Ligand-free silver(I)-catalyzed intramolecular friedel-crafts alkylation of arenes with allylic alcohols

Article abstract of DOI:10.1002/adsc.200800628

The silver-catalyzed direct activation of allylic alcohols as electrophilic partners for intramolecular Friedel-Crafts alkylation of arenes is described. Use of silver triflate (AgOTf; 10 mol%) enabled functionalized 1-vinyl-1,2,3,4-tetrahydronaphthalenes

Full text of DOI:10.1002/adsc.200800628

COMMUNICATIONS
DOI: 10.1002/adsc.200800628
Ligand-Free Silver(I)-Catalyzed Intramolecular Friedel–Crafts
Alkylation of Arenes with Allylic Alcohols
Marco Bandini,a,* Astrid Eichholzer,a Peter Kotrusz,a Michele Tragni,a
Stefano Troisi,a and Achille Umani-Ronchia,*
a
Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Universitꢀ di Bologna, Via Selmi 2, 40126 Bologna,
Italy
Fax : (+39)-051-209-9456; phone: (+39)-051-209-9751; e-mail: marco.bandini@unibo.it or achille.umanironchi@unibo.it
Received: October 13, 2008; Revised: December 16, 2008; Published online: January 15, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800628.
Abstract: The silver-catalyzed direct activation of
allylic alcohols as electrophilic partners for intramo-
lecular Friedel–Crafts alkylation of arenes is de-
scribed. Use of silver triflate (AgOTf; 10 mol%)
enabled functionalized 1-vinyl-1,2,3,4-tetrahydro-
naphthalenes and a 4-vinyl-1,2,3,4-tetrahydroisoqui-
noline to be isolated in good yields with high levels
of regiochemistry, under a ligand-free regime.
the only by-product of the reaction). However, to-
gether with these favourable features, the poor leav-
ing group character and the strongly coordinating
property of the hydroxy group have significantly lim-
ited the direct activation of alcohols in catalytic FC
processes so far.
In this context, preliminary communications were
confined to reactive electron-activated indoles,[8] and
aromatic allylic alkylation was only very recently ex-
Keywords: alkylation; aromatic compounds; cataly-
sis; Friedel–Crafts reaction; silver
tended to simple arenes. In particular, goldACTHNGUTERNN(UG III) chlo-
ride proved to be efficient in the intermolecular alky-
lation of alkoxy-substituted arenes (large excess) in
the presence of highly reactive secondary allylic alco-
hols.[9]
As a part of our continuing effort in developing
Polycyclic aromatic compounds have risen to promi- new catalytic systems for aromatic functionaliza-
nence in modern organic chemistry due to their pres- tion,[6e,10] we decided to undertake a survey of transi-
ence in countless natural occurring compounds.[1] As a tion metal salts with the aim of discovering suitable
consequence, the demand for novel, mild and high candidates for the intramolecular FC allylic alkylation
yielding synthetic routes to their preparation is grow- of benzene-like arenes with direct activation of alco-
ing.[2]
Friedel–Crafts (FC) alkylation is probably the most
hols (Figure 1).[11]
Our working hypothesis concerned the use of metal
reliable and powerful protocol for the elaboration of complexes/salts featuring dual functions such as Lewis
aromatic rings[3] and over the last few years, a great acidity and late-transition metal properties.[12] In this
deal of attention has been devoted toward the design case, in fact, both the carbon-carbon double bond and
and development of new catalytic protocols with the the hydroxy group could be alternatively or simulta-
aim of minimizing hazardous waste production.[4]
neously activated for the cyclization. Obviously, mod-
In this context, metal-catalyzed intramolecular al- erate oxophilicity is required in order to preclude the
lylic alkylations[5] deserve particular attention as they
enable the creation of benzylic stereocenters in func-
tionalized polycyclic aromatic compounds. Although
several catalytic aromatic alkylations, employing allyl
carbonates/acetates[6] and halides,[7] have been report-
ed, the new frontier in the catalytic allylic alkylation
of arenes concerns the use of allyllic alcohols as elec-
trophilic partners. The great deal of interest for such
unusual alkylation agents relies in their ready availa-
bility, atom economy and eco-sustainability (water is molecular FC-type allylic alkylation of arenes.
Figure 1. Working hypothesis for the metal-catalyzed intra-
Adv. Synth. Catal. 2009, 351, 319 – 324
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
319

Marco Bandini et al.
COMMUNICATIONS
ˇ
Inspired by the pioneering investigation of Kocov-
deactivation of the catalyst via irreversible coordina-
´
tion to the hydroxy moiety.
sky and co-workers on the [(acac)2MoACHTUNGRTNEN(UNG SbF6)2]-cata-
It is worthy of note that, although intramolecular lyzed intermolecular alkylation of phenol and ani-
protocols are generally favoured with respect to the sole,[14] we initially tested the readily available [MoCl4
intermolecular analogous, catalytic intramolecular FC
alkylations are cumbersome, due to the unfeasible use
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
of the large excesses of one reaction partner, usually conversions (56%, entry 1, Table 1) to 4a were ob-
employed in bimolecular routes. tained in refluxing dioxane after 48 h. The addition of
(Z)-Allylic alcohols 3 appeared good candidates to AgOTf to form cationic Mo species did not signifi-
assess the scope of the process. In fact, a library of cantly improve the chemical outcome (conversion:
substrates carrying different aromatic decorations, can 58%, entry 2). On the contrary, the addition of anhy-
be readily synthesized starting from the corresponding drous HCl (5 mol%), as a Brønsted co-catalyst, led to
aldehydes in four steps. Knoevenagel condensation 4a in 73% yield (entry 3). However, in all the cases,
and chemoselective reduction of the resulting C=C the concomitant formation of lactone 5a accompanied
double bond[6e] led to 1 that were condensed with the the desired FC alkylation product, in considerable
(Z)-allyllic bromide 2,[13] followed by removal of the amounts (17–30%).
TBDMS group (Scheme 1).
Iridium(I) catalysis was also considered due to its
capability of effecting intermolecular regio- and ste-
reoselective alkylation of indoles with allyl acetates.[15]
Interestingly, the combination of [Ir
ACHTUNGTRNEN(UNG COD)Cl]2/
PACHTUNGTR(NUNENG furyl)3 and AgOTf exhibits good catalytic activity,
furnishing 4a in 70% yield (5a: 10%). A higher con-
version (80%) was obtained with the addition of 5
mol% of HCl and finally, the crucial role of the coun-
teranion was verified by the reaction metathesis with
AgSbF6 (entry 6). However, the narrow generality in
Scheme 1. Synthesis of the acyclic precursors 3.
Table 1. Optimization of the catalytic system for the intramolecular FC allylic alkylation of 3a.[a]
Entry
Catalyst [mol%]
[MoCl4A(CH3CN)2] (10)
Additive [%]
Conversion [%][b] 4a/4a’/5a
1
2
3
4
5
6
7
8
G
E
–
56/0/30
58/0/27
73/0/17
70/0/10
80/0/5
10/0/50
0/63/28
60/10/10
5/0/0[d]
83/0/0[e]
89/0/0[e]
A
U
AgOTf (20)
AgOTf (20)/HCl (5)
AgOTf (10)
AgOTf (10)/HCl (5)
AgSbF6 (10)
AgOTf (30)
AgOTf (20)
AgOTf (10)
AgOTf (10)
–
A
CHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
N
RuCl3/dppb (10/10)[c]
PtCl2/dppb (10/10)[c]
AuCl (10)
9
10
11
ACHTUNGTRENNUNG
[a]
All the reactions were carried out under nitrogen atmosphere for 48 h, under reflux with anhydrous solvents. Best sol-
vents were as follows: Mo, Ir, Pt, Ru!dioxane, Au, Ag!dichloroethane.
Determined by GC-MS.
dppb=1,4-bis(diphenylphosphino)butane.
Large amount of alcohol 3a was recovered unreacted.
[b]
[c]
[d]
[e]
Isolated yield after flash chromatography. Reaction time 18 h.
320
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 319 – 324

Ligand-Free Silver(I)-Catalyzed Intramolecular Friedel–Crafts Alkylation
COMMUNICATIONS
Table 2. Silver triflate-catalyzed intramolecular FC allylic al-
kylation of aryl alcohols 3.[a]
scope was a major concern of Ir catalysis. Analogues
aspects affected also the Pt and Ru catalysis, with the
latter species (RuCl3/AgOTf) furnishing the regioiso-
mer 4a’ as the predominant product (entry 7).
Among the metal salts displaying dual functions
Au(I)[16] and Ag(I) species have risen to prominence
in catalytic organic transformations.[17]
Interestingly, in our model reaction, [(PPh3)AuCl]/
AgOTf (10 mol%) led to 4a exclusively in 83% yield
(entry 10). More importantly, focusing on operational
simplicity and costs of the process,[18] we were delight-
ed to discover that the sole AgOTf (10 mol%), even
in absence of stabilizing phosphine ligands, provided
4a in 89% yield (DCE, reflux, 18 h, entry 11).
Entry Aryl group
Product 4
Yield of 4 [%][b]
of 3
A
B
Worthy of mention is the divergent catalytic behav-
iour of gold and silver salts towards the presence of li-
gands. In particular, while ligand-free gold(I) catalysis
led to unsatisfactory chemical outcomes (Table 1,
entry 9), the addition of phosphines to AgOTf gener-
ated a general decrease in both reaction rate and che-
moselectivity (PPh3: yield=12%; BINAP: yield=
72%, ee=0%).
1
2
90
85
70
67
55
58
Contrary to the well recognized efficiency of Au(I)
and AuACHTUNGTRENNUNG(III) salts in catalytic functionalizations of
3
4
(14:1)[c] (3:1)[c]
arenes,[19] the potentialities of the slightly softer and
low toxic silver salts, in Friedel–Crafts alkylation reac-
tions, have not been explored to date.[20]
With optimal reaction conditions in hands, we fur-
ther surveyed the scope of the process, by subjecting
a range of acyclic aromatic precursors (3b–m) to ring
closure, in the presence of AgOTf (10 mol%). Our re-
sults are summarized in Table 2.
Firstly, the configuration of the carbon-carbon
double bond of the acyclic precursor does not affect
significantly the reaction outcome, with the (E)-3a al-
cohol leading to a comparably high isolated yield
(90%, entry 1). Interestingly, both electron-rich (3b–d,
i, m) and electron-neutral arenes (3e–h, l) reacted
smoothly under the best conditions furnishing the cor-
responding functionalized 1-vinyl-1,2,3,4-tetrahydro-
naphthalenes 4 in moderate to good yields. High re-
gioselectivity towards 1,4-ring-closure (14:1) was also
obtained with substrate 3c (entry 3).
For a better evaluation of the catalytic performan-
ces of AgOTf (method A), a series of comparative ex-
periments was carried out with [(PPh3)AuCl]/AgOTf
(10 mol%) (method B, Table 2).
In all cases higher isolated yields in 4 were obtained
by ligand-free silver catalysis with the exception for
alcohol 3j in which the methylthio group probably de-
activated the coordinatively unsaturated silver salt
through irreversible coordination (entry 10).
77
73
60
47
5
6
80
52
7
76
56
55
60
<5
50
63
8
9
Exemplary is the case of the ferrocenyl-containing
alcohol 3k that proved to be inert under gold-catalysis
and provided the ring closing derivative 4k in moder-
ate yield (44%) and good diastereoselection with
AgOTf (entry 11).[21]
10
53
Adv. Synth. Catal. 2009, 351, 319 – 324
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
321

Marco Bandini et al.
COMMUNICATIONS
Table 2. (Continued)
Entry Aryl group
Product 4
Yield of 4 [%][b]
of 3
A
B
44
[e]
11
12
–
(80:20)[d]
Scheme 2. Silver-catalyzed synthesis of tetrahydroisoquino-
line 7 via intramolecular FC allylic alkylation.
73[f]
28[f]
ly, the desired cyclized compound 7 was isolated in
72% yield after flash chromatography (Scheme 2).
Cationic Ag(I) salts are known to readily interact
[20,24]
ꢀ
with multiple C C bonds.
Moreover, although to
a less extent, the oxophilicity of silver salts has been
exploited in organic reactions as well.[25] These as-
pects, combined with the possible formation of highly
reactive allylic carbocationic intermediates, call for
activation modes depicted in Scheme 3.
13
65
28
[a]
All the reactions were carried out under nitrogen atmos-
phere for 48 h, in DCE under reflux, unless otherwise
specified. 10 mol% of catalyst was used.
To gain insight into the plausible coordination
mode of Ag(I)OTf, the optimal ring closing condi-
tions were applied to O-TBDMS-3f (3f’) in which the
possible coordination to the hydroxy group, should be
denied by the presence of the sterically demanding
tert-butyldimethylsilyl group (Scheme 4).
Here, although a conclusive mechanistic elucidation
still remains elusive, the comparable isolated yields in
4f, obtained with 3f and corresponding silyl ether 3f’
[b]
Isolated yield after flash chromatography. In most cases
the complete consumption of
amount of the corresponding lactones 5 (10–25%) was
formed.
3 occurred. Variable
[c]
[d]
1
The regioisomeric ratio was determined by H NMR and
GC-MS analyses.
The diastereomeric ratio was determined by 1H NMR
analysis. Reaction carried out with 20 mol% of catalyst.
Reaction time 72 h.
[e]
[f]
No reaction.
Reaction time 16 h.
The potentiality of the optimized catalytic system
for the synthesis of pharmacologically active tetrahy-
droisoquinolines[22] was then surveyed. To this aim,
the readily available allylic alcohol 6[23] was treated
with AgOTf (10 mol%) in DCE for 48 h. Interesting- Scheme 4.
Scheme 3. Hypothetical coordination modes of silver to allyl alcohols 3: i) Ag-olefin interaction; ii) allyl carbocationic inter-
mediates.
322
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 319 – 324

Ligand-Free Silver(I)-Catalyzed Intramolecular Friedel–Crafts Alkylation
COMMUNICATIONS
[4] For recent review on catalytic FC-alkylations reactions
see: a) M. Bandini. E. Emer, S. Tommasi, A. Umani-
Ronchi, Eur. J. Org. Chem. 2006, 3527–3544; b) T. B.
Poulsen, K. A. Jørgensen, Chem. Rev. 2008, 108, 2903–
2915.
(80% and 76%, respectively) calls for a coordination
of the silver(I) to the C=C bond being active during
the ring closing event (Scheme 3, path i).
In conclusion, a new, mild and environmentally
friendly silver-catalyzed strategy for the intramolecu-
lar alkylation of aromatic rings, through direct activa-
tion of alcohols, is described. Further investigations
addressing the inertness of electron-deficient arenes
and the development of an enantioselective variant
are currently underway in our laboratories.
[5] Z. Lu, S. Ma, Angew. Chem. 2008, 120, 264–303;
Angew. Chem. Int. Ed. 2008, 47, 258–297.
[6] Pd catalysis: a) M. Bandini, A. Melloni, A. Umani-
Ronchi, Org. Lett. 2004, 6, 3199–3202; b) H. Y.
Cheung, W.-Y. Yu, F. L. Lam, T. T.-L. Au-Yeung, Z.
Zhou, T. H. Chan, A. S. C. Chan, Org. Lett. 2007, 9,
4295–4298; Ir-catalysis: c) W.-B. Liu, H. He, L.-X. Dai,
S.-L. You, Org. Lett. 2008, 10, 1815–1818; Mo-catalysis:
d) A. V. Malkov, S. L. Davis, I. R. Baxendale, W. L.
Experimental Section
ˇ
´
Mitchell, P. Kocovsky, J. Org. Chem. 1999, 64, 2751–
2764; e) M. Bandini, A. Eichholzer, P. Kotrusz, A.
Umani-Ronchi, Adv. Synth. Catal. 2008, 350, 531–536;
Ru-catalysis: f) I. Fernꢂndez, R. Hermatschweiler, F.
Breher, P. S. Pregosin, L. F. Veiros, M. J. Calhorda,
Angew. Chem. 2006, 118, 6535–6540; Angew. Chem.
Int. Ed. 2006, 45, 6386–6391; I2 catalysis: g) Z. Liu, L.
Liu, Z. Shafiq, Y.-C. Wu, D. Wang, Y.-J. Chen, Tetrahe-
dron Lett. 2007, 48, 3963–3967.
Typical Procedure for the Silver-Catalyzed
Intramolecular Allylic Alkylation to Afford 4a
To a flamed-dried Schlenk tube equipped with a magnetic
stirring bar, 1 mL of anhydrous ClCH2CH2Cl (DCE) and
1.3 mg of AgOTf (5 mmol) were added. The tube was cov-
ered with an aluminium foil then a solution of (Z)-3a in
DCE (50 mmol, 0.05M) was added at once and the reaction
mixture refluxed for 18 h in the dark. The volatiles were
eliminated under reduced pressure and the crude residue di-
rectly purified through flash chromatography to afford a
white wax; yield: 89%. 1H NMR (200 MHz, CDCl3): d=
1.21 (t, J=7.2 Hz, 3H), 1.29 (t, J=7.2 Hz, 3H), 1.98 (dd,
J1 =11.8 Hz, J2 =13.8 Hz, 1H), 2.59 (dd, J1 =4.0 Hz, J2 =
11.8 Hz, 1H), 3.10 (d, J=16.0 Hz, 1H), 3.25 (d, J=16.0 Hz,
1H), 3.45–3.56 (m, 1H), 3.82 (s, 3H), 3.87 (s, 3H), 4.12–4.28
(m, 4H), 5.16–5.26 (m, 2H), 5.67–5.87 (m, 1H), 6.63 (s, 1H),
6.67 (s, 1H); 13C NMR (50 MHz, CDCl3): d=14.1 (2C),
34.7, 35.2, 41.4, 53.7, 55.9 (2C), 61.4, 61.7, 111.4 (2C), 116.3,
125.6, 128.1, 141.7, 147.6, 147.8, 170.8, 171.8; LC-MS: m/z=
363.0 (M+1), 747.2 (2M+Na); anal. calcd. for (C20H26O6):
C 66.28, H 7.23; found: C 66.25, H 7.20.
[7] R. Hayashi, G. R. Cook, Org. Lett. 2007, 9, 1311–1314.
[8] a) M. Kimura, M. Futamata, R. Mukai, Y. Tamaru, J.
Am. Chem. Soc. 2005, 127, 4592–4593; b) M. Yasuda,
T. Somyo, A. Baba, Angew. Chem. 2006, 118, 807–810;
Angew. Chem. Int. Ed. 2006, 45, 793–796; c) B. M.
Trost, J. Quancard, J. Am. Chem. Soc. 2006, 128, 6314–
6315; d) A. B. Zaitsev, S. Gruber, P. S. Pregosin, Chem.
Commun. 2007, 4692–4693; e) U. Jana, S. Maiti, S.
Biswas, Tetrahedron Lett. 2007, 48, 7160–7163; f) J. S.
Yadav, B. V. Subba Reddy, S. Aravind, G. G. K. S. Nar-
ayana Kumar, A. Srinivas Reddy, Tetrahedron Lett.
2007, 48, 6117–6120; g) Usui, S. Schmidt, M. Keller, B.
Breit, Org. Lett. 2008, 10, 1207–1210.
[9] W. Rao, P. W. H. Chan, Org. Biomol. Chem. 2008, 6,
2426–2433.
[10] a) M. Bandini, A. Melloni, A. Umani-Ronchi, Angew.
Chem. 2004, 116, 560–566; Angew. Chem. Int. Ed.
2004, 43, 550–556, and references cited therein; b) M.
Bandini, A. Eichholzer, A. Umani-Ronchi, Mini Rev.
Org. Chem. 2007, 4, 115–124, and references cited
therein.
[11] The use of mixture of Brønsted acids (TFA/AcOH) as
the solvent in intramolecular FC alkylation of corre-
sponding acetates has known to provide 4 in good
yields: a) S. Ma, J. Zhang, Tetrahedron Lett. 2002, 43,
3435; b) S. Ma, J. Zhang, Tetrahedron 2003, 59, 6273–
6283.
Acknowledgements
This work was supported by M.I.U.R. (Rome), Fondazione
del Monte 1473, and University of Bologna. M.T. thanks
NewChem S.p.A. for the financial support.
References
[1] For general review see: a) K. Tatsuta, S. Hosokawa,
Chem. Rev. 2005, 105, 4707–4729.
[2] a) G. Dyker, Angew. Chem. 1999, 111, 1808–1822;
Angew. Chem. Int. Ed. 1999, 38, 1698–1712; b) G.
[12] During the preparation of the manuscript Nishizawa
et al. reported on the use of HgACHTNUTRGNEUNG(OTf)2 as promoter for
arylenes cyclization: K. Namba, H. Yamamoto, I.
Sasaki, K. Mori, H. Imagawa, M. Nishizawa, Org. Lett.
2008, 10, 1767–1770.
ꢀ
Dyker, in: Handbook of C H transformations, Wiley-
VCH, Weinheim, 2005.
[13] W. Oppolzer, R. Moretti, C. Zhou, Helv. Chim. Acta
[3] a) G. A. Olah, in: Friedel–Crafts and Related Reactions,
Wiley, New York, 1963; b) G. A. Olah, in: Friedel–
Crafts Chemistry, Wiley, New York, 1973; c) G. A.
Olah, R. Krishnamurti, G. K. S. Prakash, in: Compre-
hensive Organic Synthesis, Vol 3, (Eds.: B. M. Trost, I.
Fleming), Pergamon, Oxford, 1991, p 293.
1994, 77, 2363–2380.
[14] A. V. Malkov, P. Spoor, V. Vinader, P. Kocovsky, J. Org.
ˇ
´
Chem. 1999, 64, 5308–5311.
[15] W.-B. Liu, H. He, L.-X. Dai, S.-L. You, Org. Lett. 2008,
10, 1815–1818.
Adv. Synth. Catal. 2009, 351, 319 – 324
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
323

Marco Bandini et al.
COMMUNICATIONS
[16] For some representative reviews of Au-catalyzed reac-
tions see: a) A. S. K. Hashmi, G. J. Hutchings, Angew.
Chem. 2006, 118, 8064–8105; Angew. Chem. Int. Ed.
2006, 45, 7896–7936; b) A. S. K. Hashmi, Chem. Rev.
2007, 107, 3180–3211; c) E. Jimꢃez-NfflÇez, A. M. Echa-
varren, Chem. Commun. 2007, 333–346; d) H. C. Shen,
Tetrahedron 2008, 64, 3885–3903; e) R. Skouta, C.-J.
Li, Tetrahedron 2008, 64, 4917–4938; f) R. A. Widen-
hoefer, Chem. Eur. J. 2008, 14, 5382–5391; g) A.
Arcadi, Chem. Rev. 2008, 108, 3266–3325; h) Z. Li, C.
Brouwer, C. He, Chem. Rev. 2008, 108, 3239–3265.
[17] Z. Shafiq, L. Liu, Z. Liu, . D. Wang, Y.-J. Chen, Org.
Lett. 2007, 9, 2525–2528, and references cited therein.
[18] Sigma–Aldrichꢄ actual prices: [(PPh3)AuCl]/AgOTf=
229.30 E/g and AgOTf (>99%)=25.30 E/g.
13596–13597; d) Z. Shi, C. He, J. Am. Chem. Soc. 2004,
126, 5964–5965; e) M. Alfonsi, A. Arcadi, G. Bianchi,
F. Marinelli, A. Nardini, Eur. J. Org. Chem. 2006,
2393–2402; f) R.-V. Nguyen, X. Yao, C.-J. Li, Org.
Lett. 2006, 8, 2397–2399; g) C. Liu, R. A. Widenhoefer,
Org. Lett. 2007, 9, 1935–1938; h) M. A. Tarselli, M. R.
Gagnꢃ, J. Org. Chem. 2008, 73, 2439–2441.
[20] S. W. Youn, J. I. Eom, J. Org. Chem. 2006, 71, 6705–
6707.
[21] Acyclic precursors carrying electron-withdrawing sub-
stituents on the aromatic ring proved to be inert under
Ag catalysis.
[22] For a comprehensive review, see: A. R. Katritzky, S.
Rachwal, B. Rachwal, Tetrahedron 1996, 52, 15031–
15070.
[23] See Supporting Information for details.
[24] C.-G. Yang, N. W. Reich, Z. Shi, C. He, Org. Lett. 2005,
7, 4553–4556.
[25] For a recent review on enantioselective silver-catalyzed
reactions see: M. Naodovic, H. Yamamoto, Chem. Rev.
2008, 108, 3132–3148.
[19] For some representative example see: a) A. S. K.
Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost, Angew.
Chem. 2000, 112, 2382–2385; Angew. Chem. Int. Ed.
2000, 39, 2285–2288; b) G. Dyker, E. Muth, A. S. K.
Hashmi, L. Ding, Adv. Synth. Catal. 2003, 345, 1247–
1252; c) Z. Shi, C. He, J. Am. Chem. Soc. 2004, 126,
324
asc.wiley-vch.de
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 319 – 324