Organic Letters
Letter
important building block for constructing diverse cyclic
compounds.8,9 Inspired by these advances and our ongoing
interest in the cyclization reactions involving palladacycles,5,10
herein we disclose a palladium-catalyzed decarboxylative
cyclization of 2-iodobiphenyls with α-bromoacrylic acids for
the synthesis of diverse dibenzofulvenes (Scheme 2c). Notably,
α-bromoacrylic acids were employed as C1 insertion units by
the cleavage of the C(vinyl)−Br bond and decarboxylation in
this reaction.
yield when the reaction was performed with 1a scaled up to 1
mmol.
Synthetic applications of products were then performed
(Scheme 3, bottom). The intermolecular oxidative ring
expansion of product 30 could successfully afford PAH 58
containing a 9,10-diarylphenanthrene core framework in 82%
yield. Diphenyldibenzofulvene 19 also underwent an intra-
molecular migration ring expansion to give 9,10-diphenylphe-
nanthrene 59 in excellent yield.
To gain insights into the reaction mechanism, we performed
several control experiments (Scheme 4). Palladacycle B-Int
We initiated our studies by investigating the reaction of 2-
iodobiphenyl 1a with α-bromoacrylic acid 2a. After extensive
screening of the reaction parameters, the anticipated
dibenzofulvene 3 was obtained in 72% yield under simple
catalytic conditions composed of Pd(OAc)2, P(o-tol)3, K2CO3,
and KOAc in DMSO at 110 °C. (See Table S1 in the SI.) The
scope of α-bromoacrylic acids 1 was subsequently examined
(Scheme 3, top). This protocol was applicable to a wide range
of α-bromoacrylic acids 1 to furnish fluorene derivatives 3−30
in moderate to excellent yields. Mono- or disubstituted alkyl
groups on the olefin terminus of α-bromoacrylic acid were well
tolerated (4−7). Cyclic dialkyl-substituted substrates contain-
ing different ring sizes, such as 5- to 12-membered rings, all
successfully participated in the [4 + 1] decarboxylative
annulation to afford dibenzofulvenes 8−10. Importantly,
bridged ring systems, including norbornane and adamantane,
could also survive to furnish dibenzofulvenes 11−13 in 56−
65% yields. Moreover, the compatibility was further demon-
strated by testing the aryl substituents. Monoaryl-, arylalkyl-,
diaryl-, and even cyclic diaryl-substituted α-bromoacrylic acids
were able to undergo the [4 + 1] annulation with 2-
iodobiphenyl 2a to afford various dibenzofulvenes 14−26 in
good to excellent yields. The optimal reaction conditions could
tolerate a series of functional groups on the benzene ring,
including Me, F, and Cl (20−24). Their electronic properties
seemed to have no obvious effect on the reactivity. It is of
particular note that the substrates with a thioxanthene ring
were reactive to deliver the heterocyclic polycyclic aromatic
hydrocarbon (PAH) 27 in moderate yield. Styryl-substituted
α-bromoacrylic acid was also a competent substrate (28).
Gratifyingly, divinylbenzene-bridged fluorenes 29 and 30 could
be formed in 75 and 73% yields by two [4 + 1] annulations.
The structure of 30 was clearly confirmed by single-crystal X-
ray crystallography. (See the SI.)
We next turned our attention to exploring the scope of 2-
iodobiaryls (Scheme 3, middle). To our delight, a large variety
of 2-iodobiphenyls 2 were subjected to the [4 + 1]
decarboxylative annulation with α-bromoacrylic acid 2a to
afford diphenyldibenzofulvenes 31−57 in good to excellent
yields. Various substituents at different positions on the
benzene ring, such as electron-donating groups (Me, OMe,
and t-Bu), modifiable halogen groups (F and Cl), and strong
electron-withdrawing groups (CF3, CHO, CO2Me, CN, and
NO2), were well tolerated, delivering products 31−48 in 62−
96% yields. Pleasingly, substrates that were diversely fused
aromatic rings and heteroaromatic rings, namely, naphthalene,
phenanthrene, pyrene, thiophene, pyridine, and indole, could
be smoothly converted into the target products 49−57 in good
to excellent yields. Remarkably, the steric effect had a vital
influence on the C−H activation step, which was demonstrated
by the regioselective formation of products 46−49. 2-
Bromobiphenyl also showed good reactivity, giving product 3
in 66% yield. Satisfactorily, product 19 was isolated in 83%
Scheme 4. Control Experiment
could react smoothly with α-bromoacrylic acids 2q, leading to
the formation of diphenyldibenzofulvenes 19 in 46% yield (eq
1). This result indicated that palladacycle B was formed in this
transformation. Subsequently, biphenylacrylic acid 60 was
synthesized and subjected to the standard reaction conditions
(eq 2). As anticipated, product 3 was obtained in 66% yield,
which suggested that a route from B to D is possible (path A in
Scheme 5). Moreover, we speculated that an alternative
carbene insertion pathway may be involved (path B in Scheme
5). Therefore, we conducted an experiment using a mixture of
α-bromoacrylic acids 2q, K2CO3, KOAc, and DMSO and
found that 1,2-diphenylethyne 61 was produced in 40% yield
(eq 3). This result revealed that carbene G was able to be
generated by the simultaneous decarboxylation and debromi-
nation of 2q.11
On the basis of our experimental results as well as previous
work,4−6 a plausible catalytic cycle for this protocol is depicted
in Scheme 5. Initially, the oxidation addition of 2-iodobiphenyl
1a to Pd(0) species produces biphenylpalladium species A,
which undergoes a C−H activation to give palladacycle B. The
result from eq 2 in Scheme 4 indicates that palladacycle B then
forms intermediate D via a successive oxidative addition and
reductive elimination process. Afterward, decarboxylation of D
leads to the six-membered palladacycle E, which is further
reductively eliminated to deliver product 19 and release Pd(0)
species. Considering this result of eq 3 in Scheme 4, the
occurrence of carbene insertion from B to E is also possible
(path B).
In summary, we have developed a new palladium-catalyzed
[4 + 1] annulation for the synthesis of diverse dibenzofulvene
derivatives by the use of α-bromoacrylic acids as C1 insertion
5746
Org. Lett. 2021, 23, 5744−5749
Zhang, Minghao
