Metal-free oxidative decarbonylative coupling of aromatic aldehydes with arenes: Direct access to biaryls

Article abstract of DOI:10.1039/c4cc10155c

A metal-free oxidative decarbonylative coupling of aromatic aldehydes with electron-rich or electron-deficient arenes to produce biaryl compounds was developed. This novel coupling was proposed to proceed via a non-chain radical homolytic aromatic substitution (HAS) type mechanism, based on the substrate scope, ortho-regioselectivity, radical trapping experiments and DFT calculation studies. With the ready availability of aromatic aldehydes and arenes, metal-free conditions should make this coupling attractive for the biaryl synthesis.

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Metal-Free Oxidative Decarbonylative Coupling of
Aromatic Aldehydes with Arenes: Direct Access to
Biaryls
Chem. Commun. Cite this: DOI:
10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Ren-Jin Tang,a Qing Heb and Luo Yang*a
www.rsc.org/
On the other hand, aldehydes are cheap and readily available
starting materials that have been directly used as precursors for
(oxidative) decarbonylative couplings catalyzed by ruthenium or
rhodium by the extensive studies of Li and co-workers since 2009
(Scheme 1b). 3b, 6 In the absence of transition metal and assisted by a
suitable oxidant, aldehydes could also be converted into the
corresponding acyl radicals for hydroacylation of olefins 7 and other
coupling reactions 8. These results inspire us to conceive the
generation of aryl radicals from aromatic aldehydes by the further
decarbonylation of these acyl radials under oxidative conditions.7b, 9
A metal-free oxidative decarbonylative coupling of aromatic
aldehydes with electron-rich or electron-deficient arenes to
produce biaryl compounds was developed. This novel
coupling was proposed to proceed via a non-chain radical
homolytic aromatic substitution (HAS) type mechanism,
based on the substrates scope, ortho-regioselectivity, radical
trapping experiments and DFT calculation studies. The ready
availability of aromatic aldehydes and arenes, metal-free
conditions should make this coupling attractive for the biaryl
synthesis.
Biaryl scaffolds are privileged structural units that have diverse Herein, we report the first metal free oxidative decarbonylative
applications in the fields of pharmaceuticals, agrochemicals, coupling of aromatic aldehydes with electron-rich or electron-
functional materials and catalysts.1 Transition metal catalyzed cross- deficient arenes to produce biaryl compounds through a radical
coupling between (pseudo)aryl halides and aryl metalloids has been pathway (Scheme 1c).
developed as arguably the most versatile and efficient method to
synthesize such biaryl motifs, but it suffers from mainly two
drawbacks: (1) it requires the pre-functionalization of both coupling
partners and produces stoichiometric metal waste; (2) it requires
expensive transition metal catalysts such as palladium, which would
increase the difficulties and costs to remove these trace transition
metal impurities from the final products after the cross-coupling at
the same time.2 Therefore, the direct biaryl synthesis from two
different C-H bonds under transition-metal-free (or even metal-free)
conditions is an ideal but challenging goal.3
For metal-free C-H arylation, homolytic aromatic substitution
(HAS) of aryl radicals with benzene derivatives has been developed
as a straightforward method to construct biaryl frameworks, however
it has been hampered by limited precursors and laborious procedures
for the generation of aryl radicals.4 Recently, breakthroughs on base
Scheme 1 Direct arene C-H transformations for biaryl synthesis.
promoted homolytic aromatic substitution have been made by Itami,
5
We first examined the oxidative decarbonylative coupling of p-
methoxycarbonyl benzaldehyde (1a)10 in benzene with TBP (di-tert-
butyl peroxide) as the initiator and oxidant. The desired biaryl
product 3a was detected by GC-MS, but in very low yield (< 2 %),
and the most of the aldehyde 1a remained intact. During the
subsequent optimization, by ordinary solvent screening, we
surprisingly found that the nitrobenzene was able to increase the
yield noticeably, and the addition of 2 equiv of nitrobenzene
afforded biaryl product 3a in 22 % yield (Table 1, entry 2). At the
beginning, the role of nitrobenzene as the additive to this reaction
was unclear so other nitro-containing compounds such as p-
nitrobenzoic acid and p-nitrobenzoyl chloride were tried, which
indeed afford better yields than nitrobenzene itself (entries 3 and 4).
Lei, Hayashi, Shi and Brase,
where aryl radicals can be
conveniently generated from aryl halides by the interaction of
chelated ligands and potassium tert-butoxide (Scheme 1a).
a R. J. Tang, Prof. L. Yang
Key Laboratory for Environmentally Friendly Chemistry and Application of
the Ministry of Education, College of Chemistry, Xiangtan University, Hunan,
411105, China. E-mail: yangluo@xtu.edu.cn; Fax: +86-731-58292251; Tel:
+86-731-59288351
b Q. He
Laboratory of Molecular Recognition and Function, Institute of Chemistry,
Chinese Academy of Sciences, Beijing, 100190, China
Electronic Supplementary Information (ESI) available: experimental
details, mechanism studies and characterisation of products.
See DOI: 10.1039/b000000x/
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It seems that nitro containing electron-deficient arenes can improve electron-deficient arenes such as 2a and slightly electron-rich arenes
this coupling so the three regioisomers of dinitrobenzene (o, m and such as 2c, but not suitable for more electron-rich anisole (2n). For
p) were tested next (entries 6-8) and o-dinitrobenzene (abbreviated the arenes (2d-2m) that was capable of generating regioisomers, the
as DNB next) afforded the best yield (60 %, entry 8). The further oxidative decarbonylative coupling subjected to afford the ortho-
fine adjustment of dosage of DNB (entries 7-9) and reaction functionalized products predominately. It is worth noting that the
temperature (entry 10) led to an optimal yield of 68 %. It is biaryl methanone byproduct (that could be generated from the acyl
noteworthy that the tert-butylhydroperoxide (TBHP) can also be radical without decarbonylation) was not detected by the GC-MS
used as the initiator and oxidant to give similar results; but dioxygen and crude 1H NMR for all of the substrates tested, which might
was ineffective (entries 11 and 12). When a mixed-solvent of imply that the decarbonylation is a rapid step in this reaction.
benzene/dichloroethane (DCE) or benzene/acetonitrile was applied
Table 2 The influence of aromatic aldehydes on the metal-free
instead of benzene alone, the yields of desired biaryl product 3a
oxidative decarbonylative coupling
dropped to 43 % and 35 %, respectively (entries 14 and 15).
Transition-metals such as copper and iron that have been widely
used as redox catalysts for aldehydes C(sp2)-H transformations11
were ineffective for this oxidative decarbonylative coupling (see
Supporting Information).
Table 1 Optimization of the metal-free oxidative decarbonylative
coupling a
yield [%] b
< 2%
22
entry
Additive (equiv)
[O] (equiv)
1
2
3
4
5
6
7
8
9
--
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBHP (2.5)
O2
NO2-C6H5 (2.0)
p-NO2-C6H4CO2H (2.0)
p-NO2-C6H4COCl (2.0)
1,2-(NO2)2-C6H4 (2.0)
1,3-(NO2)2-C6H4 (2.0)
1,4-(NO2)2-C6H4 (2.0)
1,2-(NO2)2-C6H4 (1.0)
1,2-(NO2)2-C6H4 (0.5)
1,2-(NO2)2-C6H4 (1.0)
1,2-(NO2)2-C6H4 (2.0)
1,2-(NO2)2-C6H4 (2.0)
1,2-(NO2)2-C6H4 (2.0)
1,2-(NO2)2-C6H4 (2.0),
1,2-(NO2)2-C6H4 (2.0),
38
54
60
56
48
64
46
Conditions: 1a-1o (0.2 mmol), DNB (1 equiv, 0.2 mmol), TBP (2.5 equiv, 0.5
mmol), benzene (80 equiv, 16.0 mmol, 1.5 mL), reacted for 12 h at 150C under
argon atmosphere unless otherwise noted. Isolated yields were reported.
10 c
11
68
63
12
< 2%
57
Table 3 The influence of arenes on the metal-free oxidative
13
TBP (2.0)
TBP (2.5)
TBP (2.5)
decarbonylative coupling a
14 d
15 e
43
35
a
Conditions: 1a (0.2 mmol), additive, TBP (2.5 equiv, 0.5 mmol), benzene (80
equiv, 16.0 mmol, 1.5 mL), reacted for 12 h at 140C under argon atmosphere
b
c
d
unless otherwise noted. Isolated yields. Reacted at 150C. Mixed-solvent of
benzene/DCE (0.75 mL/0.75 mL). e Mixed-solvent of benzene/ acetonitrile (0.75
mL/0.75 mL).
Under the optimized conditions (Table 1, entry 10), the substrate
scope and limitation of this oxidative decarbonylative coupling of
aldehydes with arenes were explored. The effect of substituents on
the aromatic aldehyde moiety is listed in Table 2. Benzaldehydes
bearing electron withdrawing or donating substituents were
successfully transformed into the desired decarbonylated biaryl
products in moderate yields, such as cyano (1b and 1c),
trifluoromethyl (1d), halo (1e-1i), alkyl (1k and 1l) and methoxyl
group (1m). Besides substituted benzaldehydes, the optimized
-
naphthaldehyde (1n and 1o) with benzene. Unfortunately, this
oxidative decarbonylative coupling failed for the aliphatic aldehydes
such as n-butanal and cyclohexane carbaldehyde; no corresponding
alkylated products was detected by the GC-MS and the aliphatic
carboxylic acids were revealed as the main product. 12
On the other hand, for different arene (C-H) coupling partners
(Table 3), the optimized reaction condition was applicable to both
2 | Chem. Commun., 2015, 00, 1-3
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a
Conditions: 1a (0.2 mmol), arene (2a-2n) (1.5 mL), DNB (1 equiv, 0.2 mmol),
speculated that DNB acted as an electron acceptor here too, although
other possible pathways could not be ruled out at this stage.
TBP (2.5 equiv, 0.5 mmol), reacted for 12 h at 150C under argon atmosphere
unless otherwise noted. Isolated yields were reported. The ratio of isomers was
determined by 1H NMR and assigned by comparing with the data reported by
literatures. b The ratio was not assigned.
The oxidative decarbonylative coupling fitted well with electron-
deficient arenes, which differentiates it from the Friedel-Crafts type
reactions and transition-metal catalyzed aromatic C-H activation via
electrophilic substitution mechanism. In the preliminary competition
reactions, the aldehydes and arenes with different electron densities
showed comparable reaction rate (Scheme 2, see Supporting
Information for details). Considering that peroxide is reported as a
7, 8b, 13
good radical initiator for aldehydes
and the obvious ortho-
selectivity demonstrated by arene coupling partners (Table 2), we
speculated this reaction proceeded through the homolytic aromatic
substitution (HAS) type pathway.
Scheme 3 Mechanistic investigation on the metal-free oxidative
decarbonylative coupling.
With the reaction of benzaldehyde (1j) and chlorobenzene (3m) as
an example, a plausible mechanism was proposed in Figure 1, which
was assisted by the DFT calculation (B3LYP/6-31+G (d) method,
see Supporting Information). First, the tert-butoxy radical abstracts
the aldehyde hydrogen atom to provide the acyl radical I, which
undergoes decarbonylation to afford phenyl radical II. Addition of
the phenyl radical to chlorobenzene gives the phenylcyclohexadienyl
radical III. Due to the “persistent radical effect”,17 the direct tertiary
hydrogen abstraction of radical III by tert-butoxy radical to realize
re-aromatization is dynamically unbeneficial. Instead, radical III
transfers an electron to DNB to produce phenylcyclohexadienyl
cation IV and DNB·-, which is further deprotonated by tert-butoxide
anion to afford the biaryl product (a cation/anion complex is formed
before the deprotonation implied by the IRC analysis of Gaussian).
DNB·- can be recovered by transferring the superfluous electron to
TBP to accelerate its heterolytic cleavage, or transferring the
electron to tert-butoxy radical to produce tert-butoxide anion. As a
side reaction, DNB·- can also be reduced to yield 2-nitroaniline 6
(Scheme 3c) and N-acyl-2-nitroaniline18 byproducts 7.
Scheme 2 Competition reactions of the oxidative decarbonylative
coupling.
Next, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl, 1.0 equiv) as
radical inhibitor was subjected to the standard procedure, which still
led to 41 % yield of product 3a (Scheme 3a), and thus a chain radical
mechanism might be unlikely. When 1,1-diphenylethene (2.0 equiv)
as radical scavenger was added in, the corresponding aryl radical
was trapped to afford triarylethene 5 in low yield (3 %, Scheme
3b),14 which further strongly supports the generation of aryl radical
during the reaction.
DNB played a critical role in this oxidative decarbonylative
coupling. By carefully balancing the DNB during the reaction, 58 %
was recovered after the coupling and small part of it was reduced
and converted to 2-nitroaniline (6) and N-acyl-2-nitroaniline (7) in
18 % and 13 % yield, respectively (Scheme 3c). The reduction of the
nitro group might arise from a single electron transfer to DNB.15
Since DNB and its two regioisomers (meta and para) were famous
for their electron-deficient character and thus used as electron
acceptors on the study of radical nucleophilic substitution
mechanism between
a
carbon anion with halobezenes,16 we
Figure 1. Free energy profiles for the proposed mechanism of this oxidative decarbonylative coupling. Calculated at the B3LYP/6-31+G (d) basis with
IEF-PCM solvation corrections for chlorobenzene at 298K.
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The phenylcyclohexadienyl cation IV is 35.7 Kcal/mol higher in
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often close to the reaction free energies using Marcus theory,19 and
thus the activation barrier of SET is roughly estimated as ΔGact
≈
SET
35.7 Kcal/mol. By comparing ΔGact
with the activation energies
SET
of transition states (TS1-TS4) obtained by DFT calculation, the rate-
determine step might be the SET step. The low KIE value (1.41)
obtained from the competition reaction between C6H6 and C6D6 with
aldehyde 1l further confirmed the deprotonation was not the rate-
determine step (Scheme 3d).
6
7
In conclusion, we have developed the first metal-free oxidative
decarbonylative coupling of aromatic aldehydes with electron-rich or
electron-deficient arenes to produce biaryl compounds. The substrate
scope, ortho-regioselectivity, radical trapping experiments and DFT
calculations supported a non-chain radical homolytic aromatic
substitution (HAS) type mechanism and DNB acted as the electron
“porter”. The ready availability of aromatic aldehydes and arenes,
metal-free conditions should make this oxidative decarbonylative
coupling attractive for the biaryl synthesis. The further improvement
and detailed mechanism are ongoing in our laboratory.
8
9
(a) K. Matcha and A. P. Antonchick, Angew. Chem. Int. Ed., 2013, 52,
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For n-butanal, the self-aldol condation by-product was also detected. A
control reaction using benzoic acid as the starting material to replace
benzaldehyde failed to afford the biphenyl product, which could
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Acknowledgements
L. Yang thanks Q. He for assistance on DFT calculations.
This work was supported by the National Natural Science
Foundation of China (21202138), Xiangtan University “Academic
Leader Program” (11QDZ20), New Teachers' Fund for Doctor
Stations, Ministry of Education (20124301120007), and Hunan
Provincial Natural Science Foundation (13JJ4047, 12JJ7002), Hunan
Provincial Excellent Young Scientist Foundation (13B114).
The authors declare no competing financial interest.
10
11
Notes and references
1
For recent reviews on applications of biaryl units, see: (a) J. Hassan, M.
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12
13
2
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3
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