Copper-catalyzed direct alkenylation of N-iminopyridinium ylides

Article abstract of DOI:10.1002/anie.200906020

(Figure Presented) A versatile Cu-catalyzed direct C-H alkenylation of N-iminopyridinium ylides, compatible with several different copper sources (including a penny), provides a powerful and inexpensive method for the synthesis of functionalized pyridine derivatives. Chemoselective functionalization of halide-containing compounds allows the synthesis of alkenyl pyridines containing reactive tethers for further functionalization

Full text of DOI:10.1002/anie.200906020

Angewandte
Chemie
DOI: 10.1002/anie.200906020
Copper Catalysis
Copper-Catalyzed Direct Alkenylation of N-Iminopyridinium Ylides**
James J. Mousseau, James A. Bull, and Andrꢀ B. Charette*
In memory of Keith Fagnou
The pyridine moiety is a privileged structure that is ubiquitous
in nature and is often a key component of pharmaceutically
active compounds.[1] As such, there has been much interest in
the synthesis of substituted pyridine derivatives. 2-Alkenyl
pyridine derivatives are of biological interest (Scheme 1) and
are important metal ligands. Furthermore, alkenyl pyridine
derivatives can be valuable precursors to alkyl pyridines and
piperidines, themselves of significant biological importance.[2]
seldom reported. Indeed, although electron-deficient arenes
have been reported, applications of these inexpensive metals
to the direct functionalization of heterocycles more often
employ electron-rich substrates.[6] Herein, we report the first
Cu-catalyzed direct alkenylation of N-iminopyridinium ylides
with inexpensive Cu salts.
Given our success with the direct arylation of N-imino-
pyridinium ylides, we first applied our optimal Pd-catalyzed
arylation conditions to alkenylation with (E)-b-styryl iodide
(2a).[7] These conditions gave the vinylated pyridinium 3a in
36% yield calculated by 1H NMR spectroscopy (Table S1,
entry 1; see the Supporting Information) and initial optimi-
zation did not lead to a significant increase in yield. It was
reasoned that the addition of a Lewis acid could increase the
reactivity of the pyridinium by coordination to the N-benzoyl
moiety. Indeed, the addition of CuBr to the Pd-catalyzed
reaction increased the yield of 3a to 63% (Table S1, entry 2).
In light of this result, we postulated that Cu alone might
promote the transformation. Gratifyingly, CuBr displayed
superior reactivity to Pd(OAc)2 (Table S1, entry 3). Decreas-
ing the ligand loading gave a higher yield (Table S1, entries 4
and 5), and the reaction was successful in the absence of an
external ligand (Table S1, entry 6), presumably because the
Lewis basic N-benzoyl moiety acts as an intramolecular
stabilizing group.
Scheme 1. Biologically important 2-alkenyl or 2-alkyl pyridine deriva-
tives.
Direct arylations have become increasingly important and
greener alternatives to traditional cross-coupling reactions.[3]
Recent work has shown that such reactions are applicable to
activated pyridinium species.[4] In contrast, comparatively
little has been published on direct alkenylation reactions; in
particular, there are few examples of direct vinylations on
activated pyridines. In these cases, symmetrical alkynes with
little functionality or relatively simple Heck acceptors were
required as the coupling partners.[5] Cognizant of these
deficiencies, we believed that N-iminopyridinium ylides
could provide access to functionalized 2-alkenyl pyridines
by a direct alkenylation reaction with functionalized vinyl
iodides.
Encouraged by these results, we optimized the reaction
without any external ligand. The alkenylation was insensitive
to the source of Cu; most CuII and CuI salts were compatible,
as well as Cu0 dust, and even a penny. In fact, the reaction
could be performed in a Cu vessel without any additional
1
metal source to provide a 74% yield calculated by H NMR
spectroscopy. CuBr2 was chosen because of its low cost and
high stability (Table S1, entry 7). The catalyst loading could
be decreased to 2.5 mol% and still provide good yields,
although 10 mol% were chosen for improved results. Aro-
matic and ethereal solvents both gave excellent results.
Chlorobenzene was chosen because of its lower volatility.
The yield of 3a increased to 85% when 1.5 equiv of the ylide 1
was used, because of statistical suppression of the formation
of the 2,6-divinylated by-product (Table S1, entries 9 and 10).
Owing to the low cost and high efficiency of K2CO3 in the
reaction, we elected to continue with this base, and 2 equiv
were sufficient to enable the reaction [Eq. (1); Bz = benzoyl].
Direct reactions often employ expensive metal catalysts
(Pd or Rh). Less costly Fe and Cu catalytic systems are
[*] J. J. Mousseau, Dr. J. A. Bull, Prof. A. B. Charette
Department of Chemistry, Universitꢀ de Montrꢀal
P.O. Box 6128 Stn Downtown, Montrꢀal, Quꢀbec, H3C 3J7 (Canada)
Fax: (+1)514-343-5900
E-mail: andre.charette@umontreal.ca
[**] This work was supported by the Natural Science and Engineering
Research Council of Canada (NSERC), Merck Frosst Canada Ltd.,
Boehringer Ingelheim (Canada), Ltd., the Canada Research Chairs
Program, the Canadian Foundation for Innovation, and the
Universitꢀ de Montrꢀal. J.J.M. is grateful to FQRNT for a
postgraduate scholarship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906020.
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The scope of the Cu-catalyzed reaction was found to be
the dipyridinium adduct in synthetically useful yields
(Table 1, entry 5). The reaction was successful with both
electron-rich (Table 1, entries 6–9) and electron-poor sub-
strates (Table 1, entries 10–12). Perhaps the most striking
feature of the reaction is its chemoselectivity. Halogen
substituents (Cl, Br, I; Table 1, entries 13–16) were tolerated
on the phenyl ring with the reaction occurring selectively on
the alkenyl iodide, and no arylated product was detected.
These halogenated compounds (3m–3q) are of particular
quite general (Table 1).[8] Unsubstituted arenes present on the
alkene reacted well (Table 1, entries 1 and 2). The excess ylide
employed in the reaction could be recovered to give 93%
yield based on recovered starting material (Table 1, entry 1).
The alkenylation tolerated substitution on the phenyl func-
tionality with very good yield (Table 1, entry 3) and also on
the alkene, where the group is less removed from the reactive
site (Table 1, entry 4). Bis(vinyl iodide) 2e also reacted to give
Table 1: Scope of the alkenyl iodide in the Cu-catalyzed reaction.[a]
Entry
Alkenyl Iodide
Product
Yield
[%][b]
Entry
12
Alkenyl Iodide
Product
Yield
[%][b]
1
81 (93)[c]
83
74
65
74
47
2
81
78
48
63
13
14
15
16
3
4
5[d]
6
7
8
9
87
71
93
75
17
18
19
20
41
53
52
30
10
11
71
89
21
22
71
41
[a] Reaction conditions: 1 (1.5 equiv), 2 (1.0 equiv), CuBr2 (10 mol%), K2CO3 (2 equiv), PhCl (0.2m), 1258C, 16–24 h. Bn=benzyl, PMB=p-
methoxybenzyl. [b] Yield of isolated product. [c] Yield based on recovered starting material. [d] 3.1 equiv of ylide 1.
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interest, as they contain reactive handles and, as such, could
be used as a foundation for the synthesis of more-complex
molecules. Substituents on the alkene with sp3 hybridization
gave the corresponding alkenylated pyridinium species in
moderate yields (Table 1, entries 18 and 19). Reactants with
long alkyl chains displayed poorer reactivity, as iodide 2t gave
compound 3t in 30% yield (Table 1, entry 20). However,
these molecules are still of interest and may be used as
scaffolds to build compounds such as CGS23113
(Scheme 1).[9] Z-Alkenes are reactive, yielding products
with trans configuration, possibly as a result of thermal
isomerization to the more stable alkene (Table 1, entries 21
and 22).
breaking event is not rate-limiting. Furthermore D exchange
studies (Table 3) clearly demonstrate a directed D incorpo-
ration to the 2,6-positions of the pyridinium ring in the
presence of Cu (Table 3, entries 2 and 4)[10] compared with
Table 3: Deuterium incorporation study.
Entry Metal/base
% D incorpora- LCMS Product
tion[a]
[M+1]
The scope of the pyridinium ylide was considered next
(Table 2). Pyridinium species with substituents at the 2-, 3-,
and 4-positions underwent the alkenylation in good yields
1
2
none
0
199.2
201.2
202.2
201.2
1
7
8
7
CuBr2 (10 mol%)
K2CO3 (2 equiv)
CuBr2 (10 mol%), K2CO3
(2 equiv)
n.a.
90
92
3[b]
4
Table 2: Scope of the pyridinium ylide in the Cu-catalyzed reaction.[a]
[a] Incorporation determined by 1H NMR spectroscopy. [b] Equal D
insertion in 2,4,6-positions.
Entry Pyridinium
Iodide
Product
Yield [%][b]
1
64
indiscriminate D incorporation when no CuBr2 was employed
(Table 3, entry 3). This result, coupled with the fact that the
direct alkenylation proceeds equally well with a range of Cu
salts in different oxidation states, led us to propose the
following catalytic cycle (Scheme 3): We believe that CuII is
2
72
76
49
3
4[c]
[a] Reaction conditions: 1 (1.5 equiv), 2 (1.0 equiv), CuBr2 (10 mol%),
K2CO3 (2 equiv), PhCl (0.2m), 1258C, 16–24 h. [b] Yield of isolated
product. [c] CuBr used in place of CuBr2.
(Table 2, entries 1–3). In the case of the 3-substituted 1b,
vinylation occurred exclusively at the 6-position. This meth-
odology could also be applied to other N-imino ylides;
quinolinium species 1d (Table 2, entry 4) afforded the
vinylated product in moderate yield. It is important to note
Scheme 3. Proposed catalytic cycle.
À
that the N N bond could be cleaved in the presence of the
alkene in very good yields (77–81%) to provide the corre-
sponding 2-alkenylated pyridines (Scheme 2).
Next, we wished to gain insight into the mechanistic
pathway of the reaction. Kinetic isotope effect (KIE) studies
reduced to CuI by the pyridinium ylide (A). In cases where
Cu0 is operative, the ylide may add into the copper to generate
a CuII intermediate that yields the reactive CuI species
through either reduction or disproportionation.[11] Next, there
is a carbonate–bromide exchange to generate CuCO3, which
is likely to be the reactive species in all cases, regardless of the
Cu source (B).[11] This species can undergo deprotonation/
metalation onto the pyridinium to generate an organocupra-
cycle stabilized by the Lewis basic iminobenzoyl moiety (C).
Oxidative addition (D) into the alkenyl iodide and subse-
quent reductive elimination (E) affords the product.
À
yielded a KIE value of 1.45, indicating that the C H bond
À
Scheme 2. Cleavage of the N N bond.
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Communications
[4] a) A. Larivꢁe, J. J. Mousseau, A. B. Charette, J. Am. Chem. Soc.
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In summary, we have described the first ligand-free Cu-
catalyzed direct alkenylation of electron-deficient heteroar-
enes. The reaction is believed to pass through a CuI/CuIII
catalytic cycle to provide a range of products in good to
excellent yields. The reaction is highly chemoselective
towards alkenyl iodides, introducing the possibility of prepar-
ing a scaffold from which a library of biologically interesting
compounds could be constructed. Work is continuing to
determine the full extent of these reactions as well as a
detailed mechanistic analysis of the process.
Received: October 26, 2009
Published online: December 22, 2009
Keywords: bioactivity · copper · heterocycles ·
homogeneous catalysis · vinylidene ligands
.
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