Visible-Light-Driven Intermolecular Reductive Ene–Yne Coupling by Iridium/Cobalt Dual Catalysis for C(sp3)?C(sp2) Bond Formation

Article abstract of DOI:10.1002/chem.201903708

A new methodology to form C(sp³)?C(sp²) bonds by visible-light-driven intermolecular reductive ene–yne coupling has been successfully developed. The process relies on the ability of the Hantzsch ester to contribute in both SET and HAT processes through a unified cobalt and iridium catalytic system. This procedure avoids the use of stoichiometric amounts of reducing metallic reagents, which is translated into high functional-group tolerance and atom economy.

Full text of DOI:10.1002/chem.201903708

A Journal of
Accepted Article
Title: Visible-Light-Driven Intermolecular Reductive Ene-Yne Coupling
via Iridium/Coblat Dual Catalysis for C(sp3)-C(sp2) Bond
Formation
Authors: Bernhard Breit and Maria González
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201903708
Link to VoR: http://dx.doi.org/10.1002/chem.201903708
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Visible-Light-Driven Intermolecular Reductive Ene-Yne Coupling
via Iridium/Cobalt Dual Catalysis for C(sp3)-C(sp2) Bond
Formation
María J. González,[a] and Bernhard Breit*[a]
A novel methodology to form C(sp3)ꟷC(sp2) bonds via visible-light-
driven intermolecular reductive ene-yne coupling has been
successfully developed. The process relies on the ability of the
Hantzsch ester to contribute in both SET and HAT processes
through a unified cobalt and iridium catalytic system. This procedure
avoids the use of stoichiometric amounts of reducing metallic
reagents which is translated into high functional group tolerance and
atom-economy.
heterogeneous metals or Grignard reagents due to their synthetic
challenge and poor stability.8,2f However, in their study, a tertiary
amine is employed as a sacrificial organic reductant in a visible-light-
driven photoredox cycle, overcoming the problem of limited
functional group tolerance. Later, Zhao, Wu et al. made use of this
protocol for the hydrocarboxylation and carboxylation of alkynes
using CO2.9 Nevertheless, despite of its potential in CꟷC bond
construction, this methodology has not been further developed.
The development of novel methodologies for selective CꟷC bond
formation in a highly atom-economical way is one of the most
relevant and evolving research areas in organic synthesis.1
Particularly, the transition-metal-catalyzed reductive coupling of
easily available, inexpensive and bench stable π components
provides a straightforward route to achieve this goal.2,1b However,
the available methodologies require stoichiometric amounts of
metallic reductants such as Zn powder, silanes, boranes or
Grignard reagents. In this context, Chen et al. reported in 2002 the
cobalt-catalyzed intermolecular reductive coupling of alkynes with
conjugated alkenes in a highly chemo-, regio-, and stereoselective
fashion (Scheme 1a).3 This pioneering result is significant since
typical cobalt-catalyzed CꟷC reactivities such as cyclotrimerization4a-
c and carbonylation4b-c did not ensue. Notwithstanding the power of
this approach, it struggles with low scope and the need of Zn powder
in (super)stoichiometric amounts as reducing agent to generate
active low-valent Co (I) species. Thus, the introduction of catalytic
activation modes that are environmentally benign and capable of
achieving a higher degree of structural diversity are highly desirable
in this topic.
Scheme 1. Cobalt-catalyzed intermolecular reductive ene-yne coupling. EWG
= electro withdrawing group.
Considering the lack of methodologies to achieve C(sp3)ꟷC(sp2)
bonds with both high atom-economy and broad functional group
tolerance,2 we envisioned that a photopromoted reaction pathway
may provide opportunities to achieve new structural diversity in an
environmentally compatible fashion. Herein, we report our findings
on an unprecedented visible-light-driven intermolecular reductive
ene-yne coupling via cobalt/iridium dual catalysis (Scheme 1b).10,11
Over the past decade, the renowned field of photochemistry is
retrieving a central role in synthetic endeavours.5 Especially, the
fast-moving area of photoredox catalysis has witnessed dramatic
developments which have enabled previously inaccessible or
inefficient transformations.6 Recently, Rovis et al. have explored the
combination of photoredox catalysis with cobalt catalysis in order to
access low-valent cobalt (I) or Co (0) intermediates for the
construction of arenes7a and in the hydroaminoalkylation of
conjugated dienes.7b-c These cobalt intermediates are traditionally
generated in situ using strong reducing conditions such as
To evaluate the feasibility of this hypothesis, easily accessible 2-
octyne (1a) and ethyl acrylate (2a) were selected as model
substrates (Table 1). At the outset of our investigations, we used the
following reaction conditions: CoBr2 (10 mol%), dppp (10 mol%),12
Ir(dF(CF3)ppy)2(dtb-bpy)PF6 (0.5 mol%) as photocatalyst (PC), N-
ethyldiisopropylamine (DIPEA) (3.0 equiv.) as sacrificial organic
reductant and CsOPiv (0.5 equiv.) as a base in MeCN under
irradiation with a 4.8 W Blue LED strip (entry 1). Under these
conditions, we observed the formation of regioisomers 3a/4a in low
yields and moderate (70:30) selectivity with 3a as the major product.
Regrettably, after testing different bases and solvents the previous
result could not be improved (see Table S1 in the supporting
information).
[a]
Dr. M.G. González, Prof. Dr. B. Breit
Institut für Organische Chemie, Albert-Ludwigs-Universität
21, 79104 Freiburg, Germany.
E-mail: bernhard.breit@chemie.uni-freiburg.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Then, we turned our attention to easily accessible and bench stable
Hantzsch esters (HEs)13 which over the last decade have emerged
as key electron14 and proton15 donors in a variety of challenging
photoredox reactions. HEs can be readily converted in to the
corresponding pyridines by means of a stepwise pathway via either
single electron transfer (SET) followed by hydrogen atom transfer
(HAT) or vice versa.16 Besides, their oxidative potential is typically
0.8ꟷ0.9 V vs. SCE,17 suggesting that they have redox properties
red
comparable to those of amines18 (E1/2
= 0.8ꟷ1.0 vs. SCE).
Gratifyingly, when using Hantzsch ester (HE) as organic reductant
together with 4-(dimethylamino)pyridine (DMAP) as base, 3a/4a
were obtained in a 73 % yield. Other commercially available
pyridines were tested proving less effective than DMAP. Increasing
the PC loading to 2.0 mol% the yield was improved to 80% without
affecting the selectivity (entry 5). Tests reactions in the absence of
CoBr2, PC, HE, DMAP and Blue LED were performed proving that
all the additives are required for the transformation (see Table S3 in
the supporting information).
Table 2. Visible-light-driven intermolecular reductive ene-yne coupling with
alkyl-alkyl substituted alkynes: Scope.
Table 1. Initial findings and reaction conditions optimization.[a],[b]
A subsequent screening of ligands in order to improve the selectivity
revealed that xantphos is a better ligand when using aryl substituted
alkynes affording isomers E/Z (see Tables S4 and S5 in the
supporting information). Thus, 1-aryl-1-akyl substituted alkynes were
studied employing xantphos as ligand (Table 3). Under these
conditions, 1-phenyl-1-propyne afforded 6a/7a in very good
combined yield and selectivity. Other tested acrylates gave the
corresponding derivatives in similar good yields and selectivities.
Interestingly, single isomer 6g was obtained when treated 1-phenyl-
1-propyne with acrylonitrile yet in a 35% yield. The reaction tolerated
aryl groups bearing methyl (6h/7h), trifluoromethyl (6i/7i), bromine
(6j/7j) and ester (6k/7k) substituents in good yields and moderate
selecivities. Other aryl groups such as naphthyl (6l/7l), thiophen
(6m/7m) and 1,3-benzodioxole (6n/7n) were also reactive in low to
moderate yields and selectivities. Different substituents at position
two like propyl (6o/7o), benzyl (6p/7p), methoxymethyl (6q), allyl
(6r/7r) and silyl ether protected alcohol (6s/7s) proved also suitable.
entry
z
reductant
DIPEA
y
base
x
yield (%)[c],[d]
1
2
3
4
5
0.5
0.5
0.5
0.5
0.5
2.0
3.0
CsOpiv
CsOpiv
0.5
0.5
3.0
6.0
3.0
3.0
13
9
HE
HE
1.5
1.5
Et3N
Pyridine
DMAP
DMAP
nr
HE
HE
HE
1.5
1.5
1.5
46
73
80
6
[a]
Optimizations were performed on a 0.2 mmol scale using 1a (1.0
equiv.), 2a (4.0 equiv.), CoBr2 (10 mol%), dppp (10 mol%) in MeCN over
[b]
a period of 16h under irradiation with a 4.8 W Blue LED strip.
For
further details on the optimization conditions see supporting information.
[c]
[d]
Isolated yield.
3a/4a were always obtained in a (70:30) selectivity
determined by 1H-NMR analysis of the crude mixture.
Remarkably,
when
using
(3-methoxyprop-1-yn-1-yl)benzene
By using these optimized conditions, the scope of this metallo-
photoredox reductive ene-yne coupling was first evaluated for alkyl-
alkyl substituted alkynes (Table 2). 2-octyne in the presence of ethyl
acrylate affords regioisomers 3a/4a in good yield and moderate
selectivity. However, when using tert-butyl acrylate both yield and
selectivity decrease (3a/4a). It should be noticed that with a bulky
substituent such as isopropyl at R1 only product 3c was obtained
albeit in lower yield (the bulkier tert-butyl substituent was unreactive
under these conditions). Symmetric alkyne 4-octyne delivered 3d
and 3e in 70% and 43% yields respectively. Silyl ether protected
alcohols were tolerated giving 3g/4g in good yield and moderate
selectivity. Remarkably, compound 4f was obtained as a single
product in a 65% yield probably due to steric hinderance. The
reaction also took place in the presence of an ester group affording
4h/3h in moderate yield and selectivity. It should be noticed that in
this case the regioselectivity shifts being 4h the major product which
can be associated to steric impediments as in 4f.
compound 6q was obtained as a single isomer.
Table 3. Visible-light-driven intermolecular reductive ene-yne coupling with 1-
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aryl-1-alkyl substituted alkynes: Scope.
The reaction was also tested with internal diaryl substituted alkynes
(Table 4). Under these conditions, tolan afforded isomers 9a/10a in
very good combined yield and moderate selectivity. Methyl acrylate
gave similar results while with bulkier R3 substituents such as butyl,
tert-butyl and phenyl the yields decreased. As expected, in the case
of unsymmetrical 1-(phenylethynyl)-4-(trifluoromethyl)benzene) the
acrylate was added regioselectively to the most electron rich carbon
(9f/10f). Remarkably, when using electron poor diaryl alkynes
isomers 9g-i and 10g-i could be separated.
Table 4. Visible-light-driven intermolecular reductive ene-yne coupling with
Scheme 2. Mechanistic studies.
In light of the experimental data as well as previous studies in
metallo-photoredox catalysis and intermolecular reductive ene-yne
coupling, a plausible mechanism is proposed in Scheme 3. First, a
SET process from the HE to the excited-state photocatalyst Ir(III)*
might occur generating the radical cation A. This reduction could be
possible given their reported reduction potentials (E1/2red [IrIII/IrII] = -
red
1.37 V vs SCE and E1/2 [HE] = -2.3 V vs SCE).7b,16 Then, the Co
(II) salt may be reduced in situ to Co (I) species by the
photocatalyst.7b Next, considering that the HE can contribute in both
SET and HAT processes,15d a hydrogen atom transfer from the
radical cation A to Co (I) could afford the Co (II)-H complex.20 Here,
it is assumed that DMAP would facilitate the deprotonation of A to
give B.15d The acrylate could undergo migratory insertion in to CoꟷH
giving the five-membered metallacycle C.21 At this stage, depending
on the nature of the alkyne two different insertion paths into the
CoꟷC bond could occur. Thus, when using alkyl-alkyl substituted
alkynes, intermediate D could be generated (Path I), whereas for
alkynes bearing one or two aromatic groups intermediate E might be
formed (Path II). From these intermediates, a subsequent reduction
of Co(III) to Co(II) by the photocatalyst7b followed by a final
protonolysis step would deliver the corresponding products and Co
(II) back into the catalytic cycle.3
internal diaryl substituted alkynes: Scope.
To gain insights into this transformation and propose a rational
reaction pathway, some control experiments were carried out
(Scheme 2). Firstly, to determine the proton source. As it is shown in
Scheme 2a, when the reaction was performed in CD3CN no
deuterated products were observed. Nevertheless, when the
deuterated HE (11) was employed the corresponding deuterated
products were obtained together with the deuterated form of the
oxidized HE (Scheme 2b). Besides, Stern-Volmer quenching
experiments reveal significant quenching interactions between the
excited state of the photocatalyst and the HE (see Figure S5 in the
supporting information). This indicates that the HE could act as both
proton source and terminal reductant. Additionally, the quantum yield
(Φ) was found to be 5%, which supports that a radical-chain
propagation mechanism is unlikely. Moreover, this is also in
agreement with on/off experiments where the catalytic system is
activated under irradiation and deactivated in darkness (see Figure
S6 in the supporting information). Then, to prove that this process
involves radicals, the reaction was conducted in the presence of the
radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
(Scheme 2c). In this case, the process was completely inhibited
which provides evidence of a possible initial electron transfer.19
Conclusions
In summary, we have reported a novel methodology to form
C(sp3)ꟷC(sp2) bonds via visible-light-driven intermolecular reductive
ene-yne coupling from commercially available and bench stable
alkynes and alkenes. This approach relies on the ability of the HE to
contribute in both SET and HAT processes through the synergistic
combination of photoredox and cobalt catalysis. The employment of
very mild reaction conditions avoiding the use of metallic reagents is
translated into a broad functional group tolerance. Besides, the
proposed approach is in line with main goals in synthesis such as
atom-economy and sustainability principles highlighting the potential
of this transformation. Further studies to expand the scope as well
as new synthetic applications are currently underway.
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Scheme 3. Proposed mechanism for the visible-light-driven intermolecular reductive ene-yne coupling via cobalt/iridium dual catalysis
[6]
For selected reviews: a) J. M. R. Narayanam, C. R. J. Stephenson,
Acknowledgements
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Pagire, O. Reise, B. König, Angew. Chem. Int,. Ed. 2018, 57,10034-10072; e)
Modern molecular photochemistry of organic molecules, N. J. Turro, V.
Ramamurthy, J. C. Scaiano (Eds.), University Science Books, 2010.
We thank the Principality of Asturias and the EU for a Maire
Curie Clarín-COFUND postdoctoral grant (ACA-1712) (M.G.).
Conflicts of interest
There are no conflicts to declare.
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Keywords: alkynes • reductive coupling • photoredox • dual
catalysis • cobalt
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a mixture of i-Pr2NEt/Hantzsch ester (1:2) is
employed as the organic reductant and H2O (1.0 equiv.) is the proton source.
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act as both reductant and proton source (contribute in both SET and HAT
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COMMUNICATION
María J. González and Bernhard
Breit*
Page No. – Page No.
Visible-Light-Driven
Intermolecular Reductive Ene-
Yne Coupling via Iridium/Cobalt
Dual Catalysis for C(sp3)ꟷC(sp2)
Bond Formation
A novel methodology to form C(sp3)ꟷC(sp2) bonds via visible-light-
driven intermolecular reductive ene-yne coupling has been developed.
This procedure avoids the use of stoichiometric amounts of reducing
metallic reagents which is translated into high functional group
tolerance and atom-economy.
This article is protected by copyright. All rights reserved.