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Palladium-Catalyzed Aryne Insertion/Arene Dearomatization Domino Reaction: A Highly Chemoselective Route to Spirofluorenes Zhijun Zuo, Hui Wang, Yunxia Diao, Yicong Ge, Jingjing Liu, and Xinjun Luan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03655 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018
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ACS Catalysis
Palladium-Catalyzed Aryne Insertion/Arene Dearomatization Domino Reaction: A Highly Chemoselective Route to Spirofluorenes Zhijun Zuo,†,§ Hui Wang,†,§ Yunxia Diao,† Yicong Ge,† Jingjing Liu,† and Xinjun Luan*,†,‡ †Key
Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xi’an, 710127, China ‡State
Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin, 300071, China
ABSTRACT: A palladium(0)-catalyzed dearomatizing [3+2] spiroannulation of naphthalene-based biaryls with arynes has been developed for the rapid construction of spirofluorene architectures. This reaction was realized by carbopalladation of aryne to generate an arylpalladium species, followed by termination with naphthalene dearomatization, which is a sharp contrast to the conventional C-H functionalization approach. Mechanistic studies revealed that the arene dearomatization might take place through a 5-exo-trig spiroannulation and distal-hydride elimination Heck-type pathway.
KEYWORDS: spiroannulation, dearomatization, arynes, spirofluorene, palladium
The fundamental discovery of convenient generation of arynes through fluoride-triggered 1,2-elimination of osilylaryl triflates,1 has led to a rapid revival in the development of transition-metal-catalyzed aryne-involved processes.2 Especially, intermolecular carbopalladation of arynes stands out as a powerful approach for the construction of diverse 1,2-functionalized arenes through multicomponent domino reactions.3 Typically, aryne undergoes carbopalladation with a catalytically generated organopalladium species A to produce the key intermediate B, which is eventually trapped by a nucleophile (or an olefin) to realize the vicinal bisfunctionalization (Scheme 1a).4 Besides the utilization of these prefunctionalized terminating agents, the incorporation of arene-based coupling partners through direct C-H functionalization has also gained very impressive achievements, affording a variety of biarylcontaining carbo- and heterocycles (Scheme 1b).5 These processes were often believed to be terminated by siteselective endo-cyclization,6 while possible Heck-type exospiroannulation through the generation of spirocyclic intermediate by breaking its -system of aromatic ring was dramatically disfavored.
carbopalladation reaction of arynes through a dearomative [3+2] spiroannulaition with biaryls derived
Remarkably, transition-metal-catalyzed dearomatization has become an efficient tool for converting planar arenes into many attractive threedimensional architectures.7 However, up to date, there was no report on such transformations involving an aryne reactant, since they often need to use the electron-rich arenes containing a hetero-atom such as oxygen and nitrogen,8 which would readily attack on the electrophilic arynes to prevent its arene dearomatization.9 Inspired by an elegant work from the You group on the iridiumcatalyzed allylic dearomatization of pyridines and pyrazines,10 and together with our persistent interest in the area,11 we herein present an unprecedented
Figure 1. Related Spirocyclic Frameworks for Organic Electronic Devices.
Scheme 1. Domino Intermolecular Carbopalladation of Arynes. a) Trapped by a common nucleophile OTf TMS
F
Ar
Ar
Ref. [4]
R
A [R-PdII-X]
PdIIX
R Nu
Nu
Ar
Ar
[or olefin]
B R = aryl, allyl, benzyl; X = halide
b) Trapped by an arene species via C-H bond arylation PdII
Ar
Ref. [5]
PdII H
Ar'
Ar'
Ar'
Ar
Ar
vs
Pd
endo-cyclization (favored)
aryne unit
Ar'
Ar
H
II
exo-spirocyclization (disfavored)
c) Trapped by an arene species via arene dearomatization (this work) Ar1 I H
CO2Et
TMS +
Ar OTf
cleaving C-H bond
or
Ar1 cat. [Pd0]
breaking aromaticity
Ar1 Ar
CsF
CO2Et minor product
C12H25
Ar
+
CO2Et major product
C12H25
n
SFN
SBAF
DSFA
C12H25 C12H25 PSIF
from naphthalenes bearing an ester group, thus leading to the rapid assembly of a series of spirofluorene derivatives (Scheme 1c). Notably, the spirofluorene skeleton is a type
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of structure that is widely employed in the development of electroluminescent and optoelectronic materials (Figure 1),12 so the pursuit of new methods for their assembly is of high interest. At the outset, we began the studies with 1a and 2a to explore the reaction conditions (Table 1). The first catalytic run provided undesired 3a in 75% yield (entry 1), which was consistent with the previous work on C-H arylation by Larock.5a Further studies were significantly encouraged by the concomitant generation of a trace amount of anticipated product 4a relying on naphthalene dearomatization. Gratifyingly, screening on solvents proved that MeCN was able to promote the desired transformation to give 4a in 38% yield (entry 4). Next, more studies demonstrated very significant ligand effect (entries 5-8), with electron-rich and bulky P(o-Anisyl)3 showing the better performance by reversing the chemoselectivity of the reaction (entry 8). Attempts on using common bases like K2CO3 and Cs2CO3 didn’t improve the reaction (entries 9-10). Much to our delight, it was found that palladium sources showed a dramatic impact for the process (entries 11-14), and 4a was formed in 85% yield with excellent chemoselectivity (entry 14). This result proved that the cinnamyl couterion was critical for avoiding that undesired C-H arylation. Moreover, the test with bromo-substrate 1a' proceeded smoothly to give 4a in 79% yield under the optimized conditions (entry 15). Importantly, it should be noted that compound 4a was always formed as a single E-isomer with regard to the C-C double bond, which was confirmed by X-ray crystallographic studies.
a good number of naphthalene-based biaryls (1b-p) underwent the desired [3+2] spiroannulation with arynes, affording spirofluorene products 4b-p in moderate to good yields. With respect to the upper aromatic ring, a series of substituents such as methyl (1b,f), methoxy (1g), Table 2. Reaction Scope.
Table 1. Optimization of the Reaction Conditions.
I
TMS
+
1a
entry
CO2Et
2a
[Pd] (10.0 mol%) ligand (20.0 mol%)
OTf K3PO4 (2.0 equiv) CsF (2.0 equiv) solvent (0.1 M) 90 ºC, 10 h
[Pd]
ligand
+
3a
CO2Et
solvent
1 2
Pd(OAc)2 Pd(OAc)2
PCy3 PCy3
THF 1,4-Dioxane
3
Pd(OAc)2
PCy3
4
Pd(OAc)2
PCy3
5
Pd(OAc)2
6
4a
CO2Et
yield (%)a 3a 4a 75 60
