Re/Mg bimetallic tandem catalysis for [4+2] annulation of benzamides and alkynes via C-H/N-H functionalization

Article abstract of DOI:10.1021/ja400020e

A rhenium-magnesium cocatalyzed [4+2] annulation of benzamides and alkynes via C-H/N-H functionalization is described. The reaction features a divergent and high level of diastereoselectivities, which are readily switchable by subtle tuning of reaction conditions. Thus, a wide range of both cis- and trans-3,4-dihydroisoquinolinones is expediently synthesized in a highly atom-economical manner. Moreover, mechanistic studies unraveled a tandem mode of action between rhenium and magnesium in the catalytic cycles.

Full text of DOI:10.1021/ja400020e

Communication
pubs.acs.org/JACS
Re/Mg Bimetallic Tandem Catalysis for [4+2] Annulation of
Benzamides and Alkynes via C‑H/N‑H Functionalization
Qiuzheng Tang,‡ Dexin Xia,† Xiqing Jin,† Qing Zhang,† Xiao-Qiang Sun,*,‡ and Congyang Wang*,†
†Beijing National Laboratory of Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
S
* Supporting Information
Scheme 1. [4+2] Annulation of Benzamide via C-H Activation
ABSTRACT: A rhenium-magnesium cocatalyzed [4+2]
annulation of benzamides and alkynes via C-H/N-H
functionalization is described. The reaction features a
divergent and high level of diastereoselectivities, which are
readily switchable by subtle tuning of reaction conditions.
Thus, a wide range of both cis- and trans-3,4-
dihydroisoquinolinones is expediently synthesized in a
highly atom-economical manner. Moreover, mechanistic
studies unraveled a tandem mode of action between
rhenium and magnesium in the catalytic cycles.
n the past few decades, C-H activation has attracted immense
Iinterest since it might significantly streamline the organic
synthesis.1 In principle, the direct transformations of C-H bonds
widely occurring in organic molecules can obviate the
preactivation of reaction substrates, diminish the generation of
undesired waste, and allow for late-stage C-H elaborations, thus
(RhI/RhIII). So far, to the best of our knowledge, the redox-
ultimately improving the atom and step economy of the whole
event.2 As a consequence, the application of C-H activation in the
synthesis of heterocyclic compounds, commonly found as key
structural motifs in a vast majority of natural products,
pharmaceuticals, and biologically active molecules,3 has emerged
as an important alternative to the traditional approaches.
neutral annulation of benzamides with alkynes, one of the most
straightforward routes to 3,4-dihydroisoquinolinones, remains
an unmet challenge (Scheme 1b, right). We surmise that
accomplishment of a nonredox catalytic cycle of active metal
species might be the major obstacle for harnessing such a process.
With this regard, we herein disclose the first redox-neutral [4+2]
annulation of benzamides with alkynes via C-H/N-H function-
alization by means of Re/Mg tandem catalysis (Scheme 1c).
Remarkably, both the cis- and trans-3,4-dihydroiso-quinolinones
are readily accessed in a highly diastereoselective fashion simply
by subtle tuning of reaction conditions, which adds further values
to this bimetallic catalyst system.
The construction of isoquinolinone derivatives is illustrative,
where both the condensation/cyclization routes4a−c and the
transition-metal-catalyzed coupling annulations,4d−f aza-Wacker-
type reactions,4g−i denitrogenative4j,k and decarbonylative4l,m
cycloadditions, etc.,4n,o requires the prefunctionalization of
starting materials. Recently, by using the C-H activation strategy,
the groups of Fagnou, Satoh and Miura, Rovis, and others have
disclosed the synthesis of isoquinolinones via Rh-,5a,j Ru-,5k−m
Pd-,5n,o or Ni-catalyzed5p oxidative annulations of benzamides
with alkynes (Scheme 1a). From the mechanistic point of view, it
should be pointed out that either external or internal oxidants are
intrinsically required to reoxidize low oxidation-state metal
species to high oxidation-state ones (e.g., RhI/RhIII, Ru0/RuII,
Pd0/PdII, etc.) in the final step of these catalytic cycles thus
achieving the key catalytic turnovers. Meanwhile, Glorius,
Guimond, and others elegantly described Rh-catalyzed oxidative
annulations of benzamide derivatives with olefins leading to 3,4-
dihydrioisoquinolinones, though the scope of olefins is limited to
terminal and cyclic ones, thus no issue of diastereoselectivity
being involved (Scheme 1b, left).6,5e Again, internal oxidants are
the prerequisites to gain the closed catalytic cycles of rhodium
As our continuous interest in rhenium catalysis,7,8 we first
examined the reaction of benzamide 1a and diphenylacetylene 2a
with rhenium catalysts. After an extensive survey of reaction
parameters,9 we delightedly found that, with a catalytic amount
of ReBr(CO)5 and PhMgBr, the annulation proceeded smoothly
with an excellent diastereoselectivity providing cis-3,4-dihydroi-
soquinolinone 4aa in 85% isolated yield (Scheme 2).
Importantly, no products were observed in the absence of either
ReBr(CO)5 or PhMgBr. With the optimized conditions in hand,
we set out to investigate the substrate scope. Both aliphatic and
aromatic substituents on the N-atom of benzamides are well
Received: January 2, 2013
© XXXX American Chemical Society
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Journal of the American Chemical Society
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ab
,
a b
,
Scheme 2. Substrate Scope for the Formation of 4
Scheme 3. Substrate Scope for the Formation of 5
a
Reaction conditions: 1 and 2 (1.0 mmol), ReBr(CO)5 (2.5 mol %,
0.025 mmol), PhMgBr (30 mol %, 1.0 M, 0.3 mmol), 120 °C, 36 h.
b
Listed in brackets are the isolated yields of pure 5 and the ratios of 5/
1
a
4 determined by H NMR analysis of the reaction mixture on 0.2
mmol scale. 130 °C. 140 °C. 48 h. THF (0.5 mL). 24 h. THF
(0.25 mL). 150 °C. Regioisomer 5ka′ was also detected, 5ka/5ka′ =
11:1. Single regioisomer.
Reaction conditions: 1 and 2 (1.0 mmol), ReBr(CO)5 (2.5 mol %,
c
d
e
f
g
h
0.025 mmol), PhMgBr (30 mol %, 1.0 M in THF, 0.3 mmol), THF
i
j
b
(4.0 mL), 150 °C, 18 h. Listed in brackets are isolated yields of pure 4
k
c
d
e
and the ratios of 4/5 (0.2 mmol scale). 48 h. 36 h. THF (0.5 mL).
f
g
h
THF (4.5 mL). THF (5.0 mL). Regioisomer 4ka′ was also
i
j
detected, 4ka/4ka′ = 9:1. THF (6.0 mL). THF (0.25 mL), 130 °C.
be quite general to a variety of benzamides and alkynes.
Specifically, despite their disparate properties, a multitude of
functional groups (Ph, OMe, CF3, CN, F, Cl, Br, I, etc.) were well
tolerant (5aa-ja). Benzamide with enhanced steric repulsion on
both meta positions gave smoothly the expected product 5na.
Notably, highly regio- and diastereoselective formation of 5ka
was found when m-benzoylbenzamide was examined. We
proposed the steric hindrance of benzoyl group governed
predominantly the observed regioselectivity. In contrast, the
annulation of m-fluorobenzamide with 2a took place solely at the
2-position, giving rise to 5oa as a single product. The observed
site selectivity is possibly attributed to the C-H bond acidity or
the Re-C bond stability in the cyclorhenation step.5k,10 Also,
furan- and thiophene-2-carboxamides are amenable to the
reaction conditions leading to 5pa and 5la, respectively. Finally,
an array of (hetero)aromatic alkynes was shown to be applicable
to this protocol giving the expected products 5ab-j successfully.
To gain insights into the reaction mechanism, a series of
experiments were conducted. First, the generation of olefin 3aa
was detected as the reaction (1a and 2a) plot showed in Scheme
4 (left).9 Specifically, olefin 3aa was initially formed in the early
stage and subsequently transformed into the cyclized product
k
l
m
THF (8.0 mL), Ratio of 6/6′ (E-isomer) is listed. 10 mmol scale
with 1 mol % ReBr(CO)5 for 36 h.
tolerant (4aab-c). Benzamides bearing either electron-donating
or -withdrawing groups on the benzene moiety reacted smoothly
with 2a affording the expected products in good to excellent
yields (4ba-fa). Of particular note, halogen functionalities,
including the Br and I groups often fragile under other transition-
metal catalysis, remain intact after reaction, which provides easy
handling for further synthetic elaborations (4ga-ja). A meta-
benzoyl substituent led to the regioselective cleavage of the less
sterically congested C-H bond (4ka). Thiophene-2-carboxamide
was also amenable to the reaction conditions (4la). Aromatic
alkynes containing an array of functionalities with varied
electronic and steric properties are easily compatible in the
reactions (4ab-g). Product 4ah derived from a heteroaromatic
alkyne was also accessed without difficulty. Remarkably, good to
excellent diastereoselectivities (up to 50:1) were generally
observed in this reaction, which shows the high fidelity of the
rhenium-magnesium bimetallic system. The double-bond
migrated product 6 rather than the expected 4ai was obtained
when 1-vinylphenylacetylene was used. Of note, only C-H
alkenylated product 3ak was formed when 4-octyne was
subjected to the reaction (Scheme 4), which gave a clue to the
possible reaction mechanism. Lastly, a gram-scale synthesis of
4aa was demonstrated (2.3 g, 73% isolated yield) with 1 mol % of
ReBr(CO)5.
Scheme 4. Probing Intermediacy and Selective Formation of 3
In the course of screening reaction parameters, we found that
the reaction concentration had a profound influence on the
diastereoselectivities of the annulation.9 Interestingly, the
formation of trans-3,4-dihydroisoquinolinone 5 was favored
when the reactions were conducted at high concentration
(Scheme 3). Moreover, the reaction temperature can be further
reduced while maintaining the reaction outcome. This proved to
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4aa with the elongation of reaction time. Interestingly,
chemoselective formation of 3aa can be obtained simply by
decreasing the amount of PhMgBr to 10 mol %, and 3aa was
isolated in 76% yield. Remarkably, only E-olefin 3aa was
observed after reaction, which showed a perfect stereocontrol in
the alkenylation process. This protocol proved quite general for
an array of benzamides and alkynes (Scheme 4, right).9 Of note,
this reaction also represents the first example of rhenium-
catalyzed C-H alkenylation of benzamide derivatives.11
Scheme 5. Competition and Deuterium-Labeling Studies
Second, the isolated olefin 3aa was then subjected to the
catalytic reaction conditions leading smoothly to 4aa and 5aa
(Table 1, entry 1), which is in contrast to the previous
a
Table 1. Cyclization of Olefin 3aa
b
b
entry
1
conditions
combined yield (%) 4aa/5aa
conditions, partial loss of deuterium (30%) on both ortho
positions of 1a-d5 was found, which suggests a deprotonation/
protonation equilibrium might exist in the reaction. (b) It was
observed that only 19% of deuterium was incorporated on the
olefinic position along with 13% deuterium loss on the ortho
position of 3aa-d in the first alkenylation step of 1a-d5 with
alkyne 2a.9 (c) The cyclization of N-deuterated 3aa-dN with
catalytic PhMgBr resulted in the regiospecific D-incorporation
on the 4-position of 4aa-d. (d) When 1a-d5 was treated with
alkyne 2e, the slight loss of deuterium on the ortho position and
the scrambling of D/H on both 3- and 4-positions of 4ae-d were
observed, which again confirm the occurrence of D/H crossover
in the reaction cascade.9
Re(CO)5Br (2.5 mol %), PhMgBr
(30 mol %)
79
1:6
c
2
3
ReBr(CO)5 (2.5 mol %)
0
−
c
PhMgBr (1.0 equiv), Re(CO)5Br
(1.0 equiv)
trace
−
4
5
PhMgBr (1.0 equiv)
PhMgBr (30 mol %)
47
73
1:6
1:23
a
Reaction conditions: 3aa (0.1 mmol), THF (0.4 mL), 150 °C, 18 h,
b
others as indicated. Determined by 1H NMR analysis of the reaction
c
mixture with mesitylene as an internal standard. 5aa was not detected.
isoquinolinone synthesis.5 To verify which metal, rhenium or
magnesium, plays a key role in the cyclization of 3aa, a set of
experiments were examined. No cyclized products were formed
with mere use of catalytic ReBr(CO)5 (entry 2). Similar results
were obtained when stoichmetric amounts of PhMgBr and
ReBr(CO)5 were added into the reaction in either one-portion or
stepwise manner (entry 3).9 However, the cyclized products
were formed in moderate yield in the sole presence of PhMgBr
(entry 4). Interestingly, the yield increased remarkably with using
just catalytic amount of PhMgBr (entry 5). We speculated that
3aa served also as a proton source thus rendering the cyclization
catalytic in magnesium. Thus, it is the metal of magnesium rather
than rhenium that catalyzes the cyclization of olefin 3aa affording
3,4-dihydroisoquinolinones eventually.
From these results, a plausible reaction mechanism is shown in
Scheme 6. With the aid of PhMgBr, amido-rhenium II is initially
Scheme 6. Plausible Bimetallic Catalytic Cycles
Third, the competition experiment between equimolar
amount of benzamide 1d and 1e, bearing a para methoxyl and
trifluoromethyl group, respectively, was examined (Scheme 5A,
left, red color). The total C-H transformation from the more
electron-deficient benzamide 1e was favored, which suggests an
electrophilic C-H activation is less likely operative in this
reaction. Furthermore, when equimolar amount of 1i and 1e,
with Br and CF3 substituents of similar σm values, respectively,5c
were examined, nearly equal amounts of C-H transformation
from 1i and 1e were observed (blue color). These results imply
that the rhenium-catalyzed C-H activation here might take place
through a deprotonative cyclometalation mechanism. In
addition, competition experiments between two electro-biased
alkynes, 2c and 2e, showed that the transformation of electron-
deficient 2e was preferred (Scheme 5A, right). Lastly, deuterium-
labeling experiments (Scheme 5B) were conducted to probe the
detailed nature of C-H bond cleavage: (a) When pentadeu-
terated benzamide 1a-d5 was solely subjected to the reaction
formed via amido-magnesium I, which then undergoes a
deprotonative cyclorhenation affording rhenacycle III.12 The
ensuing coordination and insertion of an alkyne gives rise to
seven-membered rhenacycle V, which further leads to inter-
mediate VI upon protonation. Transmetalation between
intermediate VI and substrate 1a results in the regeneration of
amido-rhenium II and formation of 3aa, which, after
deprotonation, suffers an intramolecular nucleophilic addition/
cyclization generating species VIII or further leading to X via
intermediate IX. Protonation of these species by 3aa would
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afford the final products and regenerate amido-magnesium VII,
thus closing the entire Re/Mg bimetallic tandem catalytic cycles.
In summary, we developed the first redox-neutral [4+2]
annulation of benzamides and alkynes via C-H/N-H function-
alization. The valuable assets of this reaction are further
represented by its high atom economy, stereodivergency, and
high diastereoselectivities. Thus, it allows for a rapid access to a
variety of cis- and trans-3,4-dihydroisoquinolinones and also
ortho-alkenylated benzamides, if needed. Mechanistic studies
reveal a rhenium-magnesium bimetallic tandem catalysis
operating in this reaction, which showcases a superb example
of the cooperation and compatibility between transition-metal
and main-group metal catalysts. Further studies to explore novel
transformations based on this concept are underway in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
wangcy@iccas.ac.cn; xqsun@cczu.edu.cn
■
Notes
The authors declare no competing financial interest.
(6) (a) Rakshit, S.; Grohmann, C.; Besset, T.; Glorius, F. J. Am. Chem.
ACKNOWLEDGMENTS
■
Soc. 2011, 133, 2350. (b) Hyster, T. K.; Knorr, L.; Ward, T. R.; Rovis, T.
̈
This work is dedicated to Prof. Zhi-Tang Huang on the occasion
of his 85th birthday. Generous financial support from the Natural
Science Foundation of China (21002103, 21272238), the Major
State Basic Research Development Program (2012CB821600),
and Institute of Chemistry, CAS are gratefully acknowledged. We
also thank Prof. Hui Chen (ICCAS) for helpful discussions.
Science 2012, 338, 500. (c) Ye, B.; Cramer, N. Science 2012, 338, 504.
(d) Li, B.; Ma, J.; Wang, N.; Feng, H.; Xu, S.; Wang, B. Org. Lett. 2012,
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(8) Xia, D.; Wang, Y.; Du, Z.; Zheng, Q.-Y.; Wang, C. Org. Lett. 2012,
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