Titanocenes as Photoredox Catalysts Using Green-Light Irradiation

Article abstract of DOI:10.1002/anie.202001508

Irradiation of Cp₂TiCl₂ with green light leads to electronically excited [Cp₂TiCl₂]*. This complex constitutes an efficient photoredox catalyst for the reduction of epoxides and for 5-exo cyclizations of suitably unsaturated epoxides. To the best of our knowledge, our system is the first example of a molecular titanium photoredox catalyst.

Full text of DOI:10.1002/anie.202001508

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: TTitanocenes as Photoredox Catalysts Using Green-Light
Irradiation
Authors: Andreas Gansäuer, Zhenhua Zhang, Tobias Hilche, Daniel
Slak, Niels Rietdijk, Ugochinyere N. Oloyede, and Robert A.
Flowers
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001508
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Angewandte Chemie International Edition
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COMMUNICATION
Titanocenes as Photoredox Catalysts Using Green-Light
Irradiation
[
a]
[a]
[a]
[a]
[b]
Zhenhua Zhang, Tobias Hilche, Daniel Slak, Niels Rietdijk, Ugochinyere N. Oloyede, Robert A.
[
b]
[a]
Flowers II* and Andreas Gansäuer*
Dedicated to the memory of Kilian Muñiz.
Abstract: Irradiation of Cp
2
TiCl
2
with green light leads to
variety,[3] they are colored and can absorb visible light, and, finally,
the electron transfer chemistry of titanocene complexes has been
electronically excited [Cp TiCl ]*. This complex constitutes an efficient
2
2
[
4]
photoredox catalyst for the reduction of epoxides and for 5-exo
cyclizations of suitably unsaturated epoxides. To the best of our
knowledge, our system is the first example of a molecular titanium
photoredox catalyst.
successfully investigated in recent years. In agreement with
previous studies,[
5a,b]
our TDDFT calculations (time-dependent
density functional theory, for details see the supporting
information)[ show that excitation results in an electron transfer
from the highest occupied molecular orbital (HOMO) contributing
to binding of the cyclopentadienyl ligands (Cp) to the mainly Ti
centered lowest unoccupied molecular orbital (LUMO). This leads
to a weakening of the bonding of the Cp ligands. Accordingly, the
6]
Due to the high abundance and low toxicity of the metal, the use
of titanium catalysts is highly attractive for the development of
sustainable reactions.[1] Additionally, titanium complexes, like
many other compounds of 3d metals, undergo facile one-electron
oxidation state changes. Therefore, they are attractive electron
transfer catalysts in radical reactions. Given these advantages, it
is surprising that in one of the most active areas of radical
chemistry, photoredox catalysis,[2] no titanium based catalysts
have been introduced. Here, we describe the first examples of
such reagents.[
slow decomposition of photoexcited Cp
2 2 2 2
TiCl and Cp TiBr occurs
[5a]
via loss of neutral C and not by loss of Cl or Br.
5
H
5
The
titanocenes exhibit an intense, long-lived charge transfer
phosphorescence at 77 K from the lowest ligand-to-metal-charge
2 2
TiCl
: ca. 800 μs).[5b-d] At
transfer (LMCT) in the solid state (Cp
5 °C, the life times are reduced even in the solid state.
2
We investigated the use of titanocene(IV) halides as photoredox
catalysts in their most common application in electron transfer
chemistry, the reductive opening of epoxides. In order to avoid the
potential problem of the short life-times of the photoexcited
2a]
2 2
species we examined the irradiation of Cp TiX in the presence of
a reductive quencher Q that also constitutes a potential ligand for
titanium. Using this approach, it should be possible to generate
●+
the desired electron transfer catalyst Cp
without complete non-radiative deactivation of the photoexcited
Cp TiX
]*. Ideally, Q●+ can be used in the radical chemistry
2
TiX together with Q
[
2
2
following reductive epoxide opening (Scheme 1).
2 2
Table 1: Optimization of the reduction of 1 with photoexcited Cp TiCl .
Scheme 1. Concept of the use of photoexcited titanocenes in synthesis
(LMCT = ligand to metal charge transfer; SET = single electron transfer).
entry
visible light
MTG
yield [%]
To this end, titanocene(IV) dihalides are especially attractive
candidates. They are readily available in a large structural
1
blue LED
blue LED
green LED
none
no
yes
yes
yes
no
n. r.
52
2
[
a]
Dr. Z. Zhang, T. Hilche, D. Slak, N. R. Rietdijk, Prof. Dr. Andreas
Gansäuer
3
85
Kekulé-Institut für Organische Chemie und Biochemie
Universität Bonn
4
n. r.
n. r.
n. r.[b]
n. r.[c]
Gerhard Domagk-Straße 1, 53121 Bonn, Germany
E-mail: andreas.gansaeuer@uni-bonn.de
U. N. Oloyede, Prof. R. A. Flowers II
Department of Chemistry
5
green LED
green LED
green LED
[
b]
6
yes
yes
Lehigh University
Bethlehem, PA 18015 USA
7
Email: rof2@lehigh.edu
[
a] conditions: Cp
2
TiCl
2
(10 mol%), 3 (3 eq.), 4 (20 mol%), epoxide 0.1 M in
TiCl . [c] no iPr NEt.
Supporting information for this article is given via a link at the end of
the document.
tetrahydrofuran (THF), r.t., two 10 W LEDs. [b] no Cp
2
2
2
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COMMUNICATION
Amines are attractive targets in this respect because they can be
oxidized to the corresponding radical cations that are potential
hydrogen atom donors.[7] Our initial results for the reduction of 1[8]
triphenylamine are not effective quenchers. The proposed
mechanism (Figure 2) features formation of Cp
2
TiCl from
photoexcited Cp TiCl by reductive quenching with DIPEA,
2
2
in the presence of N,N-diisopropyl ethyl amine 3 (iPr
base, DIPEA) are summarized in Table 1.
When only Cp TiCl and 3 are irradiated with blue light in the
2 2
presence of 1, no 2 is formed (entry 1). This indicates that 3 is
not a good enough hydrogen atom donor to reduce the radical
formed through opening of 1. In the presence of a hydrogen atom
transfer catalyst (HAT catalyst) methyl thioglycolate 4 (MTG),[9]
2
NEt, Hünig’s
epoxide opening to yield the β–titanoxy radical A that is reduced
via the hydrogen atom transfer (HAT) cycle involving MTG (4).
Because of the absence of vibrations due to an OH group in the
IR spectrum of the reaction solution 2 is not formed in the reaction.
We propose the hemiaminal (B) as initial product.
•+
0.2
2500
2000
1500
1000
500
52% yield of 2 could be obtained (entry 2). Changing the source
0.18
0.16
0.14
0.12
of light to a green light emitting diode (LED) lead to a noticeable
improvement in the yield of 2 (85%, entry 3). We propose that the
lower wavelength of irradiation bypasses catalyst decomposition.
The control experiments (entries 4-6) show that the presence of
0
.1
0.08
0.06
0.04
Cp
to 2.
Next, we examined the influence of the amine and the counter ion
X of Cp TiX
on the performance of the reaction (Table 2).[10] Of
2 2
TiCl , MTG, DIPEA, and light is mandatory for conversion of 1
0.02
0
0
2
2
4
50
500
550
600
650
700
the other anions, only the use of –Br results in a photoexcitation
and a good yield of 1 (74%) in the presence of 3. Titanocenes with
Wavelength/nm
X = F, OMs or O
photoexcitation (See SI for the calculated spectra). With X = –Cl,
NEt leads to a noticeable lower yield than 3 and, curiously, in the
presence of iPrNMe no product is formed at all.
2 3
CCF require light of higher energy for
in THF at -78 oC.
2 2
Figure 1: Absorption and emission spectrum of Cp TiCl
3
2
Table 2: Influence of the amine and the anion X on the reduction of 1.
entry
X
F
amine
yield [%]
n. r.
1
2
3
4
5
6
iPr
2
iPr
2
iPr
2
iPr
2
NEt
NEt
NEt
NEt
Br
74
OMs
n. r.
O
2
CCF
3
n. r.
Cl
NEt
3
49[b]
n. r.
Cl
iPrNMe
2
Figure 2: Proposed mechanism for epoxide reduction in the presence of
photoexcited Cp TiCl
[
a] conditions: Cp
2
TiX
2
(10 mol%), amine (3 eq.), 4 (20 mol%), epoxide 0.1 M in
2
2
.
1
THF, r.t., two 10 W green LEDs. [b] by H-NMR spectroscopy against an internal
standard (1,3,5-trimethoxylbenzene).
We investigated the substrate scope of the epoxide reduction next
(Table 3). Monosubstituted, 1,2- and 1,1-disubstituted, as well as
trisubstituted epoxides are suitable substrates for our reaction.
The diastereoselectivities of radical reduction (entries 3-5) are
typical for radical reactions and rule out different mechanisms of
epoxide opening. Gratifyingly, monosubstituted epoxides are
opened with much higher regioselectivity than in the Mn or Zn
based systems.[4l]
Arguably, the synthetically most important applications of radical
chemistry are cyclizations. We investigated examples of the most
prominent reaction in this respect, the 5-exo cyclization.[11,12] Initial
experiments were carried out under the conditions of the reductive
epoxide opening. However, with 21 the desired product was
In Figure 1 the absorption and emission spectrum of a solution of
o
0
.01 mM Cp
2
TiCl
2
in tetrahydrofuran (THF) at -78 C is shown.
We attribute the emission to transition from the first triplet state of
photoexcited Cp TiCl
.[5a,b] At room temperature the emission is
2
2
not observed suggesting that the lifetime of the transition is
transient. The success of the reductive epoxide opening
demonstrates that the triplet state does have a lifetime in THF at
room temperature that is long enough for our purposes. The
bimolecular quenching constant for photoexcited Cp
2 2
TiCl and
DIPEA was determined by Stern-Volmer analysis and determined
4
o
to be 1 x 10 at -78 C. This data clearly demonstrates reduction
by DIPEA (see SI for details). Interestingly, other amines including
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COMMUNICATION
obtained in a messy reaction with low yield. We attribute the
failure of the conditions to a competition of the reduction of the
thiyl radical by 3•+ (the HAT-cycle of Figure 2) with an addition of
the thiyl radical to the double bond of 21. This process is well
known as key-step in ‘thiol ene reactions’ and will result in a
diminished efficiency of radical reduction.[13] The addition of
are typical for radical reactions and identical to those performed
with Cp
2
TiCl formed by reduction of Cp
2
TiCl
that is irradiated with
2
with Zn or Mn.[11b,12]
In summary, we have shown that Cp
2
TiCl
2
green light constitutes an efficient photocatalyst for epoxide
derived radical chemistry. A key aspect of our approach is the use
2 2 2
NEt as oxidative quencher for [Cp TiCl
]*. The activation of
of iPr
iPr
NEt+• for HAT is crucial for closing the catalytic cycles. To the
PhSiH
catalyst to n-octyl thioglycolate 23 (OTG) resolved this issue
Scheme 2) and lead to 22 in 80% yield.[14] A yield of 75% was
3
(2.5 eq.) to the reaction mixture and a change of the HAT
2
best of our knowledge, we have presented the first example of
photoredox chemistry catalyzed by a molecular titanium complex.
(
obtained with 4 as HAT catalyst. The use of 0.5 eq. of PhSiH
gave 22 but in lower yields.
3
also
2 2
Table 4: Optimization of the cyclization of 21 with photoexcited Cp TiCl .
2 2
Table 3: Scope of the epoxide reduction by photoexcited Cp TiCl .
entry
PhSiH
3
[eq.]
yield [%]
1
2
3
4
-
40[b,c]
75
0.5
2.5
2.5
80
68[c]
[
2 2 3
a] conditions: Cp TiCl (10 mol%), 3 (3 eq.), 23 (20 mol%), PhSiH (0.5-2.5 eq.),
epoxide 0.05 M in THF, r.t., two 10 W green LEDs. [b] by 1H NMR spectroscopy
against an internal standard (1,3,5-trimethoxybenzene). [c] MTG (4) used.
[
a] conditions: Cp
r.t., two 10 W green LEDs. [b] 17 (0.05 M in THF), 50°C, 94:6 mixture of 1-and
-dodecanol. [c] 19 (0.05 M in THF), 50 ℃, 96:4 mixture of 1,5- and 1,4-diol.
TBS = tBuMe Si.
2 2
TiCl (10 mol%), 3 (3 eq.), 4 (20 mol%), epoxide 0.1 M in THF,
2
2
Scheme 2: Examples of 5-exo cyclizations with photoexcited Cp
2 2
TiCl . [a] 1 eq.
The other examples show that tetrahydrofurans can be readily
accessed with our method. Alkynes are suitable radical
acceptors. Once again, the diastereoselectivity of the reactions
PhSiH
3
.
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Keywords: catalysis • epoxide • light • radical • titanocene
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Z. Zhang, T. Hilche, D. Slak, N. Rietdijk,
U. N. Oloyede, R. A. Flowers II*,
A. Gansäuer*
Page No. – Page No.
Titanocenes as Photoredox Catalysts
Using Green-Light Irradiation
2 2
The lights are green for using Cp TiCl as photoredox catalyst! This first use of a
titanium complex in photoredox catalysis enables efficient catalytic radical chemistry
with epoxides under very mild conditions and under ragent control.
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