Metal-free radical aromatic carbonylations mediated by weak bases

Article abstract of DOI:10.1039/c7ob01572k

We report a new method of metal-free alkoxycarbonylation. This reaction involves the generation of aryl radicals from arenediazonium salts by a very weak base (HCO₂Na) under mild conditions. Subsequent radical trapping with carbon monoxide and alcohols gives alkyl benzoates. The conditions (metal-free, 1 equiv. base, MeCN, r.t., 3 h) tolerate various functional groups (I, Br, Cl, CF₃, SF₅, NO₂, ester). Mechanistic studies indicate the operation of a radical aromatic substitution mechanism.

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ARTICLE TYPE
Metal-Free Radical Aromatic Carbonylations Mediated by Weak Bases
Denis Koziakova and Axel Jacobi von Wangelina,*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
We report a new method of metalꢀfree alkoxycarbonylation. This reaction involves the generation
of aryl radicals from arenediazonium salts by a very weak base (HCO2Na) under mild conditions.
Subsequent radical trapping with carbon monoxide and alcohols gives alkyl benzoates. The
conditions (metalꢀfree, 1 equiv. base, MeCN, r.t., 3 h) tolerate various functional groups (I, Br, Cl,
10 CF3, SF5, NO2, ester). Mechanistic studies indicate the operation of a radical aromatic substitution
mechanism.
Introduction
Benzoates are key structural motifs of numerous fine
chemicals, agrochemicals, pharmaceuticals, materials, and
15 natural products.1 The transition metalꢀcatalyzed alkoxyꢀ
carbonylation of aryl electrophiles with carbon monoxide and
alcohols is the most versatile synthesis method for benzoates
(Scheme 1a).2 On the other hand, related metalꢀfree
carbonylations are rare.3 Lei and coꢀworkers reported a radical
20 tertꢀbutoxycarbonylation of aryl iodides mediated by
KOtBu/phenanthroline (Scheme 1b).4 This reaction is most
likely initated by singleꢀelectron transfer from extended
heteroaromatic radical anions that form upon baseꢀmediated
thermal condensation. However, severe limitation arise from
25 the exclusive production of tertꢀbutyl benzoates, the harsh
conditions, and the little mechanistic insight.5 Our group and
Xiao et al. simultaneously developed photoredoxꢀcatalyzed
alkoxycarbonylations of arenediazonium salts with wide scope
under mild conditions.6 We believed that a robust and
30 versatile method of alkoxycarbonylation would enable the
reaction of easily accessible aryl electrophiles with various
primary, secondary, and tertiary alcohols under mild
conditions with the aid of a cheap initiator. Arenediazonium
salts constitute an especially versatile class of aryl electroꢀ
35 philes that display distinct reactivity from aryl halides, can
easily be prepared from anilines, and undergo ready
degradation to aryl radicals in the presence of reductants and
bases.7 The formation of aryl radicals from arenediazonium
salts with very weak bases under ambient conditions was
40 demonstrated in the context of several radical aromatic
substitution reactions.7b,8 The application of arenediazonium
salts to radical alkoxycarbonylations were recently realized in
photoredoxꢀcatalyzed processes (Scheme 1c).6 We surmised
that the sequential combination of both reactivity concepts, i)
45 the mild generation of aryl radicals from arenediazonium salts
and very weak bases and ii) the facile trapping of aryl radicals
with carbon monoxide and alcohols, would allow for an
expedient method of alkoxycarbonylation of arenes and
obviate the need for radical initiators, metal catalysts, inert
50 conditions, or irradiation equipment (Scheme 1d).9
Scheme 1 Alkoxycarbonylation of aryl electrophiles.
Method development
Following earlier work on radical aromatic substitutions,6a,10
55 we investigated the efficacy of weak carboxylates as
initiators11 of radical carbonylations. We aimed at the use of
inorganic bases as they are cheap, environmentally benign,
and can be easily removed from the organic products. The
initial model reaction of the white crystalline 4ꢀnitrobenzeneꢀ
60 diazonium tetrafluoroborate (1) with CO and tertꢀbutanol
cleanly afforded the tertꢀbutyl ester 2 (Table 1). It is
important to note that transition metalꢀcatalyzed alkoxyꢀ
carbonylations mostly fail to deliver tertꢀbutyl esters due to
the steric bulk of the alcohol. Coꢀsolvents were added to assist
65 the dissolution of the arenediazonium salt 1. A survey of
inexpensive bases showed highest activity and selectivity with
stoichiometric amounts of sodium formate which resulted in
minimal hydrodediazonation toward 3.10b No reaction was
observed in the absence of base. Importantly, the reaction
70 displayed no sensitivity toward air and moisture. The
optimized conditions involved equimolar sodium formate and
1 (~0.1 M) in a mixture of tꢀbutanol/acetonitrile (10/1) under
pressurized CO (50 bar) for 3 h at room temperature to give
tꢀbutyl 4ꢀnitrobenzoate (2) in 80% yield (entry 1).
75
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Table 1 Optimization experiments.a
Table 2 Carbonylation of arenediazonium salts with tertꢀbutanol.a
Aryl
Yield [%]
Aryl
Yield [%]
4ꢀO2NꢀC6H4
4ꢀBrꢀC6H4
4ꢀClꢀC6H4
4ꢀIꢀC6H4
75
63
66
42
50
61
56
51
4ꢀMeOꢀC6H4
3ꢀBrꢀC6H4
3ꢀClꢀC6H4
44
52
60
49
54
<5
<5
54
Entry Base
Solvent (mL) Equiv. tBuOH Yield 2 / 3 [%]
1
HCO2Na
MeCN (0.4)
MeCN (0.4)
MeCN (0.4)
MeCN (0.4)
MeCN (0.2)
MeCN (3)
100
100
100
100
100
1
80 / 9
2
AcONa
BzONa
K2CO3
71 / 19
71 / 10
71 / 8
3
2ꢀMeOꢀC6H4
2ꢀF3CꢀC6H4
2ꢀO2NꢀC6H4
2ꢀMeO2CꢀC6H4
1ꢀnaphthyl
4
4ꢀF3CꢀC6H4
4ꢀMeꢀC6H4
4ꢀMeO2CꢀC6H4
4ꢀF5SꢀC6H4
5
HCO2Na
HCO2Na
NaH
76 / 9
6
8 / 3
7
MeCN (0.4)
MeCN (0.2)
MeCN (0.2)
MeCN (0.2)
AcMe (0.2)
DMF (0.2)
H2O (0.2)
100
66
24 / 15
61 / 17
22 / 18
3 / 26
8b
9b
10b
11b
12b
13b
14b
a
HCO2Na
nBuLi
66
a Conditions: arenediazonium tetrafluoroborate (0.9 mmol), HCO2Na (0.9
40 mmol), tBuOH (9 mL), MeCN (1.2 mL), CO (50 bar), r.t., 3 h.
H2NNMe2
HCO2Na
HCO2Na
HCO2Na
HCO2Na
66
66
58 / 18
26 / 25
18 / 20
40 / 18
66
66
MeCN (0)
66
Conditions: 4ꢀNitrobenzenediazonium tetrafluoroborate (0.3 mmol),
base (0.3 mmol), r.t., CO (50 bar), 12 h, GC yields vs. internal 1ꢀdodeꢀ
canenitrile; b CO (30 bar).
5
The optimized conditions were applied to a set of diverse
arenediazonium salts (Table 2). Halides, esters, CF3, SF5, and
naphthyl substituents were tolerated. Especially the high
chemoselectivity in the presence of aryl halide entities under
the reaction conditions is a major advantage over metalꢀ
10 catalyzed protocols. With 2ꢀnitro and 2ꢀcarboxymethyl
substituents, only very little product formation was observed.
Under similar conditions, methanol, ethanol, and iꢀpropanol
were reacted as simplest representatives of nꢀ and sꢀalcohols
(Scheme 2). Generally, alkoxycarbonylations of 4 afforded the
15 highest yields with MeOH, EtOH, and iPrOH while the more
electrophilic 1 showed higher reactivity with tBuOH. Despite
the good solubility of arenediazonium salts in these alcohols,
the addition of acetonitrile as coꢀsolvent was shown to lower
the rate of the hydrodediazonation pathway.10b Generally, the
20 yields of ethyl and iꢀpropyl esters were slightly lower than
those of the methyl esters which is a consequence of the
higher propensity of the former to engage in hydrogen atom
transfer reactions. Likewise, other secꢀalcohols gave the
benzoates in moderate yields and the corresponding ketones
25 from competing alcohol oxidation (Scheme 3). For shortꢀchain
alcohols, the resultant oxidized byꢀproducts are volatile and
therefore require no laborious separation from the benzoates.
In the case of tertiary alcohols, H atom transfer operated
exclusively from the αꢀposition of acetonitrile. Deuterium
30 labelling in the methoxycarbonylation of 4ꢀmethoxybenzeneꢀ
diazonium tetrafluoroborate (4) with d4ꢀmethanol, sodium d1ꢀ
formate, and d3ꢀacetonitrile, respectively, revealed the
significant deuterium atom transfer from the solvent in
methoxycarbonylations (Scheme 4). This trend was supported
35 by experiments with d3ꢀMeCN, tꢀBuCN, and d6ꢀacetone as coꢀ
solvents which enhanced the selectivity of the alkoxyꢀ
carbonylation.
Scheme 2 Methyl, ethyl, and iꢀpropyl benzoate syntheses.
Scheme 3 Alkoxycarbonylations with secꢀalcohols.
2
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was already reported earlier.7b,8 Nucleophilic attack of the
20 formate onto the diazonium terminus most likely generates a
diazoformate adduct which decomposes to an aryl radical.8
Consistent with this ionic initiation step, the use of sodium
trifluoroacetate instead of sodium formate gave identical
reactivity with 1, whereas the much less electrophilic 4 was
25 unreactive with the less nucleophilic NaO2CCF3. With the
radical trap 2,2,6,6ꢀtetramethyl piperidinyloxyl (TEMPO),
rapid formation of the aryl and aroyl adducts was observed
under reaction conditions. We postulate that the employed
base affects the acidꢀbase equilibrium of the alcohol toward
30 the more nucleophilic alkoxide. However, acidꢀcatalysis could
enhance the electrophilicity of the aroyl radical. The direct
attack of the alcohol/alkoxide onto the aroyl radical
distinguishes this mechanism from carbonylations of organoꢀ
halides (mostly iodides) which operate via the acyl halide
35 intermediate.9 Both reactivity patterns can most effectively be
provided by a buffered system comprising of a weak base.
The potentially good reducing power of the intermediate aroyl
radical and dioxaꢀsubstituted benzyl radical suggests the
operation of a radical chain mechanism.13 Besides the initial
40 aryl radical formation, the base sodium formate could induce
(minor) deprotonation to give the alcoholate R’Oꢀ and/or the
intermediate ketylꢀtype radical anion. The consumption of the
base was indirectly proven by a steady decrease of the pH
over the course of the reaction (to pH 4 upon completion).
Scheme 4 Competing hydrodediazonation by αꢀH atom transfer.
Nucleophilicity !
detected by ESI-MS (Ar = 4-MeOPh)
A methodological extension of the alkoxycarbonylation was
realized with tethered alkenes and alkynes (Scheme 5).
Reactions of 2ꢀallyloxybenzenediazonium tetrafluoroborate
afforded 2,3ꢀdihydrobenzofuranꢀ3ꢀyl acetates by a sequence
of aryl radical formation, intramolecular 5ꢀexo ring closure,
and alkoxycarbonylation. The related propargyl derivative
cleanly gave the benzofuranꢀ3ꢀyl acetate after acidꢀmediated
5
10 olefin isomerization during workup.6b The latter reaction
constitutes an inexpensive alternative to the palladiumꢀ
catalyzed Heck olefination of the same substrate.12
Increase of pH
Buffered system
45
Scheme 6 Postulated mechanism of the formateꢀmediated
alkoxycarbonylation of arenediazonium salts.
Conclusion
We have developed a new alkoxycarbonylation protocol that
50 affords alkyl benzoates from arenediazonium salts, carbon
monoxide, and alcohols. This method provides a useful
alternative to metalꢀcatalyzed carbonylation reactions. The
competition of hydrogen atom transfer can be reduced by the
choice of alcohols and solvents. The radical reaction
55 mechanism is most likely initiated by the ionic reaction
between the aryl electrophile and sodium formate under mild
conditions. The intermediacy of aryl and aroyl radicals was
shown by radical trapping experiments. Further applications
of the concept of weak baseꢀinitiated radical cascade
60 processes to other substrates are actively being pursued in our
laboratories.
Scheme 5 Sequential cyclizationꢀalkoxycarbonylation reactions.
15 Key mechanistic studies were performed that support the
notion of a radical pathway involving intermediate aryl and
acyl radicals (Scheme 6). The initiation of the arenediazonium
ion degradation to reactive aryl radicals by very weak bases
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J.ꢀR. Chen and W.ꢀJ. Xiao, Angew. Chem. Int. Ed., 2015, 54,
2265−2269.
Experimental Section
7
General procedure for base-induced alkoxycarbonylation. A
20 mL vial was charged with a magnetic stir bar, the areneꢀ
diazonium salt (0.9 mmol), sodium formate (0.9 mmol),
acetonitrile (1.2 mL for reactions with tꢀBuOH, 9 mL for
other alcohols), the alcohol (9 mL of tꢀBuOH, 45 mmol of
other alcohols) and capped with a snapꢀon lid. The lid was
punctured with a needle and transferred to a highꢀpressure reactor
(ParrTM). The reactor was sealed, placed on a magnetic stirring
10 plate, and slowly pressurized with CO up to 50 bar. After 3 h,
the gas was slowly released and the vial retrieved. The
reaction was quenched with water (5 mL) to give an emulsion
which was extracted with ethyl acetate (3×5 mL), and the
organic phases were washed with brine (5 mL) and dried
15 (MgSO4). The solvents were evaporated in vacuo, and the
residue was purified by flash column chromatography (SiO2,
pentane/ethyl acetate mixtures 100/0 to 100/20).
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Acknowledgements
D.K. thanks the Bayhost program of the State of Bavaria and
20 the Fonds der Chemischen Industrie (FCI) for doctoral
fellowships. Inspirations for this work came from the
Graduate College “Photocatalysis” (GRK 1626) of the
Deutsche Forschungsgemeinschaft (DFG). We thank Dr.
Michal Majek for intellectual support.
9
25 Notes and references
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Table of contents:
A radical aromatic substitution mechanism initiated by very weak bases effects the synthesis of
benzoates under pressurized CO atmosphere.