Palladium-Catalyzed Cascade Reaction of o-Bromobenzaldehydes with N-Sulfonylhydrazones: An Efficient Approach to the Naphthalene Skeleton

Article abstract of DOI:10.1002/adsc.201801637

A new strategy for the construction of the naphthalene backbone is described. The reaction essentially starts from two simple aldehydes. The key step is enabled by a palladium-carbene migratory insertion. After that, a sequence of reversible allylic alkylation and intramolecular condensation takes place to give the substituted naphthalene derivatives. Additional manipulations on the sulfonyl group in the product via palladium-catalyzed Kumada coupling were also investigated. (Figure presented.).

Full text of DOI:10.1002/adsc.201801637

Title: Palladium-Catalyzed Cascade Reaction of o-
Bromobenzaldehydes with N-Sulfonylhydrazones: An Efficient
Approach to Naphthalene Skeleton
Authors: Heng Zhang, Yinghua Yu, Shenlin Huang, and Xueliang
Huang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801637
Link to VoR: http://dx.doi.org/10.1002/adsc.201801637

10.1002/adsc.201801637
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium-Catalyzed
Cascade
Reaction
of
o-
Bromobenzaldehydes with N-Sulfonylhydrazones: An Efficient
Approach to the Naphthalene Skeleton
Heng Zhang,a,c Yinghua Yu,a,c Shenlin Huang,b and Xueliang Huang*a,c
a
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure
of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
Phone and fax: (+86)-591-63173587; e-mail: huangxl@fjirsm.ac.cn
College of Chemical Engineering, Jiangsu Key Lab of Biomass-Based Green Fuels and Chemicals, Nanjing Forestry
b
University, Nanjing, 210037, P. R. China.
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
c
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
final product naphthalen-1-ol (Scheme 1b). In this
communication, we would like to describe our efforts
toward this goal which leads to an unexpected
Abstract. A new strategy for the construction of the
naphthalene backbone is described. The reaction essentially
starts from two simple aldehydes. The key step is enabled
by a palladium–carbene migratory insertion. After that, a discovery of
a
cascade reaction of o-
sequence of reversible allylic alkylation and intramolecular
condensation takes place to give the substituted
naphthalene derivatives. Additional manipulations on the
sulfonyl group in the product via palladium-catalyzed
Kumada coupling were also investigated.
bromobenzaldehydes with α, β-unsaturated N-
sulfonylhydrazones enabled by migratory insertion of
palladium carbene. This reaction is efficient for the
synthesis of a range of naphthyl sulfones and the
expected naphthalen-1-ol is not observed (Scheme
1c). Naphthalene unit is an ubiquitous skeleton in
chemical and pharmaceutical industries as well as
optical and electronic materials.[6] Compared with the
existing methods on build up the naphthalene
backbone,[7] the current work uses simple aldehydes
as the reactants, giving the corresponding substituted
naphthalenes in complete regioselectivity with
relatively broad substrates scope, thus making this
strategy unique and attractive.
Keywords: Palladium carbene; migratory insertion;
naphthalene; Kumada coupling
Cascade reactions are considered as powerful
methods to construct structurally complex molecules
from simple and readily available feedstocks, while
omitting tedious procedures to isolate synthetic
intermediates, thus rendering it economic and
environmentally benign.[1] Diazo compounds are
easily accessible, possessing tunable reactivity. They
are often employed as carbene precursors to
participate in a range of classical transformations.[2]
As demonstrated by the seminal work of van Vranken,
Barluenga, Wang, and others, diazo compounds have
also proved to be versatile in a variety of transition-
metal-catalyzed cross-coupling reactions.[3,4]
Recently, we have reported a palladium-catalyzed
intermolecular acylation of aryl diazo esters with o-
bromobenzaldehydes, in which diazo esters act as
modular three-atom units to build up the key seven-
membered palladacycles I (Scheme 1a).[5] To test the
synthetic potential of this concept, we envisioned that
replacement of carbonyl group with proper C-C
double bond may give
a
seven-membered
carbopalladacyle II, which upon reductive
elimination would afford a transient ketone III.
Aromatization of III would lead to the formation of
Scheme 1. Palladium Carbene Enabled Annulations.
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10.1002/adsc.201801637
Advanced Synthesis & Catalysis
To begin our study, we chose readily available o-
the palladium catalyst, the ligand, and the base were
all crucial for the reaction to occur.
bromobenzaldehyde 1a and N-tosylhydrazone 2a as
model substrates. The reaction was initially carried
With the optimized reaction conditions in hand
(Table 1, entry 14), we examined the scope of this
reaction by variation of the sulfonyl groups in
hydrazones. The results are summarized in Scheme 2.
The reactions of hydrazones bearing substituents on
the benzene ring could be varied and both electron-
withdrawing groups (F, Cl, and Br) and electron-
donating groups (Me, OMe and NHAc) all proceeded
efficiently to lead to the corresponding products in
moderate to good yields. It is noteworthy that benzyl
hydrazone, naphthyl hydrazone and thienyl hydrazine
were also identified as suitable substrates for the
reaction, delivering the products in moderate yields.
In order to test the practicality of the current catalytic
system, the reaction was carried out on gram-scale for
1a (6 mmol, 1.11 g), and the desired product 3aa was
afforded in 72% yield (1.55 g).
out in DMF at 60 °C for 24
h
using
Pd2dba3·CHCl3/PPh3 as the precatalyst, K2CO3 as
base (Table 1, entry 1). After careful analysis of the
reaction mixture, naphthyl sulfone 3aa was isolated
in 9% yield, and the designed naphthalene-1-ol was
not observed. Further inspection of the reaction
conditions revealed that solvent had a notable effect
on the reaction yield. Trace amount of product
isolated when DMF was replaced by toluene, DMSO
or chloroform (Table 1, entries 2-4). Continuing
evaluation of the reaction medium led to identify that
MeCN was the best choice of solvent, and 3aa was
obtained in 48% yield (Table 1, entry 6). A brief
examination of the effects of base showed that K2CO3
was superior to others (Table 1, entries 6-11).
Subsequently, several palladium precatalysts
combined with different phosphine ligands were also
tested. When the reaction was carried out in presence
of [Pd(allyl)Cl]2 and PPh3 for 6 h, and product 3aa
could be isolated in 77% yield (Table 1, entry 14).
Pd(PPh3)4 could also catalyze this reaction efficiently
(Table 1, entry 13).
Scheme 2. Substrate scope of substituted N-sulfonyl
hydrazones.[a]
Table 1. Optimization of Reaction Conditions[a]
[a]Reaction conditions: Table 1, entry 13. [b]0.5 mmol
hydrazone 2 were employed. [c]5 mol% [Pd(allyl)Cl]2 and
30 mol% PPh3 were added. [d]The reaction was carried out
on 6 mmol scale.
Next the scope of the aldehydes and N-
tosylhydrazones with different substituents on the
aromatic rings were evaluated (Scheme 3). With
respect to o-bromo benzaldehyde, a series of
substituents, including Me, OMe, OTs, Cl, Br on the
phenyl ring were all compatible, giving the
corresponding products in moderate to good yields
(5da-5ha). The structure of 5ca was further
established by X-ray diffraction.[8] The toleration of
halide groups may offer new opportunity for further
transition metal-catalyzed cross coupling reactions. A
set of N-tosylhydrazones with different substituents
and functional groups, including substituted
aromatics and heteroaromatic ring, could react
smoothly with benzaldehyde 1a to afford the
corresponding naphthyl sulfone in moderate to good
yields. Substituents on para, ortho, meta-position of
[a]The reaction was carried out for 24 h. [b]Isolated yield.
[c]BnNEt3Cl (0.2 mmol) was added. [d]The reaction was
completed in 6 h.
Other palladium sources including Pd(OAc)2 and
Pd(MeCN)2Cl2 proved to be less efficient for the
current transformation (Table 1, entries 11 and 14).
Then a variety of mono- and bidentate phosphine
ligands were verified, and the yields of 3aa were not
improved. Finally, control experiments showed that
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10.1002/adsc.201801637
Advanced Synthesis & Catalysis
the phenyl ring were all tolerated. Furanyl-substituted
and naphthyl-substituted N-tosylhydrazones were
also participated in the transformation, providing the
products in 66% and 49% yields respectively.
sulfone could not be observed from the reaction of
phenyl bromide with N-tosylhydrazones (Scheme 5b).
According to the structure of the product obtained,
we could conclude that sulfone 6aa, which was
generated through the reaction of sulfinate anion with
intermediate E, would further undergo intramolecular
Aldol type condensation, giving 3aa ultimately. A
separate experiment revealed that treatment of
sulfone 6aa with 3 equivalent of K2CO3 in MeCN led
to the formation of 3aa in nearly quantitative yield.
The exact reason to explain the complete
regioselectivity is not clear at the current stage. But
one could envision that the formyl group on the
phenyl ring is the key for the reactivity and
regioselectivity. It could not only activate phenyl
bromide to facilitate elementary step of oxidative
addition, but also make the carbenic carbon atom in E
slightly more electrophilic. Thus the sulfinate anion
would add to this carbon center more selectively.
Base triggered Aldol condensation of sulfone 6aa for
the formation of thermodynamically stable 2-
sulfonylnaphthalene 3aa might be decisive for the
observed regioselectivity. To verify this assumption,
we have prepared allyl acetate 6ab and subjected it to
the reaction with sodium 4-methylbenzenesulfinate
(TsNa) under standard conditions, the naphthyl
sulfone 3aa was isolated in 58% yield. The
corresponding allyl sulfone 6aa and its regioisomer
6aa' were not observed. If the reaction was carried out
under otherwise identical conditions but without the
addition of K2CO3 (Scheme 5c, for details on the
procedure see SI), besides 3aa, allyl sulfone 6aa and
its region isomer 6aa' were obtained in 8% and 26%
yields, respectively. Notably, the reaction of 6aa'
under standard conditions also gave 3aa in 51%
isolated yield (Scheme 5d). Taken all these data
together, we believe that the sulfonylation of
intermediate E is reversible, and the formation of
thermal dynamically stable 3aa is the driving force to
make 6aa' convert to its regioisomer 6aa under
optimized conditions.
Scheme 3. Substrate scope of aldehydes and N-
tosylhydrazones [a]
[a]Reaction conditions: Table 1, entry 13. [b]0.5 mmol
hydrazone 2 were employed. [c]5 mol% [Pd(allyl)Cl]2 and
30 mol% PPh3 were added.
According to previous reports[9] and the results
obtained here, a plausible catalytic cycle for current
transformation for 2-sulfonylnaphthalene synthesis
could be envisioned (Scheme 4). As depicted, the
reaction is supposed to start with the oxidative
addition of low valent palladium catalyst to o-
bromobenzaldehyde 1a. The resulting palladium(II)
species A would further react with α, β-unsaturated
diazo compound B to give palladium carbene
intermediate C. Migratory insertion of C would
eventually afford η1-allylpalladium intermediate D,
which may exist as a fast equilibrium with η3-
allylpalladium species E. Diazo compound B was
generated in situ via Bamford-Stevens reaction[10] of
N-tosylhydrazone 2a with K2CO3. At the meantime,
one equivalent of sulfinate anion would be released.
It is worthwhile to mention that Liang and coworkers
have reported a palladium-catalyzed allyl sulfone
synthesis through the reaction of aryl iodides and N-
sulfonylhydrazone.[9h] Under the current conditions,
the reaction of phenyl iodide 1l with N-
tosylhydrazone 4b gave the corresponding allyl
sulfones as a mixture of regio isomers in a ratio of 1:1
(Scheme 5a). Interestingly, the corresponding allyl
Scheme 4. Plausible reaction pathway. Ligands were
omitted for clarity.
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10.1002/adsc.201801637
Advanced Synthesis & Catalysis
product tolyl group was not observed from the
reaction mixture.
In conclusion, we have reported a palladium-
catalyzed cascade reaction for the synthesis of
naphthyl sulfones from o-bromobenzaldehydes and
N-sulfonylhydrazones. The reaction goes through a
sequence
of
palladium–carbene
migratory
insertion/allylic alkylation and base-promoted aldol
condensation. Compared with previous work reported
in the literature, the retrosynthetic orthogonality of
current strategy to construct naphthalene backbone is
remarkable. As shown by mechanistic studies, the
sulfonylation of the allylpalladium intermediate is
reversible. The formation of a thermodynamically
stable naphthalene ring renders the whole process
regioselective.
Additional
experiments
have
uncovered an efficient palladium-catalyzed Kumada
coupling of naphthyl sulfones with Grignard reagents.
Scheme 5. Control experiments.
Experimental Section
An oven-dried Schlenk tube under argon atmosphere was
charged with o-bromo aryl formaldehydes 1 (0.2 mmol, 1.0
equiv), arylvinyl N-tosylhydrazones 2 (0.4 mmol or 0.5
mmol, 2.0 equiv or 2.5 equiv), [Pd(allyl)Cl]2 (2.5 mol% or
5.0 mol%), PPh3 (15 mol% or 30 mol%), K2CO3 (0.60
mmol, 3.0 equiv), and CH3CN (2 mL). The mixture was
stirred at 60 °C and the progress of the reaction was moni-
tored by TLC. Upon completion, the resulting mixture was
cooled to room temperature and filtered through Celite
with EtOAc as the eluent. The solvents were evaporated
under reduced pressure and the residue was purified by
flash chromatography on silica gel to afford the products.
Acknowledgements
We acknowledge financial support by the NSFC (21502190,
21502094, 21871259), the NSF of Fujian (2017J01031), the
Hundred-Talent Program, and the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB20000000).
Scheme 6. Synthetic manipulations on 3aa.
To further demonstrate the operational simplicity, a
one-pot, three-component, two-step reaction of
cinnamaldehyde with tosyl hydrazide and o- References
bromobenzaldehyde was carried out in 6 mmol scale,
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and the target naphthalene 3aa was obtained in 67%
overall yield. Molecules containing aryl sulfone motif
often exhibit unique biological and chemical
properties,[11] and the sulfonyl groups are valuable for
other reactions, for example, serve as useful
precursors for C-C bond formation via fragment
coupling or Julia olefination. Representative
downstream transformations of the products were
conducted (Scheme 6). As can be seen, the sulfonyl
group could be removed with an excess amount of
SmI2-THF under mild conditions,[12] and 2-
phenylnaphthalene 7 was obtained in high yield.
Furthermore, 3aa has also proved to a good
electrophile to participate in palladium-catalyzed
Kumada coupling reactions.[13] As such, 3aa could
react with a variety of Grignard reagents in presence
of catalytic amount of Pd(PPh3)2Cl2. Thus, the tosyl
group on the naphthalene ring was converted to other
functionalities with ease (8-11, Scheme 6). It is
worthwhile to mention that the possible coupling by-
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10.1002/adsc.201801637
Advanced Synthesis & Catalysis
COMMUNICATION
Palladium-Catalyzed Cascade Reaction of o-
Bromobenzaldehydes with N-Sulfonylhydrazones:
An Efficient Approach to Naphthalene Skeleton
Adv. Synth. Catal. Year, Volume, Page – Page
Heng Zhang,a,c Yinghua Yu,a,c Shenlin Huang,b and
Xueliang Huang*a,c
6
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