Allylic alcohol synthesis by Ni-catalyzed direct and selective coupling of alkynes and methanol

Article abstract of DOI:10.1039/d1sc02625a

Methanol is an abundant and renewable chemical raw material, but its use as a C1 source in C-C bond coupling reactions still constitutes a big challenge, and the known methods are limited to the use of expensive and noble metal catalysts such as Ru, Rh and Ir. We herein report nickel-catalyzed direct coupling of alkynes and methanol, providing direct access to valuable allylic alcohols in good yields and excellent chemo- and regioselectivity. The approach features a broad substrate scope and high atom-, step- and redox-economy. Moreover, this method was successfully extended to the synthesis of [5,6]-bicyclic hemiacetals through a cascade cyclization reaction of alkynones and methanol.

Full text of DOI:10.1039/d1sc02625a

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Allylic alcohol synthesis by Ni-catalyzed direct and
selective coupling of alkynes and methanol†
Cite this: Chem. Sci., 2021, 12, 9372
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Herong Chen, Zhijun Zhou and Wangqing Kong
Methanol is an abundant and renewable chemical raw material, but its use as a C1 source in C–C bond
coupling reactions still constitutes a big challenge, and the known methods are limited to the use of
expensive and noble metal catalysts such as Ru, Rh and Ir. We herein report nickel-catalyzed direct
coupling of alkynes and methanol, providing direct access to valuable allylic alcohols in good yields and
excellent chemo- and regioselectivity. The approach features a broad substrate scope and high atom-,
step- and redox-economy. Moreover, this method was successfully extended to the synthesis of [5,6]-
bicyclic hemiacetals through a cascade cyclization reaction of alkynones and methanol.
Received 12th May 2021
Accepted 5th June 2021
DOI: 10.1039/d1sc02625a
rsc.li/chemical-science
To address the sustainability issues in the production of new
chemicals, the development of new catalytic processes that are
free of by-products and using abundant renewable feedstocks is
one of the most important challenges facing chemists today.
The simplest alcohol, methanol, is very abundant, with a total
annual production capacity of approximately 110 million metric
tons per year,1 and is an important C1-feedstock in the chemical
industry. Beller2 and Milstein3 made fundamental develop-
ments in catalytic dehydrogenation reactions of methanol.4
Krische and coworkers pioneered the study of Ir-catalyzed direct
C–C coupling of methanol with reactive p-unsaturated reac-
tants (1,3-dienes, 1,3-enynes and allenes).5 The groups of Glo-
rius,6 Donohoe,7 Obora,8 Andersson9 and others10 demonstrated
the direct methylation of ketones or amines using methanol.
Despite these achievements, the catalytic C–C bond coupling
reactions with methanol are still extremely rare and are limited
to the use of precious and noble metal-catalysts such as Ru, Rh
or Ir.11 The development and use of cheap and abundant metal
catalysts for methanol activation is uphill and remains an
important eld that urgently needs to be developed.
On the other hand, allylic alcohols are highly versatile
building blocks in organic synthesis and the pharmaceutical
industry, and much effort has been devoted to their synthesis.
Among them, nickel-catalyzed reductive coupling of alkynes
and aldehydes represents an effective and powerful method.
However, this method generally requires the use of stoichio-
metric reducing reagents that are air-sensitive, metallic or
pyrophoric (e.g. ZnR2, BEt3, and R3SiH, Scheme 1a).12 The direct
cross-coupling of alcohols and alkynes to synthesize allylic
The Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072, P.
R. China. E-mail: wqkong@whu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
spectroscopic data, and NMR spectra. CCDC 2058189. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/d1sc02625a
Scheme
reaction with alkynes.
1 Synthesis of allylic alcohols by Ni-catalyzed coupling
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alcohols without the use of any reductant or oxidant represents energy of methanol in the dehydrogenation process (DH ¼
a
signicant advancement (Scheme 1b).13 However, this +84 kJ molꢀ1) is signicantly higher than that of higher alcohols
approach still poses many limitations that will require consid- or even ethanol (DH ¼ +68 kJ molꢀ1).17
erable effort to overcome. (1) Alkynes are limited to dialkyl
Herein we report the nickel-catalyzed direct and regiose-
alkynes, and poor regioselectivities were observed for unsym- lective hydrohydroxymethylation of alkynes for the rst time
metrical alkynes, which greatly limits the scope of application of using methanol as a C1-feedstock, providing a broad and effi-
the reaction. (2) Alcohols are restricted to active benzyl alcohols cient approach for the synthesis of high added-value allylic
and higher alcohols. The direct cross-coupling of alkynes with alcohols in a high atom-, step- and redox-economic manner. In
methanol has not yet been reported.
addition, a cascade cyclization reaction of alkynones and
Although the alkyne–paraformaldehyde reductive coupling methanol has also been developed for the synthesis of [5,6]-
has been developed,14 the paraformaldehyde was itself prepared bicyclic hemiacetals in good yields and excellent regio- and
from synthesis gas (through methanol). Therefore, the devel- diastereoselectivity (Scheme 1c).
opment of a new strategy for the direct coupling of alkynes and
In our initial experiments, we chose unsymmetrical internal
methanol without the use of any reductant or oxidant is still of alkyne 1a as a model substrate to optimize the reaction condi-
great value, but also extremely challenging: (1) alkynes are tions (Table 1). As expected, no reaction occurred under the
reactive and could rapidly dimerize to 1,3-dienes15 or cyclo- previously reported reaction conditions using Ni(COD)2/L1(IPr)
trimerize to aromatic ring derivatives in the interaction with as the catalyst (entry 1).13a Even if the reaction temperature was
nickel.16 (2) Unsymmetric alkynes could result in a mixture of increased to 100 ꢁC, only a trace amount of allylic alcohol
regioisomers that are difficult to separate. (3) The activation product 2a was observed (entry 2), which indicates that the use
Table 1 Optimization of reaction conditionsa
Entry
Ligand
Additive
Yieldb (2, %)
Yieldb (3, %)
Yieldb (4, %)
Yieldb (5, %)
1c,d
2c
3
4
5
6
7
8
9
10
11
12
13
14
L1
L1
L2
L3
L4
L5
L6
Cy3P
PPh3
L4
L4
L4
—
—
—
—
—
—
—
—
—
A1
A2
A3
A4
A3 g
No reaction
6
6
15
5
9
6
<2
13
<2
22
18
20
20
43
23
27
30 (14/1)e
3
40 (14/1)e
30 (7/1)e
No reaction
Complicated
10
4
<2
<2
<2
<2
59
<2
<2
<2
10
<2
<2
6
38 (14/1)e
60f (14/1)e
No reaction
54f (14/1)e
L4
L4
8
6
7
a
Reactions conditions: 1a (0.2 mmol), Ni(COD)2 (10 mol%), ligand (20 mol%), tBuOK (12 mol%), additive (1 equiv.) in toluene (1 mL) and MeOH (3
mL) in a sealed tube at 100 ꢁC. b Determined by GC analysis using adamantane as the internal standard. Without tBuOK. d Room temperature.
c
e
f
Regioselectivity (2a/2a0). Isolated yield. g 0.2 equivalent.
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Scheme 2 Substrate scope of alkynes for the synthesis of allylic alcohols. Reactions were carried out with 1 (0.2 mmol), Ni(COD)2 (10 mol%), L4
(10 mol%), tBuOK (12 mol%), and methyl methacrylate (0.2 mmol) in toluene (1.0 mL) and MeOH (3.0 mL) in a sealed tube at 100 ꢁC. Isolated yields
are given. a The reaction was conducted with Ni(COD)2 (15 mol%), L4 (15 mol%), and tBuOK (18 mol%). b ((4-Bromophenyl)ethynyl)trimethylsilane
was used. c The reaction was conducted with toluene (0.5 mL) and MeOH (1.5 mL).
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of methanol in the catalytic C–C coupling reactions is indeed
In addition, this transformation is not restricted to aryl-
a big challenge. Various N-heterocyclic carbene ligands (L2–L6) substituted alkynes. As shown in Scheme 2, oct-4-yne and
were investigated (entries 3–9). We found that the selectivity of cyclododecane were coupled with methanol to produce allylic
allylic alcohol 2a is challenged by a number of side reactions, alcohols 2z and 2aa in 87% and 54% yields, respectively. To
such as the hydrogenation (3a), dimerization (4a) and trimeri- further evaluate the inuence of the electronic properties of the
zation (5a) of alkyne 1a. L4 is the most effective, providing 2a substituents on the regioselectivity, we tested the hydro-
with the highest yield (40%) and excellent regioselectivity (14/1), hydroxymethylation reaction of unsymmetrical dialkyl-
but an appreciable quantity of dimerization and trimerization substituted alkynes bearing benzyloxy or dibenzylamino
by-products 4a and 5a was still obtained (entry 5). Krische14 groups at the propargylic position. To our delight, the corre-
reported that PCy3 could promote the reductive coupling of sponding allylic alcohols 2ab–2ad were obtained in moderate
alkynes and paraformaldehyde, but we found that it is not yields, with remarkably high regioselectivity (>20/1). However,
effective for alkyne–methanol coupling (entry 8).
Many examples have reported that olens can affect the
outcomes of transition metal-catalyzed cross coupling reactions
through increased activity, stability, or selectivity.18 More
recently, Montgomery et al.19 found that adding electron-
decient olens to NHC–Ni(0) complexes can improve their
catalytic performance. Inspired by this discovery, we examined
various acrylates A1–A4 (entries 10–13). Excitingly, the addition
of methyl methacrylate (A3) can indeed signicantly improve
the chemoselectivity of the reaction, providing the allylic
alcohol 2a in 60% isolated yield and a more than 14/1 ratio of
regioisomers (entry 12). The structure of acrylates has a great
inuence on the reaction outcome, indicating that they may act
as additional ligands to coordinate with the nickel catalyst,
thereby suppressing these undesired dimerization or cyclo-
trimerization side reactions. However, by-products formed by
the reductive coupling of acrylates and alkynes have also been
observed (see Section 3 in the ESI†).20 It is worth mentioning
that stoichiometric acrylate additives are not necessary. As
shown in entry 14, even if 0.2 equivalent of A3 was used, 54% of
the target product 2a can be obtained, thus showing the
subtleties of our catalytic system.
With the optimized reaction conditions in hand, we turned
our attention to explore the substrate scope of alkynes (Scheme
2). We were pleased to nd that various unsymmetrical aryl–
alkyl alkynes were coupled with methanol to provide the cor-
responding allylic alcohols 2a–2q in moderate to good yields
and high regioselectivities. Various functional groups, such as
uorine (2b), triuoromethyl (2c), chlorine (2e), allyl (2f),
bromine (2g), amine (2h–2j) and amide (2k and 2l) could all be
well-tolerated. Heteroaromatic ring-substituted alkynes, such as
5-indole,21 2-dibenzothiophene and 2-dibenzofuran could also
proceed smoothly to furnish allylic alcohols 2n–2p in 40–63%
yield. It is worth mentioning that complex biologically active
molecules such as estrone derivatives, could also be successfully
incorporated into the desired product 2q in 62%, thus
demonstrating the robustness and generality of this method-
ology for late-stage modication of complex biologically active
molecules. Terminal alkynes were also found to be compatible
with the reaction conditions, providing the corresponding
products 2r–2s in moderate yields and excellent regioselectivity
(>20/1). Symmetric diarylalkynes bearing electron-donating or
electron-withdrawing groups were applicable to the reaction
(2t–2y). Strikingly, both 1,2-di(furan-2-yl)ethyne and 1,2-
di(thiophen-2-yl)ethyne were competent substrates and fur-
Scheme 3 Substrate scope of alkynones for the synthesis of [5, 6]-
bicyclic hemiacetals. Reactions were carried out with 6 (0.2 mmol),
Ni(COD)2 (15 mol%), IMes (15 mol%), LiF (10 mol%), and methyl
methacrylate (0.2 mmol) in toluene (1.5 mL) and MeOH (0.5 mL) in
a sealed tube at 40 ꢁC. Isolated yields are given.
nished the desired allylic alcohols 2x–2y in good yields.
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the regioselectivity of this reaction was decreased by the alkyne
bearing a benzyloxy group at the homopropargylic position.
To expand the potential synthetic applications of the trans-
formation, we investigated the hydrohydroxymethylation of 1,3-
enynes. The corresponding dienol 2af was obtained, which was
selectively hydrohydroxymethylated on the alkyne but not on
the alkene moiety. 1,6-Enyne was also compatible to give the
corresponding allylic alcohol 2ag in 42% yield with >20/1
regioselectivity. This strategy can serve as a powerful supple-
ment to the previous method reported by Krische et al.,22 in
which alcohols were reacted with alkenes to obtain the corre-
sponding homopropargylic alcohols.23
Alkynone substrates were also tested, but the expected
product was not detected due to their sensitivity to base. Aer
slightly modifying the reaction conditions, we were pleased to
nd that various [5,6]-bicyclic hemiacetals 7 could be obtained
in good yields with excellent regio- and diastereoselectivities
through the cascade cyclization reaction of alkynones 6 with
methanol (Scheme 3). We rst explored the inuence of the
substituents (R1) at the terminus of the triple bond. A variety of
para-substituted aromatic rings at the alkyne terminus could
undergo tandem cyclization to provide the target hemiacetals
7b–7g in 54–78% yields. The structure of 7a was conrmed by
an X-ray crystal diffraction study. The aryl groups with substit-
uents at the meta and ortho position were also found to be
compatible, leading to the corresponding products 7h–7j in 56–
74% yields. Moreover, various (hetero)aryl rings such as naph-
thalene (7k), benzodioxan (7l), 3,4-dihydrobenzodioxine (7m),
thiophene (7n), dibenzofuran (7o), dibenzothiophene (7p),
indole (7q) and pyridine (7r) at the terminal of the triple bond
could be successfully incorporated into the desired products in
good yields. Strikingly, estrone was also compatible with this
transformation to afford the desired product 7s in 65% yield.
However, no desired product was observed when the methyl
substituted alkynone substrate was used. We then investigated
the inuence of the substituents (R2) at the 2-position of the
cyclopentane-1,3-diones. Ethyl, benzyl, and allyl were all well
tolerated leading to the corresponding [5,6]-bicyclic hemi-
acetals 7t–7w in moderate yields.
Scheme 4 Deuterium-labelling experiments.
formaldehyde intermediate is the rate-determining step of this
transformation.
On the basis of these experimental results and previous
observations, a possible reaction mechanism is proposed in
Scheme 5. The reaction is initiated by reducing alkyne to alkene
and simultaneously oxidizing methanol to formaldehyde, as
evidenced by the detection of catalytic amounts of alkene 3.
Oxidative cyclization of acrylate-coordinated NHC–Ni(0) A 17
with alkyne and formaldehyde gives oxa-nickelacycle interme-
diate B. Subsequent protonation of nickelacycle species B with
methanol affords the vinylnickel intermediate C, which can
undergo b-H elimination to generate vinyl nickel hydride
species D and formaldehyde.25 Reductive elimination of D will
furnish allylic alcohol 2 and the catalytically active Ni(0) catalyst
A. Further nucleophilic addition of the hydroxyl group to one of
To provide a deeper insight into the reaction mechanism,
deuterium-labelling experiments were performed. 1a was reac-
ted with CH3OD under our standard reaction conditions;
however, no incorporation of deuterium was detected in
product 2a (Scheme 4a), revealing that the hydroxyl of methanol
is not the proton source. This result is different from the
previous report by Zhou et al.,24 in which the Ni(0) catalyst
underwent oxidative addition to the O–H bond of methanol to
form methoxyl nickel hydride species and then migratory
insertion into unsaturated bonds. Further investigation using
CD3OD as solvent provided 2a-D in 41% yield, in which 99% of
the deuterium was incorporated into the olenic position, but
the reaction rate is obviously slowed down (Scheme 4b). We also
conducted the kinetic isotope effect (KIE) experiment. The
intermolecular competition reaction between 1a and CD3OD or
CH3OH under standard reaction conditions provided a KIE (kH/
kD) value of 6.1 (Scheme 4c). Taken together, these results may
indicate that the dehydrogenation of methanol to form the key Scheme 5 Proposed reaction mechanism.
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the ketone carbonyl groups will produce [5,6]-bicyclic hemi-
acetal 7. We speculate that the acrylate is used as an additional
ligand, thereby inhibiting the alkyne dimerization to 1,3-dienes
or cyclotrimerization to aromatic ring derivatives.
D. Shen, D. L. Poole, C. C. Shotton, A. F. Kornahrens,
M. P. Healy and T. J. Donohoe, Angew. Chem., Int. Ed.,
2015, 54, 1642.
8 S. Ogawa and Y. Obora, Chem. Commun., 2014, 50, 2491.
9 X. Quan, S. Kerdphon and P. G. Andersson, Chem.–Eur. J.,
2015, 21, 3576.
10 (a) Y. Li, H. Li, H. Junge and M. Beller, Chem. Commun., 2014,
50, 14991; (b) E. A. Jo, J. H. Lee and C. H. Jun, Chem.
Commun., 2008, 5779; (c) F. Li, J. Xie, H. Shan, C. Sun and
L. Chen, RSC Adv., 2012, 2, 8645; (d) C. Sun, X. Zou and
F. Li, Chem.–Eur. J., 2013, 19, 14030.
Conclusions
In summary, a nickel-catalyzed direct coupling of alkynes and
methanol is developed for the rst time, providing direct access
to high added-value allylic alcohols in good yields and excellent
chemo- and regioselectivity. This transformation features
a wide substrate scope and high atom-, step- and redox- 11 (a) A. Quintard and J. A. Rodriguez, ChemSusChem, 2016, 9,
economy. In addition, a cascade cyclization reaction of alky-
nones and methanol has also been developed for the synthesis
of [5,6]-bicyclic hemiacetals.
28; (b) B. G. R. Berendt, K. Polidano and L. C. Morrill, Org.
Biomol. Chem., 2019, 17, 1595; (c) T. Irrgang and R. Kempe,
Chem. Rev., 2019, 119, 2524.
12 For reviews on Ni-catalyzed alkyne–carbonyl reductive
couplings, see:(a) S. Saito and Y. Yamamoto, Chem. Rev.,
2000, 100, 2901; (b) J. Montgomery, Acc. Chem. Res., 2000,
33, 467; (c) J. Montgomery, Angew. Chem., Int. Ed., 2004, 43,
3890; (d) R. M. Moslin, K. Miller-Moslin and T. F. Jamison,
Chem. Commun., 2007, 4441; (e) Y. Nakao and T. Hiyama,
Pure Appl. Chem., 2008, 80, 1097; (f) M. Jeganmohan and
C. H. Cheng, Chem.–Eur. J., 2008, 14, 10876; (g)
H. A. Malik, R. D. Baxter and J. Montgomery, Nickel-
Catalyzed Reductive Couplings and Cyclizations, in
Catalysis without Precious Metals, ed. R. M. Bullock, Wiley-
VCH, Weinheim, 1st edn, 2010, pp. 181–210; (h) Formation
of C–C bonds via catalytic hydrogenation and transfer
hydrogenation. J. Moran and M. J. Krische, in Sustainable
Catalysis, ed. K. K. Hii, M. T. Williams, P. J. Dunn and M.
J. Krische, John Wiley and Sons, New York, 2012, pp. 363–
408; (i) J. M. Ketcham, I. Shin, T. P. Montgomery and
M. J. Krische, Angew. Chem., Int. Ed., 2014, 53, 9142; (j)
B. Sam, B. Breit and M. J. Krische, Angew. Chem., Int. Ed.,
2015, 54, 3267; (k) E. P. Jackson, H. A. Malik,
G. J. Sormunen, R. D. Baxter, P. Liu, H. Wang,
A. R. Shareef and J. Montgomery, Acc. Chem. Res., 2015, 48,
1736; (l) E. A. Standley, S. Z. Tasker, K. L. Jensen and
T. F. Jamison, Acc. Chem. Res., 2015, 48, 1503.
Data availability
The ESI include experimental detail, NMR data and HRMS data.
Author contributions
W. K. conceived and designed the experiments. H. C. and Z. Z.
performed the experiments and prepared the ESI. W. K. directed
the project and wrote the manuscript.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for nancial support from Wuhan
University, the “1000-Youth Talents Plan”, and the National
Natural Science Foundation of China (No. 21702149).
Notes and references
1 (a) P. G. Cifre and O. Badr, Energy Convers. Manage., 2007, 48,
519; (b) http://www.methanol.org/Methanol-Basics.aspx.
2 M. Nielsen, E. Alberico, W. Baumann, H. J. Drexler, H. Junge,
S. Gladiali and M. Beller, Nature, 2013, 495, 85.
3 (a) R. Langer, I. Fuchs, M. Vogt, E. Balaraman, Y. D. Posner,
L. J. W. Shimon, Y. B. David and D. Milstein, Chem.–Eur. J.,
13 (a) K. Nakai, Y. Yoshida, T. Kurahashi and S. Matsubara, J.
Am. Chem. Soc., 2014, 136, 7797; (b) E. L. McInturff,
K. D. Nguyen and M. J. Krische, Angew. Chem., Int. Ed.,
2014, 53, 3232; (c) Y. Cai, J. Zhang, F. Li, J. Liu and
S. L. Shi, ACS Catal., 2019, 9, 1.
2013, 19, 3407; (b) C. Gunanathan and D. Milstein, Acc. 14 C. C. Bausch, R. L. Patman, B. Breit and M. J. Krische, Angew.
Chem. Res., 2011, 44, 588. Chem., Int. Ed., 2011, 50, 5687.
4 R. E. R. Lugo, M. Trincado, M. Vogt, F. Tewes, G. S. Quinones 15 (a) T. Wu, J. Chen and Y. Wu, Org. Lett., 2011, 13, 4794; (b)
¨
and H. Grutzmacher, Nat. Chem., 2013, 5, 342.
G. Zhang, Y. Xie, Z. Wang, Y. Liu and H. Huang, Chem.
Commun., 2015, 51, 1850; (c) Y. Liu, G. Zhang and
5 (a) J. Moran, A. Preetz, R. A. Mesch and M. J. Krische, Nat.
Chem., 2011, 3, 287; (b) K. D. Nguyen, D. Herkommer and
M. J. Krische, J. Am. Chem. Soc., 2016, 138, 14210; (c)
M. Holmes, K. D. Nguyen, L. A. Schwartz, T. Luong and
M. J. Krische, J. Am. Chem. Soc., 2017, 139, 8114.
6 N. Ortega, C. Richter and F. Glorius, Org. Lett., 2013, 15,
1776.
˜
H. Huang, Org. Lett., 2017, 19, 6674; (d) S. Canellas,
`
`
J. Montgomery and M. A. Pericas, J. Am. Chem. Soc., 2018,
140, 17349; (e) Q. Liang, K. Hayashi and D. Song, ACS
Catal., 2020, 10, 4895; (f) H. Olivier-Bourbigou,
P. A. R. Breuil, L. Magna, T. Michel, M. F. E. Pastor and
D. Delcroix, Chem. Rev., 2020, 120, 7919.
7 (a) L. K. M. Chan, D. L. Poole, D. Shen, M. P. Healy and 16 For selected reviews, see: (a) S. Saito and Y. Yamamoto,
T. J. Donohoe, Angew. Chem., Int. Ed., 2014, 53, 761; (b)
Chem. Rev., 2000, 100, 2901; (b) K. P. C. Vollhardt, Angew.
© 2021 The Author(s). Published by the Royal Society of Chemistry
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View Article Online
Chemical Science
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´
Chem., Int. Ed., 1984, 23, 539; (c) J. A. Varela and C. Saa, 21 Trace amount of the methylated product at the C3-position
Chem. Rev., 2003, 103, 3787; (d) Y. Yamamoto, Curr. Org.
of indole was observed, see: S. Chen, G. Lu and C. Cai, RSC
Adv., 2015, 5, 70329.
´
Chem., 2005, 9, 503; (e) A. F. Orsino, M. Gutierrez del
Campo, M. Lutz and M. E. Moret, ACS Catal., 2019, 9, 2458. 22 (a) R. L. Patman, V. M. Williams, J. F. Bower and
17 (a) M. Qian, M. A. Liauw and G. Emig, Appl. Catal., A, 2003,
238, 211; (b) W. H. Lin and H. F. Chang, Catal. Today,
2004, 97, 181.
18 J. B. Johnson and T. Rovis, Angew. Chem., Int. Ed., 2008, 47,
840.
M. J. Krische, Angew. Chem., Int. Ed., 2008, 47, 5220; (b)
L. M. Geary, J. C. Leung and M. J. Krische, Chem.–Eur. J.,
2012, 18, 16823; (c) K. D. Nguyen, D. Herkommer and
M. J. Krische, J. Am. Chem. Soc., 2016, 138(16), 5238; (d)
L. M. Geary, S. K. Woo, J. C. Leung and M. J. Krische,
Angew. Chem., Int. Ed., 2018, 57, 461.
19 (a) D. P. Todd, B. B. Thompson, A. J. Nett and J. Montgomery,
˜
J. Am. Chem. Soc., 2015, 137, 12788; (b) A. J. Nett, S. Canellas, 23 (a) J. R. Kong, M. Y. Ngai and M. J. Krische, J. Am. Chem. Soc.,
Y. Higuchi, M. T. Robo, J. M. Kochkodan, M. T. Haynes,
J. W. Kampf and J. Montgomery, ACS Catal., 2018, 8, 6606;
2006, 128, 718; (b) Y. T. Hong, C. W. Cho, E. Skucas and
M. J. Krische, Org. Lett., 2007, 9, 3745.
(c) Z. Zhou, J. Chen, H. Chen and W. Kong, Chem. Sci., 24 L. Xiao, L. Cheng, W. Feng, M. Li, J. Xie and Q. Zhou, Angew.
2020, 11, 10204. Chem., Int. Ed., 2018, 57, 461.
20 C. Wang, P. Lin and C.-H. Cheng, J. Am. Chem. Soc., 2002, 25 A. Herath, W. Li and J. Montgomery, J. Am. Chem. Soc., 2008,
124, 9696.
130, 469.
9378 | Chem. Sci., 2021, 12, 9372–9378
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