Hypervalent-Iodine-Mediated Carbon–Carbon Bond Cleavage and Dearomatization of 9H-Fluoren-9-ols

Article abstract of DOI:10.1002/anie.201913373

A transition-metal-free synthesis of spiro compounds from 9H-fluoren-9-ols mediated by hypervalent iodine is reported. In this reaction, an unprecedented β-carbon elimination of tertiary alkoxyliodine(III) to form new diaryliodonium salts is proposed. The

Full text of DOI:10.1002/anie.201913373

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Accepted Article
Title: Hypervalent Iodine-Mediated Carbon-Carbon Bond Cleavage and
Dearomatization of 9H-Fluoren-9-ols
Authors: Zhenhua Gu, Ruixian Deng, Shuming Zhan, and Chunyu Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913373
Angew. Chem. 10.1002/ange.201913373
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10.1002/anie.201913373
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Hypervalent Iodine-Mediated Carbon-Carbon Bond Cleavage and
Dearomatization of 9H-Fluoren-9-ols
Ruixian Deng, Shuming Zhan, Chunyu Li and Zhenhua Gu*
Dedication ((optional))
Abstract:
A
transition-metal free, hypervalent iodine-mediated
intensively investigated. Conversely, alkoxyl radicals usually
generate more stable C(sp3)-carbon or other relatively stable
radicals instead of aryl, vinyl or alkynyl radicals (Scheme 1b).[6]
Narashaka, Chiba, Zhu, Zuo, Knowles and many other groups
found that cyclopropanols, cyclobutanols, or even cyclohexanols
underwent -carbon scission by employing silver, manganese or
other transition-metal catalysts with or without oxidants (eq. 3).[7]
Alternatively, tertiary hydroperoxide would generate oxygen
radicals, which also underwent -carbon scission to deliver alkyl
radicals. Chen and co-workers reported that cyclic iodine(III)
reagent (CIR)/photoredox generated acyl radicals via -carbon
scission of alkoxyl radicals (eq. 4).[8]
synthesis of spiro compounds from 9H-fluoren-9-ols has been
reported. In this reaction, an unprecedented -carbon elimination of
tertiary alloxyliodine(III) to form new diaryliodonium salts was
proposed. The obtained phenols intermediates underwent oxidative
dearomatization to furnish a class of oxo-spiro compounds. This
domino reaction significantly increased the complexity of these
molecules and showed excellent regio- and stereoselectivity.
The development of new reaction modes, by revolutionizing
synthetic routes and attaining higher efficiency, is always a hot
research area in synthetic chemistry. The carbon-hydrogen and
carbon-carbon activation reactions are of the two research areas
that fit the above criteria.[1-2] Though carbon-carbon cleavage
reaction was ever viewed as destructive reaction and more
challenging than carbon-hydrogen activation, recently it is going
to attract considerable attentions from the synthetic chemists.
Beyond the strained ring systems (three- or four-membered rings)
and coordination-directed C-C bond activation, the carbon-carbon
cleavage of tertiary alcohols by releasing ketones is of value to
organic synthesis.
Herein we report a hypervalent iodine-mediated domino
reaction for the synthesis of spiro-ethers from 9H-fluoren-9-ols
(Scheme 1c).[9-11] It disclosed an unprecedentedly hypervalent
iodine promoted aryl-C(sp3) bond cleavage, whose mechanism
might be different from the above discussed metal-involved -
elimination or radical -scission. The ring-opening products
quickly underwent oxidative dearomatization/cyclization to
produce oxo-spiro scaffold.[12]
There are two major strategies to realize the carbon-carbon
cleavage of tertiary alcohols (Scheme 1). The first one is the -
carbon elimination of alkoxylmetal species. Generally, the
reactions favoured forming unsaturated C(sp2)- or C(sp)-metal
intermediates instead of C(sp3)-metal species if both C(sp2)-,
C(sp)-carbon and C(sp3)-carbon exist at the -position.[3] The
driving force of these -carbon elimination reactions is either
releasing strain energy of small rings or increasing entropy by
releasing small molecules, i.e. acetone. For instance, Murakami
and Wang groups disclosed Rh-catalyzed ring-openings of
benzocyclobutanol,
where
the
intermediate
[2-(2-
oxoethyl)phenyl]rhodium was chemoselectively formed by
releasing the ring strain energy of benzocyclobutanol (eq. 1).[4]
The alkoxylpalladium(II) underwent -carbon elimination to
produce new aryl-palladium species (eq. 2).[5] The second one is
the -carbon scission of tertiary alkoxyl radicals, which has been
[a]
R. Deng, S. Zhan, Chunyu Li and Prof. Dr. Z. Gu
Department of Chemistry, Center for Excellence in Molecular
Synthesis, and Hefei National Laboratory for Physical Sciences at
the Microscale
University of Science and Technology of China
96 Jinzhai Road, Hefei, Anhui 230026 (China)
E-mail: zhgu@ustc.edu.cn (ZG)
Scheme 1. Strategies for Carbon-Carbon Cleavages of Tertiary Alcohols
Supporting information for this article is given via a link at the end
of the document.((Please delete this text if not appropriate))
In our previous studies it was found that the bis(ortho)-
substituents increased the distortion of fused biaryl systems.
Thus, these molecules showed higher reactivity than the non-
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Table 2. Substrate Scope[a]
distorted ones in ring-opening reactions.[13] Our investigation
started with 4,5-dimethyl-9-phenyl-9H-fluoren-9-ol (1a), and we
anticipated to get the ring-opening/bromination products (2a) in
the presence of CuBr and bis(trifluoroacetoxy)iodobenzene
(PIFA).[14] Surprisingly, the reaction of 1a with PIFA and KBr under
the catalysis of CuBr afforded a trace amount of bromination
product 3a, which was detected by GC-Mass (Table 1, entry 1).
However, oxo-spiro compound 2a was isolated in 30% yield,
which was regarded as the product of ring-opening/oxygenation,
followed by PIFA-mediated oxidative dearomatization. There was
no trans-isomer being detected. It is obvious to see that the
complexity from 1a to 2a was significantly increased. Considering
the fascinating oxo-spiro skeleton and interesting mechanism of
carbon-carbon cleavage, we started to optimize this
transformation. In the absence of CuBr, neither phenyliodine(III)
diacetate (PIDA) nor PIFA, PhI=O produced desired carbon-
carbon cleavage product (entries 2-4). The addition of Fe(acac)3
promoted this reaction to furnish 2a in 32% yield (entry 5). MeOH
as the co-solvent was deleterious for this transformation (entry 6),
while 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP)[15] dramatically
improved the yield to 92% (entry 7). Interestingly, it was found that
Fe(acac)3 was not necessary when HFIP was used as the co-
solvent (entry 8). At last, some of classic oxidants were
investigated. PIDA and PIFA produced acceptable yields of 2a.
However, H2O2 and m-CPBA gave unknown mixtures, while
NaIO4 was an ineffective oxidant (entries 9-13).
Table 1. The Reaction Condition Optimization[a]
entry
1[b]
2
oxidant
PIFA
Additive
solvent
DCM
Yield of 2a/%
30
CuBr, KBr
[a] Reaction Conditions: tertiary alcohol 1 (0.10 mmol) and PhI=O (0.25 mmol)
in a mixed solvent of HFIP (1.0 mL) and CH2Cl2 (1.0 mL) at r.t. for 2 h. [b] An
unknown compound was isolated.
PIDA
-
DCM
NR
NR
NR
32
3
PIFA
-
DCM
4
PhI=O
PhI=O
PhI=O
PhI=O
PhI=O
PIDA
-
DCM
With the optimal conditions in hand, we next tested the
substrate scope of this reaction (Table 2). First, the substituent
effect on the 9-aryl ring was investigated. The ortho-substituent
decreased the efficacy of this transformation, however, decent
yields still were achieved (2b and 2c). The meta- and para-
substituents had marginal effect on the yields, except the 3,5-
diemthylphenyl derivative (2d-2f). The diastereochemistry of the
products was determined to be cis by single crystal X-ray
diffraction analysis of compound 2f.[16] The reaction was less
sensitive to the electron-neutral or electron-withdrawing groups
on the 9-aryl ring: all the reactions delivered excellent yields
(compare 2g-2k with 2a). However, strong electron-donating
groups, i.e., para-methoxyl, resulted in the decomposition of the
substrate (2l). Pleasingly, the C=C double of styrene structure
was also tolerable, and the yield of isolated product 2m was still
satisfying. Further studies indicated that the substrates bearing
2-naphthyl or heterocycles, including 2-dibenzo[b,d]furan, 2-
dibenzo[b,d]thiophene and 3-methylthiophene underwent this
ring-opening/dearomatization reaction uneventfully (2n-2q).
Second, the scope regarding the substituted biaryls was then
5[c]
6[c]
7[c]
8
Fe(acac)3
DCM
Fe(acac)3
DCM/MeOH
DCM/HFIP
DCM/HFIP
DCM/HFIP
DCM/HFIP
DCM/HFIP
DCM/HFIP
DCM/HFIP
trace
92
Fe(acac)3
-
-
-
-
-
-
99
9
76
10
11
12
13
PIFA
85
[d]
H2O2
-
m-CPBA
NaIO4
complex
NR
[a] Reaction Conditions: 1a (0.10 mmol) and oxidant (0.25 mmol) in the
indicated solvent at r.t. under open air. [b] CuBr (0.01 mmol), KBr (0.25 mmol)
and NaH (0.10 mmol) was added. The reaction conducted under nitrogen
atmosphere. [c] Fe(acac)3 (0.01 mmol) was added. The reaction was performed
under nitrogen atmosphere. [d] An unidentified product was isolated.
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10.1002/anie.201913373
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studied. With para-methyl, phenyl, as well as bromo substituted
biaryl compounds, the reactions proceeded smoothly to deliver
the corresponding products 2r-2t in 77-99% yields.
gave 2u and 2v as a single isomer, which was unambiguously
confirmed by single crystal X-ray diffraction analysis of 2u.[17] It
indicated that the reaction favoured breaking naphthyl-C(sp3)
bond. Surprisingly, the reaction of 1w, bearing an adjacent methyl
group to cyclopentanol ring, selectively cleaved the more steric
C(aryl)-C(sp3) bond to provide 2w in high yield. Remarkably,
replacing adjacent methyl group with chlorine atom (1x), the
reaction gave a reversed regioselectivity with the ratio of 2x:2x’
being 10:1 (CCDC 1957816 for 2x). The 2-methoxyl substituted
benzo[c]fluoren-7-ol (1y) also favoured cleaving the C-C bond
attached to aryl ring containing ortho-methoxy group (2y:2y’= 5:1),
albeit the yields dropped (CCDC 1957817 for 2y). Interestingly,
by introducing meta-methoxy group to one to the aryl rings of 9H-
fluoren-9-ol (1z), the reaction produced 2z in good yield as a
single isomer, favouring dearomatization of the phenyl ring
without methoxy substituent. Predictable, the reaction of 1aa or
1ff afforded a mixture of regio-iomsers in a ratio of 1:1. The
reactions of the substrates bearing 4-chloro group selectively
cleaved the carbon-carbon bonds without connecting the
chlorobenezene rings (2bb-2dd) in excellent yields and
regioselectivity. The 4-fluoro substituted 9H-fluoren-9-ol (1ee)
displayed same regioselectivity as its chlorinated analogue. The
remarkable regio-selectivity of these non-symmetric substrates
further demonstrated the utilities of this transformation, though,
unfortunately the detailed mechanism regarding these
chemoselectivities was unclear currently.
Table 3. Substrate Scope[a]
Scheme 2. Anomalies and Synthetic Applications
[a] Reaction Conditions: tertiary alcohol 1 (0.10 mmol) and PhI=O (0.25 mmol)
in a mixed solvent of HFIP (1.0 mL) and CH2Cl2 (1.0 mL) at r.t. for 2-12 h.
For comparison, substrates with less distorted biaryl
structures were synthesized. Under standard conditions, 4,5-
difluoro- and 4,5-hydro-9H-fluoren-9-ols 1gg and 1hh were inert,
which were possibly due to the lack of strong torsional strain to
promote the C-C bond cleavage/ring-opening (Scheme 2, eq. 1).
Another notable merit of this transformation is the excellent
chemoselectivity of non-symmetric substrates (Table 3).
Satisfyingly,
the
reaction
of
11-methyl-7-phenyl-7H-
benzo[c]fluoren-7-ol 1u or its chlorinated analogue 1v selectively
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The binaphthyl derivative 1ii decomposed and no desired oxo-
spiro ketone 2ii was detected (Scheme 2, eq. 2). A mixture of 1a
and silylenol ethers treated with Zn(OTf)2 resulted in converting
6). Currently, the way of homolysis of 7 delivering two radicals,
followed by -scission, and radical-radical recombination, or Kita-
type radical cation could not be ruled out.[26]
the hemiketal structure to
a
tertiary ether with high
diastereoselectivities (dr 10:1 and >25:1) (Scheme 2, eq. 3). The
structure of 4b was further conformed by single crystal X-ray
analysis.[18] In the aqueous acidic medium, 1a underwent
nucleophilic ring-opening to produce a biphenyl product 5 in
excellent yield (Scheme 2, eq. 4). The asymmetric version of this
transformation by using stoichiometric chiral bulky iodine(III)
reagent showed low efficacy, along with only 25% ee (Scheme 2,
eq. 5).[19]
In conclusion, we reported a hypervalent iodine-promoted
highly efficient synthesis of oxo-spiro compounds 2 from 9H-
fluoren-9-ol derivatives. The reaction involved an unprecedented,
mechanistic interesting hypervalent iodine-involved formal -
carbon elimination of alkoxyiodine(III) compounds. In the
presence of PhI=O, the phenol intermediates underwent
subsequent oxidative dearomatization to give the oxo-spiro
products in high yields and stereoselectivity. The investigation of
detailed mechanism is still ongoing in our laboratory.
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (21622206, 21871241), Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB20000000), and the Fundamental Research Funds for the
Central Universities (WK2060190086).
Keywords: hypervalent iodine • carbon-carbon bond cleavage •
ring-opening • spiro compounds • dearomatization
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dearomatization to deliver 1H-isobenzofuran-2-ium type product
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8 was still in its speculative stage. Fortunately, the treatment of
phenol 6 with PhI=O independently also gave 2a in 48% yield (eq.
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10.1002/anie.201913373
Angewandte Chemie International Edition
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Ruixian Deng, Shuming Zhan and
Zhenhua Gu*
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Hypervalent Iodine-Mediated Carbon-
Carbon Bond Cleavage and
Dearomatization of 9H-fluoren-9-ols
A hypervalent iodine(III)-mediated carbon-carbon cleavage reaction of 9H-fluoren-
9-ols was reported. The reaction proceeded via an unprecedented hypervalent
iodine-mediated carbon-carbon cleavage, followed by reductive elimination and
dearomatization. Thus, a class of spiro compounds were synthesized in a highly
efficient and stereoselective manner.
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