Cobalt-catalyzed intermolecular hydroacylation of olefins through chelation-assisted imidoyl C-H activation

Article abstract of DOI:10.1021/acscatal.5b00581

A low-valent cobalt catalyst generated from cobalt(II) bromide, a diphosphine ligand, and zinc powder promotes intermolecular hydroacylation of olefins using N-3-picolin-2-yl aldimines as aldehyde equivalents, which affords, upon acidic hydrolysis, ketone products in moderate to good yields with high linear selectivity. The reaction is applicable to styrenes, vinylsilanes, and aliphatic olefins as well as to various aryl and heteroaryl aldimines. The cobalt catalysis features a distinctively lower reaction temperature (60 °C) compared with those required for the same type of transformations catalyzed by rhodium complexes (typically 130-150°C).

Full text of DOI:10.1021/acscatal.5b00581

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Letter
Cobalt-Catalyzed Intermolecular Hydroacylation of Olefins
through Chelation-Assisted Imidoyl C-H Activation
Junfeng Yang, Yuan Wah Seto, and Naohiko Yoshikai
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 15 Apr 2015
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Cobalt-Catalyzed Intermolecular Hydroacylation of Olefins through
Chelation-Assisted Imidoyl C–H Activation
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Junfeng Yang, Yuan Wah Seto, and Naohiko Yoshikai*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371, Singapore
KEYWORDS: cobalt, C–H activation, hydroacylation, aldimines, alkenes.
ABSTRACT: A low-valent cobalt catalyst generated from cobalt(II) bromide, a diphosphine ligand, and zinc powder promotes
intermolecular hydroacylation of olefins using N-3-picolin-2-yl aldimines as aldehyde equivalents, which affords, upon acidic
hydrolysis, ketone products in moderate to good yields with high linear selectivity. The reaction is applicable to styrenes,
vinylsilanes, and aliphatic olefins as well as to various aryl and heteroaryl aldimines. The cobalt catalysis features a distinctively
lower reaction temperature (60 °C) compared with those required for the same type of transformations catalyzed by rhodium
complexes (typically 130–150 °C).
The catalytic hydroacylation of unsaturated hydrocarbons
offers an atom- and step-economical route to ketones from
readily available aldehyde substrates.1 While such
transformations may be achieved with various catalytic
systems through different mechanistic manifolds, the most
extensively studied is the rhodium(I)-catalyzed hydroacylation
that goes through oxidative addition of the aldehydic C–H
bond to Rh, migratory insertion of the unsaturated substrate
into Rh–H, and C–C bond-forming reductive elimination. In
light of analogous reactivities of rhodium and its group 9
congener, cobalt,2 and much lower cost of the latter, the use of
low-valent cobalt catalysts in the same hydroacylation
manifold appears feasible and attractive. Nevertheless, reports
on cobalt-catalyzed hydroacylation have been sporadic.3,4,5 In
the late 1990s, Brookhart demonstrated the catalytic activity of
formal intermolecular hydroacylation using an N-3-picolin-2-
yl aldimine,9,10 which is reported herein (Scheme 1c).
Scheme 1. Cobalt-Catalyzed Olefin Hydroacylation via C–
H Activation
a
Cp*Co(I) complex toward intermolecular olefin
hydroacylation (Scheme 1a).4 Although the reaction is notable
in that it does not require any chelation assistance, only
vinylsilanes can be used as olefins. Dong and coworkers
recently disclosed an intermolecular hydroacylation reaction
of 1,3-dienes using an in situ-generated cobalt(I)–diphosphine
catalyst,5 while the reaction is proposed to go through
aldehyde/diene oxidative cyclization rather than aldehydic C–
H activation.
The seminal work of Suggs and the following extensive
studies of Jun and others have shown the utility of N-3-
picolin-2-yl and related aldimines, either preformed or in situ-
generated, as aldehyde equivalents in Rh-catalyzed
hydroacylation, which allow one to avoid undesirable
decarbonylation by the formation of five-membered chelate
intermediates upon C–H oxidative addition.9-10 After futile
attempts on cobalt–diphosphine-catalyzed hydroacylation
using parent benzaldehyde and simple olefins (e.g., styrene, 1-
octene), we turned our focus on this class of substrates.
Extensive screening of reaction conditions using benzaldimine
1a and styrene 2a as model substrates led us to achieve the
desired transformation using a cobalt catalyst generated from
CoBr2 (10 mol %), 1,1'-bis(diisopropylphosphino)ferrocene
(dippf; 10 mol %), and Zn powder (20 mol %) in acetonitrile
Recently, we reported enantioselective intramolecular
hydroacylation reactions of olefins (Scheme 1b) and ketones
using low-valent cobalt catalysts generated from cobalt(II)
salts, chiral diphosphine ligands, and metal reductants,6 which
display efficiencies and selectivities comparable to those of
the rhodium-catalyzed variants.7,8 With the modularity of the
cobalt–diphosphine catalytic system as well as the limitation
of the Cp*Co(I) catalytic system with respect to the scope of
olefins, we became interested in the feasibility of
intermolecular olefin hydroacylation under cobalt–
diphosphine catalysis. While our attempts on hydroacylation
using simple aldehydes have not been successful, we have
established a new cobalt–diphosphine catalytic system for the
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at 60 °C, affording the ketimine product 3aa in 85% GC yield
Page 2 of 5
15
2 equiv of 2a
45
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after 18 h (Table 1, entry 1). The GC analysis indicated the
presence of a trace amount of a branched isomer of 3aa, which
was later confirmed by the analysis of hydrolyzed products
(vide infra). Reactions of aldimines prepared from other 2-
aminopyridine derivatives with 2a under identical conditions
afforded the corresponding adducts in much lower yields (see
the Supporting Information). Note also that, unlike the case of
rhodium(I) catalysis,9-10 the reaction of benzaldehyde and 2a in
the presence of either catalytic or stoichiometric amounts of 2-
amino-3-picoline failed to give the desired hydroacylation
product.
aThe reaction was performed on a 0.1 mmol scale. bDetermined
by GC using n-tridecane as an internal standard.
With the Co–dippf catalytic system in hand, we explored the
scope of the present hydroacylation reaction. First, aldimine
1a was subjected to the reaction with various olefins (Table 2).
A variety of styrene derivatives 2a–2k participated in the
hydroacylation reaction to afford, upon acidic hydrolysis, the
linear ketone products 4aa–4ka in moderate to good yields
(entries 1–11). The reaction was typically accompanied by the
formation of a minor amount of branched isomer with the
linear-to-branched ratio ranging from 11:1 to > 20:1. The
reaction of 2a could be performed on a 5 mmol scale without
decrease in the yield and the regioselectivity (entry 1). Like
the styrene derivatives, 3-vinylthiophene also afforded the
desired hydroacylation product 4al in 56% yield (entry 12).
Vinylsilane, allylsilane, and alkyl olefins are also amenable to
the hydroacylation reaction, affording the linear adducts 4am–
4ap as the exclusive regioisomeric products (entries 13–16).
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The reaction efficiency is substantially influenced by the
backbone and the phosphorus substituents of the diphosphine
ligand, as evident from the lower catalytic activities obtained
with other ferrocenyldiphosphines, diphenyl ether- or
xanthene-based diphosphines (entries 2–7). The use of Mn
powder as the reductant resulted in a significantly lower yield
(entry 8), while In powder was entirely ineffective (entry 9).
The cobalt precatalyst also had a significant impact on the
reaction efficiency. No desired product was obtained using
CoCl2 (entry 10), while CoI2 promoted the reaction to only a
moderate extent (entry 11). Replacement of acetonitrile with
other solvents such as toluene and THF completely shut down
the hydroacylation (entries 12 and 13). Attempts to lower the
catalyst loading or the equivalence of 2a resulted in significant
decrease in the product yield (entries 14 and 15).
Table 2. Scope of Olefins
Table 1. Effect of Reaction Conditions on Co-Catalyzed
Addition of Aldimine 1a to Styrenea
entry
1
R
product
4aa
4ab
4ac
4ad
4ae
4af
yield (%)b
78 (79)d
l:bc
17:1
Ph
2
4-MeC6H4
4-t-BuC6H4
4-MeOC6H4
4-Me3SiC6H4
4-FC6H4
3-MeC6H4
3-MeOC6H4
3-FC6H4
2-MeOC6H4
2-FC6H4
3-thienyl
SiMe3
70
>20:1
>20:1
3
71
e
4
80
–
5
70
11:1
6
69
>20:1
15:1
7
63
4ag
4ah
4ai
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63
13:1
9
85
>20:1
>20:1
19:1
10
11
12
13
14
15
16
77
4aj
73
4ak
4al
entry
1
deviation from "standard conditions"
none
yield (%)b
56
19:1
e
85
20
0
61
–
4am
4an
4ao
4ap
e
2
dppf instead of dippf
CH2SiMe3
n-C6H13
38
–
e
3
dtbpf instead of dippf
DPEphos instead of dippf
DPEphos-Cy instead of dippf
Xantphos instead of dippf
tBuXantphos instead of dippf
Mn instead of Zn
71
–
e
4
16
68
17
0
(CH2)2CO2Et
33
–
aThe reaction was performed on a 0.3 mmol scale. bIsolated
yield. cThe ratio of linear and branched isomers determined by 1H
NMR analysis. d5 mmol scale. eNo branched isomer was detected.
5
6
7
Chart 1 summarizes examples of alkenes that failed to
participate in the reaction with 1a, thus illustrating the
limitation of the present hydroacylation. Among vinylarenes,
4-chlorostyrene (2q) and 4-vinylpyridine (2r) did not give the
desired products at all, with near complete recovery of 1a. The
latter alkene may have interfered with the reaction by the
coordination of the pyridyl group to the catalyst. On the other
hand, the failure of the former alkene is not clearly
rationalized at this moment, in light of the fact that an aryl
8
18
0
9
In instead of Zn
10
11
12
13
14
CoCl2 instead of CoBr2
CoI2 instead of CoBr2
Toluene instead of MeCN
THF instead of MeCN
5 mol % of CoBr2 and dippf
0
32
0
0
29
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chloride moiety in the aldimine reactant can be tolerated (vide During the reaction optimization and the exploration of the
1
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7
8
infra). Other unsuccessful alkenes include sterically hindered
α-methylstyrene (2s) and norbornene (2t), readily
isomerizable allylbenzene (2u) and allyl phenyl ether (2v),
alkyl olefins containing hydroxy (2w), sulfonamide (2x), and
alkyl bromide (2y) moieties, and electron-deficient ethyl
acrylate (2z).
substrate scope, we noted the presence of a substantial
induction period in the CoBr2/dippf/Zn catalytic system. Thus,
no hydroacylation product was observed for the initial several
hours (typically 4–8 h). This observation appeared to be
correlated with the change of the appearance of the reaction
mixture. Thus, the characteristic blue color of Co(II) turned
dark brown only gradually over several hours. Thus, the
induction period may be associated with slow reduction of the
Co(II) precatalyst to a catalytically active Co(I) species under
the heterogeneous conditions.11
Chart 1. Unsuccessful Alkene Substrates
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To gain insight into the hydroacylation pathway, we
performed experiments using a deuterium-labeled aldimine
1a-d (Scheme 2). The reaction of 1a-d with styrene or
vinyltrimethylsilane was performed under the standard
conditions for a shorter reaction time of 9 h to achieve a
moderate conversion, in order to analyze deuterium contents
in both the recovered aldimine and the hydroacylation product.
With either of the olefin substrates, we observed a decreased
deuterium content in the recovered aldimine (45% D and 83%
D for the reactions with styrene and vinyltrimethylsilane,
respectively). The 1H NMR analysis of the hydroacylation
product of styrene showed a partial deuteration of the β-
position (1.69H) as well as a slight deuteration of the α-
position (1.90H). By contrast, the hydroacylation product of
vinyltrimethylsilane featured substantial deuteration of the β-
position (1.55H) with no apparent deuteration of the α-
position (2.00H). Note that, as expected from the substantial
H/D scrambling, we did not observe qualitatively significant
difference between the reactivities of 1a-d and 1a.
Next, reactions of various aldimines with styrene (2a) were
examined (Table 3). A variety of aldimines 1b–1i derived
from para- and meta-substituted benzaldehydes underwent
addition to 2a to afford the desired hydroacylation products
4ba–4ia in 70% or higher yields with l:b ratios of 15:1 or
higher (entries 1–8), except for the one derived from 4-
cyanobenzaldehyde, which exhibited modest reactivity and
slightly lower regioselectivity (entry 5). The catalytic system
is sensitive to substitution at the ortho-position of the aldimine
substrate, as aldimines derived from ortho-substituted
benzaldehydes such as ortho-tolualdehyde failed to afford the
hydroacylation products (data not shown). While aldimines
derived from 2-thiophene- and 3-thiophene carboxyaldehydes
smoothly participated in the hydroacylation to 2a, curiously,
they exhibited lower regioselectivities, affording the products
4ja and 4ka with l:b ratios of 3:1 and 5:1, respectively (entries
9 and 10). Note that alkyl aldimines were not examined,
because attempts to prepare such aldimines in a pure form
from the corresponding aldehydes were unsuccessful.
Scheme 2. Deuterium-Labeling Experiment
Table 3. Scope of Aldimines
aDetermined by 1H NMR analysis of the crude product.
bIsolated yield. cDetermined by 1H NMR analysis.
entry
R
product
4ba
4ca
yield (%)b
l:bc
1
4-t-BuC6H4
4-MeOC6H4
4-FC6H4
4-ClC6H4
4-NCC6H4
3-MeC6H4
3-MeOC6H4
3-FC6H4
2-thienyl
3-thienyl
73
82
72
72
39
73
74
70
83
70
>20:1
16:1
>20:1
>20:1
9:1
On the basis of the above observations as well as the
analogy with the common mechanism of the rhodium-
catalyzed hydroacylation,1,9-10 we propose a catalytic cycle
outlined in Scheme 3. First, reduction of CoBr2 with zinc in
the presence of dippf would give rise to a low-valent cobalt
species A, which is presumably in the Co(I) oxidation state.11
The species A then undergoes pyridine-assisted oxidative
addition of the imidoyl C–H bond to give a cobaltacycle
intermediate B.12 Migratory insertion of the olefin into the Co–
H bond of B would occur in linear or branched fashion,
leading to diorganocobalt intermediates C or C', respectively.
Reductive elimination of C gives the major linear isomer of
the hydroacylation product, while the minor branched isomer,
which is formed with styrene derivatives, should arise from
C'. The erosion of the deuterium content of the recovered
aldimine (Scheme 2) can be rationalized by H/D exchange
between the aldimine and the olefin through reversible C–H
2
3
4da
4ea
4
5
4fa
6
15:1
>20:1
>20:1
3:1
4ga
4ha
4ia
7
8
9
10d
4ja
5:1
4ka
aThe reaction was performed on a 0.3 mmol scale. bIsolated
yield. cThe ratio of linear and branched isomers determined by 1H
d
NMR analysis. Linear (59%) and branched (11%) isomers were
separated by silica gel chromatography.
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(1) (a) Jun, C.-H.; Jo, E.-A.; Park, J.-W. Eur. J. Org. Chem. 2007,
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1
2
3
4
5
6
7
8
the use of a large excess (5 equiv) of the olefin, the proportion
of the deuterated olefin molecules to the total olefin molecules
should be low regardless of the extent of H/D exchange, which
accounts for the low deuterium incorporation into the
hydroacylation product. Note that the branched insertion
pathway appears to operate to some extent with styrene but
does not with vinyltrimethylsilane. The propensity of the
styrene derivatives to give the minor branched products may
be ascribed to increased stability of the putative benzylcobalt
intermediate (C', R' = aryl). 12b,13,14
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Scheme 3. Proposed Catalytic Cycle (Pic = 3-picolin-2-yl)
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Barnhart, R. W.; Wang, X.; Noheda, P.; Bergens, S. H.; Whelan, J.;
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In summary, we have demonstrated that
a cobalt–
E.-A.; Jun, C.-H. Eur. J. Org. Chem. 2006, 2504.
(f) Jo, E.-A.; Jun, C.-
diphosphine complex serves as a viable alternative catalyst for
the intermolecular formal hydroacylation reaction of olefins
using N-3-picolin-2-yl aldimines as aldehyde equivalents. The
cobalt catalysis proceeds at a distinctively lower reaction
temperature (60 °C) than typically required in the rhodium
catalysis of the same type of hydroacylation (130–150 °C).9-10
In light of the tremendous success of rhodium-catalyzed
hydroacylation using chelating aldehydes,1,8c,9-10,15 the present
results may hold promise for further development of cobalt
catalysts for chelation-assisted hydroacylation. Further studies
toward the extension of the scope of cobalt-catalyzed
hydroacylation and related C–H functionalization reactions are
currently underway.
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ASSOCIATED CONTENT
The following file is available free of charge on the ACS
Publications website at DOI: 10.1021/csXXXX.
Experimental details and compound characterization data (pdf)
Int. Ed. 2004, 43, 340.
(b) Osborne, J. D.; Randell-Sly, H. E.; Currie,
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AUTHOR INFORMATION
Org. Lett. 2011, 13, 3900.
(g) von Delius, M.; Le, C. M.; Dong, V. M.
Corresponding Author
J. Am. Chem. Soc. 2012, 134, 15022. (h) Castaing, M.; Wason, S. L.;
Estepa, B.; Hooper, J. F.; Willis, M. C. Angew. Chem., Int. Ed. 2013, 52,
13280.
* E-mail: nyoshikai@ntu.edu.sg
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by Nanyang Technological University
and JST, CREST.
REFERENCES
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