Cobalt-Catalyzed Cross-Coupling of Functionalized Alkylzinc Reagents with (Hetero)Aryl Halides

Article abstract of DOI:10.1002/anie.201914490

A combination of 10 % CoCl₂ and 20 % 2,2′-bipyridine ligands enables cross-coupling of functionalized primary and secondary alkylzinc reagents with various (hetero)aryl halides. Couplings with 1,3- and 1,4-substituted cycloalkylzinc reagents proceeded diastereoselectively leading to functionalized heterocycles with high diastereoselectivities of up to 98:2. Furthermore, alkynyl bromides react with primary and secondary alkylzinc reagents providing the alkylated alkynes.

Full text of DOI:10.1002/anie.201914490

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Accepted Article
Title: Cobalt-Catalyzed Cross-Coupling of Functionalized Alkylzinc
Reagents with (Hetero)Aryl Halides
Authors: Ferdinand Hermann Lutter, Lucie Grokenberger, Philipp
Spieß, Jeffrey Michael Hammann, Konstantin Karaghiosoff,
and Paul Knochel
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914490
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10.1002/anie.201914490
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Cobalt-Catalyzed Cross-Coupling of Functionalized Alkylzinc
Reagents with (Hetero)Aryl Halides
Ferdinand H. Lutter‡, Lucie Grokenberger‡, Philipp Spieß, Jeffrey M. Hammann, Konstantin
Karaghiosoff and Paul Knochel*[a]
Abstract: A combination of 10% CoCl2 and 20% 2,2'-bipyridine
ligands enables the cross-coupling of functionalized primary and
secondary alkylzinc reagents with various (hetero)aryl halides. The
coupling with 1,3- and 1,4-substituted cycloalkylzinc reagents
proceeded diastereoselectively leading to functionalized heterocycles
with high diastereoselectivities of up to 98:2. Furthermore, alkynyl
bromides react with primary and secondary alkylzinc reagents
providing the alkylated alkynes.
reaction leading to 3a in 51% yield (entry 6). However, CoCl2 also
proved to be a suitable catalyst for this transformation affording
the desired alkylated heterocycle 3a in 52% yield (entry 7).
Various ligands were tested to further improve the reaction
outcome (entries 8-12). Thus, using the unsubstituted 2,2'-
bipyridine led to the best coupling yield of 66% (entry 8).
Increasing the amount of ligand furnished 3a in 75% isolated yield
(entry 12). Variation of the reaction solvent, the amounts of metal
species or the catalyst loading did not further improve the yield.[9]
At this point we verified that no other metal contaminants are
responsible for this catalysis. Using CoCl2 (99.99% purity) in
combination with a new stirring bar[10] and reaction vessel
afforded the pyridine derivative 3a in 82% yield (entry 13). With
these results in hand the scope of this cross-coupling reaction
was examined.
The transition-metal-catalyzed construction of new C-C bonds is
of utmost importance in modern organic chemistry, and finds wide
application in academic and industrial processes.[1] Especially,
Negishi cross-couplings are among the most versatile methods
for the formation of carbon bonds to create highly functionalized
scaffolds.[2] Organozinc reagents combining both, the low toxicity
of zinc salts as well as a high functional group tolerance,
represent an attractive class of organometallic reagents for cross-
couplings. Albeit, various examples of palladium- [2,3] or nickel-
catalyzed[2,3e,4] Csp2-Csp3 cross-couplings using alkylzinc
reagents have been reported, the search for cheaper and more
abundant alternative catalytic systems is highly desirable. Cobalt-
salts have been found to display several beneficial
characteristics.[5] In comparison to palladium, cobalt is a fairly
cheap metal and for many transformations no sophisticated
ligands are required for an efficient catalysis.[5] Additionally,
several reported protocols showed, that cobalt salts are especially
well suited catalysts for various types of reactions utilizing
organozinc reagents as nucleophilic coupling partners[5] including
acylations[6], cross-coupling reactions[7], or aminations[8].
Using this beneficial combination, we herein wish to report a
cobalt-catalyzed cross-coupling of functionalized primary and
secondary alkylzinc reagents with a variety of aryl, heteroaryl and
alkynyl halides.
In a preliminary experiment, 6-chloronicotinonitrile (1a) was
treated with (2-(1,3-dioxan-2-yl)ethyl)zinc chloride (2a) under
various conditions (Table 1). In the absence of a catalyst, the
desired coupling product 3a could not be detected (entry 1).
Various metal halides such as MnCl2, CuCl2, FeCl2 or CrCl2 were
tested. However, no catalytic activity was observed for this cross-
coupling (entries 2-5). As expected, NiCl2 was able to catalyze the
Table 1. Optimization of the reaction conditions for the cross-coupling of 1a with
alkylzinc reagent 2a.
Catalyst
Ligand
Yield of 3a
Entry
[%][a]
0
1
2
-
-
MnCl2
CuCl2
FeCl2
CrCl2
NiCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
-
0
3
-
0
4
-
0
0
5
-
-
6
51
7
-
52
8
bipy[b]
dtbbpy[c]
neocuproine
TMEDA
bipy[b]
bipy[b]
66
9
63
10
11
12[d]
13[d]
65
39
80 (75)[e]
82
[f]
CoCl2
[a]
Ferdinand H. Lutter, Lucie Grokenberger, Philipp Spieß, Jeffrey M.
Hammann, Konstantin Karaghiosoff and Paul Knochel
Ludwig-Maximilians-Universität München, Department Chemie
Butenandtstrasse 5-13, Haus F, 81377 München (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
[a] Reactions were performed on a 0.25 mmol scale. Yields were determined by
GC-analysis. Tetradecane (C14H30) was used as internal standard. [b] 2,2'-
Bipyridine. [c] 4,4'-Di-tert-butyl-2,2'-dipyridyl. [d] 20 mol% of bipy was used.
[e] Isolated yield of the reaction performed on a 1.00 mmol scale. [f] CoCl2
(99.99% purity) was used.
[‡]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document
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10.1002/anie.201914490
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N-heterocyclic halides of type 1 were coupled with various
functionalized alkylzinc reagents of type 2 (Scheme 1). Thus, the
reaction of 1a with (3-phenylpropyl)zinc chloride afforded 3b in
73% yield. Also, the corresponding bromopyridine was used
leading to coupling products 3c and 3d in 62-75% yield. Several
alkylzinc reagents bearing various functional groups were
excellent substrates for this cross-coupling. Zinc organometallics
containing nitrile groups, masked amines, and acetates were
successfully coupled furnishing the alkylated pyridines 3e-3g in
66-87% yield.
The reactions of zinc species derived from natural products such
as (1R)-(−)-nopol and (S)-citronellol with ethyl 6-chloronicotinate
afforded 3h and 3i in 76-83% yield. Also, using 2-halonicotinic
esters in combination with zinc reagents bearing a heterocyclic or
an alkyne moiety coupled smoothly leading to 3j and 3k in 78-
83% yield. Furthermore, other N-heterocyclic halides, such as
quinoline, isoquinoline, quinazoline, and pyrimidine derivatives
were successfully cross-coupled with various functionalized
alkylzinc reagents furnishing products 3l-3s in 58-95% yield.
However, the reaction with less activated heterocyclic halides led
to poor coupling results.[11]
Scheme 1. Compounds of type 3 obtained by the Co-catalyzed reaction of, N-
heterocyclic halides of type 1 with primary alkylzinc reagents of type 2.[a]
Scheme 2. Compounds of type 3 obtained by the Co-catalyzed reaction of, aryl
halides of type 1 with alkylzinc reagents of type 2.[a]
[a] Reactions were performed on a 0.5 mmol scale. Yields are determined from
the purified and analytical pure product. [b] 20% CoCl2, 40% dtbbpy and
1.9 equiv. of the corresponding alkylzinc reagent were used. [c] Dtbbpy was
used instead of bipy. The reaction was performed at rt.
Next, this cobalt catalyzed cross-coupling was extended to
various electron-deficient aryl halides as electrophilic coupling
partners (Scheme 2). Thus, (2-(1,3-dioxan-2-yl)ethyl)zinc chloride
(2a) was coupled with 4-bromo-2-fluorobenzonitrile and ethyl 4-
iodobenzoate furnishing 3t-u in 66-82% yield. Benzophenone
was successfully alkylated in ortho- and para-position,
respectively, starting from the corresponding halide, leading to 3v
and 3w in 70-85% yield. The cross-coupling of a zinc reagent
containing
an
ester
moiety
with
a
functionalized
chlorobenzophenone led to 3x in 73% yield. Also, cyclopropylzinc
chloride was used in this procedure, affording the benzophenones
3y and 3z in 70-71% yield.
Encouraged by the results with the secondary cyclopropylzinc
reagent, we examined the cross-coupling of various substituted
six-membered cycloalkylzinc reagents. In the past, several
diastereoselective Csp3-Csp2 Negishi-type cross-couplings using
palladium[12] and nickel salts[13] have been reported. Also, a
cobalt-catalyzed version applying bis-arylzinc reagents is
known.[7e] However, this methodology only allows the coupling of
1,2-substituted cycloalkyl iodides with (hetero)aryl zinc reagents
[a] Reactions were performed on a 0.5 mmol scale. Yields are determined from
the purified and analytical pure product. [b] 20% CoCl2, 40% dtbbpy and
1.9 equiv. of the corresponding alkylzinc reagent were used.
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10.1002/anie.201914490
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COMMUNICATION
in a diastereoselective manner. To overcome this limitation, we
approached the problem by using substituted cycloalkylzinc
species with heteroaryl halides as coupling partners. Previous
studies have shown that the carbon-zinc bond is prone for an easy
epimerization in the presence of metal salts. [12a-b] Thus, a highly
diastereoselective cross-coupling is only enabled by a fast
transmetalation of the thermodynamically more stable alkylzinc
species to the transition-metal catalyst.[12a]
To evaluate the scope of a diastereoselective cross-coupling
using substituted cyclohexylzinc reagents, 2-methylcyclo-
hexylzinc iodide was coupled with 6-bromonicotinonitrile. A short
screening revealed that a catalytic system of 10% CoCl2 and 20%
4,4'-di-tert-butyl-2,2'-dipyridyl in acetonitrile led to the best yield
and diastereomeric ratio.[9] Hence, the coupling of various 1,3-,
and 1,4-functionalized cycloalkylzinc reagent with N-heterocyclic
bromides was examined (Scheme 3).
bulkier iso-propyl residue led to 3ab in 63% yield and an improved
diastereomeric ratio of 96:4. Additionally, this zinc reagent was
coupled with 2-bromopyrimidine furnishing 3ac (52% yield,
d.r. = 94:6[14]). Also, 1,4-substituted cyclohexylzinc reagents
could be used in this protocol. Thus, the cross-coupling of zinc
reagents bearing an ester or a pyrrole substituent with a
trifluoromethylated bromopyridine led to the corresponding trans-
1,4-bifunctionalized cyclohexanes 3ad and 3ae in 51-54% yield
(d.r. = 80:20-98:2[15]). Bromopyrimidine derivatives were coupled
with functionalized cyclohexyl reagents affording 3af-3ah in 64-
73% yield and diastereomeric ratios of up to 98:2.
Scheme 4. Diastereoselective cobalt-catalyzed cross-coupling of 2-
bromopyridine with zinc organometallics 2b and 2c derived from cholesterol and
nootkatone derivatives.[a]
Scheme 3.
Diastereoselective
cobalt-catalyzed cross-coupling
of
heteroaromatic bromides of type 1 with 1,3- and 1,4-substituted secondary
alkylzinc reagents of type 2 leading to products of type 3. [a]
[a] Reactions were performed on a 0.5 mmol scale. Yields are determined from
the purified and analytical pure product. The diastereomeric ratio (d.r.) was
determined by GC analysis. The major diastereomer is shown.
Remarkably, 2-bromopyrimidine could be coupled with complex
alkylzinc reagents prepared from steroid and sesquiterpene
derivatives (Scheme 4). The reaction of cholesterylzinc chloride
2b furnished 3ai in 78% yield and a diastereomeric ratio of 98:2.
Also, the corresponding coupling using zinc reagent 2c derived
from a reduced nootkaton derivative proceeded in a highly
diastereoselective fashion leading to 3aj in 52% yield (d.r. = 98:2).
Finally, this cobalt-catalyzed cross-coupling was further
extended to alkynyl bromides (Scheme 5). (Bromoethynyl)-
benzene (4a) reacted smoothly with (2-(1,3-dioxan-2-yl)ethyl)zinc
chloride (2a) affording the alkylated alkyne 5a in 55% yield.
Interestingly, the coupling of the TIPS protected alkyne 4b with
the 1,4 phenyl substituted cyclohexylzinc reagent 2d furnished
the 1,4-trans-alkynylated cyclohexane derivative 5b in 54% yield
and d.r. = 99:1.
[a] Reactions were performed on a 0.5 mmol scale. Yields are determined from
the purified and analytical pure product. The diastereomeric ratio (d.r.) was
determined by GC analysis. The major diastereomer is shown.
The reaction of 6-bromonicotinonitrile with 3-methylcyclohexyl-
zinc iodide led to the thermodynamically more stable cis-1,3-
disubstituted cyclohexane 3aa in 80% yield and d.r. = 91:9.
However, using the corresponding zinc reagent bearing the
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Scheme 5. Cobalt-catalyzed cross-coupling of alkynyl bromides with primary
and secondary alkylzinc reagents.[a]
Keywords: catalysis • cobalt • cross-coupling •
diastereoselectivity • zinc
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chloronicotinonitrile (1a) with (2-(1,3-dioxan-2-yl)ethyl)zinc
chloride (2a) 2.0 equiv. of TEMPO were added.[9] However, the
coupling product 3a was afforded in a similar yield, compared to
the standard conditions without the radical trapping agent. This
indicates that this new cobalt-catalyzed cross-coupling might not
proceed via radical intermediates.
In conclusion, we have reported a cobalt-catalyzed cross-
coupling of various substituted primary and secondary alkylzinc
reagents with aryl and heteroaryl halides. Couplings using 1,3-
and 1,4-functionalized cyclohexylzinc reagents proceeded with
high diastereoselectivities of up to 98:2. Furthermore, this
procedure allowed the coupling of primary and secondary
alkylzinc reagents with alkynyl bromides. Further mechanistic
investigations are currently underway in our laboratories.
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Acknowledgements
We would like to thank the Deutsche Forschungsgemeinschaft
and the Ludwig-Maximilians-Universität München for financial
support. We also thank Albemarle Lithium GmbH (Frankfurt) and
the BASF AG (Ludwigshafen) for the generous gift of chemicals.
We also thank Benedikt Nißl for the preparation of starting materials.
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Conflict of Interest
For further details see ESI.
The authors declare no conflict of interest.
[10] E. O. Pentsak, D. B. Eremin, E. G. Gordeev, V. P. Ananikov, ACS Catal.
2019, 9, 3070-3081.
[11] For unsuccessful substrates see ESI.
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[14] The stereochemistry of 3ac was determined by NOESY-NMR
spectroscopy (see ESI).
[15] The stereochemistry of 3ae was determined by crystal structure analysis
(see ESI).
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Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
A Cheap and Broad Scope Co-upling. We describe a cobalt-catalyzed cross-
coupling reaction of functionalized primary and secondary alkylzinc reagents with a
variety of aryl and heteroaryl halides. Cross-coupling reactions using secondary
cyclohexylzinc reagents proceed in a high diastereoselective fashion furnishing the
corresponding products in up to 98:2 d.r. Furthermore, the procedure enables the
coupling of alkynyl bromides with primary and secondary zinc reagents.
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