Cyclic Iodine Reagents Enable Allylic Alcohols for Alkyl Boronate Addition/Rearrangement by Photoredox Catalysis

Article abstract of DOI:10.1002/cjoc.201800461

All-carbon quaternary centers are prevalent in bioactive small molecules. However, their efficient construction remains a formidable synthetic challenge. Here we report cyclic iodine(III) reagents enable the synthesis of cyclopentanones, cyclohexanones, and dihydrofuranones bearing α-quaternary centers by photoredox catalysis. The reaction proceeds by the formation of the novel cyclic iodine(III) reagent-allylic alcohol complex, which enables the first alkyl boronate addition and semi-pinacol rearrangement of allylic alcohols with dual alcohol and olefin activation. The reaction is suitable for gram scale synthesis and is transformable to alcohols, olefins, oximes, and lactones with an α-quaternary center in one step.

Full text of DOI:10.1002/cjoc.201800461

Cyclic Iodine Reagents Enable Allylic Alcohols for Alkyl Boronate
Addition/Rearrangement by Photoredox Catalysis
Mingshang Liua,b, Hanchu Huanga, and Prof. Dr. Yiyun Chena,b
*
ABSTRACT All-carbon quaternary centers are prevalent in bioactive small molecules. However, their efficient construction remains a formidable
synthetic challenge. Here we report cyclic iodine(III) reagents enable the synthesis of cyclopentanones, cyclohexanones, and dihydrofuranones bearing
α-quaternary centers by photoredox catalysis. The reaction proceeds by the formation of the novel cyclic iodine(III) reagent-allylic alcohol complex, which
enables the first alkyl boronate addition and semi-pinacol rearrangement of allylic alcohols with dual alcohol and olefin activation. The reaction is suitable
for gram scale synthesis and is transformable to alcohols, olefins, oximes, and lactones with an α-quaternary center in one step.
KEYWORDS allylic alcohol, cyclic iodine reagents, quaternary all-carbon center, photoredox catalysis, alkyl boronate
All-carbon quaternary centers are prevalent in bioactive small
molecules. However, their efficient construction remains
reagent-allylic alcohol complex, which enables alkyl boronate
addition/rearrangement to allylic alcohols for the synthesis of
ketones with quaternary centers (Scheme 1c).
a
formidable challenge for synthetic chemists.1 The allylic alcohols
are versatile synthetic building blocks, and the semi-pinacol
rearrangement of allylic alcohols provides a valuable entry to
synthesize ketones with α-quaternary centers.2 The transition
metals are known to coordinate and activate allylic alcohols for
the radical addition/semi-pinacol rearrangement reactions,3 and
the use of heteroatom and stabilized carbon radicals have been
reported in these reactions (Scheme 1a).3c, 3f-h, 4
We started the investigation by studying the interaction
between tertiary allylic alcohol 1 and cyclic iodine(III) reagents
acetoxylbenziodoxole (BIOAc) 2. Upon mixing in chloroform, the
ligand exchange reaction occurred on the cyclic iodine(III) center,
and the new benziodoxole-vinyl cyclobutanol complex 3 was
isolated as an air-stable white solid (eq 1). The X-ray
crystallography of 3 showed a new O-I bond formation on the
cyclic iodine(III) center. As a result, both the hydroxyl and olefin
bond lengths were elongated in the new complex (1.43 and 1.33
Å in 1 to 1.46 and 1.34 Å in 3). The IR spectra of 3 were measured
and indicated that the alkene stretch shifted to lower
wavenumbers compared to vinyl cyclobutanol 1. We also
performed the cyclic voltammetry experiments and found the
oxidation potential of 3 (Ep = 1.38 V vs SCE in MeCN) was
significantly decreased compared to that of 1 (Ep = 1.76 V vs SCE
in MeCN). These crystallography, spectrometry and cyclic
voltammetry experiments suggest the coordination to the CIR
significantly affected both the hydroxyl and olefin groups of allylic
alcohols.9
Scheme 1. Transition Metal and Cyclic Iodine Activa-tion of Allylic Alcohols
Hypervalent iodine reagents are widely used oxidants in
organic synthesis, among which the cyclic iodine(III) reagents (CIR,
such as hydroxybenziodoxoles) demonstrate different and unique
transition-metal-like reactivity.5 Recently, the CIRs are reported to
form the CIR-benzyl alcohol complexes under photoredox
catalysis conditions for alkoxyl radical generation.6 We
hypothesize the allylic alcohols may also be activated by CIR.
However, the CIR-allylic alcohol complex is unknown, and the
reactivity of CIRs with allylic alcohols remains unexplored
(Scheme 1b). Alkyl boronates are readily available and stable alkyl
radical precursors,7 and the concerted addition/rearrangement of
alkyl boronates to allylic alcohols will enable the synthesis of
ketones with sterically congested quaternary centers.8 In this
letter, we report the discovery of a novel cyclic iodine(III)
We next tested the reaction of alkylboronic acid 4 with vinyl
III/II
cyclobutanol 1 under [Ru(bpy)3](PF6)2 7 (E0
= 1.29 V vs SCE in
1/2
MeCN) photoredox catalysis conditions. Among various alkyl
radical generation methods,10 the potassium persulfate and
noncyclic iodine(III) reagents gave no cyclopentanone product 5
amid little conversion (entries 1-3, Table 1).11 In contrast, the use
of BIOAc 2 gave the desired cyclopentanone 5 in 67% yield, which
suggested the critical benziodoxole coordination. In addition, the
a State Key Laboratory of Bioorganic and Natural Products Chemistry, Center for
Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry,
University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032 China.
b School of Physical Science and Technology, ShanghaiTech University, 100 Haike
Road, Shanghai 201210 China.
E-mail: yiyunchen@sioc.ac.cn
+Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday.
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201800461
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o-iodobenzoate addition adduct 6 was observed in 4% yield (entry
experiments and observed BIOAc 2 quenched the photoexcited
[Ir(dtbbpy)(ppy)2]PF6 8* most effectively, while the alkylboronic
acid 4 was ineffective (Scheme 2c).
4).11 The solvent optimization using acetonitrile improved the
yield of 5 to 75%, together with an increased 8% yield of 6 (entry
IV/III
5). The use of [Ir(dtbbpy)(ppy)2]PF6 8 (E0
= 1.21 V vs SCE in
1/2
Based on the mechanistic investigations above, we propose
that [Ir(dtbbpy)(ppy)2]PF6 is photoexcited to Ir(III)* and oxidized
by CIR-OAc or its resulting CIR radical to Ir(IV) (Scheme 2d).17 The
Ir(IV) oxidizes the alkyl boronates for alkyl radical generation,18
which then undergoes radical addition to the new CIR-allylic
alcohol complex, the suitable radical acceptor upon CIR
coordination. After the subsequent oxidation to carbocation and
polar rearrangement, the substituted ketone with an
-quaternary center is generated. The CIRs have triple roles for
the desired reaction pathway: i) the redox-active reactivity for the
alkyl radical formation and the cation intermediate formation, ii)
the activation of the allylic alcohol, and iii) the in situ protection
of alcohols for the inhibition of the epoxide formation.19
MeCN) gave 5 in an optimal 89% yield (81% isolated yield, entry
IV/III
6).12-13 In contrast, the use of fac-Ir(ppy)3 9 (E0
= 0.77 V vs
1/2
SCE in MeCN) with low oxidation potential was not effective
(entry 7).8d We further tested different acetoxylbenziodoxole
derivatives (3,4-difluoro acetoxylbenziodoxole (3,4-F-BIOAc) 10
and 3,4-dimethoxy acetoxylbenziodoxole (3,4-MeO-BIOAc) 11 and
6c, 14
observed similar reaction outcomes (entries 8-9).6b,
The
photocatalysts and light irradiation were both required for the
reaction (entries 10-11).
Table 1 Optimization of the Alkyl Boronate Addition to Allylic Alcohols
entry
conditions
conversion yield 5 yield 6
1
[Ru(bpy)3](PF6)2 7, K2S2O8, CH2Cl2
<10%
<5%
0
2
3
[Ru(bpy)3](PF6)2 7, PhI(OAc)2, CH2Cl2
<5%
0
0
[Ru(bpy)3](PF6)2 7,
PhI(O2CCF3)2,
<5%
0
0
CH2Cl2
4
5
6
7
8
9
[Ru(bpy)3](PF6)2 7, BIOAc 2, CH2Cl2
[Ru(bpy)3](PF6)2 7, BIOAc 2, CH3CN
[Ir(ppy)2(dtbbpy)]PF6 8, BIOAc 2, CH3CN
fac-Ir(ppy)3 9, BIOAc 2, CH3CN
entry 6, 3,4-F-BIOAc 10
>95%
>95%
>95%
<5%
78%
67%
4%
8%
0
75%
89% (81%)
0
75%
77%
0
0
1%
0
entry 6, 3,4-MeO-BIOAc 11
79%
10 entry 6, no [Ir]
<5%
<5%
0
11 entry 6, no hv
0
0
Scheme 2. Mechanistic Investigations and Proposals
a Reaction conditions: 4 (0.3 mmol, 1.5 equiv), 1 (0.20 mmol, 1 equiv), and
photo-catalysts [Ru] (0.004 mmol, 0.02 equiv) or [Ir] (0.002 mmol, 0.01
equiv) in 2.0 mL solvents under nitrogen with 4 W blue LED irradiation at
25 oC for 36 h, unless otherwise noted. b Conversions and yields were
determined by 1H NMR analysis, and isolated yields are in parentheses. BI
= benziodoxole.
We next explored the alkyl boronate scope for the reaction
(Scheme 3). The gram scale reaction of the model substrates 1
and 4 gave cyclopentanone 5 in 1.29 g in 70% yield. Various
cyclohexyl, cyclopentyl, and tosylpiperidinyl boronates all worked
well to give substituted cyclopentanones 17-19 in 61-76% yields.
The noncyclic alkyl boronates reacted smoothly to give 20-21 in
63-65% yields. Notably, tertiary alkyl boronates reacted
uneventfully to give 22-24 with two quaternary centers in 63-72%
yields. We also prepared different vinyl cyclobutanol derivatives,
which could be obtained easily in two steps from commercially
available methyl ketones.4a These various aryl substitutions did
not affect the reaction to give 25-31 in 63-80% yields. The vinyl
cyclobutanols substituted with electron-rich methoxyl groups
yielded 32 and 33 in 34% and 58% yields, respectively, together
with o-iodobenzoate addition adducts. In this regard,
3,4-MeO-BIOAc 11 gave 32 and 33 in improved 70% and 70%
yields, respectively.20
To gain mechanistic insights, we added TEMPO 12 to the
optimal reaction conditions and observed the inhibition of the
reaction together with the alkyl-TEMPO addition adduct 13 in 21%
yield (Scheme 2a).11 In addition, the radical scavenger
9,10-dihydroanthracene slowed down the formation of 5 to 26%
yield.15 We also use the cyclopropyl methylene trifluoboronate 15
as the radical clock and observed the ring-opening addition
product 16 in 32% yield, which is consistent with the radical
addition mechanism. We next used the silver salts / persulfates or
Fukuzumi catalyst conditions to generate alkyl radicals from
alkylboronic acids, and did not observe the desired product 5
(Scheme 2b).16 In contrast, the use of benziodoxole-vinyl
cyclobutanol complex 3 gave 5 in 60% yield, which suggests the
essential role of benziodoxole coordination for alkyl boronate
additions. We also performed the luminescence quenching
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addition provided the oxime 54 in 71% yield; and the
Baeyer-Villiger oxidation resulted in the ring-expanded
dioxan-2-one 55 in 70% yield. It is worth noting that
dihydrofuranones with sterically congested quaternary centers
are difficult to synthesize by other methods, and subsequent
derivation provides an expedient synthetic route for these
structurally diverse and difficult-to-approach molecules.
Scheme 3. Substrates Scope of Vinyl Cyclobutanolsa
Scheme 5. Post-Synthetic Transformations of Dihydrofuranones
We then challenged the vinyl oxetanol 34 for alkyl boronate
addition/rearrangements, which are difficult substrates for
previously reported transition-metal-catalyzed reactions to
synthesize dihydrofuranones (Scheme 4).4a-h Various alkyl
boronates reacted smoothly to give dihydrofuranones with
-quaternary centers 35-40 in 51-73% yields, including the gram
scale reaction of 35 in 0.96 g in 66% yield.21 Methyl, phenyl,
chloride, and fluoride substitutions on the vinyl oxetanols were all
tolerated and did not affect the reactions, providing 41-46 in
60-80% yields. The reaction is not limited to strained vinyl
cyclobutanols, and vinyl cyclopentanol 47 reacted uneventfully to
give substituted cyclohexanones 48-51 in 71-75% yields.4a
Conclusions
In conclusion, we have developed the first CIR-enabled ketone
synthesis with -quaternary centers by alkyl boronate
addition/rearrangement reactions under photoredox conditions.
The cyclic iodine(III) reagents formed the novel CIR-allylic alcohol
complexes, which are critical to the alkyl boronate addition, polar
rearrangements, and in situ alcohol protection, and provided
complementary reactivity comparing to transition metals. The
tunable reactivity of CIRs with alcohols for alkoxyl radical
generation or alkyl radical addition to allylic alcohols demonstrate
the versatile reactivity of CIR. Further investigations are underway
in our laboratory.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
Acknowledgement
Financial support was provided by National Natural Science
Foundation of China 21472230, 21622207, 91753126, National
Basic Research Program of China 2014CB910304, Strategic
Priority Research Program of the Chinese Academy of Sciences
XDB20020200.
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Manuscript received: XXXX, 2017
Revised manuscript received: XXXX, 2017
Accepted manuscript online: XXXX, 2017
Version of record online: XXXX, 2017
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Entry for the Table of Contents
Page No.
Cyclic Iodine Reagents Enable Allylic
Alcohols
for
Alkyl
Boronate
Addition/Rearrangement by Photoredox
Catalysis
Here we report cyclic iodine(III) reagents enable the synthesis of cyclopentanones,
cyclohexanones, and dihydrofuranones bearing α-quaternary centers by photoredox catalysis. The
reaction proceeds by the formation of the novel cyclic iodine(III) reagent-allylic alcohol complex,
which enables the first alkyl boronate addition and semi-pinacol rearrangement of allylic alcohols
with dual alcohol and olefin activation.
Mingshang Liu, Hanchu Huang, and Prof.
Dr. Yiyun Chen *
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