Facile access to 2,2-diaryl 2: H -chromenes through a palladium-catalyzed cascade reaction of ortho -vinyl bromobenzenes with N -tosylhydrazones

Article abstract of DOI:10.1039/d0ob00978d

A palladium-catalyzed cascade reaction of ortho-vinyl bromobenzenes with N-tosylhydrazones has been developed, which provides a facile approach to 2,2-disubstituted 2H-chromenes. The migration of palladium from the aryl to vinyl position is crucial, as the in situ produced vinyl palladium intermediate could further react with diazo compounds to generate the reactive species for the sequential annulation. This journal is

Full text of DOI:10.1039/d0ob00978d

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Facile access to 2,2-diaryl 2H-chromenes through
a palladium-catalyzed cascade reaction of ortho-
vinyl bromobenzenes with N-tosylhydrazones†
a,b
Cite this: DOI: 10.1039/d0ob00978d
Received 12th May 2020,
Accepted 21st June 2020
Heng Zhang,a,b Yinghua Yua and Xueliang Huang
*
DOI: 10.1039/d0ob00978d
rsc.li/obc
A palladium-catalyzed cascade reaction of ortho-vinyl bromoben- and DFT calculations, a pathway involving palladium carbene
zenes with N-tosylhydrazones has been developed, which provides migratory insertion enabled 1,4-palladium shift6,7 was
a
facile approach to 2,2-disubstituted 2H-chromenes. The proposed.
migration of palladium from the aryl to vinyl position is crucial, as Recently, ortho-vinyl bromobenzenes 1 have been demon-
the in situ produced vinyl palladium intermediate could further strated to be competent substrates to undergo the 1,4-palla-
react with diazo compounds to generate the reactive species for dium shift from the aryl to vinyl position. Based on this reac-
the sequential annulation.
tion mode, several elegant transformations have been discov-
ered by Lin and Feng (Scheme 1c).7c–g Wang and coworkers
found that 1 could react with (trimethylsilyl)diazomethane in
the presence of a palladium catalyst to give 1H-indenes in a
straightforward manner.8 Inspired by these remarkable
advances, and as a consequent study on our research interests
in palladium carbene-participated 1,4-palladium shift, we won-
dered whether the reaction of 1 with tosylhydrazone 2 could
Introduction
Palladium-catalyzed cross-coupling reactions using diazo com-
pounds as coupling reagents are versatile for the construction
of new chemical bonds.1 Among them, the reaction of appro-
priate organic halides with diazo compounds has been estab-
lished as a powerful alternative approach to generate an η3-
allylpalladium intermediate to further participate in Tsuji–
Trost-type allylic alkylations.2,3 As part of our continuing
research interest in diazo chemistry,4 we have recently demon-
strated a strategy on bridging C–H activation of aldehyde
enabled by migratory insertion of a palladium carbene inter-
mediate.5 In this specific reaction, the oxygen atom of the car-
bonyl group in the diazo compound could be considered as a
tethered nucleophile. After a sequence of C–H activation/
reductive elimination, products bearing an isocoumarin
scaffold could be obtained (Scheme 1a). Consequently, we
have further applied such a strategy for seven-membered
lactone synthesis.5d This time tosylhydrazones with an ortho-
phenol tether were employed as the diazo precursors
(Scheme 1b). According to deuterium labeling experiments
aKey Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for
Excellence in Molecular Synthesis, Fujian Institute of Research on the Structure of
Matter, Fujian College, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: huangxl@fjirsm.ac.cn; Fax: (+86)-591-63173587; Tel: (+86)-591-63173587
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China
†Electronic supplementary information (ESI) available. CCDC 1975385, 1975387,
1978345 and 1978346. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/d0ob00978d
Scheme 1 Merging the 1,4-Pd shift with migratory insertion of a palla-
dium carbene intermediate.
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offer a facile approach to 2,2-diarylsubstituted 2H-chromene, Increasing the loading of the catalyst led to a full conversion of
which has potential applications in the area of photochromic 1a, and 3aa was obtained in 79% yield upon isolation (entry
materials.9,10 It is worthwhile to mention that Wang has 12). A brief examination of the effects of the base showed that
reported an elegant work on 2H-chromene synthesis by the K2CO3 was still the best choice. Other palladium precatalysts
reaction of vinyl bromide with tosylhydrazone 2 (Scheme 1d), were also examined but delivered unsatisfactory yields except
in which there were no examples of 2,2-disubstituted 2H-chro- [Pd(cinnamyl)Cl]2. Finally, control experiments showed that
mene preparation.11,12 Herein we describe our recent endeavor the palladium source, ligand and base were all crucial for the
towards this goal, which could be considered as a rare example reaction to occur.
of cascade reactions on 1,4-palladium shift and Tsuji–Trost-
With the optimized reaction conditions in hand (Table 1,
type allylic alkylation involving a palladium carbene intermedi- entry 12), a variety of N-tosylhydrazones, easily prepared from
ate, and further complement Wang’s work on 2H-chromene the corresponding aldehydes, were employed to investigate the
synthesis (Scheme 1e).
substrate scope of this reaction. As seen from the results pre-
sented in Scheme 2, aromatic N-tosylhydrazones bearing Me,
OMe, OTBS, F, Cl, or COOMe substituents are all suitable sub-
strates for this reaction, giving the corresponding products in
moderate to good yields (3aa–3am), thus indicating that the
reaction is compatible with both electron-withdrawing and
electron-donating groups as aromatic substituents. On the
other hand, the reactions with ortho substituted tosylhydra-
zones also succeeded to afford the corresponding cyclized pro-
ducts (3al and 3am), which revealed that the method shows
good tolerance to steric hindrance effects. However, naphthyl
N-tosylhydrazone 2m gave the products with low regio-
selectivity with 4am as the major product. Hydrazones derived
from estrone and methyl tyrosinate could also provide the
desired products with good yields and chemoselectivities (3an
and 3ao). The structure of 3ac was further established by X-ray
diffraction.
Results and discussion
We initiated our investigation by performing the reaction of
aryl bromide 1a and N-tosylhydrazone 2a using [Pd(allyl)Cl]2
as the palladium source and dppb (1,4-bis(diphenylphospha-
nyl)butane) as the ligand. To our delight, the anticipated reac-
tion indeed occurred with K2CO3 as the base in THF at 110 °C,
and product 3aa was isolated in 46% yield, along with a trace
amount of isomer 4aa (Table 1, entry 1). A variety of mono-
and bidentate phosphine ligands proved to be less efficient,
providing 3aa in lower yields and lower conversion rates
(entries 2–7). Testing the effects of solvents revealed that
2-MeTHF was the most effective for this transformation, result-
ing in an increase of the yield of 3aa to 52% (entry 10).
Table 1 Optimization of reaction conditionsa
[%]b
Entry
[Pd]/L
T/°C
Solvent
Base
1a
3aa
4aa
1
2
3
4
5
6
7
8
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppb
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppe
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppp
2.5 mol% [Pd(allyl)Cl]2/15 mol% PPh3
2.5 mol% [Pd(allyl)Cl]2/15 mol% L1
2.5 mol% [Pd(allyl)Cl]2/15 mol% L2
2.5 mol% [Pd(allyl)Cl]2/15 mol% L3
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppb
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppb
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppb
2.5 mol% [Pd(allyl)Cl]2/7.5 mol% dppb
5 mol% [Pd(allyl)Cl]2/15 mol% dppb
5 mol% [Pd(allyl)Cl]2/15 mol% dppb
5 mol% [Pd(allyl)Cl]2/15 mol% dppb
10 mol% Pd(OAc)2/15 mol% dppb
110
110
110
110
110
110
110
100
100
100
100
100
100
100
100
100
THF
THF
THF
THF
THF
THF
THF
THF
Dioxane
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
2-MeTHF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
CsF
16
21
43
34
32
23
39
27
13
35
11
0
51(46)c
25
15
29
35
38
27
50
36
3
1
<1
<1
<1
<1
<1
3
<1
3
3
9
10
11d
12
13
14
15
16
52
43
87(79)c,e
36
8
16
0
0
<1
<1
2
K3PO4
K2CO3
K2CO3
68
59
5 mol% [Pd(cinnamyl)Cl]2/15 mol% dppb
0
79(78)c,e
2
a The reaction was carried out for 20 h. b Determined by 1H NMR using N,N-dimethylpyridin-4-amine as an internal standard. c Yield of the iso-
lated product. d n-Bu4NBr (0.02 mmol) was added. e The reaction was completed in 6 h.
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Scheme 2 Substrate scope of N-tosylhydrazones. a Reaction con-
ditions: Table 1, entry 12; yield of the isolated product. b Isolated as a
pure isomer, the ratio of 3/4 > 15/1. c Isolated as an inseparable mixture
of 3 and 4 with the ratio shown in parentheses. d 5 mol% [Pd(cinnamyl)
Cl]2 replaced 5 mol% [Pd(allyl)Cl]2. e 10 mol% [Pd(cinnamyl)Cl]2 and
30 mol% dppb were used. f Reaction conditions: 5 mol% Pd(PPh3)2Cl2,
dioxane (0.1 M), 300 mol% K2CO3, 100 °C, 10 h.
Scheme 4 Deuterium labeling experiments and the plausible reaction
pathway.
ponding products in moderate to good yields with high selecti-
vity. The shift of substituents to the ortho-position led to
reduced selectivity, presumably due to the steric effects (3ha
and 3ia). It is noteworthy that substrates with the phenyl ring
replaced by a naphthyl or 3-pyridinyl group were also identi-
fied to be suitable for the current reaction, delivering the pro-
ducts with moderate yields and good selectivity (3ja and 3ka).
Replacement of the phenyl ring R with a methyl or ester group
shut down the selectivity. The transformation efficiency was
diminished due to other unidentified side reactions (3la and
3ma), indicating a significant influence of the R groups on the
reaction. Bromides bearing different substituents on both aro-
matic rings were viable substrates. They could react with
N-tosylhydrazone 2a to afford the corresponding products in
moderate yields and with good selectivities (3pa and 3qa).
To understand the mechanistic details of this cascade reac-
tion, several deuterium labeling experiments were carried out
(Scheme 4a). First, the reaction of the deuterated arylbromide
d2-1a under standard reaction conditions gave the corres-
ponding products with a low incorporation of deuterium at the
ortho-position of the phenyl ring and 62% incorporation of
Scheme 3 Substrate scope of ortho-vinyl bromobenzenes. a Reaction
conditions: Table 1, entry 12; yield of the isolated product. b The ratio of
3/4 > 15/1. c Isolated as an inseparable mixture of 3 and 4 with the ratio
in parentheses. d 5 mol% of [Pd(cinnamyl)Cl]2 was used as the
precatalyst.
Next, the scope of ortho-vinyl bromobenzenes 1 was evalu- deuterium at the double bond carbon. Second, we tested the
ated (Scheme 3). If the R group was a phenyl ring, a series of reaction with 1a by adding 8 equiv. of D2O, and found 34%
substituents on the meta- or para-position, including Me, incorporation of deuterium at the ortho-position of the phenyl
OMe, CF3, Cl, and F, were all compatible, giving the corres- ring and 21% incorporation of deuterium at the double bond
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selective allylic substitution. The merging of the palladium
carbene migratory insertion chemistry with the 1,4-palladium
shift has offered a new way to generate an η3-allylpalladium
intermediate, which further provides new opportunities to
expand the scope of Tsuji–Trost-type allylic alkylations.
Conflicts of interest
Scheme 5 Derivatizations of 2H-chromene 3aa. Reaction conditions:
(a) (i) Salicylaldehyde (10 mmol), TsNHNH2 (10 mmol), 2-MeTHF (30 mL),
There are no conflicts to declare.
rt,
2 h, (ii) [Pd(allyl)Cl]2 (0.25 mmol), dppb (0.75 mmol), K2CO3
(15 mmol), 1a (5 mmol), 2-MeTHF (20 mL), 100 °C, 20 h; (b) 3aa
(0.2 mmol), NBS (0.6 mmol), acetone/H2O (2 mL/0.4 mL), rt, 16 h; (c) (i)
4-MeC6H4SH (0.4 mmol), NCS (0.4 mmol), DCM (3 mL), 0 °C, 30 min, (ii)
3aa (0.2 mmol), 0 °C–rt, 2 h.
Acknowledgements
We acknowledge financial support by the NSFC (21871259 and
21901244), the Hundred-Talent Program, and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB20000000).
carbon. On the basis of these observations and the previous
studies,7 a plausible reaction pathway of this reaction is out-
lined in Scheme 4b. As shown, the reaction is supposed to
start with the oxidative addition of ortho-vinyl bromobenzene
1a to generate PdII species A, which undergoes a C–H acti-
vation step and furnishes five-membered palladacycle B.13
Then protonation of the palladacycle occurs to generate inter-
mediate C, accomplishing the 1,4-palladium shift from the
aryl to vinyl position. During this process, the proton concen-
tration in the reaction medium is relatively high as a conse-
quence of base-assisted N-tosylhydrazone decomposition,
which probably results in low deuterium incorporation ratios
at the ortho-position of the phenyl ring in the deuterium label-
ing experiment. C reacts with diazo substrate 2a to form PdII
carbene species D, followed by migratory insertion to give η1-
allylpalladium intermediate E, which exists in a fast equili-
brium with η3-allylpalladium species F. E may also undergo a
β-hydrogen elimination to afford intermediate G14 with the
palladium hydride species coordinating with the allene skel-
eton. Reinsertion of the palladium hydride species to the
double bond in G gives rise to intermediate H. Finally, the
product 3aa is formed through an intramolecular Tsuji–Trost-
type allylic substitution; meanwhile the isomer 4aa is obtained
as a result of reductive elimination of intermediate H.
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graphic data for this paper.
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