Rhodium-catalyzed C-H activation of hydrazines leads to isoquinolones with tunable aggregation-induced emission properties

Article abstract of DOI:10.1039/c5cc05239d

Using an internally oxidizing directing group (DG) strategy, we report a Rh^III-catalyzed synthesis of isoquinolones via C-H activation/annulation of benzoylhydrazines and alkynes. Tunable double cascade cyclization of benzoylhydrazines with two equivalents of alkynes led to tetracyclic amides. These N-heterocycles demonstrated adjustable AIE properties.

Full text of DOI:10.1039/c5cc05239d

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DOI: 10.1039/C5CC05239D
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Rhodium-Catalyzed C–H Activation of Hydrazines Leads to
Isoquinolones with Tunable Aggregation-Induced Emission
Property†
Received 00th January 20xx,
Accepted 00th January 20xx
Bole Yu,‡a Ying Chen,‡a Mei Hong,a Pingping Duan,a Shifeng Gan,b Hui Chao,*c Zujin Zhao,*b and
DOI: 10.1039/x0xx00000x
Jing Zhao*a,d
www.rsc.org/
Using an internally oxidizing directing group (DG) strategy, we
report
RhIII-catalyzed synthesis of isoquinolones via C–H
a
activation/annulation of benzoylhydrazines and alkynes. Tunable
double cascade cyclization of benzoylhydrazines with two
equivalents of alkynes led to tetracyclic amides. These N-
heterocycles demonstrated adjustable AIE properties.
Aggregation-induced emission (AIE) refers to a photophysical
phenomenon, in which the nonemissive molecules are induced to
emit by aggregate formation.1 The restriction of intramolecular
rotation (RIR) in the aggregated state plays a key role in inducing
luminescence. A number of luminogenic molecules with benzene
rotors have been found to show pronounced AIE effect.2 For
example, Tang et al. first discovered hexaphenylsilole (HPS) and
tetraphenylethene (TPE) (Scheme 1) as AIE molecules in which
multiple phenyl groups rotated against the core stator in dilute
solutions, non-radiatively dissipating the decay energy. Although
AIE research has been rapidly developing in recent years, the stator
stators, core structures containning heteroatoms have also been
investigated.3 However, the potential functionality of the stator
cores are typically overlooked. New synthetic strategies for the
incorporation of functionally tunable heteroatoms into the AIE
molecular structure library are highly desirable.4
Scheme 1 N-heterocycles as new AIE molecules.
our group has synthesized several N-heterocyclic scaffolds with the
aid of N-containing directing groups.6 Notably, the function
availability of the N-heterocycles enabled proliferation controll of
reaction cascades, resulting in molecules with multiple phenyl
groups. We hypothesized that these phenyl groups might serve as
rotators for builing AIE materials. To explore N-heterocycles as
potential AIE molecules, we were attracted to recent examples on
internal-oxidant directing, RhIII-catalyzed C–H bond activation of
arenes. In those cases, the oxidative cleavage of N–O and N–N
bonds in the directing groups (DGs) have yielded diverse N-
heterocycles such as indoles and isoquinolones.7 These N-
heterocycles, as demonstrated in this study, are excellent AIE
luminogens when properly substituted, whose photoluminescent
properties have rarely been reported before (Scheme 1).
We started our synthetic efforts from readily available
benzoylhydrazines (1). Inspired by the landmark work by Glorius
and Cheng groups,7d,7o we hope to utilize the redox neutral N–N
bond cleavage ability in the hydrazine acetyl DG to escort the RhIII-
catalyzed intermolecular coupling with alkynes (2) for generating
isoquinolones (3). Compared to the several previously reported
ingenious designs to access isoquinolones through N–O bond
Metal-catalyzed C–H activation has been widely explored to
convert cheap and abundant raw materials into complex structures
with various functional groups.5 Ultilizing C–H activation strategy,
a.Shenzhen Key Lab of Nano-Micro Material Research, School of Chemical Biology
and Biotechnology, Shenzhen Graduate School of Peking University, Shenzhen,
518055, China. E-mail: jingzhao@pkusz.edu.cn
b.Guangdong Innovative Research Team, State Key Laboratory of Luminescent
Materials and Devices, South China University of Technology, Guangzhou 510640,
China. E-mail: mszjzhao@scut.edu.cn
c. MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry
and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-
mail: ceschh@mail.sysu.edu.cn
d.Institute of Chemistry and BioMedical Sciences, State Key Laboratory of
Pharmaceutical Biotechnology, School of Life Sciences, Nanjing University,
Nanjing, 210093, China
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data for all new compounds. CCDC1055195 (3ak), CCDC1055196
(
4ah), CCDC1055197 (4ai) and CCDC1055198 (4aj). For ESI and crystallographic
data in CIF or other electronic format see DOI:10.1039/b000000x/
‡ These authors contributed equally.
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clevage, our approach is more cost-effective. This is, to the best of
our knowledge, the first time that RhIII-catalyzed redox-neutral C–H internal oxidant features improved reactiviDtyOaI:n1d0.1s0e3le9c/Cti5vCitCy,05a2s3w9Dll
activation/cyclization strategy is used for AIE study. as a broader scope, compared to that with an external oxidant.
Generally, the C–H activation synthetic methodology with an
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In preliminary experiments, benzoylhydrazine (1a) was treated Taking this advantage, we prepared a series of isoquinolones 3,
with [(Cp*RhCl2)2] (2.5 mol%), CsOAc (25 mol%), and which allowed us to further explore their structure-property
diphenylacetylene (2a, 1.2 equiv) in MeOH under N2 at 70 °C for 16 relationship. As shown in Table 1, benzoylhydrazine derivatives
h, but no reaction occured (Table S1, entry 1). When HOAc (1.2 bearing electron-donating substitution groups such as methoxyl
equiv) was added, we could obtain product 3aa in 38% yield (1b), methyl (1c, 1i) or electron-withdrawing substitution groups
without any external oxidants (Table S1, entry 2). A lower yield was such as trifluoromethyl (1h) participated well in this reaction and
isolated when a less polar solvent such as MeCN or DCE was used gave desired products in good to excellent yields ranging from 77%
(Table S1, entry 3, 4). Adjusting the reaction temperature, as well as to 95%. The reaction of 2a with halo-substituted benzoylhydrazines
the catalyst and additive amount, improved the yield to 86% (Table (1d-g, 1j-l) also afforded the corresponding isoquinolones in yields
S1, entry 5-9). Next, we examined different additives and found ranging from 67% to 96%. With the same substitution group, the
that when PivOH (4 equiv) and NaOAc (50 mol%) were used in place yield was much higher for the para-substituted benzoylhydrazines
of HOAc and CsOAc, the yield further increased to 95% (Table S1, than those with the ortho-positions occupied (3ca > 3ia, 3da > 3ja,
entry 10-12). Slightly lowering the catalyst loading from 5 mol% to 4 3ea > 3ka, 3fa > 3la). Fused benzene rings were also tolerated, and
mol% and shortening the reaction time from 16 h to 12 h did not the reaction of 2a with 2-naphthoichydrazide (1m) resulted in a
impact the reaction yield (Table S1, entry 13, 14). The yield moderate yield of 70%. As for the coupling partner, both
obtained with [Cp*Rh(OAc)2] as the catalyst was similar to those symmetrical and unsymmetrical alkynes were tolerated. Besides 2a,
obtained with the [(Cp*RhCl2)2]/NaOAc system (Table S1, entry 15), symmetrical diphenylacetylenes with p-Me, p-tBu, p-F, p-Cl and p-
thus indicating that [Cp*Rh(OAc)2] might be the active catalyst. CF3 substitution groups (2b-f), as well as di(β-naphthyl)acetylene
Eventually we set the standard reaction conditions to be (2g) and 3-hexyne (2h) reacted well with 1a under the standard
[(Cp*RhCl2)2] (4 mol%), NaOAc (50 mol%), and. PivOH (4 equiv) in reaction conditions to afford isoquinolones (3ab-h) in yields ranging
MeOH under N2.
from 60% to 70%. For unsymmetrical alkynes, the present reaction
showed excellent regioselectivity. Thus, the reaction of alkyl-
(phenyl)alkynes (2i-k) with 1a gave single regioisomeric products
(3ai-k) in yields ranging from 65% to 74%. The structures of the
regioisomers were confirmed by 1D-NOESY study or X-ray
crystallography.
Table 1 Substrate Scope of Benzoylhydrazines and Alkynes a,b
The structures of 3 include an unprotected amide which,
according to the previous reports,8 might undergo additional C–H
functionalization cascades that would allow direct access to
structurally more complex polycyclic heterocyles in one pot.
Therefore, we investigated the one-pot double cascade reactions
of 1a with the same or different alkynes. Enouragingly, 5,6,13-
triphenyl-8H-dibenzo[a,g]quinolizin-8-one derivatives
4
were
produced in moderate to high yields ranging from 45% to 72%,
(Table 2). As a confirmation of our previous observation, the
progress of the current reaction cascades were also fully controlled
by oxidants and temperature. After careful screening of the
reaction conditions for the subsequent ring closing metathesis, we
chose to use 2.2 equiv. of the second alkyne 2, 2.2 equiv. AgTFA,
and reaction temperature of 100 °C. Both aryl,aryl- and alkyl,alkyl-
disubstituted alkynes were tolerated. X-ray crystallography data
proved the assembly of tetracyclic amides 4, as well as the
configurations of single regioisomers obtained with unsymmetrical
internal acetylenes.
Isotope experiments were carried out for probing the catalytic
reaction mechanism. First, the substrate 1a was subjected into the
deuterated PivOH and CH3OH, affording the corresponding product
with 92% deuterium incorporation. This suggested that C–H bond
activation was reversible [Fig. S1a, Eq. (1)]. When the standard
reaction was carried out in the deuterated conditions, we obtained
the desired product without any D-substitution at C-8 position [Fig.
S1a, Eq. (2)]. This result showed that the alkyne insertion was
irreversible. The parallel experiments gave the ratio of KH/KD=3.0,
indicating that C–H bond cleavage was involved in the
aReaction conditions: 1 (0.2 mmol), 2 (0.24 mmol) in MeOH (1 mL),
bisolated yields.
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Table 2 Substrate Scope in One-Pot Double Cascade a,b
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DOI: 10.1039/C5CC05239D
Fig. 1 (a) Emission spectra of 3aa in THF/water mixtures with
different water fractions with an excitation wavelength of 310 nm
(concentration: 10 M); (b) Fluorescent photographs of 3aa with
(0% and 95% water) under a hand-held UV lamp with an excitation
wavelength of 365 nm; (c) SEM image of the nanoaggregates
formed with 95% water.
With a wealth of structures at hand, we then measured their
photophysical spectra to systematically investigate their AIE
behaviors. The absorption spectra of the compounds 3 and 4 in THF
solutions (10 M) are given in Fig. S2 and S3. Most compounds 3
showed absorption maxima at ~310 nm except for 3ma (338 nm),
3ag (321 nm) and 3ah (339 nm). The absorption maxima of
compounds 4 are located at ~395 nm, being red-shifted in
comparison with those of compounds 3. The photoluminescence
(PL) spetra and fluorescence quantum yields () in THF (10 M) and
in powders are given in Fig. S4, S5 and Table S2. Most compounds
show weak emission in THF solutions but enhanced emission in
powders, indicating they are AIE-active. The powders of 3 and 4
show blue and green emissions, respectively. The emission
wavelengths of these powdery products could be further tuned by
different substituents, which, in our case, were 389-467 nm for 3
and 500-550 nm for 4. The emission enhancements (Table S2),
calculated as the fluorescence quantum yield of the powders over
that of the THF solutions (10 M), are as high as 21.8 and 36.3, for 3
and 4, respectively.
aReaction conditions: 1 (0.2 mmol), 2a (0.21 mmol), 2 (0.40 mmol)
in MeOH (1 mL). bisolated yields.
rate-determining step[Fig. S1a, Eq. (3)].9
The impact of substituion groups on the AIE activity is further
studied taking compounds 3 as models. As a basis, the  value of
3aa in powders was 4.4 times higher than that in THF solution.
Different substituents on the isoquinolone skeleton of the 3,4-
diphenyl isoquinolones (3ba-3ma) greatly affected the values of
emission enhancement. In general, the para-positioned such as
methoxy as in 3ba and fluoro in 3da contributed to a marked AIE
activity, showing more than 20 times emission enhancement. The
electron-withdrawing substitution groups, however, exhibited no
obvious contributions to the AIE activity. For instance, 3ha bearing
trifluoromethyl barely gave a 1.1-fold enhancement. With the same
substitution group, when the para-positions of the isoquinolone
moiety was occupied, their emission enhancements were higher
than the coresponding ortho-substitituted products (3ca > 3ia, 3da
> 3ja, 3ea > 3ka). With halo-substituted group as Br or I, the
emission of products (3fa, 3ga and 3ia) was weak both in solution
and in powders, due to the heavy atom effect.
A plausible mechanism was proposed (Fig. S1b). First, the active
catalyst Cp*Rh(OAc)2 was generated via anion exchange. Then C–H
activation of benzoylhydrazine with Cp*Rh(OAc)2 afforded a five-
membered cyclorhodium intermediate A, which was followed by
alkyne insertion, thus affording the seven-membered species B.
Then, reductive elimination allowed the C–N bond formation of
intermediate C. Subsequently, a fast oxidative addition of CpRh(I) to
N–N bond produced intermediate D, which was finally protonated
by acid to yield the desired product 3 and regenerated the
catalyst.7d,7o The mechanism of RhIII-catalyzed oxidative annulation
of 3 with alkynes has been investigated,8 and a RhIII/RhI/RhIII
catalytic cycle has been proposed for the reaction.10
Gratifyingly, the obtained 3,4-diphenyl isoquinolone 3aa exhibits
AIE behaviors. Adding anti-solvent of water into the THF solution of
3aa induced solute aggregation, and at the same time, increased
the photoluminescence of 3aa at an emission wavelength of 394
nm. Compared to THF solution of 3aa, the emission increased 12
fold for the suspension of 3aa in a mixed solvent of 95% water and
5% THF. The solids of 3aa emitted blue light and were of rectangle
prism shape with size of 300-500 nm as revealed by SEM images
(Fig. 1).
Altering the substitution groups in the rotors (3ab-3ak) also show
significant impact on their AIE property. Isoquinolones 3aa-3af with
two benzene rotors gave obviously higher emission enhancements
than 3ai or 3aj with one benzene rotor. 3ag with two naphthyl
rotors shows a low emission enhancement because of a higher 
value in THF solution and a lower one in the powders, caused by
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41, 3878; (d) S. D. Xu, T. T. Liu, Y. X. Mu, Y. F. Wang, Z. G. Chi,
lowered rotational motions of naphthyl rings and increased
intermolecular interactions between larger naphthyl rings in the
aggregated state, respectively. When the aromatic rotors are
replaced by the alky chains, such as ethyl chains in 3ah, the non-
radiative decay channel of the excited state is blocked to a larger
extent, resulting in a higher  value in solutions. Meanwhile, owing
to the planar conformation of 3ah, intermolecular interactions are
promoted, leading to a decreased  value in aggregated state. Both
effects work collectively, making 3ah exhibit aggregation-caused
quenching effect (ACQ). 3ma also shows ACQ effect because fusing
a phenyl ring to isoquinolone can effectively enlongate the
conjugation of the stator, resulting in red-shifted absorption and
emission, and improved  value in solution. The large planar core is
prone to form intermolecular interaction, rendering a decreased 
value in the aggregated state. Such ACQ effect, however, is
allievated in compounds 4 because of the twisted conformation of
the stator as disclosed by their crystal structures (Fig. S6-S8).
C. C. Lo, S. W. Liu, Y. Zhang, A. Lien and J. RV.ieXwuA,rtiAclenOgnelwine.
DOI: 10.1039/C5CC05239D
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prepare isoquinolones by coupling readily available
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synthetic strategy, we successfully assembled new AIE molecules 3
with isoquinolone skeleton and 4 containing tetracyclic amides. The
broad substrate scope and the high reaction efficiency allowed us
to freely adjust the substitutions on the stator as well as the
chemical structures of the rotors. Our prelimanry results on the
structure−property relationships suggested the following trends:
(1) molecules with two benzene rotors showed more pronounced
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We expect that this work will help researchers to identify and
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of
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Shenzhen
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