Copper-catalyzed tri- or tetrafunctionalization of alkenylboronic acids to prepare tetrahydrocarbazol-1-ones and indolo[2,3-a]carbazoles

Article abstract of DOI:10.1039/d0gc01514h

We describe a cascade strategy for tri- or tetrafunctionalization of alkenylboronic acids to prepare diverse tetrahydrocarbazol-1-ones and indolo[2,3-a]carbazoles in good yields withN-hydroxybenzotriazin-4-one (HOOBT) and arylhydrazines as oxygen and nitrogen sources, respectively. Mechanistic studies reveal that the domino reaction undergoes the copper-catalyzed Chan-Lam reaction, [2,3]-rearrangement, nucleophilic substitution, oxidation and sequential [3,3]-rearrangement over five steps in a one-pot reaction. The reaction shows a broad substrate scope and tolerates a wide range of functional groups. More importantly, the reaction is easily performed at gram scales and the product is purified by simple extraction, washing, and recrystallization without flash column chromatography. The present protocol features easily available starting materials, high site-marked functionalization, five-step cascade in one pot, multiple C-C/C-O/C-N bond formation, and diversity of indole motifs.

Full text of DOI:10.1039/d0gc01514h

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Copper-catalyzed tri- or tetrafunctionalization of
alkenylboronic acids to prepare tetrahydrocarbazol-
1-ones and indolo[2,3-a]carbazoles†
Cite this: DOI: 10.1039/d0gc01514h
Hong-Yan Bi,‡ Cheng-Jing Li,‡ Cui Wei,‡ Cui Liang and Dong-Liang Mo
*
We describe a cascade strategy for tri- or tetrafunctionalization of alkenylboronic acids to prepare diverse
tetrahydrocarbazol-1-ones and indolo[2,3-a]carbazoles in good yields with N-hydroxybenzotriazin-4-
one (HOOBT) and arylhydrazines as oxygen and nitrogen sources, respectively. Mechanistic studies reveal
that the domino reaction undergoes the copper-catalyzed Chan–Lam reaction, [2,3]-rearrangement,
nucleophilic substitution, oxidation and sequential [3,3]-rearrangement over five steps in a one-pot
reaction. The reaction shows a broad substrate scope and tolerates a wide range of functional groups.
More importantly, the reaction is easily performed at gram scales and the product is purified by simple
extraction, washing, and recrystallization without flash column chromatography. The present protocol
features easily available starting materials, high site-marked functionalization, five-step cascade in one
pot, multiple C–C/C–O/C–N bond formation, and diversity of indole motifs.
Received 3rd May 2020,
Accepted 20th July 2020
DOI: 10.1039/d0gc01514h
rsc.li/greenchem
organic synthesis, which can introduce two groups into an
alkene in one step to access more complex functional architec-
Introduction
The cascade reaction serves as one of the most important syn- tures.5 Although the use of alkenylboron reagents toward the
thetic strategies to access nitrogen heterocycles of great com- construction of various new bonds is well established, direct
plexity, and has attracted much attention because of its advan- functionalization of alkenylboron reagents would be a facile
tage of multiple bond formation from simple or easily avail- strategy to access diverse functionalized molecules.6 In 2012,
able starting materials in a single step.1 It improves the syn- Anderson and co-workers developed a powerful dioxygenation
thetic efficiency, simplifies operations, reduces waste gene- of alkenylboronic acids to prepare α-oxygenated ketones
ration, and saves time, labor and energy, which have great through a copper-mediated etherification, [3,3]-rearrangement
significance for green and sustainability issues of synthetic and hydrolysis sequence (Scheme 1A).7 In 2016, Wang and
chemistry.2 Organoboron compounds are highly reactive inter-
mediates and versatile building blocks with numerous syn-
thetic applications, primarily attributed to their versatility of
the efficient construction of novel C–C and C–heteroatom
bonds in the metal-catalyzed cross-coupling reaction.3 In par-
ticular, alkenylboron reagents have received much attention
towards the preparation of complex molecules due to the rich
transformations of the double bond in the coupling products.4
Functionalization of alkenes is a powerful and efficient tool for
State key laboratory for Chemistry and Molecular Engineering of Medicinal
Resources, Ministry of Science and Technology of China; School of Chemistry and
Pharmaceutical Sciences, Guangxi Normal University, 15 Yu Cai Road, Guilin,
541004, China. E-mail: moeastlight@mailbox.gxnu.edu.cn
†Electronic supplementary information (ESI) available: Experimental details and
characterization of all compounds and copies of 1H and 13C NMR for new com-
pounds. CCDC 1871679. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d0gc01514h
Scheme 1 Regio-controlled
functionalization
of
alkenylboron
‡H.-Y. Bi, C.-J. Li, and C. Wei contributed equally to this work.
reagents.
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Table 1 Optimization of the reaction conditionsa
Entry Cat.
Solvent
Base
3aa Yield % b 4aa Yield % b
1
2
3
4
5
6
7
8
Cu(OAc)2 DCE
Pyr
Pyr
Pyr
Pyr
Pyr
36
12
5
16
9
16
9
10
28
68
52
12
43
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
<5
40
66
CuCl2
CuSO4
DCE
DCE
Cu(OTf)2 DCE
CuI DCE
Cu(OAc)2 Toluene Pyr
Fig. 1 Selected examples of indole alkaloids.
Cu(OAc)2 THF
Pyr
Pyr
Pyr
Pyr
Pyr
NEt3
DMAP
co-workers reported
a novel one-pot difunctionalization
Cu(OAc)2 MeOH
Cu(OAc)2 DMSO
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
Cu(OAc)2 MeCN
9
of alkenyl-MIDA-boronates to synthesize halogenated and
trifluoromethylated α-boryl ketones (Scheme 1B).8 Both these
strategies have shown their attractive and powerful efficiencies
toward the preparation of functionalized molecules, but
simple target molecules were obtained. Therefore, tri- or tetra-
functionalization of alkenylboronic acids to more complex
targets in one pot is still challenging and desirable to be
explored.
10
11c
12
13
14
15d
16e
17e, f
18e,g
19e,h
Cs2CO3 <5
Pyr
Pyr
Pyr
Pyr
Pyr
60
79
10
20
<5
N–O bond compounds have attracted much attention
recently because they efficiently serve as O- or N-reagents in
the oxidation reaction,9 C–H amination10 and aminoxygena-
tion of alkenes.11 During the studies of N–O bond coupling
with alkenylboronic acids by the Chan–Lam reaction12 and N–
O bond rearrangement in our group,13,14 we proposed an
efficient cascade strategy for tri-/tetrafunctionalization of alke-
nylboronic acids to access indole scaffolds using HOOBT or
arylhydrazines as O- or N-sources, respectively, through
copper-catalyzed cross-coupling, [2,3]-rearrangement, nucleo-
philic substitution, oxidation and [3,3]-rearrangement over five
steps in a one-pot reaction (Scheme 1C). Moreover, the
functionalization can be easily controlled since indole alka-
loids are formed in a site-marked process which are not easily
obtained from traditional strategies, such as condensation of
diketones with arylhydrazines15 and condensation of β-keto-
acids or α-hydroxymethylene ketones with aryl diazonium salts
by the Japp–Klingermann reaction.16 Meanwhile, indole and
carbazole are normally occurring scaffolds in a variety of
indole alkaloids and pharmaceuticals (Fig. 1).17 Consequently,
the development of a new cascade strategy to prepare these
important indole heterocycles in a green manner is valuable.18
a Reaction conditions: 1a (0.9 mmol, 3.0 equiv.), HOOBT (0.3 mmol),
cat. (20 mol%, unless noted otherwise), base (3.0 equiv.), solvent
(3.0 mL), rt, 18–24 h, under air; then adding 2a (0.36 mmol, 1.2 equiv.)
and HOAc (5 mL), 80 °C, 10–24 h. b Isolated yield. c Cu(OAc)2
(10 mol%), 36 h. d Pyr (1.5 equiv.). e Na2SO4 (400 mg). f Performed
under N2 instead of air. g 2a (2.0 equiv.). h 2a (3.0 equiv.).
CuSO4, Cu(OTf)2, and CuI, afforded 3aa in lower yields
(Table 1, entries 2–5). Solvent screening showed that toluene,
THF, MeOH, and DMSO delivered product 3aa in less than
28% yield (Table 1, entries 6–9). Pleasingly, the yield of 3aa
was dramatically improved to 68% in MeCN (Table 1, entry
10). Decreasing the amount of Cu(OAc)2 to 10 mol% reduced
the yield of 3aa to 52% (Table 1, entry 11). The influence of the
base was also investigated (Table 1, entries 12–14). Using
either organic bases, such as NEt3 and DMAP, or inorganic
bases, such as Cs2CO3, in the reaction did not give 3aa in
higher yields than using pyridine. When the reaction was per-
formed using 1.5 equiv. of pyridine, 3aa was obtained in 60%
yield (Table 1, entry 15). Compared to entry 10, an obvious
increase in the yield of 3aa was observed in the presence of
Na2SO4 (Table 1, entry 16). The yield of 3aa decreased to 10%
under N2 instead of air (Table 1, entry 17). Interestingly, when
the amount of phenylhydrazine 2a was increased to 2.0 equiv.,
the yield of 3aa was only 20% accompanied by the tetrafunctio-
Results and discussion
Our investigations started with cyclohexenylboronic acid 1a as nalized product indolo[2,3-a]carbazole 4aa in 40% yield
the model substrate and HOOBT and phenylhydrazine 2a as O- (Table 1, entry 18). Further increasing the amount of 2a to 3.0
and N-sources, respectively (Table 1). A 36% yield of the tri- equiv. selectively afforded 4aa in 66% yield without the obser-
functionalized product tetrahydrocarbazol-1-one 3aa was vation of 3aa (Table 1, entry 19). Therefore, the optimal con-
obtained using Cu(OAc)2 as a catalyst and pyridine (pyr) as a ditions for trifunctionalization of alkenylboronic acid 1a to
base in 1,2-dichloroethane (DCE) under air for 24 h, and then prepare tetrahydrocarbazol-1-one 3aa were Cu(OAc)2 (20 mol%)
followed with 2a and HOAc at 80 °C for 18 h (Table 1, entry 1). and pyr (3.0 equiv.) with Na2SO4 as an additive in MeCN at
However, other copper(II) or copper(I) salts, such as CuCl2, room temperature under air for 18 h, and then adding phenyl-
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hydrazine 2a (1.2 equiv.) and HOAc at 80 °C for 12 h (Table 1, pound 3 was determined by X-ray diffraction analysis of 3ga.19
entry 16). The optimal conditions for preparing the tetrafunc- Linear alkenylboronic acids 1h and 1i were reacted under the
tionalized product indolo[2,3-a]carbazole 4aa included increas- trifunctionalization conditions delivering indole products 3ha
ing the amount of phenylhydrazine 2a to 3.0 equiv. (Table 1, and 3ia in only 10% and 5% yields, respectively. To our
entry 19).
delight, adding 1.0 equiv. of polyphosphoric acid (PPA) under
With the optimized conditions for the formation of tetrahy- the standard conditions furnished 3ha and 3ia in 45% and
drocarbazol-1-ones in hand, we turned to study the scope of 53% yields, respectively. Most of these scaffolds were not easily
trifunctionalization of alkenylboronic acids with various aryl- obtained by conventional condensation of unsymmetrical 1,2-
hydrazines. As shown in Table 2, a wide range of arylhydra- diketones with phenylhydrazine via the Fischer indole
zines 2 were subjected to the trifunctionalization conditions synthesis.
and they smoothly afforded diverse tetrahydrocarbozol-1-ones
Next, the scope of tetrafunctionalization of alkenylboronic
3ab–3aj in moderate to good yields. Arylhydrazines with elec- acids to prepare indolo[2,3-a]carbazoles was also investigated.
tron-donating groups at the para-position gave better yields As shown in Table 3, symmetrical indolo[2,3-a]carbazoles 4bb–
than those with electron-withdrawing groups (3ab and 3ac vs. 4dd with methoxy, methyl, and chloro groups were obtained in
3af and 3ag). Unfortunately, arylhydrazines with strong elec- moderate to good yields when a single arylhydrazine was used.
tron-withdrawing groups at the para-position, such as NO2, Compared to the traditional strategy by condensation of cyclo-
CO2Me, and CN (not shown in the table), did not afford the
corresponding tetrahydrocarbozol-1-ones even though the
temperature was increased to 120 °C or 1.0 equiv. of PPA was
Table 3 Substrate scope for tetrafunctionalization to prepare indolo
[2,3-a]carbazoles 4a,b
added as an additive. When arylhydrazine 2h with a 3-methyl
group and 2i with a 3-bromo group were subjected to the tri-
functionalization conditions, products 3ah and 3ai were
obtained in 89% and 75% yields with 2 : 1 and 1 : 1 regio-
selectivity of [3,3]-rearrangement, respectively. Various alkenyl-
boronic acids 1 were compatible for the trifunctionalization
protocol affording the corresponding desired products, tolerat-
ing both six- to eight-membered rings and various substituents
on the six-membered rings (3ba–3ga). The structure of com-
Table 2 Substrate scope for the trifunctionalization to prepare tetrahy-
drocarbazol-1-ones 3 a,b
a Reaction conditions: 1 (0.9 mmol, 3.0 equiv.), HOOBT (0.3 mmol), Cu
(OAc)2 (20 mol%), pyr (0.9 mmol, 3.0 equiv.), MeCN (3.0 mL), Na2SO4
a Reaction conditions: 1 (0.9 mmol, 3.0 equiv.), HOOBT (0.3 mmol), Cu (400 mg), rt, 18–24 h; then adding ArNHNH2 2 (0.9 mmol, 3.0 equiv.),
(OAc)2 (20 mol%), pyr (0.9 mmol, 3.0 equiv.), MeCN (3.0 mL), Na2SO4 HOAc (5 mL), 80 °C, 10–24
h
under air or adding Ar1NHNH2
(400 mg), rt, under air, 18–24 h; then, adding arylhydrazine 2 (0.45 mmol, 1.5 equiv.) and Ar2NHNH2 (0.45 mmol, 1.5 equiv.).
(0.36 mmol, 1.2 equiv.), HOAc (5 mL), 80 °C, 10–24 h. b Isolated yield. b Isolated yield. c Prepared by condensation of cyclohexane-1,2-dione
c Regioselectivity of [3,3]-rearrangement. d PPA (1.0 equiv.) was added.
with arylhydrazines. d Regioselectivity of [3,3]-rearrangement.
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hexane-1,2-dione with arylhydrazines in a one-pot process, the
yields of 4cc and 4dd were only 14% and 46%, respectively. We
were pleased to find that two different arylhydrazines were
added in sequence in a one-pot process affording unsymmetri-
cal indolo[2,3-a]carbazoles 4ab, 4ad, 4af, 4ah and 4bd in good
yields ranging from 50% to 64%, which are difficult to be
achieved by traditional methods. It is noteworthy that com-
pounds 4dd and 4ad were the natural products, Tjipanazole D
and Tjipanazole I, which were prepared in lower yields from
commercial cyclohexenylboronic acid 1a with the corres-
ponding arylhydrazines in one pot. When cycloheptenylboro-
nic acid 1b and cyclooctenylboronic acid 1c were reacted with
2a under the tetrafunctionalization conditions, products 4ba
and 4ca were obtained in 67% and 66% yields, respectively,
which were prepared for the first time. When substituted cyclo-
hexenylboronic acids 1d, 1f, 1g, and 1j were reacted under the
tetrafunctionalization conditions, the obtained products were
Scheme 3 Proposed mechanism.
dependent on the substituted groups of six-membered ring. hydrazone 6 was subjected to PPA/HOAc conditions, the
Cyclohexenylboronic acid 1d with two methyl groups at the desired product 3ha was obtained in 59% yield (Scheme 2-3),
4-position of the six-membered ring furnished product 4da in which was in accordance with the yield of 3ha in Table 2.
63% yield, suggesting that the reaction involved a 1,2-
Based on the experimental results, a possible mechanism
rearrangement of the methyl group. Cyclohexenylboronic acid for the formation of tetrahydrocarbazol-1-one 3 and indolo
1f with a phenyl group at the 4-position and 1g with a methyl [2,3-a]carbazole 4 was proposed (Scheme 3). Initially, a copper-
group at the 5-position resulted in products 4fa and 4ga in mediated cross-coupling reaction of HOOBT with alkenylboro-
68% and 63% yields, respectively. However, cyclohexenylboro- nic acids 1 affords O-vinylation intermediate A, which facili-
nic acid 1j with a 6-methyl group afforded product 4ja contain- tates a [2,3]-rearrangement to produce α-nitrogenated ketone
ing a quaternary carbon center in 65% yield. This tetrafunctio- 5.20 Compound 5 is attacked by phenylhydrazine 2a and
nalization provides a facile approach to access indolo[2,3-a]car- undergoes nucleophilic substitution, affording compound 7
bazoles or analogs in a one-pot reaction from commercial and releasing benzotriazinone. Further oxidation of 7 by air
cyclic alkenylboronic acids.
under heating conditions yields hydrazone 6, which undergoes
To better understand the formation mechanism of tetrahy- isomerization and [3,3]-rearrangement via intermediate B to
drocarbozol-1-ones 3 and indolo[2,3-a]carbazoles 4 from alke- afford tetrahydrocarbazol-1-one 3. Alternatively, compound 3
nylboronic acids, control experiments were performed could also undergo condensation with excess phenylhydrazine
(Scheme 2). Alkenylboronic acid 1h was reacted with HOOBT 2a, isomerization and [3,3]-rearrangement via intermediate C
for 12 h affording product 5h in 87% yield (Scheme 2-1), to give intermediate D. Finally, oxidation of D by air results in
which might be formed by the Chan–Lam reaction and compound 4. It is worth noting that α-nitrogenated ketone 5
sequential 2,3-rearrangement. When compound 5h was shows a high site-marked reactivity for nucleophilic substi-
reacted with phenylhydrazine 2a in HOAc at 80 °C for 4 h, in tution instead of condensation with the ketone group.
addition to hydrazone 6 that was afforded in 49% yield, com-
To show the utility of this tri-/tetrafunctionalization of alke-
pound 7 was detected by HRMS and indole 3ha was obtained nylboronic acids, gram-scale reactions were performed. As
in 12% yield accompanied by benzotriazin-4-one in 75% yield shown in Scheme 4-1, 3.0 g of 1a (24 mmol) was reacted with
(Scheme 2-2). These results showed that compound 7 might be 1.3 g of HOOBT (8 mmol) and phenylhydrazine 2a (1.0 g,
the intermediate in the formation of hydrazone 6. When 9.6 mmol) under the trifunctionalization conditions affording
3aa in 75% yield (1.11 g) by flash column chromatography.
Interestingly, the purification of compound 3aa could also be
achieved through extraction, washing and sequential recrystal-
lization without flash column chromatography to afford 1.05 g
of 3aa in 71% yield. As shown in Scheme 4-2, indolo[2,3-b]car-
bazole 4aa was obtained in 61% yield (1.2 g) by flash column
chromatography under the tetrafunctionalization conditions
while the extraction, washing, and sequential recrystallization
procedure delivered 4aa in 65% yield (1.33 g). This easy purifi-
cation procedure shows a green approach to access tetrahydro-
carbozol-1-ones and indolo[2,3-a]carbazoles.
To demonstrate the advantages of the current approach for
Scheme 2 Mechanistic studies.
preparing tetrahydrocarbozol-1-ones, an array of indole, indo-
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can be a facile strategy to access various tetrahydrocarbozol-1-
ones and indolo[2,3-a]carbazoles in good yields. The reaction
showed a broad substrate scope and tolerated various functional
groups. Moreover, the reaction was easily performed at gram
scales with a simple purification procedure and the tetrahydro-
carbozol-1-one can be converted into various indole, indoline,
and indolenine scaffolds. We anticipate that this high site-
marked indole formation from alkenylboronic acids compared
with conventional Fischer indole synthesis substrates could
offer a new approach to indole scaffold design.
Experimental
General procedure for preparing tetrahydrocarbozol-1-ones 3
Scheme 4 Gram-scale preparation of 3aa and 4aa.
A 25 mL reaction flask was charged with alkenylboronic acids
1 (0.9 mmol, 3.0 equiv.) N-hydroxybenzotriazin-4-one (HOOBT)
(0.3 mmol), Cu(OAc)2 (5.4 mg, 20 mol%) and Na2SO4 (400 mg)
under an air atmosphere, MeCN (3.0 mL) and pyridine (72 μL,
0.9 mmol, 3.0 equiv.) were then added. The reaction mixture
was stirred vigorously at room temperature for 18–24 h until
HOOBT disappeared (monitored by TLC). Then, arylhydrazine
2 (0.36 mmol, 1.2 equiv.) and HOAc (5 mL) were added to the
reaction mixture and stirred at 80 °C for 10–24 h. After this,
the reaction was quenched with H2O (10 mL) and the reaction
mixture was extracted with EtOAc (3 × 10 mL). Then, the com-
bined organic layers were washed with NaHCO3 (10 mL) and
brine (10 mL), dried over Na2SO4 and filtered. The solvent was
removed under reduced pressure and the crude product was
purified by flash column chromatography (the crude residue
was dry loaded with silica gel, 1/20 to 1/6, ethyl acetate/pet-
roleum ether) to provide compound 3.
Scheme 5 Applications of 3aa to diverse indole scaffolds. Conditions:
a(i) NH2OH, AcONa, H2O, (ii) POCl3, MeCN, 80 °C; b(i) NaH, 3-bromopro-
pyne, DMF, (ii) NH2OH, AcONa, (iii) AuCl3 (5 mol%), CHCl3, rt; c(i)
NH4OAc, NaBH3(CN), MeOH, 60 °C, (ii) NIS, AgOTf, PhNH2, DCM; d(i)
NH4OAc, NaBH3(CN), MeOH, 60 °C, (ii) NIS, AgOTf, 4-MeOC6H4OH,
DCM.
Conflicts of interest
There are no conflicts of interest to declare.
line, and indolenine scaffolds was prepared. As shown in
Scheme 5, azepine amide 8 was afforded in 63% yield by
a two-step synthetic sequence involving condensation and
Beckmann rearrangement. An alkylation/condensation/gold-
catalyzed 6-exo-dig cyclization sequence on 3aa delivered tri-
cyclic indole-fused pyrazine N-oxide 9 in 69% yield. Reduction
of the carbonyl group in 3aa with NaBH3CN and NH4OAc, fol-
lowed by indole oxidation and PhNH2 attack on the C3-posi-
tion afforded indolenine 10 in 52% yield. Similarly, furo[2,3-b]
indoline 11 was obtained in 45% yield when PhNH2 was
replaced by 4-methoxyphenol.
Acknowledgements
Financial support from the National Natural Science
Foundation of China (21871062), the Natural Science
Foundation of Guangxi (2016GXNSFFA380005), the Student
Innovation Training Program (201910602136), the State Key
Laboratory for Chemistry and Molecular Engineering of
Medicinal Resources (CMEMR2017-A01), the Ministry of
Education of China (IRT_16R15), the “Overseas 100 Talents
Program” of Guangxi Higher Education, and the “One
Thousand Young and Middle-Aged College and University
Backbone Teachers Cultivation Program” of Guangxi is greatly
appreciated.
Conclusions
We have identified an efficient one-pot cascade process of tri- or
tetrafunctionalization of alkenylboronic acids with HOOBT and
arylhydrazines as oxygen and nitrogen sources, respectively,
through the Chan–Lam reaction/2,3-rearrangement/nucleophilic
substitution/oxidation/[3,3]-rearrangement over five steps that
Notes and references
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19 CCDC 1871679 (3ga) contains the supplementary crystallo-
graphic data for this paper.†
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