Photoinduced Single-Electron Transfer as an Enabling Principle in the Radical Borylation of Alkenes with NHC–Borane

Article abstract of DOI:10.1002/anie.201913398

A photoinduced SET process enables the direct B?H bond activation of NHC–boranes. In contrast to common hydrogen atom transfer (HAT) strategies, this photoinduced reaction simply takes advantage of the beneficial redox potentials of NHC–boranes, thus obviating the need for extra radical initiators. The resulting NHC–boryl radical was used for the borylation of a wide range of α-trifluoromethylalkenes and alkenes with diverse electronic and structural features, providing facile access to highly functionalized borylated molecules. Labeling and photoquenching experiments provide insight into the mechanism of this photoinduced SET pathway.

Full text of DOI:10.1002/anie.201913398

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Accepted Article
Title: Photoinduced Single-Electron-Transfer Process as an Enabling
Principle in Radical Borylation of Alkenes with NHC-borane
Authors: Peng-Ju Xia, Dan Song, Zhi-Peng Ye, Yuan-Zhuo Hu, Jun-An
Xiao, Hao-Yue Xiang, Xiao-Qing Chen, and Hua Yang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913398
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10.1002/anie.201913398
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Photoinduced Single-Electron-Transfer Process as an Enabling
Principle in Radical Borylation of Alkenes with NHC-borane
Peng-Ju Xia, Dan Song, Zhi-Peng Ye, Yuan-Zhuo Hu, Jun-An Xiao, Hao-Yue Xiang,* Xiao-Qing Chen,*
and Hua Yang*
Abstract: A photoinduced protocol for the direct B-H bond
activation of NHC-borane via a SET process was unveiled for the
first time. Contrast to the common HAT strategies, this
photoinduced activation pathway simply takes advantage of the
beneficial redox potentials of NHC-boranes, thus avoiding the
involvement of extra radical initiators. The resulting NHC-boryl
radical was easily apt to the borylation of a wide range of α-
trifluoromethyl alkenes and alkenes with varied electronic and
structural features, enabling facile accesses to divergent highly
functionalized borylated molecules. The included labelling and
photoquenching experiments provide insights into the mechanism
of this photoinduced SET pathway.
substitution of disulfides and radical addition reactions, etc.
Recently, Wang and co-authors reported a series of elegant
works based on NHC-boranes for radical borylation/cyclization of
1,6-enynes
and α-borylation of α,β-unsaturated carbonyl
compounds respectively.7 Concurrently, Curran and Taniguchi
documented borylative lactonization of substituted propargyl
acetates with an NHC-borane through radical pathway.8
Thereafter, Curran reported a new route to α-NHC-boryl ketones,
which experienced a radical chain mechanism starting from
trifluoromethyl radical.9
Organoboron compounds have been playing a vital role in modern
synthetic chemistry over the past decades, particularly as regents
and building blocks in a wide range of C-Y (Y = C, N, O, etc) bond-
forming.1 To this end, numerous borylation methods in a
convenient manner have been extensively exploited.2 Among
these established approaches, owing to the inherent capability of
undergoing fast transpositions for odd-electron species, the
boron-centered-radicals are rapidly emerging as a prosperous
subject in boron chemistry.3
Pioneered by Roberts’s elegant work4, substantial boryl
radical-mediated processes have been developed, most of which
employed amine- and phosphine-boranes as well as N-
heterocyclic carbene (NHC) boranes.4-6 Noticeably, B−H bonds of
NHC-boranes are essentially weaker than those of the
uncomplexed boranes.5 In fact, N-heterocyclic carbene (NHC)
boranes as new reliable sources of boryl radicals have recently
drawn increasing attention.5-6 The fruitful chemical space of the
distinct NHC-borane derived boryl radicals were well documented
by Curran, Lacꢀte, Malacria, and Fensterbank,6 including the
deoxygenation of xanthates, radical polymerization with electron-
deficient alkenes, reduction of organic halides and alkyl nitriles,
radical polymerization with electron-deficient alkenes, homolytic
Scheme 1. The state of the art in generation of NHC-boryl radicals.
Despite with the impressive advances in the synthetic
applications of NHC-boryl radicals, exploration of conceptually
distinct approaches to the generation of boryl radical obviously
received insufficient attention. In general, all the reported
pathways generating NHC-boryl radicals comprehensively relied
on hydrogen atom transfer (HAT) processes, usually requiring the
use of potentially toxic radical initiators,6 such as 2,2’-
azoisobutyronitrile (AIBN), di-tert-butyl hyponitrite, di-tertbutyl
peroxoide (DTBP), with elevated temperature. Otherwise, thiyl
4-
and decatungstate W10O32 were utilized as the relatively
harmless hydrogen atom transfer agents under irradiation of light
(Scheme 1a).10 In 2018, Zhu and Xie’s group realized the first
catalytic inverse hydroboration of imines with N-heterocyclic
carbene boranes, where thiyl radical was necessary for the
formation of vital NHC-boryl radical. Despite these impressive
advances, noticeably, the existing generation pathways
thoroughly depend on HAT processes.10a In this context, we
questioned if the B-H bond in NHC-borane could be directly
scissored to deliver the corresponding boryl radical via a single-
electron-transfer (SET) process without being triggered by any
HAT agents, which has never been harnessed ever since. On
[*]
P.-J. Xia, Z.-P. Ye, Y.-Z. Hu, D. Song, Prof. H.-Y. Xiang, Prof. X.-Q.
Chen, Prof. H. Yang
College of Chemistry and Chemical Engineering, Central South
University, Changsha 410083, P. R. China
Prof. J.-A Xiao
Guangxi Key Laboratory of Natural Polymer Chemistry and Physics,
Nanning Normal University, Nanning Normal University, Nanning
530001, Guangxi, P. R. China
Prof. X.-Q. Chen, Prof. H. Yang
Key Laboratory of Hunan Province for Water Environment and
Agriculture Product Safety, Central South University, Changsha
410083, P. R. China
E-mail: hyangchem@csu.edu.cn; xqchen@csu.edu.cn;
xianghaoyue@csu.edu.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/
basis of the BDE (74.5 kcal/mol)10a of B-H bond for NHB-borane
p/2
ox
1a and its oxidation potential (E
= +0.34 V, vs. SCE. in MeCN,
see Supporting Information for details), we rationalized that it
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10.1002/anie.201913398
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stands a chance for directly generating NHC-boryl radical via a
photoinduced SET process. Given our ongoing interests and
previous works in photocatalysis11, we devoted our efforts to
successfully realizing the first photoredox-catalyzed direct
oxidation of NHC-boryl via the SET strategy, enabling facile
for this transformation (Table 1, entries 11-12). The title
reaction for 1a with 2a was performed at gram-scale under the
standard reaction conditions, which proceeded smoothly to
give the corresponding product 3a in 77% yield.
Scheme 2. Scope with respect to α-trifluoromethylalkylene [a, b]
accesses
to
a
series
of
gem-difluoroallylboronates,
aliphaticboronates, α-borylated products, β-borylated products
and monofluoroalkeneboronate (Scheme 1b). More importantly,
this innovative pathway for directly activating B-H bond in NHC-
boranes offers new and convenient avenues for synthetic
methodology development.
Table 1. Optimization of the reaction conditions [a]
Entry
1
Variation from the “standard conditions”
none
Yield (%)[b]
83 (77c)
2
3
4
5
KOAc instead of K2HPO4
CsF instead of K2HPO4
DABCO instead of K2HPO4
71
39
61
38
[Ir(dFCF3ppy)2dtbbpy]PF6 instead of
Ir(ppy)2dtbbpyPF6
6
Ru(bpy)3PF6 instead of [Ir(ppy)2dtbbpyPF6
0
7
MeCN instead of THF as the solvent
DMF instead of THF as the solvent
1,4-dioxane instead of THF as the solvent
CH2Cl2 instead of THF as the solvent
No light
62
26
35
trace
0
8
9
10
11
12
No photocatalyst
0
[a] Standard conditions A: NHC-BH3 1a (0.3 mmol, 1.5 equiv.), α-
trifluoromethylnaphthalene 2a (0.2 mmol, 1.0 equiv.), K2HPO4 (0.3 mmol, 1.5
equiv.), Ir(ppy)2dtbbpyPF6 (2 mmol%), THF (2 mL), 30 W blue LED, argon
atmosphere, rt, 24 hours. [b] Yield of isolated product. [c] Performed at 4 mmol
scale of 2a
Initially, we speculated that α-trifluoromethyl alkenes
should be mechanistically competent NHC-boryl radical
receptors as the ultimate reductive quenching step could be
facilitated by E1cB-type fluoride elimination.12 It is worth
mentioning that the borylation of α-trifluoromethyl alkene has
never been achieved by the traditional HAT strategy. We
initiated this part of the investigation by monitoring the reaction
of 1,3-dimethylimidazol-2-ylidene borane (1a) with α-
trifluoromethylnaphthalene (2a) in the presence of
photocatalyst and base (Table 1). Unless otherwise noted, all
the experiments were conducted at room temprerature in THF
under irradiation of 30 W blue LED. Gratifyingly, upon
optimizing the reaction conditions (see Table S1 in Supporting
Information), gem-difluoroallylboronate 3a was furnished in
83% yield (Table 1, entry 1). Other bases, such as KOAc, CsF
and DABCO, were found unhelpful for the reaction (Table 1,
entries 2-4). Among the screened catalysts, Ir(ppy)2dtbbpyPF6
was proven to be the optimal choice whereas using
[Ir(dFCF3ppy)2dtbbpy]PF6 and Ru(bpy)3PF6 significantly
reduced the yield of 3a (Table 1, entries 5-6). Using MeCN,
DMF and 1,4-dioxane in place of THF resulted in less efficient
reactions (Table 1, entries 7-9). When CH2Cl2 was used as the
solvent, only a trace amount of 3a was detected (Table 1, entry
10). As predicted, light and photocatalyst were indispensable
[a] Standard condition A: LB-BH3
1 (0.3 mmol, 1.5 equiv.), α-
trifluoromethylalkylene 2 (0.2 mmol, 1.0 equiv.), K2HPO4 (0.3 mmol, 1.5
equiv.), Ir(ppy)2dtbbpyPF6 (2 mmol %), THF (2 mL), 30 W blue LED, argon
atmosphere, rt, 24 hours. [b] Yield of isolated product.
With the optimal conditions in hand, we continued to
investigate the substrate generality for this photoredox-
catalyzed transformation with a range of α-trifluoromethyl
alkenes. As shown in Scheme 2, products 3 with assorted
substituents on the aromatic ring were isolated with moderate
to excellent yields. Obviously, the presence of a -OMe group
at the meta-position of phenyl moiety gave better yield than
that substituted at ortho-position (Scheme 2, 3c vs 3d). The
electronic characteristics of substituents at the para-position
of phenyl moiety slightly affected the efficiency of this
proccess, with the desired products 3e-3n being obtained in
good to excellent yields, except for methyl or fluoro substituent.
Specifically, hydroxymethyl group survived well from this
radical-based process (3g). Unfortunately, the employemnt of
non-conjugated alkenes was unable to achieve the
corresponding product. Moreover, this strategy was applicable
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[a] Standard condition B: NHC-BH3 1a (0.3 mmol, 1.5 equiv.), alkenes 4 (0.2
mmol, 1.0 equiv.), Ir(ppy)2dtbbpyPF6 (2 mmol%), MeCN (2 mL), 30 W blue
LED, argon atmosphere, rt, 48 hours.[b] Yield of isolated product.
as well to the disubstituted substrate (3o) and the substrate
containing
a heterocyclic moiety (3p). Pleasingly, the
substitution pattern on the NHC moiety slightly impacted the
reaction efficiency while the ethyl and benzyl analogues (3q
and 3r) were consistently obtained in good yields. At this stage,
we were curious about the performance of other Lewis boranes
in this process and DMAP-BH3, Et3N−BH3, Ph3P−BH3 were
next evaluated. Interestingly, only DMAP-BH3 was proven to
be the viable partner, successfully delivering the desired
product 3s in comparably lower yield. Et3N−BH3 and Ph3P−BH3
only rendered fairly complex reactions and the corresponding
products were undetected.
Subsequently, we shifted our attention to apply this
strategy to the hydroboration of other common alkenes. As
shown in Scheme 3, a range of alkenes worked well to afford
the corresponding boryl products. Regardless of the electron-
donating or electron-withdrawing substituents on the phenyl
moiety, the title reaction proceeded smoothly and consistently
provided the corresponding products in satisfactory yields (5a-
5i). Moreover, the olefin 4j bearing a thiophene ring was also
suitable substrate, giving the desired product 5j in moderate
To gain mechanistic insights into the process, control
experiments were extensively carried out (Scheme 4). The
radical inhibition experiments were conducted by adding
2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO) under the
standard reaction conditions, and the reactions were
significantly inhibited along with the formation of radical
adducts I (detected by HRMS) (Scheme 4a). To reveal more
details of the radical mechanism, the electron paramagnetic
resonance (EPR) was then performed to detect the capture of
the radical species by PBN and DMPO (see Supporting
Information for details). The EPR signals clearly indicate the
formation of spin-adduct products and the corresponding
radical adduct. These results suggest that a NHC-boryl radical
pathway is presumably involved in this photocatalytic
transformation. By replacing 1a with NHC-BD3, the desired
hydroboration product 6 was obtained in 45% yield, with a ratio
of [H] > 99% at the β-position of boron atom (Scheme 4b),
which hints us that a chain propagation-type mechanism was
unlikely involved. To further verify this speculation, light/dark
experiments were also investigated. As predicted, the product
formation was only observed during the periods with constant
irradiation (see Supporting Information for details).
Additionally, we noticed that the reaction was obviously
decelerated by using NHC-BD3, allowing us to speculate that
the electron of the B-H bond was robbed directly by the excited
photocatalysts to deliver the NHC-boryl radical along with the
loss of a proton. Further deuterium-labelling experiment by
adding D2O was performed and the corresponding deuterated
product was detected (Scheme 5c), which provides a solid
support for the source of the proton. When trifluoromethyl
alkylene 7 was used as a radical receptor, 8 and 9 were
obtained simultaneously as a mixture (Scheme 4d).This
information reveals that a carbanion intermediate might be
involved during this transformation.
yield.
Interestingly,
in
the
case
of
gem-
diphenylmethylenecyclopropane 4k, the cyclopropyl group
was unscathed in the desired product 5k (CCDC 1943134).
Not surprisingly, 2-vinylnaphthalene 4l can be hydroborated
smoothly as well, funishing the desired product 5l in 55% yield.
Moreover, it is attractive to observe that the α-borylcarbonyl
molecules and β-borylcarbonyl molecule (5m, 5n vs 5o) were
obtained chemoselectively from the corresponding activated
alkenes, further demonstrating the versatility of this developed
boration protocol.
Scheme 3. Scope with respect to alkenes[a, b]
Scheme 4. Control experiments.
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10.1002/anie.201913398
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catalyzed pathway for generating boryl radical offers a new
dimensionality for the activation of Lewis boranes.
To better discuss the interaction between the LB-BH3 and
the
excited
photocatalyst,
the
Stern-Volmer
plot
measurements for different LB-BH3 were performed (see the
Supporting Information). The excited photocatalyst can be
quenched by NHC-BH3 as well as DMAP-BH3, albeit with a
lower quenching rate constant (kq). The kq value for DMAP-BH3
(kq =7.00 * 103 M-1S-1) is larger than that of NHC-BH3 (kq =1.08
* 103 M-1S-1), but still at the same order of magnitude. The low
order of magnitude of kq value indicates that a reaction-
controlled process might be involved. On the other hand, the
PPh3-BH3 is unable to quench the excited photocatalyst, which
explains why PPh3-BH3 did not give the desired product.
Acknowledgements
We gratefully acknowledge the financial support from the National
Natural Science Foundation of China (21576296, 21776318,
21676302 & 81703365), Natural Science Foundation of Hunan
Province (2017JJ3401), Central South University.
Conflict of interest
Based on the above-mentioned results, Stern−Volmer plot
ox
measurements and the redox potentials of NHC-BH3 (E1/2
+0.34 V, see the Supporting Information),
=
The authors declare on conflict of interest.
a
possible
photoredox cycle is proposed in Scheme 5. Initially, the
excitation of the photocatalyst resulted in the excited
*Ir(ppy)2(dtbbpy)PF6, which was quenched by NHC-BH3 to
generate IrII and radical cation A. The subsequent
deprotonation of the radical cation A produced the NHC-boryl
radical B. Then this highly reactive immediately was rapidly
captured by the alkenes to generate radical species C, which
was then quickly reduced by IrII to form carbanion intermediate
D with the concurrent regeneration of potocatalyst, thus
completing the photocatalytic cycle. In the case of CF3-styrene
Keywords: NHC-borane · SET process · borylation · α-trifluoromethyl alkenes
· alkenes
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In summary, by ingeniously taking advantage of the redox
feature of NHC-BH3, a unique mode of photoinduced SET
process for its direct B-H bond activation has been first
established. This visible-light-driven direct activation mode
offers beneficial features such as streamlined synthetic
process and versatile photochemical reactivities. A wide range
of alkenes as the radical receptors were explored, including α-
trifluoromethylalkenes, alkenes and unsaturated carbonyl
compounds, readily affording a series of boron compounds.
The versatility and scalability of this transformation would
greatly broaden synthetic utilizations of NHC-BH3 in the
domain of photochemistry. More importantly, this photoredox-
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
A unique photoinduced
protocol for the direct B-H
bond activation of NHC-
borane via a SET process
is unveiled for the first
time. A wide range of
radical receptors are
compatible with this
protocol.
Peng-Ju Xia, Dan Song, Zhi-
Peng Ye, Yuan-Zhuo Hu,
Jun-An Xiao, Hao-Yue
Xiang,* Xiao-Qing Chen,*
and Hua Yang*
Page No. – Page No.
Photoinduced Single-
Electron-Transfer Process
as an Enabling Principle in
Radical Borylation of
Alkenes with NHC-boranes
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