Carbocation Lewis Acid TrBF4-Catalyzed 1,2-Hydride Migration: Approaches to (Z)-α,β-Unsaturated Esters and α-Branched β-Ketocarbonyls

Article abstract of DOI:10.1021/acs.orglett.9b03005

Carbocation Lewis acid TrBF₄-catalyzed 1,2-hydride migration of α-alkyldiazoacetates themselves or in situ-generated cross-coupling adducts of aldehydes and α-alkyldiazoacetates has been developed, affording (Z)-α,β-unsaturated esters and α-branched β-ketocarbonyls, respectively, in good yields and with high regioselectivities.

Full text of DOI:10.1021/acs.orglett.9b03005

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Carbocation Lewis Acid TrBF4‑Catalyzed 1,2-Hydride Migration:
Approaches to (Z)‑α,β-Unsaturated Esters and α‑Branched
β‑Ketocarbonyls
Wansong Shang, Depeng Duan, Yongjun Liu,* and Jian Lv*
Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science, MOE, State Key Laboratory Base of
Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology,
Qingdao 266042, China
S
* Supporting Information
ABSTRACT: Carbocation Lewis acid TrBF4-catalyzed 1,2-
hydride migration of α-alkyldiazoacetates themselves or in
situ-generated cross-coupling adducts of aldehydes and α-
alkyldiazoacetates has been developed, affording (Z)-α,β-
unsaturated esters and α-branched β-ketocarbonyls, respec-
tively, in good yields and with high regioselectivities.
iazocarbonyl compounds have been versatile intermedi-
ates in diverse organic syntheses that can easily generate
electron-deficient metal carbenes (Scheme 1A, I) in the
(Scheme 1A, II).2 However, the reactions were limited in
the scope of α-alkyl diazoacetates (such as R = H, Me, and Et).
Recently, the reactive free carbene intermediates were also
generated via photolysis of phenyldiazoacetates upon visible
light irradiation (Scheme 1A, III).3a−d In addition, Meggers
and co-workers found that α-diazo carboxylic esters could also
serve as precursors for carbon-centered radicals under
photoredox conditions.3e Thus, the development of more
efficient catalytic strategies for a number of reactions with
diazocarbonyls activated by Lewis acid is of great significance
in organic synthetic chemistry.
D
Scheme 1. New Catalytic Strategies of Diazocarbonyl
Activation
Since the discovery of the first trityl cation in 1901,4 stable
carbocations as non-metal Lewis acid catalysts have been
widely applied in organic synthesis.5−7 However, carbonyl
compounds have mainly been used as substrates in carbocation
Lewis acid catalysis (Scheme 1B). Recently, we developed
latent carbocation catalysis with chiral phosphate as a
counteranion of the trityl cation, which efficiently promoted
the asymmetric Friedel−Crafts reaction, HDA reaction, and
carbonyl-ene reaction.8 In addition, we also reported the
tritylium salt TrBArF, in situ-generated by Ph3CBr and
NaBArF-catalyzed redox-neutral α-C(sp3)H arylation of
amines9 and Diels−Alder reaction of β,γ-unsaturated α-
ketoesters with anthracenes.10 In fact, Ph3C+-mediated C−H
oxidations via a hydride abstraction mechanism are also well-
known. Pre-prepared electrophilic 1-triphenyl-methylpyridi-
nium salt by the use of stoichiometric tritylium salt, such as
TrBF4 and TrClO4, and pyridine has also been seldom
reported to promote the nucleophilic heteroaromatic sub-
stitution with sodium diisopropylphosphonate to yield
diisopropylpyridyl-4-phosphonates and triphenylmethane.11
Recently, Liu and Lou reported that a stoichiometric tritylium
salt as an oxidant promoted the reaction of N-allyl carbamates
presence of transition metal catalysts and further undergo a
number of chemical transformations such as cyclopropanation,
X−H (X = C, O, S, or N) insertion, ylide formation, and
formal carbene dimerization.1 In addition, Lewis base catalysis
was reported to promote (3+2) cycloaddition of nucleophilic
diazocarbonyls activated by triphenylphosphine to react with
β-trifluoromethyl enones via a zwitterionic intermediate
Received: August 23, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03005
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
or THF with different nucleophiles at ambient temperature.12
In a way overriding the hydride abstraction process by the
tritylium cation, we herein present a tritylium salt TrBF4 as a
carbocation Lewis acid catalyst to achieve a 1,2-hydride
migration of an α-alkyldiazoacetate itself or in situ-generated
cross-coupling adduct of an aldehyde and an α-alkyldiazoace-
tate through an ion-pair hydrogen bonding intermediate I
formed by a diazocarbonyl and TrBF4 (Scheme 1C).
In fact, Rh(II)-promoted 1,2-hydride or 1,2-alkyl migration,
which is commonly regarded as a competing side reaction with
α-alkyldiazoacetate-derived metal carbenes reactions,13a,b is
also a useful approach to activated olefins or 1,3-dicarbo-
nyls.13c,d For example, Hudlicky14 and Wang15 developed
Rh2(OAc)4-mediated 1,2-hydride and 1,2-alkyl migration of
various α-diazo carbonyls to give α,β-enones and α-aryl-β-
enamino esters, respectively. Due to poor stereoselective
control, Lewis acid-catalyzed 1,2-hydride migration with
alkyldiazoacetate to afford activated olefins has not been
successfully achieved.
(Table 1, entry 7). Furthermore, screening of various solvents
was carried out with the tritylium salt TrBF4 (10 mol %) at
room temperature. The best result was obtained in DCM
(Table 1, entry 6), whereas others resulted in either low
activity or moderate Z selectivity (Table 1, entry 6 vs entries
9−14). In addition, the yield and Z selectivity of 2a did not
decrease as the dosage of carbocation TrBF4 was decreased to
5 mol % by extending the reaction time to 60 min (Table 1,
entry 15).
Next, the other commonly employed pre-prepared trityl salt
TrClO4 was also examined in CH2Cl2 at room temperature
(Scheme 2, BF4 vs ClO4). The different efficiency observed for
Scheme 2. Counteranion Effect in Carbocation Lewis Acid
a
Catalysis
First, a 1,2-hydride migration of ethyl benzyldiazoacetate 1a
has been chosen as a model reaction to be investigated with
different Lewis acid catalysts. As we expected, the common
Lewis acids, such as In(III), Bi(III), Ni(II), and Fe(III), have
very weak activity (Table 1, entries 1−4, respectively).
a
General conditions: 1a (0.1 mmol) and carbocations (10 mol %) in
b
CH2Cl2 (0.2 M) at room temperature for 10 min. Determined by 1H
a
Table 1. Optimization of Reaction Conditions
NMR analysis with an internal standard, 1,3,5-trimethyloxylbenzene.
c
1
Ratio determined by H NMR of the reaction mixture.
two carbocation Lewis acids prompted us to further explore
the effect of the counterion on the 1,2-hydride migration.
Accordingly, a number of tritylium salts were examined by
mixing Ph3CCl with a variety of silver salts in CH2Cl2.
b
c
entry
catalyst
solvent
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
yield (%)
Z:E
1
2
3
4
5
6
7
In(OTf)3
Bi(OTf)3
Ni(OTf)2
FeBr3
Cu(OTf)2
TrBF4
TrBr
TrBF4
TrBF4
TrBF4
22
35
3
68:32
44:56
17:83
66:34
38:62
94:6
−
Interestingly, compared with the small tetrahedral BF4 ion, a
carbocation Lewis acid with the larger SbF6 , PF6 , and OTf−
(Tf = trifluoromethanesulfonate) counterions resulted in either
low activity or poor Z selectivity. Moreover, several tritylium
salts with the larger tetrahedral anion were also examined by
mixing Ph3CBr with different Lewis acids, such as InBr3 and
FeBr3, giving only trace product 2a. In addition, non-
coordinating anionic (such as BArF−) tuning, a strategy widely
applied in the catalysis with tritylium salts, a Lewis acid, and a
transition metal, worked against us. Interestingly, we were able
to isolate a trace of a byproduct characterized as a hydride
abstraction product, triphenyl methane, only in the catalysis of
the carbocation TrBArF.
−
−
17
89
99
trace
98
85
13
20
16
83
55
99
−
d
8
86:14
90:10
67:33
65:35
42:58
34:66
52:48
94:6
9
CHCl3
Et2O
10
11
12
13
14
TrBF4
TrBF4
TrBF4
TrBF4
toluene
CH3CN
acetone
CH3CH2OH
DCM
e
15
TrBF4
a
To define the roles of the counteranion in the carbocation
catalysis, deuterium labeling experiments were carried out by
using H NMR (Figure S1 and Scheme 3). When compound
1a and fully deuterated compound 1a-d7 (1:1) were subjected
to catalysis of tritylium salt [Ph3C][BF4], product 2a and fully
General conditions: 1a (0.1 mmol) and catalyst (10 mol %) in
b
solvent (0.2 M) at room temperature for 10 min. Determined by 1H
NMR analysis with an internal standard, 1,3,5-trimethyloxylbenzene.
1
c
d
Ratio determined by 1H NMR of the reaction mixture. With
e
Na2CO3 (0.5 equiv). With TrBF4 (5 mol %) for 60 min.
Scheme 3. Deuterium Labeling Experiments
Cu(OTf)2 could catalyze this reaction in good yield, albeit
with lower selectivity [Z:E = 38:62 (Table 1, entry 5)]. We
were extremely excited to find that a carbocationic Lewis acid
TrBF4 could promote this reaction in high yield and with high
Z selectivity [Z:E = 94:6 (Table 1, entry 6)]. Nevertheless, the
model reaction, conducted in the presence of inorganic
Na2CO3 (0.5 equiv) at room temperature in CH2Cl2,
proceeded smoothly to give the desired olefin 2a in 98%
yield and with good Z selectivity [Z:E = 86:14 (Table 1, entry
8)], which excluded the possibility of a free acid catalyzing this
reaction. When TrBr was used, the reaction did not work
a
b
E-Selective isomer. Z:E = 1:2.
B
DOI: 10.1021/acs.orglett.9b03005
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
deuterated product 2a-d7 were obtained in almost equal
amounts, giving the desired olefin 2a in 85% yield and with
good Z selectivity (Z:E = 83:17) after reaction for 10 min. To
our surprise, a small amount of product E-2a-d6 was observed
by 1H NMR (Scheme 3, entry 1), suggesting that E isomer 2a
may be not formed via a 1,2-hydride transfer process.
However, the use of BArF− as a counteranion was found to
facilitate hydride abstraction by the tritylium cation, and much
more product 2a-d6 (Z:E = 2:1) was observed; however, only
30% of the α,β-unsaturated ester (Z:E = 22:78) was obtained
after 360 min (Scheme 3, entry 2). The results also indicate
that hydride abstraction by TrBArF may result in a significant
decrease in the reaction rate by suppressing 1,2-hydride
transfer.
Scheme 5. Substrate Scope of 1,2-Hydride Migration of α-
Diazoacetate 1
a b
,
On the basis of the results presented above and our
preliminary mechanistic study, a plausible reaction pathway for
carbocation-catalyzed hydride migration is depicted in Scheme
4. Initially, 1,2-hydride migration of compound 1a in the
Scheme 4. Proposed Mechanism
presence of TrBF4 forms intermediate IA, which then
preferentially converts to intermediate IIA via ion-pair
−
hydrogen bonding between the fluorine atoms on [BF4 ]
anions and the Cβ-hydrogen (Ha) on ethyl benzyldiazoacetate
1a (Scheme 4).16 As illustrated in IIA, the ion pair of the
−
benzyl carbocation, the BF4 anions, and the trityldiazene
moiety are cis-coplanar in the cyclic transition state, and IIA is
converted to Z-2a by releasing one molecule of N2 and TrBF4
via an intramolecular syn elimination. Critical intermediate IA
can be detected by ESI-MS analysis of the reaction mixture,
which verifies the tritylium catalytic nature of this reaction
(Figure S2).
a
General conditions: 1 (0.4 mmol) and TrBF4 (5 mol %) in CH2Cl2
b
c
(0.2 M) at room temperature for 60 min. Isolated yield. Eight
millimole scale of 1a.
With the optimal reaction conditions and mechanism
established, the scope of carbocation Lewis acid-catalyzed
1,2-hydride migration was next explored with TrBF4 (5 mol %)
in dichloromethane (DCM) at room temperature. Different
ester groups on the diazoester part were tolerated to give α,β-
unsaturated esters with high to excellent Z selectivity [up to
Z:E = 94:6 (Scheme 5, entries 1−5)]. A variety of aryl groups
(R) in the diazoesters were also tested, and electron-
withdrawing (F, Cl, and Br) or electron-donating (CH3)
groups at the ortho, meta, or para position of the phenyl group
can be equally applied with good to high yields and in high Z
selectivity [up to Z:E = 99:1 (Scheme 5, entries 6−14)].
Gratifyingly, the large α′- or β′-naphthyl (Scheme 5, entry 15
or 16, respectively) and biphenyl-substituted α-isopropyl
diazoesters also worked well, giving moderate to good yields
and excellent Z selectivity [up to Z:E = 99:1 (Scheme 5, entries
17 and 18)]. As we expected, the reactions of 2-heteroaromatic
and alkyl-substituted α-benzyl diazoesters provided only Z-
selective product 2s and 2t in moderate to high yields. Then,
we turned our attention to phenylallyl-substituted diazoacetate
derivative 1u, and the resulting (2Z,4E)- and (2E,4E)-2,4-
dienoates 2u were isolated in 35% and 16% yields, respectively.
In particular, the reaction with allyl diazoacetate 1v also
proceeded smoothly to give the desired Z-selective olefin 2v in
39% yield. It was noted that in this case (4+1) cycloadduct 2v′
was detected by NMR in <10% yield and with only single
diastereoselectivity (Scheme 5, entry 22). Furthermore, when
ethyl 2-diazo-2-phenylacetate 1w was used, (Z)-2,3-diphenyl-
but-2-enedioic acid diethyl ester 2w was obtained in 88% yield
(Scheme 5, entry 23). Likewise, (Z)-chalcone 4 was also
obtained by the carbocation TrBF4-catalyzed 1,2-hydride
migration of α-diazoketone 3 (Scheme 6).
In addition, we found that TrBF4 (10 mol %) could catalyze
the Roskamp reaction17 of α-alkyldiazoacetates 1 and
aldehydes 5 in DCM at room temperature (Scheme 7), giving
the expected 1,3-dicarbonyls 6 in moderate to excellent yields.
The significant electronic effect of the aryl group (R) has been
observed. Electron-neutral (4-H), electron-withdrawing (4-Br),
C
DOI: 10.1021/acs.orglett.9b03005
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
respectively (Scheme 7, entries 6 and 7, respectively).
Moreover, n-decyl and allylic diazoester also proceeded
smoothly to give the expected 1,3-dicarbonyls 6ta and 6va in
49% and 37% yields, respectively. A number of the aromatic
aldehydes 5b−f could react with benzyldiazoacetate 1a, giving
moderate to good yields (Scheme 6, entries 10−14,
respectively). Heterocycles, such as thiophene and indole
groups, can also be incorporated to deliver 1,3-dicarbonyls 6ag
and 6ah in 67% and 38% yields, respectively (Scheme 6,
entries 15 and 16, respectively). Interestingly, ferrocene-
substituted aldehydes also worked well, leading to a 44%
yield (Scheme 7, entry 17). In addition, the reaction with
cinnamaldehyde derivative 5j also proceeded smoothly to give
the desired 1,3-dicarbonyl 6aj in 63% yield (Scheme 6, entry
18). In this reaction, a reactive intermediate may be diazo
cation int-1 followed by 1,2-hydride transfer to furnish product
6aa (Scheme 6, entry 19). However, an α,β-epoxy ester from
an aldehyde 3a and a diazoester 1a via diazo cation int-2 was
not obtained at all (Scheme 6, entry 20).
Scheme 6. 1,2-Hydride Migration of α-Diazoketone 3
Scheme 7. Substrate Scope of the Roskamp Reaction of α-
a b
,
Diazoacetates 1 and Aldehydes 3
In summary, we have developed a carbocation Lewis acid
TrBF4-catalyzed 1,2-hydride migration of α-alkyldiazoacetates,
affording (Z)-α,β-unsaturated esters in good yields and with
high Z selectivity. Salient features of the current reaction
include the 1,2-hydride migration versus hydride abstraction in
the presence of tritylium Lewis acid catalysis, uniformly high Z
selectivity, and the harnessing of a weak ion-pair hydrogen
bonding for Z/E selectivity control. Moreover, it was found
that TrBF4 also catalyzed the Roskamp reaction of α-
alkyldiazoacetates and aldehydes to give various α-branched
β-ketocarbonyls in good yields. Further studies are currently
underway to illuminate the mechanistic details and to extend
the carbocation Lewis acid catalysis to other reactions.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03005.
Experimental details and spectroscopic data for all new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: yongjunliu@qust.edu.cn.
*E-mail: lvjian@iccas.ac.cn, lvjian@qust.edu.cn.
ORCID
a
General conditions: 1 (0.45 mmol), 3 (0.3 mmol), and TrBF4 (10
Yongjun Liu: 0000-0002-7615-3842
Jian Lv: 0000-0001-7641-5411
Notes
mol %) in CH2Cl2 (0.2 M) at room temperature for 120 min.
b
c
Isolated yield. Eight millimole scale of 3a. Fc = ferrocene.
The authors declare no competing financial interest.
and electron-donating (4-OCF3) groups gave similar results
[64% yield for 6aa, 55% yield for 6ha, and 56% yield for 6na
(Scheme 7, entries 1, 2, and 5, respectively)]. ortho- and meta-
substituted derivatives, such as 1k and 1j, also worked well,
providing yields much better than that of its para-subsituted
counterpart 1h (Scheme 6, entries 3 and 4 vs entry 2).
Gratifyingly, α′-naphthyl 1p and heteroaromatic 3-furanyl 1s-
substituted diazoacetate could react with benzaldehyde 2a,
giving products 6pa and 6sa in 59% and 24% yields,
ACKNOWLEDGMENTS
■
The authors thank the Natural Science Foundation of China
(21472193), the Key Research and Development Program of
Shandong Province (2019GGXI02036), and the Qingdao
University of Science & Technology for financial support. J.L.
is supported by the Taishan Scholarship Project of Shandong
Province (tsqn201812075).
D
DOI: 10.1021/acs.orglett.9b03005
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(12) (a) Xie, Z.; Liu, L.; Chen, W.; Zheng, H.; Xu, Q.; Yuan, H.;
Lou, H. Angew. Chem., Int. Ed. 2014, 53, 3904−3908. (b) Wan, M.;
Meng, Z.; Lou, H.; Liu, L. Angew. Chem., Int. Ed. 2014, 53, 13845−
13849.
(13) (a) Taber, D. F.; Sheth, R. B.; Joshi, P. V. J. Org. Chem. 2005,
70, 2851−2854. (b) Liu, S.; Jiang, J.; Chen, J.; Wei, Q.; Yao, W.; Xia,
F.; Hu, W. Chem. Sci. 2017, 8, 4312−4317. (c) Taber, D. F.; Herr, R.
J.; Pack, S. K.; Geremia, J. M. J. Org. Chem. 1996, 61, 2908−2910.
(d) Ikota, N.; Takamura, N.; Young, S. D.; Ganem, B. Tetrahedron
Lett. 1981, 22, 4163−4166.
(14) Hudlicky, T.; Olivo, H. F.; Natchus, M. G.; Umpierrez, E. F.;
Pandolfi, E.; Volonterio, C. J. Org. Chem. 1990, 55, 4767−4770.
(15) Jiang, N.; Qu, Z.; Wang, J. Org. Lett. 2001, 3, 2989−2992.
(16) Ion-pair hydrogen bonding has been observed in the p-
ammonium phenol or imidazolyl ionic liquid system in nonpolar
environments. See: (a) Samet, M.; Buhle, J.; Zhou, Y.; Kass, S. R. J.
REFERENCES
■
(1) For select reviews, see: (a) Ye, T.; Mckervey, M. A. Chem. Rev.
1994, 94, 1091−1160. (b) Doyle, M. P.; Forbes, D. C. Chem. Rev.
1998, 98, 911−935. (c) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem.
Rev. 2007, 107, 3117−3179. (d) Davies, H. M. L.; Manning, J. R.
Nature 2008, 451, 417−424. (e) Zhang, Y.; Wang, J. Chem. Commun.
2009, 5350−5361. (f) Zhao, X.; Zhang, Y.; Wang, J. Chem. Commun.
2012, 48, 10162−10173. (g) Zhu, S.-F.; Zhou, Q.-L. Acc. Chem. Res.
2012, 45, 1365−1377. (h) Davies, H. M. L.; Lian, Y. Acc. Chem. Res.
2012, 45, 923−935. (i) Guo, X.; Hu, W. Acc. Chem. Res. 2013, 46,
2427−2440. (j) Gillingham, D.; Fei, N. Chem. Soc. Rev. 2013, 42,
4918−4931. (k) Ford, A.; Miel, H.; Ring, A.; Slattery, C. N.; Maguire,
A. R.; McKervey, M. A. Chem. Rev. 2015, 115, 9981−10080.
(l) Chanthamath, S.; Iwasa, S. Acc. Chem. Res. 2016, 49, 2080−2090.
(m) Cheng, Q.-Q.; Deng, Y.; Lankelma, M.; Doyle, P. M. Chem. Soc.
Rev. 2017, 46, 5425−5443.
Am. Chem. Soc. 2015, 137, 4678−4680. (b) Lin, J.; Lu, R.-q. Journal of
̈
Jinggangshan University (Natural Science) 2013, 34, 39−46.
(2) Li, Y.; Wang, H.; Su, Y.; Li, R.; Li, C.; Liu, L.; Zhang, J. Org. Lett.
2018, 20, 6444−6448.
(3) For a review on photochemical reactions of diazo compounds,
(17) For a review on the Roskamp reaction, see: (a) Candeias, N. R.;
Paterna, R.; Gois, P. M. P. Chem. Rev. 2016, 116, 2937−2981. For
some selected examples of the Roskamp reaction, see: (b) Holmquist,
C. R.; Roskamp, E. J. J. Org. Chem. 1989, 54, 3258−3260. (c) Gao, L.;
Kang, B. C.; Hwang, G.-S.; Ryu, D. H. Angew. Chem., Int. Ed. 2012,
51, 8322−8325. (d) Hashimoto, T.; Miyamoto, H.; Naganawa, Y.;
Maruoka, K. J. Am. Chem. Soc. 2009, 131, 11280−11281. (e) Li, W.;
Wang, J.; Hu, X.; Shen, K.; Wang, W.; Chu, Y.; Lin, L.; Liu, X.; Feng,
X. J. Am. Chem. Soc. 2010, 132, 8532−8533. (f) Li, W.; Liu, X.; Tan,
F.; Hao, X.; Zheng, J.; Lin, L.; Feng, X. Angew. Chem., Int. Ed. 2013,
52, 10883−10886. (g) Li, W.; Tan, F.; Hao, X.; Wang, G.; Tang, Y.;
Liu, X.; Lin, L.; Feng, X. Angew. Chem., Int. Ed. 2015, 54, 1608−1611.
́
see: (a) Ciszewski, Ł. W.; Rybicka-Jasinska, K.; Gryko, D. Org. Biomol.
Chem. 2019, 17, 432−448. For some selected examples, see:
(b) Xiao, T.; Mei, M.; He, Y.; Zhou, L. Chem. Commun. 2018, 54,
8865−8868. (c) Jurberg, I. D.; Davies, H. M. L. Chem. Sci. 2018, 9,
5112−5118. (d) Hommelsheim, R.; Guo, Y.; Yang, Z.; Empel, C.;
Koenigs, R. M. Angew. Chem., Int. Ed. 2019, 58, 1203−1207.
(e) Huang, X.; Webster, R. D.; Harms, K.; Meggers, E. J. Am. Chem.
Soc. 2016, 138, 12636−12642.
(4) (a) Norris, J. F. Am. Chem. J. 1901, 25, 117−122. (b) Kehrmann,
F.; Wentzel, F. Ber. Dtsch. Chem. Ges. 1901, 34, 3815−3819.
(5) (a) Mukaiyama, T.; Kobayashi, S.; Shoda, S.-i. Chem. Lett. 1984,
13, 907−910. (b) Mukaiyama, T.; Kobayashi, S.; Shoda, S.-i. Chem.
Lett. 1984, 13, 1529−1530. (c) Mukaiyama, T.; Kobayashi, S.;
Murakami, M. Chem. Lett. 1984, 13, 1759−1762. (d) Mukaiyama, T.;
Kobayashi, S.; Murakami, M. Chem. Lett. 1985, 14, 447−450.
(e) Kobayashi, S.; Murakami, M.; Mukaiyama, T. Chem. Lett. 1985,
14, 953−956. (f) Mukaiyama, T.; Nagaoka, H.; Murakami, M.;
Ohshima, M. Chem. Lett. 1985, 14, 977−980. (g) Kobayashi, S.;
Murakami, M.; Mukaiyama, T. Chem. Lett. 1985, 14, 1535−1538.
(h) Ohshima, M.; Murakami, M.; Mukaiyama, T. Chem. Lett. 1985,
14, 1871−1874. (i) Kobayashi, S.; Matsui, S.; Mukaiyama, T. Chem.
Lett. 1988, 17, 1491−1494. (j) Yanagisawa, M.; Mukaiyama, T. Chem.
Lett. 2001, 30, 224−225.
(6) (a) Roy, S. R.; Nijamudheen, A.; Pariyar, A.; Ghosh, A.;
Vardhanapu, P. K.; Mandal, P. K.; Datta, A.; Mandal, S. K. ACS Catal.
́
2014, 4, 4307−4319. (b) Bah, J.; Franzen, J. Chem. - Eur. J. 2014, 20,
́
1066−1072. (c) Bah, J.; Naidu, V. R.; Teske, J.; Franzen, J. Adv. Synth.
Catal. 2015, 357, 148−158. (d) Veluru Ramesh Naidu; Bah, J.;
Franzen, J. Eur. J. Org. Chem. 2015, 2015, 1834−1839. (e) Naidu, V.
R.; Ni, S.; Franzen, J. ChemCatChem 2015, 7, 1896−1905.
́
(f) Moosavi-Zare, A. R.; Zolfigol, M. A.; Rezanejad, Z. Can. J.
Chem. 2016, 94, 626−630. (g) Lyons, D. J. M.; Crocker, R. D.;
Enders, D.; Nguyen, T. V. Green Chem. 2017, 19, 3993−3996.
(h) Liu, J.; Xu, J.; Li, Z.; Huang, Y.; Wang, H.; Gao, Y.; Guo, T.;
Ouyang, P.; Guo, K. Eur. J. Org. Chem. 2017, 2017, 3996−4003.
(7) (a) Riant, O.; Samuel, O.; Kagan, H. B. J. Am. Chem. Soc. 1993,
115, 5835−5836. (b) Taudien, S.; Riant, O.; Kagan, H. B. Tetrahedron
Lett. 1995, 36, 3513−3516. (c) Sammakia, T.; Latham, H. A.
Tetrahedron Lett. 1995, 36, 6867−6870. (d) Chen, C.-T.; Chao, S.-D.;
Yen, K.-C.; Chen, C.-H.; Chou, I.-C.; Hon, S.-W. J. Am. Chem. Soc.
1997, 119, 11341−11342. (e) Magdziak, D.; Pettus, L. H.; Pettus, T.
R. R. Org. Lett. 2001, 3, 557−559. (f) Pommerening, P.; Mohr, J.;
Friebel, J.; Oestreich, M. Eur. J. Org. Chem. 2017, 2017, 2312−2316.
(8) Lv, J.; Zhang, Q.; Zhong, X.; Luo, S. J. Am. Chem. Soc. 2015, 137,
15576−15583.
(9) Zhang, Q.; Lu, J.; Luo, S. Acta Chim. Sinica 2016, 74, 61−66.
̈
(10) (a) Zhang, Q.; Lv, J.; Li, S.; Luo, S. Org. Lett. 2018, 20, 2269−
2272. (b) Zhang, Q.; Lv, J.; Luo, S. Beilstein J. Org. Chem. 2019, 15,
1304−1312.
(11) Redmore, D. J. Org. Chem. 1976, 41, 2148−2150.
E
DOI: 10.1021/acs.orglett.9b03005
Org. Lett. XXXX, XXX, XXX−XXX