Remarkable Influence of Cobalt Catalysis on Epoxide Ring-Opening with Sulfoxonium Ylides

Article abstract of DOI:10.1055/s-0036-1588814

Cobalt demonstrates a remarkable ability to catalytically divert the course of epoxide to oxetane ring expansion via reaction with a sulfoxonium ylide. An expanded survey of transition-metal catalysts has confirmed that cobalt salts uniquely instead deliv

Full text of DOI:10.1055/s-0036-1588814

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2017, 49, A–E
special topic
A
en
M. L. Jamieson et al.
Special Topic
Synthesis
Remarkable Influence of Cobalt Catalysis on Epoxide Ring-Opening
with Sulfoxonium Ylides
Megan L. Jamieson
other
transition
metals
Nicola Z. Brant
CoII or III
catalyst
Margaret A. Brimble*
Daniel P. Furkert*
or
no catalyst
OH
olefination
O
ring expansion
R
O
School of Chemical Sciences, The University of Auckland,
23 Symonds St, Auckland 1010, New Zealand
d.furkert@auckland.ac.nz
R
R
O
O
7 examples
up to 99% yield
S
Me
S
Me
Me
Me
stereochemistry retained
broad substrate scope
KOtBu, tBuOH
60 °C, 4 h
KOtBu, tBuOH
60 °C, 4 h
Published as part of the Special Topic Cobalt in Organic Synthesis
Received: 22.03.2017
Accepted after revision: 05.04.2017
Published online: 08.05.2017
ative agent of this diverted reactivity. Rigorous removal of
cobalt salts from 1 by Kugelrohr distillation restored the ex-
pected ring-expansion pathway to give oxetane 3. Control
experiments also showed that re-submission of oxetane 3
to the reaction conditions gave no reaction, and therefore
this compound was not an intermediate en route to 4. Our
initial findings suggested that traces of other cobalt salts
were similarly able to completely divert sulfoxonium ylide
mediated ring expansion to homoallylic alcohols. In order
to further confirm this finding, a wide range of transition-
metal catalysts have been screened for their influence on
the reaction, under the optimised reaction conditions (Ta-
ble 1). These data were observed to strongly corroborate
the initial observation.
DOI: 10.1055/s-0036-1588814; Art ID: ss-2017-c0181-st
Abstract Cobalt demonstrates a remarkable ability to catalytically di-
vert the course of epoxide to oxetane ring expansion via reaction with a
sulfoxonium ylide. An expanded survey of transition-metal catalysts has
confirmed that cobalt salts uniquely instead deliver homoallylic alcohol
products from epoxides, with retention of the original epoxide stereo-
chemistry. The reaction is an unusual example of cobalt-catalysed
epoxide ring-opening by a carbon nucleophile. A tandem Corey–
Chaykovsky/epoxide olefination sequence giving homoallylic alcohols
from aldehydes is further demonstrated along with preliminary mecha-
nistic analysis. This communication summarises current understanding
and ongoing studies into this intriguing new cobalt-mediated reactivity.
Key words cobalt catalysis, diverted reactivity, epoxide olefination,
O
carbon–carbon bond formation, oxetane, Jacobsen HKR
No catalyst
O
BnO
BnO
O
2
3
1
S
Cobalt-based catalysts have been used to carry out a
vast array of diverse carbon–carbon bond forming transfor-
mations,1 and have recently found successful application in
the rapidly expanding field of selective C–H activation.2
Currently, development of cobalt chemistry is encouraged
by the availability, low toxicity, and chemical versatility of
the metal, enabling discovery of novel and highly atom-
efficient processes. One particularly well studied example,
the use of cobalt(salen) complexes for Jacobsen hydrolytic
kinetic resolution (HKR) of epoxides has become a mature
mainstay of asymmetric total synthesis due to its wide sub-
strate scope, scalability, and reliability in enantioselective
preparation of epoxides,3 including industrial applications.4
Recently, we reported that the known ring expansion of ho-
mochiral epoxide 1 using ylide 2 to give oxetane 3,5 was
found to unexpectedly deliver homoallylic alcohol 4 as the
sole product (Scheme 1).6 Careful investigation of the re-
agents eventually implicated traces of cobalt catalyst 5 (Fig-
ure 1) remaining from prior HKR of the epoxide as the caus-
(76%)
Me
Me
OH
CoII or CoIII (5 mol%)
BnO
O
2
4
S
(84%)
Me
Me
Scheme 1 Traces of cobalt salts divert the known ring expansion of ep-
oxide 1 to oxetane 3, instead delivering homoallylic alcohol 4. Reagents
and conditions: trimethylsulfoxonium iodide (3 equiv), t-BuOK (3 equiv),
t-BuOH, 60 °C, 4 h.
A number of catalysts that were examined either par-
tially or completely suppressed the reaction of 1 to 3, re-
turning unchanged starting material, epoxide 1 (Table 1,
entries 1–5). The majority of transition-metal-derived cata-
lysts did not affect the normal course of the ring expansion
reaction significantly, and complete conversion into oxe-
tane 3 was observed (Table 1, entries 6–12). Remarkably, all
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

B
M. L. Jamieson et al.
Special Topic
Synthesis
try 13), but the corresponding anhydrous salt gave full con-
version to 4. The presence of a ligand did not appear to af-
fect the catalytic behaviour of the cobalt catalyst (Table 1,
entries 16–18). Both Jacobsen HKR cobalt(II) salen pre-cata-
lyst and cobalt(III) salen 5 catalyst were effective.
N
O
N
CoIII
tBu
O
tBu
X
tBu
tBu
The cobalt-catalysed epoxide olefination reaction
proved to be broadly applicable among a range of substrates
(Table 2) giving generally high yields of homoallylic prod-
ucts. Sterically demanding examples exhibited lower con-
version rates, however (Table 2, entries 3–5). As might be
expected, the original epoxide stereochemistry was re-
tained in the products, confirmed by HPLC and Mosher es-
ter analysis (Table 2, entries 1, 3, and 6). Given that the re-
action conditions used in Table 2 are very similar to those
of the Corey–Chaykovsky epoxidation of aldehydes,7 the po-
tential of a tandem epoxidation–olefination sequence was
also evaluated (Table 3).8 Overall, the transformation
proved successful, giving clean conversion into homoallylic
alcohols from both aliphatic and aryl aldehydes (Table 3,
entries 1–4) and ketones (Table 3, entries 5 and 6) although
a large excess of ylide and very long reaction times were
found to be necessary to achieve this.
5
(X = OAc, OTs)
Figure 1 Jacobsen CoIII(salen) hydrolytic kinetic resolution (HKR) cata-
lyst
Table 1 Catalyst Influence on Epoxide Reactivity with Sulfoxonium
Ylide 2a
OH
O
Catalyst
O
BnO
BnO
BnO
O
2
S
epoxide
Me
Me
oxetane
alcohol
4
1
3
tBuOH
60 °C, 4 h
Entry
Catalyst
Epoxide 1b,c Oxetane 3c Alcohol 3c
(%) (%) (%)
1
2
BF3·OEt2
100
Table 2 Cobalt-Catalysed Epoxide Olefination (Selected Examples)a
(PPh3)3RhCl
Ni(acac)2
ZnBr2
100
100
80
3
Entry Epoxide
Alcohol
Yield (%)
[ee (%)]
4
20
5
Ru(acac)
FeCl3 + bipy
Sc(OTf)3
FeCl3
50
50
98d
100
100
100
100
100
100
47d
OH
O
1
2
3
92 [99]
99
BnO
BnO
BnO
6
7
O
OH
8
BnO
9
CuCl
OH
O
51 [90]b
10
11
12
13
14
15
16
17
18
CuCl2
TBDPSO
TBDPSO
OH
PdCl2
O
MnCl2
4
5
44
35
CoCl2·H2O
CoCl2
21d
100
72d
100
100
89d
CoBr2
BnO
OMe
BnO
OMe
O
OH
CoBr2 + bipy
CoII(salen)
CoIII(salen)OAc 5
OEOM
OEOM
6c
90 [99]
a Conditions: catalyst (5 mol%), trimethylsulfoxonium iodide (3 equiv),
t-BuOK (3 equiv), 60 °C, 4 h.
O
b Unreacted starting material.
HO
c Conversion estimated by 1H NMR unless otherwise indicated.
d Isolated yield.
OBn
O
OBn
O
7
82
O
cobalt salts investigated completely diverted the ring ex-
pansion to give homoallylic alcohol 4 (Table 1, entries 13–
18). The only exception was hydrated cobalt(II) chloride
that partially promoted the diverted reaction (Table 1, en-
OH
a Reaction conditions: CoII(salen) (5 mol%), trimethylsulfoxonium iodide (3
equiv), t-BuOK (3 equiv), t-BuOH, 60 °C, 4 h.
b Epoxide starting material also 90% ee.
c EOM = ethoxymethyl.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

C
M. L. Jamieson et al.
Special Topic
Synthesis
Table 3 Tandem Corey–Chaykovsky-Epoxide Olefinationa
O
BnO
1
Entry Aldehyde
Alcohol
Timeb (d) Yield (%)
O
S
Me
2
O
OH
OH
Me
1
2
5
5
89
83
O
No catalyst
cyclisation
O
BnO
BnO
SMe2
– DMSO
O
6
O
3
O
S
Me
CoII/CoIII
(5 mol%)
2
Me
O
OH
OH
O
O
MeO
MeO
elimination
BnO
3
4
5
6
61
68
BnO
SMe2
– DMSO
4
7
OMe
OMe
OH
Scheme 2 Mechanistic overview of cobalt-catalysed epoxide olefina-
O
tion
Br
Cl
Br
Cl
ber of similar catalytic systems for closely related ring-
opening transformations of epoxides involving other het-
eroatomic nucleophiles have also been investigated.11 In
contrast, the application of related systems involving cobalt
and/or salen-based catalysts to carbon nucleophiles is
largely restricted to carbonylative epoxide openings, al-
though these reactions are highly useful and have been rel-
atively well studied.12
O
HO
5
6
6
6
69
85
OH
O
In the context of these latter studies particularly, the
successive addition of two equivalents of ylide 2 to epox-
ides under cobalt catalysis observed in this study is poten-
tially significant, as both an important extension to known
reactivity patterns and a mechanistic probe. In a series of
reports detailing further improvements in catalytic systems
for epoxide carbonylation, the Coates group propose a co-
operative mechanism involving two metal centres. Coordi-
nation of the epoxide oxygen to a Lewis acid is suggested to
promote epoxide opening by a low-valent cobalt species,
followed by insertion of one of the carbonyl ligands into the
Co bond (Scheme 3, A).13 Interestingly, this catalytic cycle
bears strong resemblance to intermediates of the bimetallic
catalytic cycle thoroughly elucidated for HKR catalyst 5 and
derivatives by Blackmond and Jacobsen (Scheme 3, B).14
This latter work also identified the strong effect of the rela-
tive concentration of poorly Lewis acidic (salen)Co–OH spe-
cies on the efficiency of hydrolytic epoxide opening. It is
plausible that a similar issue explains the observed partial
ability of CoCl2·H2O to divert the course of the epoxide ring
expansion (Table 1, entry 13).15
a Reaction conditions: CoII(salen) (10 mol%), trimethylsulfoxonium iodide (10
equiv), t-BuOK (10 equiv), t-BuOH, 60 °C.
b Time to complete conversion by NMR.
Overall, the reaction mechanism likely follows the out-
line shown in Scheme 2. Initial ring-opening of epoxide 1 by
sulfoxonium ylide 2 leads to intermediate betaine 6, that
then cyclises to give ring-expanded oxetane 3, with the loss
of dimethyl sulfoxide. In contrast, in the presence of cobalt
salts, the reaction is diverted and 6 selectively undergoes a
second addition of ylide 2, to afford homologated betaine 7
that in this case undergoes elimination to form homoallylic
alcohol 4, similarly with the elimination of dimethyl sulfox-
ide. Interestingly, the seemingly plausible products of the
converse processes, i.e. elimination from 6, or cyclisation
from 7, have not been observed in these studies. In the tan-
dem Corey–Chaykovsky/epoxide olefination sequence, the
epoxide formation obviously requires an initial addition of
ylide 2 to an aldehyde, although this extra step appears to
significantly decrease the overall efficiency of the process.
The exact mechanistic details of the strong influence of
cobalt salts on the reaction course remain unclear at pres-
ent. Jacobsen cobalt(salen) HKR catalysts, including 5, have
been demonstrated to accomplish asymmetric opening of
meso epoxides and kinetic resolution of racemic terminal
epoxides by the enantioselective addition of heteroatomic
nucleophiles including azides9 and carboxylates.10 A num-
Sulfoxides are well-known ligands for transition metals
and many metal–sulfoxide complexes are known.16 This
might suggest that coordination of ylide 2 to cobalt could
play a crucial role in delivering the nucleophile to an epox-
ide already activated by a second cobalt centre.17
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

D
M. L. Jamieson et al.
Special Topic
Synthesis
chromatography. Petroleum ether = PE. NMR spectra were recorded at
rt in CDCl3 on a spectrometer operating at 400 MHz (1H). Epoxide
substrates were prepared by literature methods.6
A Coates13
LnMO
R1
B Blackmond/Jacobsen14
Co(CO)4
R2
Ts
CO
O
Co
Olefination of Epoxides (Tables 1 and 2); General Procedure A
(GPA)
O
LnM
O
R
LnMO
R1
Co(CO)4
OH
A stirred suspension of epoxide (1 equiv), (S,S)-N,N′-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediaminocobalt(II) or test catalyst
(0.05 equiv), trimethylsulfoxonium iodide (3 equiv), and t-BuOK (3
equiv) in t-BuOH (10 mL/mmol epoxide) was heated at 60 °C for 4 h.
The mixture was allowed to cool to rt, quenched by the addition of
sat. aq NaHCO3 solution and extracted with Et2O. The combined or-
ganic extracts were dried (anhyd MgSO4) and concentrated in vacuo.
R1
R2
Co
L
R2
Co(CO)4
Scheme 3 Bimetallic cooperativity proposed in epoxide opening inter-
mediates
Tandem Corey–Chaykovsky/Epoxide Olefination (Table 3); General
Procedure B (GPB)
Alternatively, sulfoxonium and sulfonium ylides form an
emerging class of transition-metal–carbene precursors that
behave as α-diazocarbonyl equivalents in C=C, C–H, and X–
H insertion reactions.18 This suggests the possibility that
association of ylide 2 with the cobalt catalyst might lead to
formation of an intermediate cobalt carbene species that
could potentially undergo insertion into the epoxide sub-
strate, following activation by a second cobalt centre. Relat-
ed catalytic cycles involving dimethylsulfoxonium ylides
have been proposed for gold- and iridium-based systems.19
Further, an unusually stable example of a high-valent car-
bene complex of the type that might be required for this
mechanism was recently prepared and characterised by X-
ray crystallography.20 Notably in this report, attempts using
the same methodology to form the corresponding iron
complex were unsuccessful.
In conclusion, it has been demonstrated that cobalt salts
catalytically divert the expected course of the reaction of
epoxides with sulfoxonium ylides. Evaluation of other tran-
sition-metal catalysts has to date shown that only cobalt-
based catalysts possess this activity, which provides syn-
thetically useful access to enantiopure homoallylic alco-
hols. The reaction is an unusual example of cobalt(salen)-
catalysed epoxide ring-opening by a carbon nucleophile.
Continuing efforts are underway to elucidate the mechanis-
tic aspects and further implications of these findings in-
cluding kinetic studies, identification of reaction interme-
diates and extended reaction scope.
A stirred suspension of aldehyde (1 equiv), (S,S)-N,N′-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediaminocobalt(II) (0.10 equiv),
trimethylsulfoxonium iodide (10 equiv), and t-BuOK (10 equiv) in t-
BuOH (10 mL/mmol epoxide) was heated at 60 °C for the time indi-
cated. Products were isolated using the workup as described for olefi-
nation of epoxides above and purified by chromatography (silica gel,
EtOAc/PE) or identified from crude NMR spectra by reference to liter-
ature data.
1-Phenylbut-3-en-2-ol (Table 3, Entry 1)
Obtained from benzaldehyde (30 mg, 0.28 mmol) using GPB and puri-
fied by chromatography (PE/EtOAc, 1:2) to give an orange oil; yield:
37 mg (89%).
1H NMR (400 MHz, CDCl3): δ = 7.38–7.26 (m, 5 H), 5.85–5.77 (m, 1 H),
5.20–5.14 (m, 2 H), 4.77–4.73 (m, 1 H), 2.55–2.50 (m, 2 H), 2.02 (d, J =
3.2 Hz, 1 H). Spectroscopic data were in agreement with those previ-
ously reported.21
Non-1-en-4-ol (Table 3, Entry 2)
Obtained from hexanal (30 mg, 0.30 mmol) using GPB and purified by
chromatography (PE/EtOAc, 12:1) to give a yellow oil; yield: 36 mg
(83%).
1H NMR (400 MHz, CDCl3): δ = 5.86–5.78 (m, 1 H), 5.15–5.11 (m, 2 H),
3.66–3.63 (m, 1 H), 2.34–2.27 (m, 1 H), 2.17–2.10 (m, 1 H), 1.47–1.22
(m, 8 H), 0.89 (t, J = 6.9 Hz, 3 H). Spectroscopic data were in agree-
ment with those previously reported.22
1-(3,5-Dimethoxyphenyl)but-3-en-1-ol (Table 3, Entry 3)
Obtained using GPB from 3,5-dimethoxybenzaldehyde (30 mg, 0.18
mmol) and purified by chromatography (PE/EtOAc, 5:1) to give a yel-
low oil; yield: 23 mg (61%).
All reactions were performed under an atmosphere of O2-free N2 us-
ing standard anhydrous techniques. Reagents were used as received
unless otherwise noted. t-BuOH was distilled from CaH2 and stored
over molecular sieves (3 Å) under a N2 atmosphere. Other solvents
were dried by passage through a column of activated alumina under
N2 using an LC Technology solvent purification system. Yields refer to
chromatographically and spectroscopically homogeneous materials,
unless otherwise stated. Reactions were monitored by TLC carried out
on silica gel plates using UV light as visualising agent and an ethanolic
solution of vanillin and ammonium molybdate and heat as develop-
ing agents. Silica gel (60, 230–400 mesh) was used for flash column
1H NMR (400 MHz, CDCl3): δ = 6.52 (d, J = 2.0 Hz, 2 H), 6.38 (t, J = 2.2
Hz, 1 H), 5.89–5.76 (m, 1 H), 5.20–5.13 (m, 2 H), 4.68 (dd, J = 7.6, 5.1
Hz, 1 H), 3.80 (s, 6 H), 2.56–2.43 (m, 2 H). Spectroscopic data were in
agreement with those previously reported.23
1-(4-Bromophenyl)but-3-en-1-ol (Table 3, Entry 4)
Obtained using GPB from 4-bromobenzaldehyde (30 mg, 0.16 mmol)
and purified by chromatography (PE/EtOAc) to give a colourless oil;
yield: 25 mg (68%).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

E
M. L. Jamieson et al.
Special Topic
Synthesis
1H NMR (400 MHz, CDCl3): δ = 7.53–7.46 (m, 2 H), 7.26–7.23 (m, 2 H),
5.83–5.73 (m, 1 H), 5.19–5.15 (m, 2 H), 4.72 (dd, J = 7.7, 5.0 Hz, 1 H),
4.66 (s, 1 H), 2.52–2.41 (m, 2 H). Spectroscopic data were in agree-
ment with those previously reported.24
(6) Jamieson, M. L.; Hume, P. A.; Furkert, D. P.; Brimble, M. A. Org.
Lett. 2016, 18, 468.
(7) (a) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353.
(b) Aggarwal, V. K.; Winn, C. L. Acc. Chem. Res. 2004, 37, 611.
(c) Wang, Z. In Comprehensive Organic Name Reactions and
Reagents; Wiley: Chichester, 2010, 156.
3-(4-Chlorophenyl)hex-5-en-3-ol (Table 3, Entry 5)
(8) For another example of a tandem Corey–Chaykovsky reaction
sequence, see: Kumar, B. S.; Venkataramasubramanian, V.;
Sudalai, A. Org. Lett. 2012, 14, 2468.
(9) (a) Larrow, J. F.; Schaus, S. E.; Jacobsen, E. N. J. Am. Chem. Soc.
1996, 118, 7420. (b) Hansen, K. B.; Leighton, J. L.; Jacobsen, E. N.
J. Am. Chem. Soc. 1996, 118, 10924.
Obtained using GPB from 4′-chloropropiophenone (50 mg, 0.30
mmol) and purified by chromatography (PE/EtOAc) to give a yellow
oil; yield: 43 mg (69%).
1H NMR (400 MHz, CDCl3): δ = 7.34–7.28 (m, 4 H), 5.60–5.49 (m, 1 H),
5.16–5.13 (m, 2 H), 2.68–2.65 (m, 1 H), 2.47 (dd, J = 13.7, 8.6 Hz, 1 H),
1.83–1.80 (m, 2 H), 0.75 (t, J = 7.4 Hz, 3 H). Spectroscopic data were in
agreement with those previously reported.25
(10) Jacobsen, E. N.; Kakiuchi, F.; Konsler, R. G.; Larrow, J. F.;
Tokunaga, M. Tetrahedron Lett. 1997, 38, 773.
(11) (a) Schaus, S. E.; Jacobsen, E. N. Tetrahedron Lett. 1996, 37, 7937.
(b) Bai, S.; Li, B.; Zhang, X.; Yang, Q.; Li, C. Chem. Sci. 2012, 3,
2864. (c) White, D. E.; Tadross, P. M.; Lu, Z.; Jacobsen, E. N. Tetra-
hedron 2014, 70, 4165.
(12) (a) Hinterding, K.; Jacobsen, E. N. J. Org. Chem. 1999, 64, 2164.
(b) Denmark, S. E.; Ahmad, M. J. Org. Chem. 2007, 72, 9630.
(c) North, M.; Quek, S. C. Z.; Pridmore, N. E.; Whitwood, A. C.;
Wu, X. ACS Catal. 2015, 5, 3398.
1-Allylcyclohexan-1-ol (Table 3, Entry 6)
Obtained using GPB from cyclohexanone (50 mg, 0.51 mmol) and pu-
rified by chromatography (PE/ EtOAc) to give a yellow oil; yield: 55
mg (85%).
1H NMR (400 MHz, CDCl3): δ = 5.94–5.83 (m, 1 H), 5.16–5.08 (m, 2 H),
2.23–2.20 (m, 2 H), 1.57–1.29 (m, 10 H). Spectroscopic data were in
agreement with those previously reported.21
(13) (a) Mahadevan, V.; Getzler, Y. D. Y. L.; Coates, G. W. Angew.
Chem. Int. Ed. 2002, 41, 2781. (b) Getzler, Y. D. Y. L.; Mahadevan,
V.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124,
1174. (c) Schmidt, J. A. R.; Mahadevan, V.; Getzler, Y. D. Y. L.;
Coates, G. W. Org. Lett. 2004, 6, 373. (d) Getzler, Y. D. Y. L.;
Mahadevan, V.; Lobkovsky, E. B.; Coates, G. W. Pure Appl. Chem.
2004, 76, 557. (e) Schmidt, J. A. R.; Lobkovsky, E. B.; Coates, G.
W. J. Am. Chem. Soc. 2005, 127, 11426.
Funding Information
Support from The University of Auckland is gratefully acknowledged
through the award of doctoral scholarships to M.L.J. and N.Z.B.
(14) (a) Nielsen, L. P. C.; Stevenson, C. P.; Blackmond, D. G.; Jacobsen,
E. N. J. Am. Chem. Soc. 2004, 126, 1360. (b) Nielsen, L. P. C.;
Zuend, S. J.; Ford, D. D.; Jacobsen, E. N. J. Org. Chem. 2012, 77,
2486.
Acknowledgment
The authors thank Dr Paul A. Hume (SCS, UoA) for helpful discussions.
(15) Note that all reactions were conducted under anhydrous condi-
tions, which was determined to be important to obtain high
conversion in all ylide reactions, see the experimental section
for details.
(16) Sipos, G.; Drinkel, E. E.; Dorta, R. Chem. Soc. Rev. 2015, 44, 3834.
(17) Weber, L. Angew. Chem. Int. Ed. 1983, 22, 516.
(18) Burtoloso, A. C. B.; Dias, R. M. P.; Leonarczyk, I. A. Eur. J. Org.
Chem. 2013, 5005.
(19) Mangion, I. K.; Weisel, M. Tetrahedron Lett. 2010, 51, 5490.
(20) Bellow, J. A.; Stoian, S. A.; van Tol, J.; Ozarowski, A.; Lord, R. L.;
Groysman, S. J. Am. Chem. Soc. 2016, 138, 5531.
(21) Guijarro, D.; Mancheño, B.; Yus, M. Tetrahedron 1992, 48, 4593.
(22) Schmidt, B. J. Org. Chem. 2004, 69, 7672.
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