012, 134, 5893. (m) Zuo, W.; Li,
Y.; Lough, A. J.; Morris, R. H. Science 2013, 342, 1080. (n) Sues, P.
E.; Demmans, K. Z.; Morris, R. H. Dalton Trans. 2014, 43, 7650.
(4) (a) Berkessel, A.; Reichau, S.; von der Hoh, A.; Leconte, N.;
Neudorfl, J. M. Organometallics 2011, 30, 3880. (b) Fleischer, S.;
Werkmeister, S.; Zhou, S. L.; Junge, K.; Beller, M. Chem. Eur. J.
2012, 18, 9005. (c) Fleischer, S.; Zhou, S. L.; Werkmeister, S.;
Junge, K.; Beller, M. Chem. Eur. J. 2013, 19, 4997. (d) Li, Y.; Yu, S.;
Wu, X.; Xiao, J.; Shen, W.; Dong, Z.; Gao, J. J. Am. Chem. Soc.
2014, 136, 4031. (e) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.;
Wan, K. Y.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136,
1367. (f) Zuo, W.; Tauer, S.; Prokopchuk, D. E.; Morris, R. H.
Organometallics 2014, 33, 5791.
The quantity of the precatalyst was measured via a stock solution
method. A concentrated stock solution was made by dissolving com-
plex 4 (22 mg, 1.97 × 10–2 mmol) in cold CH2Cl2 (6.08 g). After all the
solid had dissolved, the solution was immediately sucked into a sy-
ringe. The solution was then divided into equal portions into several
20 mL vials, such that each portion had 0.2 g of the stock solution, and
then CH2Cl2 was removed in vacuo. These operations led to a precata-
lyst quantity of 6.48 × 10–4 mmol in each vial. The base was prepared
by dissolving t-BuOK (10 mg, 0.089 mmol) in i-PrOH (1.02 g, 1.30 mL).
i-PrOH (6.63 g, 8.44 mL), substrate (3.95 mmol) and a clean stir bar
were added to the vial that contained the precatalyst and the result-
ing solution was stirred for 5 min, or until all the precatalyst had dis-
solved. The base stock solution (0.015 g or 0.06 g, 2 or 8 equiv base)
was added to a vial that contained i-PrOH (0.546 g or 0.501 g, respec-
tively), and the mixed solution was then added to the catalyst solu-
tion to start the catalytic reaction. Samples (0.1 mL) were taken via
syringe and injected into Teflon-sealed GC vials prepared with wet,
aerated i-PrOH to quench catalysis.
Acknowledgment
NSERC is thanked for a Discovery Grant to R.H.M. S.A.M. thanks Digi-
tal Specialty Chemicals Ltd. for an OGSST Scholarship.
Supporting Information
Supporting information for this article is available online at
(5) Xu, Y.; Docherty, G. F.; Woodward, G.; Wills, M. Tetrahedron:
Asymmetry 2006, 17, 2925.
http://dx.doi.org/10.1055/s-0034-1380147.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
(6) Brands, K. M. J.; Payack, J. F.; Rosen, J. D.; Nelson, T. D.;
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Song, Z. J.; Tschaen, D. M.; Hansen, K.; Devine, P. N.; Pye, P. J.;
Rossen, K.; Dormer, P. G.; Reamer, R. A.; Welch, C. J.; Mathre, D.
J.; Tsou, N. N.; McNamara, J. M.; Reider, P. J. J. Am. Chem. Soc.
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2015, 47, 1775–1779