Ti(O iPr)4-Enabled Dual Photoredox and Nickel-Catalyzed Arylation and Alkenylation of Cyclopropanols

Article abstract of DOI:10.1021/acs.orglett.1c01795

Readily available from esters or ketones, cyclopropanols are inclined to undergo diverse ring-opening transformations. Their one-electron oxidation is a conventional way to β-carbonyl radicals. However, despite this fact, their application as a coupling partner in dual photoredox and nickel-catalyzed reactions with organic halides remains underdeveloped. Here, we report that the Ti(OiPr)4 additive enables this elusive cross-coupling with aryl and alkenyl bromides leading to β-substituted ketones.

Full text of DOI:10.1021/acs.orglett.1c01795

pubs.acs.org/OrgLett
Letter
Ti(OiPr)4‑Enabled Dual Photoredox and Nickel-Catalyzed Arylation
and Alkenylation of Cyclopropanols
Nastassia Varabyeva,§ Maryia Barysevich,§ Yauhen Aniskevich, and Alaksiej Hurski*
Cite This: Org. Lett. 2021, 23, 5452−5456
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ABSTRACT: Readily available from esters or ketones, cyclopropanols are
inclined to undergo diverse ring-opening transformations. Their one-electron
oxidation is a conventional way to β-carbonyl radicals. However, despite this
fact, their application as a coupling partner in dual photoredox and nickel-
catalyzed reactions with organic halides remains underdeveloped. Here, we
report that the Ti(OiPr)4 additive enables this elusive cross-coupling with aryl
and alkenyl bromides leading to β-substituted ketones.
ditions, these reactions generally tolerate sensitive functional
groups and, moreover, can be deployed for the formation of an
asymmetric stereocenter.3 The scope of the radical precursors
used in the coupling is exceptionally broad. After the
pioneering reports on the arylation of organotrifluoroborates,
aliphatic carboxylic acids, and N,N-dimethylaniline by Mo-
lander4 and MacMillan and Doyle,5 various alternative
coupling partners were utilized, including alkylsylicates,6
monoalkyl oxalates,7 dihydropyridines,8,9 alkyl halides,10
alkanes,11 oxiranes,12 aziridines,13 cycloalkanone oxyme-
carboxylates,14 N-hydroxyphthalimide esters,15 Katritzky
salts,16 alkylsulfinate salts,17 xanthate esters,18 boracene-based
alkyl borates,19 and linear20 and cyclic alcohols.21 Being a
source of β-ketoradicals 3,22 cyclopropanols 1 can also undergo
the photoredox/nickel dual catalyzed reaction to provide β-
substituted ketones (Scheme 1B).21 However, the scope of
cyclopropanols that were engaged in this coupling is rather
narrow. Shenvi reported arylation and alkenylation of tricyclic
silyloxycyclopropanes 4 promoted by an iridium photocatalyst
and a nickel complex.21a One of the obtained β-substituted
ketones 5 was further efficiently applied in a concise synthesis
of natural alkaloid GB-22.21a Another example of photoredox-
initiated ring-opening arylation was described by Rueping.21b
The presence of a PMP group in 6 was crucial because the
formation of alkoxy radical 2 was initiated by the one-electron
oxidation of this moiety. Here, we report that limitations in the
scope of photoredox and nickel dual catalyzed cyclopropanol
arylation and alkenylation can be overcome when the reaction
ickel-catalyzed coupling of the photochemically gen-
1
Nerated alkyl radicals with aryl, alkenyl, or alkyl halides
has recently emerged as a powerful tool for C−C bond
construction (Scheme 1A).2 Proceeding under mild con-
Scheme 1. Reactions of Cyclopropanols under Dual
Photoredox and Nickel Catalysis
Received: May 29, 2021
Published: June 25, 2021
https://doi.org/10.1021/acs.orglett.1c01795
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5452−5456
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a
is carried out in the presence of Ti(OiPr)4 as an additive.
Under these conditions, diverse β-substituted ketones 10 were
obtained from cyclopropanols 8 and aryl or vinyl bromides 9.
Transformation of cyclopropanol 1 (Eox = +1.66 V)23 to
oxycyclopropyl radical 2 requires a relatively strong oxidant.
For comparison, the reduction potential of the excited
photocatalysts commonly used in the cooperative photoredox
and nickel-catalyzed reactions lies between +0.77 and +1.35
V.2a Thus, either a photocatalyst with stronger oxidative
properties or an alternative way of oxycyclopropyl radical 2
generation was required to initiate the cross-coupling.
Recently, we found that cyclopropanols undergo one-electron
oxidation by the photoexcited acridimium salts [E1/2(P*/P−) =
+2.08 V],24 but our attempts to use them in the ring-opening
arylation of 8 with 9 were unsuccessful. Nevertheless, we found
that addition of Ti(OiPr)4 to the reaction mixture enables the
reaction even when 4CzIPN [E1/2(P*/P−) = +1.35 V]25 is
employed as a photocatalyst. The highest yield in the arylation
of 11a with p-bromoanisole (12a) was achieved when the
reaction was performed in acetone in the presence of the
photocatalyst, the nickel chloride bipyridine complex,
potassium carbonate as a base, and titanium isopropoxide as
an additive. Changing the solvent to acetonitrile and DMA led
to a slight decrease in yield, but in THF, the arylation was
significantly less efficient. When di-tert-butylbipyridine, bato-
phenanthroline, neocuproine, or dimethoxybipyridine was used
as an alternative ligand, the yield of 13a decreased compared to
that under the standard conditions. While inorganic salt K3PO4
can be employed as a base of choice, the reaction in the
presence of 2,6-lutidine afforded the product in a low 23%
yield. The yield of 13a in the reaction promoted by the
Ti(OtBu)4 additive with bulk tert-butoxide ligands was slightly
lower than in the presence of Ti(OiPr)4. Trimethyl borate also
promoted the cross-coupling, though significantly less
efficiently. No reaction was observed when aluminum
isopropoxide was used as an additive or when the arylation
was carried out in the absence of Ti(OiPr)4 or a photocatalyst
(Table 1).
Having optimized the reaction conditions, we next
investigated the scope of the reaction (Scheme 2). Aryl
bromides with donor or acceptor functional groups as well as
an ortho substituent reacted with 11a giving desired β-
arylketones 13a−f in 46−58% yields. Silylated hydroxyl group,
alkenyl, and diethylacetal units in the cyclopropanol substrate
were tolerated, but formation of 13h and 13i was slightly less
efficient. The reaction of the 1,2-disubstituted cyclopropanol
afforded arylketone 13j as a single β-branched regioisomer. In
contrast to the palladium-catalyzed arylation that leads to α-
branched products,26 the investigated radical reaction
proceeded with the cleavage of a more substituted bond of
the three-carbon ring. Next, cross-coupling of cyclopropanols
with vinyl halides was investigated. Generally, yields of the
alkenylation were better when the reaction was carried out in
the presence of neocuproine instead of a bipyridine ligand. The
reaction between cyclopropanol 11a and 2-bromoalkenes
provided ketones 14a and 14b in 84% and 55% yields,
respectively. Alkenylation of 11a with vinyl triflate also
proceeded efficiently to produce β-cyclohexenylketone 14c.
During the synthesis of γ,δ-unsaturated ketones 14d and 14e
from 2-alkyl-substituted vinyl bromides and cyclopropanol
11a, the bipyridine ligand was more favorable than neo-
cuproine and the products were isolated in 45% and 46%
yields, respectively. Formation of Z-alkenylketone 14e was
Table 1. Optimization of the Reaction Conditions
yield of 13a
b
entry
reaction conditions
standard conditions
(%)
1
60
28
49
51
47
47
21
30
56
23
47
19
0
2
THF instead of acetone
3
MeCN instead of acetone
4
DMA instead of acetone
5
6
7
8
NiCl2·DME and dtbbpy instead of NiCl2·bpy
BPhen·NiCl2·2DMF instead of NiCl2·bpy
NiCl2·DME and neocuproine instead of NiCl2·bpy
NiCl2·DME and dMeObpy instead of NiCl2·bpy
K3PO4 instead of K2CO3
9
10
11
12
13
2,6-lutidine instead of K2CO3
Ti(OtBu)4 instead of Ti(OiPr)4
B(OMe)3 instead of Ti(OiPr)4
Al(OiPr)3 instead of Ti(OiPr)4 or no Ti(OiPr)4 or
no 4CzIPN
a
Reaction conditions: 11a (0.1 mmol), 12a (0.2 mmol), photo-
catalyst (0.005 mmol), NiCl2·bpy (0.005 mmol) or NiCl2·DME
(0.005 mmol), ligand (0.005 mmol), additive (0.2 mmol), base (0.3
mmol), solvent (1 mL), blue LEDs (2 × 20 W), 15 h. Abbreviations:
4CzIPN, 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile; bpy, 2,2′-
bipyridine; dtbbpy, 4,4′-di-tert-butyl-2,2′-bipyridine; BPhen, 4,4′-
diphenyl-2,2′-bipyridine; DME, 1,2-dimethoxyethane; DMA, N,N-
dimethylacetamide; dMeObpy, 4,4′-dimethoxy-2,2′-bipyridine.
b
1
Crude H NMR yield with CH2Br2 as the internal standard.
accompanied by a slight isomerization of the double bond,
which caused a decrease in the Z:E ratio to 8:1. A decrease in
the isomeric purity was more significant in the course of the
reaction affording products 14g and 14h, which bear the
alkene unit conjugated with an electron-rich aromatic ring.
After completion of the coupling, 14g was isolated as a 6:1 E/Z
mixture. This ratio further decreased to 1.7:1 when the
reaction time was increased to 48 h. Ketone product 14f with
the unsubstituted benzene ring was obtained as a single E
isomer. Aryl chloride units were inert under the reaction
conditions, and ketone 14h was prepared from the
corresponding vinyl bromide in a good 61% yield. Then,
diverse 1-mono- and 1,2-disubstituted cyclopropanols were
tested in the reaction with 2-bromopropene. The substrate
bearing two hydroxycyclopropyl groups underwent smooth
coupling giving diketone 14i in 51% yield. Silylated and
unprotected hydroxyl groups, the alkenyl unit, and acetal
protecting groups were tolerated, and corresponding products
14j−n were isolated in 41−75% yields. The reaction
conditions were mild enough for the preparation of chiral
alkenylketones 14o and 14p that contain sensitive α-stereo-
centers. Alkenylation of 1,2-disubstituted cyclopropanols
provided regioisomerically pure β-branched products 14q−s
in 41−64% yields. These reactions proceeded more efficiently
in the presence of the bipyridine ligand. Finally, β-
isopropenylcycloheptanone 14t was obtained from the bicyclic
cyclopropanol containing an aryl substituent at C1 in 57%
yield.
To gain insight into the role of Ti(OiPr)4 in the ring-
opening cross-coupling, additional control experiments were
carried out (Scheme 3A). First, we investigated the reaction
between cyclopropane 11a and Ti(OiPr)4 in acetone-d6. A fast
exchange between the isopropoxide and cyclopropyloxy ligands
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a,b
Scheme 2. Scope of the Reaction
a
Reaction conditions A: 11 (0.25 mmol), 12 (0.5 mmol), 4CzIPN (0.0125 mmol), NiCl2·bpy (0.0125 mmol), Ti(OiPr)4 (0.5 mmol), K2CO3
b
(0.75 mmol), acetone (2.5 mL), blue LEDs, 15 h. Reaction conditions B: same as reaction conditions A but NiCl2·DME (0.00625 mmol) and
c
d
neocuproine (0.00625 mmol) were used instead of NiCl2·bpy (0.0125 mmol). Reaction run on a 1 mmol scale. With 1% NiCl2·DME and 1%
neocuproine.
leading to a 1:3 mixture of 11a and 15 was observed. Initially,
we assumed that the formed titanium cyclopropoxide complex
15 could undergo one-electron oxidation by the photoexcited
4CzIPN* more efficiently than free cyclopropanol 11a.
However, voltammograms of 11a and its 1:1 mixture with
Ti(OiPr)4 were nearly identical, suggesting that the additive
played another role in the reaction. Alternatively, Ti(OiPr)4
could serve as a co-catalyst that assists in the formation of
cyclopropyloxy nickel(III) complexes.27 Homolytic cleavage of
the RO−NiIII bond could provide cyclopropyloxy radicals,
which would further undergo ring opening and cross-
coupling.28−30 Bearing in mind that the formation of RO−
NiIII intermediates is possible even in the absence of a
photocatalyst and this reaction proceeds more efficiently under
irradiation with 390−395 nm LEDs,27b we carried out
additional control experiments without 4CzIPN. When blue
LEDs were used as a source of light, the alkenylation of 11a by
2-bromopropene gave product 14a in 10% yield, while no
reaction was observed in the absence of Ti(OiPr)4. Under
irradiation with purple LEDs, the yield increased significantly
to 38% and 1% of the product was formed in the absence of
Ti(OiPr)4. On the basis of these experiments, we proposed a
catalytic cycle in which 4CzIPN* oxidizes the nickel complex
rather than cyclopropanol or its titanium alkoxide while
Ti(OiPr)4 assists in the formation of RO−NiIII intermedates
(Scheme 3B). After the oxidative addition of aryl or vinyl
bromide to 16, aryl nickel complex 17 would undergo
oxidation by photoexcited 4CzIPN* and ligand exchange
with titanium alkoxide 18 to provide NiIII complex 19. Next,
the NiIII−O bond in 19 would break, providing NiII complex
20 and oxycyclopropyl radical 21. Ring opening of 21 would
give β-ketoradical 22, which would further react with 20 to
afford NiIII complex 23. Reductive elimination from 23 would
lead to coupling product 24 and NiI species 25, reduction of
which would close the catalytic cycle.
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Authors
Scheme 3. Control Experiments and a Plausible Catalytic
Cycle
Nastassia Varabyeva − Institute of Bioorganic Chemistry,
National Academy of Sciences of Belarus, Minsk 220141,
Belarus
Maryia Barysevich − Institute of Bioorganic Chemistry,
National Academy of Sciences of Belarus, Minsk 220141,
Belarus
Yauhen Aniskevich − Belarusian State University, Minsk
220030, Belarus
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01795
Author Contributions
§N.V. and M.B. contributed equally to this work.
Notes
This work previously appeared as a preprint: Varabyeva, N. A.;
Barysevich, M. V.; Aniskevich, Y.; Hurski, A. Ti(OiPr)4-
Enabled Dual Photoredox and Nickel-Catalyzed Arylation and
Alkenylation of Cyclopropanols. chemRxiv 2021, 10.26434/
chemrxiv.14573148.v1.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for the financial support from the
Belarusian Foundation for Fundamental Research (Projects
X20M-036 and X20PTI-008). The authors thank Alesya
Dmitrochenko and Dr. Vitaly Syakhovich (National Anti-
Doping Laboratory) and his team for the HRMS analysis and
Denis Shklyaruck (Belarusian State University) for the
determination of the enantiomeric purity of ketone 14p by
chiral GC.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01795.
Experimental procedures and compound characteriza-
tion (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Alaksiej Hurski − Institute of Bioorganic Chemistry, National
Academy of Sciences of Belarus, Minsk 220141, Belarus;
orcid.org/0000-0002-8599-2621; Email: AHurski@
iboch.by
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13865.
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