Visible Light-Enabled sp3-C?H Functionalization with Chloro- and Bromoalkynes: Chemoselective Route to Vinylchlorides or Alkynes

Article abstract of DOI:10.1002/chem.202001259

An unprecedented direct atom-economic chemo- and regioselective hydroalkylation of chloroalkynes and an sp³-C?H alkynylation of bromoalkynes was achieved. The reaction partners are unfunctionalized ethers, alcohols, amides, and even non-activated hydrocarbons. We found that a household fluorescent bulb was able to excite a diaryl ketone, which then selectively abstracts a H-atom from an sp³-C?H bond. The product of a formal alkyne insertion into the sp³-C?H bond was obtained with chloroalkynes, providing valuable vinyl chlorides. The photo-organocatalytic hydrogen atom transfer strategy gives rise to a broad range of diversely functionalized olefins. When bromoalkynes are applied in the presence of a base, a chemoselectivity switch to an alkynylation is observed. This reaction can even be performed for the alkynylation of unactivated sp³-C?H bonds, in this case with a preference of the more substituted carbon. Accompanying quantum chemical calculations indicate a vinyl radical intermediate with pronounced linear coordination of the carbon radical center, thus enabling the formation of both diastereoisomers after H-atom abstraction, suggesting that the (Z)-diastereoisomer is preferred, which supports the experimentally observed (E/Z)-distribution.

Full text of DOI:10.1002/chem.202001259

Accepted Article
Accepted Article
Title: Visible Light-Enabled sp3-C-H Functionalization with Chloro-
and Bromoalkynes: Chemoselective Route to Vinylchlorides or
Alkynes
Authors: Tapas Adak, Marvin Hoffmann, Sina Witzel, Matthias
Rudolph, Andreas Dreuw, and A. Stephen K. Hashmi
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001259
Link to VoR: https://doi.org/10.1002/chem.202001259
01/2020

10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
Visible Light-Enabled sp3-C-H Functionalization with Chloro- and
Bromoalkynes: Chemoselective Route to Vinylchlorides or
Alkynes
Tapas Adak,[a] Marvin Hoffmann,[b] Sina Witzel,[a] Matthias Rudolph,[a] Andreas Dreuw, [b] and A.
Stephen K. Hashmi *[a,c]
Dedication ((optional))
[a]
[b]
[c]
M.Sc. Tapas Adak, M.Sc. Sina Witzel, Dr. Matthias Rudolph, and Prof. Dr. A. Stephen K. Hashmi
Organisch-Chemisches Institut
Heidelberg University
Im Neuenheimer Feld 270, 69120 Heidelberg
E-mail: hashmi@hasmi.de
M. Sc. Marvin Hoffmann and Prof. Dr. Andreas Dreuw
Theoretical and Computational Chemistry, Interdisciplinary Center for Scientific Computing (IWR)
Heidelberg University,
Im Neuenheimer Feld 205A, 69120 Heidelberg (Germany).
Email: andreas.dreuw@iwr.uni-heidelberg.de
Prof. Dr. A. Stephen K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University, Jeddah 21589 (Saudi Arabia)
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: An unprecedented direct atom-economic chemo- and
regioselective hydroalkylation of chloroalkynes and an sp3-C,H
alkynylation of bromoalkynes was achieved, reaction partners are
unfunctionalized ethers, alcohols, amides, and even non-activated
hydrocarbons. We found that a household fluorescent bulb was able
to excite diaryl ketone, which then selectively abstracts a H atom from
an sp3-C-H bond. The product of formal alkyne insertion into the sp3-
C-H bond was obtained with chloroalkynes, providing valuable vinyl
chlorides. The photo-organocatalytic hydrogen atom transfer strategy
gives rise to a broad range of diversely functionalized olefins. When
well as Buchwald-Hartwig amination reactions.[5] Due to the
immense importance of this building block, strategies to
synthesize vinyl halides are of high value for organic chemists.
While vinyl chloride [6] is the most useful vinyl halide which is used
on a scale of millions of tons per year for polymerization to PVC,
also substituted vinyl chlorides and vinyl bromides are most
frequently used in cross coupling and as precursors for Grignard
and organolithium reagents.[7]
The most common strategies to access olefins are the use of
elimination reactions, Wittig olefinations,[8b] or Peterson
olefinations [8a,c] or the Julia-Lythgoe reaction.[8d] These often are
carried out under harsh reaction conditions, require multistep
syntheses, and are often incompatible with sensitive functional
groups.[8] In terms of olefination reactions, transition metal-
catalyzed (Au, Cu, Ag, Co, Ni, Rh, Ir, Fe) [9] additions to alkynes
as synthetic access to alkenes are well established (Scheme 1),
but halovinylations are still underdeveloped. The observation can
bromoalkynes are applied in the presence of
a base, a
chemoselectivity switch to an alkynylation is observed. This reaction
also can even be performed for the alkynylation of unactivated sp3-C-
H bonds, in this case with a preference of the more substituted carbon.
Accompanying quantum chemical calculations indicate a vinyl radical
intermediate with pronounced linear coordination of the carbon radical
center, thus enabling the formation of both diastereoisomers after H-
atom abstraction, suggesting that the (Z)-diastereoisomer is preferred, rationalize that with transition metal-catalyzed reactions with
which supports the experimentally observed (E/Z)-distribution.
haloalkynes usually an alkyne product is formed by the insertion
of the metal into the carbon-halogen bond[10] (Scheme 1).
Stephenson and other groups recently reported an elegant
access to vinyl halides. It is based on halogenation reactions of
alkynes (and alkenes) via halide atom transfer pathways.[11]
Duan’s group reported a palladium-catalyzed chlorine atom
transfer to alkynes for chlorovinylations, but the reactions show
low selectivity.[12] Hoveyda’s group reported cross-metathesis
reactions of alkenyl halides.[13] These few examples of halo-
olefination by Cl/Br-atom transfer to alkynes exist, but
haloolefinations by a sp3-C-H atom transfer to haloalkynes are still
unknown. While the majority of metal-free photo-catalyzed
reactions are focusing on the connection of radicals to a sp2-C
Introduction
Aryl alkenes are ubiquitous structural motives used for the
synthesis of bioactive compounds and drugs.[1] Aryl alkenes can
also serve as subunits for molecular motors.[2] Hetero atom-
functionalized derivatives are also of considerable importance for
pharmaceutical chemistry.[3] A key building block for downstream
transformations of aryl alkenes are haloarylethenes, the halogen
can be further transformed in diverse coupling reactions (e.g.
Suzuki-Miyaura-, Heck-, Stille- or Sonogashira couplings)
[4]
as
centre,[14] we envisioned that the direct coupling of
a
1
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
photochemically generated radical to the sp-carbon of an alkyne
might allow the formal insertion of the chloroalkyne into an alkyl
C-H bond. This process would directly deliver synthetically useful
chloroolefins in perfect atom-economy. To the best of our
knowledge, this process was never evaluated with haloalkynes,
despite the attractive target. Recently reported sp3-C-H atom
abstraction reactions with normal alkynes to indicate that such a
process might be feasible, but typically these reactions require a
over-stoichiometric amount of an oxidant or a radical initiator.[15]
For example, in 2015, Wang’s group reported the eosin-Y-
catalyzed vinylation of THF with alkynes by using a large excess
of tBuOOH (4 equivs) as radical initiator as well as oxidant and
two equivs of formic acid in the presence of 4 Å molecular sieves.
Furthermore, the reaction was limited to THF only, while other C-
H coupling partners (dioxane, diethyl ether, methanol, etc.) or
even diphenyl acetylene under these harsh conditions led to
decomposition. An Ir-based photocatalyst[16] is well-known for
direct C-H atom abstractions via single electron transfer. But this
catalyst is quite expensive, in most cases needs a base and
additional additive. In 2017, Wu’s group developed a direct
oxidant- and additive-free sp3-C-H functionalization by using a
Ni/Ir-catalyzed hydroalkylation of internal alkynes under blue LED
irradiation.[17] Unfortunately, the regioselectivity was poor. Later,
Ar
A
X
Path A
unfavourable
A = C, O, N
Path A
A
Path B
X
A
+
Photocatalyst
Visible light
Ar
Ar
X
Path B
favourable
X = Cl, Br
X
A
Ar
Can the elimination
of the halogenatombe
prevented?
X
A
A
Ar
Ar
Well-knownstrategy
Our new strategy
MacMillan’s group also reported
a Ni/Ir-catalyzed 1,1,3,3-
Scheme 2. Our strategy
tetramethylguanidine-assisted decarboxylative hydroalkylation of
terminal alkynes.[18]
An sp3-C-H atom transfer promoted by visible light irradiation and
using a diarylketone catalyst is a challenging strategy for the
development of new chemical transformations in synthetic
chemistry.[20] We hypothesized that a household fluorescent bulb
could activate a C=O photocatalyst via a n→π* excitation, which
then could form a reactive short-lived radical species by
abstracting a sp3-C-H atom next to the hetero (O, N) atom
(Scheme 2) of ethers/amides or alcohols and even hydrocarbons.
This reactive species could then undergo radical addition
reactions with the alkyne. The assumed role of the carbonyl group,
which acts as radical initiator as well as H-abstractor, is in line with
pioneering photoreactions like the Norrish-Yang cyclization[21] or
related studies by Breslow.[22]
a) Previous work:
reactivity of alkynes under metal and metal-free conditions
metals (Cu, Ag, Fe, Rh, Ir, Ni)
or metal -ree
Y
Y
+
A
A
Y
A
Y
Y = H, R, Ar, ester
A = O, N
Ar
metals (Au, Ni, Ag, Cu, Ir, Pd)
+
Ar
X
A
X = Cl, Br, I
alkynyl
chloride
Y
b) This work:
A
Cl
Ar
metal-free
+
alkynyl
A
X
bromide
Ar
Results and Discussion
A
base
We started with the chloroalkyne 2a as a test substrate in THF
(0.05 M) together with benzophenone (15 mol %) as photocatalyst
(Table 1, entry 1) under 23 W CFL light irradiation. Based on the
2D NMR analysis (see supporting information, COSY, NOESY,
HMBC) we could confirm that the product originated of an attack
of the THF radical at the sp-C center adjacent to the chlorine atom
(Scheme 2, Path B), delivering the corresponding vinyl halide (4a)
in 76% in a 3:1 (Z/E) diastereomeric distribution. Interestingly, no
dechlorination to the alkyne took place, which might be explained
by the mild conditions. As expected, benzaldehyde and acetone
showed no catalytic activity under these conditions (entries 2-3).
After exploring various photocatalysts (see SI), the isolated yield
improved to 93% in the presence of a dichloro-substituted
benzophenone (3d) as a photosensitizer (Table 1, entry 4). Other
photocatalysts, like xanthone and 9-fluorenone, were less
efficient (entries 5-6). Other known transition metal photocatalysts
such as Ru(bpy)3(PF6)2 showed no significant photocatalytic
activity for this reaction. Acetonitrile or acetone as co-solvent did
not improve the yield (entries 8-9) and the addition of base slightly
Scheme 1. Previous work and our assumption
Radical additions to vinyl halides are known.[19] Thus the question
arises, if after the formation of the desired vinyl halides via a
radical pathway these vinyl halides would further react to
saturated products. We assumed that by avoiding peroxide-type
radical initiators with an aldehyde-/ketone-based H-atom
abstractor, a selective formation of vinyl chlorides might be
possible (Scheme 2).
2
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
decreased the yield of the product (entry 12). The use of a blue
LED or sunlight as alternative light sources gave a lower yield
(entry 11). No reactivity was observed in the dark at 60 oC (entry
15). The chlorovinylation reaction was sensitive to oxygen but not
to moisture. However, a lower yield (71%) and a drop in
diastereoselectivity (1.3:1; Z/E) was observed, when water was
used as co-solvent (THF/H2O= 1:1). Therefore, dry solvents were
needed to increase the stereoselectivity. It required no further
additives and precautions for our protocol.
energies explained the lower reactivity of ethers and amides
compare to THF.[20d,21] Furthermore, it is noteworthy that no C-O
and C-N coupled product was observed for free alcohols and
amides.
Table 2. Substrate scope with respect to the sp3-C-H bond
O
Cl
Photocatalyst (15 mol%)
Light, RT
+
A
A
Cl
Cl
Cl
4a-m
1a-p
2a
photocatalyst (3d)
A = O, N
O
Table 1. Optimization of the reaction conditions
Ether
O
Cl
Ph
O
Ph
O
Ph
3d
Catalyst
(15 mol%)
Cl
Cl
Cl
+
O
O
4b (68%, 1.4:1)b
Light, RT
4a (93%, 3:1)
4c (72%, 2.2:1)
Cl
4a
MeO
MeO
1a
2a
MeO
Ph
O
Ph
O
Ph
Cl
EtO
Ph
Cl
Cl
Cl
Entry
Catalyst
Yield (%)a
(Z/E)b
4d (40%, 6.6:1)
4d' (32%, 10:1)
4e (58%, 2:1)
4f ( 75%, 1.4:1)
1
2
Benzophenone instead of 3d
Benzaldehyde instead of 3d
Acetone instead of 3d
76 (3:1)
Ph
HO
Ph
Alcohol
Amide
HO
Ph
Cl
HO
Ph
HO
Ph
Cl
0
Cl
Cl
3
0
4j (71%, 3:1)c
4g (88%, 2.5:1)
4h (87%, 2.5:1)
4i (82%, 3:1)
4
4,4′-Dichlorobenzophenone (3d)
9-flurenone instead of 3d
Xanthone instead of 3d
Ru(bpy)3(PF6)2 instead of 3d
CH3CN as solvent
96 (93c, 3:1)
52 (2.8:1)
59 (2.9:1)
0
O
O
5
O
N
Ph
H
N
Ph
N
H
Ph
6
Cl
Cl
Cl
4k (62%, 3:1)b
7
4l (68%, 1.1:1 )b
4m (58%, 2:1 )b
8d
9d
10
11
56
Reaction conditions: All reactions were carried out with 2a (0.2 mmol),
3d (15 mol%) in 0.05 M solvent under irradiation of two 23 W CFL
bulbs at room temperature for 3-16 hours. Isolated yields and (Z/E)
isomer ratios are reported. bReaction carried out for 72 hours.
cReaction carried out for 48 hours. Major isomers are shown.
Acetone as solvent
64
Water as solvent
71 (1.3:1)
42 / 60 (3:1)
Blue LED/ Sunlight instead of white
CFL bulb
12
13
14
15
O
K2HPO4 used as a base
No light
45
0
For the scope with respect to the alkyne, a broad range of alkynes
was studied. Excellent regioselectivities and yields were obtained
for various chloroalkynes (entries 5a-5n). Chloroalkynes with
either electron-donating or electron-withdrawing groups were
tolerated, providing slightly higher yields in the case of donor-
substituted arylchloroalkynes. Excellent yield (76-84%) and
regioselectivity were also observed with phenylchloroalkynes
bearing halides at the aryl system (5e-5g). In the case of an
electron-deficient ester, keto, or trifluoromethyl group (5h-5j),
yields were also high. Ortho- and meta- substituted chloroalkynes
well participated in our reaction in 70-84% yield (5l-5n).
Naphthalene motif also worked well but lower selectivity (5d, 5k).
To explore the reactivity of our reaction protocol, we further
studied the reaction with other alkyne systems. Gratifyingly,
diphenylacetylene (5o) was very reactive to provide 90% of the
hydroalkylated product in a 1.3:1 (Z/E) ratio. A bromoalkyne
provided a lower yield (5p). An alkyne ester (5q) and terminal
alkynes (5s-5v) also successfully provided the desired vinylated
products, while the reaction was not efficient for an iodoalkyne
(5r) as starting material. Interestingly, as observed for
chloroalkynes the THF radical attacked the carbon next to the
ester for the alkynyl ester substrate while a related previous study
showed a different reactivity (Table 4, 5q).[15a,17]
No catalyst
At 60 oC temperature in the dark
0
0
O
O
O
Cl
Cl
O
(3d)
4,4'-dichlorobenzophenone
(3c)
(3e)
(3f)
xanthone
benzophenone
9-flurenone
Reaction conditions: unless otherwise noted, all reactions were
carried out as follows: 2a (0.2 mmol), catalyst (15 mol %) in degassed
THF (0.05 M) under irradiation of 23 W CFL bulb at room temperature
for 16 h. aThe yield was determined from the crude 1H NMR spectra
by using 1,3,5-trimethoxybenzene as an internal standard. bThe (Z/E)-
ratio (confirmed by 2D NMR analysis) was determined by the crude
1H NMR spectra. Isolated yield after 3 hours reaction time with two
c
23 W CFL bulb. d1:1 ratio of solvent and THF.
With optimized condition in hand, we next explored the scope
regarding the ether component (Table 2). Cyclic ethers such as
THF, dioxane, and oxetane were tolerated well (entries 4a-4c). It
also converted acyclic ethers with alkyl chains to the
corresponding vinyl derivatives in good yield (58-75%, 4d-4f).
with dimethoxyethane, we observed both methyl and methylene
reactivity, giving regioisomers 4d and 4d’ in a combined yield of
72% yield. Gratifyingly, alcohols were also found to be reactive
substances for the radical coupling reaction in yields ranging from
71-88% (4g-4j). No C-O addition product was detected. Benzylic
alcohol reacted with the benzylic position in 71% yield (entry 4i).
Amides as substrates delivered the chlorovinylated products
adjacent to the nitrogen atom (4k-4m) in good yield (58-68%) after
prolonged reaction time. The higher C-H bond dissociation
Table 3. Substrate scope with respect to the alkynes
3
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
a) Internal alkynes
product. We were delighted to find that even weak bases allow
this (see SI), the simple KOAc enabled us to obtain the alkyne
products. This alkynation of ethers with a metal-free simple and
straightforward photocatalytic system in the absence of any
external oxidant offers attractive advantages over the already
existing strategies.[15,10] Bromoalkynes with both electron-rich and
electron-poor substituents provided the alkynated products (59-
71%, Table 4, 7a-e) in the presence of KOAc. Only recently Lei’s
group reported a trimetal-catalyzed (Ni/Cu/Ag) oxidative coupling
of hydrocarbons with alkynes by using DTBP.[23] Related
couplings, for example with a cyclohexyl group (7d), under our
photocatalytic conditions, could be conducted completely metal-
and oxidant-free. More interestingly, in the previous studies, the
major product was the insertion product into the secondary CH
bond,[23,20e] while we observed activation of the tertiary CH bond
(C3:C2 = 10:1) which nicely complements the existing strategy
(Table 4, 7e).
O
X
Photocatalyst (15 mol%)
O
+
Cl
5a-r
Light, RT
O
Cl
Cl
R
3d
Photocatalyst (
)
R
2b-2r
1a
X = Cl, Br, COOEt, Ph
O
O
O
O
Cl
Cl
Cl
Cl
OMe
OMe
5a (95%, 3.1:1 )
5b (96%, 3.1:1)
5c (91%, 3:1)
5d (72%, 1.2:1)
O
O
O
O
Cl
Cl
Cl
Cl
F
Cl
Br
COOMe
5h (71%, 2:1)
F
5g (80%,2.9:1)
5e (76%, 2.8:1)
5f (84%, 2.8:1)
O
O
O
O
Cl
Cl
Cl
Cl
CF3
Ph
5i (77%, 1.5:1)
5j (79%, 2:1)
5k (59%, 3:1)
5l (70%, 3:1)
Table 4. Scope with bromo alkynes
CF3
O
R1
R1
O
O
O
Cl
Ph
Cl
X
CF3
3d (15 mol%)
KOAc, Light, RT
X = -Br 5p (39%,20:1)
-COOEt 5q (91%, 1.3:1)
-I 5r (0%)
+
5m (84%, 3.8:1)
5n (72%, 4:1)
5o (90%, 1.3:1)
A
1a
Br
A
3b
7a-7e
b) terminal Alkynes
A = O, C
COMe
Cl
Cl
OMe
Br
O
O
O
O
O
O
O
7a (71%)
7b (66%)
7c (64%)
5s (81%, 1.4:1)
5t (80%, 1.4:1)
5u (84%, 2:1)
5v (63%, 2:1)
1
2
3
Reaction conditions: all reactions were carried out with 2a (0.2 mmol),
3d (15 mol%) in 0.05 M THF under irradiation with two 23 W CFL
bulbs at room temperature for 3-16 h. Isolated yields and (Z/E) are
reported. Major isomers are shown.
7d (61%)
7e (59%, rr [C(3):C(2) = >10:1]
Reaction conditions: 3b (0.1 mmol), 3d (15 mol%), KOAc (1.5 equiv)
in THF (0.05 M) under irradiation of light without further optimization.
Next, we tested if even non-activated alkanes would be suitable
substrates. It is noteworthy that cyclohexane could be activated
with a 53% yield (Scheme 3, 6a) for the formal insertion product.
Gratifyingly, in the case of 3-ethyl pentane (Scheme 4, 6b) the
tertiary position (the most stable radical) was addressed (C3/C2;
15:1). While the diastereoselectivity for 6a was low, the more
bulky alkyl group in 6b led to high diastereoselectivity.
A proposed mechanistic scenario is depicted in Scheme 4. To
back-up our proposal, several control experiments were carried
out. Different deuterium labeling experiments were conducted in
order to understand the proton transfer to the vinylic position. A
mixture of THF and THF-d8 as the coupling partner delivered both
the THF and THF-d8 incorporated (Scheme 4a) products. It was
noticed that the crossover product of 4a and 4a’ was obtained
with THF and THF-d8. The kinetic study (competing and parallel)
showed a high KH/KD value of 4.8, which indicates that the
hydrogen abstraction is the rate-determining step. We could
confirm that THF acts as the hydrogen source (Scheme 4b). In
the case of THF-d8 as a coupling partner, greater than 99 %
deuterium incorporated product was isolated. No exchange
processes were detected if THF and D2O (1:1; v/v) were used and
only minor traces of deuterium were incorporated in the product.
This indicates that the vinyl radical formed by the addition of a
THF radical to the chloroalkyne may abstract the H-atom directly
from THF only, not from the diarylalcohol intermediates that are
formed by the photocatalyst (III, Scheme 5). In this case, an H/D
exchange to the alcoholic group would be likely. Moreover, H/D
exchange indicated reaction may follow a proton-coupled electron
transfer (PCET) pathway (Scheme 5) to complete the catalytic
cycle. Next, the addition of TEMPO as radical scavenger
confirmed the radical pathway. Indeed, a THF-TEMPO adduct
Scheme 3. Scope with non-activated alkanes
Standard conditions
6a
(53%, Z:E = 1.4:1)
Cl
Ph
2
Cl
R
1
3
Cl
6b
(56%, Z:E = 4:1)
Standard conditions
regio-selectivity
C(3):C(2)= >15:1
Me
Reaction conditions: All reactions were carried out under standard
conditions.
We assumed that the low yield of the vinylbromide 5p (Table 3a)
was caused by a subsequent reaction of 5p with the radicals.
Thus we tried to avoid this by an HBr elimination to the alkynylated
4
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
(Scheme 4d) was detected (9% crude NMR yield). Interestingly,
tetrahydrofuranyl radicals generated by the treatment of THF with
di-tert-butyl peroxide (DTBP) as oxidant, did not provide the
desired product 3a with chloroalkyne 2a. The desired product was
neither observed in the absence of light nor with the photocatalyst
alone. The light on-off experiment also confirmed the light-
dependency of the reaction (see supporting information).
that, the reaction may follow either a direct HAT transfer pathway
or a competing radical chain mechanism (quantum yield (φ) =
3.87). In this competitive pathway, the short-lived
tetrahydrofuranyl radical undergoes a radical addition reaction to
chloroalkyne (2a), resulting in a nearly linear vinyl radical
intermediate V (supported by DFT study, see below), which next
delivers the (Z/E) isomeric distribution by the abstraction of the
hydrogen atom from THF which propagates the radical chain. In
addition, the THF radical could also reoxidize intermediate III,
which could regenerate the photocatalyst by HAT.
Scheme 4. Preliminary mechanistic study
(90% H)
a) Kinetic study
D
D
D
(90% H)
D
D
H
D
D
std. conditions
2a
D
D
D
D
D
+
O
+
D
H
Ph
O
Scheme 5. Plausible reaction mechanism
O
D
O
Cl
D
Cl
4a'
m/z = 215.10769
Ph
K
H / KD = 4.8
4a
m/z = 208.06372
1a
1a-d8
(1:1)
b) H/D exchange reaction
D
(>99% H)
D
D
(>99% D)
D
D
H
Std. Conditions
THF-d8
Std. Conditions
THF:D2O (1:1)
D
D
D
O
2a
O
Ph
Cl
Ph
Cl
Std.
Conditions
THF
Std.
Conditions
THF-d8: H2O (1:1)
D
O
3d, D2O
Light, rt
(100% H)
Ph
D
H
D
4a
D
D
(> 98%D)
O
D
D
D
Cl
Cl
Ph
c) Reaction study by Generation of THF radical using DTBP
In order to shed light on the position and diastereoselectivity of
the reaction, quantum chemical calculations were performed.
Starting from the separated reactants, two attack vectors were
scanned in a relaxed potential energy surface scan procedure
with steps of 0.1 Å (Figure 2). The respective transition state, as
well as product structures, were confirmed via frequency
calculation. The resulting transition state for the attack at the
halide side of the alkyne (V) is favored by about 3 kcal/mol
compared to the transition state for the radical addition next to the
phenyl substituent (V’). The resulting product geometry of V is
even more energetically favored by 11 kcal/mol, which leads to
the assumption that during the reaction every molecule will
ultimately arrive at V driven by thermodynamics. The respective
(E)-diastereoisomer of V’ lies merely 0.4 kcal/mol below the (Z)-
isomer and therefore also offers no considerable alternative in the
reaction route.
DTBP
2a
H
O
O
Ph
Cl
d) Radical trapping experiment
Cl
Standard conditions
TEMPO
O
H
N
+
O
O
O
Ph
Cl
4a (0%)
1a
2a
8 (9%)
Our tentative mechanistic pathway is based on the experimental
results depicted in Scheme 5. A weak absorption peak was
observed in the range of 320-380 nm in THF at the UV-visible
absorption spectrum of 4,4’-dichlorobenzophenone (Fig. 1),
indicating that the reaction is indeed initiated by photoexcitation
of 3d.
Figure 1. UV-VIS absorption spectra for photocatalyst 3d (red)
and chloroalkyne 2a (blue) in THF.
Based on the absorption spectra and accompanying TDDFT
calculations, the absorption band of 3d
above 300 nm
Figure 2. Reaction profile for the two possible attack positions of
the THF radical (pbe0-D3/pcseg-2/CPCM(THF)).[24]
corresponds to an n→π* transition into the first excited singlet
state S1 (see SI for corresponding electron/hole densities) and
subsequently undergoes intersystem crossing to the energetically
close triplet excited state (T3) due to a rather small singlet-triplet
gap of 0.14 eV as well as an enhanced spin-orbit coupling
element of about 22 cm-1 (see SI for the coupling elements to the
other excited triplet states). The diradical intermediate II is
generated by the relaxation into the corresponding n→π* triplet
excited state (T1), which then abstracts the α-H-atom from THF
delivering the highly reactive THF radical intermediate (IV). After
The rationale of the preference for V lies in the lower steric
hindrance as well as in the resulting resonance interaction of the
unpaired electron with the aromatic ring as seen by the
delocalization of the spin density (Figure 3). Furthermore, as a
result of the delocalization interaction with the aromatic ring, the
resulting product (V) possesses a pronounced linear character
with a bond angle of 162 degrees allowing the overlap of the
5
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
singly occupied orbital of the carbon with the π-system of the
aromatic ring as seen by the delocalization of the spin density.
Figure 3. Optimized structures of V and V’ (pbe0-D3/pcseg-
2/CPCM(THF)) and their respective spin density (isovalue = 0.02)
Conducting a Natural Bond Orbital (NBO) analysis[25] shows the
singly occupied NBO of the carbon atom to be of almost pure p-
character (94 %) compared to the usual sp2-type orbital in the
case of the constitutional isomer V’. The resonance interaction
and subsequent stabilization of the singly occupied orbital on the
carbon, where the radical is mainly located at, is highlighted by
the excessive-high amount of stabilization energy in total of 76
kcal/mol, compared to a total of 15 kcal/mol stabilization energy
for the singly occupied orbital where the unpaired electron is
located at in the case of V’.
Also, important to note is the non-existence of a stable (E)-isomer
of V, although the linear character of (V) and the p-type orbital of
the carbon atom allows for the H-abstraction reaction to occur on
both sides resulting in (E)- and (Z)-isomer products from only one
stable radical diastereoisomer. Though the resulting (Z)-products
after H-abstraction is still thermodynamically favored, the (E)-
isomer only exhibits a marginal higher free energy of around 1.6
kcal/mol (Figure 4).
Figure 5. The difference in Gibbs free energy of the
diastereoisomers of 5k (a) and 5j (b) (pbe0-D3/pcseg-
2/CPCM(THF)).
Furthermore, the extended π-system of 5k results in
a
pronounced increase in resonance stabilization energy of the
radical orbital of about 38 kcal/mol (in total 114 kcal/mol) as well
as an accompanying increase in linear character as indicated by
a bond angle of 179° (Figure 6). Since the π systems of the two
phenyl units within 5j are not fully overlapping due to the relative
twisted orientation to each other, there is only a slight increase in
resonance stabilization energy of the radical orbital by 12 kcal/mol
(total of 88 kcal/mol) as well as in bond angle to 166°.
Figure 6. Radical intermediates of 5k and 5j (pbe0-D3/pcseg-
2/CPCM(THF)).
Gram Scale Synthesis
Figure 4. Comparison of the Gibbs free energy for the two
resulting diastereoisomers following H-atom abstraction (pbe0-
D3/pcseg-2/CPCM(THF)).
To proof the practicability of the protocol we tried to scale up the
reaction to the gram scale. This was possible without any
significant decrease in yield (Scheme 6). The reaction of
chloroalkyne 2a (1.36 g, 10 mmol) and photocatalyst (15 mol%)
in THF (0.05 M) under irradiation of two 23 W CFL bulbs gave
86% (1.78 g) of the desired product. 8 mol% of the catalyst could
be recycled.
In order to shed light on the influence of a modification to the
substituent, the thermodynamics of the two diastereoisomers for
two examples, 5j and 5k, were calculated (Figure 5). The
qualitative trend of the increase in thermodynamic preference of
the (Z) isomer in the case of 5k is in alignment with the
experimental findings of enhanced diastereoselectivity as is the
experimentally observed lowering in diastereoselectivity goes
hand in hand with a lowered thermodynamic preference (Z)
isomer for 5j.
Cl
Photocatalyst 3d (15 mol %)
+
O
O
Light, RT
Cl
1a
2a
4a
(86%, 3:1)
Scheme 6. Gram scale chlorovinylation of THF.
Conclusion
In summary, we developed an efficient photocatalytic synthetic
strategy for the visible light promoted hydroalkylation of
6
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10.1002/chem.202001259
Chemistry - A European Journal
RESEARCH ARTICLE
chloroalkynes/alkynes via direct sp3-C-H functionalization of
ethers, amides, alcohols, and even unactivated alkanes by using
a household fluorescent light bulb. Cheap and readily available
diaryl ketones were used as photocatalyst as well as H-atom
abstractors. The hydroalkylation of alkynes via sp3-C-H bond
functionalization was achieved in an atom economic and redox-
efficient manner. Especially for the synthesis of vinyl halides from
haloalkynes, this process is remarkable as the radical approach
opens an orthogonal pathway compared to transition metal-
catalyzed strategies that are used for alkynylations. Nevertheless,
this process was also feasible if bromoalkynes were used in the
presence of a base. This radical alkynylation sp3-C-H bond offers
an attractive alternative to existing strategies. The mechanistic
studies indicate that the solvent/reactant act as a hydrogen
source. To the best of our knowledge, this is the first access to
chlorovinylated products of ethers and amides. The reaction is
highly regioselective and chemoselective under such conditions.
Due to the immense importance of vinyl halides as the key
building block, this new attractive access from readily available
starting materials in an atom-economic manner is of high value to
the chemical community.
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Keywords: Hydroalkylation • C-H functionalization •
photoorganocatalysis
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Entry for the Table of Contents
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Triplet ketone Catalysis
A simple triplet ketone catalysis, generated by household visible light bulb irradiation, open up a vinylation path with sp3-C-H
substrate whereas dehalogenation takes place with bromoalkyne provided alkyne products. This methodology allows selective
intermolecular C-C bond formation.
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