Rhodium(III)-Catalyzed Cascade Redox-Neutral C–H Functionalization and Aromatization: Synthesis of Unsymmetrical ortho-Biphenols

Article abstract of DOI:10.1002/adsc.201601296

An efficient rhodium(III)-catalyzed coupling reaction of N-aryloxyacetamides with 6-diazo-2-cyclohexenones through a cascade redox-neutral C–H functionalization and aromatization has been developed. This novel and scalable transformation provides a straightforward way to construct unsymmetrical ortho-biphenols with broad substrate scope under mild and redox-neutral conditions. The synthetic utility of this approach is demonstrated in the late-stage functionalization of bioactive compounds and the synthesis of an optically active ortho-biphenol. (Figure presented.).

Full text of DOI:10.1002/adsc.201601296

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DOI: 10.1002/adsc.201601296
Rhodium(III)-Catalyzed Cascade Redox-Neutral C–H
Functionalization and Aromatization: Synthesis of Unsymmetrical
ortho-Biphenols
Zhiyong Hua and Guixia Liua,*
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Science, 345 Lingling Road, Shanghai 200032, Peopleꢀs Republic of China
E-mail: guixia@sioc.ac.cn
Received: November 24, 2016; Revised: February 25, 2017; Published online: && &&, 0000
Supporting information for this article can be found under http://dx.doi.org/10.1002/adsc.201601296.
Abstract: An efficient rhodium(III)-catalyzed cou-
pling reaction of N-aryloxyacetamides with 6-diazo-
2-cyclohexenones through a cascade redox-neutral
C–H functionalization and aromatization has been
developed. This novel and scalable transformation
provides a straightforward way to construct unsym-
metrical ortho-biphenols with broad substrate scope
under mild and redox-neutral conditions. The syn-
thetic utility of this approach is demonstrated in the
late-stage functionalization of bioactive compounds
and the synthesis of an optically active ortho-biphe-
nol.
ed.[7] However, these approaches require selective
ortho-halogenation of phenols and multistep synthe-
sis, thus limiting their efficiency.
Transition metal-catalyzed C–H functionalization
provides a straightforward route to construct complex
organic frameworks from readily accessible sub-
strates.[8] Recently, the redox-neutral strategy employ-
ing an oxidizing directing group[9] has been emerged
in the field of C–H functionalization. This strategy
avoids the use of stoichiometric amounts of an exter-
nal metal oxidant and allows for a step-economical
and waste-reducing transformation under mild condi-
tions.[10] In our own efforts,[11] we developed a traceless
oxidizing directing group ONHAc for the redox-neu-
tral C–H functionalization of ArONHAc towards an
efficient synthesis of ortho-functionalized phenols
(Scheme 1a).[11,12] We envisioned that this powerful
strategy could be explored for the synthesis of unsym-
metrical ortho-biphenols.
Keywords: ortho-biphenols; C–H functionalization;
diazo compounds; redox-neutral conditions; rhodi-
um
a-Diazocyclohexenones are versatile molecules
ortho-Biphenols (2,2’-dihydroxy-1,1’-biaryls) are im- having both diazo and a,b-unsaturated carbonyl
portant structures featured in numerous natural and groups. They have been used as phenolic precursors
synthetic substances with a wide array of interesting in transition metal-catalyzed aromatization cascade
biological properties.[1] Furthermore, their derivatives reactions.[13] Inspired by these elegant works, we con-
are useful ligands or catalysts in catalysis.[2] Therefore, ceive a-diazocyclohexenones as an alternative arylat-
the development of practical protocols to access struc- ing agent for the synthesis of unsymmetrical o-biphe-
turally diverse ortho-biphenols is highly significant. nols. While most approaches for the construction of
Although the oxidative phenolic coupling offers ortho-biphenols are concentrated on the direct cou-
a direct approach to ortho-biphenols,[3] the synthesis pling of two phenolic units, few have been shown to
of unsymmetrical ortho-biphenols remains elusive[4] introduce the ortho-phenolic group from non-aromat-
due to homocoupling and poor regioselectivity. Un- ic precursors. Herein, we disclose a rhodium-catalyzed
symmetrical ortho-biphenols can be constructed by in- cascade reaction involving redox-neutral C–H func-
direct oxidative coupling of two phenols, in which one tionalization and aromatization from N-aryloxyaceta-
coupling partner is oxidized to the quinone derivative mides and 6-diazo-2-cyclohexenones (Scheme 1c).
and then couples with the other phenol under organo- This process delivers unsymmetrical ortho-biphenols
catalysis.[5] In addition, studies on the synthesis of with diverse substitutions, some of which are difficult-
non-C2-symmetical ortho-biphenols via transition ly accessed by traditional methods. The reaction pro-
metal-catalyzed C–X/C–M cross-coupling[6] or intra- ceeds under mild and redox-neutral conditions, and is
molecular C–H/C–X coupling have also been report- readily scalable.
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Table 1. Optimization of the reaction conditions for the
model reaction.
Entry[a]
Base
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
CsOAc
NaOAc
KOAc
Cs2CO3
K2CO3
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
–
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
MeCN
THF
CH2Cl2
acetone
ClCH2CH2Cl
ClCH2CH2Cl
83
60
60
88
76
73
25
50
80
58
80
73
9
10
11[c]
12
[a]
Reaction conditions: 1a (0.2 mmol, 1.0 equiv.), 2a
(0.4 mmol, 2.0 equiv.), [Cp*RhCl2]2 (2.5 mol%) and base
(25 mol%) in solvent (0.1M).
Isolated yields.
1.25 mol% of [Cp*RhCl2]2 were used.
Scheme 1. Our strategy for the synthesis of unsymmetrical
ortho-biphenols.
[b]
[c]
Our initial experiments were carried out with N-
phenoxyacetamide (1a) and 6-diazo-3-methylcyclo-
hex-2-enone (2a) as the model substrates to evaluate ditions (Scheme 2). When diazo compound 2a was
the feasibility of the proposed cascade strategy. To used as the coupling partner, a diverse array of N-
our delight, when the reaction was performed with arylACTHNUGRTNEUNGoxyacetamides 1 could participate in the reaction
the commonly used [RhCp*Cl2]2/CsOAc catalytic smoothly, providing the corresponding unsymmetrical
system in dichloroethane at 308C, the desired 2,2’-bi- 2,2’-biphenols in moderate to high yields. Substrates
phenol 3aa was obtained in a yield of 83% (Table 1, bearing para, meta and ortho substituents were com-
entry 1). Screening of bases (entries 2–6) demonstrat- patible with the reaction conditions (3aa–3fa). For
ed that Cs2CO3 was the most effective, affording the meta-trifluoromethyl substrate, C–H functionalization
desired product in 88% yield (entry 4). Other bases, took place at the less hindered position selectively
such as NaOAc, KOAc, K2CO3 and K3PO4, also deliv- furnishing a single isomer (3fa). In contrast, the meta-
ered the desired product, albeit in relatively lower methoxy-substituted substrate gave a mixture of two
yields. Changing the solvent from dichloroethane to regioisomers (3ga and 3ga’), probably because the
MeCN, THF, acetone and chloromethane did not methoxy group could play the role of a secondary di-
cause any improvement (entries 7–10). It is notewor- recting group leading to the formation of 3ga’. Nota-
thy that the reaction tolerates a lower catalyst load- bly, the hindered di-meta-substituted aromatic com-
ing. The reaction proceeded smoothly even with pounds (1h and 1i) delivered the desired products
1.25 mol% of catalyst giving the product in 80% yield (3ha and 3ia) in high yields. Halogen-substituted N-
(entry 11). Fine-tuning the amounts of Cs2CO3 and phenoxyacetamides, including the para-bromo-substi-
the ratios of both substrates[14] did not further im- tuted one, underwent the cascade reaction in high
prove the yield (for a more detailed optimization yields (3ja–3la) without any dehalogenation products.
study, see the Supporting Information). Notably, this Besides the commonly encountered functional groups,
transformation could run smoothly even in the ab- strong electron-withdrawing functional groups, such
sence of a base (entry 12), which may indicate that C– as ester and nitro groups, were also tolerated (3ma
H activation of N-phenoxyacetamide could occur and 3na). Furthermore, N-naphthoxyacetamide was
without the assistance of a base. Nonetheless, control found to be a suitable substrate for this transforma-
experiments showed that no reaction happened in the tion, thus allowing the synthesis of the desired prod-
absence of a rhodium catalyst.
uct 3oa in high yield (90%).
The versatility of the rhodium(III)-catalyzed redox-
We then examined differently decorated 6-diazo-2-
neutral C–H functionalization/aromatization cascade cyclohexenones with the protocol (Scheme 3). Various
process was probed under the optimized reaction con- 6-diazo-2-cyclohexenones efficiently coupled with 1a
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diazo substrates could give the corresponding prod-
ucts in high yields (3aa–3af), the substrate bearing
a substituent adjacent to the diazo group reacted
rather sluggishly (3ag, 17%).
The broad substrate scope and mild reaction condi-
tions prompted us to apply the present method in the
late-stage functionalization of complex bioactive com-
pounds. Remarkably, the derivatives of l-tyrosine and
estrone (1p and 1q) underwent the coupling with 2a
smoothly to afford the desired 2,2’-biphenol scaffolds,
which are difficult to be accessed by the traditional
synthetic methods [Eqs. (1) and (2)]. Furthermore, we
also tested whether our approach could be used to
synthesize optical active ortho-biphenols from chiral
5-substituted 6-diazo-2-cyclohexenone via central-to-
axial chirality transfer. The reaction of optical active
diazo 2h with 1o provided the desired orhto-biphenol
with high ee value (98%) albeit in a poor yield [20%
yield, Eq. (3)], along with a complex mixture of side
products. Finally, a relatively large-scale reaction was
conducted. With 2 mol% of Rh catalyst under the
standard conditions, the desired ortho-biphenol could
be produced conveniently on a gram scale without
any obvious decrease in yield [Eq. (4)].
Scheme 2. Scope of the N-aryloxyacetamides 1.
Scheme 3. Scope of 6-diazo-2-cyclohexenones 2.
to deliver the expected ortho-biphenols. We observed
To gain some insights into the reaction mechanism,
that the reactivity of the diazo compounds was influ- deuterium-labelling experiments were conducted.
enced by the position of the substituents. While most When the reaction between 1a and 2a was performed
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in the presence of D2O, 3aa was obtained in a good which undergoes aromatization and reductive elimi-
yield and no deuterium incorporation was observed nation to furnish one o-phenolic group and generate
[Eq. (5)]. Next, the kinetic isotope effect was studied the Rh(I) species (intermediate D). Intramolecular
in competition experiments and a KIE of 1.9 was de- oxidative addition of the N–O bond to Rh(I) would
termined [Eq. (6)]. Based on these results, we pro- give the intermediate E, which upon protonlysis will
pose that the C–H activation might be irreversible deliver the product 3 and regenerate the Rh(III) cata-
under the reaction conditions and the C–H bond lyst. For 5-substituted diazo substrates (2g and 2h),
cleavage process might not be involved in the rate-de- the steric congestion around the diazo group may
termining step.
result in the low reactivity.[13a,e] It is also possible that
the substituent adjacent to the diazo group may par-
tially generate the trans-H intermediate B, which im-
pedes the subsequent b-H elimination and results in
the observed low yields.
In conclusion, we have developed a rhodium-cata-
lyzed cascade C–H functionalization/aromatization
coupling of N-aryloxyacetamides with 6-diazo-2-cyclo-
hexenones under mild and redox-neutral conditions.
This process provides an efficient way for the synthe-
sis of unsymmetrical ortho-biphenols with broad sub-
strate scope. Moreover, this practical transformation
could be used in the late-stage functionalization of
bioactive compounds and the synthesis of an optical
active ortho-biphenol. Further investigations on the
synthetic application of this reaction are in progress.
A plausible mechanism for this redox-neutral C–H
functionalization/aromatization cascade process is Experimental Section
proposed (Scheme 4). The reaction is presumably ini-
tiated by a facile directed C–H activation to afford General Procedure
the intermediate A. According to the literature,[12a,c]
Without any particular precaution to exclude oxygen or
the rhodacyclic intermediate A may react with the
diazo compound 2 through metal carbene formation
and migratory insertion to produce the intermediate
B accompanied by the release of N2.[15–17] Subsequent
b-H elimination produces the intermediate C,[18]
moisture, N-aryloxyacetamide 1 (0.3 mmol, 1 equiv.), diazo
compound 2 (0.6 mmol, 2 equiv.), [Cp*RhCl2]2 (2.5 mol%,
4.8 mg) and Cs2CO3 (24.5 mg, 0.075 mmol, 25 mol%) were
weighed into a 10-mL vial equipped with a stirring bar.
Then DCE (3 mL, 0.1M) was added. The reaction mixture
was stirred at 308C for 10 h. Afterwards, the reaction mix-
ture was diluted with EtOAc and transferred to a round-
bottom flask. Silica gel was added to the flask and volatiles
were evaporated under reduced pressure. The purification
was performed by flash column chromatography on silica
gel.
Acknowledgements
We thank the National Basic Research Program of China
(2015CB856600), the National Natural Science Foundation of
China (21202184, 21232006, 21572255), and the Chinese
Academy of Science for financial support.
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Rhodium(III)-Catalyzed Cascade Redox-Neutral C–H
Functionalization and Aromatization: Synthesis of
Unsymmetrical ortho-Biphenols
7
Adv. Synth. Catal. 2017, 359, 1 – 7
Zhiyong Hu, Guixia Liu*
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