An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition: Construction of gem-Bis(boryl)alkanes

Article abstract of DOI:10.1002/anie.201903721

A series of in situ formed alkenyl diboronate complexes from alkenyl Grignard reagents (commercially available or prepared from alkenyl bromides and Mg) with B₂Pin₂ (bis(pinacolato)diboron) react with diverse alkyl halides by a Ru photocatalyst to give various gem-bis(boryl)alkanes. Alkyl radicals add efficiently to the alkenyl diboronate complexes, and the adduct radical anions undergo radical-polar crossover, specifically, a 1,2-boryl-anion shift from boron to the α-carbon sp² center. This transformation shows good functional-group compatibility and can serve as a powerful synthetic tool for late-stage functionalization in complex compounds. Measurements of the quantum yield reveal that a radical-chain mechanism is operative in which the alkenyl diboronates acts as reductive quencher for the excited state of the photocatalyst.

Full text of DOI:10.1002/anie.201903721

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Accepted Article
Title: An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition for
the Construction of gem-Bis(boryl)alkanes
Authors: Zhuangzhi Shi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903721
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10.1002/anie.201903721
Angewandte Chemie International Edition
Photocatalysis
C−N Bond Activation
An Olefinic 1,2-Boryl-Migration Enabled by Radical Addition:
Construction of gem-Bis(boryl)alkanes
Binlin Zhao+, Zexian Li+, Yixiao Wu, Yandong Wang, Jiasheng Qian, Yu Yuan, and Zhuangzhi Shi*
Abstract: A series of in situ-formed alkenyl diboronate complexes
from alkenyl Grignard reagents (commercially available or
prepared from alkenyl bromides and Mg) with B2Pin2
(bis(pinacolato)diboron) reacting with diverse alkyl halides by a Ru
photocatalyst to give various gem-bis(boryl)alkanes is reported.
Alkyl radicals add efficiently to the alkenyl diboronate complexes,
and the adduct radical anions undergo radical-polar crossover,
namely, a 1,2-boryl-anion shift from boron to the α-carbon sp2
center. This transformation shows good functional group
compatibility and can serve as a powerful synthetic tool for late-
stage functionalization in complex compounds. Measurements of the
quantum yield reveal that a radical chain mechanism is operative in
which the alkenyl diboronates acts as reductive quencher for the
excited state of the photocatalyst.
Alkylboronic acids and their esters are of paramount importance to
all facets of chemical science and have a broad spectrum of
applications ranging from material science to drug discovery and
organic synthesis.[1] Traditional methods for accessing these
compounds by hydroboration of alkenes[2] or nucleophilic addition
to Grignard or organolithium reagents with electrophilic boron
species are well developed,[3] and recently emerging methods
involving cross-couplings, radicals, and rearrangements promise to
provide expanded access to these motifs.[4] Within this general
context, 1,2-metalate rearrangement of boronate complexes has been
developed[5] with several notable advances reported in the past few
years.[6] In particular, the rearrangement of vinylboronate complexes
for the generation of alkylboronates has attracted considerable
attention from many research groups (Figure 1a). As early as 1967,
Zweifel and coworkers for the first time demonstrated the 1,2-
alkyl/aryl migration by electrophilic halogenation of vinylboronate
complexes.[7] Half a century later, Morken et al. reported an
important breakthrough for 1,2-alkyl/aryl migration[8] by
carbopalladation of vinylboronate complexes with different coupling
partners.[8a] In 2017, three groups independently extended this
elegant chemistry to the radical-polar crossover process using vinyl
boronate complexes and alkyl halides triggered by Et3B/air,[9]
visible-light[10] or nickel catalysts.[11]
Figure 1. 1,2-Metalate rearrangement of alkenyl boronate
complexes.
aldehydes and ketones[15]. Very recently, Ingleson and coworkers
demonstrated a 1,2-metalate rearrangement of a vinyl diboronate
complex induced by soft boranes including BPh3 and 9-Ph-BBN
(Figure 1b).[16] This transformation is conceptually related to the
Zweifel reaction, but the use of such boron “ate” complex can make
Bpin act as a migrating group. Notably, this Lewis acid-induced
reaction was sensitive to the steric hindrance, and the addition of
BPh3 to a complex derived from 1-propenylmagnesium bromide and
B2Pin2 led in trace amount of the intramolecular rearrangement
product. Inspired by radical-polar crossover and photochemistry[17-
18], herein, we report a mild and operationally simple strategy for the
rapid construction of gem-bis(boryl)alkanes through photocatalyzed,
visible light-mediated addition of alkenyl diboronate complexes
with alkyl halides and the subsequent cascade 1,2-boryl migration,
greatly expanding the practicality of these boron complexes (Figure
1c). Salient features of our findings include a broad range of alkenyl
diboronate complexes, diverse alkyl halides with good functional
group compatibility, and late-stage diversification nature products.
Among various organoboron compounds, gem-bis(boryl)alkanes
have drawn increasing attention in recent years due to their unique
reactivity in both catalytic and noncatalytic processes to access
value-added chemicals.[12] The 1,2-metalate rearrangement to such
products has been demonstrated,[13] either with N2 as the leaving
group in the gem-diborylation of carbenoids[14] or with OBpin (Bpin
= pinacolborane) as the leaving group in the gem-diborylation of
[]
B. Zhao[+], Z. Li[+], Y. Wang, and Prof. Dr. Z. Shi
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, China
For an initial examination of the 1,2-boryl migration reaction,
[(vinyl)B2Pin2)]MgBr (3a) was first generated in situ by treating the
vinyl magnesium bromide (1a) and B2pin2 (2a) in THF
(tetrahydrofuran) at -78 °C for two hours, as confirmed by 11B NMR
spectroscopy (broad signal at 37.4 ppm for the sp2 boron and a sharp
peak at 6.1 ppm for the sp3 boron). After solvent removal without
any further purification, a solution of commercially available
substrate 2-bromoacetophenone (4a) and the photocatalyst were
added to the vessel and the mixture was irradiated with visible-light
irradiation (Table 1). The reaction was found to be facile, with 1.0
equiv of 4a, 2.0 equiv of 3a, 0.5 equiv of TBAB
E-mail: shiz@nju.edu.cn
Y. Wu, J. Qian, and Prof. Dr. Yu Yuan
College of Chemistry and Chemical Engineering, Yangzhou
University, Yangzhou 225002, China
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification
number(s) for the author(s) for this article can be found under
http://www.angewandte.org.
1
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10.1002/anie.201903721
Angewandte Chemie International Edition
Table 1. Reaction development.[a]
Table 2. Substrate scope of alkyl halides.[a]
Yield of
Entry
Variation from the standard conditions
5aa (%)[b]
1
2
none
PC = Ru(bpy)3Cl2
PC = Ir(ppy)3
78 (75[c])
65
8
3
4
PC = Ir(ppy)2(dtbbpy)PF6
-
48
0
5
6
in the dark
0
7
in THF
66
68
69
52
54
70
8
Without KI
9
Without TBAB
Without KI and TBAB
1a/2a/4a = 1:1.1:1
1a/2a/4a = 1:1.1:2
10
11
12
[a] Reaction conditions: 1a (0.40 mmol, 2.0 equiv), 2a (0.44 mmol, 2.2
equiv), in THF (0.5 mL), -78 °C, 2 h, under Ar; then 1 mol% of
Ru(bpy)3(PF6)2, 4a (0.20 mmol, 1.0 equiv), rt , 24 h, in CH3CN, blue
LEDs. [b] Determined by 1H NMR analysis using CH2Br2 as an
internal standard. [c] Isolated yield.
(tetrabutylammonium bromide) and 1.0 equiv of KI in the presence
of Ru(bpy)3(PF6)2 (1 mol %), in MeCN after 24 h at room
temperature affording the 1,2-boryl migration product 5aa in 78%
yield (entry 1). Other photocatalysts such as Ru(bpy)3Cl2, Ir(ppy)3
and Ir(ppy)2(dtbbpy)PF6 were also effective for this transformation
albeit with lower yields (entries 2-4). Control experiments showed
that the reaction could not be conducted in the absence of the
photocatalyst (entry 5) and only under irradiation with blue LEDs
(entry 6). If the THF was not removed for the second step, only 66%
yield of 5aa could be formed (entry 7). Probably due to the in situ
formed alkyl iodides from the corresponding bromides with KI
using TBAB as a phase-transfer catalyst in this system, subjecting
4a without KI or/and TBAB in reaction conditions would lower the
yields of this transformation (entries 8-10), Finally changing the
ratio of the three components 1a, 2a and 4a to 1:1.1:1 (entry 11) or
1:1.1:2 (entry 12) decreased the yield as well.
With the optimized reaction conditions in hand, we next
examined the scope of this 1,2-boryl-migration reaction (Table 2).
Cross-coupling reactions of in situ-formed complex 3a with many
alkyl halides
4 were first examined. In most cases, this
rearrangement reaction proceeded efficiently to give the
corresponding gem-bis(boryl)alkanes in moderate to good yields. 2-
bromoacetophenone derivatives 4b-f and 2-acetonaphthone 4g
proved to be good substrates, leading to the corresponding ketones
5ab-5ag in 67-80% yields. Moreover, aliphatic ketones 4h-I were
also successfully employed. In particular, primary (4j), secondary
(4k-l) and tertiary (4m-n) aliphatic esters could all be employed as
the radical precursors, showing that the reaction tolerated the full
spectrum of steric demand. Substrates bearing amide (4o-p) and
cyano groups (4q) could also be utilized in this photoredox protocol.
Besides that, the reaction of 2-(iodomethyl)benzo[d]oxazole (4r)
with complex 3a also gave the migration product 5ar in modest
yield. Fluorine-containing compounds have been widely used in
biologically active compounds and functional materials. All mono-,
di-, and multi-fluoroalkylation processes were compatible with this
transformation (5as-v).
[a] 1 (0.40 mmol, 2.0 equiv), 2 (0.44 mmol, 2.2 equiv), in THF (0.5
mL), -78 °C, 2 h, under Ar; then 1 mol% of Ru(bpy)3(PF6)2, 4 or 6
(0.20 mmol, 1.0 equiv), TBAB (0.10 mmol, 0.5 equiv), KI (0.2 mmol,
1.0 equiv), rt , 24 h, in CH3CN, blue LEDs, isolated yield; [b] 1 (0.20
mmol, 1.0 equiv), 2 (0.22 mmol, 1.1 equiv), in THF (0.3 mL), -78 °C, 2
h, under Ar; then 1 mol% of Ru(bpy)3(PF6)2, 4 (0.60 mmol, 3.0 equiv),
TBAB (0.10 mmol, 0.5 equiv), KI (0.2 mmol, 1.0 equiv), rt , 24 h, in
CH3CN, blue LEDs, isolated yield; [c] Using iodoalkanes 4, without
TBAB and KI.
Having identified complex 3a as an ideal reagent for the
generation of gem-bis(boryl)alkanes under exceptionally mild of this
2
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10.1002/anie.201903721
Angewandte Chemie International Edition
catalytic system. Toward this aim, the derivatives of
pharmaceuticals and natural products that contain an alkyl bromide
reaction conditions, we decided to demonstrate the synthetic utility
were selected as the coupling partners for the olefinic 1,2-boryl-
migration. For example, drostanolone derivative 6a with a sensitive
chiral α-carbonyl methyl group proceeded readily to produce 7aa
without any epimerization. Nopol derivative 6b with a double bond
could also be tolerated (67% yield). Derivatives of (+)-α-Tocopherol,
D-glucose and lanosterol were shown to be compatible with our
system (7ac-ae). Mestranol derivative 6f with a terminal alkyne
group could be transformed to provide gem-bis(boryl)alkane 7af in
47% yield. Secondary amide 6g derived from rosin amine proceeded
smoothly to afford the desired product 7ag in 66% yield. Finally, to
further highlight the robustness of this strategy, stereospecific
functionalization at α position of estradiene (6h) was achieved
product 7ah in modest yield.
could react with Mg and B2pin2 (2a), and further coupled with 4a
under the standard conditions, providing the corresponding product
5fa-ga in 59% and 53% yield, respectively. The current method is
noteworthy with regard to the bulky complexes 3h-i from alkenyl
bromide 1h'-i', and the products 5ha-ia formed in modest yields.
Given the versatility and robustness features of this chemistry, we
were intrigued to delineate its mode of action. Quantum yield
measurements of the reaction between 3a and iodoacetonitrile (4q)
gave a value of Φ = 49.8[19] (see Supporting Information),
suggesting that a radical chain mechanism was operative.[20]
A
possible mechanism is proposed in Figure 2. The highly reducing
Ru+ is generated by single electron transfer (SET) from a sacrificial
amount of the [(vinyl)B2Pin2)]MgBr (3a, Ep/2 = +0.21 V vs. SCE) to
the excited Ru2+ species (ERu
= + 0.77 V vs. SCE), and the
II*/Ru
I
byproduct 3a′ was observed in the crude reaction mixtures.[21] Then,
the Ru+ (ERu = -1.33 V vs. SCE) species undergoes single electron
I/II
To further explore the scope of this rearrangement, other alkenyl
diboronate complexes were then investigated (Scheme 1a). The
complex derived from alkenyl magnesium chlorides such as 1a'
could afford the desired product 5aa in a slightly lower yield. The
effect of increasing steric hindrance at the β-vinylic carbon atom by
using the adduct 3b in situ generated from Grignard reagent 1b and
B2pin2 (2a) was also explored. To our delight, the subsequent
reaction of 2-bromoacetophenone (4a) with 3b under this system
could give the desired 1,2-boryl migration product 5ba in 43%
yield. Furthermore, α-substituted alkenyl Grignard reagents such as
isopropenylmagnesium bromide (1c) could be employed to produce
an internal geminal bis(boronate) 5ca in 65% yield. Of note, the
complex 3d from di-substituted alkenyl Grignard reagents 1d was
tolerable as well. However, a bulky β, β'-dimethyl substituted
Grignard reagent 1e was failed in this reaction. Besides these
commercially available Grignard reagents, our studies also
investigated the subjecting “ate” complexes derived from alkenyl
bromides and Mg (Scheme 1b). Alkenyl bromides bearing
substituents like ethyl (1f') and phenyl (1g') at α-vinylic position
transfer with alkyl halides 4/6 (for 4q, Ep/2 = -1.24 V vs. SCE)
leading to the formation of the reactive electrophilic radical A and
the regeneration of the Ru2+ catalyst. Radical A undergoes addition
to the electron-rich complex 3a to form boronate radical B. This
electron-rich radical anion then undergoes facile single-electron
oxidation with another molecule of alkyl halides 4/6, thus triggering
a rapid 1,2-boryl migration to afford the products 5/7. The process
also releases a new alkyl radical A, thereby feeding a radical chain
propagation pathway.[22]
Figure 2. Plausible mechanism.
In summary, we show here that carbon radicals add efficiently to
the alkenyl diboronate complexes leading to a 1,2-boryl migration
from boron to the α-carbon sp2 center. Together, these synthesis
methods cover a vast substrate scope providing a novel and efficient
multi-component coupling reaction based on ready accessible
alkenyl magnesium bromides, B2Pin2 and alkyl halides. Owing to
this impressive pool of gem-bis(boryl)alkanes that bear diverse
functionalities, our efficient and cost-effective technique promises to
be a powerful and reliable tool for total synthesis and development.
Acknowledgments
We thank the “1000-Youth Talents Plan” and the NSF of China
(Grants 2167020084) for their financial support and the “Innovation
& Entrepreneurship Talents Plan” of Jiangsu Province. This work
was also supported by a program A for Outstanding PhD candidate
of Nanjing University.
Keywords:
Rearrangement
·gem-
diborylation ·radical ·photocatalysis ·crossover
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10.1002/anie.201903721
Angewandte Chemie International Edition
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10.1002/anie.201903721
Angewandte Chemie International Edition
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Photocatalysis
Binlin Zhao, Zexian Li, Yixiao Wu,
Yandong Wang, Jiasheng Qian, Yu
Yuan, and Zhuangzhi Shi*___ Page –
Page
An Olefinic 1,2-Boryl-Migration Enabled
by Radical Addition: Construction of
gem-Bis(boryl)alkanes
B & Mg: A mild catalytic system was developed for the preparation of gem-
bis(boryl)alkanes via an olefinic 1,2-boryl-migration. This method has good
functional group compatibility and can serve as a powerful synthetic tool for late-
stage functionalization in complex compounds.
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