Catalytic application of F8BINOL: asymmetric oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1016/S0022-328X(00)00172-8

The effect of fluorine substitution at the 5, 5', 6, 6', 7, 7', 8 and 8' positions of 2,2'-dihydroxy-1,1'-binaphthyl (BINOL) on its catalytic activity in titanium-mediated sulfide oxidation with tert-butyl hydrogen peroxide and cumyl hydrogen peroxide was

Full text of DOI:10.1016/S0022-328X(00)00172-8

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 603 (2000) 98–104
Catalytic applications of F BINOL: asymmetric oxidation of
8
sulfides to sulfoxides
L. James P. Martyn, Subramanian Pandiaraju, Andrei K. Yudin *
Department of Chemistry, Uni6ersity of Toronto, 80 St. George St., Toronto, ON, Canada M5S 3H6
Received 22 November 1999; received in revised form 1 February 2000
Abstract
The effect of fluorine substitution at the 5, 5%, 6, 6%, 7, 7%, 8 and 8% positions of 2,2%-dihydroxy-1,1%-binaphthyl (BINOL) on its
catalytic activity in titanium-mediated sulfide oxidation with tert-butyl hydrogen peroxide and cumyl hydrogen peroxide was
examined. Introduction of fluorines into the BINOL scaffold was found to increase the electrophilic character of the Lewis acidic
titanium center of the catalyst. Moreover, under otherwise identical conditions, BINOL and 5,5%,6,6%,7,7%,8,8%-octafluoro-2,2%-dihy-
droxy-1,1%-binaphthyl (F BINOL) of the same absolute configuration led to opposite sense of chiral induction in the sulfoxidation
8
process. © 2000 Published by Elsevier Science S.A. All rights reserved.
Keywords: Catalytic applications; BINOL; Sulfides; Sulfoxides
1. Introduction
2,2%-Dihydroxy-1,1%-binaphthyl (BINOL, 1) and re-
lated C symmetrical ligands possessing axial chirality
2
have found wide utility in asymmetric catalysis [1].
Over the years, several modifications of the BINOL
scaffold aimed at changing its steric as well as elec-
tronic properties have appeared [2]. For instance, par-
We recently designed a new class of chiral poly-
fluoroaryl ligands based on 1 [3]. Substitution of hydro-
gens by fluorines at the 5, 5%, 6, 6%, 7, 7%, 8 and 8%
positions of 1 was found to induce considerable elec-
tronic perturbation of the aromatic system in the result-
ing 5,5%,6,6%,7,7%,8,8%-octafluoro-2,2%-dihydroxy-1,1%-bi-
tially
hydrogenated
BINOL
was
used
in
enantioselective alkylation of aldehydes [2a], conjugate
addition of diethylzinc to enones [2b], and ring-opening
of epoxides [2c]. Incorporation of bromine atoms at the
6
and 6% positions of 1 was shown to increase the
enantioselectivity of the corresponding titanium cata-
lysts in glyoxylate ene reactions [2d]. Bulky triarylsilyl
groups at the 3 and 3% positions of 1 led to increased
enantioselectivities in the asymmetric Diels–Alder pro-
cesses [2e]. In the area of oxidation catalysis, the 3,3%-
dinitrooctahydrobinaphthol ligand was applied in the
titanium-catalyzed asymmetric oxidation of methyl-p-
tolylsulfide [2f].
naphthyl (F BINOL, 2). The electron-deficient nature
8
of the aromatic rings was found to raise the oxidative
stability of 2 compared with 1 (E% =2.07 V in 2 vs. 1.47
2
V in 1) as well as to increase the acidity of the ring-
bound hydroxyl groups (pK% 9.28 in 2 vs. 10.28 of 1).
a
This electronic alteration in turn led to a dramatic
increase in configurational stability of homochiral 2
with electronic effects playing a decisive role.
We reasoned that besides changing the physicochem-
ical characteristics of the BINOL ligand, the structural
consequences of fluorine substitution should also trans-
late into modulated binding to metals as well as reac-
*
Corresponding author. Fax: +1-416-9465042.
E-mail address: ayudin@chem.utoronto.ca (A.K. Yudin)
tants in the F BINOL-mediated processes, especially in
8
0022-328X/00/$ - see front matter © 2000 Published by Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00172-8

L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
99
the cases where early transition metals are involved.
The latter are well known to exhibit diverse aggregation
equilibria in the presence of a variety of ligands [4].
Therefore, the ligands with modulated coordination
ability can give rise to the catalyst systems possessing
novel properties including increased catalytic activity.
2). This change is consistent with the formation of the
i
C2 symmetrical 1:1 adduct 4 between Ti(O Pr) and
4
13
F BINOL. The C spectrum features a resonance at
8
159.0 ppm, which is very close to the value of 158.8
ppm reported for the corresponding 1:1 adduct 3 in the
i
Ti(O Pr) /BINOL system in CDCl (Fig. 1) [7]. Thus,
4
3
Since the fluorine atom is only 0.27 A
,
larger than the
despite a tenfold decrease in acidity of the hydroxyl
groups in BINOL upon fluorine substitution, binding
to the oxophilic titanium atoms still takes place. The
hydrogen atom [5a], one could anticipate only a small
increase in the barrier to axial torsion in BINOL upon
fluorine substitution. The complexes obtained from
fluorinated versions of these and related ligands can be
considered sterically similar to their hydrogenated
counterparts, but drastically different in terms of elec-
tronics. Notably, perfluorinated ligands are known to
afford stable porphyrin-derived catalysts of high
turnover in catalytic oxidation processes [6]. In the
present paper, we report the results of our investiga-
tions of the catalytic asymmetric sulfoxidation with
19
i
F-NMR spectrum obtained by mixing Ti(O Pr) and
4
F BINOL in a 1:2 ratio featured an additional set of
8
eight peaks corresponding to the formation of a new
species having C symmetry [8].
1
The replacement of aromatic hydrogens for fluorines
is known to increase barriers to axial torsion in substi-
tuted biphenyls. For example, fluorination of the 4 and
5 positions of 9,10-dihydrophenanthrene raises the tor-
−
1
sion barrier from 4.1 to 10.3 kcal mol
[6]. The
i
F BINOL/Ti(O Pr) . These catalytic experiments indi-
torsion angle between the tetrafluoronaphthol planes in
the molecular structure of R-(−)-2 (79.7°) is only 1.4°
larger than in the parent protio-derivative R-(−)-1
(78.3°) [3,9]. This indicates that fluorine substitution at
the 5, 5%, 6, 6%, 7, 7%, 8 and 8% positions of BINOL has
fairly insignificant steric influence on the torsion angle
and the observed effect on the catalytic activity of 2
must be primarily electronic in nature. The desired
conformational flexibility, one of the most important
characteristics of BINOL allowing it to coordinate a
wide variety of metals [1], is preserved in 2, which is
8
4
cate that fluorine substitution is responsible for im-
proved levels of enantioselectivity in the concentration
range where minimum by-product sulfone is formed. In
addition, reversal in the sense of chiral induction in the
asymmetric sulfoxidation process takes place.
2. Results and discussion
The racemic form of 2 was prepared in five steps and
0% overall yield from the commercially available
evident from the NMR characterization of the
3
i
F BINOL/Ti(O Pr) adducts.
chloropentafluorobenzene [3]. The diastereomeric
bis[(−)-menthoxycarbonyl] derivatives were obtained
by reacting racemic 2 with excess (−)-menthyl chloro-
formate in quantitative yield. Subsequent deprotection
furnished the R- and S-enantiomers of 2 in 99+% ee
each.
8
4
To illustrate the utility of the F BINOL-derived cata-
8
lysts in asymmetric catalysis, we chose sulfide oxidation
as a model reaction. Enantiomerically pure sulfoxides
constitute an important target in asymmetric synthesis
[10]. At the beginning, we compared the activities of
(
R)-F BINOL and (R)-BINOL ligands in titanium-cat-
Our investigations commenced with the NMR studies
8
alyzed asymmetric oxidation of p-tolyl sulfide with
tert-butyl hydrogen peroxide (TBHP) [10b]. The results
of this study are summarized in Table 1. Gratifyingly,
aimed at characterizing the titanium/F BINOL ad-
8
i
ducts. Upon addition of Ti(O Pr) (one equivalent) to
4
the 0.133 M solution of F BINOL (one equivalent) in
8
the catalyst produced in the (R)-F BINOL/Ti system
CDCl , the reaction mixture turned yellow–orange and
8
3
(
10 mol%) was found to catalyze the sulfoxidation
resonances at −148.3, −149.1, −156.6, and −161.8
1
9
process with higher enantioselectivity than the (R)-
BINOL/Ti species at 0.05 M concentration. Uemura’s
sulfoxidation protocol [10] leads to high enantioselectiv-
ities at high concentrations, albeit at the expense of a
significant production of the sulfone by-product, which
decreases the yield of the sulfoxide. Due to the highly
ppm in the F-NMR spectrum, corresponding to free
F BINOL, disappeared. A new set of peaks at −143.8,
8
−
149.8, −158.6, and −162.4 ppm was observed (Fig.
i
electrophilic character of F BINOL, Ti(O Pr) /
8
4
F BINOL-catalyzed reactions tend to produce more
8
sulfone than the BINOL-mediated reaction arising
from over oxidation. The lack of enantioselectivity for
the (R)-BINOL/Ti species (Table 1, entry 2) suggests
that the species responsible for enantioselective oxygen
atom transfer is not produced in the 1:1 system. To our
biggest surprise, the opposite sense of induction was
Fig. 1. 13C chemical shifts of the 2 and 2% carbons in BINOL/
i
i
Ti(O Pr)4 and F BINOL/Ti(O Pr) adducts.
8
4

100
L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
Fig. 2. 19F-NMR spectra of (a) F BINOL in CDCl (0.133 M); (b) 1:1 adduct between F BINOL and Ti(O Pr) .
i
8
3
8
4
observed upon fluorine substitution: (R)-BINOL gave
scope of the F BINOL-based catalysts. It is conceivable
8
preferentially the R-sulfoxide, whereas (R)-F BINOL
that different aggregation is a likely cause for the
observed change in enantioselectivity. Notably, reversal
of asymmetric induction in the titanium-mediated oxi-
8
produced an excess of the S-enantiomer. This intrigu-
ing observation warrants further investigation into the
nature of the catalytically active species as well as the
dation of sulfides upon introduction of the CF sub-
3

L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
101
stituents into the backbone of 1,2-diarylethane-1,2-diols
was documented [11]. In addition, introduction of the
ortho-nitro groups into BINOL was previously found
to reverse the enantioselectivity of the sulfoxidation [2f].
We also compared the reactivities of BINOL and
Although the initial rates were not measured, Fig. 3
clearly demonstrates that the electronic influence of
fluorines dictates higher activity of the F BINOL-based
8
titanium catalyst.
The use of a 1:2 ratio of Ti(O Pr) –F BINOL at
i
4
8
F BINOL by generating the conversion/time diagrams
room temperature (r.t.) constitutes optimal reaction
conditions. Experiments were conducted in order to
determine the optimal amount and the nature of the
oxidant in the sulfoxidation of methyl-p-tolyl sulfide
(Table 2). The largest increase in % ee was obtained
when cumyl hydrogen peroxide was substituted for
tert-butyl hydrogen peroxide. Optimized reaction con-
ditions call for the use of cumyl hydrogen peroxide (1.1
equivalents) and chloroform as solvent. In this case,
8
i
i
for the BINOL/Ti(O Pr) and F BINOL/Ti(O Pr) oxi-
4
8
4
dations at 0.05 M substrate concentration and 5 mol%
catalyst loading. Fig. 3 features comparison of the
catalyzed oxidation of methyl-p-tolyl sulfide to methyl-
p-tolyl sulfoxide. The build-up of the sulfoxide was
monitored by gas chromatography over 8 h. Ethyl
phenyl sulfone was chosen as an internal standard in
both runs due to its resistance to further oxidation.
Table 1
i
i
Comparison of the BINOL/Ti(O Pr) and F BINOL/Ti(O Pr) catalyzed oxidation of methyl-p-tolyl sulfide to methyl-p-tolyl sulfoxide
4
8
4
Entry
Solvent
Ti:ligand ratio
Temperature (°C)
Time (h)
(R)-F BINOL % ee
(R)-BINOL % ee
8
1
2
3
4
5
CH Cl2
1:2
1:1
1:2
1:2
1:4
0–RT
0–RT
0–RT
−50 to −5
0–RT
4
4
3
20
5
61 (S)
0
31 (S)
60 (S)
64 (S)
7 (R)
0
22 (R)
0 (R)
17 (R)
2
CH Cl2
2
CCl4
CH Cl2
2
CH Cl2
2
Fig. 3. Comparison of the BINOL/F BINOL catalyzed oxidation of methyl-p-tolyl sulfide to methyl-p-tolyl sulfoxide (ꢀ: F BINOL reaction; ":
8
8
BINOL reaction).

102
L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
Table 2
Comparison of titanium sources, oxidants and ligand:titanium ratios
although better yields were observed due to the lower
amounts of sulfone produced.
Experiments to establish the substrate scope were
also carried out (Table 3). Several commercially avail-
Entry
Titanium
source
Oxidant
Ti:F BINOL
% ee
8
ratio
1:4
1:4
1:2
1:2
1:2
1:2
able sulfides were subjected to the Ti/F BINOL cata-
8
i
1
2
3
4
5
6
Ti(O Pr)4
2.2 equivalents
64 (S)
67 (S)
64 (S)
75 (S)
0
lyzed sulfoxidation protocol. The highest levels of ee
were observed with the sulfide moiety bearing a methyl
substituent. The oxidation of electron deficient systems,
known to proceed slowly, was sluggish in our case
accounting for the low conversion of methyl 2-nitro-
phenyl sulfide. Longer reaction times were required in
order to drive this reaction to completion. The attempt
to optimize this process by decreasing the reaction
temperature resulted in lower enantioselectivity. Ex-
panding on the observation that carrying out the reac-
tions at −20°C reduced the %ee, we carried out the
reactions at r.t. Having obtained reasonably good enan-
tiomeric excess at the 0.05 mmol scale, the process was
then scaled up to 1 mmol and isolated yields for
selected substrates were determined (Table 4).
a
TBHP
i
Ti(O Pr)4
1.1 equivalents
TBHP
1.1 equivalents
TBHP
i
Ti(O Pr)4
i
Ti(O Pr)4
2.2 equivalents
b
CHP
TiCl4
TiCl4
2.2 equivalents
TBHP
2.2 equivalents
CHP
0
a
TBHP, tert-butyl hydrogen peroxide.
CHP, cumyl hydrogen peroxide.
b
maximum enantioselectivity of 76–86% was observed
for the oxidation of phenyl methyl sulfide (Table 3,
entries 10–13). Kagan and co-workers had previously
established that water is essential to ensure rapid
turnover in the titanium/diethyl tartrate oxidation sys-
tem [10a]. In our case, water was also found to be
crucial and, according to the optimized protocol, two
equivalents of water with respect to sulfide are neces-
sary for the turnover to take place. As far as alternative
sources of titanium(IV), titanium(IV) chloride was
tested but did not lead to the production of the catalyt-
ically active species, presumably due to the premature
hydrolysis of titanium(IV) chloride under the reaction
conditions [12]. An increase in the ligand-to-titanium
ratio (Table 2, entry 1) was found to have little to no
effect on the %ee of the sulfoxide. Similarly, a decrease
in the amount of oxidant showed no change in the %ee,
3
. Conclusions
In summary, fluorine substitution, which confers re-
markable configurational stability on homochiral
,5%,6,6%,7,7%,8,8%-octafluoro-2,2%-dihydroxy-1,1%-binaph-
5
thyl, also has an interesting effect on the catalytic
activity and selectivity of the titanium-based catalysts.
F BINOL efficiently catalyzes the titanium-mediated
8
sulfoxidation process. The most intriguing difference
between the F BINOL and BINOL systems is the rever-
8
sal in the sense of chiral induction upon fluorine substi-
tution. Most importantly, the tenfold increase in acidity
of the ring-bound hydroxyl groups still allows coordi-
nation to Ti(IV) centers. Similar trends should operate
Table 3
i
Substrate scope of the sulfoxidation reaction catalyzed by F BINOL/Ti(O Pr)
8
4
Entry
R
R%
Solvent
CH Cl
Oxidant
Reaction time (h)
Temperature (°C)
% ee (S)
1
2
3
4
5
6
7
8
9
4-MePh
Phenyl
2-BrPh
4-BrPh
2-NO Ph
Ph
Me
Me
Me
Me
Me
Et
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
2.2 equivalents TBHP
1.1 equivalents CHP
1.1 equivalents CHP
1.1 equivalents CHP
1.1 equivalents CHP
18
18
18
18
18
18
18
18
18
4.5
4.5
4.5
4.5
−20
−20
−20
−20
−20
−20
−20
−20
−20
0
35
41
0
2
2
2
2
2
2
2
2
2
2
CH Cl
2
CH Cl
2
CH Cl
54
51
17
NR
14
NR
76
82
86
77
2
CH Cl
2
2
CH Cl
2
Ph
Ph
CH Cl
Bn
CH Cl
2
2
CH Cl
2
Ph-SO CH -
4-MePh
4-BrPh
4-MePh
4-BrPh
Me
Me
Me
Me
Me
CH Cl
2
2
2
10
11
12
13
CHCl3
CHCl3
CHCl3
CHCl3
0
23
23

L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
103
Table 4
Isolated yields for selected sulfoxides in the BINOL- and F BINOL-mediated oxidations
8
Entry
R
F BINOL
BINOL
8
Time (h)
% Yield
% ee (S)
Time (h)
Yield
% ee (R)
1
2
3
Me
Br
H
18
18
18
55
77
74
80
76
70
42
42
42
69
74
66
3
14
0
for other substituents at the 2 and 2% positions as well as
the coordinated metals, eventually providing novel cat-
alysts with modulated reactivities [13]. Reactions that
involve highly acidic and/or oxidative conditions [14]
under which 1 could become configurationally or struc-
turally unstable seem to be particularly interesting.
Noteworthy, both enantiomers of 2 can be readily
prepared from commercially available starting
materials.
i-propanol as eluent on a Daicel Chiralpak AS column
(flow rate 1 ml min ).
−
1
(b) 1 mmol scale. Into an oven-dried 10 ml flask was
placed F BINOL (4.3 mg, 0.1 mmol). CHCl (5 ml) was
8
3
i
added, followed by the addition of Ti(O Pr) (14.7 ml,
4
0.05 mmol). Water (36 ml, 2 mmol) was added and the
reaction mixture was allowed to stir for 15 min. Methyl
p-tolyl sulfide (134 ml, 1 mmol) was then introduced,
followed by the addition of cumene hydrogen peroxide
(
80%) (220 ml, 1.2 mmol). The reaction was allowed to
stir for 4–5 h at r.t. The reaction was then quenched
with 1 ml of water and the resulting mixture was
allowed to stir for additional 5 min. Celite was added to
the reaction mixture and the solvent was concentrated
under reduced pressure. The product, adsorbed on
Celite, was added to the top of a silica gel column and
was chromatographed with 4:1 hexanes–diethyl ether.
4
. Experimental
4.1. General
Anhydrous dichloromethane was obtained using the
method described by Grubbs [15]. All sulfides as well as
tert-butyl hydrogen peroxide (TBHP) and cumyl hy-
drogen peroxide (CHP) were purchased from Aldrich
Chemical Company. Column chromatography was car-
ried out using 230–400 mesh silica gel. H-NMR spec-
tra were referenced to residual CHCl and F-NMR
After the F BINOL and sulfone by-product had eluted,
8
the eluent was changed to pure ether and the sulfoxide
was obtained as an analytically pure material in 55%
1
1
yield. H-NMR (400 MHz, CDCl ): l 7.55 (d, J=8.1
3
19
Hz, 2H), 7.34 (d, J=8.4 Hz, 2H), 2.70 (s, 3H), 2.42
3
spectra were referenced to CFCl3.
(s, 3H).
4.2. Methyl-p-tolyl sulfoxide
4.3. Methyl 4-bromophenyl sulfoxide
1
(
a) 0.05 mmol scale. Into an oven dried 1 ml flask
H-NMR (400 MHz, CDCl ): l 7.68 (d, J=8.6 Hz,
3
was placed F BINOL (2.2 mg, 0.5 mmol). CHCl (1 ml)
2H), 7.66 (d, J=8.6 Hz, 2H), 2.72 (s, 3H).
8
3
i
was added, followed by the addition of Ti(O Pr) (0.74
4
ml, 0.25 mmol) via syringe. The reaction turned dark
red–orange. Water (1.8 ml, 0.1 mmol) was added and
the resulting mixture was allowed to stir for 15 min.
Methyl-p-tolyl sulfide (6.7 ml, 0.05 mmol) was intro-
duced, followed by the addition of cumene hydrogen
peroxide (80%) (11 ml, 0.06 mmol). The reaction was
allowed to stir for 4–5 h at r.t. The mixture was then
quenched with 0.5 ml of water and was allowed to stir
for an additional 5 min. Subsequent filtration through a
plug of Celite was followed by concentration under
reduced pressure. The residue was taken up in HPLC
grade hexane and chromatographed using 7:3 hexane–
4.4. Methyl phenyl sulfoxide
1
H-NMR (400 MHz, CDCl ): l 7.66 (d, J=8.1 Hz,
3
2H), 7.54 (m, 3H), 2.73 (s, 3H).
i
4.5. NMR study of the F BINOL/ Ti(O Pr)
8
4
equilibrium
To (R)-F BINOL (29 mg, 0.067 mmol) was added
8
0.5 ml of CDCl in an NMR tube at r.t. Subsequent
3
i
addition of Ti(O Pr) (20 ml, 0.068 mmol) resulted in
4
color change to yellow–orange and the formation of

104
L.J.P. Martyn et al. / Journal of Organometallic Chemistry 603 (2000) 98–104
trolled Synthesis of Fluoroorganic Compounds, Wiley, New
York, 1999. (c) T.H. Tang, M. Nowakowska, J.E. Guillet, I.G.
Csizmadia, J. Mol. Struct. 232 (1991) 133. (d) H. Jendralla, C.H.
Li, E. Paulus, Tetrahedron: Asymmetry 5 (1994) 1297. (e) M.
Schlosser, D. Michel, Tetrahedron 52 (1996) 99 and refs therein.
(f) A. Garc ´ı a Mart ´ı nez, J. Os ´ı o Barcina, A. de Fresno Cerezo,
R.G. Rivas, J. Am. Chem. Soc. 120 (1998) 673. (g) E.P. Kundig,
B. Bourdin, G. Bernardinelli, Angew. Chem. Int. Ed. Engl. 33
the 1:1 adduct 4 along with isopropanol (two equiva-
lents) observed by NMR spectroscopy. H-NMR (400
1
MHz, CDCl ): l 7.73 (d, J=8.8 Hz, 2H), 6.84 (d,
3
1
3
J=8.8 Hz, 2H); C-NMR (100 MHz, CDCl ): l 158.9,
3
1
(
1
44.7 (dm, J=253 Hz), 143.3 (dm, J=253 Hz), 140.2
dm, J=252 Hz), 137.2 (dm, J=249 Hz), 122.1, 120.9,
20.6, 116.3 (d, J=13 Hz), 115.8.
(
1994) 1856. (h) E.P. Kundig, C.M. Saudan, G. Bernardinelli,
Angew. Chem. Int. Ed. Engl. 38 (1999) 1220.
6] T.G. Traylor, S. Tsuchiya, Inorg. Chem. 26 (1987) 1338.
7] K. Mikami, S. Matsukawa, T. Volk, M. Terada, Angew. Chem.
Int. Ed. Engl. 36 (1997) 2768.
[
[
Acknowledgements
We thank the National Science and Engineering Re-
search Council (NSERC), Canada Foundation for In-
novation (CFI), the Research Corporation, the
Connaught Fund, and the University of Toronto for
financial support.
[8] Experiments aimed at understanding the differences in aggrega-
tion preferences of F BINOL versus BINOL are in progress: L.
8
James P. Martyn, S. Pandiaraju, A.K. Yudin unpublished data.
[
9] F. Toda, K. Tanaka, H. Miyamoto, H. Koshima, I. Miyahara,
K. Hirotsu, J. Chem. Soc. Perkin Trans. 2 (1997) 1877.
[
10] (a) F. Di Furia, G. Modena, R. Seraglia, Synthesis (1984) 325.
b) H.B. Kagan, in: I. Ojima (Ed.), Catalytic Asymmetric Syn-
(
thesis, VCH, New York, 1993, p. 203. (c) N. Komatsu, M.
Hashizume, T. Sugita, S. Uemura, J. Org. Chem. 58 (1993) 4529.
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A.S.C. Chan, Tetrahedron: Asymmetry 9 (1998) 1179. (f) T. Iida,
N. Yamamoto, S. Matsunaga, H.G. Woo, M. Shibasaki, Angew.
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3] A.K. Yudin, L.J.P. Martyn, S. Pandiaraju, J. Zheng, A. Lough,
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4] T.J. Boyle, N.W. Eilerts, J.A. Heppert, F. Takusagawa,
Organometallics 13 (1994) 2218.
5] (a) B.E. Smart, in: R.E. Banks (Ed.), Organofluorine Com-
pounds: Principles and Commercial Applications, Plenum Press,
New York, 1994 (Chapter 3). (b) V.A. Soloshonok, Enantiocon-
(
(
f) Y. Yamanoi, T. Yamamoto, J. Org. Chem. 62 (1997) 8560.
g) C. Bolm, O.A.G. Dabard, Synlett (1999) 360 and refs therein.
[
[
11] S. Superchi, M.I. Donnoli, C. Rosini, Tetrahedron Lett. 39
1998) 8541.
[
(
i
12] TiCl4 and Ti(O Pr) can give rise to different adducts with
4
BINOL-derived ligands: N.W. Eilerts, J.A. Heppert, M.L.
Kennedy, F. Takusagawa, Inorg. Chem. 33 (1994) 4813.
13] For studies related to electronic effects of substituents on chiral
ligands in asymmetric catalysis, see: (a) A. Schnyder, L. Hinter-
mann, A. Togni, Angew. Chem. Int. Ed. Engl. 34 (1995) 931. (b)
T.V. RajanBabu, A.L. Casalnuovo, J. Am. Chem. Soc. 118
(1996) 6325. (c) M. Palucki, N.S. Finney, P.J. Pospisil, M.L.
G u¨ ler, T. Ishida, E.N. Jacobsen, J. Am. Chem. Soc. 120 (1998)
948.
[
[
[
[
[14] Oxidation potential of F BINOL (2.07 V vs. Ag/AgCl) is 0.6 V
8
more positive than that of BINOL (1.47 V vs. Ag/AgCl).
[15] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J.
Timmers, Organometallics 15 (1996) 1518.
.
.