Chiral Diarylmethanes via Copper-Catalyzed Asymmetric Allylic Arylation with Organolithium Compounds

Article abstract of DOI:10.1021/acs.orglett.5b03396

A highly enantioselective copper/N-heterocyclic carbene catalyzed allylic arylation with organolithium compounds is presented. The use of commercial or readily prepared aryllithium reagents in the reaction with allyl bromides affords a variety of chiral diarylvinylmethanes, comprising a privileged structural motif in pharmaceuticals, in high yields with good to excellent regio- and enantioselectivities. The versatility of this new transformation is illustrated in the formal synthesis of the marketed drug tolterodine (Detrol).

Full text of DOI:10.1021/acs.orglett.5b03396

Letter
pubs.acs.org/OrgLett
Chiral Diarylmethanes via Copper-Catalyzed Asymmetric Allylic
Arylation with Organolithium Compounds
Sureshbabu Guduguntla, Valentín Hornillos,* Romain Tessier, Martín Fananas-Mastral,†
́
̃
and Ben L. Feringa*
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 Groningen, AG, The Netherlands
S
* Supporting Information
ABSTRACT: A highly enantioselective copper/N-heterocy-
clic carbene catalyzed allylic arylation with organolithium
compounds is presented. The use of commercial or readily
prepared aryllithium reagents in the reaction with allyl
bromides affords a variety of chiral diarylvinylmethanes,
comprising a privileged structural motif in pharmaceuticals,
in high yields with good to excellent regio- and enantiose-
lectivities. The versatility of this new transformation is illustrated in the formal synthesis of the marketed drug tolterodine
(Detrol).
he enantioselective synthesis of diarylmethane tertiary
using organometallic reagents.2a,8d−g We envisioned that an
Tstereogenic centers, a structural motif that is present in asymmetric allylic arylation (AAAr) with highly reactive
aryllithium reagents, as presented here, would provide a viable
and attractive alternative to access these chiral structures. In the
case of copper, the use of the corresponding alkyl nucleophiles
has been well established, and AAA reactions of a wide range of
alkyl metal reagents and allylic systems have been reported.8a−e,9
In contrast, the introduction of less reactive aryl groups continues
to provide major challenges, and several groups embarked on the
development of a general and efficient catalytic system for the
formation of chiral diarylmethanes based on this trans-
formation.10 High regio- and enantioselectivity was demon-
strated by Hoveyda and co-workers using chiral bidentate N-
heterocyclic carbenes (NHC) for the AAAr with aryldialkylalu-
minum reagents,10a derived from the corresponding organo-
lithium compounds. Bidentate NHC have also been employed by
the group of Hayashi in the allylic substitution with less reactive
arylboronates and allyl phosphates.10b,c Additionally, aryl
Grignard reagents have been employed by Tomioka and co-
workers using chiral monodentate N-heterocyclic carbenes.10d,e
Recently, we reported that organolithium compounds, among
the most widely used reagents in organic synthesis, can be
directly used as nucleophiles in copper-catalyzed AAA with a
variety of allyl systems.11 The use of Taniaphos or monodentate
phosphoramidites as chiral ligands in dichloromethane and n-
hexane as solvent and cosolvent, allowed us to control the high
reactivity of these compounds and obtain excellent regio- and
enantioselectivities in AAA reactions. Disappointingly, the
reaction with PhLi under these conditions consistently led to
poor regioselectivities.11aAs aryllithium compounds are com-
mercially available or readily accessible by lithium−halogen
exchange12 and, moreover, they are often employed as precursors
many natural products and pharmaceuticals, has attracted
considerable attention in recent years.1 Examples of compounds
bearing this subunit include podofilox (Condylox),1b nomifen-
sine, CDP-840,1c (+)-sertraline (Zoloft),1d and (R)-tolter-
odine,1e the latter being a drug with blockbuster status. Catalytic
asymmetric synthesis methods to access these compounds
comprise both stereospecific and enantioselective transforma-
tions.2−7,10 The first approaches, based on chiral starting
materials, include a nickel-catalyzed cross-coupling of 1,1-diaryl
ethers described by the group of Jarvo3a and a stereoretentive
rhodium-catalyzed decarbonylation of enantioenriched β,β-
diarylpropionaldehydes reported by the group of Carreira.3b
Catalytic enantioselective strategies include Friedel−Crafts
reactions,4a iridium-catalyzed asymmetric hydrogenation of 1,1-
diarylalkenes,4b,c a cooperative rhodium/phosphoric acid-cata-
lyzed asymmetric arylation of α-aryl-α-diazo compounds with
aniline derivates,4d and a copper-catalyzed enantioselective
electrophilic arylation of allylic amides with diaryliodonium
salts.4e Another attractive approach has been reported by Fu and
co-workers, where racemic benzylic alcohols were converted into
1,1-diarylalkanes using an enantioselective nickel-catalyzed
cross-coupling protocol.4f
Transition-metal-catalyzed 1,4-addition of organoboron com-
pounds to substituted electron-deficient styrenes has also been
shown to be effective in accessing this structural motif, in
particular using a rhodium-based catalyst.5 Additionally, the
catalytic enantiotopic group selective cross-coupling of achiral
geminal bis(pinacolboronates)6 and the recently developed
additions of malonates7a or boron reagents7b,c to quinone
methides provide useful chiral gem-diarylmethines and boronic
esters derivates.
Diarylmethane stereogenic centers can also be accessed via
Received: November 26, 2015
metal-catalyzed arylation of aryl-substituted allyl electrophiles
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03396
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
for other organometallic compounds (Al, B, Zn), the develop-
ment of a general AAAr method using these reagents is highly
desirable.
complex derived from L6 gave the same result, avoiding the use
of NaOtBu and simplifying the procedure (entry 7).
Having established optimal conditions, we next investigated
the substrate scope and generality of this arylation reaction by
using PhLi; the results are summarized in Scheme 1.
Herein, we report the first regio- and enantioselective method
for the copper-catalyzed AAAr with aryllithium compounds to
afford optically active diarylvinylmethanes with excellent regio-
and enantioselectivities (SN2′:SN2 up to 98:2, er up to 97:3).
The reaction between allyl bromide 1a and commercially
available PhLi, in the presence of catalytic amounts of CuBr·
SMe2 and chiral ligands, was used for the initial optimization
(Table 1).11a PhLi was diluted with hexane and added over 2 h to
Scheme 1. Substrate Scope for the Cu-Catalyzed
a
Enantioselective Allylic Arylation
a
Table 1. Screening of Different Ligands
b
c
entry
L
[Cu]
2a:3a
2a, er
1
2
3
4
5
6
7
L1
CuBr·SMe2
CuBr·SMe2
CuBr·SMe2
CuBr·SMe2
CuBr·SMe2
CuBr·SMe2
10:90
70:30
47:53
37:63
63:37
97:3
n.d.
n.d.
n.d.
n.d.
94:6
97:3
97:3
L2
L3
L4
L5
L6
a
Conditions: allyl bromide 1 (0.2 mmol) in CH2Cl2 (2 mL). PhLi (0.3
CuClL6
97:3
mmol, 1.8 M solution in dibutyl ether diluted with hexane to a final
concentration of 0.4 M) added over 2 h. All reactions gave full
conversion. 2/3 ratios and conversions determined by GC−MS and
1H NMR spectroscopy. Er determined by chiral HPLC after
conversion to the corresponding primary alcohol using a hydro-
boration−oxidation procedure (see the SI). The absolute config-
uration of 2a was assigned by comparing the sign of the optical
rotation with the literature value (ref 10d). (4R,5R)-L6 was used
instead. 5 mmol (1.2 g) scale reaction using 3 mol % of catalyst.
a
Conditions: allyl bromide (0.2 mmol) in CH2Cl2 (2 mL). PhLi (0.3
mmol, 1.8 M solution in dibutyl ether diluted with hexane to a final
concentration of 0.4 M) added over 2 h. All reactions gave full
b
conversion. 2a/3a ratios and conversions determined by GC−MS
c
and 1H NMR spectroscopy. Determined by chiral HPLC after
b
conversion to the corresponding primary alcohol using a hydro-
boration−oxidation procedure (see the SI).
c
d
a solution of allyl bromide in CH2Cl2 at −80 °C. As the use of
chiral phosphorus ligands, which provided high selectivity for
alkyllithium reagents,11 did not lead to satisfactory results (entry
1, Table 1 and results not shown), we decided to evaluate a series
of strong σ-donating NHCs. The use of chiral bidentate NHC−
Cu catalysts, in situ prepared by deprotonating imidazolinium
salt L210a and structurally related imidazolinium salts L3 and L4,
led to low or moderate regioselectivities (entries 2−4, Table 1).13
We then examined sterically demanding chiral monodentate
NHC ligands, and an improved regio- and good enantioselec-
tivity were observed when the catalyst derived from imidazoli-
nium salt L510d was used (entry 5, Table 1). To our delight, the
catalyst derived from L6, having o-tolyl moieties, led to a major
increase in regioselectivity toward the branched product 2a (b,l =
97:3) with excellent enantioselectivity (97:3 er, entry 6). A
possible rationale is that the use of bulkier aryl substituents on
the N atoms enhances the reductive elimination step favoring the
SN2′ product.14 Importantly, the isolated air-stable CuCl−NHC
The presence of chloro or bromo substituents at the aromatic
ring of the substrate were well tolerated, affording the
corresponding diarylvinylmethanes in high yields and selectiv-
ities and providing synthetically useful functionalities for further
transformations (2a−d). Importantly, no evidence of lithium−
halogen exchange was observed, highlighting the high chemo-
selectivity of the reaction. Trifluoromethylated and fluorinated
compounds, which are very important in the agrochemical and
pharmaceutical industries,15 were also suitable substrates
furnishing the corresponding gem-biaryl products with excellent
selectivities (2e−g). High selectivities were also obtained when
electron-donating substituents (1h, 1j, and 1k) or sterically
demanding substrates such as 1-naphthyl-substituted allyl
bromide (1i) or compounds 1j and 1k were used with this
Cu−NHC-based catalyst system. Arylation of compound 1l was
accomplished with good regio- and enantioselectivity, providing
2l, which is an advanced intermediate in the synthesis of
B
DOI: 10.1021/acs.orglett.5b03396
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
sertraline,10d a major pharmaceutical for the treatment of
depression. Compound 2k bearing m-methyl and o-methoxy
substituents at the aryl ring was also prepared with excellent
regio- (97:3) and enantioselectivity (96:4) serving as precursor
for the synthesis of (R)-tolterodine (see below). Importantly,
when this reaction was performed on a larger scale (5 mmol, 1.2
g), using a lower catalyst loading (3 mol %), product 2k was still
obtained with the same selectivities without erosion of yield.
Allylic bromides bearing a phenol ether or protected amine
provided highly functionalized chiral building blocks 2m and 2n,
with excellent yields and regioselectivity although the
enantioselectivity decreased slightly. The use of a dioxolane-
containing allylic bromide 1o led to the diastereoselective
formation of valuable 1,2-hydroxyallyl moiety 2o with excellent
stereocontrol for the anti-isomer.16
high regio- and enantioselectivity (entry 6, Table 2, and 2p,
Scheme 2). Under these conditions, aryllithiums bearing
a
Scheme 2. Scope of Aryllithium Compounds
We next explored the scope of the reaction with respect to the
aryllithium component using 1a as the electrophilic counterpart.
However, to our surprise, no conversion was observed when p-
tolyllithium or (p-methoxyphenyl)lithium solutions, prepared in
THF via bromide−lithium exchange using t-BuLi, were
employed in the reaction under previously optimized conditions
(entries 1 and 2, Table 2).
Table 2. Screening of Different Conditions for the
a
Conditions: allyl bromide (0.2 mmol) in CH2Cl2 (2 mL). R2Li (0.4
Preparation of Reactive Homemade Aryllithium
mmol) was diluted with hexane to a final concentration of 0.4 M and
added over 2 h. All reactions gave full conversion. 2/3 ratios and
conversions determined by GC−MS and H NMR spectroscopy. Er
determined by chiral HPLC after conversion to the corresponding
a
Compounds
1
primary alcohol using a hydroboration−oxidation procedure (see the
SI).
electron-donating methoxy- and alkyl groups as well as
electron-withdrawing −CF3 substituents participate in the
reactions with allyl bromides 1a,f in good to excellent yields
and regio- and enantioselectivities (Scheme 2). A limitation
found for this Cu−NHC-based catalytic system is that the use of
o-methoxy-substituted phenyllithium suffered from diminished
enantioselectivity as seen for compound 2v.
To finally demonstrate the efficiency and applicability of the
present methodology, we performed the synthesis of chiral
alcohol 4k,6 a precursor of (R)-tolterodine (Detrol). Here, the
catalytic allylic arylation of 2k with phenyllithium is followed by a
one-pot hydroboration−oxidation to afford advanced inter-
mediate 4k in 64% yield (96:4 er) (Scheme 3). (R)-Tolterodine
is a potent competitive muscarine receptor antagonist for the
treatment of urinary incontinence and cystitis.1e
entry
1
R1
lithiation cond (final conc, M)
conv (%)/2:3, 2a er
OMe t-BuLi, −30 °C to rt, 1 h
0
1:2 THF/pentane (0.57)
2
3
4
Me
Me
t-BuLi, −30 °C to rt, 1 h
0
0
1:2 THF/pentane (0.57)
t-BuLi, −30 °C to rt, 1 h
1:2 Et2O/pentane (0.57)
Me
Me
Me
Li, Et2O, rt, 2 h (1.5)
Li, Et2O, rt, 2 h (1.5)
0
b
5
47/95:5
6
n-BuLi, −30 °C to rt, 1 h
>99/85:15, 2a 97:3
4:3 Et2O/hexane (0.69)
a
Conditions: allyl bromide (0.2 mmol) in CH2Cl2 (2 mL). RLi (0.4
mmol) was diluted with hexane to a final concentration of 0.4 M and
was added over 2 h. 2a/3a ratios and conversions determined by GC−
1
MS and H NMR spectroscopy. Er determined by chiral HPLC after
Scheme 3. Conversion of (R)-2k into (R)-4k, a Synthetic
Intermediate of Tolterodine
conversion to the corresponding primary alcohol using a hydro-
boration−oxidation procedure (see the SI). 1-Chloro-4-methylben-
zene was used instead.
b
Changing THF to less coordinating Et2O as solvent led to the
same result (entry 3, Table 2). As the use of t-BuLi to effect
lithium−halogen exchange generates 1 equiv of 2-methylpro-
pene, which may coordinatively interfere with the Cu catalyst, we
decided to use a different method for the lithiation. Lithium
metal17 in combination with p-bromotoluene was still unsat-
isfactory, although the use of p-chlorotoluene allowed us to reach
47% conversion in the corresponding AAAr reaction (entry 5).
Finally, we found that the use of n-BuLi and Et2O as solvent,
which avoids SN2 reaction18 of the resulting ArLi and n-BuBr,
allowed us to obtain the desired product with full conversion and
In summary, the highly enantioselective Cu-catalyzed direct
allylic arylation using organolithium compounds has been
described. The use of readily available aryllithium reagents in
combination with allyl bromides and use of a copper−NHC
catalyst are key factors for the success of this reaction. The only
stoichiometric waste produced in this novel transformation is
LiBr. The use of n-BuLi was found to be essential for the
C
DOI: 10.1021/acs.orglett.5b03396
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
■
Feringa, B. L. Chem. Rev. 2008, 108, 2824. (c) Alexakis, A.; Backvall, J. E.;
̈
S
* Supporting Information
́ ́
Krause, N.; Pamies, O.; Dieguez, M. Chem. Rev. 2008, 108, 2796.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03396.
(d) Langlois, J.-B.; Alexakis, A. In Transition Metal Catalyzed Allylic
Substitution in Organic Synthesis; Kazmaier, U., Ed.; Springer-Verlag:
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: v.hornillos.gomez.recuero@rug.nl.
*E-mail: b.l.feringa@rug.nl.
(9) Selected examples: (a) Tissot-Croset, K.; Polet, D.; Alexakis, A.
́
Angew. Chem., Int. Ed. 2004, 43, 2426. (b) Lopez, F.; van Zijl, A. W.;
Present Address
Minnaard, A. J.; Feringa, B. L. Chem. Commun. 2006, 409.
(c) Kacprzynski, M. A.; May, T. L.; Kazane, S. A.; Hoveyda, A. H.
Angew. Chem., Int. Ed. 2007, 46, 4554. (d) Langlois, J.-B.; Alexakis, A.
Angew. Chem., Int. Ed. 2011, 50, 1877. (e) Shido, Y.; Yoshida, M.;
Tanabe, M.; Ohmiya, H.; Sawamura, M. J. Am. Chem. Soc. 2012, 134,
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517, 351.
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Ed. 2010, 49, 8370. (b) Shintani, R.; Takatsu, K.; Takeda, M.; Hayashi,
T. Angew. Chem., Int. Ed. 2011, 50, 8656. (c) Takeda, M.; Takatsu, K.;
Shintani, R.; Hayashi, T. J. Org. Chem. 2014, 79, 2354. (d) Selim, K. B.;
Matsumoto, Y.; Yamada, K.; Tomioka, K. Angew. Chem., Int. Ed. 2009,
48, 8733. (e) Selim, K. B.; Nakanishi, H.; Matsumoto, Y.; Yamamoto, Y.;
Yamada, K.; Tomioka, K. J. Org. Chem. 2011, 76, 1398.
†Department of Organic Chemistry and Center for Research in
Biological Chemistry and Molecular Materials (CIQUS),
University of Santiago de Compostela, 15782 Santiago de
Compostela, Spain.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Netherlands Organization for Scientific Research (NWO−
CW), the Royal Netherland Academy of Arts and Sciences
(KNAW), and The Ministry of Education Culture and Science
(Gravitation program 024.601035) are acknowledged for
financial support.
(11) (a) Per
́
ez, M.; Fananas-Mastral, M.; Bos, P. H.; Rudolph, A.;
́
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Harutyunyan, S. R.; Feringa, B. L. Nat. Chem. 2011, 3, 377. (b) Fananas-
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Mastral, M.; Per
Feringa, B. L. Angew. Chem., Int. Ed. 2012, 51, 1922. (c) Bos, P. H.;
Rudolph, A.; Perez, M.; Fananas-Mastral, M.; Harutyunyan, S. R.;
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Feringa, B. L. Chem. Commun. 2012, 48, 1748. (d) Perez, M.; Fananas-
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ez, M.; Bos, P. H.; Rudolph, A.; Harutyunyan, S. R.;
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