Regioselective formation of α-vinylpyrroles from the ruthenium-catalyzed coupling reaction of pyrroles and terminal alkynes involving C-H bond activation

Article abstract of DOI:10.1021/jo100269y

The cationic ruthenium catalyst Ru₃(CO)₁₂/NH ₄PF₆ was found to be highly effective for the intermolecular coupling reaction of pyrroles and terminal alkynes to give gem-selective α-vinylpyrroles. The carbon isotope effect on the α-pyrrole carbon and the Hammett correlation from a series of para-substituted N-arylpyrroles (- = -0.90) indicate a rate-limiting C-C bond formation step of the coupling reaction.

Full text of DOI:10.1021/jo100269y

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and electronic nature of the arene substituents.3 The direct
Regioselective Formation of r-Vinylpyrroles
from the Ruthenium-Catalyzed Coupling Reaction
of Pyrroles and Terminal Alkynes Involving
C-H Bond Activation
oxidative arylation of indoles and quinoline N-oxides has also
been achieved by using Pd catalysts.4 Cationic Ru-allyl and -
vinylidene complexes have been successfully utilized as catalysts
for allylation and alkenylation of indoles and pyridine deriva-
tives, respectively.5 A novel regioselective insertion of alkynes
to both Ar-H and Ar-CN bonds of N-protected 3-cyanoin-
doles and related heteroarenes has been achieved by using
Ni-phosphine catalysts.6 Though the gem-selective oxidative
coupling reaction of indolizines and alkenes has recently been
accomplished by using Pd catalysts with bidentate nitrogen
ligands,7 lack of generally reliable cis- and gem-selective vinyla-
tion methods continues to be a major problem in catalytic C-H
alkenylation methods for pyrroles and related nitrogen arene
compounds, since the formation of trans-selective vinyl pro-
ducts is normally favored for these catalytic reactions.
Ruili Gao and Chae S. Yi*
Department of Chemistry, Marquette University, Milwaukee,
Wisconsin 53201-1881
chae.yi@marquette.edu
Received February 19, 2010
While exploring the scope of the ruthenium-catalyzed coup-
ling reactions involving C-H bond activation, we have recently
developed a number of regioselective cyclization methods from
the coupling reaction of arylamines and pyrroles with terminal
alkynes by using the cationic ruthenium catalytic system Ru3-
(CO)12/NH4PF6.8 Here we report a highly regioselective forma-
tion of R-gem-vinylpyrroles from the ruthenium-catalyzed inter-
molecular coupling reaction of pyrroles and terminal alkynes.
The cationic ruthenium catalyst Ru3(CO)12/NH4PF6 was
found to be highly effective for the intermolecular coupl-
ing reaction of pyrroles and terminal alkynes to give gem-
selective R-vinylpyrroles. The carbon isotope effect on the
R-pyrrole carbon and the Hammett correlation from a series
of para-substituted N-arylpyrroles (F = -0.90) indicate a
rate-limiting C-C bond formation step of the coupling
reaction.
The treatment of N-methylpyrrole (1.0 mmol) with 4-ethy-
nylanisole (2.0 mmol) in the presence of Ru3(CO)12/NH4PF6
(1:3, 3 mol % Ru) in benzene (3 mL) at 95 °C for 8 h cleanly
produced the R-gem-vinylpyrrole product 1a (eq 1). The
product was isolated in 99% yield after a simple silica gel
column chromatography (CH2Cl2/hexanes) and was fully
characterized by both spectroscopic methods and elemental
analysis. The initial survey of ruthenium catalysts showed
that both Ru3(CO)12 and NH4PF6 are essential for the cata-
lytic activity. Other selected neutral and cationic ruthenium
C-Vinylated pyrroles are important building blocks for
forming porphyrins and related nitrogen macrocycles as well
as for serving as precursors for photoactive polymeric mate-
rials.1 Compared to the traditional arene substitution
methods using stoichiometric reagents, transition-metal-cata-
lyzed C-H bond activation methods have been shown to
exhibit a number of salient features such as increasing effi-
ciency and reducing wasteful byproducts in introducing the
vinyl group directly to pyrroles and related heteroarene
compounds.2 Pd catalysts have been found to be particularly
versatile in mediating C-H oxidative coupling reactions of
substituted pyrroles, pyridines, and indoles, where the regio-
selectivity hasoften beenfound to be dictated by both the steric
catalysts, such as RuCl3 3H2O, (PPh3)3RuHCl, [(COD)-
3
RuCl2]x (PCy3)2(CO)RuHCl, and [(PCy3)2(CO)(CH3CN)2-
RuH]þBF4-, were not effective in giving the coupling pro-
duct under the similar reaction conditions.
The scope of the coupling reaction was explored by using
the Ru3(CO)12/NH4PF6 catalytic system (Table 1). Both
N-alkyl- and N-arylpyrroles were found to react smoothly
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Published on Web 04/13/2010
DOI: 10.1021/jo100269y
r
2010 American Chemical Society

Gao and Yi
JOCNote
TABLE 1. Coupling Reaction of Pyrroles and Indoles with
Terminal Alkynesa
deuterium-labeling pattern was examined from the treat-
ment of N-phenylpyrrole with DCtCPh (2.5 equiv) and
Ru3(CO)12/NH4PF6 (1:3, 3.0 mol % Ru) in benzene (3 mL)
at 95 °C. The coupling product 1c showed 67% D on the
vinyl as well as 40% on the R-pyrrole positions as determined
by both 1H and 2H NMR (eq 2). Extensive H/D exchange at
the β-carbon of pyrrole may be due to a direct metalation by
the ruthenium catalyst during or after the coupling reaction.
Conversely, the treatment of R,R-dideuterated N-phenylpyr-
role with HCtCC6H4-p-OMe (2 equiv) yielded the product
with an extensive H/D exchange on both vinyl (33% D) and
R-pyrrole positions. The extensive H/D exchange pattern on
the vinyl positions is indicative of rapid and reversible
alkynyl and R-pyrrole C-H activation steps.
The deuterium isotope effect study also supported the
notion of rapid H/D exchange steps. The rate of the reaction
of N-phenylpyrrole and 2,5-d2-N-phenylpyrrole with 4-ethy-
nylanisole at 95 °C led to a virtually identical kobs=0.14 h-1
,
which translated to kCH/kCD=1.1 ( 0.2. Similar experiments
from the reaction of N-phenylpyrrole with HCtCPh and
DCtCPh also gave a negligible isotope effect of kCH/kCD
=
1.1 ( 0.1 (Figure S2, Supporting Information).9 These
results indicate that the R-C-H bond activation of pyrrole
is not the rate-limiting step for the catalytic reaction.
To discern the rate-limiting step of the coupling reaction,
we next measured the carbon isotope effect from the coup-
ling reaction of N-phenylpyrrole with HCtCPh by employ-
ing Singleton’s isotope measurement technique at natu-
ral abundance.10 The most pronounced carbon isotope effect
was observed on the R-pyrrole carbon when the 13C ratio of
unreacted N-phenylpyrrole isolated at 75% conversion was
compared to that of the virgin sample (13C(recovered)/13C(virgin)
at CR=1.019, average of 3 runs) (eq 3) (Table S1, Supporting
Information). This result is consistent with the C-C bond rate-
limiting step of the coupling reaction.
To examine the electronic influence on the pyrrole sub-
strate, the Hammett plot was constructed from the correla-
tion of the relative rates with σp for a series of para-substi-
tuted N-arylpyrroles p-X-C6H4NC4H4 (X=OMe, CH3, H,
Cl, F), which led to F=-0.90 (Figure 1). The promotional
effect by electron-releasing group is indicative of a nucleo-
philic nature of the pyrrole group. An analogous correlation
from the reaction of N-phenylpyrrole with para-substituted
arylalkynes p-Y-C6H4CtCH (Y = OMe, CH3, H, Br, F)
resulted in a similar electronic promotional effect, but with a
considerably less negative Hammett F value of -0.42. In this
case, the negative F value suggests a considerable cationic
character on the internal alkynyl carbon, which is stabilized
by the electron-releasing group of the aryl substituent. These
results are consistent with the notion that the C-C bond
formation step is promoted by a nucleophilic pyrrole sub-
strate via a cationic transition state.
aReaction conditions: pyrrole/indole (1.0 mmol), alkyne (2.0 mmol),
Ru3(CO)12/NH4PF6 (1:3, 3 mol % Ru), benzene (3 mL), 95 °C, 12-15 h.
with aryl-substituted terminal alkynes to give the coupling
products, in which arylalkynes with a para electron-donating
group were found to promote the coupling reaction. Neither
aliphatic-substituted terminal alkynes nor internal alkynes
gave the coupling products under the similar reaction condi-
tions. The regioselective R-insertion products 1q and 1r were
obtained for 3-methylindole substrate (entries 17 and 18),
while the analogous coupling reaction of N-methylindole led
to the 2:1 coupling products 1s and 1t, resulting from regio-
selective insertion to the β-carbon (entries 19 and 20). The
molecular structure of 1d was also established by X-ray
crystallography (Figure S3, Supporting Information).9
We performed the following kinetic experiments to gain
mechanistic insights on the coupling reaction. First, the
(10) (a) Singleton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117,
9357–9358. (b) Frantz, D. E.; Singleton, D. A.; Snyder, J. P. J. Am. Chem.
Soc. 1997, 119, 3383–3384.
(9) See the Supporting Information for the experimental details.
J. Org. Chem. Vol. 75, No. 9, 2010 3145

Gao and Yi
JOCNote
in forming sterically less demanding Ru-vinyl species.11
The migratory insertion of alkynes constitutes one of the
well-known organometallic elementary reactions,12 and the
C-C bond forming rate-determining step has been pro-
posed in other ruthenium-catalyzed arene C-H coupling
reactions.13
In summary, the cationic ruthenium catalyst Ru3(CO)12/
NH4PF6 was found to be highly effective for mediating the
regioselective intermolecular coupling reaction of pyrroles
and alkynes to give R-gem-vinylpyrroles. Both carbon iso-
tope effect and Hammett study support a mechanism of the
coupling reaction involving rate-limiting C-C bond forma-
tion step. The catalytic coupling reaction provides a reliable,
atom-economical method for directly introducing synthe-
tically useful gem-vinyl group to pyrroles and indoles.
FIGURE 1. Hammett plots of the coupling reaction of para-sub-
stituted p-X-C6H4NC4H4 (X = OMe, CH3, H, Cl, F) with PhCtCH
(b) and the reaction of N-phenylpyrrole with p-Y-C6H4CtCH (Y =
OMe, CH3, H, Br, F) (9).
Experimental Section
SCHEME 1
Representative Procedure of the Catalytic Reaction. In a
glovebox, Ru3(CO)12 (22 mg, 0.030 mmol), NH4PF6 (16 mg,
0.10 mmol), pyrrole (1.0 mmol), and an alkyne (2.0 mmol,
2 equiv) were dissolved in 3 mL of benzene solution in a
25 mL Schlenk tube equipped with a magnetic stirring bar.
The tube was brought out of the glovebox and was stirred in an
oil bath set at 95 °C for 12-15 h. The tube was cooled to room
temperature, and the crude product mixture was analyzed by
GC-MS. The solvent was removed under a rotary evaporator,
and analytically pure organic product was isolated by a column
chromatography on silica gel (CH2Cl2/hexanes).
2-[1-(4-Methoxyphenyl)ethenyl-1-methylpyrrole (1a) was
synthesized from the reaction of N-methylpyrrole (81 mg) with
4-ethynylanisole (265 mg) following the general procedure. For 1a:
1H NMR (400 MHz, acetone-d6) δ 7.2-6.9 (m, 4H), 6.7-6.1 (m,
3H), 5.46 (d, J=1.6 Hz), 5.15 (d, J=1.6 Hz), 3.81 (s, 3H), 3.30 (s,
3H); 13C{1H} NMR (100 MHz, acetone-d6) δ 160.5, 142.4, 134.6,
134.4, 129.1, 124.4, 114.8, 113.8, 110.9, 107.9, 55.4, 35.1; HREI
(m/z) calcd for C14H15NO (Mþ) 213.1154, found 213.1144.
A plausible mechanistic rationale for the coupling reaction
is illustrated on the basis of these results (Scheme 1). We
propose that the catalytically active cationic Ru-pyrrolyl
species 2 is initially formed from an R-C-H insertion of
pyrrole followed by the elimination of an arylalkene. In sup-
port of this hypothesis, the formation of styrene (3%) was
observed from the catalytic coupling reaction of N-phenyl-
Acknowledgment. Financial support from National Insti-
tutes of Health, General Medical Sciences (R15 GM55987) is
gratefully acknowledged.
1
pyrrole with PhCtCH as detected by both H NMR and
Supporting Information Available: Experimental details,
characterization data of the products, and X-ray crystallo-
graphic data of 1d. This material is available free of charge via
the Internet at http://pubs.acs.org.
GC-MS.9 Both negligible kH/kD isotope effect and exten-
sive H/D exchange patterns on the coupling reaction are con-
sistent with a rapid and reversible C-H bond activation step.
The observation ofthe pronouncedR-carbon isotopeeffecton
the pyrrole substrate provides strong evidence for the rate-
limiting C-C bond formation step. A relatively high negative
Hammett F value from the correlation of para-substituted
N-arylpyrroles indicates that the C-C bond formation step
is promoted by the nucleophilic nature of the R-metalated
pyrrolyl species 2. The formation of gem-selective pyrrole
product 1 can readily be rationalized by invoking a regiose-
lective insertion of pyrrole to the internal alkynyl carbon
(11) As the reviewers pointed out, alternative mechanistic pathways
involving cationic Ru-acetylide or direct metalation of pyrrole substrate
can also be considered (for example, see recently proposed mechanisms in
ref 8). While these mechanistic paths cannot readily explain the gem-selective
formation of the coupling products, they could not be rigorously ruled out on
the basis of available data.
(12) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 5th Ed.; Wiley: Hoboken, NJ, 2009. (b) Spessard, G. O.; Miessler, G. L.
Organometallic Chemistry, 2nd ed.; Oxford University Press: New York, 2010.
(13) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826–834 and
references cited therein.
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