Synthesis of Difluorinated Dihydrobenzo[de]chromenes via Rh(III)-Catalysed C-H Couplings of 1-Naphthols with Gem-Difluoromethylene Alkynes

Article abstract of DOI:10.1002/adsc.202001475

The Rh(III)-catalysed C?H couplings of 1-naphthols with gem-difluoromethylene alkynes have been realized for the direct construction of difluorinated dihydrobenzo[de]chromenes with broad substrate/functional group compatibility and good regio-/chemoselect

Full text of DOI:10.1002/adsc.202001475

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DOI: 10.1002/adsc.202001475
Synthesis of Difluorinated Dihydrobenzo[de]chromenes via
Rh(III)-Catalysed C-H Couplings of 1-Naphthols with Gem-
Difluoromethylene Alkynes
Liping Li,a Xiuhua Zhong,a Jiali Xu,a Hui Gao,a Zhi Zhou,a,* and Wei Yia,*
a
Key Laboratory of Molecular Target & Clinical Pharmacology and the State Key Laboratory of Respiratory Disease,
School of Pharmaceutical Sciences & the Fifth Affiliated Hospital,
Guangzhou Medical University, Guangzhou, Guangdong 511436, People‘s Republic of China
Fax: (+86)-20-37104196);
phone: (+86)-20-37104196
E-mail: zhouzhi@gzhmu.edu.cn; yiwei@gzhmu.edu.cn
Manuscript received: November 26, 2020; Revised manuscript received: January 15, 2021;
Version of record online: January 29, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202001475
fluorine
effects
in
tuning
the
reaction
Abstract: The Rh(III)-catalysed CÀ H couplings of
1-naphthols with gem-difluoromethylene alkynes
have been realized for the direct construction of
difluorinated dihydrobenzo[de]chromenes with
broad substrate/functional group compatibility and
good regio-/chemoselectivity and in an atom-eco-
nomic manner. The unique fluorine effects account
for the unconventional reaction path involving intra-
molecular 1,3-hydrogen migration process to afford
the exo-methylene fragment.
chemoselectivity.[6a–b] For example, in 2017 Loh and
Feng pioneered the Rh(III)-catalysed hydroarylation of
gem-difluoromethylene alkynes with N-pivaloxyl aroy-
lamides proceeded via an unconventional Lossen
rearrangement owing to the introduction of fluorine
atoms adjacent to the alkyne moiety (Scheme 1a).[6b]
Inspired by the seminal report, up to date, a tremen-
dous amount of CÀ H activation strategies have been
extensively explored by the groups of Huang, Wang,
Rovis and us to extend the utility of gem-difluorometh-
ylene alkynes and clarify the unique fluorine
effects.[6c–g,7] Interestingly, diverse distinguished reac-
tion modes involving one or two folds CÀ F bond
cleavage could be realized by simple modification of
the reaction conditions. Notwithstanding the advances
made, the use of fluorinated CPs is still rarely
Keywords: CÀ H functionalization; difluorinated di-
hydrobenzo[de]chromenes;
1-naphthols;
gem-
difluoromethylene alkynes; Rh(III) catalysis
Fluorine-containing structures represent privileged re-
sources for the discovery of advanced materials and
biologically active molecules.[1] Consequently, contin-
ued efforts have been devoted to develop efficient and
practical approaches including transition-metal (TM)
catalysed transformations for introducing the fluori-
nated fragments into the organic compounds.[2] Despite
the notable advances on TM-catalysed direct CÀ H
fluorination[3] and fluoroalkylation,[4] the development
of novel strategy using fluorine-containing building
blocks (e.g. fluorinated alkenes,[5] alkynes[6] etc) as
versatile coupling partners (CPs) in TM-catalysed CÀ H
functionalization exhibited profound potential for real-
izing intriguing fluorine incorporation and innovative
reaction manifolds. Among the documented fluorinated
CPs, gem-difluoromethylene alkynes have gained great
interest of chemical scientists due to their distinctive
Scheme 1. TM-catalysed
difluoromethylene alkynes.
CÀ H
couplings
with
gem-
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investigated in the field of TM-catalysed CÀ H func- together with classic [4+2] annulation product 3aa’
tionalization in comparison with the nonfluorinated (Table 1, entry 1). Of note, the CÀ F bond cleavage
reactants.
process reported with gem-difluoromethylene alkynes
On the other hand, it was well-known that, in terms was not observed in this transformation. The screening
of the high efficiency and rich diversity, TM-catalysed of various Rh(III) catalysts bearing a variety of
direct CÀ H functionalization has emerged as a modified Cp ligands revealed that higher reactivity
frequently-used strategy for synthesizing polycyclic could be realized with electron-donating Cp ligands,
ring-fused heterocycles through the direct couplings of and the acetate anion coordinated Cp*Rh(OAc)2
diversified substrates and CPs,[8] and among them, further improved the reactivity to afford 3aa in 60%
natural 1-naphthols have been proved to be versatile yield (entries 2–8). Other TM-catalysts were also
substrates for the construction of various oxygen- examined and gave inferior results due to either poor
containing fused ring compounds such as dibenzo[c,h] reactivity or specific selectivity for assembling 3aa’
xanthenes, naphtho[1,8-bc]pyrans and benzo[de] (see Table S1 in the supporting information for details).
chromenes via TM-catalysed and OH-directed C8-H Further exploration demonstrated that the solvent was
activation/cyclization.[9] Based on these, we envisaged crucial in tuning the chemoselectivity of 3aa/3aa’, in
that the use of fluorinated CPs might divert the which the dioxane was an optimal solvent to give a
reaction mode from conventional 1-naphthols-involved ratio of 66/7 (entries 9–12). Other tested bases were
reaction pathways owing to the unique influence of the less effective and furnished the desired product 3aa in
fluorine effect. In continuation of our interest in relatively lower yields (entries 13–16). After a number
Rh(III)-catalysed CÀ H functionalization with gem-
difluoromethylene alkynes, therefore, we would like to
disclose herein the synthesis of difluorinated dihydro-
benzo[de]chromene derivatives via Rh(III)-catalysed
Table 1. Optimization of reaction conditions.[a]
CÀ H couplings of 1-naphthols. Compared to the [4+
2] annulation of 1-naphthols with nonfluorinated
alkynes (Scheme 1b), this transformation features
fluorine effect-induced unprecedented 1,3-hydrogen
shift process, thus affording the dihydrobenzo[de]
chromene framework via intramolecular SN2’-type
substitution (Scheme 1c). Considering the widespread
existence of the dihydrobenzo[de]chromene scaffold in
natural products (Figure 1) and bioactive molecules,[10]
this protocol presents a straightforward synthetic route
for such structures and should have profound utility
potentials.
We commenced our investigation by testing the
model reaction between 1-naphthol 1a and gem-
difluoromethylene alkyne 2a under the [Cp*RhCl2]2-
catalysed conditions. In the presence of CsOAc, the
Entry Catalyst
Base
Solvent Yield[b] (%)
3aa 3aa’
1
2
[Cp*RhCl2]2
CsOAc DCE
41
53
59
55
11
<5
57
60
12
24
16
21
18
11
9
20
17
33
7
[Cp*iÀ PrRhCl2]2 CsOAc DCE
3
4
5
6
[Cp*CyRhCl2]2
[Cp*BnRhCl2]2
[CpIndRhCl2]2
[Cp*ERhCl2]2
[Cp*2PhRhCl2]2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
Cp*Rh(OAc)2
CsOAc DCE
CsOAc DCE
CsOAc DCE
CsOAc DCE
CsOAc DCE
CsOAc DCE
CsOAc MeOH
CsOAc dioxane 66
CsOAc toluene 57
CsOAc CH3CN 33
NaOAc dioxane 59
°
desired CÀ H couplings took place in DCE at 80 C,
7
8[c]
9[c]
10[c]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
delivering the unprecedented dihydrobenzo[de]
chromene 3aa bearing an exocyclic double bond
23
42
12
9
8
8
KOPiv
K2CO3
K3PO4
dioxane 58
dioxane 45
dioxane 49
17[c,d] Cp*Rh(OAc)2
CsOAc dioxane 75
<5
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), catalyst
°
(5 mol%) and base (1 equiv) in solvent (0.2 M) at 80 C for
24 h under air.
[b] 1H-NMR yield.
[c] 10 mol% of Cp*Rh(OAc)2 was used.
[d] The reaction was conducted in dioxane (0.1 M) on 0.2 mmol
scale; isolated yield of 3aa was reported.
Figure 1. Representative
dihydrobenzo[de]chromene-derived
natural products.
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of trials to screen the routine experimental parameters formation of the corresponding difluorinated products
including the reaction temperature, additives, concen- 3la–na, albeit with relatively inferior reactivity.
tration and the proportion of substrates, we were
pleased to identify the optimal reaction conditions for protocol for the assembly of diverse difluorinated
the synthesis of difluorinated dihydrobenzo[de] dihydrobenzo[de]chromenes, variety of gem-
chromene 3aa with good reactivity and chemoselectiv- difluoromethylene alkynes were then examined. As
ity (entry 17). summarized in Scheme 3, all the tested gem-
To further probe the compatibility of the developed
a
With the optimal conditions in hand, we next difluoromethylene alkynes participated in this Rh(III)-
investigated the substrate scope with regard to a variety catalysed annulation smoothly and showed specific
of 1-naphthols (Scheme 2). As a result, various 1- selectivity for assembling the dihydrobenzo[de]
naphthols bearing commonly encountered substituents chromene framework equipping with an exocyclic
including methyl (3ba), halogens (3ca-fa and 3ha), double bond. Of note, the inseparable mixtures of two
nitro (3ga) and ester (3ia) group were well tolerated to configurations with different Z/E ratios were obtained
afford the desired difluorinated dihydrobenzo[de] for long chain alkyl-substituted alkyne substrates,
chromenes regardless of the electronic properties and among which the (5-cyclopropyl-3,3-difluoropent-4-
the substituted positions. The structure of compound yn-1-yl)benzene resulted in the ring-opening product
3ea was further confirmed by X-ray diffraction 3ae. Remarkably, the gem-difluoromethylene alkynes
analysis (CCDC 2044552).[11] Phenanthrol and 1,6- derived from the natural products cyclamen aldehyde
dihydroxynaphthalene were also viable reactants to and citronellal were also tolerated to yield the desired
deliver the desired difluorinated dihydrobenzo[de] dihydrobenzo[de]chromenes in decent isolated yields
chromene derivatives (3ja and 3ka). In addition, (3am–3ap), illustrating that this transformation was
further examination of a series of heteroaromatics broadly applicable for the rapid construction of such
(e.g., isoquinolin-5-ol, 4-hydroxycoumarin and 4- complexes.
hydroxy-1-methyl-2-quinoline) led to the streamlined
A series of experimental mechanistic studies were
then carried out to probe the origin of the unconven-
tional chemoselectivity and the detailed reaction path-
way (Scheme 4). Control experiment indicated that
3aa and 3aa’ could not be transformed into each other
Scheme 2. Substrate scope of 1-naphthols.[a]
Scheme 3. Scope of gem-difluoromethylene alkynes.[a]
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yielded the corresponding benzo[de]chromene deriva-
tive 4 specifically. Besides, the coupling of 1a with (3-
fluorohex-4-yn-1-yl)benzene turned out to be compli-
cated (eq 5). Taken together, these results testified to
the crucial role of the gem-difluoromethylene group in
controlling the regio- and chemoselectivity. In addi-
tion, only catalytic amount of 3aa was formed when
the reaction was conducted under an atmosphere of N2,
suggesting an external oxidant was inevitable for the
catalyst regeneration. However, the use of classical
metal oxidants including silver or copper salts was less
effective compared with the standard conditions (eq 6).
Based on our experimental observations and prece-
dent literatures,[6,7,9b–d] we proposed the plausible
catalytic cycle in Scheme 5. Initially, the Cp*Rh-
(OAc)2-mediated and the hydroxyl group-directed
CÀ H activation of 1a proceeded to afford the rhoda-
cycle A, followed by the regioselective alkyne
insertion to give intermediate B. Subsequently, an
intramolecular 1,3-H migration[12] occurred to yield the
intermediate C. It should be noted that, the unique
fluorine effects diminish the migratory aptitude of the
alkenyl group, thus furnishing the intermediate C via a
1,3-H migration instead of the CÀ O reductive elimi-
nation for the formatiom of the product B’. From
intermediate C, the direct CÀ O bond reductive elimi-
nation led to the formation of dihydrobenzo[de]
chromene product 3aa along with the Cp*Rh(I)
species (path I). Alternatively, equilibrium of the η3
Scheme 4. Experimental mechanistic studies.
under the standard conditions, implying two different allylic rhodium species D from intermediate C
mechanisms were involved for delivering these skel- followed by sequential protonolysis and intramolecular
etons (eq 1). The treatment of 1-naphthol 1a in the SN2’-type attack of the naphtholate oxygen via the
absence of Rh(III) catalyst in CD3OD resulted in intermediate E to the allylic rhodium moiety[13] cannot
prominent deuterium incorporation at both C2 and C4 be ruled out at this stage (path II). Finally, the
position due to the base-mediated keto-enol tautomer- regeneration of the active Cp*Rh(III) catalyst was
ism process, while no obvious deuteration was achieved under an atmosphere of air[14] in the presence
observed at C8 position. By contrast, the deuterium- of HOAc.
labelling experiment in the presence of Cp*Rh(OAc)2
Further derivatizations of dihydrobenzo[de]
showed approximately 73% deuteration at the C8 chromene 3aa were performed under different con-
position of recovered 1a (eq 2). These results provided ditions (eq 7). Delightfully, the treatment of 3aa with
adequate evidence to support the Rh(III)-catalysed and
the hydroxyl group-directed C8-H activation process of
1-naphthol. Moreover, the difluorinated dihydrobenzo
[de]chromene product 3aa could be obtained smoothly
in CD3OD and no obvious deuterium incorporation
was observed at either exocyclic alkenylic or allylic
position, demonstrating that the allylic CÀ H was not
originated from the external source (eq 3). Further
investigation revealed that the coupling of 1a with
CD3-substituted gem-difluoromethylene alkyne 2a-d3
occurred smoothly to deliver the corresponding dihy-
drobenzo[de]chromene 3aa-d3 bearing prominent deut-
eration at the allylic position, implying a formal
intramolecular 1,3-H migration process (eq 4). To gain
more insight into the fluorine effects in this trans-
formation, further control experiment with 1-phenyl-
hex-4-yn-3-one being the CP was conducted and
Scheme 5. Proposed catalytic cycle.
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triethylsilane in the presence of TMSOTf led to the
formation of the reduced product cis-5. Additionally,
the exposure of 3aa to DDQ furnished an unprece-
dented tetracyclic skeleton 6, thus further strengthen
the synthetic utility of the developed protocol for
assembling intriguing difluorinated fused rings.
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difluoromethylene alkynes 2 (0.2 mmol, 1 equiv.), Cp*Rh-
(OAc)2 (10 mol%) and CsOAc (1 equiv.) in dioxane (0.1 M)
°
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We thank NSFC (21877020, 22007020), Guangdong Natural
Science
Funds
for
Distinguished
Young
Scholar
(2017 A030306031), Natural Science Foundation of Guang-
dong Province (2019 A1515010935) and National College
Students’ Innovation Training Program (202010570033) for
financial support on this study.
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