Olefin Hydroarylation Catalyzed by a Single-Component Cobalt(-I) Complex

Article abstract of DOI:10.1021/acs.orglett.1c00258

A single-component Co(-I) catalyst, [(PPh3)3Co(N2)]Li(THF)3, has been developed for olefin hydroarylations with (N-aryl)aryl imine substrates. More than 40 examples were examined under mild reaction conditions to afford the desired alkyl-arene product in good to excellent yields. Catalysis occurs in a regioselective manner to afford exclusively branched products with styrene-derived substrates or linear products for aliphatic olefins. Electron-withdrawing functional groups (e.g., -F, -CF3, and -CO2Me) were tolerated under the reaction conditions.

Full text of DOI:10.1021/acs.orglett.1c00258

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Letter
Olefin Hydroarylation Catalyzed by a Single-Component Cobalt(-I)
Complex
Benjamin A. Suslick and T. Don Tilley*
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ABSTRACT: A single-component Co(-I) catalyst, [(PPh3)3Co(N2)]Li-
(THF)3, has been developed for olefin hydroarylations with (N-aryl)aryl
imine substrates. More than 40 examples were examined under mild
reaction conditions to afford the desired alkyl-arene product in good to
excellent yields. Catalysis occurs in a regioselective manner to afford
exclusively branched products with styrene-derived substrates or linear
products for aliphatic olefins. Electron-withdrawing functional groups
(e.g., -F, -CF3, and -CO2Me) were tolerated under the reaction
conditions.
a
Scheme 1. Imine Coupling Partners 1A−1X
ecent developments in metal-mediated C−H bond
Ractivation chemistry have enabled new catalytic trans-
formations to generate value-added products.1−4 Late-stage
C−H functionalizations present attractive alternatives to
molecular elaboration methods such as cross-coupling, which
require sacrificial, costly, and often toxic reagents. In particular,
olefin and alkyne hydroarylations have provided atom-
economical routes for formation of C−C bonds.5−7 The
Murai group7,8 described the first hydroarylation precatalyst,
RuH2(CO)(PPh3)3, and subsequent developments involving
heavier, late-transition metal catalysts (i.e., with Rh,9−14
Ir,15−21 Pd,22−26 and Pt27−31) illustrated the synthetic value
of hydroarylation as a convenient transformation for C−H
bond diversification. However, given the low abundance and
high cost of such metals,32 it is important to develop efficient
catalysts based on environmentally benign first-row metals.
Indeed, recent advances toward this goal highlight Ni33−35 and
Co4,36−49 catalysts as suitable alternatives.
a
The catalytically cleavable C−H bonds are labeled with 1′, 2′, 3′, or
6′.
understood; moreover, the addition of excess reactive,
nucleophilic co-reagents limits the functional group tolerance.
Studies in this laboratory on the mechanism of alkyne
hydroarylations catalyzed by cobalt led to identification of a
well-defined Co(-I) complex, [(PPh3)3Co(N2)]Li(THF)3
(Co-Li), as a single-component hydroarylation catalyst with
Cobalt catalysts have emerged as a versatile platform for
ortho-directed C−H bond derivatization (eq 1).4,36−49 Current
Co-catalyzed ortho hydroarylations typically require in situ
catalyst generation, whereby CoX2 salts are treated with an
excess of nonstandard Grignard (e.g., Me3CCH2MgBr or
Me3SiCH2MgBr) and triaryl phosphine [e.g., P(3-Cl-C6H4)3]
additives.41−48 For these complex catalytic mixtures, the nature
of the active species and its mode of action are not well
Received: January 22, 2021
Published: February 9, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00258
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1495−1499
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a,b
Scheme 2. Olefin Hydroarylations with (N-Aryl)aryl Imines Catalyzed by Co-Li
a
Reaction conditions: 1 (0.1 M, 1 equiv), 2 (0.1 M, 1 equiv), and Co-Li (10 mol %) in toluene at either 25 or 80 °C under N2. 1H NMR yields (%)
b
are reported for 25 and 80 °C as the reaction temperatures and colored blue and red, respectively. Isolated yields were determined from catalysis
c
d
at 25 or 80 °C and are given in parentheses. The ratio of regioisomers resulting from 2′- or 6′-C−H activation is given in square brackets. The
e
ratio of regioisomers resulting from 1′- or 3′-C−H activation is given in square brackets. The ratio of branched (3Va) to linear (4Va) products is
f
given in curly brackets. An excess of ethylene or propylene (1 atm) was added.
a
philic functional groups (e.g., organic carbonyls), this catalyst
provides excellent yields without generation of undesired side
products. The precatalyst Co-Li was conveniently prepared on
a gram scale by a synthetically straightforward three-step
sequence, as described in earlier work from this laboratory.49
Herein, the use of Co-Li as a versatile catalyst for olefin
hydroarylations is described.
The two general catalytic conditions employed in this study
were based on initial optimizations described for the alkyne
hydroarylation system.49 Catalytic reactions were examined
under dilute substrate concentrations (0.1 M) in toluene with
10 mol % catalyst loading at either 25 or 80 °C.
Table 1. Imine Directing Group Scope
substrate
Ar
1H NMR yield (%)
1M
5
4-(OMe)C6H4 (PMP)
Ph
92
78
63
17
0
6
7
8
9
4-(CF3)C6H4
2-(OMe)C6H4
2-(tBu)C6H4
3,5-(tBu2)C6H3
72
a
Reaction conditions: (N-aryl)aryl imine 1M or 5−9 (0.1 M, 1
equiv), 2a (0.1 M, 1 equiv), and Co-Li (10 mol %) in toluene at 80
°C under N2.
On its own, efficient precoordination of acetophenone to the
metal center does not occur; competent catalysis requires a
better directing group, such as an N-coordinating imine. The
imine substrates [1A−1X (Scheme 1)] were prepared by
condensation of the corresponding substituted acetophenone
with p-anisidine. The acid sensitivity of Co-Li necessitated
preparation of the imine substrate prior to catalysis. Treatment
of an equimolar mixture of acetophenone and p-anisidine with
catalytic quantities of Co-Li resulted in rapid N−H
deprotonation to generate the inactive complex, (PPh3)3Co-
(N2)H, as has been discussed in a previous report.49 The
identity of the solvent does not affect the catalytic efficacy in
several advantages over systems that require in situ catalyst
generation.49 In the solid state, Co-Li remains catalytically
active after storage for several months under an inert
atmosphere at ambient temperatures; freshly prepared samples
of Co-Li in benzene-d6 remain identical over such periods
1
(determined by H NMR spectroscopy). In contrast, mixtures
of CoX2, phosphine, and Grignard reagents exhibit limited
storability (hours) before catalyst degradation occurs. Perhaps
the most important advantage to the single-component catalyst
Co-Li is that reactive, nucleophilic activators are no longer
required. Thus, for hydroarylation substrates bearing electro-
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a
containing substrate 1P afforded a mixture of products
resulting from 2′- and 6′-C−H bond cleavage in a ratio of
20:1. In contrast, hydroarylations with arene substrates bearing
3′-Me (1R), 3′-CF3 (1S), or 3′-CO2Me (1T) substitution
afforded only one regioisomer functionalized at position 6′.
The poorest regioselectivity occurred with naphthyl ethani-
mine 1U, which possessed a slight preference for C−H
activation at the 1′ site by a ratio of ∼5:1.
Scheme 3. Plausible Olefin Hydroarylation Mechanism
Heterocyclic aryl imines were tolerated, albeit in poor yields.
Interestingly, benzofuran 1V was the only substrate that
afforded a mixture of branched and linear products (3Va and
4Va, respectively) in a ratio of 9.8:1.0. Pyridyl- and thiophenyl-
derived substrates were unreactive, likely due to the formation
of stable N−E (E = S or N) chelates with the Co metal center.
In contrast, 1W bearing a 3′,4′-(methylenedioxy)phenyl
skeleton underwent quantitative hydroarylation to form 3Wa.
A comprehensive list of inactive substrates tested in this study
is given in Chart S1.
Substitutional effects in the styrene coupling partner were
explored with substrates 2a−2m; most of these olefins
provided quantitative yields with the methyl ester containing
imine 1M. Nonquantitative yields occurred with only styrene
derivatives bearing ortho (2b and 2g) or halide (2g, 2h, and 2i)
substituents. These examples are likely limited by either
sterically congested metal centers or competitive C−X bond
activations, respectively.
Other, non-styrene-derived olefins underwent catalytic
hydroarylations, as evidenced by the formation of products
4Mn−4Mr. Such olefins exclusively provided linear products,
which is observed in other hydrofunctionaliza-
tions.15,19,20,40,42,47 Good yields were observed with ethylene
(2n) and vinyl silanes 2q and 2r. Unfortunately, the olefin
scope possesses several limitations. Proximal steric bulk
prohibits efficient catalysis by inhibiting olefin coordination,
as evidenced with tert-butyl ethylene (2p). Internal olefins do
not undergo catalysis (e.g., cis-β-methylstyrene or cis-2-
butene). Long chain olefins (e.g., 1-octene) undergo rapid
isomerization to form internal alkenes, thereby rendering the
substrate inert.
Finally, the identity of the aryl moiety on the N-coordinating
directing group affected the catalytic yields (Table 1).
Electron-rich N-aryl groups led to higher yields than electron
deficient directing groups (i.e., 1M > 5 > 6). The steric
environment proximal to the N-coordinating imine appeared
to dictate coordination to the metal center. Ortho-substituted
aryl groups (7 and 8) provide a sterically inaccessible imine,
thus precluding catalytic activity. These steric effects are
somewhat mitigated if the steric bulk is distal to the N atom
(9). Overall, p-methoxyphenyl (PMP) as the donor group
provided the best yields.
A plausible mechanism for olefin hydroarylations catalyzed
by Co-Li is given in Scheme 3. A mechanism similar to that
described for alkyne hydroarylations is likely operative.49 As a
hydroarylation precatalyst Co-Li is converted to an active
species via an initial coordination of the olefin (A), which
displaces PPh3 and N2 ligands. Subsequent coordination of the
imine affords B, which may undergo a CMD-like proton
transfer (T.S.BC) to form a species akin to C. Reductive
elimination from C produces the observed hydroarylation
product and regenerates the active catalyst.
a
Li counterions have been omitted for the sake of clarity.
this system as determined with alkyne-based substrates.49 The
(N-aryl)aryl imine scope with styrene derivatives (2a−2m) or
non-aromatic vinyl compounds (2n−2r) elucidated the
generality of catalysis with Co-Li, as described in Scheme 2.
For the sake of clarity, branched products (3) and linear
products (4) are encoded with two letters corresponding to
the imine (first, uppercase) and olefin (second, lowercase)
coupling partners. With 2a as the olefin coupling partner, only
branched products 3Aa−3Xa were formed except in one case
(vide infra). Generally, catalysis proceeded in higher yields at
ambient temperatures.
Study of (N-aryl)aryl imine substrates possessing various
para substituents (1B−1O) revealed several substitutional
effects. Aliphatic or aromatic groups (1B−1F) distal to the N-
1
coordinating imine did not affect the product yields (by H
NMR spectroscopy). The observed yields of products 3Ga−
3Ia, which were derived from substrates bearing halides,
decreased as a function of C−X bond strength (i.e., F > Cl ≫
Br). Catalyst decomposition with substrates bearing weak C−X
bonds may occur by LiX elimination. Indeed, there is
precedent for rapid bond activations of this type with anionic
metal fragments, as best illustrated by the reactivity of the Fp−
anion with alkyl halides.50 Such competing and nonproductive
C−X activations irreversibly decompose the catalyst, thereby
limiting the halide scope to F or Cl derivatives. Both electron-
rich (1B−1F, 1K, and 1L) and electron-poor (1G, 1H, 1J, and
1M) substrates were tolerated in good to excellent yields. In
particular, substrate 1M illustrated the advantage of this single-
component catalyst; 1M quantitatively converted to the
hydroarylation product 3Ma despite its reactivity with
Grignard reagents. Substrates 1N and 1O proved to be
problematic due to competitive binding through the N-
coordinating substituent.
Catalysis with substrates bearing meta substituents (1P−
1W) examined the regioselectivity of C−H cleavage (Scheme
1). With 1P−1T, two distinct C−H bonds ortho to the imine
directing group (i.e., 2′- or 6′-C−H) exist. The 3′-fluorine-
In conclusion, a versatile, single-component Co(-I) catalyst
for olefin hydroarylations has been applied to more than 40
substrates in good to excellent yields, without undesired side
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product generation. This easily prepared and highly storable
catalyst provides a new route for C−C bond formation with
substrates bearing electrophilic functional groups that are
incompatible with nucleophilic co-reagents. Generally high
yields were observed under mild conditions for substrates
bearing a variety of electronic and steric environments. Only
branched products existed in the catalysis with styrene-derived
olefins; in contrast, linear products were exclusively observed
with vinyl silane substrates. Given the well-defined nature of
the Co(-I) catalyst, it may be possible to directly tune the
reactivity through modification of the phosphine ligand or the
countercation. One such avenue of interest is the development
of an enantioselective catalyst; this may be achieved through
the use of P-chiral phosphine ligands or by the addition of
chiral information to the participatory Li cation in the form of
a chiral crowning reagent. Further developments are currently
ongoing in this laboratory.
1106400). Thomas J. O’Connor, Dr. Addison Desnoyer,
Harrison Bergman, Dr. Gavin Kiel, Dr. Ryan Witzke, Dr. D.
Dawson Beattie, Jaruwan Amtawong, Rex Handford, and T.
Alex Wheeler (all from University of California, Berkeley) are
thanked for insightful discussions.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
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Experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
T. Don Tilley − Department of Chemistry, University of
California, Berkeley, California 94720, United States;
Chemical Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, California 94720, United States;
orcid.org/0000-0002-6671-9099; Email: tdtilley@
berkeley.edu
Author
Benjamin A. Suslick − Department of Chemistry, University of
California, Berkeley, California 94720, United States;
Chemical Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, California 94720, United States;
orcid.org/0000-0002-6499-3625
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was primarily funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
Chemical Sciences, Geosciences, and Biosciences Division,
under Contract DE-AC02-05CH11231. The authors acknowl-
edge the National Institutes of Health (Grants SRR023679A,
1S10RR016634-01, and S10OD024998) and the National
Science Foundation (Grant CHE-0130862) for funding of the
College of Chemistry NMR facility (University of California,
Berkeley). The QB3/Chemistry Mass Spectrometry Facility
(University of California, Berkeley) is acknowledged for help
and access to the LTQ-FT ESI-HRMS (SN 06053F) and
AutoSpec EI-HRMS (SN P749, property ID 20081000740)
instruments. Dr. Hasan Celik is thanked for help with high-
field, variable-temperature NMR spectroscopy experiments.
B.A.S. acknowledges the NSF-GRFP for a fellowship (DGE
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