Diversity-Oriented Synthesis through Rh-Catalyzed Selective Transformations of a Novel Multirole Directing Group

Article abstract of DOI:10.1002/cctc.201500410

In the context of transition-metal-catalyzed C-H functionalization, directing-group strategy was developed for the improvement of chemical reactivity and selectivity. Recently, to avoid the inherent limitations of traditional mono-role directing groups, a dual-role oxidizing-directing-group strategy was developed, in which the directing group acts both as directing group and oxidant. Herein, we report a multirole directing group, which possesses multiple reactive sites, exhibits unique reactivity and selectivity, and leads to four different types of products from a single starting material through rhodium-catalyzed C-H activation/alkyne annulation reactions. The excellent product diversity and regio- and redox selectivity were well controlled by the tuning of solvents and oxidants. Chemical multitasking: A novel N-N bond containing a multirole directing group is developed, which enables the synthesis of four different products selectively from the same starting material through Rh^III catalysis. Key features include ready availability, multiple reactive sites, high redox selectivity and controllability, and multifarious transformations toward important scaffolds and structural diversification. Phth=phthaloyl; DCE=1,2-dichloroethane.

Full text of DOI:10.1002/cctc.201500410

DOI: 10.1002/cctc.201500410
Full Papers
Diversity-Oriented Synthesis through Rh-Catalyzed
Selective Transformations of a Novel Multirole Directing
Group
Bo Su,[a] Jiang-bo Wei,[a] Wen-lian Wu,[b] and Zhang-jie Shi*[a]
In the context of transition-metal-catalyzed CÀH functionaliza-
tion, directing-group strategy was developed for the improve-
ment of chemical reactivity and selectivity. Recently, to avoid
the inherent limitations of traditional mono-role directing
groups, a dual-role oxidizing-directing-group strategy was de-
veloped, in which the directing group acts both as directing
group and oxidant. Herein, we report a multirole directing
group, which possesses multiple reactive sites, exhibits unique
reactivity and selectivity, and leads to four different types of
products from a single starting material through rhodium-cata-
lyzed CÀH activation/alkyne annulation reactions. The excellent
product diversity and regio- and redox selectivity were well
controlled by the tuning of solvents and oxidants.
Introduction
Diversity-oriented synthesis is an important access to the rapid
build-up of molecular complexity and diverse synthesis, which,
in turn, offers great opportunities for the discovery of new
drugs and exploration of complex biological processes. General
requirements for diversity-oriented synthesis include high func-
tional-group tolerance, multiple reactive sites, and/or good
controllability and selectivity. In the context of transition-
metal-catalyzed CÀH activation chemistry,[1] one important
strategy to increase the selectivity and controllability is the in-
troduction of a directing group (DG), which can direct the
metal catalyst to one specific reactive site among many others
and thus largely increase the selectivity.[2] However, the utiliza-
tion of DGs also has some disadvantages,[3] including 1) the DG
only plays a directing role at the CÀH activation stage and
always leaves a chemical trace in the product, 2) the introduc-
tion and removal of the DG increases the number of synthetic
steps, and 3) further manipulation of the DG is always difficult
and impractical, which largely limits the structure diversity of
the product. To address these limitations, the development of
multirole DGs has been paid more attention in the last several
years. For example, although traditional oxidative cross-cou-
pling/cyclization reactions have been widely applied in the
construction of various heterocycles, such as indoles,[4] pyrro-
les,[4b,d,5] pyridones,[6] isoquinolines,[7] and isoquinolones,[6a,8] the
DG plays solely a directing role, and a stoichiometric amount
of external oxidants are indispensable for the regeneration of
the catalysts (Scheme 1A). To address these drawbacks,
Fagnou and co-workers,[9] Tan and Hartwig,[10] Glorious and co-
workers,[11] and other groups developed the dual-role oxidiz-
ing-directing-group (DGox) strategy (Scheme 1B),[12] in which
DGox acts both as directing group and oxidant. Among the pre-
[a] Dr. B. Su, J. Wei, Prof. Z. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS)
and Key Laboratory of Bioorganic Chemistry
and Molecular Engineering of Ministry of Education
College of Chemistry and Green Chemistry Center
Peking University
Beijing 100871 (P.R. China)
Fax: (+86)10-6276-0890
E-mail: zshi@pku.edu.cn
Homepage: http://www.chem.pku.edu.cn/zshi/
[b] Dr. W. Wu
Merck Research Laboratories
2015 Galloping Hill Road, Kenilworth, NJ 07033 (USA)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500410.
This publication is part of a Special Issue on Carbon in the Catalysis
Community. Once the full issue has been assembled, a link to its Table of
Contents will appear here.
Scheme 1. Evolution of the CÀH activation strategy. Phth=phthaloyl.
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viously reported DGox variants, NÀO bond containing function-
al groups have been well investigated because of their ready
cleavability.[9–,12c–e,g–o,13] Relative to the NÀO bond, the NÀN
counterpart was considered to be more difficult to cleave as
a result of the elevated bond strength.[9b] However, some
unique features make the NÀN bond containing DG much
more appealing as an alternative DGox: 1) The trivalent nitro-
gen atom can provide more chances to tune the properties
(for example, reactivity, redox controllability of the DGox, regio-
and chemoselectivity) by adjusting the substituents on it;
2) with well-controlled cleavage of the NÀN bond, such
a group may play multiple roles as a directing group, internal
oxidant, and base; and, last but not least, 3) the intact NÀN
bond offers another access to diversify the reaction pathway
to construct different scaffolds from the same starting materi-
als. Although some attention has been paid to the develop-
ment of novel NÀN containing DGox compounds,[14] to the best
of our knowledge, there is no such case in which the NÀN
bond containing DGox exhibited advantages over the previous
NÀO bond containing DGox counterparts. Herein, we first
report a novel NÀN bond containing DGox derived from benzo-
hydrazine derivatives, which reacts with internal alkynes under
Rh catalysis and results in some novel unparallel reactivities
and unique site selectivity (Scheme 1C).
Table 1. Optimization of the RhIII-catalyzed redox-neutral CÀH activation/
annulation isoquinolinone synthesis.[a]
Entry
Catalyst
([mol%])
Additive
([mol%])
Solvent
Yield[b]
[%]
1
2
3
4
5
6
7
8
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (3.0)
[RhCp*Cl2]2 (4.0)
[RhCp*Cl2]2 (5.0)
CsOAc (25)
CsOAc (50)
CsOAc (75)
DCE
DCE
DCE
60
76
75
79
88
90
90
61
0
trace
0
0
72
76
CsOAc (100) DCE
CsOAc (50)
CsOAc (50)
CsOAc (50)
DCE
DCE
DCE
DCE
[RhCp*(MeCN)3](SbF6)2 (5.0) CsOAc (50)
9
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
[RhCp*Cl2]2 (2.5)
CsOAc (50)
CsOAc (50)
CsOAc (50)
CsOAc (50)
CsOAc (50)
–
DCE
10
11
12
13[c]
14[d]
DMF
MeOH
tAmylOH
DCE
DCE
[a] Reactions were carried out by using RhIII catalyst, additive, 1a
(0.1 mmol), and 2a (0.15 mmol) in solvent (1 mL) for 24 h at T=808C
under N2. [b] Yield of isolated product. [c] 2a (0.12 mmol) was used.
[d] 2a (0.20 mmol) was used.
Results and Discussion
We introduced a highly electron-withdrawing phthaloyl group
at the distal nitrogen atom of benzohydrazine. In our design,
the phthaloyl group not only increases the acidity of the proxi-
mal NÀH group but also weakens the NÀN bond. We initially
investigated the reactivity of 1a under external-oxidant-free
conditions. Preliminary experimental results showed that com-
pound 1a and diphenylacetylene (2a) could be transformed
into the desired CÀH activation/annulation product 3a in 60%
yield if [Cp*RhCl2]2 (2.5 mol%; Cp*=1,2,3,4,5-cyclopentadienyl)
and CsOAc (25 mol%) were employed in 1,2-dichloroethane
(DCE) at T=808C under N2 (Table 1, entry 1). After extensive
screening, the optimal reaction conditions were found to be
[Cp*RhCl2]2 (4.0 mol%) and CsOAc (50 mol%) in DCE at T=
808C under N2 (Table 1, entry 6), which afforded 3a in 90%
yield.
The substrate scope was then surveyed (Scheme 2). Various
substituted benzohydrazines 1 reacted smoothly with dipheny-
lacetylene (2a), and those bearing electron-donating groups
gave relatively higher yields. Notably, different substituents (for
example, free amine in 3c, acetyl in 3d, various halogens (in-
cluding iodine) in 3g–3j, and cyan in 3k) and a thiophene (in
3l) were well tolerated. As a result of the bulkiness of the di-
recting group, the reaction was sensitive to steric effects. If
meta-substituted (3m–3o) benzoyl hydrazines were employed,
single regioisomers were isolated. Unfortunately, ortho-substi-
tuted (Me and MeO) substrates did not react. The scope of the
accepted diarylacetylenes was also examined. Both electron-
rich and electron-poor aryl-substituted alkynes gave good
yields, and electron-rich substrates showed higher reactivities.
Unsymmetrical alkynes gave a mixture of regioisomers (3u). If
Scheme 2. Reaction scope for the formation of NÀN cleaved product 3. Re-
actions were carried out by using 1 (0.1 mmol), 2 (0.15 mmol), [RhCp*Cl2]2
(4 mol%), and CsOAc (50 mol%) in DCE (1 mL) for 24 h at T=808C under N2.
Yields refer to isolated products.
a methyl group was introduced at the 2-position of the aryl
group, the regioselectivity increased (3v).
To test the redox versatility of the newly developed oxidiz-
ing directing group and with the aim of keeping the valuable
NÀN bond intact, we next introduced an external oxidant to
the reaction system. It was found that, if Cu(OAc)2 (2.0 equiv.)
was employed in DCE, the desired product 4a was obtained in
69% yield (Table 2, entry 5). Interestingly, besides compound
4a, the oxygen-directed product 4aa and cascade cyclization
product 5a were also detected. To our satisfaction, these trans-
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Table 2. Optimization of the oxidizing controllable directing group direc-
tedRhIII-catalyzed diversified products syntheses.[a]
Entry
Oxidant
([equiv.])
Solvent
Yield of
4a+4aa[b] [%]
Yield of
5a[b] [%]
1
2
3
4
5
6
7[c]
8[c]
9
10
11[e]
12[e]
13[e]
14[e]
15[e]
16[e]
17[e]
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (0.2)
AgOAc (2.2)
AgOAc (2.2)
AgOAc (2.2)
AgOAc (4.4)
AgOTs (2.2)
THF
trace
trace
trace
trace
32
25
12
10
trace
–
29
41
63[d]
58[d]
trace
trace
trace
trace
trace
MeOH
MeCN
DMF
9+28
18+35
69+14
66+18
78+8
(76+7)[d]
(7+61)[d]
6+58
–
DCE
toluene
toluene
toluene
MeCN
PhCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
–
trace
trace
trace
trace
AgF (2.2)
Scheme 3. Reaction scope for the formation of NÀN intact product 4. Reac-
tions were carried out by using 1 (0.1 mmol), 2 (0.15 mmol), [RhCp*Cl2]2
(2.5 mol%), and Cu(OAc)2 (20 mol%) in toluene (1 mL) for 12 h at T=808C
under O2. Yields refer to isolated products. [a] AgOAc (2.0 equiv) was used in
MeCN (1 mL) for 12 h at T=808C under air.
AgNO3 (2.2)
AgOTFA (2.2)
AgOTf (2.2)
trace
[a] Reactions were carried out by using RhIII catalyst (2.5%), oxidant, 1a
(0.1 mmol), and 2a (0.15 mmol) in solvent (1 mL) for 12 h at T=808C
under air. [b] NMR spectroscopy yield with 1,3,5-trimethoxybenzene as an
internal standard. [c] The reaction was conducted under O2. [d] Yield of
isolated product. [e] 2a (0.30 mmol) was used. OTs =toluene-4-sulfonate;
TFA=trifluoroacetic acid; OTf=trifluoromethanesulfonate.
formations can be well controlled by slightly changing the re-
action conditions. The formation of nitrogen-directed product
4a was favored in nonpolar solvents (Table 2, entries 5 and 6),
whereas compounds 4aa and 5a were preferentially formed in
polar solvents (Table 2, entries 3 and 4). If the reaction was car-
ried out in toluene under O2, both the oxygen-directed prod-
uct 4aa and the cascade cyclization product 5a were largely
suppressed (Table 2, entry 7). Importantly, if the loading of
Cu(OAc)2 was reduced to catalytic amounts (Table 2, entry 8),
comparable results were achieved. Interestingly, if Cu(OAc)2
was replaced with AgOAc (Table 2, entry 9), compound 4aa
became the major product. With an increase in the amount of
2a to 3.0 equivalents, cascade cyclization product 5a was ob-
tained as the major product in synthetically useful yields
(Table 2, entry 11), and it could be easily separated from com-
pounds 4a and 4aa. Other silver salts gave inferior results
(Table 2, entries 13–17).
Scheme 4. Reaction scope for the formation of cascade cyclization product
5. Reactions were carried out by using 1 (0.1 mmol), 2 (0.30 mmol),
[RhCp*Cl2]2 (2.5 mol%), and AgOAc (2.2 equiv) in MeCN (1 mL) for 12 h at
T=808C under air. Yields refer to isolated products. p-Tol=p-tolyl; m-
Tol =m-tolyl.
Various benzoylhydrazines 1 and diarylacetylenes 2 were
tested for the synthesis of 4 (Scheme 3). Similar reactivity was
observed to that in previous studies. Differently substituted
benzoylhydrazine derivatives 1 reacted with various alkynes 2
good yield (for a detailed procedure, see the Supporting Infor-
smoothly, to give the N-directed products 4 with an intact NÀ mation).
N bond in moderate to good yields. The structures of com-
pounds 4aa and 4d were unambiguously confirmed by X-ray
analysis.[15] Through treatment with hydrazine hydrate in etha-
nol, the phthaloyl group in 4a could be easily be removed in
Several different compounds 1 and 2 were also examined
for the corresponding cascade cyclization to produce 5. Most
of them worked well to produce 5 in moderate to good yields
(Scheme 4). Notably, if the thiophene-2-carbohydrazide deriva-
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Under external-oxidant-free conditions, intermediate II might
have undergone migratory insertion to give intermediate
IV,[9,12a,14a] which then afforded product 3a after protonation in
the nonpolar solvent (path C). However, in the presence of an
AgI species, sequential CÀH activation occurred (path D) to fur-
nish intermediate V, which gave product 5a after the second
alkyne insertion and protonation processes.
Conclusions
We have developed a novel NÀN bond containing oxidizing di-
recting group. To the best of our knowledge, this is a unique
directing group, which displays a range of advantages, includ-
ing readily availability, multiple reactive sites, high redox selec-
tivity and controllability, and multifarious transformations
toward important scaffolds and structural diversification. This
directing group enables NÀN bond cleavage, NÀN intact N-di-
rected and O-directed annulations, and cascade cyclizations
highly selectively from the same single starting material
through RhIII catalysis. The features of redox controllability of
the directing group and great product diversity were accessed
by tuning of the oxidants and solvents. We hope that this
chemistry can inspire more structure-diversity-oriented synthe-
sis with new developments of directing groups in the field of
CÀH activation.
Scheme 5. Control experiments. cod=cycloocta-1,5-diene; coe=cyclo-
octene.
tive was used, compound 5g was isolated in 90% yield, with
only a trace amount of other byproducts.
To probe the process of the formation of 5a, some control
experiments were done (Scheme 5). If 4a was subjected to the
standard reaction conditions to form 5a in the presence of
either RhIII or RhI catalysts, no 5a was detected. If 3a was treat-
ed with the RhIII catalyst in the presence of 2a, 5a was ob-
tained in 86% yield. These results implied that 3 might be
a specific intermediate for the formation of 5.
Based on the observations and previous studies, an illustra-
tive mechanism was proposed to explain the product diversity
(Scheme 6). All product formation was initiated by the directed
Experimental Section
Compounds
1 (0.1 mmol), 2 (0.15 mmol), [RhCp*Cl2]2 (4.0%,
2.5 mg), and CsOAc (50%, 9.6 mg) were added to a 50 mL dry
schlenk tube with a stirring bar under N2. The tube was degassed
and refilled with N2. DCE (1.0 mL) was injected and the mixture
was stirred at T=808C for 24 h. After the reaction mixture had
cooled to room temperature, the solvent was removed and com-
pound 3 was obtained by column chromatography purification
(silica gel, CH2Cl2/EtOAc).
Acknowledgements
Support of this work by the “973” Project from the MOST of
China (2013CB228102 and 2015CB856600), NSFC (nos. 21332001
and 21431008), and MSD China Postdoctoral Research Fellow
Program is gratefully acknowledged.
Scheme 6. Proposed mechanism for the formation of diversified products.
Keywords: alkynes · CÀH activation · directing groups · redox
chemistry · rhodium
CÀH activation of substrate 1. In the nonpolar solvent, N-di-
rected aza-cyclorhodium species I was formed preferentially
(path A) and then coordinated with alkyne 2a; this was fol-
lowed by migratory insertion to afford the seven-membered
rhodium intermediate II. After reductive elimination, product
4a with the NÀN bond intact was formed, and the resulting
RhI species was reoxidized to the RhIII catalytic species by cop-
per(II) under O2. In polar solvents, O-directed CÀH activation
was favored to give intermediate III (path B), and the isomer
4aa was produced through a similar process to that for 4a.
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Received: April 14, 2015
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Published online on August 10, 2015
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