Copper-Photocatalyzed Hydroboration of Alkynes and Alkenes

Article abstract of DOI:10.1002/anie.202101874

The photocatalytic hydroboration of alkenes and alkynes is reported. The use of newly-designed copper photocatalysts with B₂Pin₂ permits the formation a boryl radical, which is used for hydroboration of a large panel of alkenes and a

Full text of DOI:10.1002/anie.202101874

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Accepted Article
Title: Copper-Photocatalyzed Hydroboration of Alkynes and Alkenes
Authors: Mingbing Zhong, Yohann Gagné, Taylor Hope, Xavier
Pannecoucke, Mathieu Frenette, Philippe Jubault, and
Thomas Poisson
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10.1002/anie.202101874
Angewandte Chemie International Edition
RESEARCH ARTICLE
Copper-Photocatalyzed Hydroboration of Alkynes and Alkenes.
Mingbing Zhong,[a] Yohann Gagné,[b] Taylor O. Hope,[b] Xavier Pannecoucke,[a] Mathieu Frenette,[b]
Philippe Jubault[a] and Thomas Poisson*[a],[c]
[a]
M. Zhong, Prof. Dr. X. Pannecoucke, Prof. Dr. P. Jubault and Prof. Dr. T. Poisson.
Normandie Univ., INSA Rouen, UNIROUEN, CNRS, COBRA (UMR 6014), 76000 Rouen, France.
E-mail: thomas.poisson@insa-rouen.fr
[b]
[c]
T O. Hope, Y. Gagné and Prof. Dr. M. Frenette.
Département de Chimie, Université du Québec à Montréal, Case postal 8888, Succursale Centre-Ville, Montréal, Québec, H3C 3P8, Canada.
Prof. Dr. T. Poisson.
Institut Universitaire de France, 1 rue Descartes, 75231 Paris, France.
Supporting information for this article is given via a link at the end of the document.
Abstract: The photocatalytic hydroboration of alkenes and alkynes is
reported. The use of newly-designed copper photocatalysts with
B2Pin2 permits the formation a boryl radical, which is used for
the defluoroborylation of gem-difluorinated alkenes,[11] a-
trifluoromethylated styrenes[12],[11a] and perfluoroarenes.[11a] With
respect to the hydroboration reaction, the inverse hydroboration
of imines was reported by Xie and Zhu,[13] while the hydroboration
of a,a-diarylethenes and b,b-diarylacrylates was described by
Xiang, Chen and Yang.[12a] Very recently, Curran reported the 1,4-
hydroboration of electron-deficient aromatic rings.[14] Importantly,
the generation of boryl radicals from a diboron species according
to a B–B cleavage, particularly from the readily available B2Pin2,
is scarce and restricted to few examples.[6],15]
hydroboration of
a large panel of alkenes and alkynes. The
hydroborated products were isolated in high yields, with excellent
diastereoselectivities and a high functional group tolerance under mild
conditions. The hydroboration reactions were developed under
continuous flow conditions to demonstrate their synthetic utility. The
reaction mechanism was studied and suggested an oxidation reaction
between an in situ formed borate and the Cu-photocatalyst in its
excited state for the boryl radical formation.
Photocatalyzed Defluoroborylation
N
N
F
BH2–NHC
Introduction
Ar
Ar
Ir
+
BH3
F
F
Blue LEDs
Organoboron compounds are linchpins in organic synthesis and
focused particular attention over the last decades. These strategic
compounds found applications as versatile building blocks in
several important transformations (e.g. Suzuki-Miyaura cross-
coupling reactions, allylation reactions…)[1] and to some extend in
medicinal chemistry (e.g. Velcade and Dutogliptin). Among the
developed reactions to build up these strategic building blocks,[2]
the celebrated hydroboration reaction, discovered by H. C. Brown
in 1956,[3] is probably the most popular one. It became a textbook
reaction in organic synthesis, that found applications in complex
natural products synthesis or in the elaboration of marketed drugs
(e.g. Nebivolol). Over the last ten years, major advances were
made thanks to the development of efficient methods catalyzed
by noble metals[4] and more recently by non-noble metals,[5] to
promote the addition of borohydride species on alkenes and
alkynes.
Ar
N
N
CF3
F
H or EWG
Ir
+
+
BH3
BH3
H or EWG
Ar
F
BH2–NHC
BH2–NHC
Blue LEDs
N
N
Fn
Ir
Fn-1
Blue LEDs
Photocatalyzed Hydroboration
BH2–NHC
N
Ar2
Ar2
Ir
+
+
BH3
BH3
N
Ar1
Ar1
Ar2
N
N
N
N
H
Blue LEDs
Ar2
H or EWG
BH2–NHC
Ir
H or EWG
Ar1
Ar1
Blue LEDs
Alternatively to these traditional approaches, the use of boryl
radicals recently emerged as a promising approach to broaden
the scope of possible transformations to build up borylated
residues.[6] Key contributions in the field from Curran, Malacria,
Lacôte and Fensternbank[7] demonstrated the high potential of the
NHC-borane species to generate the corresponding boryl radical
from its reaction with a radical initiator (e.g. AIBN or peroxide).
The addition of a boryl radical to an unsaturated C–C bond is a
very recent research area, pioneered by Lalevée,[8] Curran and
Taniguchi,[9] and further extended by others.[6],[10] Very recently,
an important milestone was reached, demonstrating the possible
generation of these NHC-boryl radical using photoredox catalysis
with Iridium complex (Scheme 1). This approach was applied to
EWG
EWG
H
BH2–NHC
EWG
N
N
Ir
+
BH3
EWG
White CFL
This work
R1 R2 or H
BPin
R1
BPin
or
Cu
or
R
R2 or H
R
B2Pin2
Blue LEDs
P Low catalyst loading P High fonctional group tolerance
P High diastereoselectivity P Extension to continuous flow
Scheme 1. Photocatalytic generation of boryl radicals: past and present work.
1
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10.1002/anie.202101874
Angewandte Chemie International Edition
RESEARCH ARTICLE
A. Design Plan - Mechanistic Analysis
B. Optimization of the Cu-Photocatalyzed Hydroboration Reactions
OH
H
a. Copper photocatalyzed hydroboration of terminal alkynes - model reaction.
O
PinB
BPin
PinB BPin
(A)
BPin
(B)
R
SET, - H
Cu-PC-1 (2.5 mol%), B2Pin2 (2 equiv.)
BPin
Ph
H
∆GDFT
- 0.5
Ph
H2O
K2CO3 (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
*Cu(II)Ln
hv
Cu(I)Ln
∆GDFT
- 34.5
1, 90%, 99:1 (E:Z)
•BPin (C) PinB OH
+
Cu(I)Ln
∆G DFT = 11.4
H
b. Copper photocatalyzed hydroboration of internal alkynes - model reaction.
R
∆GDFT = - 38.3
Cu-PC-2 (2.5 mol%), B2Pin2 (2 equiv.)
BPin
OH
Ph
SET, + H
BPin
Ph
Ph
H
PinB BPin
(A)
R
tBuOK (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
Ph
BPin
(D)
R
2, 85%, 1:14 (E:Z)
C. Newly Designed Copper-Based Photocatalysts
c. Copper photocatalyzed hydroboration of alkenes - model reaction.
NEt
2
λabs = 397 nm (ε = 2607 M-1cm-1
E0 = (CuI/Cu0) = - 1.56 V
)
Et2N
λabs = 330 nm
Ph2
P
E0 = (CuI/Cu0) = - 1.81 V
E0 = (CuII/CuI) = + 0.83 V
E0 = (CuI*/Cu0) = + 1.34 V
E0 = (CuII/CuI*) = - 2.37 V
P
N
N
N
N
Cu-PC-1 (2.5 mol%), B2Pin2 (2 equiv.)
HN
E0 = (CuII/CuI) = + 1.02 V
Cu
O
P
Et2N
Cu
O
H
BPin
3, 97%
P
E0 = (CuI*/Cu0) = + 1.21 V
E0 = (CuII/CuI*) = - 1.75 V
Ph
Ph
NEt2
PF6
Ph2
PF6
K2CO3 (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
Cu-PC-1
Cu-PC-2
Scheme 2. A. Design plan and mechanistic analysis with DFT-calculated reaction energies in kcal/mol (R = Ph) for the proposed chain reaction. B. Development
of the Cu-photocatalyzed hydroboration reactions. C. Newly designed Cu-photocatalysts Cu-PC-1 and Cu-PC-2. Potentials are reported vs SCE.
Besides, the advent of photocatalysis using visible light expended
possibilities to generate radicals and culminated in the discovery
of new reaction manifolds.[16],[17] As part of our program to develop
new transformations using copper and copper-photocatalysts as
inexpensive and sustainable alternatives to noble metal-based
photocatalysts (Ir, Ru…),[18] we sought to develop an original and
straightforward strategy to generate boryl radicals using the
inexpensive and bench stable B2Pin2, under visible light
irradiation. Herein, we present a copper-photocatalyzed formation
of a boryl radical (i.e. •BPin) from B2Pin2, for the hydroboration of
alkynes and alkenes with high levels of chemo-, regio- and
stereoselectivities.
To ascertain our hypothesis, we studied the reaction of
phenylacetylene with B2Pin2 in the presence of Cu-
a
photocatalyst, a base in a CH3CN/H2O mixture under blue light
irradiation (405 nm LEDs). After an extensive set of optimization
reactions,[20] we found that 2.5 mol% of the newly designed
complex Cu-PC-1 in the presence of K2CO3 as a base in a 9:1
mixture of CH3CN/H2O furnished the hydroborated product 1 in
an excellent 90% yield and a complete E selectivity (Scheme 2B,
eq. a). The hydroboration reaction with diphenyl acetylene was
also developed using slightly modified conditions (Scheme 2B, eq.
b). The use of the novel catalyst Cu-PC-2 (2.5 mol%) with tBuOK
as a base under blue light irradiation gave the hydroborated
product 2 in 85% yield and a 14:1 E/Z selectivity. Finally, the
optimized conditions used for the reaction with phenylacetylene
permitted the hydroboration of styrene, providing the alkyl boronic
ester 3 in 97% yield (Scheme 2B, eq. c). In all cases, control
experiments demonstrated that the reactions occurred exclusively
in the presence of the Cu-catalyst and light, precluding a reaction
Results and Discussion
To address this novel reaction manifold, we conjectured that the
cleavage of the B–B bond of the B2Pin2 could be readily achieved
through the in situ formation of a borate intermediate [B2Pin2]OH¯
(A), in the presence of a Lewis base (e.g. HO-), according to an
oxidation reaction (Scheme 2A). Such a proposal, conceptually
related to the oxidation of NHC-borane,[7] might lead to the
generation of the corresponding boryl radical. According to this
scenario, a well-designed Cu-photocatalyst in its excited state
would accept an electron from the borate species A. This
oxidation followed by proton loss will promote the B–B bond
cleavage and form radical anion B. Species B would be in
equilibrium with a putative boryl radical C which could itself be in
equilibrium with available Lewis bases, e.g., MeCN.20 BPin radical
C could add on an unsaturated C–C bond (e.g. alkene or alkyne)
forming a new carbon centered radical D. DFT calculations predict
the possible addition of boryl radicals to alkenes and alkynes to
be kinetically and thermodynamically favorable.[19] The final
product might be formed by two proposed paths from radical
species D: (left path) SET from the reduced Cu-photocatalyst,
closing the photoredox catalytic cycle or (right path) by reacting
with borate species A. The right path will regenerate radical anion
B as the boryl radical precursor that can continue a chain reaction
is globally favorable according to DFT calculations.[20]
catalyzed by
a
Cu-species resulting from
a
ligands
reorganization.[20]
With these optimized protocols in hand, we explored the scope of
the reactions (Scheme 3). First, a large panel of aromatic
substituted terminal alkynes was tested. The reaction proved to
be tolerant to a broad range of halogens and functional groups
(electron-donating and electron-withdrawing), whatever the
substitution pattern and the hydroborated products were isolated
in good to excellent yields and perfect diastereoselectivities in
favor of the E-isomers (4-30). The reaction did not require the
protection of alcohols and amines (9, 10 & 12). The compatibility
of the method with various functional groups highlights this
process as orthogonal to classical transformations to from C–C
and C–heteroatom bonds. Heterocyclic derivatives were also
suitable substrates, giving the vinyl boronic esters 21 and 22 in
good yields. Alkyl-substituted alkynes were reacted and the
corresponding hydroborated products (23-27) were obtained in
good to excellent yields. Interestingly, the presence of a iodine
substituent was well tolerated (28 & 29) as no reduction of the C–
I bond was observed.[18b]
2
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10.1002/anie.202101874
Angewandte Chemie International Edition
RESEARCH ARTICLE
Cu-Photocatalyzed hydroboration of terminal alkynes
Cu-PC-1 (2.5 mol%), B2Pin2 (2 equiv.)
BPin
R
H
R
K2CO3 (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
BPin
BPin
BPin
BPin
BPin
BPin
MeO
BPin
MeO
1, 87%, 91%[a],[b]
4, 90%
5, 92%
6, 89%
7, 92%
8, 93%
78%[a],[c]
BPin
BPin
H2N
BPin
BPin
BPin
HO
BPin
H2N
S
F
9, 78%, (17:1)[d]
BPin
10, 75%
11, 91%
12, 92%
13, 77%
14, 69%
BPin
BPin
BPin
F
BPin Cl
BPin
F3C
Cl
Br
MeOOC
HO
15, 77%
16, 83%
17, 71%
18, 85%
24, 65%
19, 80%
20, 77%
26, 75%
BPin
BPin
BPin
BPin
BPin
S
N
22, 88%, (7:1)[d]
21, 72%
23, 79%
25, 91%
HO
BPin
O
BPin
O
BPin
BPin
Bdan
I
I
31, 91%[e]
27, 72%
28, 92%
29, 90%
30, 80%
Cu-Photocatalyzed hydroboration of internal alkynes
Cu-PC-2 (2.5 mol%), B2Pin2 (2 equiv.)
BPin
Ar
Ar
R
tBuOK (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
R
BPin
Bdan
BPin
BPin
BPin
Ph
34, 88%
Cu-Photocatalyzed hydroboration of alkenes
Ph
Ph
n-Pr
NC
2, 85%, (14:1)[d]
32, 81%
33, 89%
35, 78%[e]
82%[a],[f]
Cu-PC-1 (2.5 mol%), B2Pin2 (2 equiv.)
H
BPin
R1
R1
R2
R2
K2CO3 (2 equiv.), CH3CN/H2O (9:1)
405 nm, argon, 7 h
BPin
BPin
BPin
BPin
BPin
BPin
BPin
MeO
NC
Ph
F
3, 97%
36, 95%
37, 90%
42, 99%
47, 85%
38, 97%
39, 90%
BPin
BPin
BPin
BPin
Cl
Br
41, 95%, 99%[a],[f]
43, 71%, 89%[a],[f]
40, 91%
44, 81%
BPin
BPin
BPin
BPin
48, 39%, 61%[a],[b]
Ph BPin
49, 79%, 85%[a],[f]
45, 43%
46, 60%
Bdan
BPin
BPin
BPin
PhO2S
51, 83%
BnO2C
52, 95%
HO
O
54, 95%[e]
50, 84%
53, 88%
Scheme 3. Applications to the hydroboration of alkynes and alkenes. [a] Reaction was performed in continuous flow, see supporting information for details. [b] 0.2
mmol scale. [c] 20 mmol scale. [d] E:Z ratio. [e] PinB–Bdan was used instead of B2Pin2. [f] 10 mmol scale.
3
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10.1002/anie.202101874
Angewandte Chemie International Edition
RESEARCH ARTICLE
A. Control experiments
B. Kinetic isotopic effect & effect of HOBPin as an additive
Ph
Ph
H or D
Standard Conditions
without B2Pin2
Standard
eq.1
eq.2
or
+ H BPin
no reaction
BPin
B2Pin2
Me +
Ph
Ph
Conditions
Me
Standard
Conditions
tBu
KIE = 5.5
O
in CH3CN/H2O (blue)
in CH3CN/D2O (red)
B2Pin2
BPin
+
N
BPin
Ph
O
TEMPO or BHT
tBu
in CH3CN/H2O + 2 equiv of HOBPin (green)
no reaction
Detected by GC/MS & HRMS
H
D
40%
35%
30%
25%
20%
15%
10%
5%
Standard
Ph
Me
BPin
BPin
Me
eq.3
+
+
Ph
Ph
Conditions
B2Pin2
Me
CH3CN/H2O
CH3CN/D2O
CD3CN/H2O
81% yield
14% yield
80% yield
:
:
:
100
27
100
0
73
0
K[B2Pin2]OtBu
or
K[B2Pin2]OH
Cu-PC-1 (2.5 mol%)
BPin
eq.4 Ph
+
Ph
0%
CH3CN/H2O (9:1)
405 nm, argon, 7 h
0
10
20
30
40
50
60
70
80
90
K[B2Pin2]OtBu, 39%
K[B2Pin2]OH, 66%
2 equiv
D. Quantum yields determination
C. CV measurements
B2Pin2 (green)
B2Pin2 + KOH (orange)
Standard
BPin
BPin
B2Pin2
Ph
+
Ph
Conditions
Φ = 0.31
1
Standard
Ph
Ph
B2Pin2
B2Pin2
Ph
Ph +
Ph
Conditions
Φ = 0.18
2
Standard
BPin
+
Ph
Conditions
Φ = 0.48
3
Scheme 4. Mechanistic study. A. Control experiments. B. Kinetic isotopic effect and the kinetic effect of HOBPin as an additive. C. Cyclic voltammetry measurements.
D. Quantum yields determination.
The reaction was selective to the alkyne motif as no reaction on
the olefin was observed when an enyne was reacted (30). With
respect to the hydroboration of internal alkynes, the reaction
proceeded well, with a complete diastereoselectivity (32-35)
offering an interesting approach to versatile building blocks. Then,
the reaction was developed on alkenes using the optimized
conditions (Scheme 2B, eq. c). Various styrene derivatives were
readily converted into the corresponding hydroborated products
(36-42) in excellent yields and functional group tolerance. Then,
alkyl substituted alkenes as well as cyclic ones were subjected to
the standard conditions, affording the boronic esters 43-50 in
moderate to excellent yields. The reaction on electron deficient
alkenes (i.e. Michael acceptor) provided the b-borylated sulfone,
ester and ketone (51-53) in excellent yields.[21]
Interestingly, our methodology could be extended to the formation
of the synthetically useful Bdan (1,8-diaminonaphtylboramide)
derivatives by using the PinB–Bdan reagent instead of B2Pin2.
Under the standard reaction conditions, the synthesis of 31, 35
and 54 was achieved in excellent yields and stereoselectivities.
To showcase the synthetic utility and the versatility of our new
methodologies we sought to develop a protocol under continuous
flow conditions. Indeed, photocatalyzed processes often suffer
from limitations regarding the increase of the reaction scale.[22]
The developed protocols were applied to the hydroboration of 1,
2, 41, 43, and 49 with similar yields on 10 or 20 mmol scale.
Interestingly, flow conditions applied to 43 and 48 allowed a
significant increase of the reaction yields, from 71% and 39% to
89% and 61%, respectively.
Finally, to get a better understanding on the mechanism of the
Cu-photocatalyzed hydroboration reaction, control experiments
were conducted (Scheme 4A). First, the reaction of styrene or
phenyl acetylene with H-BPin, instead of B2Pin2, gave no product,
showing that H-Bpin is not a reactive species in the process (eq.1).
Then, the addition of TEMPO (5 equiv) and BHT to the reaction
of styrene with B2Pin2 was performed, giving no desired product
in both reactions. However, we have been able to detect in both
reactions the products resulting from the trapping of the putative
boryl radical by TEMPO and BHT (eq. 2).[20] These two reactions
support the possible involvement of a boryl radical species during
the process. The hydroboration of phenylpropyne was performed
under standard conditions with either CD3CN or D2O as the co-
solvent (eq. 3). The deuterated product was only observed when
D2O was used instead of H2O, demonstrating that the H atom is
coming from H2O.[23] However, taking into account the Bond
Dissociation Energy (BDE) of both acetonitrile and water, a direct
H abstraction from the solvent is unlikely,[24] hence these results
could support a possible H abstraction from a transient borate
species. Subsequently, we studied the kinetic isotopic effect (KIE,
4
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10.1002/anie.202101874
Angewandte Chemie International Edition
RESEARCH ARTICLE
Scheme 4B). Parallel reactions revealed
a KIE = 5.5,
INSA Rouen Normandie, Labex SynOrg (ANR-11-LABX-0029)
and Innovation Chimie Carnot (I2C). M.Z. thanks the China
Scholarship Council (CSC) for a doctoral fellowship. T.P. thanks
the Institut Universitaire de France (IUF) for support and the
Agence National pour la Recherche (ANR-CE07-0004-1) for
funding. M.F. and Y.G. acknowledge NSERC for funding in the
form of a Discovery Grant (RGPIN-2016-06773) and a graduate
scholarship, respectively. We are grateful for technical support
from NanoQAM and to Gaussian and Compute Canada
(www.computecanada.ca) for computational resources. Mrs.
Rosy Valdez is warmly acknowledged for preliminary
investigations regarding the batch to flow extension of the
reactions. Dr. M. Durandetti is gratefully acknowledged for her
help regarding CV measurements. Dr. T. Gallavardin is gratefully
acknowledged for his help regarding UV/Vis measurements.
demonstrating that the H abstraction after the boryl radical
addition to the alkyne is the rate determining step. The precise
species responsible for the rate limiting H-atom donation to the
carbon-centered radical is still unknown. Using DFT calculations,
we obtained activation energies that were prohibitively high (>25
kcal/mol) for H-atom transfers from several boron candidates,
however, the reaction is thermodynamically favored using borate
A as shown in Scheme 2A. Interestingly, the addition of HOBPin
(2 equiv.) to the reaction media provided a significant increase of
the reaction rate.[20] This result confirms a plausible H abstraction
from a transient boron species formed in the reaction media. Then,
to demonstrate the involvement of a borate intermediate,
reactions were carried out with the preformed K[B2Pin2]OtBu and
K[B2Pin2]OH in the absence of base (eq. 4). Pleasingly, the
hydroborated product was isolated in 39% and 66% yield,
respectively. These results highlight the involvement of the borate
species along with its ability to react under our standard
conditions.
Keywords: Photocatalysis • Copper • Boryl Radical •
Hydroboration • Continuous Flow
Moreover, the analysis of the B2Pin2:KOH mixture (1:1) in MeCN
by cyclic voltammetry showcased an irreversible oxidation wave
[1]
For selected reviews, see: a) Carreras, A. Caballero, P. J. Pérez, Chem.
Asian J. 2019, 14, 329–343; b) J. W. B. Fyfe, A. J. B. Watson, Chem
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2483; d) L. Xu, S. Zhang, P. Li, Chem. Soc. Rev. 2015, 44, 8848–8858;
e) A. J. J. Lennox, G. C. Lloyd-Jones, Chem. Soc. Rev. 2014, 43, 412–
443; f) G. Duret, R. Quinlan, P. Bisseret, N. Blanchard, Chem. Sci. 2015,
6, 5366–5382; g) C. Ollivier, P. Renaud, Chem. Rev. 2001, 101, 3415–
3434; h) S. Namirembe, J. P. Morken, Chem. Soc. Rev. 2019, 48, 3464–
3474. For the importance of boron-containing building blocks in medicinal
chemistry, see: i) S. D. Roughley, A. M. Jordan, J. Med. Chem. 2011, 54,
3451–3479.
o
(E1/2 = +0.25 V vs SCE, Scheme 4C).[20] This observation
demonstrated the possibility of the borate species K[B2Pin2]OH to
be oxidized by the catalyst in its excited state (vide supra) and
support its involvement in the formation of the boryl radical (•BPin).
Additional light ON-OFF reactions suggested that the formation of
1 occurred only in the presence of light.[20] To evaluate the impact
of chain reactions on the mechanism, quantum yields (F) of the
reactions were determined (Scheme 4D).[25] The obtained values
of 0.31, 0.18 and 0.48 corroborate a photocatalytic process that
most likely includes short lived chain propagation as shown in
Scheme 2A. Nanosecond laser-flash photolysis experiments of
the copper photocatalyst Cu-PC-1 using a 355 nm laser pulse
gave no clear transient absorption signals (data not shown).
Combined with very weak emission spectra, these observations
indicate that Cu-PC-1 has a short-lived excited state (<10 ns).
While the photocatalyst’s short lifetime will limit the efficiency of
electron transfer reactions with neighboring molecules, the
possibility of a chain length increases the photon efficiency of this
reaction.
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In conclusion, we reported herein the unprecedented
photocatalytic hydroboration reactions using B2Pin2 as alternative
to NHC-borane species, by means of original copper
photocatalysts. The reaction was applied to a broad range of
alkynes and alkenes with an excellent functional group tolerance.
The methodology was extended to continuous flow conditions to
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RESEARCH ARTICLE
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6
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10.1002/anie.202101874
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RESEARCH ARTICLE
Entry for the Table of Contents
Insert graphic for Table of Contents here.
R1
R2
Cu-Photocatalyst
B2Pin2
BPin
R1
BPin
or
or
R
R2
Blue LEDs
R
P Inexpensive copper catalyst
P Low catalyst loading
P 54 Examples
P High diastereoselectivities
P High functional group tolerance P Mechanistic study
P Extension to continuous flow P Boryl radical intermediate
We report herein the photocatalytic hydroboration of alkenes and alkynes using newly-designed copper photocatalysts with B2Pin2.
The hydroborated products were obtained under mild conditions in high yields, excellent diastereoselectivities, a high functional
group tolerance and developed under continuous flow conditions to demonstrate their synthetic utility. Finally, the reaction
mechanism was also studied.
7
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