Photoredox-Controlled β-Regioselective Radical Hydroboration of Activated Alkenes with NHC-Boranes

Article abstract of DOI:10.1002/anie.202005749

In this Communication, we report an unprecedented β-regioselective radical inverse hydroboration (compared with ionic hydroboration) of α,β-unsaturated amides with NHC-BH₃ enabled by photoredox catalysis. Density functional theory (DFT) calculations show that the unique photoredox cycle is a key factor to control the β-regioselective radical hydroboration, by lowering the energy barrier in comparison with other pathways. This protocol provides a general and convenient route to construct a wide range of structurally diverse β-borylated amides in synthetically useful yields under mild conditions.

Full text of DOI:10.1002/anie.202005749

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Accepted Article
Title: Photoredox Controlled β-Regioselective Radical Hydroboration of
Activated Alkenes with NHC-Boranes
Authors: Jin Xie
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10.1002/anie.202005749
Angewandte Chemie International Edition
COMMUNICATION
Photoredox Controlled -Regioselective Radical Hydroboration of
Activated Alkenes with NHC-Boranes
Congjun Zhu, Jie Dong, Xueting Liu, Liuzhou Gao, Yue Zhao, Jin Xie*, Shuhua Li*, and Chengjian
Zhu*
Dedicated to the 70th anniversary of Shanghai Institute of Organic Chemistry
Abstract: In this communication, we report an unprecedented -
regioselective radical inverse hydroboration (compared with ionic
hydroboration) of  -unsaturated amides with NHC-BH3 enabled by
the photoredox catalysis. The density functional theory (DFT)
calculations show that the unique photoredox cycle is a key factor to
control the -regioselective radical hydroboration, which makes the
energy barrier is lower than other pathways. This protocol provides a
general and convenient route to construct a wide range of structurally
diverse β-borylated amides in synthetically useful yields under mild
conditions.
are always obtained due to the matched electronic demand in the
transition state. As our continuing efforts in radical borylation, we
wondered if the generated NHC-boryl radical by mild photoredox
catalysis can be used for inverse radical hydroboration of
electron-deficient alkenes to furnish synthetically valuable -
borylated compounds (compared with ionic hydroboration,
Scheme 1b, lower part). During the process of this work, Wang
and co-workers reported an elegant radical -borylation of -
unsaturated carbonyl compounds with NHC-boranes using azodi-
iso-butyronitrile (AIBN) as the radical initiators at 80 oC (Scheme
1b).[12] However, in some case,
a significant amount of
regioisomeric products can be formed dependent on the
electronic effect of substituents (  ranges from >99:1 to 62:38).
The organoboron are versatile building blocks for cross-coupling
in synthetic chemistry and also important structural motifs in
medicine and materials.[1] The direct hydroboration of alkenes
with boranes has been esteemed as a direct and efficient method
for the construction of C-B bonds although these compounds are
always regarded as the reactive intermediates for the preparation
of alcohols.[2] In general, the regioselectivity for hydroboration of
olefin should meet the electronic demand in the transition states,
in which the boron motif adds to the relatively less electronic-rich
position (Scheme 1a).
N-heterocyclic carbene (NHC) boranes are air-stable and
easily handling organoborane reagents.[3] During the past decade,
the groups of Curran, Malacria, Fensterbank, Lacôte and Wang,
have successfully shown the promising chemical space in the
exploration of NHC-boryl radicals in both organic synthesis and
main-group element radical chemistry.[4-10] In 2018, our group
developed the first inverse radical hydroboration of imines with
NHC-ligated boranes to construct -amino organoboron.[11a] In the
ionic hydroboration of activated alkenes, the -borylated products
C. Zhu, J. Dong, Dr. Y. Zhao, Prof. Dr J. Xie, and Prof. Dr C. Zhu
State Key Laboratory of Coordination Chemistry, Jiangsu Key
Laboratory of Advanced Organic Materials, Chemistry and
Biomedicine Innovation Center (ChemBIC), School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing 210023
(China)
Email: xie@nju.edu.cn; cjzhu@nju.edu.cn; shuhua@nju.edu.cn
Prof. Dr. C. Zhu
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Shanghai 200032, China
Prof. Dr. C. Zhu
College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou 450001, China
X. Liu, L. Gao, Prof. Dr S. Li
Institute of Theoretical and Computational Chemistry
School of Chemistry and Chemical Engineering, Nanjing University
Nanjing 210023 (China)
Scheme 1. The state-of-the-art of ionic and radical hydroboration of alkenes
with boranes.
A
series of electron-deficient alkenes were initially
investigated by means of synergistic photocatalysis and thiol
catalysis based on the success of our inverse hydroborylation of
imines (Scheme 2).[11] Much to our surprise, the highly electron-
deficient alkenes, such as (vinylsulfonyl)benzene (1a) and 2-
benzylidenemalononitrile (3a), cannot occur the radical
hydroboration process. The use of ethyl (E)-but-2-enoate (4a)
also failed to give the desired product under our photoredox
conditions, which is indeed a competent substrate in Wang’s
reaction conditions despite of moderate regioselectivity.[12a]
Fortunately, when −unsaturated amide (7a) was employed,
the -borylated amide can be obtained in 62% yield.[13] It is
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
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10.1002/anie.202005749
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interesting to find that this kind of NH-free amide functional group
(7a) is a crucial factor for a successful radical hydroboration.
MeCN as the solvent under the irradiation of 36 W compact
fluorescent light (CFL) at room temperature (Table 1, entry 1). The
desired product (8a) can be obtained in 77% yield under the
standard conditions. The use of other photocatalysts decreased
the reaction yield (entries 2-4, Table 1). Switching the light source
from CFL to blue LEDs led to a significant lower yield (entry 5,
t
Table 1). Interestingly, without BuSH still 27% yield can be
obtained (entry 6, Table 1). We speculated that in this case the
amide motif in 7a may act as HAT catalyst role. Provided by
stronger bond dissociation energies (BDE) of N-H (7a) than S-H
bonds,[14] we performed the density functional theory calculations
and it was found that electrophilic thiyl radical was slightly easier
than amidyl radical to abstract one hydrogen atom from NHC-BH3
(6.3 kcal mol-1 versus 7.7 kcal mol-1), giving rise to the
corresponding NHC-boryl radical (See Supporting Information for
details). According to initial investigation, if protected amide
substrate (6a in Scheme 2) was employed, the reaction did not
occur (See Supporting Information for details). The control
experiments indicated that both light and photocatalyst were
important factors for the reaction (entries 7 and 8, Table 1).
Scheme 2. Initial investigation of radical hydroboration of electron-deficient
alkenes. Bn = benzylic group
Table 1: Optimization of reaction conditions.[a]
Entry
Variation of standard conditions
None
Yield [%][b]
77
1
2
3
4
5
6
7
8
Ru(bpy)3Cl2
ND
4CZIPN
ND
Eosin Y
trace
25
blue LEDs instead CFL
without tBuSH
without light
27
ND
Without photocatalyst
ND
[a] Standard reaction conditions: Ir(ppy)2(dtbbpy)PF6 (1 mol%), tBuSH (20
mol%), NaH (20 mol%), 1a (0.2 mmol), 2a (0.4 mmol), MeCN (1.0 mL), 36 W
CFL bulb, 24 h, ambient temperature. [b] Yield of isolated product. ND means
not detected; CFL means compact fluorescent lamps; LEDs means light
emitting diode strips.
Scheme 3. Reaction scope with regard to α,β-unsaturated alkenes. [a] Standard
conditions: Ir(ppy)2(dtbbpy)PF6 (1 mol%), tBuSH (20 mol%), NaH (20 mol%), 1a
(0.2 mmol), 2a (0.4 mmol), MeCN (1.0 mL), 36 W CFL bulb, 24 h, ambient
temperature. [b] The diastereoselectivity of product cannot be determined by
NMR due to the complex peak overlap. [c] BnSH instead BuSH. brsm means
the isolated yields are calculated based on the recovered starting materials.
After getting these initial results, we focused our attention on
optimizing the reaction conditions (See Supporting Information for
details). The optimized reaction conditions include of 1 mol% of
Ir[ppy]2(dtbbpy)PF6 as the photocatalyst, 20 mol% of 2-
methylpropane-2-thiol (tBuSH) as the hydrogen-atom transfer
(HAT) organocatalyst together with 20 mol% NaH as the base and
t
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In general, a wide array of amide bearing substituents with
different electronic and steric properties are competent substrates
(Scheme 3). The electron-donating and electron-withdrawing
substituents on ortho-, meta- and para-positions of phenyl rings
could smoothly occur this radical hydroboration in exclusive
regioselectivity (8a–n). The position of substituents on phenyl ring
have little influence on the yield (8d, 8g, 8h). The alkyl substituted
amide can also deliver the desired products in synthetically useful
yield and excellent regioselectivity (8o–x). Importantly, the
introduction of heterocyclic motif into the amides can tolerate the
reaction conditions well (8n, 8s, 8t, 8v). A series of biologically
important compounds derived amides were subjected to this
protocol and they can readily proceed the regioselective
hydroboration process (8u–x). Subsequently, a wide range of
readily available unsaturated alkenes were evaluated. It has a
very broad scope (8y–ii) independent of the substitution
structures and electronic effect. The long-chain alkyl-substituted
(8y–cc) unsaturated amides can give rise to the desired products
in moderate yield. Interestingly, 1H-pyrrole-2,5-dione is also a
good substrate for radical hydroboration (8dd). Besides, the cyclic
unsaturated amides can occur this transformation to afford the
desired -borylated amides in moderate yields and
diastereoselectivity (8ee–ii). Of note, the terminal alkene motif
hardly influences the reaction selectivity and it can keep intact well
during the reaction process (8gg).
the presence of AIBN and thiol with heating conditions,[12a] the
overwhelming -regioselectivity (=5.7:1) was detected (Eq. 2).
Scheme 5. DFT studies: Gibbs free energy (in kcal mol-1) profile for the
reaction between the radical 9 and 7a.
To gain more insight into this selectivity, DFT calculations
with the M06 functional[16] was performed, for the model reaction
between the NHC-boryl radical 9 and 7a (computatisonal details
are provided in the supporting information). Our DFT calculations
indicated that the addition of the NHC-boryl radical to the -
position of 7a requires a lower activation energy (10.3 kcal mol-1)
than that to the -position (13.2 kcal mol-1). The subsequent
photoreduction of the -addition intermediate (7a–Int to 8a–Int1)
is readily feasible with an single-electron transfer (SET) energy
barrier as low as 4.2 kcal mol-1. The following protonation step to
form the product 8a involves a free energy barrier of 6.0 kcal mol-
1, and is exothermic by 22.3 kcal mol-1. This result indicates that
the -addition pathway under photoredox condition is both
thermodynamically and kinetically favorable. In contrast, our
theoretical study suggests that the photoreduction of -addition
intermediate (7a’–Int to 8a’–Int1) is thermodynamically
unfavorable. The calculated results are in accord with the
observed -regioselectivity under photoredox catalysis (Scheme
5, Eq. 1).
Our calculations also showed that under thermal (radical
chain process) conditions,[12a] the -addition step is slightly
favorable than the -addition step, but the subsequent hydrogen-
atom transfer (HAT) of the -addition intermediate from thiol
involves a barrier of 14.4 kcal mol-1. In contrast, the HAT of the -
addition intermediate involves a free energy barrier of 12.7 kcal
mol-1. Thus, under thermal conditions, the formation of −addition
product 8a is kinetically more favorable, which is consistent with
the experimental result (: = 5.7:1, Scheme 5, Eq. 2).
Scheme 4. Scope with respect to the NHC-boranes. Standard conditions:
Ir(ppy)2(dtbbpy)PF6 (1 mol%), tBuSH (20 mol%), NaH (20 mol%), 1a (0.2 mmol),
2a (0.4 mmol), MeCN (1.0 mL), 36 W CFL bulb, 24 h, ambient temperature.
Finally, different kinds of NHC-boranes were explored and
the results are summarized in Scheme 4. Excitingly, the
substituents on NHC have little effect on the reaction efficiency.
The NHC-boranes (2a–f) tested could provide the -borylated
products in 61–77% yields.
The 2,2,6,6-tetramethyl-1-piperidyloxy (TEMPO) and 1,4-
dinitrobenzene can completely inhibit the hydroboration,
suggesting that a radical process may be likely. Both the NHC-
boryl radical and amidyl radical can be successfully trapped by
TEMPO (See Supporting Information for details).[15] In addition, in
our radical hydroboration process, a full -regioselectivity was
obtained (Scheme 5a). However, for the same substrate (Scheme
5, Eq. 1), when it was subjected to Wang’s reaction conditions in
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grams desired products (8a) can be obtained under standard
conditions without compromise the reaction efficiency. To our
delight, treatment of product (8a) with 2 M HCl in the presence of
pinacol can afford -boronate carboxylic acid (14) in 67% yields
(Scheme 7).
In conclusion, an unprecedented photoredox radical
hydroboration of -unsaturated amides with NHC-BH3 has been
developed by means of photoredox catalysis. The DFT
calculation are performed to explain the corresponding - or -
regioselectivity under photoredox catalysis or normal radical
chain conditions. Interestingly, we found that the photoredox
catalysis is an important factor to control -regioselective radical
hydroborylation compared to previous work. This protocol
provides a general and convenient route to construct a wide range
of structurally diverse -borylated amides in synthetically useful
yields under mild conditions. The excellent functional group
compatibility, exclusive regioselectivity and mild reaction
conditions enable a promising protocol in radical boron chemistry.
Further application of this strategy and the exploration of inverse
hydroboration strategy for the synthesis of enantiopure -
borylated amides is underway in our laboratory.
Scheme 6. Proposed mechanism. Gibbs free energy (in kcal mol-1)
Base on our DFT calculations and control experiments
(results are provided in the SI), a plausible mechanism is
proposed
in
Scheme
6.
First,
the
photoexcited
*Ir[ppy]2(dtbbpy)PF6 undergoes single-electron oxidation of
nitrogen anion (10) to form an amidyl radical (11),[17] which can
abstract one hydrogen atom from NHC-BH3 (2a) (Gǂ= 7.7 kcal
mol-1) to form the NHC-boryl radical species (9) (path a). On the
other hand, the resulting amidyl radical (11) can also abstract one
hydrogen atom from tBuSH (Gǂ= 15.1 kcal mol-1) to generate
electrophilic thiyl radical, and the thiyl radical subsequently
abstracts one hydrogen atom from NHC-BH3 (2a) (Gǂ= 6.3 kcal
mol-1) to form the NHC-boryl radical species (9) (path b).[10b,11a]
Two paralleled pathways may account for a higher yield in the
presence of thiol (27% vs 77%). Subsequently, the radical
addition of the NHC-boryl radical to the unsaturated carbon-
carbon double bond can afford radical intermediate (12), which
further accepts one electron from Ir(II) to complete the photoredox
cycle and generates intermediate (13), which can get one proton
Acknowledgements
We thank the National Natural Science Foundation of China
(21971111, 21971108, 21702098, 21732003, 21672099,
21833002 and 21673110), the Natural Science Foundation of
Jiangsu Province (Grant No. BK20190006), “ Innovation &
Entrepreneurship Talents Plan” of Jiangsu Province, “Jiangsu Six
Peak Talent Project”, "1000-Youth Talents Plan” and start-up
funds from Nanjing University for financial support. Professor
Yong Liang at Nanjing University, Professor Xiang-Ai Yuan at
Qufu Normal University, Mr. Jie Han at Theoretical and
Computational Chemistry Deparment of Heidelberg University
and Dr. Guoqiang Wang at Technische Universität Berlin are
warmly acknowledged for their helpful suggestion and discussion
on DFT calculation section. All theoretical calculations were
performed at the High-Performance Computing Center (HPCC) of
Nanjing University.
t
from (7a) or BuSH to afford the desired product 8a. Indeed, the
photoredox catalysis is a key factor to achieve -regioselective
radical hydroborylation of unsaturated amides compared to
Wang’s work.[12a]
Keywords: Regioselective hydroboration • Photoredox catalysis
• Radical borylation • Umpolung
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COMMUNICATION
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10.1002/anie.202005749
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Congjun Zhu, Jie Dong, Xueting Liu,
Liuzhou Gao, Yue Zhao, Jin Xie*,
Shuhua Li* and Chengjian Zhu*
Page No. – Page No.
Photoredox Controlled -
Regioselective Radical Hydroboration
of Activated Alkenes with NHC-
Boranes
Light Does Different: An unprecedented photoredox-controlled regioselective
radical inverse hydroboration (compared with ionic hydroboration) of  -
unsaturated amides with NHC-BH3 has been developed. This protocol provides a
general and convenient route to construct a wide range of structurally diverse β-
borylated amides in synthetically useful yields under mild conditions.
researcher Twitter usernames: XIE Group NJU @GroupXie
This article is protected by copyright. All rights reserved.