Visible-Light-Induced Radical Carbo-Cyclization/ gem-Diborylation through Triplet Energy Transfer between a Gold Catalyst and Aryl Iodides

Article abstract of DOI:10.1021/jacs.0c03197

Geminal diboronates have attracted significant attention because of their unique structures and reactivity. However, benzofuran-, indole-, and benzothiophene-based benzylic gem-diboronates, building blocks for biologically relevant compounds, are unknown. A promising protocol using visible light and aryl iodides for constructing valuable building blocks, including benzofuran-, indole-, and benzothiophene-based benzylic gem-diboronates, via radical carbo-cyclization/gem-diborylation of alkynes with a high functional group tolerance is presented. The utility of these gem-diboronates has been demonstrated by a 10 g scale conversion, by versatile transformations, by including the synthesis of approved drug scaffolds and two approved drugs, and even by polymer synthesis. The mechanistic investigation indicates that the merging of the dinuclear gold catalyst (photoexcitation by 315-400 nm UVA light) with Na2CO3 is directly responsible for photosensitization of aryl iodides (photoexcitation by 254 nm UV light) with blue LED light (410-490 nm, λmax = 465 nm) through an energy transfer (EnT) process, followed by homolytic cleavage of the C-I bond in the aryl iodide substrates.

Full text of DOI:10.1021/jacs.0c03197

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Article
Visible Light-Induced Radical Carbo-Cyclization/gem-Diborylation Through
Triplet Energy Transfer between a Gold Catalysts and Aryl Iodides
Lumin Zhang, Xiaojia Si, Frank Rominger, and A. Stephen K. Hashmi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2020
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Journal of the American Chemical Society
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Visible Light-Induced Radical Carbo-Cyclization/gem-
Diborylation Through Triplet Energy Transfer between a Gold
Catalysts and Aryl Iodides
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Lumin Zhang,†,‡ Xiaojia Si,†,‡ Frank Rominger,† and A. Stephen K. Hashmi†,‡*
†Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
‡ Chemistry Department, Faculty of Science, King Abdulaziz University (KAU), Jeddah 21589, Saudi Arabia
KEYWORDS. photochemistry • energy transfer • cyclization/gem-diborylation • aryl iodides
ABSTRACT: Geminal diboronates have attracted significant attention because of their unique structures and reactivity. However,
benzofuran-, indole- and benzothiophene-based benzylic gem-diboronates, building blocks for biologically relevant compounds, are
unknown. A promising protocol using visible light and aryl iodides for constructing valuable building blocks, including
benzofuran-, indole- and benzothiophene-based benzylic gem-diboronates, via radical carbo-cyclization/gem-diborylation of
alkynes with a high functional group tolerance is presented. The utility of these gem-diboronates has been demonstrated by a ten
gram scale conversion, by versatile transformations, by including the synthesis of approved drug scaffolds and two approved drugs,
and even by polymer synthesis. The mechanistic investigation indicates that the merging of the dinuclear gold catalyst
(photoexcitation by 315-400 nm UVA light) with Na2CO3 is directly responsible for photosensitization of aryl iodides
(photoexcitation by 254 nm UV light) with blue LEDs light (410-490 nm, λmax = 465 nm) through an energy transfer (EnT) process,
followed by homolytic cleavage of the C-I bond in the aryl iodide substrates.
available building blocks, have been successfully used in
INTRODUCTION
Contemporary organic chemistry is
photochemistry.8 It is well recognized that aryl radical could
be produced either by direct photoexcitation (irradiation with
UV light) (Fig. 1, B) or by single electron transfer (SET)
process in presence of photocatalysts. However, another
possibility, an energy transfer (EnT)9 mediated by visible light,
has remain relatively undeveloped: upon excitation by visible
light, the excited state of photosensitizer can subsequently
transfer its excited state energy to the ground state of aryl
halides, leading to the triplet state of the aryl halides. The
reactive triplet state of organic compounds would be
efficiently and selectively accessed by EnT process (visible
light) while overcoming the issues (selectivity, applicability,
functional groups tolerance etc.) of direct photoexcitation (UV
light) and providing opportunities for inventing new reactions.
a
particularly
interdisciplinary science, it also is of importance for biology,
physics and materials science. This inspired us not only to
pursue merely molecular transformations but also constructing
valuable structures for drug discovery and materials synthesis.
Recently, geminal diboronates1 have attracted significant
attention as important building blocks. Due to the empty p-
orbital at the boron atoms, the gem-C-B bonds are activated
and α-carbanionic intermediates are stabilized. It has been
successfully applied in cross-coupling reactions,2 two-fold
functional groups conversion and even asymmetric
transformations.3 So far, several novel methods have been
developed to access geminal diboronates, even benzylic
diboronates.4
However,
benzofuran-,
indole-
and
Among many research groups in the field of
photochemistry,10 our group has dedicated considerable efforts
to the development of novel methodologies relying on light-
driven processes,11 especially to photochemistry involving
dinuclear bis(diphosphine) complex [Au2(μ-dppm)2]Cl2 (dppm
= bis(diphenylphosphino)methane), a bench stable complex
possessing unique photophysical properties as a long-lived
excited state (around 850 ns in MeCN) and a distinctive inner-
sphere oxidation quenching (exciplex) pathway.12 Generally,
the ground state of [Au2(μ-dppm)2]Cl2 inherently relays on an
UVA light source for its photoexcitation (Fig. 1, C), only
single electron transfer (SET) process between the
photoexcited state of the [Au2(μ-dppm)2]Cl2 and aryl/alkyl
halides has been reported. Until recently, our group has
explored a highly efficient C-C coupling by desulfurizing
gold-catalyzed radical reaction13 which shows the possibility
benzothiophene-based benzylic gem-diboronates, building
blocks for biologically relevant compounds, are unknown. The
most ideal and efficient strategy for the construction of these
biologically relevant building blocks would be carbo-
cyclization/borylation. However, only cyclized vinylboronic
esters were obtained, either by using stoichiometric amount of
n-BuLi, followed by carbo-cyclization of a alkynylchloride5 or
through carbo-borylation of arylzinc reagents by cobalt and
chromium catalysts under harsh conditions (Fig. 1, A).6
Since light-induced radical carbo-cyclization7 is among the
most powerful and versatile methods for the constructing
mono- and polycyclic systems with the advantage of high
functional group tolerance, mild reaction conditions, a high
level of regio- and stereoselectivity, we assumed that this
could be an ideal protocol. Aryl halides, widely commercial
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Page 2 of 9
Bpin
of using blue LEDs’ light as light source for new reactions.
Bpin
Blue LEDs (max = 465 nm)
I
1
2
3
4
5
6
7
8
The red shift of the absorption wavelength originates from the
combination of [Au2(μ-dppm)2]Cl2 and ligand (Ph3P or
mercaptan). Inspired by this work, we were wondering
whether new gold complexes, formed in situ, would bring us
new physical properties and chemical reactivity by counter
anion exchange, 14 coordination15 or aggregation. 16

1.0 mol% Au2( -dppm)2(OTf)2
B2Pin2
2aa
+
O
O
Na2CO3 (0.5 eq)
1aa
, 1.0 eq
, 1.2 eqs
Variation
no variation
3aa
MeCN (0.2 mL), 15 h
Entry
Conversion [%]
Yield [%]
1
2
90
0
100
0
no base
no light
3
4
5
0
0
0
0
0
0
RESULTS AND DISCUSSION
Herein, benzofuran-, indole- and benzothiophene-based
benzylic gem-diboronates building blocks, have selectively
no Au2(-dppm)2(OTf)2
9
NaOTf (10 mol%)
10
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been obtained via
a
visible light-induced carbo-
6
7
8
dppm (10 mol%)
0
0
0
0
cyclization/gem-diborylation protocol, catalyzed by dinuclear-
gold(I) complexes in the presence of half an equivalent of
sodium carbonate (Fig. 1, D). The necessary triplet state of
aryl iodides was accessed by an energy transfer process rather
than the widely developed single electron transfer pathway.
no light, heated at 50 o
C
70
89
Au2(-dppm)2Cl2
9
Ph3PAuCl
[Ir(ppy)2(dtbbpy)]PF6
[Ru(bpy)3](PF6)2
Mes-Acr-Me+BF-
CFL light
0
0
0
0
0
0
0
0
10
11
12
1) Zn (2.0 eqs)
A
LiCl (1.0 eq)
CoBr2 (10%)
400 oC, 15 min
Bpin
2)
Me
1)
Me
Cl
Cl
B(OH)2
1) n-BuLi (1.1 eqs)
I
I
THF
MeOBpin
+
2) iminopyridine (10%)
CrCl3(thf)3 (20%)
TMSCl (1.2 eqs)
13
14
78
52
100
85
X
X
2) B(Oi-Pr)3 (1.1 eqs)
O
O
4 examples
26 - 50% yields
X = O, NTs
UVA ( max = 365 nm )

72%, 1 example
60 o
C
Reactions were run with 0.2 mmol scale, % yields and %
conversions were calculated based on 1H NMR spectroscopy
using 1,3,5-trimethoxybenzene (0.1 mmol) as an internal standard.
C
B

I
I
UVA light (315-400 nm)
[Au2(-dppm)2]Cl2
Via SET
UV light (254 nm)
R1
Products
R1
Substrates
singlet state
triplet state
With the optimal condition in hand, we next determined the
scope of this chemoselective and stereoselective cascade
cyclization/gem-diborylation protocol. As shown in Scheme 2,
either terminal alkynes (3aa, 90% yield) or mono/bis-alkyl-
substituted alkynes (3ab-3ae, 60-88% yields) all react very
D
[Au2(-dppm)2](OTf)2 + Na2CO3
R2
Bpin
Bpin
R2
Blue LEDs (465 nm)
in MeCN
1.0 mol% [Au2(-dppm)2](OTf)2
I
Blue LEDs (465 nm)
R3
R1
R1
X
B2Pin2 (1.2 eqs)
X
R3
Aryl-I 
Aryl-I
Na2CO3 (0.5 eq.)
MeCN (1.0 M)
X = O; S; NBoc
triplet state
singlet state
well. Furthermore,
a diverse array of both electron-
Simple, easily accessible
substrates
high functional groups tolerance
valuable building block
Energy Transfer
Photosensitization
withdrawing and electron-donating substituents on the phenyl
groups (acyl, carboxylic acid esters, methoxy, tert-butyl,
trifluoromethyl) gave the corresponding cyclization/gem-
diborylation products (3af-3ak) in good yields (59-85%).
Interestingly, other halogens (fluorine, chlorine, bromine)
were also tolerated and the products (3al-3ao) can be isolated
in excellent yields (75-88%). The reaction is also worked well
for an amide substituent (3ap, 70%). For R2 being a phenyl
group, carbo-cyclization/iodination product 3aq is the major
product which could be caused by steric hindrance or
electronic effects. After constructing complex benzofurans, we
continued to expand this protocol to the synthesis of
benzothiophenes. As expected, terminal and non-terminal
alkynes provide cyclization/gem-diborylation products (3ar-
3at) in 67%-87% yield. Also indoles could be converted, both
terminal and alkyl-substituted alkyne substrates were
successfully converted to the corresponding cyclization/gem-
diborylation products (3au and 3av) in good yields (80% and
75%). Scaffolds related to the approved drug Almotriptan,
3aw and 3ax, were obtained by this protocol in 77% and 82%
yield.
Figure 1. Previous studies and this work; A: Previous
contributions on carbo-cyclization/borylation of alkynes; B:
Direct photoexcitation for reactive triplet state of aryl iodides. C:
Previous reports on the light source for dinuclear gold as
photocatalyst: UVA (inherently) and mechanistic pathway (single
electron transfer); D: This work.
The carbo-cyclization/gem-diborylation product 3aa was
accessible in 90% isolated yield from 1aa (1.0 eq.) and B2Pin2
(1.2 eqs) in the presence of [Au2(μ-dppm)2](OTf)2 (1.0 mol%)
and Na2CO3 (0.5 eq.) in MeCN (1.0 M) by the simple
exposure to blue LED light for 15 h (Scheme 1, entry 1). Our
control experiments indicate that Na2CO3, blue LED light and
[Au2(μ-dppm)2](OTf)2 are all critical for the reaction (entries
2-4). In the absence of [Au2(μ-dppm)2](OTf)2, no desired
product was formed by merely adding each of the components
of catalyst (entries 5-6). The possibility of a thermal reaction
has been excluded by operating the standard reaction at 50 °C
in the absence of light (entry 7). No conversion was observed
with the mononuclear gold catalyst Ph3PAuCl (entry 9). Other
widely used photosensitizers resulted in no conversion (entries
10-12). This reaction also proceeded well under CFL (compact
fluorescent lamp) light (entry 13). Only 52% yield of the
desired product was obtained under UVA light irradiation
(entry 14).
Scheme 2. Scope of cascade gem-bis(borylations)
Scheme 1. Control experiments
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R2
R2
Bpin
Bpin
Blue LEDs
I
1.0 mol% Au2(-dppm)2(OTf)2
R1
R3
1
B2Pin2
R1
+
X
R3
X
Na2CO3 (0.5 eq)
2
MeCN (0.2 mL), 15 h
1, 1.0 eq
2, 1.2 eqs
3, X = O; NBoc; S
3
4
5
6
Benzofurans
Me
Bpin
Et
Et
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
iPr
Bpin
Me
O
O
O
O
O
3aa, 90% (0%)
3ab, 82%
3ac, 88%
3ad, 80%
3ae, 60%
7
8
9
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
O
Bpin
Bpin
Bpin
Bpin
MeOOC
tBu
F3C
O
O
O
MeO
O
O
3af, 83%
3ag, 82%
3ah, 85%
3ai, 59%
3aj, 83%
10
11
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Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
nbutyl
Bpin
Bpin
Bpin
F
Cl
Br
O
O
F3
C
O
O
O
Br
3ak, 73%
3al, 88%
3am, 85%
3an, 78%
3ao, 75%
Benzothiophenes
Bpin
Bpin
Me
Bpin
Bpin
Ph
Bpin
Bpin
Bpin
I
Bpin
MsHN
nbutyl
S
S
O
O
S
Cl
3ap, 70%
3aq, 52%, Z/E =1.0: 1.2
3ar, 72%
3as, 87%
3at, 67%
Indoles
Bpin
Et
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Bpin
Cl
O
S
N
O
N
N
N
N
Boc
Boc
Boc
Boc
3au, 80%
3av, 75%
3aw, 77%
3ax, 82%
Reactions were run in 0.2 mmol scale, isolated yields.
Next, the synthetic utility of the products were also
investigated. As shown in Figure 2A, both a one gram and a
ten gram scale reaction with the easily accessible substrate 1aa,
using either 1.0 mol% of [Au2(μ-dppm)2](OTf)2 (89% yield) or
0.1 mol% of [Au2(μ-dppm)2](OTf)2 (65% yield). The X-ray
structure of 3aa is shown in Figure 2B; it is worth to mention
that this compound remain intact after storing in the fridge at –
20 °C in 5 g scale for ten months. Furthermore, versatile
transformations of 3aa were also explored (Fig. 2C). 3aa was
successfully converted to compounds 4 or 5 by deborylation
and protonation.3f Compounds 6 and 7 were generated by
nucleophilic substitution of the carbanion intermediate in the
presence of bases.3f 3aa was oxidized to benzyl alcohol 8 or
aldehyde 9 by different oxidants. In addition, 3aa can undergo
a metal-catalyzed cross-coupling reaction to result compounds
10 with a palladium catalyst.2 The 1,2-addition to carbonyl
compounds afforded secondary alcohols 112f or 122g in the
presence of LiTMP or AgOAc.
Finally, we successfully applied this gem-diboronate
building block in the synthesis of the approved drugs
Sertaconazole 15 and Dronedarone 18 and also utilized in the
synthesis of the polymer 19. Our synthetic building block 3at
undergo oxidation, chlorination of the hetero-benzylic alcohol
and nucleophilic substitution in the presence of a base, to
afford the approved drug 15 in good yields (Fig. 2D). The
approved drug 18 could be accessed from gem-diboronates
3ap by oxidation to the aldehyde 16 and a subsequent 1,2-
carbonyl addition to afford secondary alcohol 17 and a final
PCC oxidation. Polymer 19 has been successfully synthesized
by a cross-coupling reaction with the number-average weight
(Mn) being 2155 g mol−1 and the polydispersity (Đ) being 1.7.
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C
A
Bpin
Bpin
Bpin
1
D
Bpin
Blue LEDs
I
2
3
4
1.0 mol% Au2(-dppm)2(OTf)2
O
B2Pin2 (1.2 eqs)
Na2CO3 (0.5 eq)
O
O
O
Ph
4
, 100%
5
, 100%
Bpin
1aa
, (1 g, 1.0 eq)
MeCN (1 M), 48 h
3aa
, 1.3 g, 89%
KOtBu
O
2
O
OH
t
Bu
5
THF/H2O
Ag
OAc
O
KO
12
, 71%
Bpin
6
7
8
9
THF/D
t Bu
Bpin
6
, 57%
Blue LEDs
I
Bpin
NaO
r
B
0.1 mol% Au2(-dppm)2(OTf)2
Bpin
Allyl-
OH
Bn
O
B2Pin2 (1.2 eqs)
Na2CO3 (0.5 eq)
MeCN (1 M), 96 h
O
Bpin
LiTMP
NaOt
Bu
1aa
O
, (10 g, 1.0 eq)
O
BnCl
NaBO
3aa
, 9.7 g, 65%
] 2
Recrystallization
3aa
3
O
10
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11
, 48%
t
Bu)
KO
.
4
3
CF3
7
, 46%
t
H
2 O
B
Bu
Pd[P(
m-CPBA
DCM
OH
OH
O
O
Stored in fridge (- 20 oC) for ten months
without decomposed
CHO
10
, 57%
8
, 95%
O
9
, 85%
3aa
D
N
N
N
N
Bpin
Cl
Cl
OH
Cl
Bpin
Et3N, MsCl
0 oC to rt
O
HO
KOt-Bu, NaBO3 . 4H2O
Cl
Cl
S
S
S
95%
98%
50% NaOH aq.
50 oC, 4h
82%
S
Cl
13
Cl
14
Cl
3at
Cl
15
Approved drug: Sertaconazole
Indication: Fungal infection
E
n-butyl
O
n-butyl
n-butyl
n-butyl
N
Bpin
Br
O
HO
O
O
Bpin
n-butyl
O
n-butyl
3
N
3
H
PCC
m-CPBA
0 o
3
MsHN
n-butyl
BuLi, -78 o
C
MsHN
MsHN
MsHN
C
DCM
88%
n-butyl
n-butyl
n-butyl
86%
O
73%
O
O
O
16
18
17
3ap
Approved drug: Dronedarone
Indication: Atrial fibrillation;
Heart arrhythmia; Angina
F
Bpin
Bpin
n-butyl
n
Pd[P(tBu)3]2
Mn = 2.1545E+3 g/mol
D = 1.7151E+0
n-butyl
Cs2CO3, THF/H2O, 100 o
82%
C
O
O
Br
3ao
19
Figure 2. Applications. A: Gram scale synthesis; B: X-ray structure of 3aa; C: Versatile transformations; D: Synthesis of approved drug
Sertaconazole; E: Synthesis of approved drug Dronedaron; F: Polymer synthesis.
After exploration of the reaction scope and demonstrating
the synthetic value of the products, we turned to mechanistic
studies. First, we tried to explore the nature of the real
photocatalysts in this transformation. Both [Au2(μ-
dppm)2](OTf)2 and aryl iodides show no absorption in the blue
LEDs’ emission wavelength (410-490 nm, λmax = 465 nm).
According to our 31P NMR detection, a new gold complex was
formed by the combination of [Au2(μ-dppm)2](OTf)2 and
Na2CO3 (Figure 3). This newly formed gold complex shows a
red shift of the absorption wavelength, overlapping with the
emission wavelength of the blue LEDs light. This newly
formed gold complex shows similar UV-vis absorption
wavelengths and 31P NMR characteristic peaks including
chemical shifts and signal multiplicities compare with the
signals detected in the reaction mixture. At this moment, we
propose that this new gold complexes could be resulted from
an aggregation of [Au2(μ-dppm)2](OTf)2 in the presence of
Na2CO3. A triplet and a multiple peaks are observed in the 31
P
NMR rather than a singlet peak, and the [Au2(μ-dppm)2](OTf)2
catalyst can be recovered from this newly formed gold
complex after evaporating the CD3CN and dissolving in
CDCl3 (as detected by 31P NMR, see SI).
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A
[Au2
]
35.66 (s)
Bpin
Bpin
1
2
3
4
Radical inhibitors
Yields [%]
I
standard condition
radical inhibitors
[Au2]+Na2CO3
TEMPO (1.0 eq)
DHA (1.0 eq)
0
34.89 (s)
O
O
37.03 (t, J = 7.7 Hz)
37.03 (t, J = 7.7 Hz)
53
30.55 (m)
1aa
3aa
[Au2]+Na2CO3+1aa+B2Pin2
34.82 (s)
30.55 (m)
5
B
6
7
(A1
)
I
(A2
)
Blue LEDs
I
1.0 mol% [Au2(-dppm)2(OTf)2]
8
9
O
Na2CO3
O
20
, 92%
Figure 3. The nature of the real photocatalysts or photosensitizers.
(A1) All of samples are measured in MeCN with 0.01 mM
concentration. See details in SI; (A2) 31P NMR spectra are
measured in CD3CN.
1aa
10
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38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
I
Bpin
Blue LEDs
I
1.0 mol% [Au2(-dppm)2(OTf)2]
Na2CO3
15 h
O
O
O
The reaction has been operated in the presence of radical
inhibitors (Scheme 3A), no desired product was formed in the
presence of TEMPO and a decreased yield (53%) was
observed with 1,9-dihydroanthracene (DHA), which suggested
a radical pathway. Besides, in the absence of B2Pin2, aryl
iodides 1aa has been converted to vinyl iodide 20 in 92%
yield (Scheme 3B) which indicates that B2Pin2 does not play
any role for the formation of aryl radical from aryl iodides.
Taking UV−vis absorption, 31P NMR peaks and our control
experiments into account, we propose the merging of [Au2(μ-
dppm)2](OTf)2 with Na2CO3 is directly responsible for the
initiation of this photoreaction under blue LEDs light
irradiation. Attempts to isolate this species failed, suggesting
the complex maybe only exist in solution. More interestingly,
in the presence or absence of B2Pin2, only carbo-
cyclization/iodination product 22 has been formed in the case
of aryl iodide 21. Even through the presence of B2Pin2 could
slight increase the efficiency of this transformation, no
detectable borylation product 23 was formed, which obviously
is inconsistent with previous reports17 for an SET process.
Besides, , aryl radical should also be produced from aryl
bromides if SET process is really proceeded in our reaction
because the inner-sphere oxidation quenching (exciplex)
pathway have no limitation of redox potentials,11,12 however
aryl bromide results in no conversion. And the possibility by
going through atom transfer radical cyclization (ATRC)
process could also be ruled out because of the bond
dissociation energy,7b,18 an alkyl radical cannot abstract an
iodine atom from an aryl iodide. And also our quantum yield
23
, not detectable
21
22
with B2Pin2
without B2Pin2
conv. 72%
conv. 55%
70%
52%
In addition of going through photoredox catalysis and
photoinitiation, we now considered the possibility to EnT
process. The direct excitation experiment with UV light (254
nm), which are often used to indicate if excited (triplet) state
intermediates are indeed present in the reaction mechanism,9,19
shown the possibility of an EnT process. As shown in Scheme
4A, direct photoexcitation of aryl iodide 21 under UV light
irradiation in the absence of gold catalyst also gave only
carbo-iodination product 22, but less efficiently. And also
direct photoexcitation of aryl iodide 1aa generated the desired
cyclization/gem-diborylation product 3aa in 30% yield after
extending the reaction time to 72 h (Scheme 4B). It is well
known that the ground state of aryl iodides could be converted
to the photoexcited triplet state by UV light,7,8 exothermically
homolytic cleavage of aryl C-I bond affords aryl free radical
and iodo free radical, resulting cyclization iodination products.
Scheme 4. Direct excitation experiments
A
I
I
Na2CO3
B2Pin2
O
O
h = 254 nm
21
22
(16%, 15 h)
results (= 35% for cyclization/gem-diborylation;  = 34%
for carbo cyclization/iodination, see SI) indicate that this
transformation is not radical chain process dominated.
B
Directly irradiation
Bpin
Bpin
h = 254 nm
Scheme 3. Radical trapping experiments and ruling out the
possibility of involving SET or ATRC radical chain
processes
Na2CO3
O
3aa
(30%, 72 h)
I
+
B2Pin2
O
Bpin
h = 465 nm
Bpin
1aa
1.0 mol% [Au2(-dppm)2(OTf)2]
Na2CO3
O
Photosensitization
3aa
(90%, 15 h)
Furthermore, vinyl iodide has been converted to the desired
product by our standard condition (Scheme 5A), without light
or gold catalysts result no conversion which indicates that
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A
further photoexcitation of vinyl iodide is involved and the
I

Gold complexes
excited state
26
1
2
3
4
5
6
dehydroborylation step occurred after the cyclization step. The
KIE value observed in Scheme 5B, indicate that
dehydrogenation is not involved in rate-determination step
(the degree of deuteration of 24 is more than 99%).
O
1aa
Blue LEDs
Energy transfer (EnT)
Photosensitization

Na2CO3
+
I
Scheme 5. Cyclization/gem-diborylation of vinyl iodide and
kinetic isotopic effect measurement
Gold complexes
[Au2(-dppm)2](OTf)2
MeCN
O
26
ground state
1aa
7
8
Triplet state
A
B
9
homolysis
5-exo cyclization
Bpin
1aa
I
Bpin
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
I
27
28
standard condition
vinyl radical
O
O
C
I
NaO2CO Bpin
31
20
3aa
(88%)
+
Bpin
I
OCO2Na
Bpin Bpin
Bpin
Bpin
NaO2CO Bpin
Variation
photosensitization
29
Yields [%]
or
I-Bpin
O
O
O
I
O
none
88%
n.r.
28
Isolated
)
30
20
(
32
-H elimination
no light
radical-radical combination
vinyl radical
Na2CO3
quenching
Bpin
no Au2[-dppm]2(OTf)2
n.r.
Bpin
Bpin
O
B
O
3aa
4
Isolated
)
, (
n-pentane
Bpin
Figure 4. Proposed mechanism
n-pentane
I
Bpin
CONCLUSIONS
standard condition
H/D
H
In conclusion, this protocol presents a significant advance in
the field of carbo-cyclization/borylation of alkynes through
energy transfer pathway under visible light irradiation. The
synthetically useful protocol provides a variety of gem-
diboronates, which not only can act as highly functionalized
substrates for methodology research, but also as useful
building blocks which simplify the synthesis of relevant
approved drugs and accelerate the process of discovering new
drugs or polymers.
O
O
D
24
25
(87%) KIE = 1.5
Taking all these results together, a possible mechanism
cycle have been proposed (Fig. 4). The combination of [Au2(μ-
dppm)2](OTf)2 with Na2CO3 in acetonitrile provides a new
gold complex. After photoexcitation, the aryl iodide leads to
photoexcited triplet state through energy-transfer process (EnT)
with newly formed gold complex (Fig. 4A), followed by
homolytic cleavage aryl-I bond and resulting aryl free radical
and iodo free radical. The resulting aryl radical 27 goes
through 5-exo-cyclization with high regio-selectivity affording
vinyl radical 28 (Fig. 4B), followed by radical-radical com
bination leading to vinyl iodide 20, which can be converted to
vinyl radical 28 again by further photosensitization (Fig.4C).
The vinyl radical 28 could be trapped by the ate-complex 29
resulting to vinyl boronate 30 and 31.20 Then the desired
produce can be accessed by electrophilic borylation of vinyl
boronate 29 with Bpin-I or Bpin-OCO2Na, resulting the
stabilized benzyl cation 32, followed by rapidly β-H
elimination. The intermediate vinyl iodide 20 has been
detected during the reaction and also been isolated. The
intermediate vinyl boronate 30 is very unstable and we have
failed to isolate it, however around 8% yield of compound 4
has been isolated after quenching the reaction mixture.21
ASSOCIATED CONTENT
The supporting information is available free of charge via the
Internet at http://pubs.acs.org.
Experimental procedures and compound characterization (PDF)
Accession Code
CCDC 1991449 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge
Crystallographic
https://www.ccdc.cam.ac.uk/structures/.
Data
Centre
via
AUTHOR INFORMATION
Corresponding Author
*hashmi@hashmi.de
ORCID
A. Stephen K. Hashmi: 0000-0002-6720-8602
Author Contributions
All authors have given approval to the final version of the
manuscript. / ‡These authors contributed equally.
ACKNOWLEDGMENT
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Journal of the American Chemical Society
tosylhydrazone into B–B and B–Si bonds: gem-diborylation and gem-
L.Z., X.S. are grateful for PhD fellowships from the
China Scholarship Council (CSC).
silylborylation of sp3 carbon. Org. Lett. 2014, 16, 448. (f) Hong, K.;
Liu, X.; Morken, J. P. Simple access to elusive α-boryl carbanions
and their alkylation: an umpolung construction for organic synthesis.
J. Am. Chem. Soc. 2014, 136, 10581. (g) Wommack, A. J.;
Kingsbury, J. S. On the scope of the Pt-catalyzed Srebnik diborylation
of diazoalkanes. An efficient approach to chiral tertiary boronic esters
and alcohols via B-stabilized carbanions. Tetrahedron Lett. 2014, 55,
3163. (h) Palmer, W. N.; Obligacion, J. V.; Pappas, I.; Chirik, P. J.
Cobalt-catalyzed benzylic borylation: enabling polyborylation and
functionalization of remote, unactivated C(sp3)–H bonds. J. Am.
Chem. Soc. 2016, 138, 766. (i) Palmer, W. N.; Zarate, C. P.; Chirik, J.
Benzyltriboronates: building blocks for diastereoselective carbon–
carbon bond formation. J. Am. Chem. Soc. 2017, 139, 2589.
1
2
3
4
5
6
7
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R2
Bpin
Bpin
R2
Blue LEDs (465 nm)
1
2
3
4
1.0 mol% [Au2(-dppm)2](OTf)2
I
R3
R1
R1
X
B2Pin2 (1.2 eqs)
X
R3
Na2CO3 (0.5 eq.)
MeCN (1.0 M)
X = O; S; NBoc
Simple, easily accessible
substrates
high functional groups tolerance
valuable building block
5
6
Application:
Scope:
24 examples, 60-90% yields
10 grams synthesis
7
8
9 transformations
2 approved drugs
Mechanistic studies:
energy transfer
1 polymer
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54
55
56
57
58
59
60
9
ACS Paragon Plus Environment