Acetic Acid Promoted Redox Annulations with Dual C-H Functionalization

Article abstract of DOI:10.1021/acs.orglett.7b01047

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-alkylquinoline-3-carbaldehydes as well as the corresponding 4-alkyl isomers and pyridine analogues. These processes involve dual C-H bond functionalization. Acetic acid

Full text of DOI:10.1021/acs.orglett.7b01047

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Letter
pubs.acs.org/OrgLett
Acetic Acid Promoted Redox Annulations with Dual C−H
Functionalization
Zhengbo Zhu and Daniel Seidel*
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United
States
S
* Supporting Information
ABSTRACT: Amines such as 1,2,3,4-tetrahydroisoquinoline under-
go redox-neutral annulations with 2-alkylquinoline-3-carbaldehydes
as well as the corresponding 4-alkyl isomers and pyridine analogues.
These processes involve dual C−H bond functionalization. Acetic
acid is used as a cosolvent and acts as the sole promoter of these transformations.
s part of our continuing efforts to develop methods for the
redox-neutral α-C−H bond functionalization of amines,1,2
Brønsted acid catalyzed variants in addition to additive-free
reactions and catalytic enantioselective versions.11
We evaluated the title reaction under a range of conditions
using 2-methylquinoline-3-carbaldehyde (1a) and 1,2,3,4-
tetrahydroisoquinoline (THIQ) as model substrates (Table
1). The optimized conditions call for using a 1:1 mixture of
A
we have reported on a range of transformations that achieve
amine annulation (Scheme 1).3−5 Specifically, aldehydes with a
Scheme 1. Redox Annulations of Amines
a
Table 1. Reaction Development
entry
deviation from optimized conditions
time (h)
yield (%)
b
1
none
1
25
2
87
complex
complex
complex
30
2
no AcOH
3
20 mol % of AcOH in PhMe
20 mol % of BzOH in PhMe
5 equiv of AcOH in PhMe
10 equiv of AcOH in PhMe
20 equiv of AcOH in PhMe
4
2
5
3
6
3
38
7
1.5
1.5
3.5
3.5
1
57
c
8
PhMe/AcOH = 3:1
84
d
9
PhMe/AcOH = 1:3
80
e
pendent nucleophilic site undergo condensations with amines
in reactions that combine reductive N-alkylation with
concurrent oxidative α-C−H bond functionalization.6 While a
broad range of polycyclic amines have been accessed with this
general method, in all cases, the nucleophilic site has been
limited to rather activated systems including anilines,3a,c,d
phenols,3e thiophenols,3f electron-rich aromatics,3b malonates,3h
nitroalkanes,3g and ketones.3i Here, we report the first examples
of redox annulations that involve alkyl azaarenes as relatively
nonactivated nucleophiles.7
At least in part due to the prevalence of the pyridine nucleus
in bioactive materials,8 the functionalization of the alkyl group
in 2-alkyl azaarenes has drawn significant attention in recent
years.9 The possibly earliest report on “Condensations of
Methylated Quinolines and Pyridines” dates back to 1883 and
describes zinc chloride promoted reactions of quinaldine and
picoline with phthalic anhydride and benzaldehyde.10 Recent
developments include transition metal, Lewis acid, and
10
11
12
AcOH as solvent
73
0.2 M conc
71
0.05 M conc
1.5
87
a
Reactions were performed on a 0.2 mmol scale. All yields correspond
b
c
to isolated yields. Corresponds to 87.5 equiv of AcOH. 44 equiv of
AcOH. 131 equiv of AcOH. 175 equiv of AcOH.
d
e
toluene and acetic acid as the reaction medium at a 0.1 M
substrate concentration. Reflux of 1a and THIQ (1.5 equiv) in
this mixture for 1 h provided desired product 2a in 87% yield.
In the absence of acetic acid, or when using catalytic amounts
of carboxylic acids, only complex reaction mixtures were
observed (entries 2−4). Upon increasing the amount of the
acetic acid promoter, the yield of 2a increased gradually with
Received: April 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01047
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
concurrent reduction in reaction time (entries 5−8). An
increase in acetic acid beyond the 1:1 mixture with toluene led
to a slight drop in yield (entry 9). However, it is notable that
the reaction can be conducted in pure acetic acid as the solvent
(entry 10). A reduction in yield was also observed at higher
substrate concentration (entry 11), whereas little change was
observed under more diluted conditions (entry 12).
1a. Finally, 1-phenyl-THIQ and the corresponding 1,2,3,4-
tetrahydro-β-carboline participated in redox annulations with
1a to provide products possessing a tetrasubstituted stereogenic
center at the site of C−C bond formation.
Compared to 2-alkyl azaarenes, the corresponding 4-alkyl
azaarenes are less activated. In fact, few previous reports have
targeted this class of compounds with regard to alkyl group
functionalization.11b,v Gratifyingly, conditions optimized for 1a
and its analogues proved to be suitable for the redox annulation
of 4-methyl azaarenes (Scheme 3). Interestingly, 4-methylpyr-
idine-3-carboxaldehyde provided higher yields than the
corresponding quinoline derivative.
The scope of the amine annulation with alkyl azaarenes is
outlined in Scheme 2.12 2-Methylquinoline-3-carbaldehydes
a
Scheme 2. Scope of the Redox Annulation.
a
Scheme 3. Redox Annulation with 4-Methylazaarenes
a
Reactions were performed on a 0.5 mmol scale. All yields correspond
b
to isolated yields. AcOH was used as the solvent.
The reaction of 1a with pyrrolidine, an amine that is less
reactive than THIQ in most redox reactions,3−5 required
modified reaction conditions (5 equiv of amine) and elevated
temperatures (eq 1). Product 5 was obtained in 59% yield. We
have previously shown that decarboxylative variants of certain
amine α-C−H bond functionalization reactions can offer
advantages with regard to reaction setup and product
yields.3b,d,13,14 Indeed, decarboxylative condensation of 1a
with proline in acetic acid provided product 5 with an
improved yield of 70% (eq 2).
The overall mechanism of the redox annulation likely shares
many features with previously reported redox transformations
(Scheme 4).6 Accordingly, acetic acid promoted condensation
of 1a and THIQ is expected to give rise to the initial formation
of N,O-acetal 6. This species can undergo loss of AcOH to form
a
Reactions were performed on a 0.5 mmol scale. All yields correspond
b
c
to isolated yields. 20 equiv of acetic acid in PhMe. AcOH was used
as the solvent.
possessing a range of electron-withdrawing or electron-
donating substituents on different ring-positions of the
quinoline core readily underwent annulation with THIQ to
provide products 2 in consistently good yields. Replacement of
the methyl group in 1a with benzyl or ethyl was also tolerated.
These reactions were moderately diastereoselective. Replace-
ment of the aldehyde in 1a with a methyl ketone also allowed
for the synthesis of the corresponding annulation product. 2-
Alkylpyridine-3-carboxaldehydes, which are typically less
reactive than their corresponding quinoline counterparts, also
participated in annulation reactions with THIQ. These
reactions were performed in acetic acid as the only solvent.
In addition, amines other than THIQ underwent reactions with
Scheme 4. Proposed Mechanism
B
DOI: 10.1021/acs.orglett.7b01047
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem., Int. Ed. 2009, 48, 2854. (b) Mahatthananchai, J.; Bode, J. W.
Acc. Chem. Res. 2014, 47, 696. (c) Ketcham, J. M.; Shin, I.;
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44, 1155.
azomethine ylide intermediate 7. The latter reengages AcOH to
form a regioisomeric N,O-acetal that can interact with acetic
acid via 8.11l,q Acetic acid promoted tautomerization to
proposed intermediate 9 is followed by ring closure with loss
of AcOH to ultimately form product 2.
(3) (a) Zhang, C.; De, C. K.; Mal, R.; Seidel, D. J. Am. Chem. Soc.
2008, 130, 416. (b) Zhang, C.; Das, D.; Seidel, D. Chem. Sci. 2011, 2,
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1024.
In summary, we have achieved redox annulations of amines
with various alkyl azaarenes. Acetic acid acts as the sole
promoter of these reactions, which proceed with dual C−H
bond functionalization.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01047.
(4) Selected examples of related intermolecular redox trans-
formations from our laboratory: (a) Ma, L.; Chen, W.; Seidel, D. J.
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(6) For detailed discussions on the mechanisms of these trans-
formations, see refs1u and 3c,e−g and the following report: Ma, L.;
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(7) During the preparation of this manuscript, a related report
appeared in which aluminum triflate (30 mol %) was used as a catalyst.
The substrate scope appears to be more limited: Li, J.; Qin, C.; Yu, Y.;
Fan, H.; Fu, Y.; Li, H.; Wang, W. Adv. Synth. Catal. 2017, 359, x.
(8) Vitaku, E.; Smith, D. T.; Njardarson, J. T. J. Med. Chem. 2014, 57,
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(11) Selected recent examples of alkyl azaarene C−H functionaliza-
tion: (a) Qian, B.; Guo, S.; Shao, J.; Zhu, Q.; Yang, L.; Xia, C.; Huang,
H. J. Am. Chem. Soc. 2010, 132, 3650. (b) Duez, S.; Steib, A. K.;
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Experimental procedures and characterization data
(PDF)
X-ray crystal structures of products 2a (CIF)
X-ray crystal structures of products 2j (CIF)
X-ray crystal structures of products 2l (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: seidel@rutchem.rutgers.edu.
ORCID
Daniel Seidel: 0000-0001-6725-111X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NIH-NIGMS (Grant No.
R01GM101389) is gratefully acknowledged. We thank Dr.
Tom Emge (Rutgers University) for X-ray crystallographic
analysis and Dr. Wazo Myint (Rutgers University) for
assistance with NMR assignments.
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