Electron-withdrawing, bipheny 1-2,2 -diol-based compounds for asymmetric catalysis

Article abstract of DOI:10.1002/ejoc.201000070

Facile synthetic routes to a chiral chloro-substituted biphenyl-2,2′- diyl hydrogen phosphate and a chiral O,O-biphenyl-2,2′-diyl phosphoramidothioate are described. The performance of these compounds as catalysts for the hydrophosphorylation of imines and the Friedel-Crafts alkylation of indole was investigated. In the latter reaction, the chlorosubstituted phosphoric acid derivative was found to rival the best Bronsted acid catalysts reported to date.

Full text of DOI:10.1002/ejoc.201000070

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000070
Electron-Withdrawing, Biphenyl-2,2Ј-diol-Based Compounds for Asymmetric
Catalysis
Elisa G. Gutierrez,[a] Eric J. Moorhead,[a] Eva H. Smith,[a] Vivian Lin,[a]
Laura K. G. Ackerman,[a] Claire E. Knezevic,[a] Victoria Sun,[a] Sharday Grant,[a] and
Anna G. Wenzel*[a]
Keywords: Organocatalysis / Asymmetric catalysis / Chiral resolution / Biaryls
Facile synthetic routes to a chiral chloro-substituted bi-
phenyl-2,2Ј-diyl hydrogen phosphate and a chiral O,O-bi-
phenyl-2,2Ј-diyl phosphoramidothioate are described. The
phosphonylation of imines and the Friedel–Crafts alkylation
of indole was investigated. In the latter reaction, the chloro-
substituted phosphoric acid derivative was found to rival the
performance of these compounds as catalysts for the hydro- best Brønsted acid catalysts reported to date.
Introduction
The Brønsted acid catalyzed activation of carbonyl
groups and imines to nucleophilic attack is one of the most
ubiquitous synthetic methods used in organic chemistry;
however, the use of chiral Brønsted acids to catalyze asym-
metric reactions has only recently been actively investi-
gated.[1] The application of relatively strong protic acids,
such as chiral phosphoric acid derivatives, is a particularly
recent development, with the majority of reports occurring
Figure 1. Examples of phosphoric acid derived catalysts.
within the past five years.[1]
Most chiral phosphoric acids employed to date have been
binaphthol derivatives.[1–3] From this extensive body of
butyl groups, and the parent diol possesses atropomeric chi-
work, two design features have emerged that effectively pro-
mote both high catalyst activity and asymmetric induction:
the incorporation of bulky 3,3Ј-disubstitution on the bi-
naphthol rings and the presence of electron-withdrawing
substituents to lower the pKa of the phosphoric acid deriva-
tive by inductive effects. Phosphoric acid derivatives 1b,[4]
1c[5] 1d,[6] and 2[7] (Figure 1) are representative examples of
catalysts that contain these design elements. Catalysts that
lack one or both of these motifs, such as 1a, have generally
been found to afford poor results.[1,4]
rality. Despite these desirable attributes, acid 3 lacks the
electron-withdrawing substitution known to be beneficial to
Brønsted acid catalyzed asymmetric catalysis. Akiyama and
co-workers have previously reported the use of 3,3Ј-ni-
trophenyl-substituted biphenyl-2,2Ј-diyl hydrogen phos-
phates to effect enantioselective Mannich reactions with
good yields and enantioselectivities.[3a] Seeking to investi-
gate an alternative substitution pattern, we sought to pre-
pare catalyst family 4 (Figure 2), where X represents elec-
tron-withdrawing moieties placed at the 5,5Ј-positions.
To investigate an alternative catalyst framework, we be-
came intrigued with the chiral biphenyl-2,2Ј-diyl hydrogen
phosphate 3, originally reported by Schrock and Hoveyda
as a ligand precursor in the preparation of enantioselective
olefin-metathesis catalysts.[8] Acid 3 retains the desired de-
sign element of bulky 3,3Ј-disubstitution in the form of tert-
[a] Joint Science Department, Claremont McKenna, Pitzer and
Scripps Colleges,
Claremont, California 91711, USA
Fax: +1-909-621-8588
E-mail: awenzel@jsd.claremont.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000070.
Figure 2. Target catalysts for investigation.
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A. G. Wenzel et al.
SHORT COMMUNICATION
Herein, we report a facile, inexpensive synthetic route to the spectrometry or 31P NMR spectroscopy after heating for
preparation of (S)-4a, as well as the preparation of the re- 24 h and aqueous workup; the starting diol was isolated
lated biphenyl-based phosphoramidothioate catalyst (R)-5. with 80% recovery.
Chiral, binaphthol-derived phosphoramidothioate catalysts
Scheme 1 shows the optimized synthetic route to (S)-(+)-
were recently reported by Yamamoto and co-workers to dis- 4a. The tert-butylation of 6 proceeded smoothly to afford
play enhanced reaction rates and greater asymmetric 7 in quantitative yield. Phenol oxidative coupling proved
induction relative to their corresponding hydrogen phos- challenging, and numerous experimental conditions were
phates.[9] The relative performance of (S)-4a and (R)-5 in investigated to boost product yield.[14] Ultimately, the reac-
asymmetric reactions, namely the hydrophosphonylation tion of 7 in the presence of tert-butyl peroxide (1.5 equiv.)
of imines[5,10,11] and the Friedel–Crafts alkylation of in refluxing chlorobenzene (0.04 ) afforded the most con-
indole,[7,12] is discussed.
sistent results; diol 8 was isolated in 40% yield. This reac-
tion was further optimized, such that we were able to obtain
higher yields (98%, following flash chromatography) using
shorter reaction times (10 min) and less peroxide reagent
(1.05 equiv.) by heating a solution of 7 (1  in chloro-
benzene) in a microwave reactor to form 8.[15]
After thoroughly investigating various amine base/sol-
vent combinations, we found optimal resolution conditions
in the crystallization of the cinchonidine salt of 4a from
toluene. Subsequent decomplexation of the acid with 2 
aqueous HCl afforded analytically pure (S)-4a. Including
the resolution, the preparative route to this catalyst employs
five synthetic steps in 36% overall yield (73% of theoretical)
and requires only a single chromatographic purification
step. A suitably refined crystal structure of (–)-(S)-8 in hex-
anes was later obtained after the methylation and reduction
of (S)-4a (93% overall yield, Scheme 2),[16] providing stereo-
chemical confirmation as well as efficient access to chiral 8
for synthetic and materials science applications.
Results and Discussion
Prior to this article, only the racemic parent diol of 4c
had previously been reported.[13] For the others, we envi-
sioned starting with commercially available phenols, such
as 6 (Scheme 1), and generating the requisite diols by acid-
catalyzed tert-butylation, followed by oxidative coupling to
form the diol precursors to 4a, 4b, and 4d, respectively. The
diols could then be phosphorylated and resolved to gener-
ate the resulting chiral acid.
Scheme 2. Preparation of chiral diol 8.
In addition to (S)-4a, we were able to prepare and char-
acterize catalyst (R)-5 by adapting a synthetic procedure
first reported by Yamamoto and co-workers.[9] Starting
from the commercially available (R)-diol and proceeding via
a phosphorochloridothioate intermediate, (R)-5 was iso-
Scheme 1. Optimized route to (+)-(S)-4a.
After extensive investigation, only the preparation of 4a lated in 45% overall yield.[16]
proved viable according to our proposed resolution route.
To investigate the performance of (S)-4a and (R)-5 rela-
We found the four-step synthesis to access the diol of 4c tive to known catalysts, we first chose to screen them in the
too low-yielding to be practicable. In the case of 4d, the hydrophosphonylation of N-benzylidene-2-methoxyaniline
oxidative coupling afforded a negligible yield, and attempts (9, Table 1). In our studies, we found 9 to produce better
to nitrate more readily prepared precursors were unsuccess- yields and more consistent results than the more commonly
ful. In addition, we observed diol phosphorylation to be used p-methoxyphenyl analogue with all catalysts screened.
highly sensitive to the electronics of the starting phenol. While modest, these results do highlight the better perform-
Several conditions were investigated (e.g. POCl3/pyridine, ance of (S)-4a relative to the other catalysts investigated. In
POCl3/NaH),[2b,8] but only diol 8 was found to afford the addition, the positive impact of chloro versus methyl substi-
desired product in high yield (98% yield; Scheme 1). By tution on both yield and enantioselectivity (Entries 1 and
comparison, in the case of 4b, only trace amounts of the 3) and the positive impact of bulky 3,3Ј-disubstitution (rela-
desired phosphoric acid derivative were observed by mass tive to Entry 5) is clearly apparent. The reaction yield
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Biphenyl-2,2Ј-diol-Based Compounds for Asymmetric Catalysis
(10%) and asymmetric induction (13% ee) of (R)-5 was Table 2. Friedel–Crafts alkylation of indole.
found to match those of the corresponding phosphoric acid
derivative (R)-3, although it should be noted that the con-
figuration of the major isomer was inverted to favor the
formation of the (S) enantiomer. The reactions for this
study were allowed to proceed for 24 h before workup and
analysis. In the case of (S)-4a, we found that the reaction
did continue towards completion with additional reaction
time, displaying little or no loss in enantioselectivity (En-
Entry Catalyst
Yield [%][a]
ee[%][b]
Configuration[c]
try 2).
1
2
3
4
5
(S)-4a
(R)-3
85
83
39
51
27
60
48
8
(S)
(R)
(R)
(R)
(R)
Table 1. Asymmetric hydrophosphonylation of 9.
(R)-5
(R)-1a
(R)-1b
6
26
[a] Isolated yield. [b] Determined by chiral HPLC on a Chiralpak
AD-H column: 20% isopropyl alcohol in hexanes, 1.0 mL/min, λ
= 254 nm; tR = 11.5 and 13.1 min. [c] Absolute configuration of
the major product isomer was determined by correlation to the
literature value.[7]
Entry Catalyst t [h] Yield [%][a] ee[%][b] Configuration[c]
1
2
3
4
5
6
(S)-4a
(S)-4a
(R)-3
24
72
24
24
24
24
45
89
24
38
37
13
13
0
(S)
(S)
(R)
(S)
–
acid derivative (S)-4a, the development of a new microwave
synthetic protocol proved instrumental in obtaining high
yields of the precursor diol. Phosphoric acid derivative (S)-
4a has demonstrated itself to be a competent catalyst. In
the case of the Friedel–Crafts addition of indole to chal-
cone, we found 4a to possess improved asymmetric induc-
tion relative to the best acid catalysts reported to date. Be-
yond its use as a catalyst, we have also shown that (S)-4a
can be readily converted into the corresponding (S)-diol in
high yield. We are currently investigating these compounds
as ligands in additional asymmetric reactions as well as in
materials science applications, such chiral dopants in ne-
matic liquid crystals.
(R)-5
10
(R)-1a
(R)-1b
10
ca. 2[d]
–
–
[a] Isolated yield. [b] Determined by chiral HPLC on an OD-H
column: 3% isopropyl alcohol in hexanes, 0.5 mL/min, λ = 254 nm;
tR = 17.7 and 21.0 min. [c] Absolute configuration of the major
product isomer was determined by correlation of the optical rota-
tion of deprotected 10 to that reported in ref.[17] [d] Determined by
1H NMR spectroscopy.
To further investigate the performance of (S)-4a and (R)-
5 relative to known catalysts, we next tested its ability to
promote the Friedel–Crafts addition of indole to chalcone
(Table 2). While several asymmetric organocatalytic exam-
ples have been developed,[11] this remains a challenging re-
action for strong Brønsted acid catalysts. The best results
to date using a phosphoric acid catalyst were reported by
Tang and co-workers in 2008 after an extensive catalyst
screen where phosphoric acid derivative 2 was found to cat-
alyze the addition of indole to chalcone in 56% ee.[7] It
should be noted that a slightly higher ee (58%) has been
reported for this reaction by using a -camphor-based sul-
fonic acid.[18]
When (S)-4a (10 mol-%) was employed in this model
Friedel–Crafts reaction, the product indole 11 was isolated
in 85% yield and 60% ee after 48 h. A slightly higher yield
(93%) was obtained after 72 h with no loss in enantio-
selectivity. We found that this product readily crystallized
from diethyl ether/hexanes: product 11 could be isolated in
90% ee and 64% overall yield after a single recrystallization
step. In contrast, phosphoramidothioate (R)-5 proved to be
a poor catalyst in this reaction: the (R) enantiomer of 11
was obtained in only 39% yield and 8% ee after 48 h.
Experimental Section
2-tert-Butyl-4-chloro-5-methylphenol (7): A flame-dried, 500-mL
flask was charged with 6 (15.0 g, 105 mmol, 1.0 equiv.). CH2Cl2
(50 mL) was then added with stirring. To this mixture, tert-butyl
alcohol (10.1 mL, 105 mmol, 1.0 equiv.) was added. The reaction
mixture was cooled to 0 °C for 5 min, and 95% aq. H2SO4 (5.9 mL,
105 mmol, 1.0 equiv.) was added. The reaction mixture was allowed
to warm to ambient temperature. After 48 h, additional tert-butyl
alcohol (10.1 mL, 105 mmol) and 95% H2SO4 (5.9 mL, 105 mmol)
were added. After an additional 48 h, saturated aq. NaHCO3 was
slowly added until gas evolution ceased (ca. 100 mL). The mixture
was then partitioned with EtOAc (100 mL). The organic layer was
washed with saturated aq. NaHCO3 (2ϫ75 mL) and brine
(1ϫ75 mL). The organic layer was then dried with anhydrous
Na2SO4, filtered, and concentrated to afford analytically pure prod-
uct as a yellow oil in quantitative isolated yield (20.8 g). TLC (sil-
ica): R = 0.54 (15% EtOAc in hexanes). IR (thin film): ν = 3554
˜
f
(w), 2958 (m), 1499 (m), 1375 (s), 1184 (s), 1169 (s), 1096 (s) cm–1.
1H NMR (300 MHz, CDCl3, 20 °C): δ = 7.19 (s, 1 H, ArH), 6.54
(s, 1 H, ArH), 4.69 (s, 1 H, OH), 2.25 (s, 3 H, CH3), 1.36 [s, 9 H,
(CH3)3] ppm. 13C NMR (75 MHz, CDCl3, 20 °C): δ = 152.4, 135.4,
134.1, 127.5, 125.4, 118.8, 34.3, 29.4, 19.3 ppm. HRMS (EI): m/z
= 198.0819 [M]+.
Conclusions
We have developed efficient routes towards the prepara-
3,3Ј-Di-tert-butyl-5,5Ј-dichloro-6,6Ј-dimethylbiphenyl-2,2Ј-diol (8):
tion of two new catalysts. In the case of chiral phosphoric In a 250-mL flask, 7 (5.0 g, 25 mmol, 1.0 equiv.) was dissolved in
Eur. J. Org. Chem. 2010, 3027–3031
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3029

A. G. Wenzel et al.
SHORT COMMUNICATION
chlorobenzene (200 mL). Di-tert-butyl peroxide (6.9 mL,
37.8 mmol, 1.5 equiv.) was then added with stirring. The flask was
capped with a reflux condenser and heated to reflux for 2 h. The
reaction mixture was then allowed to slowly cool to ambient tem-
perature. The volatiles were removed, and the resulting residue was
purified by silica gel chromatography (100% hexanes) to afford the
desired product as a white solid in 40% isolated yield (3.95 g). Mi-
crowave procedure: In a 0.2–5 mL microwave reaction vial, 2-tert-
butyl-4-chloro-5-methylphenol (431 mg, 2.17 mmol, 1.0 equiv.) was
dissolved in chlorobenzene (1.5 mL). Di-tert-butyl peroxide
(416 µL, 2.28 mmol, 1.05 equiv.) was then added. The vial was
capped and placed in the microwave reactor. After thoroughly mix-
ing for 5 min, the reaction mixture was heated to 200 °C for an
additional 10 min. Upon cooling, the reaction mixture was directly
purified by flash chromatography on silica gel to afford the desired
product in 98% isolated yield (841 mg). TLC (silica): Rf = 0.26
quinoline ArH), 7.83 (d, J = 8.2 Hz, 1 H, quinoline ArH), 7.74–
7.68 (m, 2 H, quinoline ArH), 7.53–7.48 (m, 1 H, quinoline ArH),
7.41 (s, 2 H, biphenyl ArH), 6.43 (br., 1 H), 5.52–5.41 (m, 1 H,
alkene CH), 5.03–4.95 (m, 2 H, alkene CH), 4.58 (br. m, 1 H, meth-
ine CH), 3.43–3.37 (m, 1 H, quinuclidine CH), 3.28–3.04 (m, 3 H,
quinuclidine CH), 2.62 (br. m, 1 H), 2.40–1.75 (m, 4 H, quinuclid-
ine CH), 1.95 (s, 6 H, ArCH3), 1.53 [s, 18 H, Ar(CH3)3], 1.50 (m,
1 H, quinuclidine CH), 1.10 (br. m, 1 H, quinuclidine CH) ppm.
To decomplex the acid salt, the solid was fully dissolved in CH2Cl2
(ca. 250 mL) and placed in a separation funnel. The organic layer
was then washed with 2  aq. HCl (4ϫ100 mL), dried with anhy-
drous Na2SO4, filtered, and concentrated to afford analytically
pure (+)-(S)-3a (2.77 g, 38% yield). [α]2D3 = +258 (c = 0.017,
CH3OH). All other analytical information matches that for the ra-
cemic acid listed above.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed procedures and characterization data for all new
compounds discussed, as well as the 1H NMR spectra for the unre-
solved/resolved salts of 4a; experimental procedures for the asym-
metric hydrophosphonylation of imines and asymmetric Friedel–
Crafts reaction of indole.
(100% hexanes). IR (pellet): ν = 3505 (s), 2969 (m), 1371 (s), 1182
˜
(s) cm–1. 1H NMR (300 MHz, CDCl3, 20 °C): δ = 7.38 (s, 2 H,
ArH), 4.87 (s, 2 H, OH), 1.96 (s, 6 H, CH3), 1.40 [s, 18 H,
(CH3)3] ppm. 13C NMR (75 MHz, CDCl3, 20 °C): δ = 150.9, 136.1,
133.5, 128.7, 126.2, 121.3, 34.9, 29.3, 16.8 ppm. HRMS (FAB): m/z
= 394.1456 [M]+.
3,3Ј-Di-tert-butyl-5,5Ј-dichloro-6,6Ј-dimethylbiphenyl-2,2Ј-diyl Hy-
drogen Phosphate (4a): To a 2-neck, 50-mL flame-dried flask
equipped with a reflux condenser 8 (2.5 g, 6.32 mmol, 1.0 equiv.)
was added under N2. Anhydrous pyridine (18 mL, 220 mmol,
35 equiv.) was added, followed by the dropwise addition of phos-
phorus oxychloride (1.2 mL, 12.7 mmol, 2.0 equiv.), with stirring.
The reaction mixture was heated to 95 °C under N2 for 24 h, and
then allowed to cool to ambient temperature. Deionized water
(5.2 mL, 288 mmol, 45 equiv.) was added, and the mixture was re-
heated to 95 °C for an additional 5.5 h. Upon cooling, the reaction
mixture was transferred to a separatory funnel by using CH2Cl2
(100 mL); 1  aq. HCl was then slowly added until the aqueous
layer was acidified to pH = 1. The layers were separated, and the
organic layer was washed with 1  aq. HCl (3ϫ60 mL), dried with
anhydrous Na2SO4, and concentrated to afford analytically pure
product as a white solid in 98% yield (2.9 g). TLC (silica): Rf =
Acknowledgments
Acknowledgment is made to the donors of the American Chemical
Society Petroleum Research Fund (PRF#46492-GB 1) for partial
support of this research. E. G. G. was supported by a Rose Hills
Foundation Summer Research Grant; C. E. K. and E. S. were
funded by Keck Foundation Summer Research Grants. Law-
rence M. Henling and Michael Day of the Caltech X-ray Crystal-
lography Facility and Dr. Mona Shahgholi at the Caltech Mass
Spectrometry Facility are gratefully acknowledged. Biotage is
thanked for the generous loan of a microwave reactor. The authors
additionally wish to thank Dr. Adam Johnson and Harvey Mudd
College for the use of their polarimeter.
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0.14 (10% CH OH in CH Cl ). IR (pellet): ν = 2967 (w), 1219 (m),
˜
3
2
2
1200 (m), 1098 (m), 1021 (s), 922.7 (s) cm–1. H NMR (300 MHz,
CD2Cl2, 20 °C): δ = 7.50 (s, 2 H, ArH), 3.23 (br., 2 H, OH), 1.98
(s, 6 H, CH3), 1.44 [s, 18 H, (CH3)3] ppm. 13C NMR (75 MHz,
CD3OD, 20 °C): δ = 149.6, 141.9, 134.9, 132.5, 131.0, 129.0, 36.2,
1
31.9, 17.8 ppm. 31P NMR (121 MHz, CD2Cl2, 20 °C):
–1.41 ppm. HRMS (FAB): m/z = 457.1100 [M + H]+.
δ =
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diyl Hydrogen Phosphate [(S)-4a]: In a 1-L flask racemic 4a (7.33 g,
16.03 mmol, 1.0 equiv.) and cinchonidine (4.81 g, 16.35 mmol,
1.02 equiv.) were heated in refluxing ethanol (170 mL) until fully
dissolved (ca. 5 min). After stirring for an additional 15 min, the
mixture was cooled, and the volatiles were removed to afford the
corresponding salt. The flask was then heated to 120 °C in an oil
bath, and toluene (ca. 350 mL) was slowly added until the mixture
was entirely homogeneous. The mixture was then allowed to slowly
cool to ambient temperature and stand for 48 h. The flask was
subsequently cooled to 0 °C for an additional 40 min before filter-
ing to isolate the resulting white crystals. The crystals were washed
with cold toluene (3ϫ7 mL) and dried in vacuo to afford the (S)-
acid/cinchonidine salt in 38% isolated yield as a single diastereomer
by 1H NMR [most notably, the peak at δ = 7.35 ppm (s, 2 H)
corresponding to the biphenyl ArH of the (R)-acid/cinchonidine
salt has disappeared].[16] 1H NMR (300 MHz, CDCl3, 20 °C): δ =
8.91 (d, J = 4.4 Hz, 1 H, quinoline ArH), 8.14 (d, J = 8.3 Hz, 1 H,
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Biphenyl-2,2Ј-diol-Based Compounds for Asymmetric Catalysis
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[15] The scope of this microwave protocol has been investigated and
will be reported shortly.
[16] See Supporting Information. CCDC-772003 contains the sup-
plementary crystallographic data for this paper. These data can
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Received: January 20, 2010
Published Online: April 28, 2010
Eur. J. Org. Chem. 2010, 3027–3031
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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