Chromium-catalyzed cyclopropanation of alkenes with bromoform in the presence of 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine

Article abstract of DOI:10.1039/d0sc00964d

Chromium-catalyzed cyclopropanation of alkenes with bromoform was achieved to produce the corresponding bromocyclopropanes. In this catalytic cyclopropanation, an organosilicon reductant, 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (1a

Full text of DOI:10.1039/d0sc00964d

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Chromium-catalyzed cyclopropanation of alkenes
with bromoform in the presence of 2,3,5,6-
tetramethyl-1,4-bis(trimethylsilyl)-1,4-
dihydropyrazine†
Cite this: DOI: 10.1039/d0sc00964d
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
*
and Kazushi Mashima
*
Hideaki Ikeda,
Kohei Nishi,
Hayato Tsurugi
Chromium-catalyzed cyclopropanation of alkenes with bromoform was achieved to produce the
corresponding bromocyclopropanes. In this catalytic cyclopropanation, an organosilicon reductant,
2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (1a), was indispensable for reducing
CrCl3(thf)3 to CrCl2(thf)3, as well as for in situ generation of (bromomethylidene)chromium(III) species
from (dibromomethyl)chromium(III) species. The (bromomethylidene)chromium(III) species are proposed
to react spontaneously with alkenes to give the corresponding bromocyclopropanes. This catalytic
cyclopropanation was applied to various olefinic substrates, such as allyl ethers, allyl esters, terminal
alkenes, and cyclic alkenes.
Received 18th February 2020
Accepted 4th March 2020
DOI: 10.1039/d0sc00964d
rsc.li/chemical-science
reactive CH2Cl2 and CH2Br2 in the presence of excess zinc
powder (Fig. 1a).8f–8i Takai et al. reported that chromium-
Introduction
catalyzed cyclopropanation of alkenes with Me3SiCHI2
proceeds in the presence of catalytic amounts of chromium
complex and excess Mn powder as a reducing reagent, from
which gem-dichromiomethane complexes (Cr2–SiMe3) are iso-
lated (Fig. 1b),9a and, similarly, Anwander et al. isolated an
iodomethyl-bridged dichromium complex by treating CrCl2
Cyclopropane is a strained three-membered carbocycle, and
a common structural motif in pharmaceutical and biologically
active compounds.1 The synthesis of cyclopropanes from easily
available starting materials is in high demand, and several
stoichiometric synthetic protocols for the C3 ring have been
developed: (1) classical reductive cyclization of 1,3-dihalopro-
panes or b-haloalkenes using metal-based reductants such as
lithium and magnesium,2 (2) cyclopropanation of alkenes using
haloform (CHX3) and a strong base in phase-transfer conditions
to afford geminal dihalocyclopropanes,3 and (3) cyclo-
propanation of alkenes using nitrogen-, phosphonium-, and
sulfur-ylides,4 in situ-generated zinc carbenoid from Zn reagents
and CH2I2 (Simmons–Smith reaction),5 and in situ-generated
chromium carbene species from excess amounts of CrCl2,
diamine ligands, and RCHI2.6 In contrast to these stoichio-
metric reactions, metal-catalyzed cyclopropanation of alkenes
using diazomethane and its derivatives is an alternative effec-
tive protocol, despite the use of explosive diazomethane deriv-
atives.7 To avoid the use of explosive compounds, the
development of metal-catalyzed cyclopropanation reactions
using non-explosive reagents was recently explored.8 Uyeda et al.
reported that some nickel and cobalt complexes serve as cata-
lysts for Simmons–Smith type reactions of alkenes with less
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, Japan. E-mail: mashima@chem.es.osaka-u.ac.jp;
tsurugi@chem.es.osaka-u.ac.jp
Fig.
1 Metal-assisted cyclopropanation of alkenes with di- and
trihalomethanes; (a) cyclopropanation with excess zinc powder, (b)
cyclopropanation with excess or catalytic amounts of chromium, and
(c) bromocyclopropanation with catalytic amounts of chromium and
organosilicon reductant 1a (This Work).
† Electronic supplementary information (ESI) available. CCDC 1954370. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc00964d
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with CHI3 as a key intermediate species for cyclopropanation to
give iodocyclopropanes.9b
Results and discussion
We have focused our attention on the versatility of a family of
organosilicon-based reductants, 1,4-bis(trimethylsilyl)-1,4-
dihydropyrazine derivatives and 1,10-bis(trimethylsilyl)-4,40-
bipyridinylidene, as stoichiometric reagents for reducing not
only transition metal complexes10 for reductive C–C bond
formation without generating any metal-based waste,10b,10d but
also main group halides for producing multiple bonds between
main group elements11 and some oxo compounds, such as
nitrobenzenes and sulfoxides, in a metal-free fashion to give
respectively anilines and thioethers.12c,12d Herein, we report that
chromium(III) trichloride with N,N,N0,N0-tetramethylethylenedi-
amine (TMEDA) served as a catalyst for the cyclopropanation of
alkenes with bromoform in combination with 2,3,5,6-
tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (1a) to
obtain synthetically useful bromocyclopropanes in high yield
(Fig. 1c).13
We then screened conditions by tuning reductants, additives,
and supporting ligands to optimize the chromium-catalyzed
cyclopropanation of allyl benzyl ether (2a) with bromoform as
a model reaction, and the results are summarized in Table 1.
When we used a 1 : 1 mixture of CrCl3(thf)3 (5 mol%) and
TMEDA (5 mol%) in the presence of 1a (2 equiv.) in 1,2-dime-
thoxyethane (DME) at 50 ꢀC for 24 h, bromocyclopropane 3a was
obtained in 98% yield with high trans (89%) selectivity (entry 1).
Cyclopropanation at 25 ꢀC resulted in a slightly lower yield
(81%) of 3a with almost the same trans selectivity (entry 1 vs. 2).
No cyclopropanation product was obtained when organosilicon
compounds 1b–d were used as the reducing reagents (entries 3–
5), although 1b–d reduced CrCl3(thf)3 to CrCl2, probably due to
coordination of the reduction byproducts, 2,5-dimethylpyrazine
(from 1b), pyrazine (from 1c), and 4,40-bipyridyl (from 1d), to the
chromium center, as conrmed by the inhibition of the catalytic
reaction when pyrazine was added under the standard
Table 1 Optimization study of reaction conditionsa
Entry
Variation from standard condition
Yield (%)b
trans : cisb
1
2
3
4
5
6
7
8
None
98 (93)c
81
0
0
0
0
0
0
0
56
7
97
7
0
8
0
0
90
89 : 11
90 : 10
N/A
N/A
N/A
N/A
N/A
N/A
N/A
87 : 13
71 : 29
90 : 10
57 : 43
N/A
>99 : 1
N/A
N/A
25 ꢀC, 24 h
1b (2 equiv.) instead of 1a
1c (2 equiv.) instead of 1a
1d (2 equiv.) instead of 1a
TDAE (2 equiv.) instead of 1a
Zn (6 equiv.) instead of 1a
Mn (6 equiv.) instead of 1a
Addition of ZnCl2 (2 equiv.)
Addition of MnCl2 (2 equiv.)
Without TMEDA
L1 (5 mol%) instead of TMEDA
L2 (5 mol%) instead of TMEDA
L3 (5 mol%) instead of TMEDA
L4 (5 mol%) instead of TMEDA
L5 (5 mol%) instead of TMEDA
L6 (5 mol%) instead of TMEDA
CrCl3(tmeda) (5 mol%) instead of CrCl3(thf)3 and TMEDA
9
10
11
12
13
14
15
16
17
18
88 : 12
a
Reaction condition: 2a (0.1 mmol), bromoform (2 equiv.), CrCl3(thf)3 (5 mol%), ligand (5 mol%), and reductant (above-mentioned amount) in 1,2-
dimethoxyethane (DME, 0.10 M) at 50 ꢀC for 24 hours. b Determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard.
c
Isolated yield. TDAE: tetrakis(dimethylamino)ethylene.
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Table 2 Scope of substratesa
conditions. Screening of several multidentate nitrogen-based
ligands revealed that TMEDA was the best ligand for this cata-
lytic reaction (entry 1 vs. 12–17; amines, phosphines, and other
ligands in ESI†). Notably, no reaction was observed when using
typical organic and inorganic reductants, such as tetrakis(di-
methylamino)ethylene (TDAE), Zn, and Mn powder (entries 6–
8). The coordination of TMEDA to the chromium center was
essentially required to produce the catalytic activity: the addi-
tion of either ZnCl2 (2 equiv.) or MnCl2 (2 equiv.) to the standard
reaction conditions resulted in no reaction (entry 9) or lowered
the yield of 3a (entry 10), respectively, due to the removal of
TMEDA from the chromium center,9a while under ligand-free
conditions, the yield of 3a decreased signicantly (entry 11).
When isolated CrCl3(tmeda) (5 mol%) was used as the catalyst,
the yield of 3a was comparable with that of the in situ CrCl3(thf)3
and TMEDA system (entry 18).
With the optimized reaction conditions in hand, we analyzed
the substrate scope of the alkenes (Table 2). Allyl phenyl ether
(2b) was converted to the corresponding bromocyclopropane 3b
in 92% yield with high trans selectivity. Other allyl aryl ethers 2c–
g with electron-withdrawing and -donating substituents on the
phenyl ring were transformed to the corresponding cyclopro-
panes 3c–g in moderate to high yields, with a cyano group or
halogen atoms at the para-position of the aryl ring remaining
intact during the cyclopropanation reaction. Reaction of CHBr3
with allyl butyl ether (2h) afforded 3h in 81% yield with a trans-
: cis ratio of 87 : 13. The carbonyl group also tolerated the
reductive conditions to produce cyclopropanes; benzoyl-
substituted alkene 2i was converted to 3i in 75% yield, while
allyl carbonate 2j, which is typically used for allylic substitution of
nucleophiles, afforded 3j in 60% yield without any decomposi-
tion of 2j. Allylamine 2k was also applicable and the corre-
sponding cyclopropylmethylamine 3k was obtained in 64% yield.
Simple a-olens, such as allylbenzene 2l, 5-hexenyl acetate 2m, 1-
octene 2n, and vinylcyclohexane 2o, gave the corresponding
cyclopropanes 3l–o in good yield. When we applied substrates
possessing two olenic moieties,
a terminal and mono-
substituted olenic part was selectively cyclopropanated to give
3p and 3q in moderate yield. Internal alkenes with cis-congu-
ration were also applicable to our catalytic system: cis-1,4-
diacetoxy-2-butene (2r) showed a moderate reactivity to give the
corresponding cyclopropane 3r in 47% yield, while some cyclic
alkenes such as cycloheptene (2s), cyclooctene (2t) and ace-
naphthylene (2u) were applicable to afford polycyclic compounds
3s, 3t, and 3u in moderate to high yields, though debromination
of initially formed bromocyclopropane might be involved for the
formation of 3u. Other olens such as styrene, 1,1-disubstituted
alkenes, acyclic internal alkenes, and dienes were not applicable
in this cyclopropanation reaction (see ESI† for the limitations of
this cyclopropanation).
a
Standard reaction condition: 2 (0.4 mmol), bromoform (0.8 mmol, 2
equiv.), CrCl3(thf)3 (0.02 mmol, 5 mol%), TMEDA (5 mol%), and 1a
(0.8 mmol, 2 equiv.) in 1,2-dimethoxyethane (DME, 4 mL) at 50 ꢀC for
b
c
24 hours. CrCl3(thf)3/TMEDA: 10 mol%. NMR yield. Isolated yields
In addition to bromoform, other trihalomethanes were
applicable to the catalytic cyclopropanation. It was noteworthy
that, in the reactions of 2a with both CHClBr2 and CHCl2Br, the
same bromocyclopropane 3a was obtained as the major product
in 84% and 66% yield, respectively, along with chlor-
ocyclopropane as a minor product, although it was much easier
to cleave the C–Br bond than the C–Cl bond (Scheme 1a).14
aer purication by ash column chromatography are noted.
Direct synthesis of iodocyclopropane was not accessible under
the optimized conditions with CHI3, while cyclopropanation
using CHBr3 in the presence of NaI (2 equiv.) produced iodo-
cyclopropane 3a0 instead of 3a in 75% yield (Scheme 1b). When
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Fig. 2 Proposed reaction pathway for chromium carbene species
from isolated dichromium complexes by Takai et al. and Anwander
et al.
were obtained upon treatment with alkenes. The related germa-
nium derivative, Cr2–GeMe3, was also isolated and used for
cyclopropanation. Anwander et al. independently observed the
formation of a gem-dichromiomethane complex (Cr2–I) from the
reaction of CrCl2 and CHI3 at low temperature.
We next conducted a stoichiometric cyclopropanation reac-
tion of alkene 2a with bromoform in the presence of excess
CrCl2 (Scheme 2). The desired cyclopropane 3a was not ob-
tained even at 80 ꢀC (Scheme 2a), although formation of the
corresponding cyclopropanes was observed when iodoform and
diiodomethane derivatives were used. Moreover, under the
catalytic conditions using 1a, the yield of 3a gradually decreased
as the catalyst loading was increased from 5 to 100 mol%
(Scheme 2b). The lower product yield caused by increasing the
amount of the chromium salt suggested that involvement of
gem-dichromiomethane species was less likely in our metal-salt
free system with 1a compared with other chromium-catalyzed
cyclopropanation developed by Takai et al.
On the basis of these ndings, we propose the reaction
mechanism shown in Scheme 3. The initial step is the activation
of bromoform by chromium(II) species A to form (dibromo-
methyl)chromium(III) species B accompanied by the formation
of an equimolar amount of chromium(III) trihalide C, which can
be reduced by 1a or in situ-generated chromium(I) halide F (vide
infra). Species B is dehalogenated by 1a to afford (bromome-
thylidene)chromium(III) D along with the elimination of Me4-
pyrazine and 2 equiv. of Me3SiX (X ¼ Cl, Br), whose reactivity is
assumed due to the reductive dehalogenation of vicinal
Scheme
1 Cyclopropanation using tri- and dihalomethanes. (a)
Reaction with CHClBr2 and CHCl2Br. (b) Reaction with CHBr3 in the
presence of NaI. (c) Reaction with Me3SiCHI2.
Me3SiCHI2 was used as a C1 source, corresponding silyl-
substituted cyclopropane 3a00 was obtained in quantitative
yield (Scheme 1c).
To elucidate the reaction mechanism, we carried out
a kinetic study for the formation of 3a, and the resulting data
were analyzed by variable time normalization analysis (see
ESI†).15 The overall reaction rate did not change with various
concentrations of chromium catalyst (0.004–0.01 M) and alkene
2a (0.08–0.12 M), giving a rate dependence of [Cr]0[2a]0, which is
in sharp contrast to the report of Takai et al. who found that
chromium-catalyzed cyclopropanation with Me3SiCHI2 obeys
rst-order dependence on the concentrations of both a chro-
mium carbene complex and 2a, giving a rate dependence of
[Cr]1[2a]1.9a Such a difference was further observed in the reac-
tion prole; no induction period was observed under the
various reaction conditions.16
Next, to understand how 1a functioned to generate a catalyti-
cally active species, we performed some control experiments.
Direct activation of CHBr3 by 1a was excluded because no signif-
icant rate acceleration was observed when a mixture of CHBr3 and
1a was pre-treated by stirring at 50 ꢀC for 1 hour before adding the
chromium catalyst (see ESI†). Although we tried repeatedly to
isolate the dichromium species having a bridging bromomethyl
group, the target complex could not be isolated and characterized,
probably due to the instability of the bromomethyl-bridged
dichromium species (see ESI†). In previous reports, however,
gem-dichromiomethane complexes (Cr2–X) was isolated as key
intermediates prior to generating reactive mononuclear carbene
species via disproportionation (Fig. 2). Takai et al. reported the
rst example of an isolated gem-dichromiomethane complex (Cr2–
SiMe3) by introducing a bulky trimethylsilyl-substituent on
a carbon atom of diiodomethane, from which silylcyclopropanes
Scheme 2 Control experiments. (a) Reaction in the presence of
excess amount of CrCl2. (b) Reaction with different catalyst loading.
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Acknowledgements
H. I. thanks the nancial support by the JSPS Research Fellow-
ships for Young Scientists. This work was supported by JSPS
KAKENHI Grant No. JP26708012 (Grant-in-Aid for Young
Scientist (A)) to H. T. and JP15H05808 (Precisely Designed
Catalysis with Customized Scaffolding) to K. M.
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Scheme 3 Proposed reaction mechanism.
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the reaction was applicable to allyl ether derivatives, allyl
carbonate, allylamine, and simple a-olens. Control experiments
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(dibromomethyl)chromium(III) species to generate mononuclear
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Conflicts of interest
The author declares no conict of interest.
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Chem. Sci.
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