Rhodium Catalysts with a Chiral Cyclopentadienyl Ligand Derived from Natural R-Myrtenal

Article abstract of DOI:10.1002/ejoc.202001029

A new chiral cyclopentadiene Cp^myrH was synthesized from the natural terpene (R)-myrtenal in 5 steps and about 40 % total yield. The key step was the reaction of vinyl-dibromocyclopropane derivative with MeLi which provided the diene Cp^myr

Full text of DOI:10.1002/ejoc.202001029

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Rhodium catalysts with a chiral cyclopentadienyl ligand derived
from natural R-myrtenal
Authors: Roman A Pototskiy, Andrey V Kolos, Yulia V Nelyubina, and
Dmitry Perekalin
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001029
Link to VoR: https://doi.org/10.1002/ejoc.202001029
01/2020

10.1002/ejoc.202001029
European Journal of Organic Chemistry
FULL PAPER
Rhodium catalysts with a chiral cyclopentadienyl ligand derived
from natural R-myrtenal
Roman A. Pototskiy,[a] Andrey V. Kolos,[a] Yulia V. Nelyubina,[a] Dmitry S. Perekalin,*[a,b]
Dedicated to academician Aziz M. Muzafarov in recognition of his strategic work for development of science in Russia and on the
occasion his 70th anniversary.
[a]
[b]
R. A. Pototskiy, A.V. Kolos, Dr. Y.V. Nelyubina, Prof. Dr. D.S. Perekalin
Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences
Vavilova 28, Moscow, 119991, Russia
Web-page: http://dmitryperekalin.blogspot.com/ E-mail: dsp@ineos.ac.ru
Plekhanov Russian University of Economics
36 Stremyannyi pereulok, Moscow, 117997, Russia
Supporting information for this article is given via a link at the end of the document
Abstract: A new chiral cyclopentadiene CpmyrH was synthesized from
the natural terpene R-myrtenal in 5 steps and about 40% total yield.
The key step was the reaction of vinyl-dibromocyclopropane
derivative with MeLi which provided the diene CpmyrH via Skattebøl
rearrangement. The reaction of CpmyrH with [(cod)Rh(OAc)]2 gave the
rhodium(I) complex (Cpmyr)Rh(cod) and subsequent oxidation with
halogens X2 gave the rhodium(III) complexes [(Cpmyr)RhX2]2 (X = Br,
I). Complex (Cpmyr)Rh(cod) in combination with benzoyl peroxide
catalyzed the C-H activation of O-pivaloyl-phenylhydroxamic acid and
its subsequent annulation with alkenes giving dihydroisoquinolinones
with high yield (70−90%) but moderate enantioselectivity (16−36% ee).
Results and Discussion
We started our investigation with the synthesis of the camphor-
based ligand 4 using the procedures similar to those developed
by Halterman and Tretyakov (Scheme 2).[26] The camphor
t
hydrazone derivative 5 reacted with 2 equivalents of BuLi to
generate the vinyl lithium species[27] 6 and following addition of
benzylideneacetone produced the bis-allyl-alcohol 7 in 30-50%
yields. Note that the use of different ketones on this stage would
allow for straightforward variation of substituents in the final
cyclopentadiene product. The yield of the alcohol 7 was
sometimes diminished by a side process of deprotonation of the
methyl group of the starting benzylideneacetone by the strongly
basic lithium reagent 6, which was indicated by the formation of
2-bornene as a byproduct. Complete purification of the compound
7 was also problematic.
Introduction
Rhodium catalysts with chiral cyclopentadienyl ligands have
recently attracted a lot of attention thanks to their successful
application in C-H activation reactions.[1–6] The most explored
catalysts of this type are the binaphthyl derivatives 1 developed
by Cramer et al. and You et al.,[7–11] the amino acid derivatives 2
developed by Antonchik and Waldmann et al.,[12] the
streptavidine-docked derivatives 3 developed by Ward and Rovis
et al.,[13,14] as well as some others[15–20] (Scheme 1). A wide
application of these catalysts is still somewhat limited by their
multi-step synthesis involving rather expensive starting materials
and reagents. In attempt to solve this problem and to expand the
library of ligands we focused our attention on complexes with
cyclopentadienes derived from cheap natural chiral terpenes.
Some complexes of this type have been synthesized
previously,[21–23] but they have been rarely applied in
catalysis.[24,25]
Scheme 2. Attempted synthesis of the chiral cyclopentadiene 4 from R-
camphor.
The next crucial step was planned to be the Nazarov cyclization
of 7 into the cyclopentadiene 4. However, it was not successful:
most of the acidic reagents (CF3COOH, Et2O·BF3, ZnCl2)
Scheme 1. Examples of rhodium catalysts with chiral cyclopentadienyl ligands.
1
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10.1002/ejoc.202001029
European Journal of Organic Chemistry
FULL PAPER
converted 7 into a mixture of products, while the only clean
reaction in the presence of TsOH·H2O produced the triene 8 in
84% yield. Attempts to convert the triene 8 into the target
cyclopentadiene 4 by strong acids were also unsuccessful. It is
worth noting that Halterman and Tretyakov have also observed
problems with Nazarov cyclization of similar camphor-derived
alcohols.[26]
the reaction of 9 with thallium ethoxide followed by the addition of
[(cod)RhCl]2 albeit in a lower yield 32%.
We then turned to the synthesis of the alternative chiral
cyclopentadiene CpmyrH (9) using the Skattebøl rearrange-
ment[28,29] as a key step (Scheme 3). Cheap natural R-myrtenal
(<100$ for 100 g) was used as a starting material. Addition of
phenylmagnesium bromide to R-myrtenal followed by oxidation
gave the ketone 10 in a good total yield 79%. Further Wittig
olefination of 10 and cyclopropanation by CHBr3/KOH produced
the di-bromo-cyclopropane 12 as a mixture of two diastereomers
with ca. 4:1 ratio. They can be separated by crystallization from
petroleum ether, but this separation was not necessary for further
synthesis. Methyl lithium converted the dibromide 12 into the
carbene intermediate, which underwent the Skattebøl
rearrangement to give the desired cyclopentadiene 9. The minor
byproduct of this reaction was presumably the allene 13, similar
to that noted by Paquette et al.[30] The amount of 13 can be
reduced to ca. 10% by conducting reaction in diluted solution with
efficient cooling. We could not fully separate this byproduct from
9 but fortunately it caused no problems in further applications and
was separated during purification of the rhodium complex (vide
infra). Although the synthesis of 9 requires 5 steps, it can be
conveniently carried out on a multi-gram scale with a reasonable
total yield of 35−40%. Neat diene 9 slowly polymerizes at room
temperature but can be stored for months in hexane solution at
−18 °C. Similar instability has been previously observed for the
related diene without a phenyl substituent.[28]
Scheme 4. Synthesis of rhodium complexes with the chiral ligand Cpmyr
.
The yellow complex 14 was stable in the solid state but notably
decomposed in solution in air within a day (brownish color
appeared). It was very soluble in non-polar solvents, so it can be
separated from the excess of the starting diene 9 and 13 only by
the column chromatography in petroleum ether. To avoid these
complications 14 can be converted into the air-stable and less
soluble rhodium(III) complexes [(Cpmyr)RhX2]2 (15a,b; X = Br, I;
90-95%) by the oxidation with molecular bromine or iodine.
Noteworthy, the complexes 15a,b can be also obtained directly
from 9 without isolation of the intermediate 14. The structures of
the compounds 14 (Figure 1; CCDC 2017672) and 15a
(supporting information; CCDC 2017673) were established by the
X-ray diffraction, which confirmed the coordination of the metal
from the less hindered side of the ligand, i.e. opposite CMe2 group.
Figure 1. Crystal structure of the complex 14 represented in 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity. Selected distances (Å):
Rh1−C19 2.119(4), Rh1−C22 2.110(4), Rh1−C23 2.126(4), Rh1−C26 2.105(5),
Rh1−C5plane 1.911.
Scheme 3. Synthesis of the chiral cyclopentadiene 9 from R-myrtenal.
The reaction of the diene 9 with RhCl3 in refluxing ethanol failed
to give the expected complex [(Cpmyr)RhCl2]2, possibly because
of the low thermal stability of the diene in the presence of acid.
Fortunately, rhodium coordination was achieved by the reaction
of 9 with [(cod)Rh(OAc)]2 according to the method recently
developed by Cramer et al (Scheme 4).[31] This way the
cyclopentadienyl complex (Cpmyr)Rh(cod) (14) was obtained in
77% yield. Remarkably, the coordination selectively occurred to
the less hindered face of the ligand Cpmyr and only one
diastereoisomeric complex was observed in the 1H NMR
spectrum of the crude product. Complex 14 was also prepared by
The catalytic performance of the complex 14 was tested in the
benchmark
C-H
activation
reaction[4]
of
O-pivaloyl
phenylhydroxamic acid 16 with alkenes (Scheme 5). We
generated the active rhodium(III) species (Cpmyr)Rh(OBz)2 by in
situ oxidation of the rhodium(I) complex 14 with benzoyl peroxide
as suggested by Cramer et al.[17] Interestingly, the complex 14
itself can be also generated in situ from the ligand 9 and
[(cod)Rh(OAc)]2 prior to the addition of (BzO)2.[31] This approach
allows fast screening of the enantioselectivity of the reactions. In
the presence of the precatalyst 14 (5 mol-%) the tested reactions
of 16 with styrene, 1-decene, and norbornene gave the expected
2
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10.1002/ejoc.202001029
European Journal of Organic Chemistry
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dihydroisoquinolinone products 17a−c in high yields 80−90%.
The yields were somewhat lower (40−70%) if 14 was generated
in situ. Reaction of 16 with norbornene was also tested in the
presence of the bromide 15a as a catalyst and gave the target
product 17c in 83% yield. Noteworthy, the reaction of 16 with 1-
decene gave a mixture of 3- and 4-octyl-substituted products 17b
with 1:2 ratio. This observation indirectly confirmed rather low
steric hindrance of the new Cpmyr ligand, since the catalysts with
bulkier cyclopentadienyl ligands typically gave 4-octyl-
dihydroisoquinolinone as the only product.[32–34] Unfortunately, the
enantioselectivities of the tested reactions with the catalyst 14
were moderate (16-36% ee). Only the minor 3-octyl-substituted
isomer of 17b was obtained with high 94% ee. Enantioselectivites
were almost the same (±1% ee) when in situ generated 14 or 15a
were used as catalysts. Similar results were also obtained when
O-Boc-phenylhydroxamic acid was used as a starting material
instead of 16.
14 seems to be less asymmetric, because it has substituents both
on the right and on the left side from the key CH2 group. This is
reflected by steric map with 48% and 41% of volume occupied by
the ligand in top left and bottom right quadrants, respectively
(compare to 47% and 34% in the catalyst 18). Probably this leads
to the less efficient enantiomeric induction (36% ee). However,
the numerical differences between the volumes occupied by
ligands (Vburried) in the complexes 14 and 18 are small. Therefore,
it seems that the steric maps can correctly predict the type of
enantiomeric product (R or S) but have limited efficiency for
prediction of enantiomeric excess in such reactions.
In attempt to explain the observed enantioselectivity we
generated the steric maps of the three catalysts 14, 18 and 19
with different chiral cyclopentadienyl ligands using the SambVca
2.1 tool (Figure 2).[35] The maps show the amount of space
occupied by the ligands around the central rhodium atom. It can
be seen, that in the case of the catalysts 18 and 19 the main steric
influence is provided by the methyl groups of the cyclopentadienyl
ligands shown in red. In the case of 14 the steric influence is
provided mainly by CH2 group of the myrtenal framework.
Interestingly, the previously reported catalysts 18 and 19 gave the
product 17a with almost the same enantiomeric excess (92 and
90% ee) but with opposite configuration.[12,17] This correlates with
very similar steric maps of opposite symmetry. Our new catalyst
Scheme 5. Catalytic performance of the new rhodium complex 14.
Figure 2. Steric maps of three chiral cyclopentadienyl ligands generated by SambVca 2.1 tool. Vburried values show the amount of space occupied by ligand in the
coordination sphere with 4 Å radius around the central metal atom. The Rh−Cp distance was set identical for all ligands to 1.77 Å (typical distance for Rh(III)
complexes).
3
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10.1002/ejoc.202001029
European Journal of Organic Chemistry
FULL PAPER
152.2, 137.3, 135.7, 135.4, 128.7, 128.6, 128.5, 127.3, 127.1, 127.0, 126.4,
73.9, 51.0, 33.1, 32.8, 27.9, 27.8, 25.4, 25.4, 19.8, 19.8, 19.7, 13.4. The
13C NMR spectrum was complex due to the presence of two diastereomers.
Only the major signals are given here. MS(EI) m/z: 282.2 [M]+, 265.2
[M−OH]+.
Conclusion
New chiral cyclopentadiene CpmyrH was synthesized from the
natural R-myrtenal in 5 steps and ca. 35-40% yield. The
advantage of using terpenes as a chirality source is their high
availability, which allows for easy multi-gram synthesis. However,
the possibilities of structural variation of their chiral elements are
limited, and the strained framework can lead to side reactions
such as acid-catalyzed rearrangements. The face-selective
coordination of rhodium to Cpmyr ligand was conveniently
achieved by reaction of the diene with [(cod)Rh(OAc)]2 precursor.
The chiral rhodium complex (Cpmyr)Rh(cod) in the presence of
(BzO)2 catalyzed C-H activation of O-pivaloyl-phenylhydroxamic
acid and its subsequent annulation with alkenes to give
dihydroisoquinolinones with high yield (70−90%) but moderate
enantioselectivity (16−36% ee). Steric maps of the chiral
cyclopentadienyl ligands can predict the efficiency of such
rhodium catalysts to a limited extent.
1,7,7-trimethyl-2-(4-phenylbuta-1,3-dien-2-yl)bicyclo[2.2.1]hept-2-ene
(8): p-Toluenesulfonic acid (3 mg, 0.018 mmol, 10 mol%) was added to
solution of alcohol 7 (50 mg, 0.18 mmol) in diethyl ether (3 ml). The mixture
was stirred at room temperature overnight. The solvent was evaporated
and the residue was purified by column chromatography (eluted with
hexane) to give triene 8 as yellowish oil (40 mg, 84%). Rf = 0.42 (hexane).
1H NMR (400 MHz, CDCl3): δ = 7.39 (d, J = 7.8 Hz, 2H), 7.30 (t, J = 7.6
Hz, 2H), 7.20 (t, J = 7.3 Hz, 1H), 6.82 (d, J = 16.0 Hz, 1H), 6.67 (d, J =
16.0 Hz, 1H), 5.94 (d, J = 2.8 Hz, 1H), 5.20 (s, 1H), 4.93 (s, 1H), 2.39 (t, J
= 3.0 Hz, 1H), 1.91 (tt, J = 7.3, 4.2 Hz, 1H), 1.59 (m, 1H), 1.13–1.04 (m,
2H), 1.01 (s, 3H), 0.95 (s, 3H), 0.81 (s, 3H). 13C NMR (101 MHz, CDCl3):
δ = 147.1, 143.6, 137.6, 133.3, 131.3, 130.8, 128.7, 127.6, 126.6, 114.8,
56.9, 55.5, 52.0, 32.0, 31.4, 25.8, 20.0, 12.5. MS(EI) m/z: 265.2 [M+H]+.
Phenyl-(1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-yl)-methanone (10):
The first step was carried out similar to the literature procedure.[39]
Bromobenzene (15.7 g, 0.10 mol) was added dropwise to magnesium
(2.67 g, 0.11 mol) in anhydrous THF (80 mL), extensive heating of the
mixture was observed. After complete addition, the mixture was stirred at
room temperature for 30 min and R-myrtenal (15.0 g, 0.10 mol) in
anhydrous THF (20 ml) was added dropwise. The mixture was refluxed for
3 h, then cooled to 0 °C and quenched with 50 mL of a saturated aqueous
solution of NH4Cl. THF was removed by evaporation under reduced
pressure and the remaining emulsion was extracted with ethyl ether. The
organic layer was washed with a saturated solution of ammonium chloride,
dried over anhydrous Na2SO4. The solvent was evaporated to give the α-
(2-borenyl)-benzyl alcohol as a yellow oil (20.7 g, 92%), which was used
without further purification in the next step.
Experimental Section
General information: Unless otherwise noted all reactions were carried
out under argon atmosphere in anhydrous solvents, which were purified
and dried using standard procedures. The isolation of products was carried
out in air. Complexes [Rh(cod)Cl]2
hydrazone 5,[27] O-pivaloyl phenylhydroxamic acid (16),[38] and O-Boc
phenylhydroxamic acid[17] were synthesized according to the published
procedures. All other reagents were obtained from commercial sources
(Acros, Aldrich, J&K Scientific, or Vekton) and used as received. High
[36]
and [Rh(cod)OAc]2,[37] as well as
resolution mass spectra were recorded on
a Bruker microTOF
spectrometer with electrospray ionization (ESI). H and 13С NMR spectra
were measured with a Bruker Avance 400 or AV-600 spectrometer at
20 °C. The chemical shifts are reported relative to residual signals of the
solvent (for CHCl3: 7.26 1H, 77.16 13C; for DMSO: 2.50 1H, 39.52 13C). The
copies of NMR spectra of the new compounds are given in the supporting
information file.
1
A mixture of α-(2-borenyl)-benzyl alcohol (7.22 g, 31.6 mmol) from the
previous step and pyridinium chlorochromate on alumina (63.2 g, 63.2
mmol, activity ~ 1 mmol/g) in dichloromethane (50 ml) was stirred for 3
hours at room temperature. The reaction mixture was filtered and the
solids were washed with diethyl ether. The solvent was evaporated, the
residue was dissolved in diethyl ether (10 ml), filtered through a silica gel
plug and evaporated again in vacuum. The product 10 was obtained as
yellow oil (6.18 g, 86%) and directly used without additional purification in
the next step. Analytically pure sample can be obtained by vacuum
distillation (150−160 °C at 2 mbar). 1H NMR (400 MHz, CDCl3): δ = 7.69
(d, J = 7.4 Hz, 2H), 7.51 (t, J = 6.9 Hz, 1H), 7.42 (t, J = 7.3 Hz, 2H), 6.42
(s, 1H), 2.99 (t, J = 5.6 Hz, 1H), 2.59–2.46 (m, 3H), 2.18 (s, 1H), 1.38 (s,
3H), 1.21 (d, J = 9.2 Hz, 1H), 0.87 (s, 3H). 13C NMR (101 MHz, CDCl3): δ
= 195.7, 148.7, 140.4, 138.4, 131.6, 129.3, 128.2, 41.5, 40.4, 37.8, 32.8,
31.4, 26.0, 21.1. HRMS (ESI) calculated for C16H18O+ [M+H]+: 227.1435,
found: 227.1430. Elemental analysis calculated for C16H18O: C, 84.91; H,
8.02; found: C, 84.60; H, 8.19.
4-phenyl-2-(1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-yl)-but-3-en-2-ol
(7): The stirred solution of hydrazone 5 (600 mg, 1.39 mmol) in anhydrous
THF (6 mL) was cooled to –78 °C and t-BuLi (2 ml, 1.42 M in pentane, 2.92
mmol, 2.1 equiv) was added dropwise (the temperature should not exceed
–50 °C). Then the cold bath was removed and the stirred orange solution
was allowed to warm to 0 °C. Nitrogen began to evolve at –20 °C, the color
of the mixture changed to dark red. The flask was kept immersed into an
ice bath (0 °C) until nitrogen evolution ceases (ca. 40 min). Then the
reaction mixture was again cooled to –78 °C and the solution of
benzylideneacetone (304 mg, 2.08 mmol) in anhydrous THF (3 ml) was
added. After 20 minutes the solution was warmed to room temperature and
then stirred overnight. Saturated aqueous solution of NH4Cl (9 ml) was
added to reaction mixture and it was opened to air. The mixture was
transferred to a separation funnel, the organic layer was collected, the
aqueous layer was extracted with Et2O (2×10 ml). The combined organic
layers were washed with brine and dried over Na2SO4. The solvent was
removed in vacuum and the residue was purified by column
chromatography (eluted first with hexane, then hexane/AcOEt (2:1). The
product 7 was obtained as a yellow oil (204 mg, 52%). It should be noted
that the yield of 7 varies significantly apparently because the side process
of deprotonation of benzylideneacetone by vinyl lithium reagent 6
becomes dominant if the temperature increases during the reaction. Rf =
0.41 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ = 7.40 (d, J = 7.7
Hz, 2H), 7.33 (t, J = 7.5 Hz, 2H), 7.24 (t, J = 7.1 Hz, 1H), 6.67 (m from two
overlapping doublets of diastereomers, 1H), 6.33 (m, from two overlapping
doublets of diastereomers, 1H), 5.96–5.86 (m, 1H), 2.34–2.27 (m, 1H),
1.98–1.82 (m, 3H), 1.58–1.54 (m, 3H), 1.22–1.18 (m, 3H), 1.08–0.98 (m,
2H), 0.88–0.84 (m, 3H), 0.77 (s, 3H). 13C NMR (101 MHz, CDCl3): δ =
6,6-dimethyl-2-(1-phenylvinyl)bicyclo[3.1.1]hept-2-ene (11): THF (60
ml) was added to the solid methyltriphenylphosphonium bromide (12.2 g,
34.1 mmol) and potassium tert-butoxide (3.83 g, 34.1 mmol) and the
mixture was stirred for 30 minutes to give a bright yellow suspension. Then
the ketone 10 (7.02 g, 31.0 mmol) was added dropwise; strong heating of
the mixture was observed. The reaction mixture was stirred for 24 hours
and opened to air. The solvent was then evaporated in vacuum and the
residue was triturated with petroleum ether (15 ml). The precipitate was
removed by filtering the petroleum ether solution through silica gel plug
and the solvent was evaporated in vacuum. The diene 11 was obtained as
yellow oil (6.2 g, 89%). This crude product can be purified further by
vacuum distillation (bp 135-140 °С at 2 mbar) to give 11 as colourless oil
(4.0 g, 58%). 1H NMR (400 MHz, CDCl3): δ = 7.47–7.15 (m, 5H, Ph), 5.46
(s, 1H), 5.15 (s, 1H), 5.01 (s, 1H), 2.52–2.43 (m, 2H), 2.33 (qt, J = 18.6,
3.2 Hz, 2H), 2.16–2.10 (m, 1H), 1.32 (s, 3H), 1.27 (d, J = 8.2 Hz, 1H), 0.91
4
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10.1002/ejoc.202001029
European Journal of Organic Chemistry
FULL PAPER
(s, 3H). 13C NMR (101 MHz, CDCl3): δ = 150.3, 147.9, 128.6, 127.9, 127.1,
123.6, 110.7, 43.8, 40.6, 37.9, 31.9, 31.6, 26.4, 20.9. MS(EI) m/z: 225.2
[M+H]+. HRMS-ESI analysis was not successful because of reluctance of
ionization of the diene 11. Elemental analysis calculated for C17H20: C,
91.01; H, 8.99; found: C, 90.81; H, 8.85.
placed on a top of a silica gel column (20×1 cm) and eluted with petroleum
ether under argon pressure (Rf = 0.9 in hexane). Yellow band was
collected and evaporated to give 114 mg (77%) of complex 14 as а yellow
solid. Similar reaction with 1 equiv. of CpmyrH gave complex 14 in 40%
yield apparently because of the side polymerization of the starting diene.
1H NMR (400 MHz, CDCl3): δ = 7.28 (d, 4H, J = 4.3 Hz, CHPh), 7.16 (m,
1H, CHPh), 5.17 (d, 1H, J = 2.6 Hz, CHCp), 4.84 (d, 1H, J = 2.7 Hz, CHCp),
3.61–3.54 (m, 2H, CHcod), 3.45 (q, 2H, J = 6.1, 4.8 Hz, CHcod), 2.85 (t, 1H,
J = 5.5 Hz), 2.75 (dt, 1H, J = 9.1, 6.1 Hz), 2.71–2.53 (m, 2H), 2.19 (m, 5H,
CH2cod + CHmyr), 1.89 (m, 5H, CH2cod + CHmyr), 1.34 (s, 3H, CH3), 0.63 (s,
3H, CH3). 13C NMR (151 MHz, CDCl3): δ = 128.84 (CHPh), 128.25 (CHPh),
127.88 (CHPh), 125.84 (CHPh), 116.39 (d, JRh-C = 3.4 Hz CHcod), 102.61 (d,
JRh-C = 3.4 Hz CHcod), 102.11 (d, JRh-C = 3.7 Hz CHcod), 81.84 (d, JRh-C = 4.0
Hz CHCp), 80.22 (d, JRh-C = 4.8 Hz CHCp), 68.25, 68.15, 66.67, 66.58, 42.19,
41.84, 40.35, 35.58, 32.69, 32.40, 27.06, 26.90 (CH3), 21.70 (CH3). HRMS
(ESI) calculated for C26H31Rh [M]+: 446.1481, found: 446.1475.
2-(2,2-dibromo-1-phenylcyclopropyl)-6,6-dimethylbicyclo-[3.1.1]-
hept-2-ene (12): A mixture of the diene 11 (4.00 g, 17.9 mmol), powdered
KOH (4.50 g, 80.4 mmol, 4.5 equiv.), phase-transfer catalyst Adogen 464
(2.40 g, 1.80 mmol, 10 mol%) and ethanol (520 µl, 9 mmol, 0.5 equiv.) was
dissolved in dichloroethane (80 ml). To the stirred mixture a bromoform
(13.6 g, 53.6 mmol, 3 equiv.) was added dropwise over 30 minutes at room
temperature. Immediately afterwards, the addition of KOH (4.50 g, 80.4
mol, 4.5 equiv.) and CHBr3 (13.6 g, 53.6 mol, 3 equiv.) was repeated, again
over 30 min. The reaction mixture was then stirred for 30 minutes, filtered
and the solid residue was rinsed with CH2Cl2 (ca. 30 ml) until its color was
pale-brown (no further color change). The combined organic fractions
were then extracted with H2O (3×80 ml). The combined aqueous layers
were extracted with CH2Cl2 (2×30 ml) and the combined organic fractions
were then dried over MgSO4. The solution was filtered through silica gel
plug and the solvent was evaporated in vacuum. The vinyldibromo-
cyclopropane product 12 was obtained as a light brown semi-solid (7.1 g,
80 %). The diastereoisomers of 12 can be separated by crystallization. A
minimal amount of petroleum ether was added to dissolve the mixture of
diastereomers. The resulting solution was kept at −30 °С until the major
diastereomer precipitated as colorless needle crystals. The spectral data
below is given for a pure diastereomer. The spectra of the initial mixture
are given in the supporting information. 1H NMR (400 MHz, CDCl3): δ =
7.48–7.15 (m, 5H, Ph), 5.60 (s, 1H), 2.65 (t, J = 5.3 Hz, 1H), 2.51 (dt, J =
8.8, 5.8 Hz, 1H), 2.40–2.23 (m, 2H), 2.21–2.08 (m, 2H), 2.02 (s, 1H), 1.39
(d, J = 8.8 Hz, 1H), 1.21 (s, 3H), 0.09 (s, 3H). 13C NMR (101 MHz, CDCl3):
δ = 148.86, 139.67, 129.81, 128.02, 127.32, 121.41, 47.09, 45.22, 40.30,
37.69, 34.34, 34.29, 32.20, 31.62, 25.97, 19.95. HRMS (ESI) calculated
for C20H27Br+ [M−Br]+: 315.0743, found: 315.0741. Elemental analysis
calculated for C18H20Br2: C, 54.57; H, 5.09; found: C, 54.37; H, 5.02.
Alternative synthesis of (Cpmyr)Rh(cyclooctadiene) (14): A pale yellow
solution of CpmyrH (8, 53.0 mg, 0.224 mmol, calculated according to 3:1
ratio of 8 and 13) in hexane (2.5 ml) was added to a solution of TlOEt (56
mg, 0.224 mmol) in hexane (2.5 ml) in a Shlenk flask protected from light
by aluminum foil. The brown precipitate was formed immediately. The
mixture was stirred for 1 h and then [Rh(cod)OAc]2 (60.0 mg, 0.112 mmol)
and benzene (0.5 ml) were added. After stirring for 24 hours, the mixture
was filtered through a short pad of celite topped with a layer of neutral
aluminum oxide under argon atmosphere. The solvent was evaporated
and the residue was eluted by petroleum ether on a silica gel column as
described above to give 14 (32 mg, 32% yield) as yellow oily solid.
[(Cpmyr)RhBr2]2 (15a): In air a solution of Br2 (50 mg, 0.315 mmol) in
hexane (4 ml) was added dropwise to a stirred solution of the complex 14
(50 mg, 0.105 mmol) in hexane (4 ml). The orange precipitate was formed
immediately. After 5 minutes of stirring the precipitate was collected by
centrifugation, washed with hexane (4×5 ml) and Et2O (2×5 ml), and dried
in vacuum to give product 15a (50 mg, 90%) as an orange powder. 1H
NMR (400 MHz, DMSO-d6): δ = 7.78–7.72 (m, 2H, CHPh), 7.47–7.39 (m,
3H, CHPh), 6.33 (d, 1H, J = 2.5 Hz, CHCp), 5.66 (d, 1H, J = 2.5 Hz, CHCp),
2.91 (dd, 1H, J = 18.2, 2.9 Hz), 2.85 (t, 1H, J = 5.4 Hz), 2.69 (dd, 1H, J =
10.7, 5.7 Hz), 2.38 (d, 1H, J = 18.3 Hz), 2.15 (d, 2H, J = 7.9 Hz), 1.40 (s,
3H, CH3), 0.76 (s, 3H, CH3). 13C NMR (101 MHz, DMSO-d6): δ = 129.42
5,5-dimethyl-3-phenyl-4,5,6,7-tetrahydro-2H-4,6-methanoindene (9):
The vinyldibromocyclopropane 12 (3.0 g, 7.57 mmol, 1.0 equiv.) was
dissolved in Et2O (60 ml). The mixture was stirred, cooled to –78 °C and
MeLi (10 ml, 3.1 M in (EtO)2CH2, 30.3 mmol, 4 equiv.) was added dropwise.
After stirring for 1.5 hours, saturated aqueous solution of NH4Cl (20 ml)
was added at –78 °C. The mixture was then opened to air, allowed to warm
to room temperature, transferred to a separation funnel and the organic
layer was separated. The aqueous layer was diluted with H2O (30 ml) and
extracted with Et2O (3×20 ml). The combined organic layers were washed
with brine and dried over Na2SO4. The solvent was removed under
reduced pressure to give the mixture of the target cyclopentadiene 9 and
the presumed isomeric vinylallene 13 (1.64 g, 91% total yield). We could
not purify this mixture further by column chromatography because of the
very low polarity of the compounds, similar Rf values and instability of the
substances towards polymerization at room temperature. The ratio of 9:13
isomers vary from 3:1 to 8:1 according to 1H NMR and GC-MS. The
position of double bonds in the cyclopentadiene 9 was assigned to match
best with the predicted spectra. 1H NMR (400 MHz, CDCl3): δ = 7.45–7.30
(m, 5H, Ph), 6.05 (s, 1H, CHCp), 3.43 (q, J = 20 Hz, 2H, CH2Cp) 3.22 (t, J =
5.3 Hz, 1H), 2.79 (q, J = 16 Hz, 2H, CH2myrtenal) 2.63 (m, 1H), 2.31 (m, 1H),
2.16 (m, 1H), 1.41 (s, 3H), 0.85 (s, 3H). The signals assigned to 13: 1H
NMR (400 MHz, CDCl3): δ = 5.45 (s, 1H), 5.12 (s, 2H), 1.36 (s, 3H), 0.96
(s, 3H). 13C NMR spectrum of the mixture was too complicated for proper
(CHPh), 129.24 (CHPh), 129.12 (CHPh), 128.75 (CHPh), 117.75 (d, JRh-C
=
5.8 Hz), 109.93, 91.28 (d, JRh-C = 7.8 Hz CHCp), 73.08 (d, JRh-C = 8.5 Hz
CHCp), 38.83, 34.08, 25.88, 25.07 (CH3), 21.35 (CH3). HRMS (ESI)
calculated for C18H19BrRh [(Cpmyr)RhBr]+: 416.9725, found: 416.9725;
calculated for C20H22BrNRh [(Cpmyr)RhBr+MeCN]+: 457.9991, found:
457.9906.
[(Cpmyr)RhI2]2 (15b): In air a solution of I2 (27 mg, 0.105 mmol) in hexane
(4 ml) was added dropwise to a stirred solution of the complex 14 (50 mg,
0.105 mmol) in hexane (4 ml). The dark precipitate was formed
immediately. After 5 minutes of stirring the precipitate was collected by
centrifugation, washed with hexane (4×5 ml) and dried in vacuum to give
product 15b (59 mg, 95%) as a dark violet powder. 1H NMR (400 MHz,
DMSO-d6): δ = 7.78–7.70 (m, 2H, CHPh), 7.42 (q, 3H, J = 4.6, 3.8 Hz,
CHPh), 6.36 (d, 1H, J = 2.6 Hz, CHCp), 5.77 (d, 1H, J = 2.5 Hz, CHCp), 3.24
(dd, 1H, J = 18.2, 3.1 Hz), 2.92 (t, 1H, J = 5.4 Hz), 2.67–2.61 (m, 1H),
2.60–2.53 (m, 1H), 2.16–2.11 (m, 1H), 2.02 (d, 1H, J = 10.3 Hz), 1.42 (s,
3H, CH3), 0.77 (s, 3H, CH3). 13C NMR (101 MHz, DMSO-d6): δ = 129.47
(CHPh), 129.40 (CHPh), 129.16 (CHPh), 128.67(CHPh), 118.01 (d, JRh-C = 5.0
+
analysis. HRMS (ESI) calculated for C20H27 [M+H]+: 237.1638, found:
Hz), 109.87 (d, JRh-C = 5.8 Hz), 97.71 (d, JRh-C = 5.9 Hz), 90.99 (d, JRh-C
=
7.1 Hz CHCp), 75.93 (d, JRh-C = 7.4 Hz CHCp), 40.02, 35.79, 25.88, 25.77
(CH3), 21.31 (CH3). Elemental analysis calculated for C18H19I2Rh: C,
36.52; H, 3.23; I, 42.87; Rh, 17.38; found: C, 36.33; H, 3.30; I, 42.40; Rh,
17.18.
237.1644.
(Cpmyr)Rh(cyclooctadiene) (14): Complex [Rh(cod)OAc]2 (83.0 mg,
0.155 mmol) and crude CpmyrH (8, 148.0 mg, 0.62 mmol, 2 equiv. per each
rhodium atom, calculated according to 3:1 ratio of 8 and 13) were dissolved
in a mixture of MeOH (2.5 ml) and toluene (2.5 ml). The resulting orange
solution was stirred for 48 hours, opened to air. A small amount of silica
gel was added and the solvent was removed in vacuum. The residue was
General procedure for catalytic transformations of O-pivaloyl-
phenylhydroxamic acid (16): Without protection from oxygen or moisture
the complex 14 (2.4 mg, 5.0 μmol) and benzoyl peroxide (1.2 mg, 5.0 μmol)
5
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10.1002/ejoc.202001029
European Journal of Organic Chemistry
FULL PAPER
were dissolved in mixture of MeOH (0.45 ml) and CH2Cl2 (0.050 ml,
improves solubility of alkenes) and stirred for 10 min. Then, O-pivaloyl
phenylhydroxamic acid (16, 44.2 mg, 0.2 mmol) was added to the mixture
and stirred for 15 minutes, followed by addition of alkene (0.40 mmol, 2
equiv.). The mixture was then stirred for 24 hours at 25 °C, the solvent was
evaporated in vacuum and the residue was purified on a silica gel column
by using CH2Cl2:EtOAc (5:1) as eluent.
Reaction phenylhydroxamic acid (16) with norbornene catalyzed by
the bromide complex 15a: Without protection from oxygen or moisture
the complex 15a (5.3 mg, 5.0 μmol) and CsOAc (9.6 mg, 50.0 μmol) were
dissolved in mixture of MeOH (0.45 ml) and CH2Cl2 (0.050 ml) and stirred
for 15 min. Then, O-pivaloyl phenylhydroxamic acid (16, 44 mg, 0.2 mmol)
and norbornene (38 mg, 0.40 mmol, 2 equiv.) were added to the mixture
and it was stirred for 2.5 hours at 25 °C. Then the solvent was evaporated
in vacuum and the residue was purified on a silica gel column as described
above. The product 17c was obtained as pale yellow solid (35 mg, 83%).
3-phenyl-3,4-dihydroisoquinolin-1-one (17a) prepared from 16 and
styrene: Yield 38 mg (85%). 1H NMR (400 MHz, CDCl3): δ = 8.11 (d, J =
7.7 Hz, 1H, CHAr), 7.46 (t, J = 7.5 Hz, 1H, CHAr), 7.42–7.32 (m, 6H), 7.18
(d, J = 7.5 Hz, 1H), 6.31 (s, 1H, NH), 4.85 (dd, J = 10.6, 5.1 Hz, 1H, CH),
3.25−3.07 (m, 2H, CH2). 1H NMR data are in agreement with those
reported previously.[38] Chiral HPLC: Chiralpak IA-3 column (4.6 × 150
mm), heptane/i-PrOH 90:10, 1.0 mL/min; tr (major) = 9.3 min, tr (minor) =
Acknowledgements
The main synthetic work was supported jointly by the Russian
Foundation for Basic Research and the Department of Science
and Technology of India (grant # 19-53-45035). X-ray diffraction
data were collected with the financial support from the Ministry of
Science and Higher Education of the Russian Federation using
the equipment of Center for molecular composition studies of
INEOS RAS. We thank Dr. Denis Chusov and Alexey Tsygankov
for help with analytical chromatography.
20
10.3 min, er 68/32. [α]D = +62.9 (c 1.0, CHCl3), which indicated
20
predominance of R-enantiomer. Literature data for pure R-isomer [α]D
=
+196.3,[17] +195.3,[40] for S-isomer [α]D20 = −159.4,[12] −161.0.[41] There is a
notable discrepancy between the reported rotation angles, however the
sign of the rotation angle for our sample clearly indicate the predominance
of R-enantiomer.
3-octyl-3,4-dihydroisoquinolin-1-one
and
3-octyl-3,4-
dihydroisoquinolin-1-one (17b) prepared from 16 and 1-decene: Total
1
yield of both isomers 38 mg (74%), ratio 1:2 according to H NMR. The
Keywords: rhodium • asymmetric catalysis • cyclopentadienyl •
isomers were separated by column chromatography and the analytical
data were obtained for pure samples. For 3-octyl-isomer: 1H NMR (400
MHz, CDCl3): δ = 8.03 (d, 1H, J = 7.7 Hz), 7.49–7.41 (m, 1H), 7.34 (t, 1H,
J = 7.6 Hz), 7.19 (d, 1H, J = 7.5 Hz), 3.68 (dd, 1H, J = 10.3, 4.0 Hz, CHN),
2.97 (dd, 1H, J = 15.5, 4.4 Hz), 2.80 (dd, 1H, J=15.5, 10.2 Hz), 1.31–1.19
(m, 13H), 0.88 (t, 3H, J = 6.9 Hz). Chiral HPLC: Chiralpak IA-3 column (4.6
× 150 mm), heptane/i-PrOH 99:1, 1.0 mL/min; tr(minor) = 3.4 min, tr
(major) = 3.8 min, er 97/3. For 4-octyl-isomer: 1H NMR (400 MHz, CDCl3):
δ = 8.05 (d, 1H, J = 7.6 Hz), 7.45 (t, 1H, J = 7.5 Hz), 7.34 (t, 1H, J = 7.5
Hz), 7.20 (d, 1H, J = 7.5 Hz), 6.72 (s, 1H, NH), 3.70 (dd, 1H, J = 12.4, 4.4
Hz, CH2NH), 3.39 (dt, 1H, J = 12.5, 4.0 Hz, CH2NH), 2.82 (tt, 1H, J = 7.6,
3.9 Hz, CH), 1.66 (q, 2H, J = 6.7 Hz), 1.25 (d, 12H, J = 9.0 Hz), 0.86 (t, 3H,
J = 6.7 Hz). Chiral HPLC: Chiralpak IA-3 column (4.6 × 150 mm),
heptane/i-PrOH 99:1, 1.0 mL/min; tr(major) = 4.2 min, tr (minor) = 4.6 min,
er 58/42. 1H NMR data are in agreement with those reported previously.[33]
C-H activation
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1H, J = 7.6 Hz), 7.23 (d, 1H, J = 7.7 Hz), 6.48–6.32 (m, 1H, NH), 3.82 (d,
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(3.2 mg, 6.0 μmol) and ligand CpmyrH (8, 2.4 mg, 10.0 μmol) were stirred
for 1 hour in a mixture of MeOH (0.45 ml) and CH2Cl2 (0.050 ml). Then
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mg, 0.20 mmol) was added, the mixture was stirred for 30 minutes,
followed by addition of alkene (0.40 mmol, 2.0 equiv.) and stirring for 24
hours at 25 °C. The volatiles were removed in vacuum and the residue
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7
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Entry for the Table of Contents
Rhodium complex with a new chiral cyclopentadiene was synthesized from the natural R-myrtenal and tested as a catalyst in C-H
activation reaction providing dihydroisoquinolinones with high yields and moderate enantioselectivity.
8
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