Aldimine-directed branched-selective hydroarylation of styrenes

Article abstract of DOI:10.1002/anie.201207958

Branching out: A simple and inexpensive cobalt/triarylphosphine catalyst promotes aldimine-directed hydroarylation of styrene with high branched regioselectivity to afford 1,1-diarylethane derivatives in good yields under mild reaction conditions. The ortho-formyl group in the hydroarylation products is amenable to dehydrative cyclization, to give fused polycyclic aromatic hydrocarbons, as well as decarbonylative removal. Copyright

Full text of DOI:10.1002/anie.201207958

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DOI: 10.1002/anie.201207958
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C H Functionalization
Aldimine-Directed Branched-Selective Hydroarylation of Styrenes**
Pin-Sheng Lee and Naohiko Yoshikai*
Efficient and selective construction of a 1,1-diarylethane core
structure has received considerable attention because of its
occurrence in a variety of pharmacologically active com-
pounds.[1] Among various synthetic approaches to 1,1-diaryl-
ethanes, hydroarylation of styrene derivatives with branched
regioselectivity is attractive for the inherently perfect atom
economy.[2–6] Such transformations can be achieved through
two mechanistically distinct modes of substrate activation:
decarbonylation to afford 1,1-diarylethanes with substitution
patterns not accessible by the Friedel–Crafts alkylation.
The addition of the aldimine 1a, derived from 1-naph-
thaldehyde and p-anisidine, to styrene (2a) serves as an
illustrative example showing the efficiency, selectivity, and
scalability of the cobalt catalysis (Scheme 1a). A catalytic
system consisting of CoBr2 (10 mol%), PPh3 (20 mol%), and
=
activation of the styrene C C bond with a Lewis acid followed
by a Friedel–Crafts-type aromatic alkylation,[2,3] and activa-
À
tion of the aromatic C H bond with a low-valent transition-
metal catalyst and subsequent insertion of styrene.[4,5,7] The
former type of reaction is applicable to electron-rich arenes
with exclusive branched selectivity, but is often accompanied
by imperfect regioselectivity with respect to the arene
substrate. In contrast, the latter type of reaction typically
leads to 1,2-diarylethanes rather than 1,1-diarylethanes as the
major products.[7] Some exceptions to this trend include the
ruthenium-catalyzed reaction of the ortho position of N-
methylaniline[4] and the nickel-catalyzed reaction of the C2
position of electron-deficient heteroarenes such as azole
derivatives.[5a–c] As such, significant limitations remain in the
synthetic scope of the hydroarylation approach to 1,1-diaryl-
ethanes.[8]
Scheme 1. Contrasting regioselectivity of cobalt- and ruthenium-cata-
lyzed aldimine-directed hydroarylation reactions of styrene. a) Cobalt
catalysis: branched selectivity. b) Ruthenium catalysis: linear selectivity
(Ref. [7h]). PMP=p-methoxyphenyl, THF=tetrahydrofuran.
Recently, we reported that cobalt/tricyclohexylphosphine
(PCy3) and cobalt/N-heterocyclic carbene (NHC) catalysts
promote the addition of 2-arylpyridine to styrene in
branched-selective and linear-selective manners, respective-
ly.[9a] The former case represents a rare example of branched-
À
selective styrene hydroarylation by chelation-assisted C H
Me3SiCH2MgCl (50 mol%) promoted this transformation on
a 10 mmol scale at 408C, and subsequent acidic hydrolysis
afforded the adduct 3aa in 84% yield with exclusive branched
selectivity, which was in sharp contrast to the linear selectivity
observed by Darses et al. for the ruthenium-catalyzed reac-
tion of a similar aldimine (1a’) with 2a (Scheme 1b).[7h]
Comparable reaction efficiencies were achieved using triar-
ylphosphine ligands bearing p-methyl and p-methoxy sub-
stituents, whereas the use of triarylphosphines with electron-
withdrawing substituents (e.g., F, Cl) and PCy3 led to lower
catalytic activities. In contrast to our previous study on the
reaction of 2-arylpyridines,[9a] NHC preligands such as
IMes·HCl (1,3-bis(2,4,6-trimethylphenyl)imidazolium chlo-
ride) were not very effective, thus affording no adducts or
a mixture of the branched and linear adducts in a low yield
(see Table S1 in the Supporting Information).
activation.[4] However, the utility of the hydroarylation
products was severely limited by the pyridyl directing group,
which is not amenable to further transformations. Herein we
report that a simple cobalt/triarylphosphine catalyst[10,11]
allows aldimine-directed, branched-selective hydroarylation
of styrene derivatives under mild reaction conditions. The
ortho-formyl group of the 1,1-diarylethane products can
either be utilized as a synthetic handle to construct polycyclic
aromatic hydrocarbons (PAHs) by Lewis acid catalyzed
dehydrative cyclization or can be removed by catalytic
[*] P.-S. Lee, Prof. N. Yoshikai
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Singapore 637371 (Singapore)
E-mail: nyoshikai@ntu.edu.sg
A variety of aromatic aldimines participated in the
addition reaction to styrene under the Co/P(p-Tol)3 catalytic
system,[12] thereby affording the corresponding 1,1-diaryl-
ethanes with high branched selectivity (Scheme 2). The ortho-
substituted aromatic aldimines, including those having an
electron-donating amino group and a pyrene skeleton,
afforded the corresponding adducts 3ba–3ga in moderate to
Homepage: http://www3.ntu.edu.sg/home/nyoshikai/yoshi-
kai_group/Home.html
[**] This work was supported by the National Research Foundation
Singapore (NRF-RF2009-05), Nanyang Technological University,
and JST, CREST.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207958.
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Scheme 2. Addition of various aromatic aldimines to styrene
(0.3 mmol scale). The branched/linear ratio was greater than 50:1.
Unless otherwise stated the yields are those of the isolated products.
1
[a] A 0.9 mmol scale. [b] The yield was determined by H NMR
spectroscopy using 1,1,2,2-tetrachloroethane as an internal standard.
[c] The reaction was performed using 2.4 equiv of styrene. The product
was obtained as an approximately 1:1 mixture of diastereomers.
[d] The product obtained as a mixture with the monoadduct (7%).
[e] The reaction was performed using 0.3 mmol of aldimine and
0.12 mmol of styrene. The yield was based on styrene.
Scheme 3. Addition of 1a to various styrene derivatives (0.3 mmol
scale). The branched/linear ratio was greater than 50:1 unless other-
wise noted. The yields are those of the isolated products. [a] The
product was obtained without acidic hydrolysis. [b] The aldimine
derived from indole-3-carboxaldehyde (1l) was used instead of 1a.
[c] The reaction was performed using 3 mmol of 1a, 1.2 mmol of 1,3-
divinylbenzene, 20 mol% of CoBr2, 40 mol% of P(p-Tol)3, and
100 mol% of Me3SiCH2MgCl. The product was obtained as an
approxiamtely 1:1 mixture of diastereomers. [d] The reaction was
performed using (Z)-trimethyl(styryl)silane.
good yields. The reaction of aldimines, having no ortho
substitutent, that is, aldimines derived from benzaldehyde,
3,5-difluorobenzaldehyde,
and
p-anisaldehyde,
with
1.2 equivalents of styrene resulted in the formation of
a mixture of mono- and dialkylation products, with the
latter being the major product. In the presence of an excess
amount of styrene (2.4 equiv), these aldimines afforded the
corresponding adducts 3ha, 3ia, and 3ja in moderate to good
yields. When the amount of styrene was limited to 0.4 equiv-
alents, an aldimine derived from m-fluorobenzaldehyde
branched adducts 3ab–aj, 3ak’, and 3al’ in moderate to
good yields with high branched selectivity, and bromo and
iodo substituents were not tolerable because of hydrodeha-
logenation. The products 3ak’ and 3al’ were isolated as
aldimine derivatives because the acidic hydrolysis resulted in
intramolecular Friedel–Crafts cyclization (see below). An
ortho substituent, especially the methoxy group, slowed the
reaction (see 3ai and 3aj). The reaction was also applicable to
1- and 2-vinylnaphthalenes and 2-vinylbenzofuran, as dem-
onstrated by the formation of the adducts 3am–3ao in good
yields. The latter olefin also afforded the adduct 3mo with
indole in 62% yield. Twofold addition of 1a to 1,3-divinyl-
benzene took place to afford the adduct 3ap as a 1:1 mixture
of diastereomers in 45% overall yield. (Z)-Trimethylsilyl-
(styryl)silane was also amenable to the present hydroaryla-
tion, thus affording the 1,1-diarylalkane 3aq, albeit in
a modest yield. Note that the reaction of the E isomer of
this vinylsilane was more sluggish under identical reaction
conditions (7% yield). a-Methylstyrene did not participate in
the reaction.
À
reacted regioselectively at the C H bond proximal to the
fluorine atom, thus affording the adduct 3ka in 84% yield.
Such regioselectivity has been frequently observed in aro-
À
matic C H functionalization reactions using cobalt and other
transition-metal catalysts.[10a,13–15] An aldimine derived from
2-naphthaldehyde reacted regioselectively at the more hin-
dered position, thus affording the adduct 3la, albeit in
a modest yield. The reaction on the C2 position of the
indole proceeded efficiently (see 3ma). Aldimines bearing
electron-withdrawing groups (p-CF3 and p-CN) did not
participate in the reaction.
We next explored the scope of styrene derivatives using
1a as the reaction partner (Scheme 3). A variety of styrene
derivatives bearing electron-donating (alkoxy, amino, and
silyl) and electron-withdrawing (chloro and fluoro) substitu-
ents participated in the reaction, thereby affording the
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Scheme 4. Influence of Grignard reagent on the branched/linear selec-
tivity of the addition of 1a to 2-fluorostyrene (2r). The yield was
1
determined by H NMR spectroscopy using 1,1,2,2-tetrachloroethane
as an internal standard.
Scheme 5. Deuterium-labeling experiment. The yields refer to the
isolated yields. The content of protons on each carbon atom was
1
determined by H NMR analysis. Ar=2-naphthyl. N.D.=Not deter-
mined.
During the exploration of styrene derivatives, we
observed an interesting effect of Grignard reagents on the
branched/linear selectivity (Scheme 4). Whereas the reaction
of 1a and 2-fluorostyrene (2r) preferentially afforded the
expected branched adduct 3ar’ under the standard reaction
conditions employing Me3SiCH2MgCl, the use of
tBuCH2MgBr instead resulted in selective formation of the
linear adduct 4ar’, albeit in modest overall yields for both
cases. Though not as drastic as this case, the reaction of other
styrenes using tBuCH2MgBr also resulted in considerable
formation of linear adducts (see Table S2 in the Supporting
Information). These observations may suggest that the
Grignard reagent not only serves as a reducing agent for the
cobalt precatalyst but also is intimately associated with the
catalytically active species to influence the product-determin-
ing step.[9,10a]
The ortho-formyl functionality in the present 1,1-diaryl-
ethane products serves as a versatile synthetic handle for
additional transformations. Given the importance of linearly
fused PAHs as building blocks for optoelectronic applica-
tions,[16] an attractive transformation of the diarylethane
products would be the construction of linearly fused PAHs
through Lewis or Brønsted acid mediated cyclization
(Scheme 6). Thus, the tetraphene derivatives 5aa, 5ad, and
5ae were obtained in good yields by In(OTf)3-catalyzed
dehydrative cyclization[17] or BF3-catalyzed, sulfonamide-
mediated deaminative cyclization.[18] Because of the elec-
tron-rich 3,4-methylenedioxyphenyl and 3,4,5-trimethoxy-
phenyl groups, the hydroarylation products 3ak’ and 3al’
In our previous study on the cobalt-catalyzed reaction of
2-arylpyridine with styrene, we proposed a catalytic cycle
À
consisting of C H oxidative addition, styrene insertion
leading to either a branched or linear alkylcobalt intermedi-
ate, and reductive elimination (see Scheme S1 in the Sup-
porting Information), where the former two steps are
reversible and the last step is rate- and regioselectivity-
determining.[9] Given this background, we were surprised to
find that the pentadeuterated benzaldimine [D5]-1h failed to
react with parent and substituted styrenes including 2a, 2c,
2k, and 4-tert-butylstyrene under the standard reaction
conditions. Only the reaction with 2-vinylnaphthalene (2n)
afforded the mono- and dialkylation products in low yields
(Scheme 5). The distribution of the deuterium atoms in the
recovered starting materials and the products was in contrast
to the extensive deuterium scrambling observed in the Co/
PCy3-catalyzed reaction of 2-(pentadeuteriophenyl)pyridi-
ne.[9a] The deuterium content at the ortho position of [D5]-
1h did not decrease at all, and the a and b positions of 2n
were only slightly deuterated. The products had a significant
deuterium content at the methyl groups, while no apparent
deuteration was observed at the methine groups. While we
speculate that the present and previous reactions share the
same mechanistic framework, the above observations suggest
that there is a significant difference in terms of the relative
rates of the elementary steps. Thus, the most rate-influencing
Scheme 6. Transformation of the hydroarylation products into poly-
cyclic aromatic hydrocarbons. The yields are those of the isolated
products. [a] Reaction conditions: In(OTf)3 (5 mol%), DCE, 1158C.
[b] Reaction conditions: p-toluenesulfonamide (1.1 equiv), BF3·OEt2
(30 mol%), toluene, RT. [c] Reaction conditions: 3n HCl, EtOAc, RT.
[d] Reaction conditions: 3n HCl, EtOAc, 708C.
À
steps of the present reaction is likely the C H oxidative
addition rather than reductive elimination.
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5an, and heteroacenes 5ao and 5mo were also obtained in
good yields.
Another notable transformation of the 1,1-diarylethane
products is catalytic decarbonylation (Scheme 7). With the
aid of the [{Ir(cod)Cl}2]/PPh3 catalytic system,[19] the formyl
groups on the hydroarylation products 3aa–3ca, 3ea, and
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products by iridium-catalyzed decarbonylation. The yields are those of
the isolated products. cod=cyclo-1,5-octadiene.
In summary, we have developed a mild and highly
branched-selective addition reaction of aromatic aldimines
to styrenes using a simple cobalt/triarylphosphine catalyst,
which has significantly expanded the synthetic scope of the
hydroarylation approach to 1,1-diarylethanes. Furthermore,
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À
arylation by C H activation are currently underway.
Received: October 2, 2012
Published online: December 6, 2012
À
Keywords: alkenes · arenes · C H activation · cobalt ·
synthetic methods
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