Arenophile-Mediated Dearomative Reduction

Article abstract of DOI:10.1002/anie.201609686

A dearomative reduction of simple arenes has been developed which employs a visible-light-mediated cycloaddition of arenes with an N-N-arenophile and in situ diimide reduction. Subsequent cycloreversion or fragmentation of the arenophile moiety affords 1,3-cyclohexadienes or 1,4-diaminocyclohex-2-enes, compounds that are not synthetically accessible using existing dearomatization reactions. Importantly, this strategy also provides numerous opportunities for further derivatization as well as site-selective functionalization of polynuclear arenes.

Full text of DOI:10.1002/anie.201609686

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609686
German Edition: DOI: 10.1002/ange.201609686
Photochemistry
Arenophile-Mediated Dearomative Reduction
Mikiko Okumura, Stephanie M. Nakamata Huynh, Jola Pospech, and David Sarlah*
Abstract: A dearomative reduction of simple arenes has been
developed which employs a visible-light-mediated cycloaddi-
tion of arenes with an N-N-arenophile and in situ diimide
reduction. Subsequent cycloreversion or fragmentation of the
arenophile moiety affords 1,3-cyclohexadienes or 1,4-di-
aminocyclohex-2-enes, compounds that are not synthetically
accessible using existing dearomatization reactions. Impor-
tantly, this strategy also provides numerous opportunities for
further derivatization as well as site-selective functionalization
of polynuclear arenes.
been extensively used in the preparation of natural products
and pharmaceuticals. In addition, osmium–arene h2-com-
plexes are known to undergo chemoselective partial hydro-
genation of the arene ligand;[7] however, the high toxicity and
cost have significantly restricted their widespread use. Finally,
a microbial reductive process involving benzoyl-coenzyme A
reductases exists,[8] but its translation to organic synthesis has
not yet been realized. Accordingly, to the best of our
knowledge, there are no known general methods for the
preparation of 1,3-dienes from arenes, even though these
highly versatile building blocks are only one reduction level
away from the parent aromatic material.[9] Herein, we report
a reductive dearomatization strategy (Scheme 1b), which
delivers 1,3-cyclohexadienes as well as cis-1,4-diaminocyclo-
hex-2-enes.
T
he dearomative strategies that convert arenes to com-
pounds of broader utility are of fundamental importance for
organic synthesis.[1] Despite significant advances that have
been made in recent years, mainly involving heteroarenes and
phenols,[2,3] there are only limited options available for the
dearomatization of non-activated benzene derivatives.[4] For
example, in the area of reductive dearomatization, the most
common transformations are dissolving-metal (Birch) reduc-
tion and dearomative hydrogenation (Scheme 1a).[5,6] The
resulting 1,4-cyclohexadiene and cyclohexane products have
Recently, we introduced a new dearomative platform
based on the application of small organic molecules, termed
arenophiles (Scheme 1b).[10] The underlying feature of this
approach is the ability of visible-light-excited arenophiles[11]
to undergo para-cycloaddition with arenes (1 + 2!I) to
provide bicycles II that are amenable to further transforma-
tions. Formally, reactions with arenophiles could be seen as
isolation of p-bonds within an aromatic starting material and
have the potential to connect olefin chemistry to the area of
dearomative functionalizations. Accordingly, this strategy
could be used for dearomative reductions, as it would provide
products that are complementary to those obtained by known
dearomative processes. Described reactions are conducted
using operationally simple conditions, under ambient atmos-
phere and have been performed on gram-scale. Moreover, the
utility of this method is demonstrated by the dearomative
diversification of 1-phenylheptane, as well as the rapid
conversion of naphthalene to naturally occurring g-hydroxy-
tetralone.
Our studies commenced with exploring the photochem-
ical reaction of benzene (1a) with N-methyl-1,2,4-triazoline-
3,5-dione (2, MTAD) and subsequent in situ reduction. Initial
experiments evaluated metal-catalyzed homogenous or het-
erogeneous hydrogenation; however, no reduced products
were observed at cryogenic conditions with cycloadducts of
type II. Next, diimide reduction was examined, as this reagent
is capable of engaging olefins at low temperatures and has
a broad functional group compatibility.[12] Accordingly, we
identified that the cycloaddition in EtOAc at ꢀ788C followed
by reduction with diimide, generated from potassium azodi-
carboxylate and acetic acid, at ꢀ508C provided the best result
and gave product 3a in 65% yield (Table 1, entry 1). Several
aspects observed during optimization are noteworthy:
1) keeping the temperature at ꢀ788C throughout the
sequence resulted in significantly lower yields of reduced
bicycle (entry 2). 2) As shown in entries 3–5, several other
solvents were suitable for this reaction, with CH2Cl2 being
Scheme 1. Reductive dearomatizations, including this work.
[*] M. Okumura, S. M. Nakamata Huynh, Dr. J. Pospech,
Prof. Dr. D. Sarlah
Roger Adams Laboratory, Department of Chemistry
University of Illinois
Illinois 61801 (USA)
E-mail: sarlah@illinois.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201609686.
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Table 1: Optimization of dearomative cycloaddition/diimide reduction.[a]
Entry
Deviation from standard conditions
Yield [%][b]
1
2
3
4
5
6
none
65[c]
36
diimide reduction at ꢀ788C
CH2Cl2 was used instead of EtOAc
acetone was used instead of EtOAc
EtCN was used instead of EtOAc
o-NO2-C6H4-SO2Cl and N2H4·H2O were
61[c]
31[c]
55[c]
10
=
used instead of KO2CN NCO2K and AcOH
7
8
9
10
HCO2H was used instead of AcOH
TFA was used instead of AcOH
with 10 equiv of AcOH
61
16
26
48
=
with 2.0 equiv of KO2CN NCO2K
[a] Reaction conditions: benzene (1a, 5.0 mmol, 10 equiv), MTAD (2,
0.5 mmol, 1.0 equiv), EtOAc (0.1m), visible light LEDs, ꢀ788C; then
=
KO2CN NCO2K (3.0 equiv), AcOH (15 equiv), ꢀ508C, 5 h. [b] Deter-
mined by GC relative to an internal standard. [c] Isolated yields after
purification by flash chromatography.
Scheme 2. Scope of the dearomative cycloaddition/diimide reduction
of mononuclear arenes. All reactions were carried out on 1.0 mmol
scale under the standard conditions (see Table 1, entry 1). Yield of the
isolated product after purification by chromatography. Ratios of 3 and
3’ were determined by 1H NMR of the crude reaction mixtures and is
shown in parenthesis. [a] MeOH was added with AcOH. [b] Reaction
was conducted in CH2Cl2. [c] Run on gram-scale. [d] 3.0 equiv of 1g
used. [e] 1.7:1 d.r. [f] Mixture with 3,6-constitutional isomer.
comparable to EtOAc. 3) Direct, hydrazine-based diimide
generation was not as effective (entry 6).[13] 4) Use of stronger
acids, as well as lower stoichiometry of acetic acid or
azodicarboxylate salt had a detrimental effect on the reaction
yield (entries 7–10).
Having identified the optimal reaction conditions for the
cycloaddition and subsequent in situ reduction, we explored
the substrate scope of this process with mononuclear arenes
(Scheme 2). Thus, on a preparative scale (1.0 mmol), the
model reaction with benzene (1a) successfully furnished
product 3a in 68% yield. Furthermore, a range of mono-
substituted arenes bearing ester (3 f, 3h, 3i, 3j, 3o, 3q),
halogen (3d, 3n, 3p), amide (3g), acetal (3k), ketal (3l), and
orthoester (3m) functional groups delivered the correspond-
ing reduced bicycles. Most notably, benzylic heteroatom-
containing functional groups (3i–3n) were well tolerated in
this process. High regioselectivity of cycloaddition and
chemoselectivity of reduction were observed in most cases;
only reaction with phenyltrimethylsilane (1e) resulted in
a nearly equal mixture of constitutional isomers 3e and 3e’.
Moreover, MTAD cycloaddition with methyl 2-phenylben-
zoate (1q) and subsequent reduction delivered products 3q
and the corresponding column-separable 3,6-constitutional
isomer in a 1.9:1 ratio. Importantly, no over-reduction was
observed in any case.[14] All reactions were run under air and
using commercial grade solvents and reagents, making this
cycloaddition/reduction protocol practical to perform. Finally,
this process can be performed on a multigram scale as
demonstrated in preparation of 3 f (2.0 g, 17.7 mmol of
MTAD used, 68% yield, 10:1).
groups were tolerated, including acetal (4k), ketal (4l),
orthoester (4m), alkyl chloride (4p), and silyl (4e and 4e’) as
well as alkyl substituents (4b and 4c). Only allyl bromide 3n
was converted to the corresponding isopropoxy ether 4n. For
the cycloreversion of phthalimide 3g, we modified this one-
pot sequence by using hydrazine and amino group re-
protection to obtain Boc-protected aminodiene 4g. Finally,
conducting this transformation on a sevenfold larger scale
proceeded smoothly and without significant decrease in yield
as exemplified with diene 4 f (0.88 g, 3.15 mmol of 3 f used,
77% yield).
In a similar fashion, we examined the fragmentation of the
arenophile moiety within the reduced bicycles 3 to deliver 1,4-
diaminocyclohex-2-ene derivatives 5 (Scheme 3, right side).
These products formally represent the dearomative bis-1,4-
hydroamination of the parent arenes, a functionalization
method for which no synthetic or biological equivalent exists.
Thus, a two-step protocol, involving urazole hydrolysis/
hydrazine benzoylation and subsequent SmI2-mediated N-N
reduction furnished the desired diamine products protected
as a benzamides.[16] Again, a variety of functionalities were
tolerated with the exception of esters and allylic bromide,
which delivered alcohol 5j, acids 5h and 5o, and ether 5n.
While free alcohols did not undergo re-protection, the
primary amino group was benzoylated during the first step
(5g). Noteworthy, substrates bearing vinyl and aliphatic
chlorides 5d and 5p were compatible with the reductive
conditions, although a slight decrease in yield was observed
with vinyl chloride 5d.
We next investigated the conversion of reduced bicycles 3
to the corresponding 1,3-cyclohexadienes 4 (Scheme 3, left
side) using one-pot hydrolysis followed by in situ oxidation of
the resulting cyclic hydrazines with CuCl2.[15] While esters
were hydrolyzed under the basic reaction conditions to
alcohols (4 f, 4i, 4j) and acids (4h, 4o, 4q), other functional
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Scheme 3. Cycloreversion and fragmentation of cycloadducts 3. Conditions for cycloreversion: 3 (0.4 mmol, 1.0 equiv), KOH (4.0 mmol, 10 equiv),
iPrOH (0.2m), 1008C, 12–24 h; then pH 7, CuCl2, (0.4 mmol, 1.0 equiv), 258C, 1 min. Conditions for fragmentation: 1) 3 (0.2 mmol, 1.0 equiv),
KOH (2.0 mmol, 10 equiv), iPrOH (0.3m), 1008C, 12–24 h; then BzCl (1.0 mmol, 5.0 equiv), rt, 4–8 h. 2) SmI2 (0.5 mmol, 2.5 equiv), MeOH
1
(0.1m), 08C to rt. Yields of isolated product after purification by flash chromatography. [a] Determined by H NMR using internal standard.
[b] Run on 3.15 mmol scale. [c] N2H4 was used instead of KOH followed by in situ protection with Boc2O.
In addition to benzene derivatives, the arenophile-medi-
ated dearomative bis-1,4-hydroamination strategy was also
applied to polynuclear arenes (Scheme 4). In all cases, the
cycloaddition proceeded with exclusive site-selectivity for the
terminal aromatic moiety. The fragmentation step was
accomplished by a one-pot hydrazinolysis[17] and Ni-catalyzed
reductive cleavage.[18] Overall, this protocol proved efficient
for several naphthalene derivatives (8a–8c)[19] and phenan-
threne (8d),[20] as well as heteroarenes such as quinoline (8e),
quinoxaline (8 f), acridine (8g), and benzo[h]quinoline (8h).
Sensitive arene substituents, such as benzylic ketal (8b) and
bromide (8e) were tolerated under the described reductive
conditions. Moreover, dearomatization of polynuclear sub-
strates can be successfully performed on gram-scale, as
demonstrated with the conversion of naphthalene (6a) to
7a (1.13 g, 10.0 mmol of MTAD used, 82% yield).
This arenophile-mediated dearomatization can provide an
expedient entry to a range of functionally diverse compounds.
In addition to 1,3-dienes, which are extensively used in
organic synthesis, dearomatized products 5 are versatile
entities that can be subjected to further synthetic manipu-
lations (Scheme 5). For example, 1-phenylheptane (1r) was
converted to the corresponding partially unsaturated diamine
derivative 5r (37% over three steps), which was oxidized to
epoxide 9 or diol 10. Moreover, Mukaiyama hydration[21]
delivered tertiary alcohol 11, and reductive olefin coupling[22]
with methyl acrylate installed a quaternary center in carbo-
cyle 12, obtained as a readily separable mixture of diastereo-
isomers. Finally, substrate-directed, catalytic hydrogenation
afforded saturated analog 13.
Scheme 4. Dearomative bis-1,4-hydroamination of polynuclear arenes.
Conditions for cycloaddition/reduction: see Table 1 for reaction con-
ditions, 6 (2.0 mmol, 2.0 equiv), MTAD (2, 1.0 mmol, 1.0 equiv).
Conditions for cycloreversion: N2H4 (20 equiv), 1008C, 12 h, then
Raney-Ni (50 mol%), EtOH (0.2m), H2 (1 atm), 508C, 8 h. Yields are
of isolated product after purification by flash chromatography. [a] Run
on gram-scale.
pounds by manipulating the cycloreversion step. For instance,
the conversion of naphthalene derivative 7a to aminoalcohol
14 was achieved through a one-pot cycloreversion/nitroso
ꢀ
Furthermore, the reduced cycloadducts based on poly-
nuclear arenes could also be elaborated to different com-
Diels–Alder reaction, and subsequent N O bond cleavage.
Similarly, the urazole moiety was replaced with an endoper-
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Scheme 5. Elaboration of dearomatized products.
oxide that underwent an in situ Kornblum–DeLaMare
rearrangement[23] to deliver g-hydroxytetralone 15. This
reaction sequence exemplifies the production of an expensive
bioactive natural product 15[24] ($577gꢀ1, Aldrich) from
a cheap feedstock naphthalene (6a, $0.03gꢀ1) in two one-
pot chemical operations.
In summary, we have developed a new strategy for
dearomative reduction of simple arenes to deliver 1,3-cyclo-
hexadienes and 1,4-diaminocyclohex-2-ene derivatives con-
taining a variety of pre-installed functional groups that have
been previously incompatible with reductive dearomatiza-
tions. The obtained products are orthogonal to those acces-
sible through known dearomatization chemistry and are
amenable to further functionalizations. From a practical
standpoint, these reactions are operationally simple and
have been conducted on gram-scale. Studies to expand the
scope as well as additional applications of the versatile
products are currently ongoing and will be reported in due
course.
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We acknowledge support from the University of Illinois and
the National Science Foundation for CAREER Award No.
CHE-1654110. M.O. thanks the Honjo International Scholar-
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Keywords: arenes · arenophiles · dearomatization ·
functionalization · photochemistry
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Manuscript received: October 3, 2016
Final Article published: && &&, &&&&
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Communications
Photochemistry
M. Okumura, S. M. Nakamata Huynh,
J. Pospech, D. Sarlah*
&&&& — &&&&
Arenophile-Mediated Dearomative
Reduction
Light it up: A dearomative strategy based
on the visible-light-promoted para-cyclo-
addition of arenophile with arenes and in
situ diimide reduction was developed
that provides access to 1,3-cyclohexa-
dienes or unsaturated 1,4-diamines.
These products are complementary to
known dearomative processes and are
amenable to further diversification.
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!