Rhodium(III)-catalyzed alkenyl C-H bond functionalization: Convergent synthesis of furans and pyrroles

Article abstract of DOI:10.1002/anie.201207995

Ring in the new: A new annulation for the efficient synthesis of substituted furans and pyrroles is reported. The Rh^III-catalyzed reaction of O-methyl α,β-unsaturated oximes with aldehydes and N-tosyl imines affords secondary alcohol and amine intermediates, respectively. Cyclization and aromatization occurs under the reaction conditions to provide access to biologically relevant furans and pyrroles in good yields. Cp=C₅Me₅, DCE=1,2-dichloroethane, THF=tetrahydrofuran. Copyright

Full text of DOI:10.1002/anie.201207995

Angewandte
Chemie
DOI: 10.1002/anie.201207995
Annulation
À
Rhodium(III)-Catalyzed Alkenyl C H Bond Functionalization:
Convergent Synthesis of Furans and Pyrroles**
Yajing Lian, Tatjana Huber, Kevin D. Hesp, Robert G. Bergman, and Jonathan A. Ellman*
Furans and pyrroles are pervasive in natural products and
pharmaceuticals.[1] For this reason, the synthesis of these five-
membered ring heterocycles has been an intense topic for
decades[2] with transition-metal-based C H bond functional-
À
ization strategies recently providing powerful annulative
approaches for their assembly.[3,4] Specifically, Glorius and
co-workers[3a] and Fagnou and co-workers[3b] have reported on
the synthesis of pyrroles by rhodium(III)-catalyzed coupling
of enamides with internal alkynes [Eqs. (1) and (2); Cp* =
C5Me5], and most recently, Lei and co-workers have reported
on the synthesis of furans by the silver acetate mediated
oxidative coupling of terminal alkynes and 1,3-dicarbonyl
compounds [Eq. (3)].[3c] Herein, we report a new type of
À
catalytic C H bond functionalization with annulation, thus
enabling the versatile and efficient synthesis of furans and
pyrroles from simple electrophilic coupling partners which
serve as very different inputs from those used previously
[Eq. (4)].[5] Additionally, in contrast to the other reported C
H-functionalization-based annulations [Eqs. (1)–(3)], no
external oxidant is required.
À
bonds of enamides to isocyanates,[10m] and very recently, Shi
À
and co-workers reported alkenyl C H bond addition to N-
Over the past several years spectacular progress has been
À
tosyl imines and activated aldehydes using a pyridyl directing
achieved on the rhodium(III)-catalyzed addition of Csp2
H
group.[10f]
bonds across alkenes and alkynes.[6–9] Recently, we and others
We initiated our investigation by exploring the rhodium-
(III)-catalyzed addition of the a,b-unsaturated oxime 1a to
ethyl glyoxylate (2a; Table 1).[11] Although the use of 5 mol%
of [{Cp*RhCl2}2] and AgSbF6 with commercially available 2a,
À
have also actively explored rhodium(III)-catalyzed C H
À
À
bond additions across polarized C N and C O multiple
bonds.[10] Our proposed annulation approach for the synthesis
of furans and pyrroles in particular requires a rhodium(III)-
À
catalyzed alkenyl C H bond addition to imines and alde-
Table 1: Screening of reaction conditions.
À
hydes. However, few examples of the addition of alkenyl C H
À
À
bonds across C N and C O multiple bonds have been
reported. We first described the addition of alkenyl C H
À
[*] Y. Lian, T. Huber, K. D. Hesp, Prof. Dr. J. A. Ellman
Department of Chemistry, Yale University
225 Prospect St., New Haven, CT 06520 (USA)
E-mail: jonathan.ellman@yale.edu
Entry
Catalyst
Catalyst
loading
(mol%)
T [8C]
t [h]
Yield
[%][b]
1[c]
2
[{RhCp*Cl2}2], AgSbF6
[{RhCp*Cl2}2], AgSbF6
[{RhCp*Cl2}2], AgSbF6
[{RhCp*Cl2}2], AgSbF6
[Cp*Rh(CH3CN)3](SbF6)2
[RhCp*Cl2]2, AgSbF6
[{RhCp*Cl2}2], AgSbF6
[{RhCp*Cl2}2]
5
5
5
5
5
75
75
75
75
75
50
50
75
75
75
20
20
5
5
5
5
5
5
5
84
>99
>99
54
87
>99
89
0
0
>99
Prof. Dr. R. G. Bergman
Department of Chemistry, University of California, Berkeley
Berkeley, CA 94720-1416 (USA)
and
Lawrence Berkeley National Laboratory
1 Cyclotron Road, Berkeley, CA 94720 (USA)
3
4[d]
5
6
7
8
9
2.5
1.25
5
20
2.5
[**] This work was supported by the NIH Grant GM069559 (to J.A.E.).
R.G.B. acknowledges funding from The Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division, U.S. Department of Energy, under Contract DE-AC02-
05CH11231. K.D.H. is grateful to the National Sciences and
Engineering Research Council of Canada (NSERC) for a postdoc-
toral fellowship.
AgSbF6
[{RhCp*Cl2}2], AgSbF6
10
5
[a] Reaction conditions: 1a (0.10 mmol), purified ethyl glyoxylate (2a;
0.20 mmol) in 0.33 mL of DCE. [b] Determined by H NMR analysis
using 1,3,5-trimethoxybenzene as an internal standard. [c] Commercially
available 50% solution of ethyl glyoxylate in toluene used. [d] Used
1.0 equiv of ethyl glyoxylate. DCE=1,2-dichloroethane.
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207995.
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which is stored as a 50% toluene solution, afforded the
desired product 3a in good yield (entry 1), a quantitative yield
was observed when 2a was used in pure form (entry 2). The
reaction time could be shortened to 5 hours without a reduc-
tion in yield (entry 3). Using 2.0 equivalents of 2a is crucial
because the yield dropped significantly when only 1.0 equiv-
alent was employed (entry 4). The use of the preformed
cationic rhodium(III) precursor [Cp*Rh(CH3CN)3(SbF6)2]
slightly reduced the yield (entry 5). A lower catalyst loading
proved to be effective. At a 2.5 mol% catalyst loading,
complete conversion into product was maintained (entry 6),
while at a 1.25 mol% loading only a slight drop in yield was
observed (entry 7). Conversion into product was not observed
when either [{Cp*RhCl2}2] or AgSbF6 alone were used
(entries 8 and 9), which suggests that a cationic rhodium(III)
substituent, R3, is not essential for the success of the reaction,
thereby enabling the preparation of trisubstituted furans
(3h,i). We also explored variation at R1 and established that
different alkyl groups work well in this transformation (3e–g).
Varying the R2 substituent demonstrated that alkyl-, aryl-
(3h), and benzyl-substituted (3i) oximes are effective sub-
strates. However, a substituent at this position is crucial
because no product was obtained when R2 was H. To enhance
the practicality of the method, we performed the reaction on
the benchtop and found that the yield was comparable to that
observed when the reaction was set up in a glovebox under
inert atmosphere conditions (see 3a).
Having demonstrated a broad scope for coupling oximes,
bearing different substitution patterns, with 2a, we next
explored the possibility of extending this transformation to
less activated aldehydes. To apply the transformation to
aromatic aldehydes, or even to aliphatic aldehydes, the
À
species is required for this C H bond functionalization
process. Although reaction of 1a with 2a proceeds to
complete conversion within 5 hours at 508C (entry 6),
a longer reaction time and a higher temperature (758C) did
not result in any reduction in yield (entry 10). These more
forcing conditions were found to be necessary for less reactive
oxime substrates.
À
reversible nature of metal-catalyzed C H bond addition to
aldehydes must be considered (Scheme 1). In contrast to the
reaction of the electron-deficient and destabilized 2a, which
Having defined an effective catalyst system and reaction
conditions for the synthesis of furan 3a in high yield, we next
explored the substrate scope with a range of a,b-unsaturated
oximes (1) having different substitution patterns (Table 2).
Annulation of cyclic and acyclic trisubstituted oximes resulted
in fully substituted furan products in good yields (3a–g), with
cyclohexyl (3a,e,g) and cyclopentyl (3b) derivatives provid-
ing access to different bicyclic frameworks. The terminal
Table 2: Oxime substrate scope with ethyl glyoxylate.[a]
Scheme 1. Proposed reaction pathways.
likely favors formation of alcohol intermediates such as those
illustrated in Scheme 1, literature reports suggest that starting
materials are generally more stable than the addition
products when less-electron-deficient aldehydes are used.[12]
For unactivated aldehydes we therefore envisioned that
capture of the initial alcohol product by cyclization and
aromatization might be necessary to drive the reaction to
completion.[13]
We initially focused on the identification of suitable
reaction conditions for the efficient coupling of 4-trifluoro-
methylbenzaldehyde (5a) and the oxime 1b (Table 3). The
use of DCE as a solvent, which we have previously found to
À
be one of the most effective for rhodium(III)-catalyzed C H
À
À
additions to polarized C N and C O multiple bonds,
provided the desired furan 7a in poor yield (entry 1). A
range of solvents with different polarities and hydrogen-
bonding capabilities were therefore evaluated because differ-
ent solvation properties might be expected to affect the
equilibrium between the starting materials and the alcohol
intermediate, and could also impact the rate of cyclization and
aromatization. A variety of alcohols with different steric
properties and acidities were first explored, with CF3CH2OH
increasing the yield to 50% (entries 2–4). Evaluation of
aprotic solvents resulted in an interesting trend. Polar and
[a] Reaction conditions: 1 (0.20 mmol), purified ethyl glyoxylate (2a;
0.40 mmol) in 0.67 mL of DCE. The yield of isolated, purified material is
reported for each product. [b] Conducted in a Schlenk flask under N2 on
the benchtop.
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Table 3: Reaction optimization for aromatic aldehydes.[a]
Table 4: Substrate scope for other aldehydes.[a]
Entry
Solvent
Yield[b] [%]
1
2
3
4
5
DCE
EtOH
iPrOH
CF3CH2OH
CH3CN
DMF
23
trace
20
50
trace
17
6
7
8
9
EtOAc
1,4-dioxane
HOAc
63
53
78
10
THF
78
[a] Reaction conditions: 1b (0.10 mmol), aldehyde 5a (0.20 mmol) in
0.33 mL of solvent. [b] Determined by 1H NMR analysis using 1,3,5-
trimethoxybenzene as an internal standard. DMF=N,N-dimethylform-
amide.
strongly coordinating solvents, such as CH3CN or DMF
resulted in very low yields (entries 5 and 6). However, the use
of moderately coordinating solvents significantly improved
the yield (entries 7–10). Among the solvents screened, HOAc
and THF were the most effective, thus providing 7a in
excellent yield (78%).[14] THF was chosen for further
experimentation because it is more easily removed than
HOAc.
Having defined an effective solvent system for the syn-
thesis of furans from the aromatic aldehyde 5a, we next
explored a wider range of aldehydes (Table 4). Good func-
tional-group compatibility was demonstrated, with aromatic
aldehydes substituted with chloro, fluoro, trifluoromethyl,
methoxy, nitro, and ester groups all proving to be effective
coupling partners. Moreover, substitution at the ortho, meta,
and para positions of the aromatic ring was found to be well
tolerated (7a–h). Electron-poor aromatic aldehydes generally
afford furan products in higher yields (7a,d,e), but unacti-
vated aromatic aldehydes are also effective substrates
(7b,c,h). Moreover, the annulation of aliphatic aldehydes
proceeds in moderate yields (7i,j), which is notable because
[a] Reaction conditions: 1 (0.20 mmol), aldehyde 5 (0.40 mmol) in
0.67 mL of THF. The yield of the isolated, purified material is reported for
each product. [b] Conducted in a Schlenk flask under N2 on the
benchtop. [c] Using 10 mol% [{Cp*RhCl2}2].
À
few prior examples of rhodium(III)-catalyzed C H bond
additions to aliphatic aldehydes have been reported.[6l] The
reaction is not limited to oxime 1b, but is also effective for
other oxime substitution patterns (7k–m).
À
proceeds by C H bond activation. The annulation is initiated
À
With successful demonstration of this new type of
annulation for the synthesis of diverse furan derivatives, we
next investigated the possibility of extending this reaction to
pyrrole synthesis. The reaction of the oxime 1a with the N-
tosyl imine of ethyl glyoxylate (8) in DCE at 908C for
16 hours afforded the pyrrole 9a in reasonable yield
(Table 5). The reaction proved to be effective for a,b-
unsaturated oximes (1) having different substituents and
substitution patterns, but preliminary attempts to extend the
reaction to N-tosyl imines of aromatic aldehydes have not yet
been successful.
by the rhodium(III)-catalyzed addition of an alkenyl C H
=
=
bond across an aldehyde C O or imine C N bond with
subsequent cyclization and aromatization. This operationally
simple process is amenable to a number of different
substituents on the a,b-unsaturated oxime. Annulations with
ethyl glyoxylate and the corresponding N-tosyl imine provide
access to 2-carboxyethyl-substituted furans and pyrroles,
respectively. Moreover, a broad range of aromatic and
aliphatic aldehydes participate in the reaction to provide
access to an extensive group of differently substituted furans.
We are continuing to investigate this approach for the
synthesis of other five-membered ring heterocycles including
azoles.
In conclusion, a new type of annulation for the synthesis
of substituted furans and pyrroles has been developed and
Angew. Chem. Int. Ed. 2013, 52, 629 –633
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Table 5: Substrate scope for pyrrole synthesis.[a]
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À
[4] For recent general reviews on C H functionalization with
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À
[5] The connectivity obtained in the initial C H functionalization
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Ed. Engl. 1979, 18, 239.
[a] Reaction conditions: 1 (0.20 mmol), imine 8 (0.40 mmol) in 0.67 mL
of DCE. The yield of the isolated, purified material is reported for each
product. Ts=4-toluenesulfonyl.
À
[6] For recent reviews on rhodium(III)-catalyzed C H functional-
ization, see: a) T. Satoh, M. Miura, Chem. Eur. J. 2010, 16, 11212;
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2012, 41, 3651.
Experimental Section
[7] For selected rhodium(III)-catalyzed oxidative coupling of
alkenes, see: a) S. Rakshit, C. Grohmann, T. Besset, F. Glorius,
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2011, 50, 1064; c) F. Wang, G. Song, Z. Du, X. Li, J. Org. Chem.
2011, 76, 2926; d) S. H. Park, J. Y. Kim, S. Chang, Org. Lett. 2011,
13, 2372; e) X. Li, M. Zhao, J. Org. Chem. 2011, 76, 8530; f) J.
Chen, G. Song, C.-L. Pan, X. Li, Org. Lett. 2010, 12, 5426; g) N.
Umeda, K. Hirano, T. Satoh, M. Miura, J. Org. Chem. 2009, 74,
7094; h) K. Ueura, T. Satoh, M. Miura, Org. Lett. 2007, 9, 1407.
[8] For selected rhodium(III)-catalyzed annulation of alkynes, see:
a) M. V. Pham, B. Ye, N. Cramer, Angew. Chem. 2012, 124,
10762; Angew. Chem. Int. Ed. 2012, 51, 10610; b) B.-J. Li, H.-Y.
Wang, Q.-L. Zhu, Z.-J. Shi, Angew. Chem. 2012, 124, 4014;
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Besset, N. Kuhl, F. Glorius, J. Am. Chem. Soc. 2011, 133, 2154;
d) N. Guimond, S. I. Gorelsky, K. Fagnou, J. Am. Chem. Soc.
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Representative procedure: In a N2-filled glovebox, [{Cp*RhCl2}2]
(3.1 mg, 0.0050 mmol, 0.025 equiv), AgSbF6 (6.9 mg, 0.020 mmol,
0.10 equiv), the indicated O-methyl oxime (0.200 mmol, 1.0 equiv),
and ethyl glyoxylate (40.8 mg, 0.400 mmol, 2.0 equiv) were added to
a screw-capped conical vial with a stir bar. DCE (0.66 mL, [O-methyl
oxime] = 0.30m) was rhen added. The vial was sealed with a cap
containing a PTFE septum and was removed from the glovebox. The
reaction vial was then placed in a temperature-controlled oil bath at
758C. After 16 h of stirring, the vial was removed from the oil bath
and was cooled to ambient temperature. The mixture was filtered
through a pad of celite and the solvent was removed under reduced
pressure. The crude residue was loaded onto a silica gel column for
chromatographic purification (hexanes/ethyl acetate 20:1).
Received: October 3, 2012
Published online: November 22, 2012
À
Keywords: annulation · C H activation · heterocycles ·
rhodium · synthetic methods
.
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À
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À
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À
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