Rh-catalyzed oxidative coupling between primary and secondary benzamides and alkynes: Synthesis of polycyclic amides

Article abstract of DOI:10.1021/jo101596d

A methodology for the high yield and facile synthesis of isoquinolones from benzamides and alkynes via the oxidative ortho C-H activation of benzamides has been developed. Ag₂CO₃ proved to be an optimal oxidant when MeCN was used as

Full text of DOI:10.1021/jo101596d

pubs.acs.org/joc
is very important due to their wide biological and photo- and
Rh-Catalyzed Oxidative Coupling between Primary
and Secondary Benzamides and Alkynes: Synthesis
of Polycyclic Amides
electrochemical applications.2 Among them, isoquinolones are
important structural motifs in various natural products.3
Recent seminal work on the synthesis of isoquinolones by the
group of Murakami4 and the group of Kurahashi and
Matsubara5 focused on nickel-catalyzed denitrogenative inser-
tion of alkynes into benzotriazin-4(3H)-ones and the decarbo-
nylative insertion of alkynes into phthalimides, respectively. In
these systems, five-membered metalacycles have been proposed
as key intermediates. We envision that an atom-economic
synthesis of isoquinolones from simple benzamides and
alkynes can be advantageous by way of chelation-assisted
C-H activation under rhodium-catalyzed oxidative condi-
tions. Recently, Fagnou,6 Satoh and Miura,7 Jones,8 and
Glorius9 have independently reported the important and useful
Rh-catalyzed oxidative coupling reactions between arenes or
heteroarenes and alkynes or alkenes, wherein the active five- or
six-membered organorhodium species were generated with the
assistance of directing groups such as amides,6b,9a carboxyls,7e
hydroxyls,7g,i imines,7a and other N atoms.7b,f,h Considering
that amides are readily available, the ortho Caryl-H activation
of secondary acetanilides has been well documented in oxida-
tive coupling with alkenes or alkynes catalyzed by Pd(II)10 and
Rh(III).6b,9a In the case of benzamides bearing N-aryl groups
such as 1, complication of selectivity of C-H activation can
arise as to which arene undergoes C-H activation. To the best
of our knowledge, no such selectivity has been examined,11
Guoyong Song,† Dan Chen,† Cheng-Ling Pan,†
Robert H. Crabtree,§ and Xingwei Li*,†,‡
†Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian, China 116023, ‡The Scripps Research
Institute, Scripps Florida, Jupiter, Florida 33458,
§
United States, and Chemistry Department, Yale University,
New Haven, Connecticut 06520, United States
xwli@dicp.ac.cn
Received August 23, 2010
(2) For selected examples, see: (a) Abet, V.; Nunez, A.; Mendicuti, F.;
Burgos, C.; Alvarez-Builla, J. J. Org. Chem. 2008, 73, 8800. (b) Ahmed, E.;
Briseno, A. L.; Xia, Y.; Jenekhe, S. A. J. Am. Chem. Soc. 2008, 130, 1118.
(c) Parenty, A. D. C.; Song, Y.-F.; Richmond, C. J.; Cronin, L. Org. Lett.
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E. J. J. Med. Chem. 2005, 48, 792.
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J. Am. Chem. Soc. 2008, 130, 16474.
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Commun. 2009, 5141. (b) Umeda, N.; Hirano, K.; Satoh, T.; Miura, M.
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Chem. 2007, 72, 5362. (d) Ueura, K.; Satoh, T.; Miura, M. Org. Lett. 2007, 9,
1407. (e) Mochida, S.; Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2009,
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12, 2068. (g) Uto, T.; Shimizu, M.; Ueura, K.; Tsurugi, H.; Satoh, T.; Miura,
M. J. Org. Chem. 2008, 73, 298. (h) Umeda, N.; Tsurugi, H.; Satoh, T.;
Miura, M. Angew. Chem., Int. Ed. 2008, 47, 4019. (i) Mochida, S.; Shimizu,
M.; Hirano, K.; Satoh, T.; Miura, M. Chem. Asian J. 2010, 5, 847.
(8) Li, L.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2008, 130,
12414.
A methodology for the high yield and facile synthesis of
isoquinolones from benzamides and alkynes via the oxidative
ortho C-H activation of benzamides has been developed.
Ag2CO3 proved to be an optimal oxidant when MeCN was
used as a solvent, and [RhCp*Cl2]2 was utilized as an efficient
catalyst. Both N-alkyl and N-aryl secondary benzamides can
be applied as effective substrates. Furthermore, primary
benzamides react with two alkyne units, leading to tricyclic
products via double C-H activation and oxidative coupling.
The reactivity of the structurally related 1-hydroxyisoquino-
line was also demonstrated, where both N- and O-containing
rhodacyclic intermediates can be generated, leading to the
construction of different O- or N-containing heterocycles.
In the past decade, catalytic activation of C-H bonds has
become an increasingly important strategy for the elaboration
of readily available simple substrates to complex products in an
atom-economic fashion.1 In this context, the construction of
nitrogen-containing heterocycles via (directed) C-H cleavage
(9) (a) Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010, 132, 9982.
(b) Rakshit, S.; Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010, 132,
9585.
(10) (a) Li, B.-J.; Tian, S.-L.; Fang, Z.; Shi, Z.-J. Angew. Chem., Int. Ed.
2008, 47, 1115. (b) Wan, X.; Ma, Z.; Li., B.; Zhang, K.; Cao, S.; Zhang, S.;
Shi., Z. J. Am. Chem. Soc. 2006, 128, 7416. (c) Boele, M. D. K.; van
Strijdonck, G. P. F.; de Vries, A. H. M.; Kamer, P. C. J.; de Vries, J. G.;
van Leeuwen, P.W. N. M. J. Am. Chem. Soc. 2002, 124, 1586. (d) Giri, R.;
Lam, J. K.; Yu, J.-Q. J. Am. Chem. Soc. 2010, 132, 686.
(11) During the preparation of this manuscript, similar studies leading to
isoquinolone synthesis catalyzed by Rh(III) were reported by the groups of T.
Rovis and M. Miura; see: (a) Hyster, T. K.; Rovis, T. J. Am. Chem. Soc. 2010,
132, 10565. (b) Mochida, S.; Umeda, N.; Hirano, K.; Satoh, T.; Miura, M.
Chem. Lett. 2010, 39, 744.
(1) For leading reviews, see: (a) Colby, D. A.; Bergman, R. G.; Ellman, J. A.
Chem. Rev. 2010, 110, 624. (b) Sun, C.-L.; Shi, Z.-J. Chem. Commun. 2010, 46,
677. (c) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J.-Q. Angew. Chem., Int. Ed.
2009, 48, 5094. (d) Ackermann, L.; Vicente, R.; Kapdi, A. R. Angew. Chem., Int.
Ed. 2009, 48, 9792. (e) Xu, L.-M.; Yang, Z.; Shi, Z.-J. Chem. Soc. Rev. 2010, 39,
712. (f) Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013. (g) Alberico, D.; Scott,
M. E.; Lautens, M. Chem. Rev. 2007, 107, 3013. (h) Satoh, T.; Miura, M. Top.
Organomet. Chem. 2008, 24, 61. (i) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev.
2002, 102, 1731. (j) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826.
(k) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698.
DOI: 10.1021/jo101596d
r
Published on Web 10/05/2010
J. Org. Chem. 2010, 75, 7487–7490 7487
2010 American Chemical Society

Song et al.
JOCNote
SCHEME 1. Metal-Mediated Synthesis of Isoquinolones
TABLE 2. Rh(III)-Catalyzed Oxidative Coupling between Secondary
Benzamides and Alkynes
TABLE 1. Screening of Rh-Catalyzed Oxidative Couplinga
cat. loading
(mol %)
solvent
(temp)
entry oxidantþ
yield (%)b
1
2
3
4
5
6
7
Ag2CO3
Ag2O
4
4
4
4
4
2
2
toluene (130 °C)
toluene (130 °C)
toluene (130 °C)
ClCH2CH2Cl (115 °C)
CH3CN (115 °C)
CH3CN (115 °C)
CH3CN (115 °C)
45
34
<5
64
Cu(OAc)2
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
97 (94c)
77
88
aReaction conditions: 1a (50.0 mg, 0.237 mmol), diphenylacetylene
(55.0 mg, 0.308 mmol, 1.3 equiv), oxidant (1.5 equiv of Ag2X or 2.2 equiv
of Cu(OAc)2, catalyst, solvent (3 mL) in a 25 mL sealed tube under
nitrogen, 12 h. b1H NMR yields with 1,3,5-trimethoxybenzene as an
internal standard. cIsolated yield. d1,2,3,4-Tetraphenyl-1,3-cyclopenta-
diene (5 mol % based on 1a) was added.
resulted in significantly lower yield. Decreasing the loading
of the [Cp*RhCl2]2 is possible with the introduction of
tetraphenylcyclopentadiene ((CpH)Ph4) as an additive.
However, a slightly lower isolated yield of 2aa was obtained
with the (Cp*RhCl2)2 (2%)/(CpH)Ph4 (5%) catalyst system
(condition B).
although the ortho C-H activation of PhC(O)NHMe has
been reported by Fagnou (Scheme 1).12 We now report the
facile synthesis of a variety of N-alkyl and N-aryl isoquinolones
via the selective activation of ortho C-H bonds of the benzyl
ring (C-phenyl) in N-alkyl and N-aryl benzamides (Scheme 1).
More importantly and unexpectedly, we successfully achieved
the double oxidative insertion of alkyne into primary benza-
mides to give unreported tricyclic amides.
We initiated our investigation on the coupling between 1a
and PhCtCPh using [Cp*RhCl2]2 (4 mol %) as the catalyst.
During the survey of the reaction conditions, essentially no
appreciable formation of isoquinolone 2aa was observed
when Cu(OAc)2 was used as the oxidant in the presence or
absence of air in several common aprotic solvents such as
1,4-dioxane, toluene, or ClCH2CH2Cl (115 to 130 °C, 12 h,
Table 1). Thus we felt that Ag(I) might be an appropriate
oxidant. Indeed, when this reaction was conducted with
Ag2CO3 as the oxidant (1.3 equiv) in MeCN or acetone,
product 2aa was obtained in nearly quantitative yield after
12 h (condition A, Table 1), which was characterized by
X-ray crystallography (see the Supporting Information).
Strong solvent effect was observed in that switching MeCN
or acetone to DMF, 1,4-dioxane, or dichloroethane all
Following the screening, the scope of this reaction was
investigated. As given in Table 2, secondary benzamides
bearing electron-donating or -withdrawing groups at both
the N-phenyl and the C-phenyl rings all react to give high
yield under condition A. Halogenated benzamides can
also be tolerated (2ma), and high selectivity was obtained
when ortho and meta substituents were introduced into
the C-phenyl ring (2ja, 2jb and 2ka, 2kb), and this high
selectivity is likely caused by efficient steric shielding of such
groups, consistent with those previously reported for coupling
reactions that involve ortho C-H activation of arenes.13 Inter-
estingly, introduction of an ortho substituent into the N-phenyl
ring has no detrimental effect on this reaction (2fa and 2ga,
2gb). Since this reaction involves no cleavage of any C-H bond
in the N-substituent, benzamides bearing both N-alkyl (2ha,
2hb) and N-benzyl (2ia, 2ib) groups are applicable, and they
successfully undergo oxidative coupling with alkynes although
lower yields were observed, together with the recovery of nearly
all the starting benzamides. In contrast to the high yield and
insensitivity to steric perturbation caused by ortho substitutents
of the N-aryls of benzamides, essentially no reaction proceeds
(12) Guimond, N.; Gouliaras, C.; Fagnou, K. J. Am. Chem. Soc. 2010,
132, 6908.
(13) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147.
7488 J. Org. Chem. Vol. 75, No. 21, 2010

Song et al.
JOCNote
for PhC(O)NHCy. In addition, the scope of the alkyne can
be extended to a dialkyl-substituted alkyne (PrCtCPr) and
unsymmetrical alkynes which give high yield and moderate to
high regioselectivity (2bc and 2bd for unsymmetrical alkynes),
and the major isomer of 2bd was unambiguously identified by
X-ray crystallography (see the Supporting Information). The
preferred regioselectivity agrees with that reported in related
rhodium-catalyzed oxidative coupling reactions.6 In addition
to benzamides, the structurally related methylmethacramide
reacts with PhCtCPh under the same conditions to give
pyridinone 4 in 68% yield (eq 1).
TABLE 3. Rh(III)-Catalyzed Double Oxidative Coupling between
Primary Benzamides and Alkynes
Since this reaction involves the cleavage of both C-H
and N-H bonds, we carried out competitive reactions to
explore the electronic effects of both aryl groups in N-aryl
benzamides. An equimolar amount of 2c, 2d, and PhCt
CPh was allowed to react under the standard conditions.
Benzamides with electron-poor groups in the N-aryl ring
react with the alkyne in slight preference (eq 2). Likewise,
the competitive reaction between benzamides 2l and 2m also
leads to the conclusion that coupling reaction is favored for
benzaimdes with electron-withdrawing groups in the C-aryl
ring (eq 3). Ideally, useful information on the cleavage of the
ortho C-H bond may be obtained from kinetic isotope effect
experiments. However, KIE obtained from inter- or intra-
molecular competitive reaction is not applicable since heating
protonated and deuterated benzamide (C6D5C(O)NHPh)
in equimolar ratio in the absence of PhCtCPh resulted in
scrambling of deuterium into the ortho sites of both benza-
mides, which indicates that the ortho C-H activation here
should be a reversible process under these conditions (see the
Supporting Information), consistent with the results obtained
for PhC(O)NHOMe.12
yield). Further exploration of the scope of primary benzamides
reveals that substrates bearing both electron-donating (6b-e)
and -withdrawing groups (6f-i) at different positions react with
PhCtCPh to give the corresponding products in high isolated
yields (71-93%). In contrast to the high selectivity of C-H
activation for a secondary benzamide bearing m-methyl groups
(2ka, 2kb, Table 2), the regioselectivity for m-CH3(C6H4)C(O)-
NH2 is not significant, and isomeric products 6c and 6c0 were
obtained in 3:1 ratio. This diminished selectivity is likely caused
by the reduced steric hindrance of the NH2 group, which makes
the cleavage of the two ortho C-H bonds less discriminating.
Interesting results were obtained in the oxidative coupling
between PhC(O)NH2 and Me—CtC—Ph. Although four
regioisomeric products can be possible, only product 6k was
obtained (83%).
When 4-octyne was used as diphenylacetylene substitute to
react with 5a, the first oxidative coupling product isoquinolone
6ab was obtained in 63% yield (eq 4). This result suggests that
double oxidative insertion of alkynes is a stepwise process,
wherein the isoquinolone acts as an intermediate. To further
explore this likelyhood, isoquinolone 7 was independently syn-
thesized12 and was allowed to react with Ph—CtC—Ph under
the same conditions (eq 5). Indeed product 6a was isolated in
92% yield, and this result supports the hypothesis that 7 is a
Noting the somewhat “spectating” behavior of the N-alkyl
and N-aryl groups in benzamides, we moved to the extreme case
by using simple primary benzamides as substrates. Surprisingly,
the expected N-H isoquinolone was not observed under the
standard conditions. Instead, a tricyclic product was obtained
and fully characterized by NMR spectroscopy (and X-ray
crystallography for 6a, see the Supporting Information) as a
result of double oxidative coupling with the incorporation of
two alkyne units, a process that theoretically requires 2 equiv of
Ag2CO3 (Table 3). Additional optimization of the conditions by
simply providing an excess of PhCtCPh and Ag2CO3 leads to
the nearly quantitative formation of the product (91% isolated
J. Org. Chem. Vol. 75, No. 21, 2010 7489

Song et al.
JOCNote
likely intermediate during the course of this double oxidative
coupling reaction.
In summary, we have developed a methodology to achieve
the catalytic ortho C-H activation of benzamides in the
C-phenyl ring under oxidative coupling conditions to give
isoquinolones. Both N-alkyl and N-aryl secondary benza-
mides can be applied as effective substrates, while primary
benzamides react differently in that two alkyne units are
oxidatively incorporated to give tricyclic products. The re-
activity of the structurally related 2-hydroxyisoquinolines
was also demonstrated, where both N- and O-containing
rhodacyclic intermediates can be generated, leading to the
construction of different O- or N-containing heterocycles.
Experimental Section
The scope of this reaction was further explored by using
commercially available 1-hydroxyisoquinoline (8a). Here the
absence of the two phenyl groups renders unlikely the formation
of N-containing rhodacycle. Instead, oxidative insertion into the
tautomeric O-H bond was observed, and product 9a was
isolated in high yield with the construction of a new six-
membered heterocycle (eq 6). A question may arise when both
N and O atoms are accessible in substituted 2-hydroxyisoqui-
nolines, a scenario similar to the selectivity of C-H activation in
the two aryl groups in N-aryl benzamides. In this context, we
carried out the oxidative coupling reaction between 8b and
PhCtCPh under the standard conditions (eq 6). Although low
yield of the analogous product 9b (29%) was isolated due to the
rather poor solubility of 8b in most common organic solvents
(8b was recovered in 65% yield), no other coupling product was
detected on the basis of NMR and LC analysis. Amide groups
are well-known in facilitating the activation of ortho C-H
bonds as in acetanilides. However, it remains a question whether
the chelation assistance was offered by the N or the O atom. Our
results strongly support that both N- and O-containing rhoda-
cyclic intermediates can be generated from C-H activation of
1-hydroxyisoquinoline (eqs 5 and 6). These results also reveal
that 1-hydroxyisoquinolines may deliver versatile reactivity in
their oxidative coupling with internal alkynes.
Synthesis of compound 2aa: 1a (106.0 mg, 0.502 mmol),
diphenylacetylene (116.3 mg, 0.653 mmol, 1.3 equiv), Ag2CO3
(208 mg, 0.752 mmol, 1.5 equiv), and [RhCp*Cl2]2 (12.4 mg,
4 mol %) were weighted into a pressure tube, and degassed
acetonitrile (5 mL) was added. After being purged by nitrogen,
the mixture was stirred at 115 °C for 12 h. The mixture was then
diluted with CH2Cl2 and filtered through Celite. All volatiles
were removed under reduced pressure. The crude product was
purified by column chromatography on silica gel to afford 2aa
1
as white microcrystals (182 mg, 94%). H NMR (400 MHz,
CDCl3) δ 8.56 (d, J = 8.0 Hz, 1H), 7.54 (t, J = 6.8 Hz, 1H), 7.48
(t, J = 6.8 Hz, 1H), 7.24 (d, J = 8.0 Hz, 1H), 7.11-7.18 (m, 5H),
6.98 (s, 4H), 6.89 (s, 5H), 2.20 (s, 3H, CH3). 13C NMR (100
MHz, CDCl3) δ 162.7, 141.3, 137.6, 137.3, 136.8, 136.5, 134.9,
132.5, 131.6, 131.0, 129.3, 129.1, 128.3, 128.0, 127.2, 127.0, 126.8
(two overlapping signals), 125.5 (two overlapping signals),
118.7, 21.1. IR 1660, 1605, 1590, 1509, 1489, 1329, 700. HRMS
(ESI) calcd for [C28H21NO þ H]þ 388.1696, found 388.1694.
Anal. Calcd for C28H21NO (387.47): C, 86.79; H, 5.46; N, 3.61.
Found: C, 86.67; H, 5.60; N, 3.70.
Acknowledgment. We gratefully acknowledge Dalian In-
stitute of Chemical Physics, Chinese Academy of Sciences for
generous financial support. We thank Xiang Zhao for perform-
ing some initial screenings.
Supporting Information Available: Detailed experimental
procedures, characterization of new compounds, X-ray crystal-
lographic data for 2aa, 2bc, and 6a, and NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
7490 J. Org. Chem. Vol. 75, No. 21, 2010