Photoinduced Aerobic Iodoarene-Catalyzed Spirocyclization of N-Oxy-amides to N-Fused Spirolactams**

Article abstract of DOI:10.1002/anie.202009175

Iodoarene catalysis is a powerful methodology that usually requires an excess of oxidant, or of redox mediator if the terminal oxidant is dioxygen, to generate the key hypervalent iodine intermediate to proceed efficiently. We report that, using the spiro-cyclization of amides as a benchmark reaction, aerobic iodoarene catalysis can be enabled by relying on a pyrylium photocatalyst under blue light irradiation. This unprecedented dual organocatalytic system allows the use of low catalytic loading of both catalysts under very mild operating conditions.

Full text of DOI:10.1002/anie.202009175

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Accepted Article
Title: Photoinduced Aerobic Iodoarene-Catalyzed Spirocyclization of N-
Oxy-Amides to N-Fused Spirolactams
Authors: Kevin Cariou and Loïc Habert
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202009175
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Photoinduced Aerobic Iodoarene-Catalyzed Spirocyclization of N-
Oxy-Amides to N-Fused Spirolactams
Loïc Habert,[a],[b] Kevin Cariou[a],[b]
*
In memory of Professor Kilian Muñiz (1970–2020)
Dr L. Habert, Dr K. Cariou*
[a]
[b]
Institut de Chimie des Substances Naturelles, LabEx LERMIT, UPR 2301
Université Paris-Saclay, CNRS
1, avenue de la Terrasse, 91198, Gif-sur-Yvette, France
Current address: Institute of Chemistry for Life and Health Sciences, Laboratory for Inorganic Chemical Biology
Chimie ParisTech, PSL University, CNRS
11, rue Pierre et Marie Curie 75005 Paris, France
E-mail: kevin.cariou@cnrs.fr
Supporting information for this article is given via a link at the end of the document.
Abstract: Iodoarene catalysis is a powerful methodology that usually
requires an excess of oxidant, or of redox mediator if the terminal
oxidant is dioxygen, to generate the key hypervalent iodine
intermediate to proceed efficiently. We report that, using the spiro-
cyclization of amides as a benchmark reaction, aerobic iodoarene
catalysis can be enabled by relying on a pyrylium photocatalyst under
blue light irradiation. This unprecedented dual organocatalytic system
allows the use of low catalytic loading of both catalysts under very
mild operating conditions.
Apart from anodic oxidation, which, more than 20 years after the
pioneering studies by Fujita,[12] currently receives increasing
attention,[13–19] iodoarene catalysis requires the use of
a
stoichiometric oxidant,[8] thus generating an equimolar quantity of
inorganic or organic waste (Scheme 1b). A more desirable
alternative is the use of molecular oxygen as a widely available
and cheap stoichiometric oxidant that would only generate
aqueous waste (Scheme 1c). Since the single electron oxidation
potential of iodobenzene is much higher (half peak potential: Ep/2
= 2.17 V vs SCE)[20] than the reduction potential of oxygen (–0.33
V vs SHE, for single electron reduction at pH = 7 ),[21] resorting to
an electron-transfer mediator[22] (ETM) is mandatory. This
strategy was eventually implemented in 2017 by Miyamoto &
Uchiyama[23] and Powers[24] who independently reported the first
examples of aerobic oxidation of iodoarenes. Both approaches
are based on the formation of a suitable oxidant during the O2-
mediated autoxidation of aldehydes,[25–27] although the protocols
slightly differ. The Miyamoto-Uchiyama protocol requires a
sterically hindered aldehyde and was applied to the glycol
scission of 1,2-diols and the Hofmann rearrangement of primary
amides with only 3 to 5 equivalents of iso-butyraldehyde and 5 to
20 mol% of pentamethyliodobenzene.[23] Powers’ method, which
was further refined for the formation of iodine(V) reagents,[28] uses
10 equivalents of acetaldehyde and 1 mol% of CoCl2 as the
initiator to promote the synthesis of (diacetoxy)iodoarenes from
the corresponding aryl iodides in acetic acid. This system was
named “oxidase catalysis” by the authors.[29] Five distinct
examples of catalytic applications were presented, necessitating
between 10 to 20 mol% of diiodobenzene and one equivalent of
iodobenzene to work efficiently. Despite the spectacular
improvement that these aerobic strategies bring to iodoarene
catalysis, yet they still require a rather large excess of aldehyde
and, in some cases, even a stoichiometric amount of the
iodobenzene catalyst.
Hypervalent iodine(III) compounds have been known for 130
years,[1] yet interest in their reactivity was very modest until the
80’s, before witnessing a dramatic surge in the 2000’s.[2–4] They
possess many advantages in terms of versatility, high selectivity
and lack of toxicity; however, their use as a stoichiometric reagent
remains a drawback. Indeed, the driving force behind their
reactivity is the final release of one equivalent of iodoarene
(Scheme 1a), such as iodobenzene. The liberation of
a
stoichiometric amount of organic waste is in total contradiction
with the need for greener and more responsible processes. In
order to circumvent these hurdles, the more general and sought
after strategy is based on the in situ re-oxidation of the iodoarene
into a reactive iodine(III) species. This approach, pioneered by
Ochiai & Miyamoto[5] and by Kita[6] in 2005, de facto establishes
iodoarenes as a particular subclass of organocatalysts,[7–9] with
many synthetic applications, including asymmetric versions.[10,11]
With the goal of developing an aerobic system that would only
require a substoichiometric amount of the O2-mediator we sought
to employ a visible-light photoredox catalyst to reoxidize the
iodoarene into an iodine(III) derivative. Since the 2011 study by
Lalevée on the use of a ruthenium photocatalyst with a
diphenyliodonium to form phenyl radicals,[30] the interplay
Scheme 1. Stoichiometric iodoarene oxidative transformation (a), iodoarene
catalysis (b) and aerobic iodoarene catalysis (c).
1
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between visible-light photoredox catalysis and hypervalent
iodine(III) derivatives has been increasingly studied.[31] However,
all these studies rely on the reduction of iodine(III) reagents to
give an aryliodide by-product while there is no report of a
photoredox oxidation of an iodoarene into an iodine(III) derivative.
The oxidation potential of aryliodides being quite high,[20] it
precludes the use of most Ru- and Ir-based complexes[32] in favor
of organic dyes[33–35] such as acridinium (Fukuzumi’s catalyst
Acr+) or pyrylium (pyry+) salts.
To explore this dual (iodoarene/photocatalyst) catalytic system,
the spirocyclization of amides into N-fused spirolactams was
chosen as a benchmark reaction. After their initial work on the
generation of nitrenium ions from chloramides,[36] this
transformation was then developed by Kikugawa in 2003 using a
stoichiometric amount bis(trifluoroacetoxy)iodo]benzene (PIFA)
in trifluoroethanol (TFE) (Scheme 2a).[37] This reaction was later
reported using anodically generated iodine(III) by Nishiyama.[38]
In 2007, Kita described a catalytic version, using 10 mol% of
iodotoluene in the presence of 1.5 equivalents of m-CPBA
(Scheme 2b)[39] while Togo used Oxone.[40] Kita later refined this
method using 2 mol% of a diiodobiaryl with 2 equivalents of
peracetic acid.[41] Finally, this reaction was one of the examples
that Powers[24] implemented in an aerobic fashion with 20 mol%
of diiiodobenzene and 10 equivalents of acetaldehyde (Scheme
2c). For our part, we set to establish the feasibility of this
transformation under aerobic conditions with an iodoarene
catalyst, which would be oxidized using an organic photocatalyst
(phot+) under irradiation (Scheme 2d).
at 455 nm (Table 1). The influence of the solvent was dramatic as
no reaction was observed in dichloroethane (DCE) or acetonitrile,
while trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP)
enabled the formation of the desired spiro-adduct in 10% and 18%,
respectively. Doubling or tripling the reaction time allowed to
double the yield to 37%. As the TPT catalyst is known for its poor
photo-stability, we used the more robust and more oxidizing
mesityl-2,6-diphenylpyrylium tetrafluoroborate (MDPT, E1/2 P*/P•
= 2.62 V vs. SCE) recently developed by Beeler.[43] This led to a
slight improvement of the yield to 43%. On the contrary, the use
of 9-mesityl-10-phenylacridinium tetrafluoroborate (Ph-Acr, E1/2
P*/P• = 2.20 V vs. SCE)[44] did not permit the reaction to proceed.
The reaction was first studied with N-Methoxy amide 1a as the
starting material using a high catalyst loading of iodobenzene (50
mol%) in the presence of 2.5 mol% of 2,4,6-triphenylpyrylium
tetrafluoroborate (TPT, E1/2 P*/P• = 2.30 V vs. SCE)[42] as the
photocatalyst, under one atmosphere of dioxygen and irradiation
Scheme 2. Spirocyclization of amides to N-fused spirolactams using
stoichiometric iodine(III) reagent (a), iodoarene catalysis (b), aerobic iodoarene
catalysis (c) and dual aerobic photo- and iodoarene catalysis (d, this work).
a
Table 1. Optimization of the reaction conditions[a]
Iodoarene (mol%)
Photocatalyst (mol%)
Solvent
Time (h)
Yield 2 (%)[b]
PhI (50)
PhI (50)
PhI (50)
PhI (50)
PhI (50)
PhI (50)
PhI (50)
PhI (50)
PhI (50)
IBA (20)
1,2-DIB (50)
Kita (20)
Kita (2.5)
TPT (2.5)
TPT (2.5)
DCE
MeCN
24
24
24
24
48
72
72
72
72
48
48
72
72
N. R.[c]
N. R.
10
TPT (2.5)
TFE
TPT (2.5)
HFIP
18
TPT (2.5)
HFIP
37
TPT (2.5)
HFIP
37
MDPT (2.5)
HFIP
43
Ph-Acr (5.0)
MDPT (2.5 x 2)[d]
MDPT (2.5 x 2)[d]
MDPT (2.5 x 2)[d]
MDPT (2.5 x 2)[d]
MDPT (2.5 x 2)[d]
HFIP
N. R.
49
HFIP
HFIP
58
HFIP
46
HFIP/DCE 10:1
HFIP/DCE 10:1
52
55
[a] All reactions were carried out with 0.28 mmol of 1a, stirred in 1.5 mL of solvent under 455 nm irradiation. [b] isolated yields. [c] no reaction. [d] The reaction was
started with 2.5 mol% of catalyst, another 2.5 mol% was added after half the reaction time.
2
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To favor the oxidation of the iodoarene, derivatives bearing a
substituent in ortho were then assayed. Using 20 mol% of
substrates 1r and 1s, which would have furnished the 4-
membered and 6-membered spiro derivatives, respectively, only
led to decomposition. Other types of substituent that could help
stabilize a putative radical intermediate on the nitrogen such as a
phthalimidate (1t),[39,41] a tosyl (1u) or a phenyl (1v) were also
iodobenzoic acid (IBA), which could lead to
a transient
benziodoxole intermediate,[45–47] spiro adduct 2a could be isolated
with a 58% yield. 1,2-Diiodobenzene, that can form µ–oxo
intermediates,[48] was less effective and a 46% yield was obtained. ineffective, and so was an alkyl group (1w) or an unprotected
Finally, Kita’s bis iodoarene catalyst[41] was employed (DCE was
added to HFIP to increase its solubility) and a 55% yield could be
obtained with a catalyst loading of only 2.5 mol%. Being almost
as efficient as with 20 mol% of IBA, these conditions were
selected as optimal since only 2.5 mol% of the iodoarene catalyst
and 5 mol% of the photocatalyst are needed to promote the spiro-
cyclization.
amide (1x). Control experiments (Scheme 4) showed that the
reaction did no proceed in the absence of either catalyst, oxygen
or light, nor in the presence of 20 mol% of a radical quench such
as BHT and TEMPO. When air was used instead of pure dioxygen,
the reaction only yielded 12% of 2a. Finally, acetic acid was used
as a highly polar and protic solvent but no reaction took place.
When the N-methoxy group was changed for an ethoxy, or a
benzyloxy group the corresponding spiro-adducts 2b and 2c were
obtained with 59% and 63% yield, respectively (Scheme
3a).While electron-rich groups were not suitable for the reaction
(vide infra), electron-poor benzyloxy groups such as
pentafluorobenzyl and para-nitrobenzyl groups were compatible
and yielded 2d (39%) and 2e (51%). The spirocyclization also
proceeded smoothly with various N-acyloxy amides, whether an
acetate (2f, 61%), a pivalate (2g, 52%), a phenyl acetate (2h,
53%), a benzoate (2i, 46%) or a meta-nitrobenzoate (2j, 47%).
More diversely substituted N-alkoxy derivatives also underwent
the spirocyclization, including the propargyl and allyl derivatives
(2k, 46% and 2l, 50%) and the chloropentanyl compound (2m,
45%). Yet for several substrates the reaction only led to a complex
mixture, although it could not be determined if it was due to a
decomposition of the starting material, the final product or both
(Scheme 3b). These included N-oxy derivatives with an electron-
rich group (1n), a hydroxyl functionality (1o), a silyl group (1p) or
a tertiary ether (1q). In contrast with what was observed by Kita,
Scheme 4. Control experiments.
These observations led us to propose a plausible mechanism
based on a dual organocatalytic system (Scheme 5). Photo-
excitation of the pyrylium Pyry+ would generate the highly
oxidizing Pyry+* (E*red vs SCE= 2.62 V)[43] that could promote the
oxidation of iodoarene A[49] into iodoaryl radical cation B and form
Pyry•. Oxidation of the latter into Pyry+ by the dioxygen would
close the photoredox catalytic cycle and generate the superoxide
ion.[50] Iodoaryl radical cation B has been proposed as an
intermediate in the anodic generation of iodine(III) reagents[19] and
could serve as a precursor to iodanyl radical C,[27] upon reaction
with the solvent. After combination with the superoxide and
protonation peroxy-iodinane D could be formed. Eventually, full
solvolysis could occur leading to iodinane E.[13,14,51] This
spirocyclization has been proposed to occur through the initial
formation of a nitrenium ion[39] but a N-centered radical, stabilized
by the alkoxy group,[52] could also be operative. This means that
all three species C, D and E could promote the transformation of
1 into 2, by triggering the formation of a reactive nitrogen
intermediate. In all cases, this would regenerate the starting
iodoarene, thus closing the second catalytic cycle. Control
experiments demonstrate that several factors are essential for
this dual catalytic system to be operative. As evidenced by the
absence of reactivity in non-fluorinated solvent, including the
highly polar and protic AcOH, HFIP solvent is essential, a feature
that has already been noted for electrochemical generation of
iodine(III) species,[17–19] presumably via multiple roles. First, its
superior solvatation properties for charged species[53] such as
arene radical cations generated in photoredox process,[54] would
favor the formation of B and C. Moreover, when apically bound to
the iodine in λ3-iodanes D or E, its strong electron-withdrawing
character would enhance the stabilization of the 3-center 4-
electron hypervalent bond.[55]
Scheme 3. Scope and limitations of the spirocyclization of amides to N-fused
spirolactams under photoinduced aerobic iodoarene catalysis.
3
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Scheme 5. Mechanistic proposal: (a) dual catalytic cycles and (b) putative structure of the solvolyzed ArI(III) species.
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have thus demonstrated that aerobic iodoarene catalysis can be
enabled by relying on a pyrylium photocatalyst under blue light
irradiation. This unprecedented dual organocatalytic system
allows the use of low catalytic loading of both catalyst under very
mild operating conditions. We are currently pursuing more
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Keywords: hypervalent iodine • pyrylium • aerobic photoredox
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5
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Entry for the Table of Contents
Lighting up iodoarene catalysis: Using the spiro-cyclization of amides as a benchmark reaction, the feasibility of photocatalytic
aerobic iodoarene catalysis was demonstrated by relying on a pyrylium photocatalyst under blue light irradiation. This unprecedented
dual organocatalytic system allows the use of low catalytic loading of both catalysts under very mild operating conditions.
Institute and researcher Twitter usernames: Dr Kevin Cariou: @CariouK; Chimie ParisTech | PSL: @ChimieParisTech; ICSN:
@ICSN_lab.
6
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