Metal-Free Oxidative Decarbonylative Coupling of Aliphatic Aldehydes with Azaarenes: Successful Minisci-Type Alkylation of Various Heterocycles

Article abstract of DOI:10.1002/adsc.201500268

A metal-free oxidative decarbonylative coupling of aliphatic aldehydes with various electron-deficient heterocycles has been developed. This reaction is supposed to be realized via a Minisci-type mechanism, based on the substrate scope, regioselectivity and radical trapping experiments. The ready availability of aliphatic aldehydes, metal-free conditions and broad substrate scope should make this method attractive for the late-stage alkylation of bioactive heterocycles.

Full text of DOI:10.1002/adsc.201500268

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DOI: 10.1002/adsc.201500268
Metal-Free Oxidative Decarbonylative Coupling of Aliphatic
Aldehydes with Azaarenes: Successful Minisci-Type Alkylation of
Various Heterocycles
Ren-Jin Tang,+a Lei Kang,+a and Luo Yanga,*
a
College of Chemistry, Xiangtan University, Xiangtan, Hunan 411105, Peopleꢀs Republic of China
Fax: (+86)-731-5829-2251; phone: (+86)-731-5829-8351; e-mail: yangluo@xtu.edu.cn
+
R.-J. Tang and L. Kang contributed equally to this work.
Received: March 17, 2015; Published online: June 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500268.
Abstract: A metal-free oxidative decarbonylative
coupling of aliphatic aldehydes with various elec-
tron-deficient heterocycles has been developed.
This reaction is supposed to be realized via a Minis-
ci-type mechanism, based on the substrate scope,
regioselectivity and radical trapping experiments.
The ready availability of aliphatic aldehydes, metal-
free conditions and broad substrate scope should
make this method attractive for the late-stage alky-
lation of bioactive heterocycles.
2
À
Scheme 1. Direct heterocyclic C(sp ) H bond alkylation.
Keywords: aldehydes; alkylation; decarbonylation;
heterocycles; metal-free conditions
tion by alkenes[8] catalyzed by late transition metals
has been developed as complementary method for
the alkylation of these azaarenes (Scheme 1, path b).
More recent developments on the addition of fluo-
roalkyl radicals to heterocycles, discovered by Baran
and co-workers,[9] have led to a renaissance in radical-
Nitrogen-rich heterocyclic compounds are abundant mediated alkylation. Alkylation of heterocycles using
3
À
in pharmaceutical and agricultural chemistry and C(sp ) H bonds as radical sources has also been re-
widely used as ligands in organic synthesis.[1] While ported,[6] which was only confined to simple symmet-
numerous preparative methods for these electron-de- rical alkanes to suppress regioisomers. Considering
ficient heterocycles have been developed, the further the difficulties and costs to remove the trace transi-
2
À
direct functionalization of the heterocyclic C(sp ) H tion metal contamination from the final products, es-
bonds is far less studied. In contrast to the facile alky- pecially for the late-stage functionalization of biologi-
lation of electron-rich arenes via Friedel–Crafts reac- cally active pharmaceuticals,[5a,10] a general method
tion, few methods are available for the alkylation of for the alkylation of heterocycles under metal-free
these azaarenes due to their innate electron-deficient conditions would be highly desired.
reactivity.[2] Among these methods, the addition of nu-
On the other hand, aldehydes are cheap and readily
cleophilic alkyl radicals to azaarenes, the so-called available bulk chemicals and have been directly used
Minisci reaction,[3–6] is a commonly used approach as precursors for (oxidative) decarbonylative cou-
(Scheme 1, path a). However, the classic Minisci reac- plings catalyzed by ruthenium or rhodium, as shown
tion has been hampered by limited precursors (car- by the extensive studies of Li and co-workers since
boxylic acids or alkyl halides), laborious procedures 2009.[11] However, in the absence of a transition metal
for the generation of alkyl radicals, and the usage of and assisted by a suitable oxidant, aldehydes can also
metal catalysts (Ag, for example) in up to stoichio- be converted into the corresponding acyl radicals for
metric amounts. In recent research the direct hetero- the hydroacylation of olefins,[12] oxidative acylation of
2
cyclic C(sp ) H bond activation and subsequent heterocycles[13] or other acylative reactions.[14] These
À
cross-coupling with alkyl (pseudo)halides[7] or inser- reports and our recent study[15] on the metal-free oxi-
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Ren-Jin Tang et al.
Table 1. Optimization of the metal-free oxidative decarbon-
dative decarbonylative coupling of aromatic alde-
hydes with benzene inspired us to conceive a similar
generation of alkyl radicals from aliphatic aldehydes
by further decarbonylation of these acyl radicals
under oxidative conditions. Actually, the decarbonyla-
tive alkylation of 4-methylquinoline by pivalaldehyde
was realized in 0.4% yield as reported by Minisci,[4d]
and much better yields were obtained in the decar-
bonylative radical NO2 substitution of b-nitrostyrene
reported by Yao et al.[16] As part of our ongoing inter-
ylative coupling.[a]
Entry [O] (equiv.)
Solvent
Temp. [8C] Yield [%][b]
À
est on the C H bond functionalization of azaar-
1
2
3
4
5
6
TBP (2.5)
TBHP (2.5)
DCP (2.5)
H2O2 (2.5)
BPO (2.5)
K2S2O8 (2.5)
benzene 130
benzene 130
benzene 130
benzene 130
benzene 130
benzene 130
57
<5
35
trace
18
26
28
38
54
52
51
67
73
82
89
67
75
65
66
54
enes,[17] herein we report a general metal-free oxida-
tive decarbonylative coupling of aliphatic aldehydes
with electron-deficient azaarenes, which was success-
fully applied to the late-stage alkylation of nitrogen-
rich natural heterocycles (Scheme 1, path c).
7
PhI(OAc)2 (2.5) benzene 130
Aldehydes can be easily transformed into acyl radi-
cals in the presence of radical initiators such as perox-
ide, dioxygen, hypervalent iodine(III) reagents and
others. While some previous reports tried to suppress
the decarbonylation side-reaction during the acylation
process, our recent study successfully combined the
aldehyde hydrogen abstracting and decarbonylation
sequences together to afford the aryl radicals, by
using TBP (di-tert-butyl peroxide) as the initiator and
oxidant. The aryl radical generated in such a way was
further proved by radical trapping experiments.
Hence we hypothesized that an alkyl radical could
also be generated in the same way, which could then
react with electron-deficient heterocycles to afford al-
kylated product through a Minisci-type mechanism.
With this in mind, the reaction between isoquino-
line (1a) and cyclohexanecarboxaldehyde (2a) was ex-
amined in the presence of TBP, which successfully af-
forded the corresponding alkylated product (1-cyclo-
hexanylisoquinoline, 3a) in medium yield (Table 1,
entry 1). During the subsequent optimization, we first
8[c]
9
TBP (2.5)
TBP (2.5)
TBP (2.5)
TBP (2.0)
benzene 120
benzene 140
benzene 150
benzene 130
benzene 130
benzene 130
10
11
12[d] TBP (2.5)
13[e] TBP (2.5)
14[e] TBP (2.5)
15[e] TBP (2.5)
16[e] TBP (2.5)
17[e] TBP (2.5)
18[e] TBP (2.5)
19[c,e] TBP (2.5)
20[c,e] TBP (2.5)
C6H5Cl
o-C6H4Cl2 130
C6H5F 130
o-C6H4F2 130
130
toluene
DCE
CH3CN
130
130
130
[a]
Conditions: 1a (0.2 mmol), 2a (3 equiv., 0.6 mmol), TBP
(2.5 equiv., 0.5 mmol), solvent (1.5 mL), reacted for 12 h
under air unless otherwise noted.
Isolated yields.
Acylation product 3a’ was detected (<5% yield).
2a (4 equiv., 0.8 mmol) was added.
[b]
[c]
[d]
[e]
2a (5 equiv., 1.0 mmol) was added.
screened several widely used radical initiators includ- a better yield of 82%, in o-dichlorobenzene it led to
ing TBP, TBHP (tert-butyl hydroperoxide), DCP (di- an optimal yield of 89%, and in other more polar sol-
cumyl peroxide), H2O2, BPO (benzoyl peroxide), vents such as DCE (dichloroethane) and CH3CN, the
K2S2O8 and PhI(OAc)2 (entries 1–7), and among them yield dropped to 66% and 54%, respectively (en-
TBP gave the best results. Next, the decarbonylation tries 19 and 20).
process was found to be dependent on the reaction
Under the optimized conditions (Table 1, entry 15),
temperature; reactions at 1308C or higher tempera- the substrate scope and limitation for this oxidative
tures would guarantee the complete decarbonylation decarbonylative coupling of isoquinoline (1a) with
and led to yields between 52–57%, while reactions at various aliphatic aldehydes were explored. The influ-
1208C afforded an obviously lower yield of 38%, ac- ence of the aliphatic aldehydes is listed in Table 2.
companied by the generation of acylation product 3a’ Both branched and linear aliphatic aldehydes were
(entries 8–10). An excess of aldehyde (2a) and TBP successfully transformed into the desired decarbony-
was required to compensate their consumption due to lated products in moderate to good yields. Among
aldehyde oxidation (to carboxylic acid and carboxylic them, the branched aldehydes (2a–2e) obviously af-
tert-butyl ester). Then, on further increasing the forded better yields than the linear ones (2f–2h),
dosage of aldehyde, the yield of alkylation product which could be explained by the fact that the
(3a) rose to 73% (entries 12 and 13). At last, the branched aldehydes have lower self-aldol condensa-
effect of solvents on this oxidative decarbonylative tion reactivity and also that the branched alkyl radi-
coupling was investigated. While the reaction pro- cals produced after decarbonylation are more stable.
ceeding in the low polarity chlorobenzene afforded Itꢀs worth noting that no alkyl chain rearrangement
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Metal-Free Oxidative Decarbonylative Coupling of Aliphatic Aldehydes
Table 2. The influence of aliphatic aldehydes on the metal-
Table 3. The influence of azaarenes on the metal-free oxida-
free oxidative decarbonylative coupling.[a]
tive decarbonylative coupling.[a]
[a]
Conditions: 1a (0.2 mmol), 2 (5 equiv., 1.0 mmol), TBP
(2.5 equiv., 0.5 mmol), ortho-dichlorobenzene (1.5 mL),
reacted for 12 h under air unless otherwise noted. Isolat-
ed yields.
1
products were detected by GC-MS and H NMR for
both the liner and branched aliphatic aldehydes
2
3
À
tested, which may imply that the C(sp ) C(sp ) bond
forming step was a fast step.
After investigating the scope of aliphatic aldehydes,
we next focused our attention on the scope of hetero-
cycles (Table 3). The scope turned out to be very
broad, and various six-membered rings, including iso-
quinolines (1b–1d), quinolines (1e and 1f), pyridines
(1g–1k), pyrazine (1l) and pyrimidine (1m) were ef-
fectively alkylated. The regioselectivity of different
heterocycles agreed well with that of the Minisci reac-
tion, and generally the most electron-deficient posi-
tions were alkylated, so pyridines could produce
a mixture of mono- and disubstituted products on
their C-2/C-6 positions (4g–4i, MAP=monoalkylation
product). By introducing one substitutent to the C-2
or C-6 position of pyridines thus leaving just one reac-
tive position, the corresponding monoalkylation prod-
uct (4j and 4k) was obtained smoothly. Similarly, pyri-
midine (1m) resulted in two regioisomers (4m) in
67% total yield and the ratio of C-2/C-4 was deter-
[a]
Conditions: 1 (0.2 mmol), 2a (5 equiv., 1.0 mmol), TBP
1
mined as 1:2.7 by integration of the H NMR spectra
(2.5 equiv., 0.5 mmol), ortho-dichlorobenzene (1.5 mL),
reacted for 12 h under air unless otherwise noted. Isolat-
ed yields.
of the crude products. For other heterocycles, substi-
tuted quinoxalines (1n–1p), quinazoline (1q), phthala-
zine (1r), benzothiazole (1s), benzoimidazoles (1t and
1u) and benzoxazole (1v) were used to react with cy-
clohexanecarboxaldehyde (2a) under the optimized
[b]
MAP=monoalkylation product.
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Scheme 2. Mechanistic investigation on the metal-free oxidative decarbonylative coupling.
conditions, which all afforded the alkylated products
(4n–4v) successfully. Gratifyingly, the bioactive caf-
feine (1w) and theophylline (1x) can also take part in
this oxidative decarbonylative coupling directly, which
showed the potential of this method for the late-stage
functionalization of electron-deficient natural hetero-
cycles.
This oxidative decarbonylative coupling fitted well
with various kinds of electron-deficient heterocycles
and the regioselectivity agreed with that of the Minis-
ci reaction. At the same time, peroxides are known as
good radical initiators for transforming aldehydes to
acyl radicals, and by-products coming from the further
decarbonylation of acyl radicals were also reported by
other research groups.[4d,16] Therefore, we speculated
that this reaction was realized through a Minisci-type
Scheme 3. Proposed mechanism for the metal-free oxidative
decarbonylative coupling.
Based on the literature reports[12] and our studies,
mechanism. To verify this speculation, further mecha- a plausible mechanism was proposed in Scheme 3,
nistic experiments were performed. First, TEMPO with the oxidative decarbonylative coupling of isoqui-
(2,2,6,6-tetramethylpiperidine-1-oxyl, 2.0 equiv.) as noline (1a) and cyclohexanecarboxaldehyde (2a)
a radical inhibitor was subjected to the standard pro- shown as an example. First, the homolytic cleavage of
cedure, which led to the total extinction of this reac- TBP forms a tert-butoxy radical, which abstracts the
tion (Scheme 2a), and the O-alkylated 1-(cyclohexyl- aldehyde hydrogen atom to provide the acyl radical I.
oxy)-2,2,6,6-tetramethylpiperidine 5 was isolated in Then, the acyl radical I undergoes decarbonylation to
17% yield. These results supported the Minisci-type afford alkyl radical II, which further adds to the elec-
mechanism and indicated the generation of alkyl tron-deficient isoquinoline (1a) to afford the radical
radicals. Furthermore, when 1,1-diphenylethene III. Due to the “persistent radical effect”,[18] the direct
(2.0 equiv.) as radical scavenger was added, the corre- tertiary hydrogen abstraction of radical III by a tert-
sponding decarbonylated alkyl radical was trapped to butoxy radical to realize re-aromatization might be
afford 6 and 6’ in 35% and 23%yields, respectively dynamically unbeneficial. Instead, radical III transfers
(Scheme 2b), which further strongly supported the an electron to the tert-butoxy radical and/or TBP to
generation of alkyl radicals by the decarbonylation of produce cation IV, which is further deprotonated by
aliphatic aldehyde during the reaction. Meanwhile, a tert-butoxide anion to afford the alkylated product
a control experiment demonstrated that the alkyl rad- 3a.
ical did not come from the corresponding aliphatic
In conclusion, we have developed a metal-free oxi-
acid through decarboxylation as in the classic Minisci dative decarbonylative coupling of aliphatic aldehydes
reaction (Scheme 3c), although the oxidation of ali- with various electron-deficient heterocycles to pro-
2
À
phatic aldehydes to aliphatic acids was possible duce C(sp ) H alkylated compounds. The substrate
during the reaction.
scope, regioselectivity and radical trapping experi-
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[4] For selected pioneering works of F. Minisci, see: a) F.
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ments supported the generation of the alkyl radical
by aldehydic hydrogen abstraction and decarbonyla-
tion sequences, therefore a Minisci-type mechanism
was proposed. The ready availability of aliphatic alde-
hydes, metal-free conditions and broad substrate
scope should make this oxidative decarbonylative
coupling attractive for the alkylation of heterocycles,
and even for the late-stage functionalization. Studies
on the further development and application of this
method are ongoing in our laboratory.
Experimental Section
Typical Procedure
An oven-dried reaction vessel was charged with isoquinoline
(1a, 0.2 mmol, 25.8 mg) and cyclohexanecarboxaldehyde (2a,
1.0 mmol, 112.2 mg), ortho-dichlorobenzene (1.5 mL), and
TBP (2.5 equiv., 0.5 mmol, 94 mL) under air. The vessel was
sealed and heated at 1308C (oil bath temperature) for 12 h.
The resulting mixture was cooled to room temperature,
transferred to a silica gel column directly and eluted with
petroleum ether and ethyl acetate (1:1) to give product 3a;
yield: 38.0 mg (89%).
[6] For Minisci-type alkylation of heterocycles with
3
À
C(sp ) H bonds as radical sources, see: a) A. P. Anton-
chick, L. Burgmann, Angew. Chem. 2013, 125, 3349;
Angew. Chem. Int. Ed. 2013, 52, 3267; b) Z. Wu, C. Pi,
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Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (21202138), Xiangtan University “Aca-
demic Leader Program” (11QDZ20), New Teachersꢀ Fund
for
Doctor
Stations,
Ministry
of
Education
(20124301120007), University Student Innovation Program,
Ministry of Education (201210530008), Project of Hunan
Provincial Natural Science Foundation (13JJ4047, 12JJ7002),
and Excellent Young Scientist Foundation of Hunan Provin-
cial Education Department (13B114).
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