Efficient cyanation of aryl bromides with K4[Fe(CN)6] catalyzed by a palladium-indolylphosphine complex

Article abstract of DOI:10.1016/j.tetlet.2011.09.088

This study describes a general palladium-catalyzed cyanation of aryl bromides using K₄[Fe(CN)₆] as the cyanide surrogate. The reactions can be successfully conducted under mild reaction conditions (at 50 °C) in mixed solvents (water/MeCN = 1:1) without any surfactant additives, and afford the desired aryl nitriles in good-to-excellent yields. Particularly noteworthy is that this system allows the mildest reaction temperature reported so far for palladium-catalyzed cyanation of aryl bromides with K ₄[Fe(CN)₆] source in general. Common functional groups, including keto, aldehyde, free amine, and heterocyclic substrates are compatible under this system. Interestingly, the phosphine ligands bearing -PCy ₂ moiety, which usually show excellent activity in aryl halide couplings, are found less effective than the corresponding ligands with -PPh₂ group.

Full text of DOI:10.1016/j.tetlet.2011.09.088

Tetrahedron Letters 52 (2011) 7038–7041
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Efficient cyanation of aryl bromides with K4[Fe(CN)6] catalyzed by a
palladium-indolylphosphine complex
⇑
Pui Yee Yeung, Chun Pui Tsang, Fuk Yee Kwong
State Key Laboratory for Chirosciences and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
This study describes a general palladium-catalyzed cyanation of aryl bromides using K4[Fe(CN)6] as the
cyanide surrogate. The reactions can be successfully conducted under mild reaction conditions (at
50 °C) in mixed solvents (water/MeCN = 1:1) without any surfactant additives, and afford the desired aryl
nitriles in good-to-excellent yields. Particularly noteworthy is that this system allows the mildest reac-
tion temperature reported so far for palladium-catalyzed cyanation of aryl bromides with K4[Fe(CN)6]
source in general. Common functional groups, including keto, aldehyde, free amine, and heterocyclic sub-
strates are compatible under this system. Interestingly, the phosphine ligands bearing –PCy2 moiety,
which usually show excellent activity in aryl halide couplings, are found less effective than the corre-
sponding ligands with –PPh2 group.
Received 3 August 2011
Accepted 16 September 2011
Available online 22 September 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Aryl nitriles are common structural motifs constituted in phar-
maceuticals,1 agrochemicals, dyes, herbicides, and insecticides
(Fig. 1). In organic synthesis, there are a myriad of nitrile organic
transformations.2,3 For instances, the synthesis of benzoic acids/es-
ters, amides, amines, aldehydes, and nitrogen-containing heterocy-
cles can be from nitrile precusors.4–8 Traditional methods of the
synthesis for arene-containing nitriles are the Rosenmund–
Braun9–12 and Sandmeyer13,14 reaction, which start from aryl
iodides/bromides and aryl diazonium salts, respectively. Stoichi-
ometric amount of CuCN is usually employed and the reaction tem-
peratures can be as high as 150–250 °C. Ammoxidation is an
industrial method for preparing simple aromatic nitriles.15,16 Tolu-
ene derivatives are used to react with oxygen and ammonia in the
presence of heterogeneous catalysts under high reaction pressure
and temperature.
cyanation has further developed for the applications in pharma-
ceutical production.18,26,32–37 In fact, palladium systems have a bet-
ter functional group tolerance, higher catalyst stability under air/
moisture, and superior catalytic activity. The palladium-catalyzed
cyanation has mostly been explored on aryl bromide substrates
with elevated temperatures in the presence of cyanides M–CN
(M = alkali metal,21,25,38,39 trimethylsilyl,40,41 Cu,42,43 Zn,37,44–47),
acetone cyanohydrin,48 potassium hexacyanoferrate(II),23,49–53
and other cyanating agents.54 Herein, we report a general palla-
dium-catalyzed cyanation of aryl bromides under mild reaction
conditions. Although the reaction temperature of 50 °C had been
reported by Ryberg37 using ZnCN source, to the best of our knowl-
edge, we are the first to show a cyanation procedure for aryl
bromides mediated by environmentally benign potassium hexacy-
anoferrate(II) at that temperature without any additives.
The limitations of these methods have been overcome later by
transition metal-catalyzed cyanations, as versatile alternatives for
accessing more complicated molecules bearing benzonitrile motifs
(Fig. 1).17–19 Nickel,20,21 copper,22–25 and palladium are the most
effective metal complexes for this purpose. Yet, nickel catalyst is
less attractive to palladium catalyst because of its high air- and
moisture-sensitivity, high catalyst loading (stoichiometric amount
of Ni complex)21 and low functional group compatibility. Although
copper catalyst is less expensive, it is not applicable for aryl chlo-
ride cyanation and less effective for aryl bromide cyanation at
low temperature.
We initially tested the palladium-catalyzed cyanation using
electronically neutral 3-bromotoluene as the benchmark substrate
in the presence of a series of commercially available ligands, as
well as our previously developed indolylphosphines L1–L4
(Scheme 1). It is interesting to show that well-known superior
ligands for cross-coupling reactions such as XPhos,55 SPhos,56
CataCXium A,57 and CataCXium PCy58 were less effective under
the stated conditions. Moreover, we were surprised that our previ-
ously developed CM-Phos,52 which was found successful in aryl
sulfonate coupling reactions, was also inferior. With our indolyl
ligand family in hand, we next probed other ligand structures
L160–L4 that could offer efficacy to this cyanation reaction. Surpris-
ingly, L1 displayed lower yield than L2 (Scheme 1). It should be
noted that L1 and L2 consisted of the same skeletal structure
except the –PR2 group. In fact, the ligand possessed –PCy2 moiety
should offer higher efficiency in general cross-coupling of aryl
In 1973, the first palladium-catalyzed cyanation protocol was
disclosed.31 Since this pioneering study, palladium-catalyzed
⇑
Corresponding author.
E-mail address: bcfyk@inet.polyu.edu.hk (F.Y. Kwong).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.088

P. Y. Yeung et al. / Tetrahedron Letters 52 (2011) 7038–7041
7039
CN
CN
retain excellent activity in some unique cross-coupling reactions.
Apart from L2, the aminophosphine L359 also showed good activ-
ity. After fine tuning of the ligand structure, the best ligand choice
was found to be L4, named as Ph-o-Andole-phos.
Me
NC
N
Me
O
Me
Me
N
O
NH
With the aid of the Pd/L4 (Pd/Ph-o-Andole-phos) catalyst sys-
tem, we further investigated reaction parameters in this cyanation
(Table 1). Palladium(0) salts were found to be more effective for
promoting this reaction (entries 1–7). Pd(dba)2 gave the best result
(entry 7). The concentration of K4[Fe(CN)6]Á3H2O showed signifi-
cant inhibition to the catalyst (entries 4, 8–12). In fact, previous
study also revealed that the concentration of CNÀ source (from
ZnCN) would suppress the cyanation efficiency.45 In our study,
the optimal stoichiometry of K4[Fe(CN)6]Á3H2O used was found to
be 0.23 mmol (entry 10). Inorganic bases were found to be essen-
tial for the fruitful performance of cyanation (entries 4, 13–15).
The best result was obtained in the presence of KOt-Bu.
N
Br
NH2
F
Escitalopram (Citalopram)
Antidepressant
Selective serotonin uptake inhibitor
Etravirine
Reverse transciptase inhibitor
CN
F3C
NC
NC
O
OH
NH
Me
O
S
N
N
O
O
N
N
Me
S
With the optimized reaction conditions in hand, we next probed
the effectiveness of this new system with a series of aryl bromides
(Table 2). A variety of para- and meta-substituted aryl bromides
with functional groups such as chloro, methoxy, keto, free amine,
and especially aldehyde were compatible under our reaction con-
ditions (entries 2–10). It should be noted that no competitive ami-
nation of 3-bromoaniline and 5-bromo-2-methylaniline was
observed from GC analysis (entries 6 and 8). Thus this catalytic sys-
tem is attractive that offers specific cyanation of aryl bromides in
the presence of other common functional groups. Heterocyclic sub-
strate, 6-bromoquinoline, furnished the desired product in excel-
lent yield (entry 12).
In summary, we have succeeded in showing the mildest
Pd-catalyzed cyanation of aryl bromides using K4[Fe(CN)6] as the
cyanide source. The new palladium/Ph-o-Andole-phos catalyst sys-
tem described here exhibits good functional group compatibility.
Keto, aldehyde, free amine, and heterocyclic groups remain intact
during the course of reaction. Particularly noteworthy is that the
phosphine ligand bearing –PPh2 moiety was found to have a better
efficacy than the ligand with –PCy2 group in this reaction. We
anticipate this unique finding is useful for further complementary
investigation of aryl halide efficiency in cyanation and related reac-
tions using K4[Fe(CN)6] as the cyanide source.
CN
F
HOOC
Bicalutamide (Casodex)
Non-steroidal antiandrogen
Antineoplastic
Febuxostat
Xanthine oxidase inhibitor
Letrozole
Antineoplastic
Aromatase inhibitor
Anti(prostate)cancer
Me
O
Me Me
N
N
Me
N
S
CN
N
S
CN
N
N
NC
Cyamemazine
Neurolepic
Tranquilizer
Fadrozole (Arensin)
Periciazine
Antipsychotic
Neuroleptic
Antineoplastic
Non-steroidal armatase inhibitor
Figure 1. Selected examples of aryl nitrile motif-containing pharmaceuticals
prepared by transition metal-catalyzed cyanation.26–30
2 mol%Pd(dba)2
Pd:L = 1:1
Me
CN
Me
Br
KOt-Bu, K4[Fe(CN)6].3H2O
MeCN/water(1:1), 50 oC, 18 h
Ligand L:
n-Bu
PCy2
i-Pr
PCy2
N
Table 1
P
PCy2
OMe
i-Pr
Effect of metal, base, and the loading of cyanide source on the cyanation of 3-
MeO
bromotoluenea
2 mol%Pd(dba)2
i-Pr
XPhos (13%)
Me
Br
Pd:L4 = 1:1
Me
CN
SPhos (34%)
CataCXium A (18%) CataCXium PCy (24%)
KOt-Bu, K4[Fe(CN)6].3H2O
MeCN/water(1:1), 50 oC, 18 h
R3
Entry Pd (mol %)
K4[Fe(CN)6] loading (mmol) Base
0.25 KOt-Bu
Yield (%)b
N
1
2
3
PdCl2 (2)
Pd(OAc)2 (2) 0.25
Pd2(dba)3 (2) 0.25
44
92
93
94
21
88
93
31
90
96
91
23
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
R2
R1
4
Pd(dba)2 (2)
0.25
CM-phos: R1 = PCy2, R2 = H,
R3 = Me,
(7%)
(20%)
(83%)
L1:
L2:
L3:
L4:
R1 = H,
R1 = H,
R1 = H,
R2 = PCy2, R3 = Me,
R2 = PPh2, R3 = Me,
R2 = H,
5
6
Pd(OAc)2 (1) 0.25
Pd2(dba)3 (1) 0.25
R3 = PPh2, (80%)
(93%)
7
8
9
10
11
12
13
14
15
Pd(dba)2 (1)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
Pd(dba)2 (2)
0.25
0.50
0.27
0.23
0.21
0.125
0.25
0.25
0.25
R1 = OMe, R2 = PPh2, R3 = Me,
Scheme 1. Ligands screening in palladium-catalyzed cyanation of aryl bromide.
Reaction conditions: 3-Bromotoluene (1.0 mmol), Pd(dba)2 (0.02 mmol), ligand
(indicated, 0.02 mmol), KOt-Bu (0.25 mmol), K4[Fe(CN)6]Á3H2O (0.25 mmol) in
MeCN/water (1:1, 2 mL total) at 50 °C for 18 h. Calculated GC-FID yields were
reported using dodecane as the internal standard. Cy = cyclohexyl.
K3PO4ÁH2O 89
K2CO3
KOAc
84
70
a
Reaction conditions: 3-Bromotoluene (1.0 mmol), Pd(dba)2 (0.02 mmol), L4
halides. Nevertheless, in our study of cyanation of aryl bromides,
the L1 bearing only –PPh2 could provide more superior activity
than L2. This finding offers us relevance to develop phosphine
ligands that may not necessarily consist of –PCy2 group, yet can
(0.02 mmol), Pd:ligand (1:1), base (as indicated, 0.25 mmol), K4[Fe(CN)6]Á3H2O (as
indicated) in MeCN/water(1:1, 2 mL total) at 50 °C for 24 h.
b
Calculated GC-FID yields were reported using dodecane as the internal
standard.

7040
P. Y. Yeung et al. / Tetrahedron Letters 52 (2011) 7038–7041
Table 2
(10 mL Â 3). The combined organic phases were concentrated un-
Palladium-catalyzed cyanation of aryl bromidesa
der reduced pressure. The crude product was purified by flash col-
umn chromatography on silica gel (230–400 mesh) to afford the
desired products.
2 mol% Pd(dba)2
Br
+ K4[Fe(CN)6].3H2O
CN
2 mol% L4
R
R
KOt-Bu, MeCN / H2O (1:1)
50 oC, 24 h
Acknowledgments
Entry
ArBr
ArCN
Yieldb
96
Me
Br
Me
CN
We thank the Research Grants Council of Hong Kong (CERG:
PolyU 5008/08P), The Hong Kong Polytechnic University Internal
Research Grant (A-PA8R), and UGC Areas of Excellence (AoE-P10/
01) for financial support.
1
2
3
OHC
O
Br
Br
OHC
O
CN
CN
89
88
Supplementary data
Me
Me
O
O
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.09.088.
4
5
92
66
Br
CN
Br
CN
References and notes
1. Kleemann, A.; Engel, J.; Kutscher, B.; Reichert, D. Pharmaceutical Substances:
Syntheses, Patents, Applications, 4th ed.; Thieme: Stuttgart, 2001.
2. Rappoport, Z. Chemistry of the Cyano Group; John Wiley & Sons: London, U.K.,
1970.
3. Larock, R. C. In Comprehensive Organic Transformations: A Guide to Functional
Group Preparations; VCH: New York, U.S.A, 1989; Vol. 819,
4. Rock, M.-h.; Marhold, A. U.S. Pat. DE 19706648 A1.
t-Bu
H2N
t-Bu
t-Bu
H2N
t-Bu
6
7
8
91
94
75
Br
CN
Cl
Br
Br
Cl
CN
CN
H2N
H2N
5. Sriram, D.; Yogeeswari, P. Medicinal Chemistry; Pearson Education: München,
Germany, 2007.
6. Cagir, A.; Jones, S. H.; Gao, R.; Eisenhauer, B. M.; Hecht, S. M. J. Am. Chem. Soc.
2003, 125, 13628.
Me
Me
MeO
Br
MeO
CN
7. Qiao, J. X.; Cheng, X.; Modi, D. P.; Rossi, K. A.; Luettgen, J. M.; Knabb, R. M.;
Jadhav, P. K.; Wexler, R. R. Bioorg. Med. Chem. Lett. 2005, 15, 29.
8. Ju, Y.; Liu, F.; Li, C. Org. Lett. 2009, 11, 3582.
9. Rosenmund, K. W. Berichte der Deutschen Chemischen Gesellschaft [Abteilung] B:
Abhandlungen 1919, 52B, 1749.
10. Braun, V.; Justus, J. Liebigs Ann. Chem. 1931, 488, 111.
11. Mowry, D. T. Chem. Rev. 1948, 42, 189.
12. Ellis, G. P.; Romney-Alexander, T. M. Chem. Rev. 1987, 87, 779.
13. Sandmeyer, T. Berichte der Deutschen Chemischen Gesellschaft 1885, 18, 1492.
14. Sandmeyer, T. Berichte der Deutschen Chemischen Gesellschaft 1884, 17, 2650.
15. Stevenson, A. C. Ind. Eng. Chem 1949, 41, 1846.
9
10
11
12
87
89
95
91
Br
CN
MeO
MeO
Br
Br
CN
CN
N
N
16. Denton, W. I.; Bishop, R. B.; Caldwell, H. P.; Chapman, H. D. Ind. Eng. Chem.
1950, 42, 796.
Br
CN
17. Sundermeier, M.; Zapf, A.; Beller, M. Eur. J. Inorg. Chem. 2003, 2003, 3513.
18. Takagi, K. In Handbook of Organopalladium Chemistry for Organic Synthesis;
Negishi, E.-i., Ed.; Wiley Interscience: New York, U.S.A., 2002; Vol. 1, p 657.
19. Littke, A. F.; Fu, G. C. Angew. Chem. Int. Ed. 2002, 41, 4176.
20. van Soolingen, J.; Brandsma, L.; Kruse, C. G. Eur. Pat. Appl. EP 613719 A1.
21. Arvela, R. K.; Leadbeater, N. E. J. Org. Chem. 2003, 68, 9122.
22. Ren, Y.; Liu, Z.; Zhao, S.; Tian, X.; Wang, J.; Yin, W.; He, S. Catal. Commun. 2009,
10, 768.
13
62
a
Reaction conditions: ArBr (1.0 mmol), Pd(dba)2 (0.02 mmol), L4 (0.02 mmol),
Pd:ligand (1:1), KOt-Bu (0.25 mmol), K4[Fe(CN)6]Á3H2O (0.23 mmol) in MeCN:water
(1:1, 2 mL total) at 50 °C for 24 h.
b
Isolated yields were reported.
23. Schareina, T.; Zapf, A.; Mägerlein, W.; Müller, N.; Beller, M. Chem. Eur. J. 2007,
13, 6249.
Experimental section
General procedures
24. Cristau, H.-J.; Ouali, A.; Spindler, J.-F.; Taillefer, M. Chem. Eur. J. 2005, 11, 2483.
25. Zanon, J.; Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc. 2003, 125, 2890.
26. Pitts, M. R.; McCormack, P.; Whittall, J. Tetrahedron 2006, 62, 4705.
27. Brennan, A. B.; Long, C. J.; Bagan, J. W.; Schumacher, J. F.; Spiecker, M. M. U.S.
Pat. Appl. Publ. US 20100226943 A1.
An oven-dried Schlenk tube with the presence of Teflon-coated
magnetic stir bar was charged with Pd(dba)2 (11.5 mg, 0.02 mmol,
2 mol %) and ligand L4 (8.4 mg, 0.02 mmol, 2 mol %). The flask was
evacuated and backfilled with nitrogen (three cycles). Pre-com-
plexation of palladium and ligand was initiated by injecting freshly
distilled CH2Cl2 (2.0 mL) and Et3N (0.1 mL) into the tube. The solu-
tion was stirred and warmed with a hair drier till the solvent con-
densed on the tube wall. The solvent was removed under high
vacuum. ArBr (1.0 mmol), KOt-Bu (0.25 mmol), and potassium
hexacyanoferrate(II) trihydrate (0.23 mmol) were then charged
successively to the tube followed by another three evacuation-
nitrogen refill cycles. Water (1.0 mL) and acetonitrile (1.0 mL) were
added. The tube was immersed into a preheated 50 °C oil bath for
24 h. The reaction was quenched by cooling to ambient tempera-
ture and added with EtOAc and water. The organic supernatant
was analyzed by GC. The organic layer was separated and the
remaining aqua medium was further extracted with EtOAc
28. Castaldi, G.; Rasparini, M.; Sillani, L. Eur. Pat. Appl. EP 2266966 A1.
29. Kil, K.-E.; Biegon, A.; Ding, Y.-S.; Fischer, A.; Ferrieri, R. A.; Kim, S. W.; Pareto, D.;
Schueller, M. J.; Fowler, J. S. Nucl. Med. Biol. 2009, 36, 215.
30. See Ref. 1 for more details about the patent information.
31. Takagi, K. Chem. Lett. 1973, 471.
32. Wang, X.; Zhi, B.; Baum, J.; Chen, Y.; Crockett, R.; Huang, L.; Eisenberg, S.; Ng, J.;
Larsen, R.; Martinelli, M.; Reider, P. J. Org. Chem. 2006, 71, 4021.
33. Challenger, S.; Dessi, Y.; Fox, D. E.; Hesmondhalgh, L. C.; Pascal, P.; Pettman, A.
J.; Smith, J. D. Org. Process Res. Dev. 2008, 12, 575.
34. Maligres, P. E.; Waters, M. S.; Fleitz, F.; Askin, D. Tetrahedron Lett. 1999, 40,
8193.
35. Beaudin, J.; Bourassa, D. E.; Bowles, P.; Castaldi, M. J.; Clay, R.; Couturier, M. A.;
Karrick, G.; Makowski, T. W.; McDermott, R. E.; Meltz, C. N.; Meltz, M.; Phillips,
J. E.; Ragan, J. A.; Ripin, D. H. B.; Singer, R. A.; Tucker, J. L.; Wei, L. Org. Process
Res. Dev. 2003, 7, 873.
36. Wallace, D. J.; Campos, K. R.; Shultz, C. S.; Klapars, A.; Zewge, D.; Crump, B. R.;
Phenix, B. D.; McWilliams, J. C.; Krska, S.; Sun, Y.; Chen, C.-y.; Spindler, F. Org.
Process Res. Dev. 2009, 13, 84.
37. Ryberg, P. Org. Process Res. Dev. 2008, 12, 540.
38. Sakakibara, Y.; Sasaki, K.; Okuda, F.; Hokimoto, A.; Ueda, T.; Sakai, M.; Takagi, K.
Bull. Chem. Soc. Jpn. 2004, 77, 1013.
39. Sundermeier, M.; Zapf, A.; Beller, M.; Sans, J. Tetrahedron Lett. 2001, 42, 6707.

P. Y. Yeung et al. / Tetrahedron Letters 52 (2011) 7038–7041
7041
40. Sundermeier, M.; Mutyala, S.; Zapf, A.; Spannenberg, A.; Beller, M. J. Organomet.
Chem. 2003, 684, 50.
41. Nakamura, H.; Shibata, H.; Yamamoto, Y. Tetrahedron Lett. 2000, 41, 2911.
42. Jia, X.; Yang, D.; Zhang, S.; Cheng, J. Org. Lett. 2009, 11, 4716.
43. Arvela, R. K.; Leadbeater, N. E.; Torenius, H. M.; Tye, H. Org. Biomol. Chem. 2003,
1, 1119.
52. (a) Yeung, P. Y.; So, C. M.; Lau, C. P.; Kwong, F. Y. Angew. Chem., Int. Ed. 2010, 49,
8918; (b) So, C. M.; Kwong, F. Y. Chem. Soc. Rev. 2011, 40, 4963.
53. (a) Yeung, P. Y.; So, C. M.; Lau, C. P.; Kwong, F. Y. Org. Lett. 2011, 13, 648; (b)
Yeung, P. Y.; Chung, K. H.; Kwong, F. Y. Org. Lett. 2011, 13, 2912.
54. Schareina, T.; Zapf, A.; Beller, M. Chem. Commun. 2004, 1388.
55. Schareina, T.; Zapf, A.; Beller, M. J. Organomet. Chem. 2004, 689, 4576.
56. Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc.
2005, 127, 4685.
44. Tschaen, D. M.; Desmond, R.; King, A. O.; Fortin, M. C.; Pipik, B.; King, S.;
Verhoeven, T. R. Synth. Comm. 1994, 24, 887.
45. Marcantonio, K. M.; Frey, L. F.; Liu, Y.; Chen, Y.; Strine, J.; Phenix, B.; Wallace, D.
J.; Chen, C.-y. Org. Lett. 2004, 6, 3723.
57. Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed. 2000, 39, 4153.
58. Zapf, A.; Beller, M. Chem. Commun. 2005, 431.
46. Chobanian, H. R.; Fors, B. P.; Lin, L. S. Tetrahedron Lett. 2006, 47, 3303.
47. Buono, F. G.; Chidambaram, R.; Mueller, R. H.; Waltermire, R. E. Org. Lett. 2008,
10, 5325.
48. Sundermeier, M.; Zapf, A.; Beller, M. Angew. Chem., Int. Ed. 2003, 42, 1661.
49. Ren, Y.; Liu, Z.; He, S.; Zhao, S.; Wang, J.; Niu, R.; Yin, W. Org. Process Res. Dev.
2009, 13, 764.
59. (a) So, C. M.; Zhou, Z.; Lau, C. P.; Kwong, F. Y. Angew. Chem., Int. Ed. 2008, 47,
6402; (b) Lee, H. W.; Lam, F. L.; So, C. M.; Lau, C. P.; Chan, A. S. C.; Kwong, F. Y.
Angew. Chem. Int. Ed. 2009, 48, 7436; (c) So, C. M.; Lau, C. P.; Kwong, F. Y. Angew.
Chem. Int. Ed. 2008, 47, 805; (d) So, C. M.; Lau, C. P.; Chan, A. S. C.; Kwong, F. Y. J.
Org. Chem. 2008, 73, 7731.
60. So, C. M.; Chow, W. K.; Choy, P. Y.; Lau, C. P.; Kwong, F. Y. Chem. Eur. J. 2010, 16,
7996.
50. Grossman, O.; Gelman, D. Org. Lett. 2006, 8, 1189.
51. Weissman, S. A.; Zewge, D.; Chen, C. J. Org. Chem. 2005, 70, 1508.