A Lewis Base Catalysis Approach for the Photoredox Activation of Boronic Acids and Esters

Article abstract of DOI:10.1002/anie.201709690

We report herein the use of a dual catalytic system comprising a Lewis base catalyst such as quinuclidin-3-ol or 4-dimethylaminopyridine and a photoredox catalyst to generate carbon radicals from either boronic acids or esters. This system enabled a wide range of alkyl boronic esters and aryl or alkyl boronic acids to react with electron-deficient olefins via radical addition to efficiently form C?C coupled products in a redox-neutral fashion. The Lewis base catalyst was shown to form a redox-active complex with either the boronic esters or the trimeric form of the boronic acids (boroxines) in solution.

Full text of DOI:10.1002/anie.201709690

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Accepted Article
Title: A Lewis Base Catalysis Approach for the Photoredox Activation
of Boronic Acids and Esters
Authors: Fabio Lima, Upendra K Sharma, Lars Grunenberg, Debasmita
Saha, Sandra Johannsen, Joerg Sedelmeier, Erik V Van der
Eycken, and Steven Victor Ley
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709690
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A Lewis Base Catalysis Approach for the Photoredox Activation of
Boronic Acids and Esters
Fabio Lima,[a] Upendra K. Sharma,[b] Lars Grunenberg,[a] Debasmita Saha,[b] Sandra Johannsen,[a] Joerg
Sedelmeier,[c] Erik V. Van der Eycken,*[b,d] and Steven V. Ley*[a]
Abstract: We report herein the use of a dual catalytic system
comprising of a Lewis base catalyst such as quinuclidin-3-ol or 4-
dimethylaminopyridine combined with a photoredox catalyst to
generate carbon radicals from either boronic acids or esters. This
system enabled a wide range of alkyl boronic esters and aryl or alkyl
boronic acids to react via radical addition with electron-deficient
olefins to efficiently form C–C coupled products in a redox neutral
fashion. The Lewis base catalyst was shown to form a redox-active
complex with either boronic esters or the trimeric form of the boronic
acids (boroxines) in solution.
electron oxidation under photoredox conditions when their vacant
p orbital is engaged in a dative bond with the n orbital of a
stoichiometric Lewis base (LB) additive (see Scheme 1, B).[11]
Lewis base catalysis was introduced as a concept by Denmark to
enhance the reactivity of electrophilic n*, π* and σ* orbitals.[12]
Based on this knowledge, we hypothesized that the use of a
catalytic amount of an organic Lewis base would be a viable
option for the photoredox activation of boronic acids and esters.[13]
Carbon-centered radicals are a synthetically powerful class of
reactive intermediates.[1] They are particularly attractive in the
context of C–C bond forming reactions,[2] overcoming problems
often associated with two electron processes.[3] By enabling
visible-light promoted single electron transfer, photoredox
catalysis has become a method of choice to perform single
electron reduction or oxidation of organic substrates and thereby
access open shell intermediates in a mild and selective fashion.[4]
A range of reductive or oxidative carbon radical precursors are
now available to generate carbon radicals in the context of a
photocatalytic cycle.[5] Oxidative carbon radical precursors are
often anionic species suffering from poor solubility in common
organic solvents. For example, extensively studied organoborates
[6] possess an electron-rich B(sp3) moiety that can be subjected to
single electron oxidation, leading to a neutral carbon radical after
C–B cleavage (see Scheme 1, A).
Despite their ubiquity as reagents in organic synthesis[7] and in
biologically active molecules[8] the use of boronic acid derivatives
to generate carbon-centered radicals remains underexplored.[9]
Due to their high oxidation potentials, they received much less
attention in this regard with few reports making use of strong
stoichiometric oxidants or anodic oxidation.[10] We recently
demonstrated that benzyl boronic esters can undergo single
Scheme 1 Photoredox activation of organoboron reagents. LB = Lewis base.
Herein, we describe a dual catalytic method to effectively form
alkyl and aryl radicals from a wide array of boronic esters or acids
by direct photoredox single electron oxidation under mild and safe
conditions, without the requirement of stoichiometric activators or
oxidants. These reactive species were further engaged in
intermolecular C–C bond forming processes to deliver desirable
C(sp3)–C(sp3) and C(sp2)–C(sp3) bonds in a redox neutral fashion.
[a]
[b]
F. Lima, L. Grunenberg, S. Johannsen and Prof. S. V. Ley
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge CB2 1EW (UK)
E-mail: svl1000@cam.ac.uk
Dr. U. K. Sharma, Dr. D. Saha and Prof. E. V. Van der Eycken
Laboratory for Organic and Microwave-Assisted Chemistry
(LOMAC), Department of Chemistry, University of Leuven (KU
Leuven), Celestijnenlaan 200F, B-3001 Leuven, Belgium
Dr. J. Sedelmeier
Scheme 2 Optimized conditions for “Giese-type” addition of boronic ester 1a
on 2a including selected Lewis base catalysts (LB). Optimization reaction
conducted with 0.1 mmol of 1a, 0.4 mmol of 2a. Yields in 3aa determined by
1H-NMR analysis of crude reaction mixture with CH2Br2 as an internal
standard. Isolated yield in parentheses.
[c]
[d]
Novartis Pharma AG, Novartis Campus, 4002 Basel, Switzerland
Prof. E. V. Van der Eycken
Peoples Friendship University of Russia (RUDN University)
Miklukho-Maklaya street 6, 117198 Moscow, Russia
The addition of electron-rich carbon-centered radicals onto
electron-deficient olefins, also known as “Giese-type” additions[14]
is an interesting method to form C–C bonds in a redox neutral
Additional data related to this publication is available at the
https://doi.org/10.17863/CAM.13394 data repository.
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10.1002/anie.201709690
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fashion and can also be used to assess the presence of the
postulated radical intermediates.[15] We initially subjected model
boronic ester 1a to an excess of methyl acrylate 2a in the
presence of 1.5 equiv. of DMAP additive and the photoredox
catalyst PC(1) that we already proved to be quenched by DMAP
activated 1a.[11] Irradiation of this mixture with blue LEDs for 24 h
readily led to 86% yield of the coupling product 3aa. Reducing
DMAP catalyst loading to 20 mol% still provided 75% yield of 3aa
(see Scheme 2), with the remaining mass balance resulting from
oligomerization due to multiple acrylate additions. Pleased by this
level of catalytic activity, we decided to investigate other Lewis
bases.
According to Denmark’s theory, n-n* interactions are the most
productive type of activation for a Lewis base catalyst to be
active,[12] so a range of commercial neutral Lewis bases with an
available non-bonding n orbital were screened at 20 mol% loading.
Strongly nucleophilic[16] quinuclidine-derived bases such as
quinuclidin-3-ol and quinuclidine were identified as productive
catalysts, leading to 80% (75% isolated) and 77% yield of 3aa
respectively. Phosphine-derived Lewis bases were also
investigated with triphenylphosphine (PPh3) showing good activity.
Control experiments revealed the necessity of blue-light,
photocatalyst, Lewis base and methanol for the successful
conversion of boronic esters (see SI for full optimization and
control experiments).
high yields, thereby expanding the range of functional groups
tolerated with this method. gem-Disubstituted olefins also reacted
in a radical conjugate addition fashion, with methyl methacrylate
(3ah), a conjugated lactam (3ai) and two cyclic enones (3aj and
3ak) selectively coupled in 58% to 68% yield. Interestingly, 2- and
4-vinyl pyridines were successfully alkylated at the β-carbon (3am
and
3an)
providing
examples
of
challenging
N-
heteroaromatics.[17] These results could be extended to a 2-
pyridyl-containing 1,1-disubstituted olefin partner (3ao),
showcasing the possibility to generate pheniramine analogs and
the potential application of the method for antihistaminic drug
discovery.[18] Finally, flavone natural products could be alkylated,
albeit in lower yields (3aq and 3ar).
Scheme 4 Scope of boronic pinacol esters. Isolated yields. Reactions carried
out using 0.2 mmol of 1a-q, 0.8 mmol of 2d, 2 mol% of PC(1) and 20 mol% of
the indicated Lewis base catalyst (LB). Irradiation supplied by a commercial
blue LED strip (14.4 W at 450 nm). *Yield determined by 1H-NMR analysis of
crude reaction mixture with CH2Br2 as an internal standard.
Scheme 3 Scope of electron deficient alkenes. Isolated yields. Ar = 4-MeO-
C6H4-. Reactions carried out using 0.2 mmol of 1a, 0.4–0.8 mmol of 2a-r, 2 mol%
of PC(1) and 20 mol% of quinuclidin-3-ol as Lewis base catalyst (LB). Irradiation
supplied by a commercial blue LED strip (14.4 W at 450 nm).
We next turned our attention to establishing the scope of boronic
ester coupling partners with methyl vinyl ketone 2d (see Scheme
4). Primary benzylic pinacol esters were selectively coupled (3ad
to 3ed) using quinuclidin-3-ol as the Lewis base catalyst.
Interestingly, primary alkyl boronic esters α to heteroatoms were
also coupled in high yields (3fd to 3hd, 86% to 91%) with
triphenylphosphine proving to be the most efficient catalyst for α-
amino products 3gd and 3hd. More sterically demanding
secondary benzylic esters required the use of DMAP as a Lewis
base catalyst, highlighting the effect of the steric hindrance on the
required initial complexation between boronic ester and Lewis
base. While methyl (3id and 3jd) and benzyl (3kd) were well
With optimized conditions in hand, we assessed the scope of
electron-deficient alkenes 2a-r (see Scheme 3). In addition to
methyl acrylate, tert-butyl and benzyl acrylates were also
successful coupled partners (3aa to 3ac). Methyl vinyl ketone was
identified as the best coupling partner with conjugate addition
product isolated in 82% yield (3ad). Pleasingly, acrolein and
acrylonitrile coupled products (3ae and 3af) were also obtained in
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10.1002/anie.201709690
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tolerated, the presence of larger isopropyl (3ld) and phenyl (3md)
groups led to less efficient coupling. Lastly, tertiary boronic esters
were explored (3nd to 3pd). Despite their well-known difficulty to
be efficiently engaged in metal catalyzed cross-couplings,[14a,15,19]
DMAP allowed a clean activation to form quaternary carbon
centers in respectable yields even from commercial and less
activated tBuBpin (3pd).
Scheme 6 Possible mechanism for the Lewis base and photoredox catalyzed
“Giese-type” addition of boronic acid derivatives.
Despite the usual harsh conditions employed to oxidize aryl
boronic acids,[10b] we found that a large number of electron-rich
aryl boronic acids could be successfully coupled to 2d under
extremely mild and redox efficient conditions. Phenyl boronic
acids substituted with nitrogen (5cd to 5ed), oxygen (5ad and 5fd)
and sulfur (5bd and 5gd) around the ring all proceeded in good
to excellent yields. Oxygen-containing heterocycles derived from
catechol could be incorporated into the substrates (5hd and 5id)
and unprotected 5- and 6-indoyl boronic acids (5jd and 5kd) were
also successfully functionalized in the presence of nucleophilic
NH and C3 residues. The enhanced reactivity observed with
boroxines relative to boronic esters encouraged us to attempt
unactivated alkyl boronic acids as starting materials. Primary alkyl
boronic acids were successfully coupled (5md to 5od) along with
secondary alkyls (5pd and 5qd), showcasing the usefulness of
the method to generate functional unstabilized alkyl radicals.[5a]
Secondary α-amino boronic acids derived from amino acids[7d]
were also well tolerated with proline-derived 5rd as well as the
peptide drug ixazomib transformed in high yield (5sd) illustrating
the potential application to late stage functionalization.
According to NMR studies, a fast, dynamic equilibrium was
observed between the boroxine (6aʹ) derived from boronic acid
(6a) or the boronic ester (6b) and the Lewis base catalyst (LB) in
the reaction solvent mixture (see SI). Cyclic voltammetry
measurements informed us that complex 7 can be single electron
oxidized (E1/2 (1a-DMAP) = +0.81 V vs. SCE) within the reductive
quenching cycle of PC(1) (E1/2 (IrIII*) = +1.2 V vs. SCE).[21] The
carbon radical thus generated (8) undergoes a radical addition
with 10 to form the intermediate radical 11, which can then be
reduced and quenched by a proton from methanol to lead to
coupling product 13.[6c] The resulting methanolate can then be
used to regenerate the LB from 9.
Scheme 5 Scope of boronic acids. Isolated yields. Reactions carried out using
0.2 mmol of 4a-s, 0.8 mmol of 2d, 2 mol% of PC(1), 20 mol% of the indicated
Lewis base catalyst (LB). Irradiation supplied by a commercial blue LED strip
(14.4 W at 450 nm).
Aryl boronic esters, on the other hand, were found to be
substantially less reactive than their activated alkyl counterparts.
We initially observed only low reactivity after 24 h of irradiation,
and therefore surveyed different aryl-B(sp2) species to found that
aryl boronic acids were more reactive than pinacol, glycol,
neopentyl and catechol esters (see SI). Our experience with
Lewis acidity of aryl boronic acids led us to propose that the
reactive species in solution was more likely to be the trimeric
boroxine than the monomeric species.[20] This was confirmed by
NMR experiments showing complexation of quinuclidin-3-ol with
boroxine instead of the corresponding free boronic acid (see SI).
This finding led us to screen a series of commercially available
boronic acids in this reaction (see Scheme 5).
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To conclude we have developed a novel set of photoredox
reaction conditions taking advantage of the Lewis acidity of
boronic esters and boroxines (from boronic acids) to generate
primary, secondary and tertiary alkyl or aryl radicals. These
intermediates could be engaged in redox neutral C–C couplings
with electron-deficient olefins forming a range of new C(sp3)–
C(sp3) and C(sp2)–C(sp3) cross-coupled products. Over 50
examples of structurally and functionally diverse products were
successfully synthesized. This new activation method should
enable the use of boronic acids and esters in a wide range of other
radical based reactions.
[5]
[6]
[7]
For carbon radicals generations using photoredox catalysis see: a)
J.-P. Goddard, C. Ollivier, L. Fensterbank, Acc. Chem. Res. 2016, 49,
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Acknowledgments
We are grateful to Novartis Pharma AG (F.L.), Erasmus
Scholarship Scheme (L.G and S.J.) and the EPSRC (S.V.L.,
Grant Nos. EP/K009494/1, EP/K039520/1 and EP/M004120/1)
for financial support. U.K.S. and D.S. are thankful to the University
of Leuven for postdoctoral funding and FWO for visiting
postdoctoral scholarship (U.K.S.) in University of Cambridge.
E.V.VdE would like to thank the Ministry of Education and Science
of the Russian Federation for financial support (agreement
number 02.a03.0008). We thank Dr. Berthold Schenkel and Dr.
Gottfried Sedelmeier for insightful discussions. We thank Merck
Rahway USA for the generous gift of the PC(1) photoredox
catalyst. We thank Jian Siang Poh for assistance with the
preparation of the manuscript and support with starting material
synthesis. Finally, we thank Mark Bajada and Bertrand Reuillard
for help with the cyclic voltammetry experiments.
[8]
[9]
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[10]
Conflicts of interest
The authors declare no conflict of interest.
Keywords: boronic acids • cross-coupling • Lewis base catalysis
• photoredox catalysis • synthetic methods
[11]
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[12]
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Jamison, Y. Slutskyy, D. W. C. MacMillan, L. E. Overman, J. Am.
Chem. Soc. 2015, 137, 11270–11273; b) T. Chinzei, K. Miyazawa, Y.
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9850.
[20]
Better reactivity of boroxines against boronic esters or acids can be
explained by their enhanced Lewis-acidity see mixed computational
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[21]
E1/2 (1a) = +1.43 V vs. SCE and E1/2 (DMAP) = +1.24 V vs. SCE
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COMMUNICATION
Fabio Lima, Dr. Upendra K. Sharma, Lars
Grunenberg, Dr. Debasmita Saha,
Sandra
Johannsen,
Dr.
Joerg
Sedelmeier, Prof. Erik V. Van der Eycken
and Prof. Steven V. Ley*
Page No. – Page No.
A Lewis Base Catalysis Approach for
the Photoredox Activation of Boronic
Acids and Esters
Boronic Acids Turn Photoactivable! We report herein the use of a dual catalytic system comprising of a Lewis base catalyst such as
quinuclidin-3-ol or 4-dimethylaminopyridine combined with a photoredox catalyst to generate carbon radicals from either boronic acids
or esters. This system enabled a wide range of alkyl boronic esters and aryl or alkyl boronic acids to react via radical addition with
electron-deficient olefins to efficiently form C–C coupled products in a redox neutral fashion. The Lewis base catalyst was shown to
form a redox-active complex with either boronic esters or the trimeric form of the boronic acids (boroxines) in solution.
This article is protected by copyright. All rights reserved.