Readily available chiral phosphine-aminophosphine ligands for highly efficient Rh-catalyzed asymmetric hydrogenation of α-enol ester phosphonates and α-enamido phosphonates

Article abstract of DOI:10.1021/jo702488j

(Chemical Equation Presented) A new class of unsymmetrical hybrid phosphine-aminophosphine ligands has been prepared from commercially available, inexpensive (S)-1-phenylethylamine through a concise synthetic procedure. These ligands are not very sensitive to air and moisture, and displayed good enantioselectivities in the Rh-catalyzed asymmetric hydrogenation of various dimethyl α-benzoyloxyethenephosphonates bearing β-aryl, β-alkyl, and β-alkoxy substituents and N-benzyloxycarbonyl α-enamido phosphonates, in which up to 97% ee was obtained. A side-by-side comparison study disclosed that these new phosphine-aminophosphine ligands showed better enantioselectivity than BoPhoz ligands.

Full text of DOI:10.1021/jo702488j

related binding sites.2 C2-symmetry reduces the number of
possible catalyst-substrate arrangements and, consequently, the
number of competing reaction pathways by a factor of 2, which
can have a beneficial effect on the enantioselectivity.1a However,
this does not mean that C2-symmetry is a necessary motif in
ligand design, and a highly dissymmetric C1-symmetry structure
will equally fulfill the above conditions. Indeed, many recent
examples have clearly demonstrated the value of unsymmetrical
hybrid ligand design for obtaining more selective and efficient
catalysts, although the number is still significantly less than that
of C2-symmetrical ligands.2,3
A combination of phosphine and aminophosphine fragments
has proved to be an effective arrangement for the construction
of unsymmetrical hybrid bidentate phosphorus ligands. In the
past few years, a few examples of phosphine-aminophosphine
species based on a chiral ferrocenylethyl backbone have emerged
as ligands for highly efficient catalytic asymmetric hydrogena-
tions.4 These phosphine-aminophosphine ligands have advan-
tages of being highly modular, stable toward air and moisture,
and easily optimized by tuning the electronic and steric
properties of phosphino and aminophosphino moieties for certain
hydrogenation reactions. The first successful phosphine-
aminophosphine ligand was known as BoPhoz, reported by Boaz
et al. in 2002, which showed excellent enantioselectivities in
the Rh-catalyzed asymmetric hydrogenation of R-dehydroamino
acid derivatives, itacontate derivatives, and R-ketoesters.4a
However, the results in the Rh-catalyzed asymmetric hydroge-
nation of enamides with these BoPhoz ligands were less
satisfactory. Yip and Chan et al. found that using the fluorinated
BoPhoz-type ligands could dramatically increase the enanti-
oselectivity in the Rh-catalyzed asymmetric hydrogenation of
enamides.4d Very recently, Chen et al. have introduced a
stereogenic phosphorus atom into Bophoz-type ligands, and the
comparative results demonstrate that P-chirality improves the
enantioselectivity when acting cooperatively with the planar
chirality and the chirality at the carbon center.4g However, in
Readily Available Chiral
Phosphine-Aminophosphine Ligands for Highly
Efficient Rh-Catalyzed Asymmetric
Hydrogenation of r-Enol Ester Phosphonates and
r-Enamido Phosphonates
Dao-Yong Wang,†,‡ Jia-Di Huang,†,‡ Xiang-Ping Hu,*,†
Jun Deng,†,‡ Sai-Bo Yu,†,‡ Zheng-Chao Duan,†,‡ and
Zhuo Zheng*,†
Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, China, and Graduate School of
Chinese Academy of Sciences, Beijing 100039, China
zhengz@dicp.ac.cn; xiangping@dicp.ac.cn
ReceiVed NoVember 20, 2007
A new class of unsymmetrical hybrid phosphine-amino-
phosphine ligands has been prepared from commercially
available, inexpensive (S)-1-phenylethylamine through a
concise synthetic procedure. These ligands are not very
sensitive to air and moisture, and displayed good enantiose-
lectivities in the Rh-catalyzed asymmetric hydrogenation of
various dimethyl R-benzoyloxyethenephosphonates bearing
â-aryl, â-alkyl, and â-alkoxy substituents and N-benzyloxy-
carbonyl R-enamido phosphonates, in which up to 97% ee
was obtained. A side-by-side comparison study disclosed that
these new phosphine-aminophosphine ligands showed better
enantioselectivity than BoPhoz ligands.
(2) (a) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.;
Studer, M. AdV. Synth. Catal. 2003, 345, 103-151. (b) Tang, W.; Zhang,
X. Chem. ReV. 2003, 103, 3029-3069.
(3) For recent examples, see: (a) Sannicolo`, F.; Benincori, T.; Rizzo,
S.; Gladiali, S.; Pulacchini, S.; Zotti, G. Synthesis 2001, 2327-2336. (b)
Yan, Y. J.; Chi, Y. X.; Zhang, X. Tetrahedron: Asymmetry 2004, 15, 2173-
2175. (c) Jia, M.; Li, X. S.; Lam, W. S.; Kok, S. H. L.; Xu, L. J.; Lu, G.;
Yeung, C.-H.; Chan, A. S. C. Tetrahedron: Asymmetry 2004, 15, 2273-
2278. (d) Hu, X.-P.; Zheng, Z. Org. Lett. 2004, 6, 3585-3588. (e) Zeng,
Q.-H.; Hu, X.-P.; Duan, Z.-C.; Liang, X.-M.; Zheng, Z. Tetrahedron:
Asymmetry 2005, 16, 1233-1238. (f) Hu, X.-P.; Zheng, Z. Org. Lett. 2005,
7, 419-422. (g) Burk, S.; Francio`, G.; Leitner, W. Chem. Commun. 2005,
3460-3462. (h) Vargas, S.; Rubio, M.; Sua´rez, A.; del R´ıo, D.; AÄ lvarez,
E.; Pizzano, A. Organometallics 2006, 25, 961-973. (i) Vallianatou, K.
A.; Kostas, I. D.; Holz, J.; Bo¨rner, A. Tetrahedron Lett. 2006, 47, 7947-
7950. (j) Zhang, W.; Zhang, X. Angew. Chem., Int. Ed. 2006, 45, 5515-
5518. (k) Zhang, W.; Zhang, X. J. Org. Chem. 2007, 72, 1020-1023. (l)
Wassenaar, J.; Reek, J. N. H. Dalton Trans. 2007, 3750-3753.
Metal-catalyzed asymmetric synthesis is an important and
challenging area of contemporary synthetic organic chemistry,
in which choosing a suitable chiral ligand is a crucial task.1 In
the past decades, a large number of chiral bidentate phosphorus-
containing ligands have been developed successfully for various
Rh-catalyzed asymmetric hydrogenations, and most of these
ligands have a C2-symmetrical structure or hold two closely
(4) (a) Boaz, N. W.; Debenham, S. D.; Mackenzie, E. B.; Large, S. E.
Org. Lett. 2002, 4, 2421-2424. (b) Boaz, N. W.; Large, S. E.; Ponasik, J.
A., Jr.; Moore, M. K.; Barnette, T.; Nottingham, W. D. Org. Process Res.
DeV. 2005, 9, 472-478. (c) Boaz, N. W.; Ponasik, J. A., Jr.; Large, S. E.
Tetrahedron: Asymmetry 2005, 16, 2063-2066. (d) Li, X.; Jia, X.; Xu, L.;
Kok, S. H. L.; Yip, C. W.; Chan, A. S. C. AdV. Synth. Catal. 2005, 347,
1904-1908. (e) Boaz, N. W.; Mackenzie, E. B.; Debenham, S. D.; Large,
S. E.; Ponasik, J. A., Jr. J. Org. Chem. 2005, 70, 1872-1880. (f) Boaz, N.
W.; Ponasik, J. A., Jr.; Large, S. E. Tetrahedron Lett. 2006, 47, 4033-
4035. (g) Chen, W.; Mbafor, W.; Roberts, S. M.; Whittall, J. J. Am. Chem.
Soc. 2006, 128, 3922-3923.
† Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
(1) (a) ComprehensiVe asymmetric Catalysis; Jacobsen, E. N., Pfaltz,
A., Yamamoto, H., Eds; Springer: Berlin, Germany, 1999. (b) Catalytic
Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York,
2000. (c) Handbook of Chiral Chemicals, 2nd ed.; Ager, D., Ed.; Taylor &
Francis: Boca Raton, FL, 2006.
10.1021/jo702488j CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/12/2008
J. Org. Chem. 2008, 73, 2011-2014
2011

TABLE 1. Rh-Catalyzed Asymmetric Hydrogenation of r-Enol
Ester Phosphonates 4 (â-Aryl), 5 (â-Alkyl), and 6 (â-Alkoxy)a
SCHEME 1. Synthesis of New Phosphine-Aminophosphine
Ligands (2)
yield
(%)b
ee (%)
entrya
ligand
substrate (R)
(confign)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Me-BoPhoz
2b
H-BoPhoz
2a
4a: R ) Ph
98
97
99
98
96
99
98
95
99
99
98
98
98
98
94
99
99
97
99
99
5
85 (S)
89
96 (S)
92
4a: R ) Ph
4a: R ) Ph
4a: R ) Ph
Me-DuPHOS
4a: R ) Ph
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
4b: R ) p-FC6H4
4c: R ) p-ClC6H4
4d: R ) p-BrC6H4
4e: R ) p-NO2C6H4
4f: R ) p-MeOC6H4
4g: R ) m-MeOC6H4
4h: R ) o-ClC6H4
4i: R ) 1-nathphyl
4j: R ) 2-thienyl
5a: R ) H
96 (+)
94 (+)
97 (+)
95 (+)
95 (S)
96 (+)
94 (+)
95 (+)
95 (+)
93 (S)
96 (S)
96 (S)
96 (S)
94 (S)
93 (+)
5b: R ) Me
5c: R ) Et
addition to these ferrocene-based ligands, there was no other
phosphine-aminophosphine skeleton reported for the use in the
asymmetric catalysis, which prompted us to develop new and
modular phosphine-aminophosphine ligands for the hydrogena-
tion of some challenging substrates. Our interest was augmented
by having access to a chiral amino-phosphine precursor, (S)-
1-[2-(diphenylphosphino)phenyl]ethylamine [(S)-DPPNH2, 1a],
an intermediate in the synthesis of a variety of our recently
introduced phosphine-phosphoramidite ligands (PEAPhos) for
highly efficient Rh-catalyzed asymmetric hydrogenation.5a It can
be prepared by a one-step transformation from commercially
available, inexpensive chiral (S)-1-phenylethylamine. As a result,
herein we report a new and readily available chiral phosphine-
aminophosphine ligand derived from (S)-1-phenylethylamine,
which promoted excellent enantioselectivities in the Rh-
catalyzed asymmetric hydrogenation of various dimethyl R-ben-
zoyloxyethenephosphonates bearing â-aryl, â-alkyl, and â-alkoxy
substituents and dimethyl R-benzyloxycarbonylamino-ethene-
phosphonates.
New phosphine-aminophosphine ligands were synthesized
as outlined in Scheme 1. As we have reported, the intermediates
(S)-DPPNH2 (1a) and (S)-DPPNHMe (1b) can be easily
prepared from commerically available, inexpensive (S)-1-
phenylethylamine (3) through a concise procedure.5 By the
treatment of (S)-DPPNH2 (1a) and (S)-DPPNHMe (1b) with
1.1 equiv of ClPPh2 in CH2Cl2 at 0 °C in the presence of Et3N
as a scavenger for HCl eliminated, the target phosphine-
aminophosphine ligands (S)-2a and (S)-2b were obtained in
good yields. To our delight, these ligands are not sensitive to
air and moisture. Even after being held at ambient temperature
in open air for 1 month, these ligands did not show any changes
in its 1H and 31P NMR spectra. This salient feature makes these
new phosphine-aminophosphine ligands highly practical for
general laboratory preparations as well as scale-up operations.
In the first set of experiments, we used the Rh-catalyzed
asymmetric hydrogenation of the challenging substrates, â-aryl-
5d: R ) (CH2)9CH3
6a: R ) OMe
6b: R ) OEt
a All reactions were performed with 1.0 mmol of substrate at room
temperature under a H2 pressure of 10 atm in 4 mL of CH2Cl2 for 24 h
unless otherwise specified. Substrate/Rh(COD)2BF4/ligand ) 1/0.01/0.011.
Full conversions were achieved in all reactions. b Isolated yields. c The ee
values were determined by HPLC on a chiral column. The absolute
configuration was determined by comparing the sign of optical rotation
with reported data or by comparison of chiral HPLC elution order with
configurationally defined examples.
R-enol ester phosphonates 4a-j, to benchmark the potential of
these newly developed phosphine-aminophosphine ligands. The
reaction was conducted in CH2Cl2 at room temperature under a
H2 pressure of 10 atm in the presence of 1 mol % of catalysts
prepared in situ from Rh(COD)2BF4 and 1.1 equiv of chiral
ligands, and the results are summarized in Table 1. The reduction
of â-aryl-R-enol ester phosphonates for the synthesis of biologi-
cally and synthetically important chiral R-hydroxy phosphonates
is of great interest, since the latter can provide convenient access
to important phosphorus analogues of phenylalanine, tyrosine,
or DOPA.6 The catalytic asymmetric hydrogenation of â-aryl-
R-enol ester phosphonates 4 has proved to be highly difficult,
and many famous biphosphine ligands such as DuPHOS, BPE,
and MiniPHOS, which are highly efficient for the asymmetric
hydrogenation of various functionalized olefins, gave unsatisfac-
tory results.7 It is only very recently that some unsymmetrical
hybrid phosphane-phosphite and phosphine-phosphoramidite
ligands were found to show excellent enantioselectivities in this
transformation.8 Therefore, the search for new chiral ligands
with properties superior to those of their predecessors for this
transformation is still highly desirable. It is found that these
(6) Drescher, M.; Li, Y.-F.; Hammerschmidt, F. Tetrahedron 1995, 51,
4933-4946.
(7) (a) Burk, M. J.; Stammers, T. A.; Straub, J. A. Org. Lett. 1999, 1,
387-390. (b) Gridnev, I. D.; Higashi, N.; Imamoto, T. J. Am. Chem. Soc.
2001, 123, 4631-4632. (c) Gridnev, I. D.; Yasutake, M.; Imamoto, T.;
Beletskaya, I. P. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5385-5390. (d)
Liu, H.; Zhou, Y.-G.; Yu, Z.-K.; Xiao, W.-J.; Liu, S.-H.; He, H.-W.
Tetrahedron 2006, 62, 11207-11217.
(5) (a) Huang, J.-D.; Hu, X.-P.; Duan, Z.-C.; Zeng, Q.-H.; Yu, S.-B.;
Deng, J.; Wang, D.-Y.; Zheng, Z. Org. Lett. 2006, 8, 4367-4370. (b) Huang,
J.-D.; Hu, X.-P.; Yu, S.-B.; Deng, J.; Wang, D.-Y.; Duan, Z.-C.; Zheng, Z.
J. Mol. Catal. A: Chem. 2007, 270, 127-131.
2012 J. Org. Chem., Vol. 73, No. 5, 2008

SCHEME 2. Rh-Catalyzed Asymmetric Hydrogenation of
r-Benzyloxycarbonylaminoethenephosphonates 11 and 12
newly developed phosphine-aminophosphine ligands, despite
their simple appearances, displayed extraordinarily high enan-
tioselectivities in this rhodium-catalyzed asymmetric hydrogena-
tion in comparison with some other bidenate phosphorus ligands.
As shown in Table 1, Me-BoPhoz was ineffective for this
hydrogenation in terms of enantioselectivity, although full
conversion and 98% isolated yield were achieved (entry 1). In
sharp contrast, ligand 2b, bearing a similar structure to that of
Me-BoPhoz, surprisingly gave the promising result of an ee
value of up to 85% (entry 2). The presence of an N-H proton
in these ligands proved to be advantageous to achieving higher
enantioselectivity. Thus, when ligand 2a with an N-H proton
on the amino moiety was applied in the reaction, the enanti-
oselectivity was further increased to 96% ee (entry 4). Under
the same hydrogenation conditions, Me-DuPHOS exhibited 92%
ee (entry 5). The solvent screening experiment revealed that
CH2Cl2 was the best reaction media in terms of activity and
selectivity.
demonstrates the potential usefulness of the present catalytic
system for the preparation of optically active R-aminophospho-
nates.
In summary, we have developed a new class of readily
available, air-stable chiral phosphine-aminophosphine ligands
and successfully applied them to the enantioselective hydroge-
nation of various R-enol ester phosphonates and R-enamido
phosphonates. The results indicated that these new phosphine-
aminophosphine ligands exhibited superior enantioselectivity to
that obtained with BoPhoz analogues. Excellent enantioselec-
tivities (93-97% ee) have been achieved in the hydrogenation
of all substrates tested, demonstrating the high potential of these
phosphine-aminophosphine ligands in the preparation of opti-
cally active R-hydroxyphosphonates and R-aminophosphonates.
Studies further investigating the scope of these ligands are
currently in progress.
A wide range of â-aryl-R-enol ester phosphonate substrates
were then examined with this methodology. The results indicated
that the reaction system had a high tolerance to the substitution
pattern and electronic properties of the substrates, and all of
substrates were hydrogenated in over 94% ee (entries 6-14).
The highest enantioselectivity was obtained in the hydrogenation
of â-(p-bromophenyl)-R-enol ester phosphonate 4d, affording
the corresponding hydrogenation product in 97% ee (entry 8).
The hydrogenation of a variety of â-alkyl- and â-alkoxy-
substituted R-enol ester phosphonates 5 and 6 was also
investigated under the reaction condition as employed in the
hydrogenation of â-aryl-R-enol ester phosphonates. As expected,
good enantioselectivities were retained in the hydrogenation of
these substrates, indicating high efficiency of this catalytic
system (entries 15-20). As reported by Burk et al., the
deprotection of the benzoyl group of the hydrogenation product,
R-benzoyloxy phosphonate, can be easily performed by treat-
ment with K2CO3 in methanol at room temperature to generate
the corresponding R-hydroxy phosphonate without loss of
optical purity in high yields.7a
Experimental Section
(S)-DPPNH2 1a and (S)-DPPNHMe 1b were prepared according
to our recently reported method.5 R-Enol ester phosphonates 4-6
and R-enamido phosphonates 11 and 12 were prepared as reported
by Burk et al.7a
General Procedure for the Synthesis of R-Phenylethylamine-
Derived Phosphine-Aminophosphine Ligands 2. Amine-phos-
phine intermediate (S)-DPPNH2 1a (305 mg, 1.0 mmol) was
dissolved in 4 mL of dried and degassed toluene, to which 0.42
mL of triethylamine (3.0 mmol) was added. The mixture was cooled
in an ice-water bath, and then chlorodiphenylphosphine (0.2 mL,
1.1 mmol, 1.1 equiv) was added dropwise. The reaction mixture
was allowed to warm to ambient temperature overnight at which
point TLC indicated no 1a. After the precipitate was filtered off,
the reaction mixture was concentrated in vacuum. The residue was
purified by column chromatography, resulting in 411 mg (84%
To further demonstrate the scope and flexibility of the present
catalytic system, we decided to apply the phosphine-amino-
phosphine ligand 2a to the hydrogenation of R-enamido
phosphonates 11 and 12. The reaction was performed under the
optimized conditions (CH2Cl2 as the reaction media, H2 pressure
of 10 atm, and room temperature for 24 h) as employed in the
hydrogenation of R-enol ester phosphonates. Although catalytic
asymmetric hydrogenation of dehydroamino acid derivatives is
one of the most studied and widely applied methods for the
enantioselective preparation of R-amino acids, there are only a
few catalytic hydrogenation methods available to access opti-
cally active R-amino phosphonic acid derivatives.7a,c,9 To our
delight, Rh/(S)-2a complex is also highly efficient for the
hydrogenation of this substrate class. As shown in Scheme 2,
both dimethyl R-benzyloxycarbonylaminoethenephosphonate
(11) and dimethyl (E)-R-benzyloxycarbonylamino-â-phenylethe-
nephosphonate (12) were hydrogenated completely to give the
corresponding R-aminophosphonates with 96% ee. This result
yield) of (S)-2a as a colorless viscous oil. [R]25 -3.6 (0.156,
D
CHCl3). 1H NMR (400 MHz, CDCl3) δ 1.33 (d, J ) 6.4 Hz, 3H),
2.37-2.40 (m, 1H), 5.12-5.18 (m, 1H), 6.87-6.88 (m, 1H), 7.13-
7.20 (m, 1H), 7.22-7.34 (m, 22H), 7.56-7.58 (m, 1H). 13C NMR
(100 MHz, CDCl3) δ 25.5, 53.2, 127.0, 128.1, 128.2, 128.3, 128.5,
128.6, 128.7, 129.4, 131.1, 131.2, 131.3, 133.7, 133.8, 133.9, 134.0,
134.2, 137.0, 142.5, 151.4, 151.6. 31P NMR (162 MHz, CDCl3) δ
-16.9, 34.0. HRMS (EI) calcd for C32H29NP2 [M+] 489.1775, found
489.1783.
(9) (a) Scho¨llkopf, U.; Hoppe, I.; Thiele, A. Liebigs Ann. Chem. 1985,
555-559. (b) Kitamura, M.; Tokunaga, M.; Pham, T.; Lubell, W. D.;
Noyori, R. Tetrahedron Lett. 1995, 36, 5769-5772. (c) Schmidt, U.; Oehme,
G.; Krause, H. Synth. Commun. 1996, 26, 777-781. (d) Kitamura, M.;
Yoshimura, M.; Tsukamoto, M.; Noyori, R. Enantiomer 1996, 1, 281-
303. (e) Holz, J.; Quirmbach, M.; Schmidt, U.; Heller, D.; Stu¨rmer, R.;
Bo¨rner, A. J. Org. Chem. 1998, 63, 8031-8034. (f) Grassert, I.; Schmidt,
U.; Ziegler, S.; Fischer, C.; Oehme, G. Tetrahedron: Asymmetry 1998, 9,
4193-4202. (g) Dwars, T.; Schmidt, U.; Fischer, C.; Grassert, I.; Kempe,
R.; Fro¨hlich, R.; Drauz, K.; Oehme, G. Angew. Chem., Int. Ed. 1998, 37,
2851-2853.
(8) (a) Rubio, M.; Sua´rez, A.; AÄ lvarez, E.; Pizzano, A. Chem. Commun.
2005, 628-630. (b) Rubio, M.; Vargas, S.; Sua´rez, A.; AÄ lvarez, E.; Pizzano,
A. Chem. Eur. J. 2007, 13, 1821-1833. (c) Wang, D.-Y.; Hu, X.-P.; Huang,
J.-D.; Deng, J.; Yu, S.-B.; Duan, Z.-C.; Xu, X.-F.; Zheng, Z. Angew. Chem.,
Int. Ed. 2007, 46, 7810-7813.
J. Org. Chem, Vol. 73, No. 5, 2008 2013

General Hydrogenation Procedure. To a solution of [Rh-
(COD)2]BF4 (4.0 mg, 0.01 mmol) in anhydrous and degassed CH2-
Cl2 (2 mL), which was placed in a nitrogen-filled glovebox, was
added phosphine-aminophosphine ligand 2a (5.4 mg, 0.011 mmol).
The reaction mixture was stirred at room temperature for 30 min,
and then a solution of R-enol ester phosphonate (1.0 mmol) in 2
mL of CH2Cl2 was added. The mixture was transferred to a Parr
stainless autoclave. The autoclave was purged three times with
hydrogen, and maintained a hydrogen pressure of 10 atm. The
hydrogenation was performed at room temperature for 24 h. After
carefully releasing the hydrogen, the solvent was removed. The
residue was filtered through a short SiO2 column to remove the
catalyst. The filtrate was concentrated under reduced pressure, and
the enantiomeric excess was determined by HPLC on a chiral
column.
Acknowledgment. We are grateful for the generous financial
support from the National Natural Science Foundation of China
(20472083).
Supporting Information Available: 1H, 31P, and 13C NMR
spectra of ligands 2a,b and the hydrogenation products 7-9, 13,
and 14, and analysis of ee values of these hydrogenation products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO702488J
2014 J. Org. Chem., Vol. 73, No. 5, 2008