Synthesis of a novel unsymmetrical bisoxazoline ligand with sp2 bridging carbon

Article abstract of DOI:10.1055/s-0029-1217760

A novel unsymmetrical bisoxazoline ligand was synthesized in one step by the Knoevenagel condensation of aldehydes with a C2-symmetric indane-derived bisoxazoline having two acidic hydrogens connected to the bridging carbon. The electronic properties of i

Full text of DOI:10.1055/s-0029-1217760

LETTER
2589
2
Synthesis of a Novel Unsymmetrical Bisoxazoline Ligand with sp Bridging
Carbon
N
R
ovel Unsymme
u
l
Bisoxa
s
zoline Lig
l
and an Yuryev, Andreas Liese*
Institute of Technical Biocatalysis, Hamburg University of Technology, Denickestr. 15, Hamburg 21073, Germany
Fax +49(40)428782127; E-mail: liese@tuhh.de
Received 30 April 2009
Currently there are two distinct approaches to obtain un-
symmetrical BOX ligands. In the first one a monosubsti-
tuted malonic acid ester is used as a precursor, which is
Abstract: A novel unsymmetrical bisoxazoline ligand was synthe-
sized in one step by the Knoevenagel condensation of aldehydes
with a C -symmetric indane-derived bisoxazoline having two acidic
2
hydrogens connected to the bridging carbon. The electronic proper- converted into the desired product by substitution of the
ties of incorporated bridge substituent due to p–p conjugation with bridge hydrogen with a second group and transformation4
oxazoline rings can affect the catalytic performance of the ligand in
of the ester groups into oxazolines (Scheme 1, route A).
asymmetric syntheses, as was shown for the Henry reaction be-
tween benzaldehyde and nitromethane.
This synthetic route is, however, quite long (up to nine in-
termediate steps) and suffers from low overall yield. An
Key words: bisoxazoline ligands, asymmetric catalysis, asymmet-
ric synthesis, transition-metal complexes, Henry reaction
alternative approach utilizes an unsubstituted C -symmet-
2
rical BOX as a starting material (Scheme 1, route B). In
this case the substituents are introduced in two steps by
consequent substitution of the two acidic hydrogens con-
5
Bisoxazoline (BOX) ligands belong to the class of ‘privi- nected to the bridge carbon. Although this reaction se-
leged’ chiral catalysts, which have been extensively ap- quence is significantly shorter, the overall yield of the
plied in asymmetric organic synthesis in the last decade.1 target compound is also low, because it is difficult to
The major area of their competence are enantioselective
C–C bond formations (e.g., cyclopropanation, aldol- or
Michael-type condensation, Diels–Alder cycloaddition)
where these ligands show excellent selectivities. Meth-
ods for the synthesis of BOX ligands from commercially
available precursors are well established and quite versa-
avoid the formation of the C -symmetric disubstituted
2
BOX derivative as a byproduct. In order to overcome this
problem we decided to remove both bridge hydrogens si-
multaneously in one step by the Knoevenagel condensa-
tion with aldehydes yielding unsymmetrical BOX ligands
2
2
with an sp bridge carbon (Scheme 1, route C). In princi-
tile allowing ‘fine tuning’ of the ligand’s structure to suit ple, such unsymmetrical ligands can be also synthesized
the particular application. Most of the reported synthetic by other methods, for example, starting from malonic acid
approaches end up with C -symmetric structures bearing derivatives already having a C=C double bond in the
2
6
identical substituents R on the bridge carbon. Such sym- bridge, but the synthetic route proposed herein could
metrical ligands are the dominant class of BOX-based cat- have an advantage in the cases when the respective alde-
alysts used in the chiral transformations. Unsymmetrical hydes and unsubstituted C -symmetrical BOX substrates
2
1
2
BOX catalysts with two different substituents R and R
are more accessible than other starting materials.
(
Scheme 1) are rather rare, but have proved to be very use-
The new approach was tested in the synthesis of unsym-
metrical indane-derived bisoxazoline (IndaBOX) ligands
ful for the immobilization of the ligands on polymeric car-
3
riers utilizing one of the substituents as a linker.
bearing aromatic as well as aliphatic substituents
,8
(
Scheme 2).7
R1
R2
R1
EtO
OEt
O
O
A
R
N
O
O
N
N
O
N
O
O
O
R1
a
N
N
B
O
O
O
O
C
N
N
N
N
1
7–73%
Scheme 1 Synthetic routes to unsymmetrical BOX ligands
Scheme 2 Synthesis of novel unsymmetrical IndaBOX ligands
The isolated yield of the synthesized ligands depended on
the nature of the aldehyde (Table 1). The reaction pro-
ceeded neatly with benzaldehydes 4 and 5 bearing strong
electron-withdrawing substituents, as depicted by their
SYNLETT 2009, No. 16, pp 2589–2592
Advanced online publication: 03.09.2009
DOI: 10.1055/s-0029-1217760; Art ID: D10709ST
x
x
.x
x
.2
0
0
9
©
Georg Thieme Verlag Stuttgart · New York

2
590
R. Yuryev, A. Liese
LETTER
9
Hammet constants. In contrast, for benzaldehydes 2 and terion; the gap between the respective pK values should
a
3
the reaction mixtures after 24 hours still contained some be decreased due to p–p conjugation between the elec-
unreacted starting material. A similar reactivity order was tron-releasing p-hydroxyphenyl moiety and the two elec-
also observed by Heitler in his study of the Knoevenagel tron-withdrawing oxazoline rings. This rationale is
condensation between substituted benzaldehydes and eth- supported by the fact, that other BOX derivatives, which
1
0
yl cyanoacetate.
have p-hydroxyphenyl groups not conjugated to oxazo-
line rings, are well soluble in low polarity solvents, such
as ethyl acetate or toluene.4
The condensation of IndaBOX with 4-hydroxybenzalde-
hyde (entry 6) gave somewhat anomalous results. The
d,12
product yield was much higher than expected from the The ligand 7 contains two bisoxazoline moieties conjugat-
Hammet constant for the OH group. Moreover, contrary ed with each other via the phenyl ring (Figure 2). This
to other products the ligand 6 obtained was practically in- molecule is attractive, not only because of the unusual to-
soluble in lower polarity solvents such as dichlo- pology, but also due to its potential application as a recy-
romethane or even ethyl acetate. Probably, it reveals that clable catalyst for asymmetric syntheses, since its
this ligand exists in the form of a zwitterion (Figure 1).
bulkiness can facilitate its recovery from reaction mix-
tures, for instance by means of solvent resistant nanofil-
tration.
Table 1 Synthesis of Unsymmetrical IndaBOX Ligands
13
Entry
RCHO
h (%)a
48
spb
In order to check whether the electronic effects of p-sub-
stituents on the phenyl group have any influence on cata-
lytic performance of the ligands, the synthesized
IndaBOX ligands were complexed with copper(II)
acetate14 and tested as catalysts in the asymmetric nitro-
1
2
3
4
5
6
7
c-C H CHO
–
6
11
4-MeOC H CHO
24
–0.27
0.23
0.66
0.78
–0.37
–
6
4
4-ClC H CHO
17
6
4
aldol (Henry) reaction, in which C -symmetric bisoxazo-
2
1
5
lines have been already successfully applied. The
condensation between benzaldehyde and nitromethane
was chosen as a model system (Table 2). The reactions
were carried out either in ethanol or 2-propanol using
THF as a co-solvent, because the complexes had limited
solubility in these alcohols. The formation of the conden-
sation product (R)-2-nitro-1-phenylethanol as well as the
ee was followed by HPLC.16
4-NCC H CHO
60
6
4
4-O NC H CHO
73
2
6
4
4-HOC H CHO
62
6
4
c
4-OHCC H CHO
30
6
4
a
Isolated yield.
b
c
Hammet constant for para-substituent in R.9
The condensation product contains two BOX moieties (Figure 2).
Table 2 Condensation of Benzaldehyde and Nitromethane in the
Presence of IndaBOX–Cu(OAc) Complexes
2
OH
O–
O
OH
NO2
ligand–Cu(OAc)2
+
MeNO2
O
O
O
O
N
N
N+
N
Ligand
i-PrOH–THF (3:2)
EtOH–THF (3:2)
H
n (mM/h)a
ee (%)
n (mM/h)
0.66
ee (%)
1
2
3
4
5
0.57
54
69
79
82
82
66
34
47
58
73
65
53
Figure 1 Plausible zwitterionic form of ligand 6
0.27
0.29
0.29
0.42
O
N
0.43
0.46
0.46
0.50
O
N
N
O
7
0.24
0.42
N
O
a
Initial reaction rates determined by HPLC analysis; [cat.] = 4 mM,
[PhCHO] = 40 mM; [MeNO ] = 3.0 M; 25 °C.
2
For ligands 2–5 the observed initial reaction rates as well
as enantiomeric excess of the resulting b-nitroalcohol
were mutually related to the electron-withdrawing power
of p-substituents. This indicates that these groups, thanks
to p–p conjugation, do affect perceptibly the remote cata-
Figure 2 Ligand 7 containing two bisoxazoline moieties
9
a
Albeit a simple phenolic OH group (pK = 10) is a much
a
weaker acid than the protonated oxazoline ring
1
1
(
pK = 5.5), militating against formation of such a zwit-
a
Synlett 2009, No. 16, 2589–2592 © Thieme Stuttgart · New York

LETTER
Novel Unsymmetrical Bisoxazoline Ligand
2591
lytic center of the respective BOX ligand. The enhance-
ment of the Henry reaction by electron-withdrawing
substituents was expected a priori as a consequence of sta-
bilization of the negatively charged transition structure
occurring after the attack on the benzaldehyde carbonyl
(5) Chollet, G.; Rodriguez, F.; Schulz, E. Org. Lett. 2006, 8,
39.
5
(
(
6) Sibi, M. P.; Chen, J. Org. Lett. 2002, 4, 2933.
7) Representative Procedure for the Synthesis of Ligands
1
–7
IndaBOX (1 mmol), aldehyde (1 mmol), and piperidinium
acetate (0.2 mmol) were dissolved in pyridine (5 mL) at
65 °C while stirring. The obtained solution was maintained
at the same temperature for 24 h. At the end the solvent is
evaporated in vacuo and the solid residue purified by column
chromatography on silica using EtOAc as an eluent. For the
synthesis of ligand 7 only 0.05 mmol of aldehyde was added,
and a EtOAc–MeOH mixture (10:1) was used as an eluent.
Ligand 6 was isolated by pouring the reaction mixture into
1
5a,b
group by nitromethane anion.
The performance of
ligand 7 was similar to ligands 2 and 3 pointing out that
the bisoxazoline moiety is a weaker electron acceptor
compared to CN or NO groups. In 2-propanol all cata-
2
lysts showed better enantioselectivity, although the reac-
tion rates were practically the same as in ethanol. The
analogous solvent effect has been also noticed in the
asymmetric Henry reaction catalyzed by C -symmetric
H O (50 mL), filtering the precipitate, and washing it on the
2
2
1
5a
bisoxazolines. Contrary to catalysts 2–5 the ligand 1,
which had a cyclohexyl group instead of phenyl, showed
relative low enantioselectivity exhibiting a comparatively
high reaction rate at the same time. It seems probable that
the complex between ligand 1 and copper(II) ion has low-
er stability compared to the others, leading to higher con-
centration in the reaction mixture of uncomplexed ligand,
which promotes parallel unselective reaction and thus de-
creases the ee value of the condensation product.
filter with i-PrOH (25 mL) and CH Cl (25 mL).
2
2
(
8) Characterization Data for Ligands 1–7
2
,2¢-(2-Cyclohexylethene-1,1-diyl)bis(8,8a-dihydro-3aH-
indeno[1,2-d]oxazole) (1)
1
H NMR (400 MHz, CDCl ): d = 7.47–7.37 (m, 2 H), 7.21–
.11 (m, 6 H), 6.43 (d, J = 10.0 Hz, 1 H), 5.65 (d, J = 7.6 Hz,
H), 5.59 (d, J = 7.9 Hz, 1 H), 5.32 (ddd, J = 7.6, 6.5, 1.2
3
7
1
Hz, 1 H), 5.24 (ddd, J = 7.9, 6.7, 1.6 Hz, 1 H), 3.35 (dd,
J = 18.0, 6.5 Hz, 1 H), 3.32 (dd, J = 18.3, 6.7 Hz, 1 H), 3.23–
3
0
1
1
.13 (m, 2 H), 1.89–1.74 (m, 1 H), 1.47–1.24 (m, 5 H), 0.99–
1
3
.58 (m, 5 H). C NMR (100 MHz, CDCl ): d = 161.8,
3
In conclusion, the synthesized series of unsymmetrical
ligands extends the toolbox of BOX-based chiral cata-
lysts. Due to p–p conjugation such ligands are interesting
model systems to study the influence of electronic effects2
on their catalytic activity. Moreover, thanks to the sp
configuration of the bridge carbon they have a ligand bite
angle larger than the other known bisoxazolines. This can
be an advantage in certain reactions, such as Diels–Alder
cycloaddition, where ligands with larger bite angle exhibit
61.1, 151.9, 142.1, 141.8, 139.8, 139.5, 128.4, 128.3,
27.4, 127.3, 125.8, 125.6, 125.2, 125.1, 118.4, 83.2, 83.0,
77.1, 76.7, 39.7, 39.5, 38.6, 31.9, 31.6, 25.6, 25.3, 25.1. MS
(EI): m/z = 424 [M].
2,2¢-[2-(4-Methoxyphenyl)ethene-1,1-diyl]bis(8,8a-
dihydro-3aH-indeno[1,2-d]oxazole) (2)
1
H NMR (400 MHz, CDCl ): d = 7.45–7.07 (m, 9 H), 6.83–
3
6
5
.67 (m, 2 H), 6.28–6.22 (m, 2 H), 5.75 (d, J = 7.7 Hz, 1 H),
.64 (d, J = 7.8 Hz, 1 H), 5.40–5.30 (m, 2 H), 3.63 (s, 3 H),
3.39 (dd, J = 18.1, 6.7 Hz, 1 H), 3.36 (dd, J = 18.1, 6.7 Hz, 1
1
7
13
higher stereoselectivity. Beside this the synthesis of
such unsymmetrical BOX ligands is attractive for immo-
bilization purposes, because it permits a convenient one
step procedure to incorporate a linker into the ligand
structure. Further studies about application of this type of
bisoxazolines in other reactions are under way and will be
reported in due course.
H), 3.25 (d, J = 17.9 Hz, 1 H), 3.16 (d, J = 18.1 Hz, 1 H).
NMR (100 MHz, CDCl ): d = 161.2, 160.4, 159.7, 141.0,
C
3
1
1
3
2
39.9, 139.2, 138.8, 130.4, 127.6, 126.8, 126.4, 124.9,
24.8, 124.4, 114.5, 112.6, 82.5, 82.2, 76.3, 76.0, 54.3, 38.6,
8.3. MS (EI): m/z = 447 [M – H].
,2¢-[2-(4-Chlorophenyl)ethene-1,1-diyl]bis(8,8a-
dihydro-3aH-indeno[1,2-d]oxazole) (3)
1
H NMR (400 MHz, CDCl ): d = 7.49–7.16 (m, 9 H), 6.80–
3
6.65 (m, 4 H), 5.75 (d, J = 7.6 Hz, 1 H), 5.68 (d, J = 7.8 Hz,
1 H), 5.36 (ddd, J = 7.2, 6.4, 0.9 Hz, 1 H), 5.33 (ddd, J = 7.2,
6.4, 1.7 Hz, 1 H), 3.38 (dd, J = 17.9, 6.7 Hz, 1 H), 3.32 (dd,
References and Notes
J = 17.0, 6.1 Hz, 1 H), 3.26 (dd, J = 17.4, 1.0 Hz, 1 H), 3.10
(
(
1) Yoon, T. P.; Jacobsen, E. N. Science 2003, 299, 1691.
2) For a review, see: (a) Desimoni, G.; Faita, G.; Jørgensen,
K. A. Chem. Rev. 2006, 106, 3561. (b) Ghosh, A. K.; Bilcer,
G.; Fidanze, S. In The Chemistry of Heterocyclic
1
3
(
d, J = 18.0 Hz, 1 H). C NMR (100 MHz, CDCl3):
d = 162.2, 160.9, 141.6, 140.7, 140.0, 139.9, 139.7, 135.2,
1
1
32.0, 130.5, 128.7, 128.5, 128.3, 127.8, 127.5, 125.9,
25.2, 125.1, 118.9, 83.7, 83.4, 77.3, 76.9, 39.5, 39.1. MS
Compounds, Vol. 60; Palmer, D. C., Ed.; John Wiley and
Sons: New York, 2004, 529. (c) Johnson, J. S.; Evans, D. A.
Acc. Chem. Res. 2000, 33, 325. (d) Ghosh, A. K.;
Mathivanan, P.; Cappiello, J. Tetrahedron: Asymmetry
1
(
4
EI): m/z = 451 [M – H].
-{2,2-Bis(8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-
yl)vinyl}benzonitrile (4)
1
H NMR (400 MHz, CDCl ): d = 7.50–7.16 (m, 9 H), 6.98–
3
998, 9, 1.
3) For a review, see: Rechavi, D.; Lemaire, M. Chem. Rev.
002, 102, 3467.
4) (a) Lee, S. S.; Hadinoto, S.; Yinga, J. Y. Adv. Synth. Catal.
006, 348, 1248. (b) Benaglia, M.; Cinquini, M.; Cozzi, F.;
Celentano, G. Org. Biomol. Chem. 2004, 2, 3401.
c) Corma, A.; Garcia, H.; Moussaif, A.; Sabater, M. J.;
Zniber, R.; Redouane, A. Chem. Commun. 2002, 1058.
d) Annunziata, R.; Benaglia, M.; Cinquini, M.; Cozzi, F.;
6
1
3
.87 (m, 4 H), 5.72 (d, J = 7.6 Hz, 1 H), 5.70 (d, J = 7.9 Hz,
H), 5.38–5.31 (m, 2 H), 3.39 (dd, J = 18.0, 6.7 Hz, 1 H),
.31 (dd, J = 16.6, 6.5 Hz, 1 H), 3.26 (dd, J = 17.0, 1.1 Hz, 1
(
(
2
1
3
H), 3.05 (d, J = 18.2 Hz, 1 H). C NMR (100 MHz, CDCl ):
3
2
d = 161.7, 160.4, 141.4, 140.4, 139.9, 139.6, 139.0, 137.9,
1
1
31.7, 129.4, 128.9, 128.6, 127.9, 127.5, 125.9, 125.3,
25.2, 121.8, 118.5, 112.2, 84.0, 83.6, 77.4, 76.9, 39.5, 39.1.
(
MS (EI): m/z = 442 [M – H].
(
2
,2¢-[2-(4-Nitrophenyl)ethene-1,1-diyl]bis(8,8a-dihydro-
Pitillo, M. J. Org. Chem. 2001, 66, 3160. (e) Orlandi, S.;
Mandoli, A.; Pini, D.; Salvadori, P. Angew. Chem. Int. Ed.
3
aH-indeno[1,2-d]oxazole) (5)
H NMR (400 MHz, CDCl ): d = 7.53–7.15 (m, 11 H), 6.98–
1
3
2001, 40, 2519.
Synlett 2009, No. 16, 2589–2592 © Thieme Stuttgart · New York

2
592
R. Yuryev, A. Liese
LETTER
(13) For selected examples on the recovery of organometallic
6
1
3
.92 (m, 2 H), 5.72 (d, J = 6.4 Hz, 1 H), 5.70 (d, J = 6.4 Hz,
H), 5.38–5.32 (m, 2 H), 3.39 (dd, J = 18.0, 6.7 Hz, 1 H),
.31 (dd, J = 17.9, 6.5 Hz, 1 H), 3.27 (dd, J = 18.0, 1.0 Hz, 1
H), 3.06 (d, J = 18.2 Hz, 1 H). C NMR (100 MHz, CDCl ):
d = 161.6, 160.3, 147.4, 141.4, 140.4, 139.9, 139.8, 139.6,
38.7, 129.6, 129.0, 128.7, 127.9, 127.5, 125.9, 125.8,
25.3, 125.2, 123.2, 122.4, 84.1, 83.6, 77.5, 76.9, 39.5, 39.1.
MS (EI): m/z = 462 [M – H].
-{2,2-Bis(8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-
catalysts by solvent resistant nanofiltration, see: (a) Beigi,
M.; Haag, R.; Liese, A. Adv. Synth. Catal. 2008, 350, 919.
(b) Greiner, L.; Laue, S.; Wöltinger, J.; Liese, A. Top. Curr.
Chem. 2007, 276, 111. (c) Scarpello, J. T.; Nair, D.;
Freitas dos Santos, L. M.; White, L. S.; Livingston, A. G.
J. Membr. Sci. 2002, 203, 71. (d) Wöltinger, J.; Drauz, K.;
Bommarius, A. S. Appl. Catal., A 2001, 221, 171.
(e) Wöltinger, J.; Bommarius, A. S.; Drauz, K.; Wandrey, C.
Org. Process Res. Dev. 2001, 5, 241.
1
3
3
1
1
4
yl)vinyl}phenol (6)
H NMR (400 MHz, DMSO-d ): d = 10.7 (s, 1 H), 7.38–7.16
m, 9 H), 6.82–6.75 (m, 2 H), 6.40–6.33 (m, 2 H), 5.65 (d,
1
(14) The ligands (1 equiv) were mixed with Cu(OAc) (1.5 equiv)
6
2
(
in CH Cl at r.t. overnight. The uncomplexed salt was
2
2
J = 7.7 Hz, 1 H), 5.56 (d, J = 7.9 Hz, 1 H), 5.43–5.31 (m, 2
H), 3.40 (dd, J = 18.1, 7.0 Hz, 1 H), 3.37 (dd, J = 18.0, 6.7
Hz, 1 H), 3.09 (d, J = 17.4 Hz, 1 H), 3.05 (d, J = 17.8 Hz, 1
removed from the resulting solutions by extraction with H O
2
(3×). The organic phases were dried over anhyd Na SO , the
2
4
solvent was evaporated in vacuo yielding the complexes as
dark solids.
H). 13C NMR (100 MHz, DMSO-d ): d = 161.6, 160.1,
6
1
1
1
59.8, 141.8, 140.6, 140.1, 139.8, 139.6, 131.4, 128.4,
28.2, 127.2, 127.1, 125.4, 125.3, 125.2, 125.1, 123.3,
15.2, 113.5, 82.8, 82.5, 76.3, 76.2, 39.1, 38.8. MS (EI):
(15) (a) Ginotra, S. K.; Singh, V. K. Org. Biomol. Chem. 2007, 5,
3932. (b) Evans, D. A.; Seidel, D.; Rueping, M.; Lam, H.
W.; Shaw, J. T.; Downey, C. W. J. Am. Chem. Soc. 2003,
125, 12692. (c) Christensen, C.; Juhl, K.; Hazell, R. G.;
Jørgensen, K. A. J. Org. Chem. 2002, 67, 4875.
(d) Christensen, C.; Juhl, K.; Jørgensen, K. A. Chem.
Commun. 2001, 2222.
m/z = 433 [M – H].
,4-Bis{2,2-bis(8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-
1
yl)vinyl}benzene (7)
1
H NMR (400 MHz, CDCl ): d = 7.49–7.14 (m, 18 H), 6.35
s, 4 H), 5.71 (d, J = 7.7 Hz, 2 H), 5.68 (d, J = 7.9 Hz, 2 H),
3
(
(16) HPLC conditions used (a) for quantification of 2-nitro-1-
phenylethanol and benzaldehyde: RP-8 column (Merck),
30 °C, 67 mM KH PO –MeOH (65:35), 2 mL/min, 225 nm;
5.36–5.28 (m, 4 H), 3.39 (dd, J = 18.0, 6.6 Hz, 2 H), 3.32–
1
3
3.22 (m, 4 H), 3.03 (d, J = 18.1 Hz, 2 H). C NMR (100
2
4
MHz, CDCl ): d = 162.3, 160.8, 141.7, 140.7, 140.4, 139.9,
(b) for determination of ee of 2-nitro-1-phenylethanol: OD-
3
1
39.7, 134.4, 129.0, 128.6, 128.5, 127.7, 127.5, 125.9,
25.8, 125.2, 125.0, 119.0, 83.6, 83.4, 77.3, 76.9, 39.6, 39.1.
RH column (Chiralcel), 5 °C, MeOH–H O (65:35), 0.2 mL/
2
1
min, 225 nm.
MS–FAB: m/z = 759 [M + H].
(17) For discussion on the influence of ligand bite angle on the
enantioselectivity in the Diels–Alder reaction, see:
(a) Lipkowitz, K. B.; Pradhan, M. J. Org. Chem. 2003, 68,
4648. (b) Lipkowitz, K. B.; Schefzick, S. J. Am. Chem. Soc.
2001, 123, 6710. (c) van Leeuwen, P. W. N. M.; Kamer,
P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. Rev. 2000, 100,
2741. (d) Davies, I. W.; Gerena, L.; Cai, D.; Larsen, R. D.;
Verhoeven, T. R.; Reider, P. J. Tetrahedron Lett. 1997, 38,
1145. (e) Davies, I. W.; Gerena, L.; Castonguay, L.;
Senanayake, C. H.; Larsen, R. D.; Verhoeven, T. R.; Reider,
P. J. Chem. Commun. 1996, 1753.
(
9) (a) Lange’s Handbook of Chemistry; Dean, J. A., Ed.;
McGraw-Hill: New York, 1999. (b) Hammett, L. P. J. Am.
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