Preparation of 5-membered rings via radical addition-translocation- cyclization (RATC) processes mediated by diethyl thiophosphites

Article abstract of DOI:10.1002/adsc.201000852

A practical method for the formation of thiophosphonates bearing functionalized monocyclic, fused bicyclic and spirocyclic residues is presented. The procedure requires the easily available terminal alkynes as starting materials as well as commercially an

Full text of DOI:10.1002/adsc.201000852

FULL PAPERS
DOI: 10.1002/adsc.201000852
Preparation of 5-Membered Rings via Radical Addition-
Translocation-Cyclization (RATC) Processes Mediated by
Diethyl Thiophosphites
Christophe Lamarque,a Florent Beaufils,a Fabrice Dꢀnꢁs,a,b Kurt Schenk,c
and Philippe Renauda,*
a
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
Fax : (+41)-31-631-8057; e-mail: philippe.renaud@ioc.unibe.ch
Facultꢀ des Sciences et des Techniques, CEISAM UMR CNRS 6230, Universitꢀ de Nantes, 2 rue de la Houssiniꢁre, BP
b
92208, 44322 Nantes Cedex 3, France
Laboratoire de Cristallographie, ꢂcole Polytechnique Fꢀdꢀrale de Lausanne, Le Cubotron – Dorigny, CH-1015 Lausanne,
c
Switzerland
Received: November 14, 2010; Revised: February 21, 2011; Published online: May 17, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000852.
Abstract: A practical method for the formation of one-pot process without any slow addition of one of
thiophosphonates bearing functionalized monocyclic, the reagents.
fused bicyclic and spirocyclic residues is presented.
The procedure requires the easily available terminal
alkynes as starting materials as well as commercially Keywords: alkynes; dialkyl phosphites; dialkyl thio-
and readily available reagents such as diethyl thio- phosphites; 1,5-hydrogen transfer; radical initiators;
phosphite. The experimental procedure consists of a radical reactions
Introduction
cient addition-cyclization processes leading to cycloal-
kane derivatives by using either dialkyl phosphites or
Organophosphorus compounds represent a very im- dialkyl thiophosphites.[8–11] Interestingly, the radical
portant class of natural and non-natural compounds, processes concern mostly addition to alkenes, the rad-
which possess important biological properties.[1] For ical addition of P radicals to terminal alkynes is less
instance, phosphonates present a structural analogy documented in the literature.[12,13]
with phosphates, and their higher stability toward hy-
Recently, we reported an efficient procedure to
drolysis makes them very attractive to act as enzyme prepare functionalized 5-membered rings involving a
inhibitors. Phosphonates show neurotoxic activity, and thiophenol-mediated radical addition-translocation-
they have found applications as antibiotics, herbicides, cyclization process (RATC) [Scheme 1, Eq. (1)].[14–16]
blood pressure regulators, antiviral and anticancer This process complements nicely the alkenyl radical
agents.[2] Therefore, the formation of carbon-phospho- translocation-cyclization
process
pioneered
by
rus bonds has attracted the attention of synthetic Curran.[17–19] Although thiophenol turned out to be
chemists for a long time.[3] The main ionic procedures extremely efficient in mediating the radical transloca-
known to date are the well-known Michaelis–Arbu- tion with most substrates, this reagent showed limita-
zov rearrangement,[4] as well as the Kabachnik– tions when a slow hydrogen-transfer was involved. In
Fields[5] and Pudovik[6] reactions. This last reaction is this case, the desired cyclized product was contami-
particularly interesting since it involves the addition nated by a mixture of uncyclized compounds (E:Z
of an organophosphorus compound containing a mixture of diastereoisomers) and benzothiophene de-
À
labile P H bond to alkenes and alkynes. This reaction rivatives resulting from the cyclization of the inter-
is known to work either via a polar or via a radical mediate alkenyl radical onto the benzene ring of thio-
mechanism depending on the substrates and the reac- phenol [Scheme 1, Eq. (2)].[15,16]
tion conditions.[7] The radical addition has been thor-
oughly investigated by Parsons who developed effi- phosphites are known to be slower reducing agents
Phosphorus P H reducing agents such as dialkyl
À
Adv. Synth. Catal. 2011, 353, 1353 – 1358
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1353

Christophe Lamarque et al.
FULL PAPERS
Scheme 2. RATC reactions with dimethyl phosphite.
Scheme 1. Scope and limitations of the thiophenol-mediated
RATC reaction.
crude product with a purity ꢀ95%. In this particular
example, neither the tin hydride procedure developed
by Curran[18] nor our thiophenol procedure[14,15,16] [see
than tributyltin hydride and thiophenol. The lower re- Scheme 1, Eq. (2)] gave satisfactory results due to the
ducing properties of dialkyl H-phosphonates are slowness of the H-atom abstraction step. The malo-
mainly due to the difference in BDE of the hetero- nate 3a, prepared by alkylation of dimethyl propargyl-
atom H bond: the S H bond in thiophenol has a malonate with bromocyclopentane,[15] gave under sim-
À
À
value[20] of about 331 kJmolÀ1 while that of the P H ilar reaction conditions the expected bicycloalkane 4a
À
bond in À1 diakyl H-phosphonate[11] is about in 93% yield [Eq. (4), dr 90:10]. Similarly, substrate
À
365 kJ.mol . The moderate reducing power of the P
5a, easily prepared by alkylation of the corresponding
H reducing agents and the high reactivity of P-cen- b-keto ester followed by decarboxylation, was effi-
tered radicals towards alkenes and alkynes motivated ciently converted into spiroketone 6a (97%, dr 70:30)
us to test them in radical addition-translocation-cycli- [Eq. (5)].[22]
zation (RATC) processes with the aim of avoiding the
Since alkyl phosphonates could not easily engage in
high dilution conditions and/or slow addition tech- subsequent olefination reactions, we decided to inves-
niques required in both the tin hydride and the thio- tigate the use of other phosphorus reagents that are
phenol processes.
known to facilitate further olefination processes. We
In a preliminary communication, we have reported report herein a detailed study of the use of thiophos-
that dialkyl phosphites are powerful reagents for radi- phites for RATC reactions. These substrates present
cal translocation-cyclization reactions starting from several advantages over the phosphites in term of re-
terminal alkynes.[21] This represents an efficient action efficacy and for further transformations of the
method for the synthesis of cycloalkyl phosphonate products.
derivatives. Three typical examples illustrating the
synthetic utility of this process for the preparation of
monocyclic, fused bicyclic and spirocyclic compounds Results and Discussion
are depicted in Scheme 2 [Eqs. (3)-(5)].
For instance, it was established that highly reprodu- Dialkyl Thiophosphite-Mediated Reactions
cible results and high yields are obtained by treating a
0.1M solution of the alkyne 1a with 5 equivalents of In an effort to develop a RATC reaction combining
dimethyl phosphite and 1 equivalent of dilauroyl per- the practicability of the dialkyl phosphite method and
oxide (DLP) in refluxing cyclohexane. The desired cy- the flexibility of the thiophenol method relative to
clized product 2a was obtained in 81% yield and no further transformation of the products, we decided to
À
trace of the product of direct reduction was observed investigate alternative P H reagents. Thiophosphites
[Eq. (3)]. Interestingly, the reaction was performed by have a BDE of 337 kJmolÀ1 positioned close to that
mixing all reagents together at the beginning. Remov- of thiophenol (331 kJmolÀ1)[21] and well below that of
al of the excess of reagent by evaporation and filtra- dialkyl H-phosphonates (365 kJmolÀ1).[11] They are
tion through a short pad of silica gel afforded a pure known to add efficiently onto alkenes and alkynes via
1354
asc.wiley-vch.de
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1353 – 1358

Preparation of 5-Membered Rings via Radical Addition-Translocation-Cyclization (RATC) Processes
Table 1. Preliminary attempts to run a diethyl thiophosphite-
mediated RATC reaction with 3b according to Eq. (6).
levels of stereocontrol. Since di-tert-butyl peroxide is
usually used for thermal initiation (typically in reflux-
ing chlorobenzene), we envisioned using irradiation
to promote the fragmentation of the peroxide. Sub-
strate 1b was used as a model substrate for this
second optimization of the diethyl thiophosphite-
mediated RATC process [Eq. (7)]. Results are sum-
marized in Table 2. The reaction was tested in the
presence of 1.5 equivalents of the thiophosphite and
À
Entry P(S) H Initiator
Solvent
ACHTUNGTNER(NUNG T 8C)
7b (endo/
exo)
(equiv.)
N
1
2
3
2
2
1.5
AIBN (50)
AIBN (50)
THF (refl.)
t-BuOH (refl.) 81% (82:18)
DTBHN (20) t-BuOH (45) 89% (95:5)
61%[a,b]
[a]
[b]
Stereoselectivity not determined.
8b (27%) was also isolated.
a radical pathway.[11,13,23] Moreover, Parsons has dem-
onstrated that thiophosphonates are very suitable for
further olefination reactions.[9,11]
The cyclopentanone 3b was used for preliminary
experiments according to Eq. (6), and the results are
collected in Table 1. The AIBN-initiated reaction was
investigated first with 2 equivalents of thiophosphite 20 mol% of di-tert-butyl peroxide under irradiation
in refluxing THF. Under these reaction conditions, with a 300 W sunlamp in different solvents (Table 2,
entries 1–3). Very low yields of 9b were obtained in
dichloromethane and dimethylformamide (Table 2,
entries 1 and 2). However, promising results were ob-
tained in benzene (60% yield, entry 3). We attributed
this unusual solvent effect to the fact that benzene
could act as a sensitizer to promote the homolytic
fragmentation of di-tert-butyl peroxide.[24] It was ex-
pected that the use of dicumyl peroxide (DCP), a rad-
ical initiator that should be more prone to homolytic
cleavage under irradiation conditions, should favour
the overall process. Indeed, this cheap initiator could
perform the tandem radical transformation equally
well at room temperature in CH2Cl2 (entry 4), ben-
zene (entry 5), and t-BuOH (entry 6), affording the
the desired bicyclic ketone 7b was obtained in 61% cyclized compound 9b in high yields (84–88%). To the
yield, along with 27% of the reduced product 8b best of our knowledge, this is the first use of this per-
(Table 1, entry 1). The formation of 8b could be oxide under photochemical conditions for small mole-
avoided by running the reaction in a less good H- cule synthesis. The loading of initiator could be re-
donor solvent. For instance, under similar reaction duced to 10 mol% for the reaction carried out in t-
conditions in refluxing t-BuOH, 7b was isolated in BuOH without any decrease in yield (entry 7). Finally,
81% yield as a mixture of stereoisomers (Table 1, the use of a slightly larger excess of the thiophosphite
entry 2). By using di-tert-butyl hyponitrite (DTBHN) (1.8 equivalents) led to 9b in excellent yield (Table 2,
as an initiator, the reaction could be carried out at a entry 8).
lower temperature, affording 7b as a single product in
89% yield and with a very good stereocontrol
(Table 1, entry 3). Interestingly, the use of di-tert-butyl Table 2. Optimization of the diethyl thiophosphite-mediated
RATC reaction with 1b according to Eq. (7).
hyponitrite allows the amount of initiator to be re-
duced from 50 mol% down to 20 mol%. This indi-
cates, that alkoxyl radicals are more suitable to ini-
tiate the reaction by abstracting an H-atom from di-
ethyl thiophosphite than the 2-cyanoprop-2-yl radicals
generated from AIBN.
In a second series of experiments, we tried to devel-
op reaction conditions that would involve a cheap and
commercially available radical initiator such as di-tert-
butyl peroxide (DTBP) while carrying out the reac-
tion at room temperature in order to ensure good
À
Entry P(S) H (equiv.) Initiator (mol%) Solvent 9b
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.8
DTBP (20)
DTBP (20)
DTBP (20)
DCP (20)
DCP (20)
DCP (20)
DCP (10)
DCP (10)
CH2Cl2 15%
DMF
–
benzene 60%
CH2Cl2 88%
benzene 88%
t-BuOH 84%
t-BuOH 89%
t-BuOH >95%
Adv. Synth. Catal. 2011, 353, 1353 – 1358
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1355

Christophe Lamarque et al.
FULL PAPERS
In order to demonstrate the scope of the reaction,
several terminal alkynes were tested and the results
are collected in Scheme 3. The reactions were carried
out in t-BuOH under two sets of conditions. In proce-
dure A, dicumyl peroxide (10 mol%) and irradiation
at room temperature in the presence of 1.8 equiva-
lents of diethyl thiophosphite was used. In procedure
B, di-tert-butylhyponitrite (20 mol%) at 458C with 1.5
equivalents of diethyl thiophosphite was employed.
Ester, sulfonyl, sulfinyl, and phenyl substituted rad-
icals [Eqs. (8), (10)-(12)] can be successfully generat-
ed and afford the desired cyclopentane derivatives in
good yields by using procedure A or B. The relative
configurations of the major isomers of 9c, 9e–9g have
not been assigned, however, related systems possess-
ing gem-diester substituents are known to give the cis-
cyclopentanes as major isomers.[18,25] The reaction of
the dioxolane 1d afforded the cyclopentanone 9d in
79% yield [Eq. (9)]. In this particular reaction, the
radical cascade process was followed by a rapid hy-
drolysis of the acetal. Fused bicyclic skeletons are
also obtained efficiently from simple alkynes, as dem-
onstrated by the cyclization of 3b and 3c, which led to
the formation of 7b and 7c in 92% and 75% yields
[Eqs. (13)-(15)]. The relative endo configuration of
the major isomer of 7b was established by an X-ray
crystal structure analysis (Figure 1).[26] Spirocyclic
Figure 1. X-ray single crystal structure analysis of compound
7b (major diastereomer).
compounds can be prepared from easily available
starting materials such as 5b, 5c and 5d according to
Scheme 3. Reaction of various terminal alkynes with diethyl
thiophosphite. Procedure A: (EtO)2P(S)H (1.8 equiv.), di-
cumyl peroxide (10 mol%), t-BuOH, sun lamp irradiation,
room temperature. Procedure B: (EtO)2P(S)H (1.5 equiv.),
t-BuON=NO-t-Bu (20 mol%), t-BuOH, 458C. Procedure
B*: benzene was used as solvent instead of t-BuOH.
1356
asc.wiley-vch.de
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1353 – 1358

Preparation of 5-Membered Rings via Radical Addition-Translocation-Cyclization (RATC) Processes
(35 mg, 0.2 mmol) in t-BuOH (10 mL) was heated at 458C
Eqs. (15)-(17). For the spirocyclic ketals 10c and 10d,
the best results were obtained by using benzene in-
stead of tert-butyl alcohol as a solvent. It is notewor-
thy that the preparation of these highly acid-sensitive
spiroketals is not possible by using our dialkyl phos-
phite procedure. Indeed, the thermal decomposition
of the peroxides employed (dilauroyl and dibenzoyl
peroxides) very likely results in the presence of car-
boxylic acid derivatives in the reaction medium,
which led to decomposition of either the starting ace-
tals or the spiroketals. The beneficial effect of ben-
zene in the cyclization of 5c and 5d is attributed to a
higher stability of the acetal functionality in non-
protic solvent.
for 16 h. The solvent was evaporated under reduced pres-
sure along with the excess of diethyl thiophosphite. The
crude product was purified by flash chromatography.
Procedure B* (thermal initiation, benzene): A stirred so-
lution of the alkyne (1.0 mmol), diethyl thiophosphite
(231 mg, 1.5 mmol) and di-tert-butyl hyponitrite (35 mg,
0.2 mmol) in benzene (10 mL) was heated at 458C for 16 h.
The solvent was evaporated under reduced pressure along
with the excess of diethyl thiophosphite. The crude product
was purified by flash chromatography.
Acknowledgements
We thank the Swiss National Science Foundation (grants 21-
67106.01 and 7SUPJ062348) for financial support.
Conclusions
References
Dialkyl thiophosphites are powerful reagents for radi-
cal addition-translocation-cyclization (RATC) reac-
tions starting from easily available terminal alkynes.
[1] S. C. Fields, Tetrahedron 1999, 55, 12237.
[2] V. M. R. dos Santos, C. L. Donnici, J. B. N. DaCosta,
J. M. R. Caixeiro, Quimica Nova 2007, 30, 159.
[3] C. Y. Yuan, S. S. Li, C. Z. Li, S. J. Chen, W. S. Huang,
G. Q. Wang, C. Pan, Y. X. Zhang, Heteroat. Chem.
1997, 8, 103; I. P. Beletskaya, M. M. Kabachnik, Mende-
leev Commun. 2008, 18, 113.
[4] A. K. Bhattacharya, G. Thyagarajan, Chem. Rev. 1981,
81, 415.
[5] E. K. Fields, J. Am. Chem. Soc. 1952, 74, 1528; M. I.
Kabachnik, T. Y. Medved, Izvest. Akad. Nauk SSSR
Ser. Khim. 1952, 540.
[6] A. N. Pudovik, I. V. Konovalova, Synthesis 1979, 81.
[7] A. N. Pudovik, I. V. Konovalova, Zhur. Obshch. Khim.
1959, 29, 3342; D. Semenzin, G. Etemad-Moghadam,
D. Albouy, O. Diallo, M. Koenig, J. Org. Chem. 1997,
62, 2414; A. R. Stiles, W. E. Vaughan, F. F. Rust, J. Am.
Chem. Soc. 1958, 80, 714; R. L. Kenney, G. S. Fisher, J.
Org. Chem. 1974, 39, 682; H. J. Callot, C. Benezra,
Can. J. Chem. 1971, 49, 500; D. Leca, L. Fensterbank,
E. Lacote, M. Malacria, Chem. Soc. Rev. 2005, 34, 858.
[8] J. M. Barks, B. C. Gilbert, A. F. Parsons, B. Upeandran,
Tetrahedron Lett. 2001, 42, 3137; M. P. Healy, A. F. Par-
sons, J. G. T. Rawlinson, Synlett 2008, 329; C. M. Jessop,
A. F. Parsons, A. Routledge, D. J. Irvine, Tetrahedron
Lett. 2004, 45, 5095.
À
As anticipated from the comparison of P H bond dis-
sociation energies, thiophosphites are more efficient
than the corresponding phosphites and only a slight
excess (1.5–1.8 equiv.) of the thiophosphite together
with substoichiometric amounts of initiators (10–
20 mol%) are necessary to ensure full conversion and
high yields. The reaction can be carried out at room
temperature or below 508C depending on the initia-
tion method. The radical initiation system involving
dicumyl peroxide and sunlamp irradiation is particu-
larly attractive and should be applicable to a broad
range of radical processes requiring initiation by al-
koxyl radicals at moderate temperature. The different
sets of reaction conditions developed showed a high
functional group tolerance. The resulting cyclic thio-
phosphonates produced by this reaction cascade are
particularly attractive for further transformations, as
recently illustrated by Parsons and co-workers in their
approach to quinuclidines.[9,10]
Experimental Section
[9] M. P. Healy, A. F. Parsons, J. G. T. Rawlinson, Org.
Lett. 2005, 7, 1597.
[10] T. A. Hunt, A. F. Parsons, R. Pratt, J. Org. Chem. 2006,
71, 3656.
General Procedures for the RATC Reaction with
Diethyl Thiophosphite
Procedure A (photochemical initiation): A stirred solution
of the alkyne (1.0 mmol), diethyl thiophosphite (277 mg,
1.8 mmol) and dicumyl peroxide (27 mg, 0.1 mmol) in t-
BuOH (10 mL) was irradiated under nitrogen with a 300 W
sunlamp (positioned at 20 cm from the pyrex flask) for 12 h.
The solvent was evaporated under reduced pressure along
with the excess of diethyl thiophosphite. The crude product
was purified by flash chromatography.
[11] C. M. Jessop, A. F. Parsons, A. Routledge, D. J. Irvine,
Eur. J. Org. Chem. 2006, 1547.
[12] J. E. Brumwell, N. S. Simpkins, N. K. Terrett, Tetrahe-
dron 1994, 50, 13533; J. E. Brumwell, N. S. Simpkins,
N. K. Terrett, Tetrahedron Lett. 1993, 34, 1215; O.
Tayama, A. Nakano, T. Iwahama, S. Sakaguchi, Y. Ishii,
J. Org. Chem. 2004, 69, 5494; P. Carta, N. Puljic, C.
Robert, A.-L. Dhimane, C. Ollivier, L. Fensterbank, E.
Lacꢄte, M. Malacria, Tetrahedron 2008, 64, 11865; P.
Carta, N. Puljic, C. Robert, A. L. Dhimane, L. Fenster-
bank, E. Lacꢄte, M. Malacria, Org. Lett. 2007, 9, 1061.
Procedure B (thermal initiation, tert-butyl alcohol): A
stirred solution of the alkyne (1.0 mmol), diethyl thiophos-
phite (231 mg, 1.5 mmol) and di-tert-butyl hyponitrite
Adv. Synth. Catal. 2011, 353, 1353 – 1358
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1357

Christophe Lamarque et al.
FULL PAPERS
[13] C. M. Jessop, A. F. Parsons, A. Routledge, D. J. Irvine,
Tetrahedron: Asymmetry 2003, 14, 2849.
[14] F. Dꢀnꢁs, F. Beaufils, P. Renaud, Org. Lett. 2007, 9,
4375; M. Lachia, F. Dꢀnꢁs, F. Beaufils, P. Renaud, Org.
Lett. 2005, 7, 4103.
[15] F. Beaufils, F. Dꢀnꢁs, B. Becattini, P. Renaud, K.
Schenk, Adv. Synth. Catal. 2005, 347, 1587.
[16] F. Beaufils, F. Dꢀnꢁs, P. Renaud, Org. Lett. 2004, 6,
2563.
[21] F. Beaufils, F. Dꢀnꢁs, P. Renaud, Angew. Chem. 2005,
117, 5407; Angew. Chem. Int. Ed. 2005, 44, 5273.
[22] C. K. Sha, C. W. Hsu, Y. T. Chen, S. Y. Cheng, Tetrahe-
dron Lett. 2000, 41, 9865.
[23] S. R. Piettre, Tetrahedron Lett. 1996, 37, 2233; T. F.
Herpin, W. B. Motherwell, B. P. Roberts, S. Roland,
J. M. Weibel, Tetrahedron 1997, 53, 15085; A. Gautier,
G. Garipova, O. Dubert, H. Oulyadi, S. R. Piettre, Tet-
rahedron Lett. 2001, 42, 5673.
[17] D. P. Curran, D. Kim, C. Ziegler, Tetrahedron 1991, 47,
6189.
[18] D. P. Curran, W. Shen, J. Am. Chem. Soc. 1993, 115,
6051.
[24] M. V. Encinas, E. A. Lissi, J. Photochem. 1982, 20, 153.
[25] I. De Riggi, J. M. Surzur, M. P. Bertrand, A. Archavlis,
R. Faure, Tetrahedron 1990, 46, 5285; M. E. Kuehne,
R. E. Damon, J. Org. Chem. 1977, 42, 1825.
[19] F. Dꢀnꢁs, F. Beaufils, P. Renaud, Synlett 2008, 2389; P.
Renaud, F. Beaufils, F. Dꢀnꢁs, L. Feray, C. Imboden, N.
Kuznetsov, Chimia 2008, 62, 510.
[20] F. G. Bordwell, X.-M. Zhang, A. V. Satish, J. P. Cheng,
J. Am. Chem. Soc. 1994, 116, 6605; Y.-R. Luo, Hand-
book of Bond Dissociation Energies in Organic Com-
pounds, CRC Press, Boca Raton, 2003.
[26] CCDC 778530 contains supplementary crystallographic
information for 7a. These data can be obtained free of
charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, U.K.; E-mail: de-
posit@ccdc.cam.ac.uk.
1358
asc.wiley-vch.de
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1353 – 1358