Facile synthesis of enantioenriched phenol-sulfoxides and their aluminum complexes

Article abstract of DOI:10.1039/c6ob00136j

Chiral phenolic p-tolylsulfoxides and t-butylsulfoxides were prepared by several short synthetic routes starting from readily available starting materials. The key synthetic step was the reaction of lithiated arenes with menthyl sulfinates or enantioselective oxidation of a t-butyl sulfide. Well-defined neutral ligand-AlMe₂ complexes were obtained by stoichiometric treatment with AlMe₃.

Full text of DOI:10.1039/c6ob00136j

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Facile synthesis of enantioenriched phenol-
sulfoxides and their aluminum complexes†
Cite this: DOI: 10.1039/c6ob00136j
Received 16th January 2016,
Accepted 14th March 2016
Yani Tang, Yuji Sun, Jiyong Liu and Simon Duttwyler*
DOI: 10.1039/c6ob00136j
www.rsc.org/obc
Chiral phenolic p-tolylsulfoxides and t-butylsulfoxides were pre- properties: (1) configurational inertness, (2) synthetic accessa-
pared by several short synthetic routes starting from readily avail- bility of both enantiomers and (3) creation of a chiral environ-
able starting materials. The key synthetic step was the reaction of ment that enables reactions with exceptional enantio-
lithiated arenes with menthyl sulfinates or enantioselective oxi- induction, based on the steric and electronic differences of the
dation of a t-butyl sulfide. Well-defined neutral ligand–AlMe2 lone pair, oxygen atom and organic group attached to
complexes were obtained by stoichiometric treatment with AlMe3.
sulfur.4–7 Furthermore, either oxygen or sulfur can act as the
coordinating atom towards metal centers when sulfoxides are
used as ligands, adding an interesting facet to their chem-
istry.8 In terms of chiral auxiliaries, the t-butane-sulfinamide
group is the most prominent example of a stoichiometric sulf-
oxide-based reagent in the synthesis of natural products and
biologically active compounds, including pharmaceuticals and
agrochemicals.9 With regard to use in catalysis, ligands
binding to transition metals based on S(O)–N, S(O)–P, S(O)–S,
S(O)–olefin and S(O)–Cp (Cp = cyclopentadienyl) coordination
have been reported. Primarily in combination with Pd, Rh and
Ru, the resulting complexes have proven to be highly effective in
enantioselective transformations such as allylic alkylations as
well as 1,2- and 1,4-addition reactions to carbonyl groups.10,11
Compared to the plethora of chiral reagents derived from
naturally occurring amines/amino acids or based on chiral
phosphines, the number of hitherto known sulfoxide frame-
works is still limited. We therefore sought to develop a class of
Introduction
The preparation of optically active compounds has been a
long-standing challenge in organic synthesis. Two main
approaches for the synthesis of enantioenriched products
starting from achiral molecules are the use of chiral auxiliaries
and enantioselective catalysis.1–3 The former requires the step-
wise introduction and removal of a chiral moiety, while the
latter relies on direct formation of a desired stereoisomer.
Both strategies can provide access to products in high yields as
well as excellent enantiomeric excess, and the choice of the
synthetic route towards a specific target will depend on many
factors, such as, obviously, the transformation to be per-
formed, the availability of an enantioselective methodology,
costs of materials involved, ease of operation, etc. The quest
for improved stereoselective reactions is a remaining challenge
in academic research and industry, and therefore the develop-
ment of new classes of reagents that are capable of transferring
stereochemical information to target compounds is still of
high significance.
compounds combining
a central phenolic functionality
flanked by a stereogenic sulfoxide moiety and an additional
substituent confining sterics (I, Fig. 1).12 From this design we
anticipated a tunable chiral cone around the central group.
Furthermore, targeting phenol-based products, a range of
The research into and increased use of chiral sulfoxides as
auxiliaries and ligands for enantioselective reactions over the
past two decades may well be termed a success story. The sulf-
oxide moiety as a stereogenic unit exhibits several favorable
Department of Chemistry, Zhejiang University, 310027 Hangzhou, P.R. China.
E-mail: duttwyler@zju.edu.cn
†Electronic supplementary information (ESI) available: Experimental procedures
and spectroscopic data. CCDC 1447653–1447656 and 1462873. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c6ob00136j
Fig. 1 General framework of chiral target compounds I based on a
stereogenic sulfoxide moiety.
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(Lewis-)acidic and (Lewis-)basic reagents should be accessible, hydrochloric acid in tetrahydrofuran afforded product 4. Each
such as phenolates, phenolate–metal complexes and phenol– of the steps proceeded in high yield, and the phenol-sulfoxide
phosphoric acids/phosphates or esters of other inorganic was obtained with 94% ee. Only in the sulfination step a slight
acids. In this work, we report on the synthesis of a series of erosion of enantiomeric excess was observed.13 Product 4
novel phenol-sulfoxide products. Chiral p-tolylsulfoxides and could also be prepared applying a two-step procedure starting
t-butylsulfoxides were prepared by several short synthetic from 1. Bromination of the free phenol with NBS (N-bromosuc-
routes. Enantioenriched compounds were prepared by reaction cinimide) gave bromophenol 5 in 78% yield (Scheme 1b).14
of lithiated arenes with menthyl sulfinates or enantioselective OH-Deprotonation/Br–Li exchange of this intermediate fol-
oxidation of a t-butyl sulfide precursor as the key step. Well- lowed by sulfination gave 4 in 56% yield. Even though this
defined neutral ligand–AlMe2 complexes were obtained by stoi- second route is shorter as it does not require protection–
chiometric treatment with AlMe3.
deprotection steps, the overall yield was found to be lower
(44% vs. 77%). Furthermore, over-bromination of 1 must be
prevented by slow addition of NBS and careful monitoring of
the reaction.15 Commercially available 2-bromo-4-methyl-
phenol (6) provided a more versatile access to phenol-sulfox-
Results and discussion
Our goal was to develop synthetic routes relying on commer-
cially available or easily accessible starting materials. Further-
more, a small number of steps and tunability of the system in
terms of the nature of R1 and R3 of I were aimed at.
At the outset of our studies, we investigated the synthesis of
phenol-oxide 4. 2-Phenylphenol (1) was first protected as its
MOM (methoxymethyl) ether, lithiated at the free ortho posi-
tion and treated with commercially available (1R,2S,5R)-
(−)-menthyl (S)-p-toluenesulfinate to introduce the chiral sulf-
oxide moiety (Scheme 1a). Subsequent deprotection with
Scheme 1 Synthesis of phenol-sulfoxide 4 by protection–sulfination–
deprotection (a) and bromination–sulfination (b).
Scheme 2 Synthesis of phenol-sulfoxides 11 and 15.
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ides I. Introduction of an aryl substituent was accomplished
by microwave-assisted Pd-catalyzed cross-coupling of 6 with
arylboronic acids (Scheme 2a).16 The coupling with 1-naphthyl-
boronic acid and 4-biphenylboronic pinacol ester was tested.
These reactions proceeded efficiently with 0.4 mol% Pd(OAc)2
and a phase-transfer catalyst in water, giving 7a and b in 92%
and 99% yield, respectively. Bromination and subsequent
MOM protection afforded products 8a,b and 9a,b in good
overall yields. The 4-methyl substituent in 7a,b prevented over-
bromination of the central ring effectively. Br–Li exchange fol-
lowed by sulfination gave 10a,b, and final deprotection
afforded phenol-oxides 11a,b in moderate to good yields and
high enantiomeric excess (97% and 98% ee, respectively). Bro-
mination of 7a,b was a necessary step; ortho-lithiation of
MOM-protected 7a,b and direct reaction with menthyl sulfi-
nate did not give satisfactory yields; control experiments in
which the putative lithiate was quenched with D2O or I2
showed little incorporation of iodine or deuterium.
Alternatively, 2,6-dibromophenol (12) allowed for a similar
synthesis of the target scaffold. MOM protection to give 13, fol-
lowed by Br–Li exchange, sulfination and deprotection yielded
compound 15 in 65% yield over three steps (Scheme 2b). With
the sulfoxide moiety already in place, 15 is a promising com-
pound to undergo Pd-catalyzed cross-coupling reactions at the
C–Br position similar to those from 6 to 7, and we are currently
investigating its potential to form phenol-sulfoxides. Crystals
suitable for X-ray diffraction were obtained for 10a and 15. The
observed absolute (S) configurations were in agreement with
inversion at the sulfur center in the sulfination step using
(−)-menthyl (S)-p-toluenesulfinate. In the solid state, both pro-
ducts prefer a conformation that results in reduced repulsion
of the tolyl ring with the OR group of the central ring (Fig. 2).
The respective torsion angles for 10a and 15 are C1–S1–C8–C9
= −78.8° and C7–S1–C6–C1 = −81.5°. Likewise, in 10a the
naphthyl moiety exhibits an orientation with a dihedral angle
of 70.8° between the best-fit planes through the naphthyl
system and the central ring; the S-tolyl and naphthyl substitu-
Scheme 3 Synthesis of phenol-sulfoxide 18 by enantioselective oxi-
dation of sulfane 16 and the X-ray crystal structure of 17; H atoms are
omitted for clarity, thermal ellipsoids are drawn at the 30% probability
level.
Introduction of a t-butyl sulfoxide group was accomplished
by sulfane formation and enantioselective oxidation
(Scheme 3). Reaction of lithiated 2 with di-t-butyl disulfide
gave sulfane 16 in 54% yield. This compound was then sub-
jected to mono-oxidation using hydrogen peroxide and the
chiral vanadium catalyst derived from VO(acac)2 (acac = acetyl-
acetonate) and Schiff base ligand L to afford 17.17 The free
phenol 18 was obtained in 95% yield and 91% ee after de-
protection; the yield over three steps for this route was 42%.18
The absolute configurations (R) were inferred from an X-ray
structural analysis of 17. In the solid state, two molecules of 17
of similar geometry were found in the asymmetric unit with
torsion angles of C7–S1–C1–C2 = 90.6°/94.1°, showing confor-
mations comparable to those found for 10a and 15.
1
ents are oriented in an anti fashion. For 10a and 11a, the H
and 13C{1H} NMR spectra indicated slow rotation about the
naphthyl–aryl bond. Because of the stereogenicity at sulfur,
the syn and anti forms gave rise to diastereomers with two sets
of signals (see the ESI† for details).
We then tested the ability of some products to form com-
plexes with Lewis acids. Bromo- and aryl-substituted 15, 4, 11b
and 18 underwent a clean reaction with AlMe3 to give 1 : 1
Fig. 2 X-ray crystal structures of 10a (a) and 15 (b); H atoms are
omitted for clarity except for O–H, thermal ellipsoids are drawn at the
30% probability level.
Scheme 4 Synthesis of aluminum-phenolate complexes 19 and 20.
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Fig. 4 X-ray crystal structure of protected bis-sulfoxide 23; H atoms
are omitted for clarity, thermal ellipsoids are drawn at the 30% prob-
ability level. ORTEP representation showing all C, O and S atoms (a); side
view with best-fit planes through the biaryl rings showing only C9 of the
tolyl rings and O2 of the MOM groups (b).
Fig. 3 X-ray crystal structure of aluminum complex 19a; H atoms are
omitted for clarity, thermal ellipsoids are drawn at the 30% probability
level.
O–S(O)–AlMe2 aluminates 19a–c and 20 in toluene at room
temperature (Scheme 4). Their identity and ligand : Al ratio
were elucidated by NMR spectroscopy and an X-ray structural
analysis of 19a. 1H, 13C{1H} and 2D (HSQC, HMBC) NMR
spectra showed no unusual features apart from line broaden-
ing of certain signals because of coupling to 27Al, which has a
quadrupole moment owing to its nuclear spin of I = 5/2.
Single crystals of 19a were obtained by slow evaporation of
a diethyl ether solution at room temperature. The sulfoxide
moiety showed coordination to the Al center through oxygen
with bond distances of O1–Al1 = 1.860(4) Å, O2–Al1 = 1.786(3)
Å and S1–O1 = 1.545(4) Å (S1–O1 = 1.501(2) Å for 15) (Fig. 3).
The Al1–O1–S1–C6–C1–O2 unit forms a puckered five-mem-
bered ring, and the observed torsion angles are O1–S1–C6–C1
= −31.5° and S1–C6–C1–O2 = −4.6°.
tions to give 22 in 90% yield (Scheme 5). Subsequent di-ortho-
lithiation and treatment with (−)-menthyl (S)-p-toluenesulfi-
nate furnished bis-sulfoxide 23 in 74% yield. Final de-
protection with concentrated aqueous HCl gave biphenol-bis-
sulfoxide 24 in 94% yield. Thus, starting from the commer-
cially available biphenol, this three-step route provided access
to the desired chiral ligand system in 63% overall yield.
Single crystals of 23 were obtained by slow evaporation of a
dichloromethane solution at room temperature. The molecular
structure exhibited C2 symmetry with bond distances of S1–O1
= 1.491(3) Å, S1–C5 = 1.810(4) Å and S1–C9 = 1.798(4) Å, com-
parable to the ones found in 17 (Fig. 4). The central biaryl
moiety showed a dihedral angle of 65.3° between the respective
best-fit planes. The observed torsion angle of C9–S1–C5–C6 =
−80.4° indicated an orientation of the sulfoxide group with
respect to the biaryl ring which is almost identical to that
found for 10a and 15.
In an extension of the synthesis of phenol-sulfoxide pro-
ducts, we also investigated the feasibility of synthesizing a bis-
sulfoxide ligand.21 Biphenol 21 was protected at both OH posi-
Conclusions
In conclusion, we have developed several strategies to prepare
phenol-sulfoxides of the general structure I based on short
syntheses, featuring moderate to good overall yields and high
enantiomeric excesses. Aluminate complexes were formed
cleanly with four of the free phenols, demonstrating their
ability to undergo complex formation with Lewis acids. In
addition, a biphenol-bis-sulfoxide system was elaborated and
obtained in good overall yield starting from commercially
available starting materials. Further research into the versati-
lity of the synthetic procedures, the potential of the phenols to
afford complexes with transition metals or esters of inorganic
acids and application of the products in catalysis is currently
ongoing in our laboratory.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (grant 107305-N11412) and the Chinese “1000 Young
Talents Plan”. We thank Prof. Dr. Nathaniel Finney from
Scheme 5 Synthesis of biphenol-bis-sulfoxide 24 starting from biphe-
nol 21.
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Tianjin University (P.R. China) for giving us a sample of the
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excess. The latter finding was attributed to a stabilized,
biphenol 21 as a substrate and synthesize ligand 24.
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