Bis(μ-oxo)-Dititanium(IV)-Chiral Binaphthyldisulfonate Complexes for Highly Enantioselective Intramolecular Hydroalkoxylation of Nonactivated Alkenes

Article abstract of DOI:10.1021/acscatal.1c01146

A series of chiral 1,1′-binaphthyl-2,2′-disulfonic acids was designed, synthesized, and applied in a highly enantioselective Ti-catalyzed intramolecular hydroalkoxylation of nonactivated alkenes. The catalyst is probably a complex between two chiral binaphthyldisulfonate ligands and a bis(μ-oxo)-dititanium(IV) core structure. The sulfonamide groups of the ligands and water are necessary for the catalysis, as they may stabilize the catalytically active complex through hydrogen bonding. Various 2-methylcoumarans were obtained in up to greater than 99% yields and up to 97% enantiomeric excess under mild conditions.

Full text of DOI:10.1021/acscatal.1c01146

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Letter
Bis(μ-oxo)−Dititanium(IV)−Chiral Binaphthyldisulfonate Complexes
for Highly Enantioselective Intramolecular Hydroalkoxylation of
Nonactivated Alkenes
Wen-Bin Xie and Zhi Li*
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ABSTRACT: A series of chiral 1,1′-binaphthyl-2,2′-disulfonic acids was
designed, synthesized, and applied in a highly enantioselective Ti-catalyzed
intramolecular hydroalkoxylation of nonactivated alkenes. The catalyst is
probably a complex between two chiral binaphthyldisulfonate ligands and a
bis(μ-oxo)−dititanium(IV) core structure. The sulfonamide groups of the
ligands and water are necessary for the catalysis, as they may stabilize the
catalytically active complex through hydrogen bonding. Various 2-
methylcoumarans were obtained in up to greater than 99% yields and up
to 97% enantiomeric excess under mild conditions.
KEYWORDS: asymmetric catalysis, hydroalkoxylation, alkene, binaphthyldisulfonic acid, titanium
ligands can be obtained with up to 37% total yield in 11 steps
(see Figures S1−S3 in the Supporting Information for details).
We initially examined the reaction of 2-allylphenol (1a)
using the (CF3)2Ph-substituted BINSA L1 along with
metals.16,26 While L1 itself enabled a complete conversion of
the substrate without any asymmetric induction (Table 1,
entry 1), the metal/ligand (M/L = 1:1) complex between
Ti(O-i-Pr)4 and L1 gave a greater than 99% yield with 39%
enatiomeric excess (ee) in dry toluene at 90 °C (Table 1, entry
2; see Table S1 in the Supporting Information for a detailed
study of L1). Unfortunately, BINSAs with or without
fluoroalkyl groups all afforded poor results (Table 1, entries
3−6).15,27 A significant improvement in ee value was observed
when using XieBINSA L6 as the ligand, especially when the
metal was applied in excess (entries 7−9). In addition, the
reaction temperature could be reduced to 75 °C, producing
74% ee, albeit with more time (entry 11). More surprisingly,
titanium alkoxides with bulkier alkyl groups gave higher ee
values, among which Ti(EHO)4 (tetrakis(2-ethylhexyoxy)-
titanium) was the best (entries 11−13). Other metal alkoxides,
such as those of Hf, Zr, La, and Pd, did not have as high an
he asymmetric hydroalkoxylation of alkenes is an
Tattractive method to construct chiral ethers, as it is
simple and intrinsically atom-economical.1 Strong Lewis and
Brønsted acids enable a very efficient hydroalkoxylation of
alkenes,2−5 but an enantioselective hydroalkoxylation catalyzed
by chiral Lewis acids has suffered from harsh reaction
conditions and hampered enantioselectivity in comparison to
that by chiral Brønsted acids (Scheme 1a−d).6−14 An
intriguing question would be whether the activity of strong
Lewis acids and a high selectivity of chiral ligands can be
combined in a single catalyst. The prevalence of triflate Lewis
acids suggests that chiral binaphthyldisulfonic acids would be
suitable candidates that preserve the Lewis acidity of the
central metals. For example, chiral 1,1′-binaphthyl-2,2′-
disulfonic acid (BINSA),15−19 which itself is a strong chiral
Brønsted acid, has been proven to be a promising chiral ligand
for a metal-mediated enantioselective catalysis.20−24 Here we
report our design of a new type of chiral sulfonic acids and
their applications in a highly enantioselective intramolecular
hydroalkoxylation under mild conditions (Scheme 1e).
The new ligands, XieBINSAs, all contain a SO2NMe2 or
SO2NEt2 moiety at the substituted aryl groups. The keys to
synthesizing XieBINSAs are multiple cross-coupling reactions,
Newman−Kwart rearrangement (NKR),25 and oxidation of S-
thiocarbamoyl compounds. The electron-withdrawing property
of the sulfonamide groups made the synthesis easier in
comparison to that for other existing sulfonic acids.17 All
Received: March 12, 2021
Revised: May 5, 2021
Published: May 10, 2021
https://doi.org/10.1021/acscatal.1c01146
© 2021 American Chemical Society
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Scheme 1. Acid-Catalyzed Asymmetric Intramolecular
Hydroalkoxylation of Nonactivated Alkenes
Table 1. Optimization of Catalytic Asymmetric
Hydroalkoxylation of 2-Allylphenol
a
b
c
entry
metal (x)
ligand
temp, °C
yield,
%
ee, %
1
2
3
4
5
6
7
8
L1
L1
L2
L3
L4
L5
L6
L6
L6
L6
L6
L6
L6
L7
L8
L9
L10
L11
L12
L13
L6
90
90
90
90
90
90
90
90
90
90
75
75
75
75
75
75
75
75
75
75
75
>99
>99
>99
96
11
15
51
89
98
36
95
98
>99
70
56
14
43
72
>99
65
0
−39
−1
−33
−2
−5
46
57
63
36
74
86
89
70
69
33
8
40
76
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (5)
Ti(O-i-Pr)4 (7.5)
Ti(O-i-Pr)4 (10)
Ti(O-i-Pr)4 (15)
Ti(O-i-Pr)4 (10)
Ti(O-i-Bu)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
Ti(EHO)4 (10)
9
10
11
12
13
14
15
16
17
18
19
20
activity or enantioselectivity (Table S2). Subsequently, several
XieBINSAs were used as the chiral ligands in an M/L ratio of
2:1 at 75 °C (entries 14−20). Although L7−L13 did not
provide as high an enantioselectivity as L6, their structural
diversity demonstrated that, in the ligand design, the
sulfonamide group and the 6,6′-alkyl groups are necessary.
Finally, the reaction produced 97% ee with a greater than 99%
yield in cyclohexane (CyH) at 75 °C (Table 1, entry 21; see
Table S2 in the Supporting Information for additional data).
Note that, because of their high water solubility, all XieBINSAs
can be recycled by extraction into alkaline methanol followed
by acidification with ion exchange resins.
During the conditions study, inconsistent results were
sometimes obtained. In particular, the reaction did not
proceed at all in the presence of activated 4 Å molecular
sieves (Table S2). This indicates that water may be involved in
the catalyst formation process, which is not uncommon for
titanium complexes.7,28−30 The effect of water was then
evaluated by adding different amounts of water to a preformed
anhydrous mixture of L6 and Ti(EHO)4. The catalysis results
indeed confirmed that the yields and enantioselectivities are
both correlated to the amount of water by showing a volcano
plot (Figure 1a; see Table S3 in the Supporting Information
for details).7 Moreover, the relative ratio between water and
Ti(EHO)4 also seriously affects the reaction rate and
enantioselectivity (see Table S4 in the Supporting Informa-
tion). When the optimal 10−12 mol % water was added, (R)-
2a was obtained in a greater than 99% yield with 97% ee in
only 24 h, which is much faster than in any previous
investigations.
−30
97
d
21
>99
a
Reaction conditions: after a mixture of the ligand (5 mol %) and
metal alkoxides (x mol %) in PhMe (0.5 mL) was stirred at 60 °C for
a catalyst formation process of 0.5 h, 1a (0.1 mmol) was added.
Entries 1−10, 90 °C, 36 h; entries 11−21, 75 °C, 48 h. See Tables S1
b
and S2 in the Supporting Information for details. Determined by 1H
c
NMR using CH2Ph2 as an internal standard. Determined by chiral
SFC analysis. Positive values represent the enrichment of (R)-2a.
d
CyH as solvent.
A matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) analysis showed the predominance of the
formula Ti2O2(XieBINSA)2, consistent with a polynuclear
bis(μ-oxo)−dimetal Ti2O2−chiral binaphthyldisulfonate com-
plex,7,28−34 from the samples prepared from a mixture of
Ti(EHO)4, water, and XieBINSAs (L6, L8, L9, and L13) in
CyH. As a comparison, similarly prepared samples of
conventional BINSAs (such as L1 and L3), which showed
no catalytic activity (Table S4), did not show signals of such
complexes (Figures S4−S6). The SO2NMe2 moiety is probably
necessary for the formation of the active metal complex. 2-
Ethylhexanol (EHOH) generated during the metal complex
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Scheme 2. Mechanistic Studies and Control Reactions
Figure 1. Water effects on yield and enantioselectivity (a) and NLE
between (R)-2a and (R)-L6 (b, x = 12).
formation seemed to promote the solubility of the catalyst.
Using Ti(O-i-Pr)4 as the metal source resulted in a cloudy
mixture and poor results (see Figure S7). In addition, a
positive nonlinear effect (NLE) was observed by using L6 with
different ee values (Figure 1b; see Table S5 in the Supporting
Information for details), indicating the active catalyst is likely
not monomeric in the chiral ligand.
The dimeric complex became a bigger puzzle in deciphering
the true catalyst, because it can only account for a 2:2:2 metal/
ligand/water (M/L/W) ratio composition, while the optimal
M/L/W ratio is 4:2:4. The excess Ti(EHO)4 may act as an
essential substrate transporter to the catalytic center by
forming a mixed alkoxide salt with the substrate, and a few
experiments support this hypothesis. First, a composition of
2:2:4 M/L/W could not catalyze the reaction (Scheme 2a),
but when the excess portion of Ti(EHO)4 was added
separately into the reaction system after the catalyst formation
process, the reaction proceeded as usual (Scheme 2b, see eq S3
in the Supporting Information for details). These experiments
suggest that the excess Ti(EHO)4 did not serve as the metal
source of the chiral metal complex. Second, the primary
alcohol substrate 3a underwent a cyclization using Ti(O-i-Pr)4
as the metal source but not Ti(EHO)4 (Scheme 2c; see Table
S6 in the Supporting Information for details), suggesting that
a shorter D−O hydrogen bond in comparison an H−O
hydrogen bond, which might disrupt the formation of the
delicate catalytically active metal complex.35
the ligand exchange product, Ti(EHO)m(ORSub
)4−m, in which
RSubOH represents the substrate, is essential and that the
higher pKa of 3a in comparison to phenol substrates prevented
its reaction with Ti(EHO)4. Third, too much Ti(EHO)4
inhibited the reaction, which could be attributed to the
With the information on the possible catalyst and
intermediates in hand, we were able to propose a tentative
catalytic cycle of the asymmetric hydroalkoxylation (Scheme
3). First, the complex [(μ-O)Ti(XieBINSA)]2 was generated
in situ from Ti(EHO)4, L6, and H2O in a 4:2:4 ratio. The
excess water most likely bridges one SO2NMe2 group of one
XieBINSA and one Ti(IV) of the Ti2O2 cluster through
hydrogen bonding. The dimeric complex may thus adopt cis-
or trans-diaqua configurations (Figure S11). The cis-diaqua
configuration seems to be the favored structure for catalysis,
since its two open coordination sites are facing the same side
and constitute a wide chiral pocket. The excess Ti(EHO)4 may
coordinate with the free SO2Me2 (Figure S11) before forming
the substrate-transporting complex titanium phenoxide A with
1a and releasing EHOH, which was clearly observed in an
NMR spectrum (Figure S8). Then, the substrate RO was
transported to the titanium metal center of the active cis-
diaqua-[(μ-O)Ti(XieBINSA)]2 as intermediate B by a ligand
exchange. Subsequently, the reaction underwent a concerted
increase in m of Ti(EHO)m(ORSub
)
that may reduce the
4−m
transportation efficiency of RSubO to the catalyst (Figure S8).
Thus, Ti(EHO)m(ORSub
could be the key intermediate
)
4−m
that transports the substrate to the dimeric catalyst.
The excess water is believed to be involved in the catalyst
formation process, because separate additions of excess water
and Ti(EHO)4 into a 2:2:2 M/L/W mixture did not give any
product (Scheme 2d; see eq S4 in the Supporting Information
for details). Since an MALDI-TOF analysis did not reveal the
incorporation of additional water into the dimeric complex, we
believe the excess water most likely interacts with the complex
through hydrogen bonding. As a further demonstration, the
reaction did not work if H2O was replaced with D2O (Scheme
2e), in which a formula of one less oxygen [Ti2O-
(XieBINSA)2] was detected as the major component in the
MALDI-TOF analysis (Figure S9). This could be attributed to
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Letter
underwent facile cyclization, giving the Markovnikov-type 2-
methylcoumaran derivative addition products 2a−2u in high
yields and ee values (Scheme 4). The para substituents of the
Scheme 3. Proposed Catalytic Cycle
Scheme 4. Catalytic Asymmetric Hydroalkoxylation of Non-
a
Activated Alkenes
transition state TS,26 in which protonation of the double bond
and C−O bond formation occur simultaneously.9,36 Finally,
the product (R)-2a was obtained through a ligand exchange
between the intermediate C and any ROH (such as EHOH or
RSubOH) in the system. The concerted TS is supported by the
following experiments: if the reaction operates under a
stepwise mechanism through any carbocation intermediate
before the enantioselectivity-determining step, substrates 3b
and 3b′ should provide the same selectivity, since they
generated the same carbocation. However, in reality they gave
the opposite selectivity of 4b under the same conditions
(Scheme 2f). Second, a reaction kinetics study revealed the
first-order kinetics of 1a (Table S7), and the reaction of 2-
allylphenol-d catalyzed by a catalyst made with H2O resulted in
a low yield (7%), unusually large kinetic isotope effect (KIE) =
66.7( 8.2), and reasonable ee (88%) (Scheme 2g; see Figure
S10 in the Supporting Information for details). The rate-
determining step likely involves the cleavage or formation of an
X−H bond (X = O or C) in a linear transition state of a
hydrogen transfer.37 Note that, since the reaction system with
all active protons replaced by deuterium did not give any
conversion (Scheme 2e), the extra KIE could be the result of
the generation of inactive Ti complexes in the presence of
deuterium.
a
Isolated yields at 48 h. Chiral SFC analysis for ee. See Section 5 in
b
the Supporting Information for details. Cy = cyclohexyl. PhMe as
solvent.
phenol did not affect the enantioselectivities, regardless of their
electronic and steric properties (2b−2k). The meta sub-
stituents on either side of the OH group also allow good
reaction results (2l−2q). Note that substrates such as 1e, 1f,
1k, 1o, and 1p suffered from poor solubility in CyH, and their
reactions had to be performed in toluene. The reaction
tolerates a variety of functional groups: halogens (1b, 1c),
nitrile (1e), ester (1f), ether (1j, 1q), phenol (1k, 1p), isolated
alkene (1k), and ketone (1o). Racemic 2-(but-3-en-2-yl)-
phenol 1r could cyclize into both diastereomers of 2r, with
89% ee of trans-2r and 85% ee of cis-2r in a 1:2.4
diasteriomeric ratio (dr) at 48 h, while 26% ee of the
remaining 1r was left. However, reactions of 2-(1-phenylallyl)-
phenol 1s, α,α-dimethyl 2-allylphenol 1t, and spirocyclic
cycloalkyl derivative 1u proceeded with low reaction rates
and poor ee values, likely due to steric hindrance according to
the mechanism, which also explains why the ortho-substituted
2-allyl-6-methylphenol 1v did not react at all (Table S8). In the
TS, the position ortho to the phenol OH is very close to the
SO2NMe2-substituted aryl ring, while the benzyl position is
Under the optimized reaction conditions, the intramolecular
hydroalkoxylation substrate scope was examined using 10 mol
% of Ti(EHO)4, 5 mol % of (R)-XieBINSA L6, and 12 mol %
of H2O as the standard catalyst. Various 2-allylphenols 1a−1u
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Letter
(4) Dunach, E.; Antoniotti, S.; Poulain-Martini, S. Electrophilic
̃
close to the SO2NMe2 moiety. Any substituents on these
positions greatly hampered the reactivity and selectivity.
In conclusion, we have developed a new type of chiral
binaphthyldisulfonic acid ligands, XieBINSAs, and discovered
that their complexes with a bis(μ-oxo)−dititanium core
structure are able to catalyze asymmetric intramolecular
hydroalkoxylation of 2-allylphenols to chiral 2-methylcoumar-
ans in excellent yields and enantioselectivities under mild
conditions. The reaction likely proceeds via a concerted
mechanism of protonation−cyclization.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01146.
■
sı
Details of the preparation of ligands and starting
materials, details of the optimization of the reactions,
details of nonlinear effect data, details of the preparation
and characterization of catalysts, details of the
deuteration experiments and kinetic studies data, details
of the experimental procedures, and characterization
data (NMR, MS, SFC analysis) (PDF)
AUTHOR INFORMATION
Corresponding Author
■
(12) Zhao, P.; Cheng, A.; Wang, X.; Ma, J.; Zhao, G.; Li, Y.; Zhang,
Y.; Zhao, B. Asymmetric Intramolecular Hydroalkoxylation of
Unactivated Alkenes Catalyzed by Chiral N-Triflyl Phosphoramide
and TiCl4. Chin. J. Chem. 2020, 38, 565−569.
Zhi Li − School of Physical Science and Technology,
ShanghaiTech University, Shanghai 201210, People’s
Republic of China; orcid.org/0000-0003-2770-6364;
Email: lizhi@shanghaitech.edu.cn
(13) Helmbrecht, S. L.; Schluter, J.; Blazejak, M.; Hintermann, L.
̈
Axially Chiral 1,1′-Binaphthyl-2-Carboxylic Acid (BINA-Cox) as
Ligands for Titanium-Catalyzed Asymmetric Hydroalkoxylation. Eur.
J. Org. Chem. 2020, 2020, 2062−2076.
Author
Wen-Bin Xie − School of Physical Science and Technology,
ShanghaiTech University, Shanghai 201210, People’s
Republic of China; Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, People’s
Republic of China; University of Chinese Academy of
Sciences, Beijing 100049, People’s Republic of China;
orcid.org/0000-0002-2396-1483
(14) Ebisawa, K.; Izumi, K.; Ooka, Y.; Kato, H.; Kanazawa, S.;
Komatsu, S.; Nishi, E.; Shigehisa, H. Catalyst- and Silane-Controlled
Enantioselective Hydrofunctionalization of Alkenes by Cobalt-
Catalyzed Hydrogen Atom Transfer and Radical-Polar Crossover. J.
Am. Chem. Soc. 2020, 142, 13481−13490.
(15) Hatano, M.; Maki, T.; Moriyama, K.; Arinobe, M.; Ishihara, K.
Pyridinium 1,1′-Binaphthyl-2,2′-disulfonates as Highly Effective
Chiral Bronsted Acid-Base Combined Salt Catalysts for Enantiose-
lective Mannich-type Reaction. J. Am. Chem. Soc. 2008, 130, 16858−
16860.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01146
(16) Garcia-Garcia, P.; Lay, F.; Garcia-Garcia, P.; Rabalakos, C.;
List, B. A Powerful Chiral Counteranion Motif for Asymmetric
Catalysis. Angew. Chem., Int. Ed. 2009, 48, 4363−4366.
Notes
The authors declare the following competing financial
interest(s): Part of the results have been filed in a patent in
which Z. L. and W.-B. X. are the inventors.
(17) Hatano, M.; Ozaki, T.; Nishikawa, K.; Ishihara, K. Synthesis of
Optically Pure 3,3′-Diaryl Binaphthyl Disulfonic Acids via Stepwise
N−S bond Cleavage. J. Org. Chem. 2013, 78, 10405−10413.
ACKNOWLEDGMENTS
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Financial support for this work was generously provided by the
National Natural Science Foundation of China (Grant No.
21673141) and ShanghaiTech University start-up funding. The
authors acknowledge support from the Analytical Instrumen-
tation Center (Grant No. SPST-AIC10112914), SPST,
ShanghaiTech University, for compound characterization.
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