Redox-Divergent Hydrogen-Retentive or Hydrogen-Releasing Synthesis of 3,4-Dihydroisoquinolines or Isoquinolines

Article abstract of DOI:10.1021/acs.orglett.6b01091

A rare Ru-catalyzed highly selective synthesis of 3,4-dihydroisoquinolines or isoquinolines is accomplished via a redox-divergent hydrogen-retentive or hydrogen-releasing fashion. Notably, high cis-selectivity of 3,4-dihydroisoquinolines is achieved. Potential applications are shown by gram-scale reactions and very concise synthesis of N-containing polycyclic aromatic compounds. Primary mechanistic investigations indicate that the sequence of the major pathway involves Ru-catalyzed C-H activation, alkyne insertion, and subsequent 6π-electrocyclization.

Full text of DOI:10.1021/acs.orglett.6b01091

Letter
pubs.acs.org/OrgLett
Redox-Divergent Hydrogen-Retentive or Hydrogen-Releasing
Synthesis of 3,4-Dihydroisoquinolines or Isoquinolines
Ke-Han He,†,§ Wei-Dong Zhang,†,§ Ming-Yu Yang,† Kai-Li Tang,†,‡ Mengnan Qu,‡ You-Song Ding,†
and Yang Li*,†
†Center for Organic Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China
‡College of Chemistry and Chemical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China
S
* Supporting Information
ABSTRACT: A rare Ru-catalyzed highly selective synthesis of
3,4-dihydroisoquinolines or isoquinolines is accomplished via a
redox-divergent hydrogen-retentive or hydrogen-releasing
fashion. Notably, high cis-selectivity of 3,4-dihydroisoquino-
lines is achieved. Potential applications are shown by gram-
scale reactions and very concise synthesis of N-containing
polycyclic aromatic compounds. Primary mechanistic inves-
tigations indicate that the sequence of the major pathway involves Ru-catalyzed C−H activation, alkyne insertion, and subsequent
6π-electrocyclization.
Scheme 1. Annulations between N−H Imines and Alkynes
oupling reactions are a cornerstone in organic chemistry as
Cthey are widely used in efficient synthesis of value-added
functional molecules.1 From classical transition-metal (TM)-
catalyzed coupling reactions to C−H transformations,2,3 several
generations of synthetic methods have been developed. Ideal
couplings should be H2-releasing with nearly 100% atom-
economy via inert R−H bond cleavage to produce H2 as the
sole byproduct.4−8 In very recent years, construction of C−C, C−
N, C−P, C−S, and S−S bonds via this dehydrogenative strategy
started to be increasingly explored5−7 as did C−Si coupling
systems.8 Despite the progress, H2-releasing couplings remain
largely under-explored. Thus, there is a strong incentive to
develop novel coupling systems.
with the reported seminal Mn catalysis (Scheme 1, eq 1),6
switchable chemoselectivity is achieved.
Heterocycles are prevalent motifs in bioactive compounds and
organic functional materials.9 Consequently, increasing efforts
have been made in their synthesis via a C−H activation
pathway.10 The strategies of these methods are oxidative or
redox-neutral processes. The former requires a stoichiometric
amount of oxidants, such as Cu(II)/Ag(I) salts, while the latter
needs an internal N−O or N−N oxidizing group, which has to be
preinstalled on the substrate and has tedious synthesis. To meet
the goals of environmental friendliness and atom-economy, the
ideal strategy of heterocycle synthesis would be a H2-releasing
coupling with oxygen/air as an oxidant.11
Due to the high activity of Ru complexes in C−H
transformations10,12 and hydrogen production,13 we anticipate
ample potential for these complexes in H2-releasing coupling
reactions. Herein, we report the first highly selective synthesis of
3,4-dihydroisoquinolines or isoquinolines via Ru-catalyzed
redox-divergent hydrogen-retentive or hydrogen-releasing pro-
cesses (Scheme 1, eqs 2 and 3). Notably, synthesis of the former
directly from alkynes is rare (Scheme 1, eq 2).14 Moreover, the
reactions feature an extraordinarily high cis-selectivity. Compared
The studies commenced with the coupling of bis(4-
methoxyphenyl)methanimine (1a) and 1,2-diphenylacetylene
(2a).6 Investigation of the reaction parameters (e.g., solvents,
reaction temperature/time, and catalyst loading) disclosed that
the annulation gave cis-dihydroquinoline 3a in 86% yield with 1
mol % of [RuCl2(p-cymene)]2 at 120 °C in 1,4-dioxane (Table
S1, entry 18). The structure of 3a was unambiguously confirmed
by X-ray crystallography (Scheme 2). The Ru catalyst proved
necessary for the reaction system (Table S1, entry 19). To the
best of our knowledge, there are few reports on the preparation of
3,4-dihydroisoquinolines from alkynes.14 The substrate scope
was next investigated under the optimized conditions. Substrates
bearing electron-donating and weak electron-withdrawing
groups on the phenyl ring of ketimines led to reactions with
high efficiency (3a−d). Introduction of strong electron-with-
drawing groups decreased the reaction yields slightly (3e,f).
Compound 3g was isolated in 49% yield as the major regioisomer
Received: April 15, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01091
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Letter
a
a
Scheme 2. Synthesis of 3,4-Dihydroisoquinolines
Scheme 3. Synthesis of Isoquinolines
a
Reaction conditions: 1 (0.20 mmol), 2a (0.24 mmol), [RuCl2(p-
b
a
cymene)]2 (1 mol %), DMSO (0.5 mL), isolated yields. With 2.5 mol
Reaction conditions: 1 (0.20 mmol), 2a (0.30 mmol), [RuCl2(p-
c
d
b
% of catalyst. With 3 equiv of alkyne. Reaction was performed at 160
cymene)]2 (1 mol %), 1,4-dioxane (2.0 mL), isolated yields. With 2.5
e
f
°C. 1H NMR yield. With 2 equiv of alkyne.
mol % of catalyst, only the regioisomer shown was isolated, and the
c
presence of the other regioisomer could not be confirmed. 1H NMR
yields. Thermal ellipsoids are drawn at 30% probability with most H
atoms omitted for clarity.
d
products in good to excellent yields (4h−o). In contrast to the
selectivity of the corresponding 3,4-dihydroisoquinolines (3n,
3n′, 3o, 3o′), higher regioselectivity was obtained (4n as the only
isomer, 4o/4o′ = 3.1:1; see SI for the single X-ray crystallography
of 4n). Alkyl phenyl-disubstituted alkynes gave two regioisomers
in good yields (4p, 4p′, 4q, 4q′). Substituted phenylacetylenes
achieved moderate yields (4r−u). Dialkyl-substituted alkyne
displayed low activity (4v). Moreover, dithienylacetylene was
compatible (4w).
and the favored, more hindered ortho-C−H activation product,
which should be attributed to the secondary directing effect of
meta-methoxy.6 The scope of the alkynes was further explored.
First, symmetric electron-rich diarylacetylenes (2h,i) gave
reaction activities lower than those of electron-poor ones (2j−
m). Electronically biased diarylacetylenes, however, coupled with
poor regioselectivity (ca. 1:1 for 3n/3n′ and 3o/3o′).
To explore the potential application of the synthetic method,
gram-scale synthesis of 3a (91%) and 4a (80%) was
demonstrated (eq 6). Furthermore, a series of useful polycyclic
Surprisingly, the chemoselectivity was totally switched from
3,4-dihydroisoquinolines to an isoquinoline as a result of H2-
releasing coupling when the solvent was replaced by DMSO
(Table S1, entry 13). Further optimization of reaction conditions
(Table S2) improved the isolated yield of 4a to 86%. [RuCl2(p-
cymene)]2 also proved necessary for this coupling (Table S2,
entry 7). The yield of H2 was 7% for a gram-scale reaction with
80% yield of 4a (eq 4). Traces of H2 were accepted by 1,2-
aromatic products (5a−g, 6a−h) were synthesized in moderate
to excellent yields (Scheme 4).17 The regioselectivity of the
imines was confirmed by the X-ray crystallography of 6d.
To gain insight into the redox-switchable chemoselectivity and
reaction mechanism, a series of experiments were conducted.
Monitoring of the synthesis of 3a (Figure 1a) showed no
apparent stable intermediates or byproducts during the reaction.
Notably, two major intermediates, 3a and bis(4-
methoxyphenyl)methanone (7a), were observed for preparation
diphenylethyne as 2.5% of cis-1,2-diphenylethene was observed
in the reaction mixture. It should be noted that when using
sulfolane as solvent under similar conditions, 4a was obtained in
59% yield with 53% yield of H2 (eq 5). These results suggested
that DMSO acted as a partial hydrogen acceptor. The low yield of
H2 is likely caused by subsequent hydrogen reduction of CO2 to
CH4 as both CO2 and CH4 have been observed. The CO2 likely
originated from the decomposition of small amounts of DMSO
by Ru catalysis [see Supporting Information (SI), Scheme
S1].13c,d,15,16 DMSO was thus used as an optimal solvent for
further investigations with higher reaction yield.
Scheme 4. Synthesis of Polycyclic Aromatic Products
The substrate scope was studied (Scheme 3). Variation of
groups on the phenyl rings of diaryl ketimines from 4-OMe to 4-
CF3 decreased the yield from 86 to 40% (4a−f). Notably, 1-
phenylpentan-1-imine gave moderate yield (4g). Substituted 1,2-
diphenylacetylenes bearing both electron-donating and electron-
withdrawing groups on the phenyl rings afforded desired
a
Reaction conditions: 1 (0.20 mmol), 2 (0.30 mmol), 1,4-dioxane (1.0
b
mL), isolated yields. 1 (0.20 mmol), 2 (0.24 mmol), DMSO (0.4
mL), isolated yields. Thermal ellipsoids are drawn at 30% probability,
and H atoms are omitted for clarity.
c
B
DOI: 10.1021/acs.orglett.6b01091
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Letter
°C (eq 8), which indicated that the reversible C−H activation is
rate-limiting.
For the preparation of isoquinolines, [D5]-1c-0 was decom-
posed to the corresponding ketone in DMSO with catalytic
[RuCl2(p-cymene)]2 in the absence of 2a (eq S11). H/D
scrambling in the intramolecular kinetic isotope experiment also
suggests the reversible C−H activation (eq S12). However, no
further information was obtained.
Additionally, a set of experiments was conducted using
intermediate 8 [(E)-(2-(1,2-diphenylvinyl)phenyl)(phenyl)-
methanimine] under similar conditions with/without
[RuCl2(p-cymene)]2 (eqs S13−S16). The yields of 3c (59%)
and 4c (24%) with catalyst (eq S13) and 3c (74%) under TM-
free conditions in 1,4-dioxane (eq S15) suggest that 6π-
electrocyclization is involved in the catalytic cycle. This
deduction is also supported by the results of experiments in
DMSO (eqs S14 and S16). The observation of 4c and traces of H2
in the absence of Ru should be attributed to DMSO as a hydrogen
acceptor (eq S16).15
Figure 1. Yield/time diagram for preparation of 3a (a) and 4a (b). Yields
were reported as H NMR yields using Cl2CHCHCl2 as an internal
standard (see Tables S3 and S4 for details).
1
of 4a (Figure 1b). This observation indicated that 3,4-
dihydroisoquinoline led to the corresponding isoquinoline via
dehydrogenation. It was further confirmed by the unidirectional
conversion of 3a to 4a (eqs S4 and S5). Moreover, intermediate
7a was confirmed in this reaction, and labeling experiments using
18O-DMSO showed that the oxygen atom of 7a originates from
DMSO. The detected ammonia suggested that the nitrogen atom
of 1a is converted into ammonia (eqs S6 and S7).
Based on these investigations, a proposed reaction mechanism
is shown in Scheme 5. Catalyst precursor [RuCl2(p-cymene)]2
Next, different factors related to the chemoselectivity were
studied. First, a comparable experiment (eq S8) with the reported
Mn catalysis (eq 1) disclosed that catalysts directly affected the
chemoselectivity. Second, a set of experiments with variable
amounts of DMSO in 1,4-dioxane showed that traces of DMSO
sharply inhibited the catalyst activity toward 3,4-dihydroisoqui-
nolines at both 120 and 140 °C (Figure S1a,b). Meanwhile, with
more than 20 equiv of DMSO in the reaction mixture, the
chemoselectivity showed an inversed tendency. Third, increasing
reaction temperature improved the selectivity toward dihydroi-
soquinolines, while the yield of the isoquinolines was essentially
unaffected (Figure S1c). Fourth, the chemoselectivity toward
isoquinolines is increased at a higher concentration, accompanied
by the reduced selectivity toward dihydroisoquinolines (Figure
S1d). These investigations show that the exhibited chemo-
selectivity is collectively effected by catalysts, solvents, reaction
temperature, and the concentration of reactants.
Scheme 5. Proposed Reaction Mechanism
Kinetic isotope experiments were then conducted. H/D
scrambling in intramolecular isotopic experiments for the
synthesis of 3c suggests the reversibility of the C−H cleavage
(eqs 7 and S9). Labeling studies offered important hints to the
produces active Ru complex 9. Then a reversible electrophilic
substitution between imine 1a and 9 affords intermediate 10. A
subsequent coordination of 2a with 10 and the following alkyne
insertion leads to 12. The σ-bond metathesis between C(sp2)−
Ru and N−H bonds in 12 leads to 4a and Ru species 13 (Scheme
5A, path a). Then 13 reacts with the proton generated in situ at
the initiating step to produce H2 with release of 9. On the other
hand, protonation of 12 affords 15 by 14 via 6π-electrocyclization
(Scheme 5A, path b). Isomerization of 15 via deprotonation of
N−H and protonation of the anion in 16 affords 3a. The cis-
selectivity should be explained by the steric factor. An alternative
pathway for the formation of 3a from 15 is by a [1,5]-sigmatropic
hydrogen shift.18 These pathways agree with the observation of
the retentive hydrogen source in 3c (see eqs 7 and S10). 4a, Ru
species 13, and the proton are generated via transition state 17
from 15. Notably, using DMSO as solvent, the possible apparent
equilibration between 1a and diaryl ketone 7a is shifted by the
subsequent steps, and 12 generation via C−H activation of
ketone 7a cannot be ruled out at this stage (Scheme 5B). The
transformation of 3a to 4a is realized via 18. As mentioned above,
reaction pathway. No deuterium incorporation at the g and h
position was observed for products [D5]-3c-1 and [D5]-3c-2
derived from [D5]-1c. This observation was further consistently
made by trace product in the coupling of [D10]-1c at 140 °C (eq
S10). These data suggest that the H(g) and H(h) should
originate from the NH group. However, essentially no formation
of [D10]-3c-1 and [D10]-3c-2 was detected at 120 °C (eq 9). This
result was in sharp contrast to the smooth coupling of 1c at 120
C
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catalytic cycle (Scheme 5C).
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In conclusion, we developed a chemoselective annulation of
N−H imines and alkynes for synthesis of 3,4-dihydroisoquino-
lines or isoquinolines using the same Ru catalyst. The valuable
assets of the couplings are exhibited by switchable chemo-
selectivity, nearly 100% atom-economy, high cis-selectivity for the
synthesis of 3,4-dihydroisoquinolines and further possible
applications. Mechanistic investigations indicate a new strategy
toward constructing various chemical bonds via H2 release. The
established method will attract wide interest in developing
various catalysis systems for these highly concise cross-couplings.
ASSOCIATED CONTENT
* Supporting Information
■
(6) He, R.; Huang, Z.-T.; Zheng, Q.-Y.; Wang, C. Angew. Chem., Int. Ed.
2014, 53, 4950−4953.
S
(7) Zhou, A.-X.; Mao, L.-L.; Wang, G.-W.; Yang, S.-D. Chem. Commun.
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The Supporting Information is available free of charge on the
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Experimental procedure, characterization data, computa-
tional details (PDF)
Copies of 1H and 13C NMR spectra (PDF)
X-ray data for 3a (CIF)
X-ray data for 4n (CIF)
X-ray data for 6d (CIF)
AUTHOR INFORMATION
Corresponding Author
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*E-mail: liyang79@mail.xjtu.edu.cn.
Author Contributions
§K.-H.H. and W.-D.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support of this work by the startup funding from Xi’an Jiaotong
University and NSFC (No. 21472145) is gratefully acknowl-
edged. We thank Prof. Dr. Zhang-Jie Shi, Peking University, Prof.
Dr. Xing-Wei Li, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, and Prof. Dr. Matthias Beller and Dr. Lin
He, Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock,
̈
̈
Germany, for helpful discussions. We thank Prof. Dr. Yan-zhen
Zheng, Xi’an Jiaotong University, for single X-ray crystallography
measurement.
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