Degenerative xanthate transfer to olefins under visible-light photocatalysis

Article abstract of DOI:10.3762/bjoc.14.283

The degenerative transfer of xanthates to olefins is enabled by the iridium-based photocatalyst [Ir{dF(CF₃)ppy}₂(dtbbpy)](PF₆) under blue LED light irradiation. Detailed mechanistic investigations through kinetics and phot

Full text of DOI:10.3762/bjoc.14.283

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Degenerative xanthate transfer to olefins under
visible-light photocatalysis
Atsushi Kaga, Xiangyang Wu, Joel Yi Jie Lim, Hirohito Hayashi, Yunpeng Lu,
Edwin K. L. Yeow* and Shunsuke Chiba*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 3047–3058.
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
doi:10.3762/bjoc.14.283
Received: 28 September 2018
Accepted: 28 November 2018
Published: 13 December 2018
Email:
Edwin K. L. Yeow* - edwinyeow@ntu.edu.sg; Shunsuke Chiba* -
shunsuke@ntu.edu.sg
This article is part of the thematic issue "Reactive intermediates part I:
radicals".
* Corresponding author
Guest Editor: T. P. Yoon
Keywords:
energy transfer; olefin; photocatalysis; radical; xanthate
© 2018 Kaga et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The degenerative transfer of xanthates to olefins is enabled by the iridium-based photocatalyst [Ir{dF(CF3)ppy}2(dtbbpy)](PF6)
under blue LED light irradiation. Detailed mechanistic investigations through kinetics and photophysical studies revealed that the
process operates under a radical chain mechanism, which is initiated through triplet-sensitization of xanthates by the long-lived
triplet state of the iridium-based photocatalyst.
Introduction
A degenerative radical transfer of xanthates to olefins has been formation of carbon radicals A that add onto olefins 2. The
developed as a robust synthetic tool to create new C–C and C–S subsequent reaction of the resulting alkyl radicals B with
bonds in a single operation [1-13]. The method is featured by xanthates 1 provides xanthate adducts 3 with generation of car-
not only its capability of introducing a wide range of carbon bon radicals A that maintain the radical chain (Scheme 1A).
substituents but also the ability of the installed xanthyl group in Peroxide initiators such as dilauroyl peroxide (DLP) are com-
being transformed into a variety of functionalities [1-14]. This monly utilized [1-14], while decomposition of DLP needs a
concept has also been of particular importance in the field of high reaction temperature and inevitably generates consider-
polymer science, known as reversible addition–fragmentation able amounts of byproducts derived from DLP that sometimes
chain transfer (RAFT) polymerization [15,16]. Mechanistically, require tedious purification of the desired products. A combina-
the degenerative transfer of xanthates 1 to olefins 2 proceeds tion of triethylborane (Et3B) and molecular oxygen can also
through a radical chain mechanism, and thus requires an initial initiate the reaction at lower temperature (e.g., room tempera-
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Beilstein J. Org. Chem. 2018, 14, 3047–3058.
ture), while the employment of Et3B is hampered due to its
pyrophoric nature under aerobic conditions as well as unde-
sired Et3B-mediated dexanthylation of α-xanthyl ketones [17-
21]. As an alternative strategy, a light-driven approach has been
developed [22-26], since the first degenerative transfer of
xanthates using S-benzoyl O-ethyl xanthate as a photo-cleav-
able initiator under tungsten lamp irradiation was reported by
Zard [25,26] (Scheme 1B). However, these protocols have thus
far adopted energy intensive light sources. Therefore, there is
still ample room for establishing new protocols to realize the
degenerative transfer of xanthates onto olefins under user-
friendly and milder reaction conditions. Herein, we report a
photocatalytic degenerative radical transfer of xanthates to
olefins using an iridium-based photocatalyst under blue LED ir-
radiation (Scheme 1C). A series of mechanistic investigations
identified that the process involves a triplet-sensitization of the
xanthates by the long-lived triplet state of the iridium-based
photocatalyst that triggers the radical chain process [27].
Results and Discussion
Over the last decade, there has been a remarkable advance in
synthetic chemistry that takes advantage of various chro-
mophores (either metallic or organic) having visible-light
charge transfer absorption [28-37]. In the area of polymer syn-
thesis, visible-light-induced RAFT polymerization of xanthates
with vinyl monomers under blue LED (light-emitting diode) ir-
radiation has been reported [38-41]. Visible-light-induced
single unit monomer insertion of the thiocarbonylthio com-
pounds has also been developed for the synthesis of the se-
quence-controlled oligomers [41-45]. For example, the group of
Boyer and Xu developed fac-Ir(ppy)3 (6)-catalyzed polymeriza-
tion of xanthate 4 with various vinyl monomers such as vinyl
acetate, providing polymers of type 5 having a high molecular
weight with a narrow molecular weight distribution. It was pro-
posed that the polymerization is initiated by single-electron
reduction of xanthate 4 by the highly reducing photo-excited
state of fac-Ir(ppy)3 (6) [46], although the details were not elu-
cidated (Scheme 2) [39,40].
Scheme 1: Degenerative radical transfer of xanthates to olefins.
Based on these backgrounds, we wondered if the degenerative
transfer of xanthates onto olefins could be facilitated by visible-
light photocatalysis under milder reaction conditions. We there- cially, the rather oxidizing [Ir{dF(CF3)ppy}2(dtbpy)](PF6) (8)
fore commenced our investigation with the reaction of ethyl resulted in full conversion of 1a, affording 3aa in 89% yield
ethoxycarbonylmethyl xanthate (1a) and 1-octene (2a) using (Table 1, entry 3). Other photocatalysts, such as Ru(bpy)3Cl2
fac-Ir(ppy)3 (6) in DMSO under blue LED irradiation (λmax = (9) [46], fluorescein (10) [48], and phenoxazine 11 [49], were
469 nm, Table 1, entry 1). As expected, the desired xanthate not optimal for the present transformation (Table 1, entries
transfer was observed, while the process efficiency was not very 4–6). It should be noted that the reaction without the photocata-
high, forming 3aa in only 58% yield with incomplete conver- lyst under visible light- and halogen lamp irradiation resulted in
sion even after stirring for 20 h. Interestingly, we found that the poorer conversion with formation of 3aa in only 10% and 30%
employment of the less reducing Ir catalysts 7 [46] and 8 [47] yield, respectively, suggesting that the photocatalyst was impor-
also worked for the process (Table 1, entries 2 and 3). Espe- tant for the degenerative transfer of xanthate 1a (Table 1,
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Beilstein J. Org. Chem. 2018, 14, 3047–3058.
Scheme 2: Photocatalytic RAFT polymerization of xanthate 4.
entries 7 and 8). On the other hand, the employment of a light excitation. When the concentration of xanthate 1a was
365 nm UV lamp in place of the blue LED gave 3aa in 75% gradually increased, a reduction in the PL intensity (I) of photo-
yield, although a slower reaction rate was observed compared to catalyst 8 was observed (Figure 1A). The Stern–Volmer plot of
the optimal reaction conditions (Table 1, entry 9).
the ratio I0/I, where I0 is the initial PL intensity in the absence
of quencher, versus concentration of 1a showed a linear rela-
In principle, visible-light-mediated photocatalysis can serve for tionship with a quenching rate kq = 1.25 × 107 M−1s−1 (see Sup-
electron transfer (for either oxidation or reduction) and/or for porting Information File 1). On the other hand, the addition of
energy transfer. We found that the reduction potential Ep/2 of 1-octene (2a, 40 mM), in place of xanthate 1a, resulted in only
xanthate 1a is −1.78 V vs SCE, which is not sufficient to be a small PL quenching of photocatalyst 8 (<8%, Figure 1B).
reduced by the photoexcited states of Ir catalysts 6–8 (E1/2 of
6*, 7* and 8* = −1.73, −1.28, and −0.89 V vs SCE, respective- The time-resolved PL lifetime decay profiles of photocatalyst 8
ly [46,47]). Apparently, photoinduced single-electron reduction (25 μM solution in degassed DMSO, 410 nm pulse excitation
of xanthate 1a by the photoexcited state of the optimal catalyst and monitoring emission at 480 nm) were recorded in the
8 is not feasible. In contrast, the triplet energy ET of xanthate 1a absence of a quencher, and in the presence of xanthate 1a and
was estimated as 57.5 kcal/mol by DFT calculation, that is close 1-octene (2a, 40 mM, Figure 1C). The lifetime profiles were de-
to those of photocatalysts 7 and 8 (ET = 60.1 kcal/mol [50]), in- scribed using a mono-exponential decay function with a life-
dicating that the process could be initiated by the triplet sensiti- time of 1.40 μs in the absence of a quencher, and 1.03 and
zation pathway [51,52]. This assumption is in agreement with 1.37 μs in the presence of xanthate 1a and 1-octene (2a), re-
the lower process efficiency (Table 1, entry 1) observed in the spectively. The decrease in the PL lifetime of photocatalyst 8 in
reaction with fac-Ir(ppy)3 (6) that possesses a lower triplet the presence of xanthate 1a suggests that they are interacting
energy (ET = 55.2 kcal/mol [50]). The optimal photocatalyst 8 with each other. On the other hand, only a very weak interac-
[47] has a longer excited state lifetime than 7 does [46], tion exists between 1-octene (2a) and the photocatalyst 8 as
suggesting that the lifetime of the excited state of the photocata- demonstrated by the insignificant PL quenching of the photocat-
lyst is a key factor for the energy transfer mechanism.
alyst [53].
To obtain a detailed mechanistic insight, steady-state photolu- The ns-transient absorption (TA) spectra of photocatalyst 8
minescence (PL) quenching of photocatalyst 8 was examined (25 μM solution in degassed DMSO) obtained using 355 nm
using xanthate 1a and 1-octene (2a) as potential quenchers pulse excitation and recorded at different delay times are shown
(Figure 1). The intensity of the PL peak of photocatalyst 8 (con- in Figure 2A. The band between 450 nm and 600 nm is attri-
centration of 8 was fixed as 25 μM solution in degassed DMSO buted to the excited 3MLCT state of the photocatalyst [53-55].
for all the samples; for details see Supporting Information The positive ΔOD feature in the UV region (<400 nm) is also
File 1) at 480 nm, arising from the radiative emission of the ascribed to the excited 3MLCT state [55]. The transient kinetic
3MLCT state of the photocatalyst, was measured using 410 nm profile probed at 480 nm decays mono-exponentially with a
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Beilstein J. Org. Chem. 2018, 14, 3047–3058.
Table 1: Optimization of reaction conditions.a
E1/2(M+/M*)
[V vs SCE]b
ET
Conv.
[%]c
Yield
[%]c
Entry
Photocat.
[kcal/mol]b
1
2
6
7
–1.73
–1.28
–0.89
–0.81
–1.42
–1.80
–
55.2
60.1
60.1
46.5
44.7
56.5
–
64
38
>99
13
34
9
58
34
3
8
90 (89)d
13
4
9
5
10
31
6
11
9
7
none
none
none
10
39
84
10
8e,f
9g
–
–
30
–
–
75
aThe reactions were conducted using 0.3–0.5 mmol of xanthate 1a, 1-octene (2a, 2 equiv) and a photocatalyst in DMSO (1 M) at <30 °C with irradia-
tion of a blue LED strip (λmax = 469 nm, 15 W/m, 1.5 m) under an argon atmosphere. bThe values were obtained from references [46-50]. cNMR
yields using 1,1,2,2-tetrachloroethane as an internal standard. dIsolated yield is stated in parentheses. eHalogen lamp (300 W) was used in place of
blue LED. fReaction was conducted at 40 °C. g365 nm UV lamp (100 W) was used in place of blue LED.
lifetime of 1.73 μs (Figure 2B); close to the lifetime of the The ns-TA spectra of photocatalyst 8 (25 μM) in the presence of
excited 3MLCT state of photocatalyst 8 measured in Figure 1C. xanthate 1a (40 mM) in degassed DMSO at various delay times
The ns-TA spectra of xanthate 1a in degassed DMSO, measured are shown in Figure 2D. The kinetic profile at 480 nm is de-
using 355 nm pulse excitation, at various delay times show a scribed using a mono-exponential decay function with a
broad band centered at ≈620 nm (Figure 2C). This band has pre- quenched lifetime of 1.27 μs. The ca. 37% decrease in the life-
viously been ascribed to the absorption of the xanthic acid time of the excited 3MLCT state of photocatalyst 8 observed in
radical formed plausibly from homolytic C–S bond cleavage of the PL lifetime decay measurement (Figure 1C) and ns-TA
the short-lived triplet state of xanthate 1a [22].
kinetic measurement (Figure 2B) can be rationalized using
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