Variability of Rhodium(III)-Catalyzed Reactions of Aromatic Oximes with Alkenes

Article abstract of DOI:10.1055/s-0040-1707961

Acetophenone oxime reacts with various alkenes in the presence of the rhodium catalyst [Cp*RhCl 2 ] 2 (2.5 molpercent; Cp? = pentamethylcyclopentadienyl) and 1,1,1,3,3,3-hexafluoropropan-2-ol as an important cosolvent. Styrene, aliphatic terminal alkenes, and strained cyclic alkenes gave the corresponding substituted dihydroisoquinolines in yields of 50-99percent. On the other hand, alkenes containing functional groups close to the double bond gave a variety of different products. The reactions of acetophenone oxime with styrene or dec-1-ene in the presence of the chiral catalyst [(C 5 H 2 t Bu 2 CH 2 t Bu)RhI 2 ] 2 provided the corresponding dihydroisoquinolines with improved regioselectivity but a low enantiomeric ratio (61:39 in both cases).

Full text of DOI:10.1055/s-0040-1707961

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2020, 31, 1117–1120
letter
1117
en
E. A. Trifonova et al.
Letter
Synlett
Variability of Rhodium(III)-Catalyzed Reactions of Aromatic Oximes
with Alkenes
Evgeniya A. Trifonovaa
main pathway
Alina A. Komarovaa
Denis Chusova,b
Dmitry S. Perekalin*a,b
OH
R
R
[Cp*RhCl2]2 (2.5 mol%)
N
N
Rn
other
pathways
0
0
0
0-0
0
0
2-0
8
6
4-
8
9
0
7
50–99%
a Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, 28 Vavilova Street, Moscow, 119991,
Russian Federation
b Plekhanov Russian University of Economics, 36 Stremyannyi
pereulok, Moscow, 117997, Russian Federation
dsp@ineos.ac.ru
COOEt
O
N
N
N
N
O
EtOOC
COOEt
This work is dedicated to the memory of Prof. Keith Fagnou,
who made a seminal contribution to rhodium(III)-catalyzed
C−H functionalization reactions, and sadly passed away ten
years ago at the age of 38.
Received: 01.03.2020
Accepted after revision: 03.04.2020
Published online: 24.04.2020
the other hand, similar reactions of oximes with alkenes are
still relatively underexplored, even though the expected
products, namely partially hydrogenated isoquinolines, are
common structural fragments of biologically active com-
pounds.11 Bergman, Ellman, and co-workers12 and the Lee
group13 have reported that O-methyl oximes react with
alkenes without ring closure to produce vinyl-substituted
derivatives (Scheme 1, equations B and C). A related enanti-
oselective reaction has been carried out by Wang and Cra-
mer, with the help of binaphthyl-substituted Rh complexes.14
DOI: 10.1055/s-0040-1707961; Art ID: st-2020-r0120-l
Abstract Acetophenone oxime reacts with various alkenes in the pres-
ence of the rhodium catalyst [Cp*RhCl2]2 (2.5 mol%; Cp* = pentameth-
ylcyclopentadienyl) and 1,1,1,3,3,3-hexafluoropropan-2-ol as an im-
portant cosolvent. Styrene, aliphatic terminal alkenes, and strained
cyclic alkenes gave the corresponding substituted dihydroisoquinolines
in yields of 50–99%. On the other hand, alkenes containing functional
groups close to the double bond gave a variety of different products.
The reactions of acetophenone oxime with styrene or dec-1-ene in the
t
t
presence of the chiral catalyst [(C5H2 Bu2CH2 Bu)RhI2]2 provided the
corresponding dihydroisoquinolines with improved regioselectivity but
a low enantiomeric ratio (61:39 in both cases).
[Cp*RhCl2]2
base
R
OR
N
N
A
+
many examples
R
R
R
Key words C–H activation, rhodium catalysis, oximes, alkenes, iso-
quinolines
[Cp*RhCl2]2
OMe
OMe
AgSbF6, Cu(OAc)2
N
N
+
B
Bergman and
Ellmann et al.
2011
Reliability is one of the most important characteristics
of a synthetic method because it permits its use in prepara-
tions of previously unknown compounds. To establish the
reliability of a method, organic chemists apply it to a vari-
ety of substrates. However, negative results that narrow the
substrate scope are frequently not reported. Here, we
would like to break away from this tendency and report not
only the expected, but also the unexpected products of the
Rh(III)-catalyzed reactions of aromatic oximes with
alkenes.
In the last decade, C−H functionalization reactions cata-
lyzed by the rhodium(III) complex [Cp*RhCl2]2 (Cp* = pen-
tamethylcyclopentadienyl) have been widely used for the
synthesis of various heterocycles.1–3 In particular, rhodium-
catalyzed reactions of aromatic oximes with alkynes to give
isoquinolines (Scheme 1, equation A) have been studied in
detail,4–7 as have similar cobalt-catalyzed examples.8–10 On
R
R
[Cp*RhCl2]2
OMe
OMe
AgSbF6, KOPiv
N
N
+
+
+
C
D
R
R
R
Li et al.
2017
O
O
R
[Cp'Rh(NCMe)3]2+
CsOAc
R
R
R
OPiv
N
R
N
Rovis et al.
2015
R
R
[Cp*RhCl2]2
K2CO3
R
R
OH
N
N
E
R = Ar, Chen
and Li et al. 2019
Rn
R = Alk, this work
Scheme 1 Some typical rhodium-catalyzed reactions of oximes
© 2020. Thieme. All rights reserved. Synlett 2020, 31, 1117–1120
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

1118
E. A. Trifonova et al.
Letter
Synlett
Several groups have shown that reactions of O-acetyl
oximes with dienes,15 vinyl acetates,16,17 vinyl aldehydes,18
or ketones,18 are accompanied by aromatization and give
isoquinolines. Similar reactions proceed without aromati-
zation in the case of activated alkenes such as ketenes19 or
urea-derived bicycle olefins.20 Rovis and co-workers found
that acrylate oximes are apparently more active than aro-
matic ones, and their O-pivaloyl derivatives can react with
various alkenes with or without aromatization (Scheme 1,
equation D).21–23 During the preparation of this manuscript,
the group of Chen and Li reported the reaction of aromatic
oximes with various styrenes to give dihydroisoquinoline
heterocycles (Scheme 1, equation E).24 However, similar re-
actions with aliphatic alkenes remained unexplored.
We started our investigation with the reaction of the
most common substrates, namely that of acetophenone ox-
ime (1) with styrene in the presence of the classical catalyst
[Cp*RhCl2]2 with K2CO3 as a base (Table 1). Optimization of
the reaction conditions revealed an unexpected solvent de-
pendence. In particular, the reaction proceeded only in
MeCN (Table 1, entry 4); product 2a was not detected in
more polar solvents (MeOH, DMF) or less polar solvents
(DCE, THF). Possibly, the coordinating ability of MeCN is re-
sponsible for this effect, although one would expect that its
coordination with rhodium would inhibit the catalysis. We
assumed that MeCN facilitates the dissociation of chloride
ligands from [Cp*RhCl2]2 to form [Cp*Rh(MeCN)3]2+ or simi-
lar species, which can then coordinate with the oxime.
However, the addition of Ag2CO3 or CsOAc had no positive
effect on the reaction. On the other hand, the addition of
1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) markedly in-
creased the reaction rate (entry 5). A similar positive effect
of fluorinated alcohols has been observed previous-
ly.[22−23,25] Cobalt and iridium catalysts [Cp*MCl2]2 (M = Co,
Ir) (entries 6 and 7), as well as simple RhCl3·4H2O (entry 8),
were found to be inactive in this reaction. The reaction also
failed to proceed if the O-acetyl or O-pivaloyl oxime deriva-
tive was used instead of 1. Possibly, the deprotonation of
OH group of the oxime 1 is important because this im-
proves its coordination with rhodium.5 Overall, under the
optimized conditions with [Cp*RhCl2]2 catalyst (2.5 mol%)
in a 10:1 MeCN–HFIP solvent mixture, the oxime 1 reacted
with styrene to give the target product 2a in 82% yield.26
Notably, the 3-substituted regioisomer was formed exclu-
sively.
Table 1 Optimization of the Reaction Solvent and Catalyst
OH
catalyst (2.5 mol%)
N
N
+
Ph
K2CO3 (100 mol%)
20 °C, 72 h
Ph
2 equiv
solvent
2a
1
Entry
Catalyst
Solvent
Yielda (%) of 2a
1
2
3
4
5
6
7
8
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*CoCl2]2
[Cp*IrCl2]2
RhCl3·4H2O
MeOH
<2
<2
<2
25
82
<2
<2
<2
THF
DCE
MeCN
MeCN + HFIP (10%)
MeCN + HFIP (10%)
MeCN + HFIP (10%)
MeCN + HFIP (10%)
a NMR yields are given for all the entries except entry 5, for which the isolat-
ed yield is given.
ratios of 2:1 to 4:1. These were separated by column chro-
matography. The 4-substituted isomers were formed pre-
dominantly; this can be explained by the need to avoid ste-
ric repulsion between the alkyl group of the alkene and the
Cp* ligand during the alkene-insertion step of the catalytic
cycle (see Supporting Information for a possible mecha-
nism). A similar regioselectivity pattern has been observed
previously in other Rh(III)-catalyzed C−H functionalization
reactions.5,27–29
[Cp*RhCl2]2
OH
R
R
(2.5 mol%)
N
N
+
K2CO3 (100 mol%)
MeCN/HFIP (10:1)
20–100 °C, 1–3 d
R
Rn
1
2–5 equiv
2b–j
N
N
N
C8H17
(CH2)4OSiiPr3
2b, 75%, 2.4:1 rr
2e, 72%, 3.9:1 rr
2h, 99%
N
N
N
N
(CH2)8COOMe
Next, we investigated the reactions of 1 with aliphatic
alkenes (Scheme 2). These were found to be less reactive
than styrene and, consequently, two to five equivalents of
the alkene and heating to 70 °C were required to achieve
full conversion of the substrate 1 in a reasonable time (1–3
days). Under these conditions dec-1-ene, methyl undec-10-
enoate, hexa-1,5-diene, and i-Pr3Si-protected hex-5-en-1-
ol gave the corresponding dihydroisoquinolines 2b–e in
yields of 53–75%. In contrast to styrene, the products were
formed as mixtures of 3- and 4-substituted regioisomers in
2c, 68%, 3.9:1 rr
2f, 89%
2i, 78%
N
N
N
N
O
COOiPr
COOiPr
2d, 53%, 2.1:1 rr
2g, 50%
2j, 97%
Scheme 2 The expected catalytic reactions of oxime 1 with alkenes;
rr = ratio of 4- to 3-substituted regioisomers
© 2020. Thieme. All rights reserved. Synlett 2020, 31, 1117–1120

1119
E. A. Trifonova et al.
Letter
Synlett
Bulky terminal alkenes, such as 3,3-dimethylbut-1-ene
and triethoxy(vinyl)silane, as well as disubstituted alkenes,
such as cyclohexene and ethyl methacrylate, did not react
with 1, apparently as a result of steric hindrance. Strained
cyclic alkenes such as methylenecyclobutane, 2,5-dihydro-
furan, norbornene, norbornadiene, as well as the cyclopen-
tadiene–DIAD adduct, were more active and gave the corre-
sponding products 2f–j in yields of 50–99%. Heating at
100 °C for 18 hours was required for full conversion of 1 in
the reaction with methylenecyclobutane. On the other
hand, all three norbornene-type alkenes reacted readily at
room temperature. Interestingly, ring opening of the DIAD
adduct14,30,31 was not observed under these conditions.
Despite these positive results, many common function-
alized alkenes did not produce the expected dihydroiso-
quinolines 2 in good yields. In particular, allyl-substituted
alkenes, such as allyl chloride, allyl bromide, allylic alcohol,
N-tosylallylamine, or cyclohex-2-en-1-one, reacted slug-
gishly with 1, eventually giving mixtures of several prod-
ucts. Even alkenes with more distant functional groups,
such as homoallylic alcohol or pent-4-enoic acid, were
problematic, possibly because intramolecular coordination
of the double bond and hydroxy group to rhodium inhibit-
ed the catalyst. The reaction of 1 with butyl vinyl ether gave
aromatic 1-methylisoquinoline (3) in 78% yield, apparently
through elimination of BuOH from the expected butoxy-
substituted dihydroisoquinoline (Scheme 3). A similar pro-
cess has been observed previously for vinyl acetates.16,17
The reaction of 1 with ethyl acrylate in MeCN resulted in
Michael addition of the hydroxy group of the oxime to the
activated double bond, giving product 4 in 89% yield. Inter-
estingly, a similar reaction in DCE as solvent gave the disub-
stituted dihydroisoquinoline 5 in 65% yield (double substi-
tution occurred even when only one equivalent of ethyl ac-
rylate was used). Apparently, the initial reaction with the
first alkene molecule gave the expected dihydroisoquino-
line with a COOMe substituent, which then formed an eno-
late and underwent Michael addition to a second acrylate
molecule. Similar processes have been reported for 3-
COOR-substituted dihydroisoquinolines.32,33 Most unex-
pectedly, the reaction of 1 with allyl acetate gave the ben-
zoxazepine derivative 6 with a seven-membered ring. This
compound was formed in both MeCN and DCE solvents, but
the later provided a cleaner reaction and a higher yield of
81%. The mechanism for the formation of this product is
unclear, although it may involve an initial ortho-allylation
of the oxime 1.12,34–36
[Cp*RhCl2]2
(2.5 mol%)
OH
OH
OH
N
N
N
N
+
+
+
+
K2CO3 (100 mol%)
MeCN/HFIP (10:1)
70 °C, 3 d
OBu
1
1
2 equiv
3, 78%
[Cp*RhCl2]2
(2.5 mol%)
O
N
K2CO3 (100 mol%)
MeCN, 70 °C, 3 d
EtOOC
4, 89%
COOEt
2 equiv
[Cp*RhCl2]2
(2.5 mol%)
COOEt
N
K2CO3 (100 mol%)
DCE/HFIP (5:1)
60 °C, 1 d
COOEt
COOEt
1
5 equiv
5, 65%
[Cp*RhCl2]2
(2.5 mol%)
OH
N
O
N
CsOAc (100 mol%)
DCE/HFIP (5:1)
70 °C, 2 d
OAc
1
5 equiv
6, 81%
Scheme 3 Unexpected catalytic reactions of the oxime 1 with alkenes
[Cp*RhCl2]2 catalyst, and six days of heating at 50 °C were
required for full conversion of 1. The reaction with dec-1-
ene proceeded faster and with markedly improved regiose-
lectivity to give the 4-substituted dihydroisoquinoline 2b
exclusively. However, both 2a and 2b were formed as mix-
tures of enantiomers in an unsatisfactory ratio of 61:39 (co-
incidentally, the er values were almost identical). The low
stereoselectivity of these reactions may be explained by in-
sufficient steric crowding, in particular by the absence of a
tBu
tBu
tBu
Rh
l2
2
OH
(2.5 mol%)
N
N
+
Ag2CO3 (5 mol%)
K2CO3 (100 mol%)
MeCN/HFIP (10:1)
50 °C, 6 d
Ph
Ph
1
5 equiv
2a, 62%, 61:39 er
single regioisomer
same catalyst
as above
OH
(2.5 mol%)
N
N
+
We have recently synthesized the chiral rhodium cata-
Ag2CO3 (5 mol%)
K2CO3 (100 mol%)
MeCN/HFIP (10:1)
70 °C, 1 d
C8H17
t
t
lyst [(C5H2 Bu2CH2 Bu)RhI2]2, in which the bulky cyclopen-
tadienyl ligand is assembled from three 3,3-dimethylbut-1-
yne molecules (Scheme 4).37,38 Application of this catalyst
in the reactions of 1 with styrene and dec-1-ene gave the
expected products 2a (62%) and 2b (69%), respectively. As a
result of steric hindrance, the reaction with styrene pro-
ceeded markedly more slowly than that in the presence of
C8H17
5 equiv
1
2b, 69%, 61:39 er
single regioisomer
Scheme 4 Reactions of 1 with alkenes in the presence of the chiral
rhodium catalyst. Addition of Ag2CO3 was required for abstraction of io-
dide ligands from the rhodium atom.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, 1117–1120

1120
E. A. Trifonova et al.
Letter
Synlett
substituent on the oxygen atom of the oxime 1, which could
deviate from the plane of the molecule.
(16) Webb, N. J.; Raw, S. A.; Marsden, S. P. Tetrahedron 2018, 74, 5200.
(17) Chu, H.; Sun, S.; Yu, J.-T.; Cheng, J. Chem. Commun. 2015, 51, 13327.
(18) Chen, R.; Qi, J.; Mao, Z.; Cui, S. Org. Biomol. Chem. 2016, 14, 6201.
(19) Yang, X.; Liu, S.; Yu, S.; Kong, L.; Lan, Y.; Li, X. Org. Lett. 2018, 20,
2698.
(20) Vijayan, A.; Jumaila, C. U.; Radhakrishnan, K. V. Asian J. Org.
Chem. 2017, 6, 1561.
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(23) Romanov-Michailidis, F.; Sedillo, K. F.; Neely, J. M.; Rovis, T.
J. Am. Chem. Soc. 2015, 137, 8892.
To conclude, we have shown that aromatic oximes react
with alkenes in the presence of [Cp*RhCl2]2 catalyst to give
substituted dihydroquinolines. At the same time, our re-
sults, as well as previous reports in the literature, indicate
that the rhodium-catalyzed reactions of oximes with
alkenes can give a variety of other products, depending on
the structures of the substrates and the conditions. Notably,
HFIP seems to be an important cosolvent that facilitates
such reactions.
(24) Zhang, X.; Ouyang, X.-H.; Li, Y.; Chen, B.; Li, J.-H. Adv. Synth.
Catal. 2019, 361, 4955.
(25) Sanjosé-Orduna, J.; Sarria Toro, J. M.; Pérez-Temprano, M. H.
Angew. Chem. Int. Ed. 2018, 57, 11369.
(26) 1-Methyl-3-phenyl-3,4-dihydroisoquinoline (2a); Typical
Procedure
Funding Information
The main synthetic work was supported by the Russian Science Foun-
dation (Grant no. 17-73-20144). The characterization of the com-
pounds was carried out by using equipment at the Center for
Molecular Composition Studies of INEOS RAS, with financial support
Acetophenone oxime (1; 27.0 mg, 0.20 mmol), [Cp*RhCl2]2 (3.1
mg, 0.005 mmol), K2CO3 (28 mg, 0.20 mmol), and styrene (0.092
ml, 0.8 mmol) were mixed in dry MeCN (0.5 mL). HFIP (0.05 mL)
was added, and argon was bubbled through the solvent for 2
min. The vial was then sealed and the mixture was stirred for 3
d at 20 °C. The vial was then opened to air and the mixture was
evaporated in vacuum. The residue was purified by column
chromatography [silica gel; hexanes–EtOAc (10:1 to 1:1 gradi-
ent)] to give a colorless oil; yield: 36 mg (82%).
from the Ministry of Science and Higher Education.
R
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si
a
n
S
c
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n
c
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F
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n
d
ati
o
n
(17-73-2
0
1
4
4)
Acknowledgment
We thank Dr. Mikhail Seliverstov for his help with several of the syn-
thetic procedures.
1H NMR (400 MHz, CDCl3):  = 7.57 (d, J = 7.4 Hz, 1 H, CHAr), 7.46
(d, J = 7.4 Hz, 2 H, CHAr), 7.40–7.35 (m, 4 H, CHAr), 7.29 (t, J = 7.2
Hz, 1 H, CHAr), 7.20 (d, J = 7.2 Hz, 1 H, CHAr), 4.60–4.50 (m, 1 H,
CH), 2.96 (dd, J = 15.8, 5.5 Hz, 1 H, CH2), 2.90–2.81 (m, 1 H, CH2),
2.51 (d, J = 2.0 Hz, 3 H, CH3). 13C NMR (101 MHz, CDCl3):  =
164.61, 144.50, 136.98, 130.95, 129.54, 128.58, 127.57, 127.26,
127.18, 127.00, 125.54, 61.01, 34.77, 23.47. HRMS (ESI): m/z [M
+ H]+ calcd for C16H16N = 222.1277; found: 222.1279.
(27) Wodrich, M. D.; Ye, B.; Gonthier, J. F.; Corminboeuf, C.; Cramer,
N. Chem. Eur. J. 2014, 20, 15409.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1707961.
S
u
p
p
orti
n
gInformati
o
n
S
u
p
p
orit
n
gInformati
o
n
References and Notes
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© 2020. Thieme. All rights reserved. Synlett 2020, 31, 1117–1120