Silver-Catalyzed Cross-Coupling of Isocyanides and Active Methylene Compounds by a Radical Process

Article abstract of DOI:10.1002/anie.201504254

Isocyanides are versatile building blocks, and have been extensively exploited in C-H functionalization reactions. However, transition-metal-catalyzed direct C-H functionalization reactions with isocyanides suffer from over-insertion of isocyanides. Repor

Full text of DOI:10.1002/anie.201504254

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International Edition: DOI: 10.1002/anie.201504254
German Edition: DOI: 10.1002/ange.201504254
Radical Reactions
Silver-Catalyzed Cross-Coupling of Isocyanides and Active Methylene
Compounds by a Radical Process
Jianquan Liu, Zhenhua Liu, Peiqiu Liao, Lin Zhang, Tao Tu, and Xihe Bi*
Abstract: Isocyanides are versatile building blocks, and have
À
been extensively exploited in C H functionalization reactions.
À
However, transition-metal-catalyzed direct C H functionali-
zation reactions with isocyanides suffer from over-insertion of
isocyanides. Reported herein is a radical coupling/isomeriza-
tion strategy for the cross-coupling of isocyanides with active
methylene compounds through silver-catalysis. The method
solves the over-insertion issue and affords a variety of
otherwise difficult to synthesize b-aminoenones and tricarbo-
nylmethanes under base- and ligand-free conditions. This
À
report presents a new fundamental C C bond-forming reac-
tion of two basic chemicals.
C
arbon–carbon bond formation is fundamentally important
in organic synthesis. Transition-metal-catalyzed functionali-
À
zation of C H bonds is emerging as a very powerful method-
À
ology for C C bond formation and has received a lot of
À
Figure 1. Isocyanides in direct C H functionalization through a cou-
pling/isomerization sequence.
attention in recent years.[1] Despite the substantial advances,
À
development of novel C C coupling reactions of two basic
chemicals is still highly appealing. Isocyanides are versatile
building blocks in organic synthesis because of the carbene-
like reactivity of the isocyano group.[2] Besides the consistent
prosperity of isocyanides in multicomponent reactions,[3] they
hydes,[9] and indoles;[10] and 3) the imidization of terminal
alkynes.[11] All these reactions require either specific align-
ment of substrate structure or special catalysts, and moreover
À
À
recently have been extensively exploited in C H functional-
Csp3 H functionalization is restricted to carbamoylation of the
ization reactions.[4] In this context, the uncontrollable over-
insertion of isocyanides is a serious issue for transition-metal-
tertiary amines.[8] Therefore, the discovery of an effective
strategy which is capable of expanding the repertoire of
À
À
catalyzed intermolecular C H functionalization (Fig-
isocyanide-involved C H functionalization, in particular for
ure 1a).[2a,5] As a result, isocyanide-involved C H function-
Csp3 H bonds, remains highly desirable.
À
À
alizations reported so far have usually appeared as an intrinsic
part of tandem cyclization reactions, and represent a valuable
methodology for assembling various hetero- and carbocyclic
compounds.[6] In stark contrast, reports on transition-metal-
After the gold rush[12] and traditional copper catalysis,[13]
silver catalysis is emerging as an active area in organic
synthesis,[14] particularly in radical reactions.[15] Herein, we
report a novel radical coupling/isomerization strategy for the
À
À
catalyzed C H functionalization for cross-coupling reactions
C C cross-coupling of isocyanides and active methylene
of isocyanides, without subsequent cyclization, are consider-
ably rare.[7–11] Hitherto, only three such reactions have been
established: 1) the cyanation of indoles and 2-aryl/-alkenyl
pyridines;[7] 2) the carbamoylation of tertiary amines,[8] alde-
compounds.[16] The over-insertion of isocyanides, which is
most frequently encountered in the transition-metal-cata-
À
lyzed direct C H functionalization with isocyanides, can be
skillfully avoided by this strategy. During this manuscript
preparation, Hong et al. reported a formal coupling/isomer-
À
ization of isocyanides and benzyl cyanides, in which a new C
C bond is formed indirectly by addition of a carboanion to an
[*] J. Liu, Z. Liu, Dr. P. Liao, Dr. L. Zhang, Prof. X. Bi
Department of Chemistry, Northeast Normal University
Renmin Str. 5268, Changchun 130024 (China)
E-mail: bixh507@nenu.edu.cn
imidoyl copper intermediate (Figure 1b).[17] In our present
work, the C C coupling event directly occurs by a radical
À
addition process with the generation of an imidoyl radical
intermediate (Figure 1c). A variety of otherwise difficult to
synthesize b-aminoenones and tricarbonylmethanes are effi-
ciently prepared in an isocyanide-dependent approach, under
base- and ligand-free conditions.
The reaction between p-methoxyphenyl isonitrile (1a)
and ethyl acetoacetate (2a) was initially investigated by
varying the metal salts and solvents to optimize the protocol
Prof. X. Bi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (China)
Prof. T. Tu
Department of Chemistry, Fudan University
220 Handan Rd, 200433 Shanghai (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504254.
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Chemie
Table 1: Optimization of the reaction conditions.[a]
Entry
Cat.
Amount
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
AgOTf
Ag2O
AgF
30 mol%
30 mol%
30 mol%
30 mol%
30 mol%
10 mol%
30 mol%
30 mol%
30 mol%
30 mol%
30 mol%
30 mol%
30 mol%
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-Dioxane
CH3CN
25
43
51
76
AgOAc
Ag2CO3
Ag2CO3
Pd(OAc)2
CuI
Mn(OAc)3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
92
48 (52)[c]
0[d]
0[d]
9
0[d]
10
11
12
13
78
21
15
trace
DCE
DMSO
MeOH
[a] Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol), 1,4-Dioxane
(2.0 mL), Ag2CO3 (0.15 mmol), 808C for 3 h. [b] Yield of the isolated
product. [c] In the presence of 10 mol% Ag2CO3; yield calculated from
the 1H NMR spectrum and the amount of unreacted substrate 1a within
parentheses. [d] Resulting in an unidentified mixture. No 3a was
1
detected by H NMR analysis of the mixture. DCE=1,2-dichloroethane,
DMSO=dimethyl sulfoxide.
(Table 1). Only the silver salts were effective in catalyzing the
coupling of the isocyanide to the active methylene group, with
Ag2CO3 being the best and thus affording the N-p-methox-
yphenyl-b-aminoenone (3a) in 92% yield (entries 1–5). The
quantity of the Ag2CO3 catalyst played a crucial and apparent
role on the efficacy of the reaction. For instance, the reaction
efficiency was sharply diminished and an incomplete con-
version was observed when the amount of Ag2CO3 used was
reduced to 10 mol% (entry 6). Other metal salts such as
Pd(OAc)2, CuI, and Mn(OAc)3 were checked for catalytic
activities, and in stark contrast, all resulted in an unidentified
mixture, in which the desired b-aminoenone (3a) was not
detected by 1H NMR analysis (entries 7–9). After establishing
Ag2CO3 as the optimal catalyst, we focused on screening of
the solvents. Aprotic and polar solvents such as CH3CN,
DCE, and DMSO have a deleterious effect and afforded 3a in
15–78% yields (entries 10–12). The protic solvent MeOH
resulted in only trace amounts of product (entry 13). We
therefore decided to use Ag2CO3 (30 mol%) in 1,4-dioxane at
808C as the optimal reaction conditions.
With the optimal reaction conditions in hand, we explored
the substrate scope with regard to different active methylene
compounds and aryl isonitriles (Scheme 1). Delightfully, the
substrate scope is quite general, and a wide variety of active
methylene compounds, having various electron-withdrawing
groups (EWGs), and aryl isonitriles can be applied to this
protocol, thus affording the corresponding b-aminoenones in
good to excellent yields. All the reactions proceeded
smoothly when open to air and were completed within 3–
5 hours. For example, when 1a reacted with a variety of active
methylene compounds, in which the methylene moiety is
activated with two EWGs which are either the same or
Scheme 1. Scope with respect to the active methylene compounds and
aryl isonitriles.
different (carbonyl, nitro, cyano, and sulfonyl) groups,
participated in the coupling reactions smoothly to afford the
target N-aryl-b-aminoenones (3b–o) in 46–96% yields. The
structure and stereochemistry of 3h and 3r were unequiv-
ocally resolved by X-ray crystallographic analysis (see the
Supporting Information for details). Except for 3k, which is
a mixture of E and Z isomers in a 4:3 ratio, other substrates
with different EWGs (EWG1 ¼ EWG2) generated single
stereoisomers such as 3b–j and 3l–m. Their stereochemistry
was identified by NOE experiments. Interestingly, acetoace-
tamides (1h and 1i) resulted in the products 3h and 3i in the
Z-isomeric form. This unusual behavior is probably a result of
the propensity of the amide group to form hydrogen bonds.[18]
It is worth noting that, in addition to the acyclic active
methylene compounds, cyclic substrates were also capable
reacting using this protocol, and afforded the corresponding
products (3p–r) in nearly quantitative yields. Although it is
a little discouraging that the benzyl cyanide, a compound
successfully utilized in Hongꢀs work,[17] did not result in the
desired product 3s (benzyl cyanide was recovered), this result
clearly suggested a different mechanism is operative in the
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current silver-catalyzed reaction. Later, the substrate scope of
this reaction was extended to other aromatic isocyanides
without difficulty. The electronic properties of the aromatic
rings had no influence on either the stereoselectivities or
yields of the products 3aa–ak, although use of 2-isocyano-9H-
fluorene resulted in a slight decrease in the yield of
corresponding product (3ak). All the products were obtained
with an E configuration, and the stereochemistry was con-
firmed with the help of an NOE experiment involving 3ak. It
is worth mentioning that synthetic methods for enamines like
3, which having two different EWGs, are rare.[19]
loading (10 mol%), albeit affording product 3i with a slightly
decreased yield. The highly functionalized alkenes generated
by this protocol offer opportunities for further synthetic
manipulation to access functionalized heterocyclic motifs. For
instance, synthetic conversion of N-aryl-b-enaminamide (3i)
into the functionalized pyrazole 6 and isoxazole 7 were
attained in good yields by reacting with hydrazine hydrate and
hydroxylamine hydrochloride, respectively.
We then turned our attention to investigating the mode of
reactivity and effect on the coupling reaction in the case of
aliphatic isocyanides (Scheme 2). Gratifyingly, an unprece-
Control experiments were performed to gain insight into
the reaction mechanism (Scheme 3). The cross-coupling
Scheme 2. The reaction of b-ketoesters with aliphatic isocyanides.[32]
Scheme 3. Mechanistic investigations.
À
dented Csp3 H carbamoylation of b-ketoesters with tert-butyl
reaction between 1a and 2-tosylacetonitrile (2n) was com-
pletely suppressed by the addition of the radical inhibitor
2,2,6,6-etramethylpiperidine-N-oxyl (TEMPO) or 2,6-di-tert-
butyl-hydroxytoluene (BHT),[23] thereby suggesting that the
reaction goes through a free radical intermediate [Eq. (1)].
However we could not isolate adducts of the scavengers
formed from radical intermediates. Deuterium-labeling stud-
ies using [D]-2n unambiguously confirmed the active meth-
ylene compounds as the source of the hydrogen atom in the
product [Eq. (2)], wherein the erosion of deuteration in [D]-
3n is probably due to traces of water. Further, the possibility
of the solvent as the hydrogen source was excluded as no [D]-
3n can be isolated from the reaction performed in [D8]1,4-
dioxane [Eq. (3)]. Meanwhile, to investigate the effect of O2
on the carbamoylation reaction, we performed the reaction of
4a and 2a under an 18O2 atmosphere, and expectedly [18O]-5a
isocyanide (4a) was discovered, and led to a range of
tricarbonylmethanes (5a–h) in moderate to high yields.
Unfortunately, the use of other aliphatic isocyanides, which
are similar in structure to 4a (e.g., 4b and 4c), resulted in an
unidentified mixture. The structure of 5a was unambiguously
confirmed by the single-crystal X-ray diffraction analysis.
À
Therefore, we have developed a novel and efficient Csp3
H
carbamoylation of active methylene compounds.[2,4,20] This
finding is of great interest as the tricarbonylmethane structure
is a key structural motif for many antibiotics.[21] Herein we
provide an extremely simple way to access such kind of
valuable compounds.[22]
A large-scale (10 mmol) synthesis was carried out under
the silver-catalyzed conditions. Pleasingly, the reaction of 1a
and 2i proceeded smoothly, even with a reduced the catalyst
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À
was obtained in 86% yield with
a
high degree of
experiment [Scheme 3, Eq. (2)]. Regarding the Csp3 H car-
18O incorporation [Eq. (4)]. Furthermore, the yield of 5a
was sharply decreased to 23% (when the reaction was run
under an N2 atmosphere), thus confirming the participation of
oxygen in the reaction.[24] Importantly, the addition of either
the radical scavenger TEMPO or BHT to the reaction
mixture of 4a and 2a under an oxygen atmosphere resulted
in an unidentified mixture, without formation of 5a, therefore
suggesting a free radical process. Further, unlike H2O acting
as the oxygen source in previous reports on isocyanide-
involved carbamoylation,[25] the reaction of 4a with 2a does
not provided [18O]-5a in the presence of H218O [Eq. (5)].
Based on the results from the experimental investigations
and related literature precedents, a plausible reaction mech-
anism (Scheme 4) is proposed by using 2a as a model
substrate. In the radical initiation stage, the carbonate anion
bamoylation of 1a with 4a, oxygenation of B leads to the
hydroperoxide compound F. Afterwards, silver-promoted
decomposition of F generates the oxyanion intermediate G
and a hydroxyl radical.[29] The so formed hydroxyl radical
could abstract a hydrogen atom from 1a to generate A and
H2O for protonation of the intermediate G to produce the
target product 5a. All the reactions belonging to any of these
two categories proceed through silver(I)-initiated autocatal-
ysis, and finally termination of the radical species A to 1a by
oxidation of Ag(s) and subsequent proton abstraction from
AgHCO3, thereby leading to the regeneration of Ag2CO3
catalyst and completing the catalytic cycle.
The reason for the divergent isocyanide-dependent reac-
tions was further elucidated by theoretical calculations.[30] As
shown in Figure 2, the intermediate BtBu shows a smaller
oxidation potential than that of BPMP (0.35 versus 0.47), thus
Figure 2. Theoretical calculations on the oxidation potentials (V) and
HOMO energies (eV) at the B3LYP/6-311G(d,p) level of theory.
Calculated for 1,4-dioxane with a polarized continuum model.
suggesting it is easily subject to one-electron removal. In
addition, BtBu has much higher HOMO energy level than does
B
PMP (À5.19 versus À5.40),[31] and further supports the easier
oxidation of BtBu, because the destabilized HOMO would lead
to one-electron oxidation more smoothly.
Scheme 4. Plausible reaction mechanism.
In conclusion, an intriguing radical coupling reaction
between active methylene compounds and isocyanides has
been developed for the first time and involves silver catalysis.
The over-insertion of isocyanides commonly observed for
of Ag2CO3 abstracts a proton from 2a, thus leading to the
generation of a carboanion and AgHCO3, followed by further
oxidation of a carboanion by an Ag+ ion to yield the radical
intermediate A.[15e,26] In this process, Ag2CO3 plays the dual
role as base and one-electron oxidant. The oxidant behavior
of Ag2CO3 was explained by the often observed silver mirror
in the reactions. Meanwhile, the complex Ag2CO3(RNC)n
may be formed[27] and react with A to give the imidoyl
radical B.[6] The fate of B, to afford either the enamine 3a or
the tricarbonylmethane 5a, is directly linked to the type of
isocyanide. When the isonitrile is aromatic, for example 1a,
there are two possible paths for B to 3a conversion. One way
is direct abstraction of a hydrogen atom from 2a by
generating a new radical intermediate (A), thus resulting in
the formation of the imine intermediate C, which quickly
isomerizes to 3a as the final product. Alternatively, 1,2-H
migration may take place to deliver a more stable tricarbo-
nylmethenyl radical (D),[28] which also abstracts a hydrogen
atom from 2a to release the intermediate E, which eventually
isomerizes to 3a. These two reaction pathways all reasonably
account for the outcome observed for the deuterium-labeling
À
transition-metal-catalyzed C H functionalization with iso-
cyanides has been skillfully avoided by using a radical
coupling/isomerization strategy. A range of otherwise difficult
to synthesize b-aminoenones and tricarbonylmethanes was
efficiently prepared in an isocyanide-dependent approach. A
radical mechanism was proposed on the basis of preliminary
mechanistic investigations. This report presents a new funda-
À
mental C C bond-forming reaction of two basic chemicals.
Studies to expand its scope and apply it to the development of
multicomponent radical reactions are forthcoming.
Acknowledgements
Many thanks to Institute of Functional Material Chemistry,
Northeast Normal University for theoretical calculations.
This work was supported by NSFC (21372038, 21202016,
21172029), the Ministry of Education of the Peopleꢀs Repub-
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Keywords: cross-coupling · isocyanides · isomerization ·
radicals · silver
How to cite: Angew. Chem. Int. Ed. 2015, 54, 10618–10622
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[32] CCDC 1413526 (5a) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
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Received: May 10, 2015
Revised: June 8, 2015
Published online: July 24, 2015
10622 www.angewandte.org
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