Bicatalytic allylation-cross-metathesis reactions as γ-carbonyl cation equivalents

Article abstract of DOI:10.1055/S-0032-1317045

The products corresponding to the reactions of arenes and γ-carbonyl cations may be obtained by a one-pot, bicatalytic process involving InCl ₃-catalyzed arene allylation and cross metathesis with electron-deficient alkenes. The process is successful with electronically neutral and electron-rich arenes, and modestly Lewis basic donor groups are tolerated with an increase in InCl3 loading from 10 mol% to 15 mol%, and in one case, 20 mol%. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/S-0032-1317045

LETTER
▌2371
letter
Bicatalytic Allylation–Cross-Metathesis Reactions as γ-Carbonyl Cation
Equivalents
Bicatalytic Allylation–Cross Metathesis
Jake R. Henkie, Sugadar Dhaliwal, James R. Green*
Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, N9B 3P4, Canada
Fax +1(519)9737098; E-mail: jgreen@uwindsor.ca
Received: 19.06.2012; Accepted: 19.07.2012
Fortunately, allylbenzene is an excellent partner in cross
metathesis (CM) for carbonyl-substituted alkenes,15
which are in turn type 2 alkenes with the Grubbs 2nd gen-
eration (G2) precatalyst.16 As a result, we considered it
Abstract: The products corresponding to the reactions of arenes
and γ-carbonyl cations may be obtained by a one-pot, bicatalytic
process involving InCl3-catalyzed arene allylation and cross me-
tathesis with electron-deficient alkenes. The process is successful
with electronically neutral and electron-rich arenes, and modestly possible that the products corresponding to reaction of
Lewis basic donor groups are tolerated with an increase in InCl3
loading from 10 mol% to 15 mol%, and in one case, 20 mol%.
γ-carbonyl cations with arenes (4, Scheme 1) could be re-
alized by a one-pot, bicatalytic allylation–cross metathe-
Key words: allylation, electrophilic aromatic substitution, indium,
metathesis, umpolung
sis process involving indium(III). Herein we report our
efforts towards this goal.
+
O
O
O
+
The creation of γ-carbonyl cation equivalents (or 1,5-
homo-Michael equivalents) (1, Figure 1) is a tactic of in-
creasing importance for functionalizing carbonyl systems
by way of umpolung synthons.1 Among methods not in-
volving rearrangement of the carbon framework,2 there
has been considerable success in the generation and use of
γ-carbonyl cationic species stabilized as propargyldi-
cobalt complexes (2)3,4 or iron allyl complexes [3, MLn =
Fe(CO)4],5,6 however, these methods require stoichiomet-
ric complexation and decomplexation steps.7 A number of
metal-catalyzed allylation (3, M = Pd, Mo, Ir) and propar-
+
R
R
R
MLn
Co2(CO)6
2
1
3
Figure 1 γ-Carbonyl cations
O
R1
R1
R1
R2
Br
R2
O
InCl3 (cat)
Grubbs II
4
gylation reactions are known, which have been8 or could Scheme 1 Allylation–cross metathesis protocol
be in principle be extended to γ-carbonyl cation equiva-
lents. Unfortunately, these have limited electrophilicity
Initial experimentation began with 1,3,5-trimethoxy-
benzene (5a, 3 equiv; Figure 2) as test nucleophile. In the
presence of allyl bromide and 4 Å molecular sieves,
10 mol% InCl3 gave conversion to 2-allyl-1,3,5-trime-
thoxybenzene at reflux in CH2Cl2 (20 h), but ceased at
90% conversion. Consequently, 15 mol% InCl3 was em-
ployed, to give complete conversion over the same peri-
od.17 At this point, addition of Grubbs II precatalyst and
methyl acrylate (6a, 3 equiv) showed complete conver-
sion to 4a at 10 mol% catalyst loading. Unfortunately, the
analogous experiments employing methyl vinyl ketone
(6b) as CM partner afforded no indication of cross me-
tathesis, but rather 1H NMR spectral analysis revealed the
presence of extensive amounts of the product of conjugate
addition of 1,3,5-trimethoxybenzene to methyl vinyl ke-
tone (7, Figure 3).
and as such are insufficiently reactive with most arenes.9
Phosphine-catalyzed γ-nucleophilic attack reactions are
known for electron-deficient alkynes or allenes,10 but
these also require strong nucleophiles for successful reac-
tion. Recently, 3,4-allenoates have shown the capability
of undergoing gold(I)- or palladium(II)-catalyzed reac-
tions with nucleophiles to give γ-functionalized alkeno-
ates; based on a limited number of cases, strongly
activated arene nucleophiles may be employed.11 In con-
trast, there have been a number of recent strong Lewis
acid catalyzed allylation and propargylation protocols.12
By far most of these and a few of metal-catalyzed
systems13 require substrates in which the allyl cation is ad-
ditionally stabilized, rather than being capable of bearing
strongly electron-withdrawing groups.
Among the Lewis acid allylation catalysts, indium(III) has
been demonstrated recently to be among the most reac-
tive, as they have the ability to induce allylation from sub-
strates with no additional cation stabilizing groups.14
In view of the apparent limitations of this protocol, a num-
ber of additives were screened in order to minimize the
conjugate addition side reactions caused either by residual
indium(III) or by the HBr liberated in the allylation pro-
cess. It was found that addition of NaHCO3 (3 equiv), pri-
or to the addition of the electron-deficient alkene and G2
precatalyst, enabled the CM reaction with methyl vinyl
ketone to proceed to give 4b with 7.5 mol% G2, and with
SYNLETT 2012, 23, 2371–2374
Advanced online publication: 24.08.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317045; Art ID: ST-2012-S0530-L
© Georg Thieme Verlag Stuttgart · New York

2372
J. R. Henkie et al.
LETTER
methyl acrylate allowed a reduction in the required product), and decreased with increasing ortho-substitu-
amount of Grubbs II precatalyst for conversion to 4a to tion, but was a substantial by-product in the case of ben-
5 mol%.
zene (5c, 10 equiv) as nucleophile and methyl acrylate
(10% of 8 relative to 4d). For this case 12 mol% of 1,4-
benzoquinone19 was added to the reaction mixture imme-
diately prior to the 5 mol% G2 precatalyst; this modifica-
tion reduced the amount of cinnamate by-product to trace
amounts (1–1.5% relative to 4d) and allowed the forma-
tion of 4d in good yield (75%).
R
R
R
R
5c R = H
5d R = Me
5a R = OMe
5b R = Me
5g
5e
5f
OMe
O
OMe
O
R
R
R
O
OMe
CO2Me
CN
O
Br
MeO
OMe
MeO
6a R = OMe
6b R = Me
6c R = H
5h R = H
5i R = OMe
6d
8
7
9
5k
5j
Figure 3 Reaction side-products
Figure 2 Selected arenes and electron-poor alkenes
The general trends with mesitylene also applied to other
arenes lacking Lewis basic functions, in that the InCl3 (10
mol%), NaHCO3 (3 equiv), CH2Cl2, room temperature
conditions were sufficient for complete allyl bromide con-
sumption (Table 1, Figure 4). Toluene (5d, 10 equiv) ulti-
mately afforded 4e (82% yield) as a mixture of
Experimentation moved at this point to mesitylene (5b),
chosen as a nucleophile representative of arenes without
Lewis basic groups. In our hands, and consistent with the
work of Cook,14a complete consumption of allyl bromide
now occurred with 10 mol% at room temperature in
CH2Cl2 (20 h), but instead of simple allylation, only the
products of allylation followed by HBr addition to the al-
kene (8, Figure 3) were observed in the 1H NMR spectrum
of the reaction mixture. Fortunately, a simple switch in
protocol to the addition of solid NaHCO3 (3 equiv, in ad-
dition to the 4 Å molecular sieves) prior to the addition of
allyl bromide reduced the amount of 8 to at most trace lev-
els, and did not appear to interfere substantially with the
allylation rate. As a result, this protocol was adopted as
the general one, with modifications in InCl3 and G2 load-
ings to give complete conversions. In most cases, the
amount of nucleophile was fixed at three equivalents, ex-
cept in the cases of arene nucleophiles sufficiently volatile
to be removed readily under vacuum. In those instances
larger excesses were employed. Under these conditions
the 1,3,5-trimethoxybenzene (5a)–methyl acrylate (6a)
combination afforded 4a in 67% yield, the 1,3,5-trime-
thoxybenzene–methyl vinyl ketone (6b) combination af-
forded 4b in 71% yield, and the mesitylene–methyl
acrylate combination gave 4c in 72% yield (Table 1).
Chromatographic separation of each allylation–cross me-
tathesis product was readily accomplished; this is in con-
trast to the simple allylation intermediates, which were
difficult to separate from the arene nucleophile. Each of
Table 1 Results of the Allylation–Cross Metathesis
Arene
(equiv)
InCl3
(mol%) (°C)
Temp
6
G2
(mol%)
4
Yield
(%)
5a (3)
5a (3)
5b (3)
5c (10)
5d (10)
5e (10)
5f (5)
15
15
10
10
10
10
10
10
10
15
15
20f
10
10
10
40
40
22
22
22
22
22
22
22
40
40
61
22
22
22
6a
6b
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6b
6c
6d
5.0
7.5
4.6
5.0a
5.4
5.0
5.0
5.0
2.8
2.4
3.2
5.0
3.7
8.9
7.3g
4a
4b
4c
4d
4e
4f
67
71
72
75
82b
72c
65
4g
4h
4i
5g (10)
5h (3)
5i (3)
61
61d
73
4j
5j (3)
4k
4l
61e
60
5k (3)
5b (3)
5b (3)
5b (3)
1
4m
4n
4o
68
4a–n was isolated as exclusively the E-isomer, while H
NMR spectra of the crude reaction products showed pre-
purification E/Z-ratios of ≥96:4.
61
63h
One other side product could be observed during this pro-
cess, during the reactions involving methyl acrylate,
which was the formation of small amounts of β-aryl-
alkenoate (cinnamate) 9. This material was believed to
stem from RuH-induced isomerization of the alkene func-
tion of the allylated arene into conjugation with the arene,
a 1,4-Benzoquinone (12 mol%) was added.
b 4e/4e′/4e′′ = 60:30:10.
c 4f/4f′ = 70:30.
d 4i/4i′ = 87:13.
e 4k/4k′ = 85:15.
f Reaction conditions: CHCl3, reflux, 2 d.
followed by its cross metathesis.18 In most cases this was g Hoveyda–Grubbs II catalyst was employed
h Z/E = 71:29.
quite minimal in amount (≤4% relative to the intended
Synlett 2012, 23, 2371–2374
© Georg Thieme Verlag Stuttgart · New York

LETTER
Bicatalytic Allylation–Cross Metathesis
2373
regioisomers (o/p/m [4e/4e′/4e′′] = 60:30:10), while m- Finally, a small number of electron-deficient alkenes oth-
xylene (5e, 10 equiv) gave a mixture of 1,2,4- and 1,2,3- er than methyl acrylate were investigated in conjunction
substitution products 4f (1,2,4-/1,2,3- [4f/4f′] = 70:30) in with mesitylene (5b) for the allylation–cross metathesis
72% yield. While p-xylene (5f, 5 equiv) and 1,2,3,4-tetra- protocol. Use of methyl vinyl ketone (6b) gave 4m (68%
methylbenzene (5g, 10 equiv) gave 4g (65% yield) and 4h yield) readily, and in contrast to the case employing 1,3,5-
(61% yield) in straightforward fashion, naphthalene (5h) trimethoxybenzene, required only 4 mol% of G2 for com-
gave 4i with a small amount of the C-2 substitution isomer pletion of the CM step. Acrolein (6c) also served as suit-
(4i′) along with the C-1 isomer (C-1/C-2 [4i/4i′] = 87:13, able partner in the allylation–cross metathesis process,
61% yield).
although an increased amount of G2 (9 mol%) was re-
quired to obtain 4n in 61% yield. Finally, the use of acry-
lonitrile (6d) and Hoveyda–Grubbs II precatalyst (HG2)
(7 mol%) enabled the formation of 4o (63% yield). As is
established in CM reactions with acrylonitrile with HG2,
the Z-isomer of 4o predominated (Z/E = 71:29).20
The other alkoxyarenes employed as nucleophiles showed
significant similarities to 1,3,5-trimethoxybenzene, in that
a higher InCl3 loading (15 mol%) in CH2Cl2 at reflux
(with 3 equiv NaHCO3) was required for complete con-
sumption of allyl bromide. Under such conditions,
3,4-methylenedioxytoluene (5i) and 2,6-dimethoxynaph- In summary, we have demonstrated that a bicatalytic, one-
thalene (5j) were converted into 4j (73% yield) and 4k pot indium(III) allylation–cross metathesis protocol can
(60% yield) following cross metathesis, the latter case afford the products corresponding to the reaction of γ-car-
predominantly as the C-1 isomer (C-1/C-3 [4k/4k′] = bonyl cations with arene nucleophiles.21 Relative to the
85:15). 1,2,4-Trimethoxybenzene (5k) reacted more slug- complementary Nicholas reaction chemistry, arenes of
gishly, possibly due to the presence of methoxy groups ca- somewhat more modest nucleophilicity, including ben-
pable of chelation to indium(III), but the use of 20 mol% zene, may be employed in the current work. In compari-
InCl3 in CHCl3 at reflux (2 d) slowly gave complete con- son to other catalytically generated γ-carbonyl cation
sumption of allyl bromide, and ultimately product 4l (60% equivalents, the current protocol may employ far less re-
yield).
active nucleophiles. Moderately Lewis basic substituents
are tolerated in the allylation step, although somewhat
more aggressive conditions are required.
R
R
R
O
R
CO2Me
CO2Me
Future work will be concerned with the use of substituted
allyls for the goal of creating products with γ-substitution,
and with other nucleophiles compatible with the bicatalyt-
ic allylation–CM protocol.
R
R
4e R = o-Me
4e' R = p-Me
4e'' R = m-Me
R 4a R = OMe
4c R = Me
4d R = H
4b R = OMe
4m R = Me
Acknowledgment
CO2Me
CO2Me
R2
We are grateful to NSERC (Canada), the Canada Foundation for in-
novation (CFI) and the Ontario Innovation Trust (OIT) for support
of this research. We wish to thank Prof. Greg Cook (North Dakota
State University) for helpful discussions on indium(III)-catalyzed
allylations and Prof. Deryn Fogg (University of Ottawa) for fruitful
discussions on cross metathesis.
R1
4g
4f R1 = Me, R2 = H
4f' R1 = H, R2 = Me
O
O
CO2Me
Supporting Information for this article is available online at
CO2Me
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
4h
4j
CO2Me
References
R
R
R
R
(1) (a) Hase, T. A. In Umpoled Synthons; Hase, T. A., Ed.;
Wiley: New York, 1987, 292. (b) Green, J. R. Synlett 2012,
23, 1271.
CO2Me
4i' R = H
4k' R = OMe
4i R = H
(2) For selected papers on carbonyl- or donor–acceptor-
substituted cyclopropanes, see: (a) Qu, J.-P.; Deng, C.;
Zhou, J.; Sun, X.-L.; Tang, Y. J. Org. Chem. 2009, 74, 7684.
(b) Lifchits, O.; Alberico, D.; Zakharian, I.; Charette, A. B.
J. Org. Chem. 2008, 73, 6838; and references therein.
(c) Hu, B.; Xing, S.; Wang, Z. Org. Lett. 2008, 10, 5481.
(d) Tang, J.; Ren, J.; Wang, Z. Tetrahedron Lett. 2008, 49,
6659. (e) Yu, M.; Pagenkopf, B. L. Tetrahedron 2005, 61,
321. (f) Harrington, P. A.; Kerr, M. A. Tetrahedron Lett.
1997, 38, 5949. (g) Danishefsky, S. Acc. Chem. Res. 1979,
12, 66.
4k R = OMe
MeO
MeO
O
H
CO2Me
CN
OMe
4l
4n
4o
Figure 4 Allylation–cross metathesis products
(3) (a) Taj, R. A.; Green, J. R. J. Org. Chem. 2010, 75, 8258.
(b) Green, J. R.; Tjeng, A. A. J. Org. Chem. 2009, 74, 7411.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2371–2374

2374
J. R. Henkie et al.
LETTER
(12) (a) Bandini, M.; Tragni, M. Org. Biomol. Chem. 2009, 7,
(c) Green, J. R. Chem. Commun. 1998, 1751.
(d) Vizniowski, C. S.; Green, J. R.; Breen, T. L.; Dalacu, A.
V. J. Org. Chem. 1995, 60, 7496.
1501. (b) Detz, R. J.; Hiemstra, H.; van Maarseveen, J. H.
Eur. J. Org. Chem. 2009, 6263.
(4) For recent reviews on the Nicholas reaction, see: (a) Diaz, D.
D.; Betancort, J. M.; Martin, V. S. Synlett 2007, 343.
(b) Teobald, B. J. Tetrahedron 2002, 58, 4133. (c) Green, J.
R. Curr. Org. Chem. 2001, 5, 809. (d) Fletcher, A. J.;
Christie, S. D. R. J. Chem. Soc., Perkin Trans 1 2000, 1657.
(e) Welker, M. E. Curr. Org. Chem. 2001, 5, 785. (f) Müller,
T. J. J. Eur. J. Org. Chem. 2001, 2021. (g) Omae, I. Appl.
Organomet. Chem. 2007, 21, 318. (h) Bromfield, K. M.;
Gradén, H.; Ljungdahl, N.; Kann, N. Dalton Trans. 2009,
5051.
(5) (a) Green, J. R.; Carroll, M. K. Tetrahedron Lett. 1991, 32,
1141. (b) Zhou, T.; Green, J. R. Tetrahedron Lett. 1993, 34,
4497. (c) Enders, D.; Finkham, M. Synlett 1993, 401.
(d) Charlton, M. A.; Green, J. R. Can. J. Chem. 1997, 75,
965. (e) Enders, D.; Jandeleit, B.; Von Berg, S. Synlett 1997,
421; and references therein. (f) Enders, D.; Jandeleit, B.; von
Berg, S.; Raabe, G.; Runsink, J. Organometallics 2001, 20,
4312.
(6) For reviews including γ-carbonyl cation equivalents and
their vinylogous homologues by way of iron dienyl cations,
see: (a) Pearson, A. J. Adv. Met.-Org. Chem. 1988, 1, 1; and
references therein. (b) Pearson, A. J. Iron Compounds in
Organic Synthesis; Academic: London, 1994, Chap. 5.
(c) Donaldson, W. A.; Chaudhury, S. Eur. J. Org. Chem.
2009, 3831. (d) Donaldson, W. A. Curr. Org. Chem. 2000,
4, 837.
(7) For stoichiometric allylmolybdenum cases, see: (a) Zhang,
Y.; Liebeskind, L. J. Am. Chem. Soc. 2005, 127, 11258.
(b) Coombs, T. C.; Zhang, Y.; Garnier-Ambland, E. C.;
Liebeskind, L. S. J. Am. Chem. Soc. 2009, 131, 876.
(8) Palladium: (a) Jackson, W. R.; Strauss, J. U. G. Tetrahedron
Lett. 1975, 2591. (b) Tsuji, J.; Ueno, H.; Kobayashi, Y.;
Okumoto, H. Tetrahedron Lett. 1981, 21, 2573. (c) Nemoto,
T.; Fukuda, T.; Matsumoto, T.; Hitomi, T.; Hamada, Y. Adv.
Synth. Catal. 2005, 347, 1504. (d) Molybdenum: Trost, B.
M.; Lautens, M. Organometallics 1983, 2, 1687. (e) Iridium:
Baeza, A.; Casas, J.; Nájera, C.; Sansano, J. M. J. Org.
Chem. 2006, 71, 3837.
(13) Miyake, Y.; Uemura, S.; Nishibayashi, Y. ChemCatChem
2009, 1, 342.
(14) (a) Kaneko, M.; Hayashi, R.; Cook, G. R. Tetrahedron Lett.
2007, 48, 7085. (b) Lim, H. J.; Keum, G.; Kang, S. B.; Kim,
Y.; Chung, B. Y. Tetrahedron Lett. 1999, 40, 1547. (c) For a
Rh(I)- or Ir(I)-based catalyst, see: Tsukada, N.; Yagura, Y.;
Sato, T.; Inoue, Y. Synlett 2003, 1431. (d) For cases via
Ru(IV) allyl intermediates, see: Fernández, I.;
Hermatschweiler, R.; Breher, F.; Pregosin, P. S.; Veiros, L.
F.; Calhorda, M. J. Angew. Chem. Int. Ed. 2006, 45, 6386.
(e) For Mo catalysis, see: Shimizu, I.; Sakamoto, T.;
Kawaragi, S.; Maruyama, Y.; Yamamoto, A. Chem. Lett.
1997, 26, 137.
(15) For recent reviews, see: (a) Prunet, J.; Grimaud, L. In
Metathesis in Natural Product Synthesis; Cossy, J.;
Arseniyadis, S.; Meyer, C., Eds.; Wiley-VCH: Weinheim,
2010, 287–312. (b) Waetzig, J. D.; Hanson, P. R.
Chemtracts 2006, 19, 157. (c) Connon, S. J.; Blechert, S.
Angew. Chem. Int. Ed. 2003, 42, 1900.
(16) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H.
J. Am. Chem. Soc. 2003, 125, 11360.
(17) In the absence of molecular sieves, lower conversions
occurred.
(18) Alcaide, C.; Almendros, P.; Luna, A. Chem. Rev. 2009, 109,
3817.
(19) Hong, S. H.; Sanders, D. P.; Lee, C. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2005, 127, 17160.
(20) (a) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics
2006, 25, 25. (b) Ledoux, N.; Linden, A.; Allaert, B.; Vander
Mierde, H.; Verpoort, F. Adv. Synth. Catal. 2007, 349, 1692.
(21) (a) Representative Procedure: To a suspension of InCl3
(0.0216 g, 0.098 mmol, 15 mol%) and 4 Å molecular sieves
(ca. 0.4 g) in CH2Cl2 (6 mL) were added 1,3,5-
trimethoxybenzene (0.328 g, 1.95 mmol), NaHCO3 (0.164 g,
1.95 mmol), and allyl bromide (55.0 μL, 0.650 mmol). The
mixture was heated to reflux for 20 h. The mixture was
removed from the heating source, at which time methyl
acrylate (0.18 mL, 2.0 mmol) and Grubbs II precatalyst
(0.0143 g, 0.0168 mmol, 2.6 mol%) were added. Following
heating to reflux for 10 h, removal of the volatiles under
reduced pressure and preparative TLC (PE–Et2O, 2:1)
afforded 4a as a colorless solid; yield: 0.1167 g (0.438
mmol, 67%). IR (neat): 3000 (m), 2949 (m), 1716 (s), 1652
(s) cm–1. 1H NMR (500 MHz): δ = 7.05 (dt, J = 15.6, 6.4 Hz,
1 H), 6.14 (s, 2 H), 5.72 (dt, J = 15.6, 1.6 Hz, 1 H), 3.82 (s,
3 H), 3.79 (s, 6 H), 3.69 (s, 3 H), 3.46 (dd, J = 6.4, 1.6 Hz, 2
H). 13C NMR (125 MHz): δ = 167.5, 160.0, 158.7, 148.4,
120.1, 106.4, 90.5, 55.6, 53.3, 51.2, 25.5. MS: m/z = 266
[M+]. HRMS (EI): m/z calcd for C14H18O5: 266.1154; found:
266.1144.
(9) (a) Kuhn, O.; Mayr, H. Angew. Chem. Int. Ed. 1999, 38, 342.
(b) Troshin, K.; Mayer, P.; Mayr, H. Organometallics 2012,
31, 2416.
(10) For selected papers, see: (a) Trost, B. M.; Li, C. J. Am.
Chem. Soc. 1994, 116, 3167. (b) Lu, X.; Zhang, C.; Xu, Z.
Acc. Chem. Res. 2001, 34, 535.
(11) (a) Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 2079.
(b) Tarselli, M. A.; Liu, A.; Gagné, M. R. Tetrahedron 2009,
65, 1785. (c) Fang, Z.; Fu, C.; Ma, S. Chem.–Eur. J. 2010,
16, 3910. (d) Fang, Z.; Fu, C.; Ma, S. Eur. J. Org. Chem.
2011, 1227. (e) See also: Zhang, M.; Jiang, H.; Dixneuf, P.
H. Adv. Synth. Catal. 2009, 351, 1488.
Synlett 2012, 23, 2371–2374
© Georg Thieme Verlag Stuttgart · New York