Cobalt-Catalyzed C?H Functionalizations by Imidate Assistance with Aryl and Alkyl Chlorides

Article abstract of DOI:10.1002/adsc.201600384

A catalytic system comprising the inexpensive cobalt(II) acetylacetonate [Co(acac)₂] and an N-heterocyclic carbene (NHC) ligand enabled versatile C?H arylations by oxazoline assistance. The broadly applicable, low-valent cobalt catalyst displayed a wide substrate scope, and even proved applicable to challenging C?H alkylations with β-hydrogen-containing primary and secondary alkyl chlorides. The power of the cobalt-catalyzed C?H arylation protocol was among others reflected by facile C?H activation at a room temperature of 23 °C, and efficient late-stage diversification, providing access to bioactive biaryls. (Figure presented.).

Full text of DOI:10.1002/adsc.201600384

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DOI: 10.1002/adsc.201600384
Very Important Publication
À
Cobalt-Catalyzed C H Functionalizations by Imidate Assistance
with Aryl and Alkyl Chlorides
Ruhuai Meia and Lutz Ackermanna,*
a
Institut fꢀr Organische und Biomolekulare Chemie, Georg-August-Universitꢁt, Tammanstraße 2, 37077 Gçttingen,
Germany
Fax : (+49)-551-396-777; e-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Received: April 10, 2016; Revised: April 30, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600384.
À
Abstract: A catalytic system comprising the inexpen- col was among others reflected by facile C H activa-
sive cobalt(II) acetylacetonate [Co(acac)2] and an N- tion at a room temperature of 238C, and efficient
heterocyclic carbene (NHC) ligand enabled versatile late-stage diversification, providing access to bioac-
À
C H arylations by oxazoline assistance. The broadly tive biaryls.
applicable, low-valent cobalt catalyst displayed
a wide substrate scope, and even proved applicable
to challenging C H alkylations with b-hydrogen-con- Keywords: alkylation; arylation; C H activation;
À
À
taining primary and secondary alkyl chlorides. The chlorides; cobalt; imidates; N-heterocyclic carbene
À
power of the cobalt-catalyzed C H arylation proto- ligands
Introduction
lines, we very recently devised a protocol for high-
À
valent cobalt(III)-catalyzed C H amidations by oxa-
[1]
zoline assistance.[12] Within our program on base
À
The arylation of otherwise unreactive C H bonds
[13]
À
has been identified as an increasingly viable alterna- metal-catalyzed C H activation,
we have now es-
tive to traditional cross-coupling chemistry,[2] since C
tablished the first cobalt-catalyzed C H arylation of
À
À
H arylations avoid multi-step syntheses of prefunc- aryl imidates, on which we report herein. Notable fea-
À
tionalized substrates and reduce undesired by-product tures of our strategy include (i) efficient C H aryla-
formation.[3] The vast majority of direct arylations has tions of synthetically meaningful aryl oxazolines, (ii)
been accomplished with catalysts of precious 4d tran- the use of challenging aryl and alkyl chlorides as elec-
À
sition metals, most notably with the aid of palladium, trophiles, (iii) C H arylations at a room temperature
rhodium and ruthenium complexes.[3] In consideration of 238C, as well as (iv) removable/modifiable imidates
of the natural abundance and low costs of 3d transi-
À
tion metals, the focus in C H activation catalysis has
shifted in the recent years towards the use of base
metal catalysis,[4] with major advances accomplished
by versatile cobalt catalysts.[5] In this context, Acker-
mann[6] and Yoshikai[7] have recently reported on C
H arylations with organic halides by low-valent cobalt
À
catalysis.[8] Despite these considerable advances,
cobalt-catalyzed C H arylations by oxazoline assis-
tance have as yet unfortunately proven elusive, al-
À
though oxazolines constitute valuable structural
motifs of various bioactive compounds.[9] Moreover,
cyclic imidates are readily available and can be con-
verted into a wide variety of valuable functional
groups,[10] which renders them valuable intermediates
in organic syntheses as well as useful ligands in metal
catalysis.[11] Given the practical importance of oxazo- Figure 1. Bioactive ortho-arylated acid derivatives I–III.
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Table 1. Optimization of the cobalt-catalyzed C H arylation
that provide expedient access to ortho-arylated car-
boxylic acid derivatives – key precursors to natural
products and bioactive compounds, such as alternariol
I, non-steroidal-specific endothelial cell proliferation
inhibitor II or non-peptidic angiotensin II receptor
antagonist III (Figure 1).[14]
by oxazoline assistance.[a]
Results and Discussion
We initiated our studies by probing various cobalt
À
salts and ligands for the desired C H arylation on ar-
yloxazoline 1a with aryl chloride 2a (Table 1). Among
a
representative set of N-heterocyclic carbene
(NHC)[15] preligands derived from benzimidazolium
salts 4a, b, triazolium salts 4c–e, and imidazolium salts
4f–4h (entries 1–9), the dicyclohexyl-substituted imi-
dazolium chloride 4g provided the best results
(entry 8). Imidazolinium salt 4i also proved compe-
tent, with a slightly diminished yield of the desired
product 3aa (entry 10). The key importance of the
cobalt salt was verified (entry 11), with the optimal
performance being achieved by Co(acac)2 (entries 8
versus 12–15). The optimal metal to ligand ratio was
determined to be 1/1 (entries 8 and 16). Interestingly,
the previously[6a,c,d] employed NHC precursors
IMesHCl and IPrHCl failed in delivering the desired
product 3aa in satisfactory yields (entries 17 and 18).
The unique power of the low-valent cobalt-catalyzed
Entry
[Co] (mol%)
Ligand (mol%)
3aa [%][b]
1
2
3
4
5
6
7
8
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
–
CoCl2(5.0)
CoBr2(5.0)
CoI2(5.0)
Co(acac)3 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
Co(acac)2 (5.0)
–
<5
19
27
34
9
20
13
81
14
65
–
4a (5.0)
4b (5.0)
4c (5.0)
4d (5.0)
4e (5.0)
4f (5.0)
4g (5.0)
4h (5.0)
4i (5.0)
4g (5.0)
4g (5.0)
4g (5.0)
4g (5.0)
4g (5.0)
4g (10.0)
IMesHCl (5.0)
IPrHCl (5.0)
4g (5.0)
À
C H arylation strategy was reflected by the exceed-
ingly mild reaction conditions.[16] Thus, the optimized
9
À
10
11
12
13
14
15
16
17
18
19
cobalt(II) catalyst allowed efficient C H activations
at a room temperature of 238C (entry 19).
74
67
73
51
56
13
4
The versatility of the optimized catalyst was subse-
quently explored with a representative set of aryl
chlorides 2 (Scheme 1). The catalytic system compris-
ing of Co(acac)2 and preACTHNUTRGENNGUliACHUTGTNRNEUGgN and ICyHCl (4g) allowed
for the conversion of aryl chlorides 2 with para-,
meta- and even sterically hindered ortho-substituents,
and proved amenable to the gram-scale preparation
of biaryl 3ai.
79[c]
[a]
Reaction conditions: 1a (0.50 mmol), 2a (0.65 mmol),
CyMgCl (2.0 equiv.), DMPU (1.0 mL), 608C, 16 h.
Yield of isolated product.
Thereafter, we tested a variety of cyclic imidates
À
1 in the room-temperature cobalt-catalyzed C H ary-
[b]
[c]
lation regime (Scheme 2). The catalyst proved to be
applicable to both electron-rich as well as electron-de-
ficient arenes 1, thereby delivering the desired prod-
The reaction was performed at 238C.
ucts 3 with excellent levels of positional selectivity. became attracted to delineating its working mode. To
Furthermore, substituted cyclic imidates 1e–g with this end, we performed a series of competition experi-
varying ring size were well tolerated, as was the ments (Scheme 4). Thus, electron-rich aryl chlorides 2
hetero
G
were less efficiently converted, indicating a kinetically
À
The broadly applicable low-valent cobalt-NHC cat- relevant C Cl cleavage step (Scheme 4a). An inter-
alyst was not restricted to C H arylations. Indeed, molecular competition experiment with aryl imidates
[6c,19]
primary and secondary C H alkylations
with 1 revealed that electron-deficient substrate 1d reacted
challenging b-hydride-containing[20] chlorides 5 and 7, exclusively (Scheme 4b). The challenging nature of
À
respectively, proved viable likewise (Scheme 3).
In consideration of the unique activity of the cobalt periment between arylating and alkylating reagents
catalyst in imidate-assisted C H functionalization, we 2a and 5a, respectively (Scheme 4c). Thus, the C H
C H alkylations was reflected by a competition ex-
À
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Scheme 2. Cobalt-catalyzed C H arylation of aryl imidates
1.
À
Scheme 1. Scope of cobalt-catalyzed C H arylation with
aryl chlorides 2.
arylated product 3aa was formed as the major prod-
uct, while only traces of the alkylated arene 6aa were
detected.
Furthermore, independent experiments with sub-
strates 1b and [D]4-1b[21] revealed a kinetic isotope
effect (KIE) of kH/kD ꢀ1.2 (Scheme 5),[22] being indi-
À
cative of a facile C H metalation event being opera-
tive.
The presence of typical radical scavengers in stoi-
chiometric amounts led to inhibition of the catalystꢃs
activity (Scheme 6), thus being suggestive of single
À
Scheme 3. Primary and secondary C H alkylation.
electron transfer (SET)-type process[8] being opera- Conclusions
tive.
Finally, the synthetic utility of our strategy was il- In summary, we have reported on the first cobalt-cat-
lustrated by the facile diversification[10,23] of the thus alyzed C H arylation by oxazoline assistance. Thus,
À
obtained biaryl imidates 3 to furnish alcohols 9, cobalt catalysts derived from N-heterocyclic carbenes
À
amide 10i, arylated benzoic acid 11a and benzochro- enabled positional-selective C H arylations with diffi-
men-6-one 12i[24] (Scheme 7). Thereby, the modifiable cult to activate aryl chlorides. Thereby, biaryloxazo-
directing group strategy[25] gave access to key structur- lines were prepared in a step-economical fashion,
al motifs of a variety of bioactive compounds.
which thus far called for the use of precious rutheni-
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Scheme 4. Intermolecular competition experiments.
À
Scheme 6. Cobalt-catalyzed C H arylation in the presence
of radical scavengers.
Experimental Section
Scheme 5. Kinetic isotope effect study.
À
Typical Procedure: Cobalt-Catalyzed C H Arylation
of Aryl Imidates 1
ACHTUNGTRENNUNG
um(II) catalysts.[26] The optimized cobalt catalysts also
A suspension of Co(acac)2 (6.4 mg, 5.0 mol%), ICyHCl 4g
(6.7 mg, 5.0 mol%), aryloxazoline 1a (80.6 mg, 0.50 mmol),
aryl chloride 2a (92.7 mg, 0.65 mmol), and DMPU (1.0 mL)
was stirred for 5 min at 08C. A solution of CyMgCl solution
in 2-methyltetrahydrofuran (1.0M, 1.0 mL, 2.0 equiv.) was
added dropwise at the same temperature. Then the mixture
was stirred at 238C for 16 h. At ambient temperature, aque-
ous NH4Cl (2.0 mL) and H2O (15 mL) were added. The re-
action mixture was extracted with MTBE (3ꢄ20 mL). The
À
proved applicable to C H alkylations with challeng-
ing alkyl chlorides. The versatile low-valent cobalt
catalyst displayed broad substrate scope, while mech-
À
anistic studies were indicative of a facile C H metala-
tion and a kinetically relevant C Cl cleavage. The
subsequent diversification of the thus obtained biary-
lated imidates gave expedient access to key structural
scaffolds of bioactive compounds.
À
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chromatography on silica gel (n-hexane/EtOAc 5:1) yielded
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1H NMR (500 MHz, CDCl3): d=7.55 (s, 1H), 7.31–7.20 (m,
4H), 6.94–6.86 (m, 2H), 4.12 (t, J=9.4 Hz, 2H), 3.90 (t, J=
9.4 Hz, 2H), 3.82 (s, 3H), 2.37 (s, 3H); 13C NMR (125 MHz,
CDCl3): d=166.3 (Cq), 158.6 (Cq), 138.5 (Cq), 136.4 (Cq),
133.6 (Cq), 131.2 (CH), 130.6 (CH), 130.1 (CH), 129.3 (CH),
127.1 (Cq), 113.4 (CH), 67.8 (CH2), 55.2 (CH3), 54.9 (CH2),
20.9 (CH3); IR (ATR): n=2934, 1647, 1608, 1487, 1244,
1176, 1034, 947, 819, 538 cmÀ1; MS (EI): m/z (relative inten-
sity)=268 (10) [M+H+], 267 (40) [M+], 266 (100), 251 (15),
222 (20); HR-MS (ESI): m/z=268.1338, calcd. for
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Acknowledgements
Generous support by the European Research Council under
the European Communityꢀs Seventh Framework Program
(FP7 2007–2013)/ERC Grant agreement no. 307535, and the
CSC (fellowship to R.M.) is gratefully acknowledged.
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Cobalt-Catalyzed C H Functionalizations by Imidate
Assistance with Aryl and Alkyl Chlorides
7
Adv. Synth. Catal. 2016, 358, 1 – 7
Ruhuai Mei, Lutz Ackermann*
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