An Enantioselective Cascade Cyclopropanation Reaction Catalyzed by Rhodium(I): Asymmetric Synthesis of Vinylcyclopropanes

Article abstract of DOI:10.1002/adsc.201600789

N-Tosylhydrazone-yne-ene substrates are satisfactorily prepared and their cyclization under rhodium catalysis is evaluated. A cascade process involving rhodium vinyl carbene formation – through carbene/alkyne metathesis – and cyclopropanation has been dev

Full text of DOI:10.1002/adsc.201600789

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DOI: 10.1002/adsc.201600789
Very Important Publication
An Enantioselective Cascade Cyclopropanation Reaction
Catalyzed by Rhodium(I): Asymmetric Synthesis of
Vinylcyclopropanes
a
a
a,
ꢀ
a
scar Torres, Anna Roglans, and Anna Pla-Quintana *
Institut de Quꢁmica Computacional i Catꢂlisi (IQCC) and Departament de Quꢁmica, Universitat de Girona (UdG),
Facultat de Ciꢃncies, C/ Maria Aurꢃlia Campmany 69, 17003 Girona, Spain
Fax : (+34)-972-418-150; phone: (+34)-972-418-275; e-mail: anna.plaq@udg.edu
Received: July 21, 2016; Revised: September 7, 2016; Published online: && &&, 0000
Supporting information for this article can be found under: http://dx.doi.org/10.1002/adsc.201600789.
Abstract: N-Tosylhydrazone-yne-ene substrates are
satisfactorily prepared and their cyclization under
rhodium catalysis is evaluated. A cascade process
effective for treating anxiety symptoms and in reliev-
ing the symptoms of withdrawal from chronic use of
[
2]
drugs, and also of bicifadine and DOV21947, which
are being studied for the treatment of chronic pain
involving rhodium vinyl carbene formation
–
[
3]
through carbene/alkyne metathesis – and cyclopro-
panation has been developed to stereoselectively
afford vinylcyclopropanes.
and depression, respectively. (À)-Nardoaristolone B,
which exhibits protective activity on the injury of neo-
[
4]
natal rat cardiomyocytes, and (+)-Sarcandralactone
[
5]
A, a vinylcyclopropane derivative, constitute further
examples of compounds containing this moiety at the
core.
Keywords: carbenes; cyclization; cyclopropanes;
enantioselectivity; rhodium
Cyclopropane-containing compounds are also rec-
ognized for their rich reactivity that, either through
ring-opening or rearrangement, allow for the synthe-
sis of a multitude of topologically different com-
[
6]
The construction of polycyclic structures with proper pounds. The synthetic applications of cyclopropane-
stereochemical control is a major challenge in organic containing compounds are enhanced when the deriva-
synthesis. Among these structures, small ring-contain- tives bear activating groups that can facilitate their
ing compounds – cyclopropane and cyclopropane-con- ring opening or offer opportunities for futher transfor-
taining polycyclic derivatives – have attracted consid- mations, as occurs in the case of vinylcyclopropanes
[
7]
erable interest due to their presence in bioactive sub- (VCP).
stances. This is the case of bicyclo[3.1.0] derivatives
(
[1]
The asymmetric cyclopropanation of olefins stands
Figure 1), which are found at the core of Eglumegad, out as one of the most versatile and straightforward
methods for the stereoselective synthesis of cyclopro-
[
8]
[9]
panes, and metal carbenes are among the most ef-
ficient cyclopropanating agents. When metal vinyl car-
benes are used for this purpose, vinylcyclopropanes
can be obtained directly (Scheme 1). There are sever-
al examples in which the decomposition of vinyl diazo
compounds has been used as a source of vinyl car-
benes, which make enantioselective cyclopropanation
[
10]
reactions possible (Scheme 1, a). However, an elec-
tron-withdrawing group adjacent to the diazo funcion-
ality is necessary to facilitate its synthesis, inhibit side
reactions and achieve high stereoselectivity.
Another strategy is the use of transition metal-cata-
lyzed ring-opening of cyclopropenes to afford transi-
tion metal vinyl carbenes under mild conditions due
[
11]
Figure 1. Examples of compounds containing a bicyclo[3.1.0]- to their intrinsic ring strain (Scheme 1, b).
When
hexane ring system.
properly substituted cyclopropenes are reacted, regio-
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[
18]
of non-stabilized diazo compounds.
Having devel-
oped a methodology to use the chiral catalyst to me-
diate carbene/alkyne metathesis, we decided to test
whether chiral vinylcyclopropanes could be obtained
by the reaction of an enyne tethered to an N-tosylhy-
drazone. In this communication we report the realiza-
tion of this goal.
We initially studied the cyclization of substrate
[
19]
1a
under the conditions that triggered cyclization in
our previous study (Scheme 2). Using dichloroethane
Scheme 1. Strategies for the synthesis of VCPs by alkene cy-
clopropanation with metal vinyl carbenes.
selective ring-opening takes place but no enantiose-
lective cyclopropanation has been achieved so far.
The transition metal-catalyzed rearrangement of
propargylic esters, providing access to metal vinyl car-
Scheme 2. Investigation of the reactivity..
benes when a 5-exo-dig cyclization takes place (also as the solvent at reflux, the reaction worked efficient-
referred to as 1,2-shift), has also been explored ly to afford a 71% yield of vinylcyclopropane
[12]
(
Scheme 1, c).
Remarkably, Toste et al. reported (1R,5S)-2a with a 78% enantiomeric excess. It is re-
the enantioselective synthesis of vinylcyclopropanes markable that the reaction works without the need to
using a gold vinyl carbene generated by transition add a base for N-tosylhydrazone activation. After
metal-induced rearrangement of a propargyl ester some effort, single crystals of the cycloadduct were
that either intra- or intermolecularly reacted with an obtained from which the solid phase structure was
[
13]
alkene.
solved and the absolute configurations of the stereo-
[20]
Finally, metal vinyl carbenes have also been ob- centres were determined. When the atropoisomeric
tained through carbene/alkyne metathesis, a process ligand of the rhodium was changed to (R)-BINAP,
by which a transition metal carbene reacts with an a 55% yield of (1S,5R)-2a was isolated with an 89%
alkyne, transferring the carbene-like character to the enantiomeric excess.
b-carbon of the alkyne, and then reacted with al-
kenes to obtain vinylcyclopropanes (Scheme 1, d).
When the carbene/alkyne metathesis–cyclopropana- moieties (Table 1).
[
14]
The reaction was then extended to substrates 1b–
[
15]
[19]
f, which differ in the units tethering their reactive
tion cascade takes place intramolecularly, polycyclic
Substrates containing sulfonamide tethers in both X
compounds can be obtained but none of the cases re- and Z positions (2b–d, Table 1) were successfully re-
[16]
ported so far involves enantioselective cyclopropa- acted under the cyclization reaction conditions giving
nation. the cycloadducts in good yields. Whereas the vinylcy-
We have recently described an enantioselective syn- clopropanes containing NTs tethers could be efficient-
thesis of sulfones through a cascade triggered by ly characterized, the low solubility of both the 4-nitro-
a base-free decomposition of N-sulfonylhydrazones benzenesulfonyl (Ns) and 2-nitrobenzenesulfonyl (2-
[
17]
featuring a two-fold carbene/alkyne metathesis. N- Ns) tethers prevented the determination of the enan-
Tosylhydrazones are used as an efficient in situ source tiomeric excess of the products obtained. A malonate
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[a]
Table 1. Scope and limitations of the process.
Scheme 3. Reactions of 1,2-disubstituted olefins.
mixture that contained a considerable amount of un-
identified by-products (Scheme 3).
The reactivity of substrate 5, which has a 1,1-disub-
stituted alkene, was then evaluated (Table 2). Only
a 29% yield of vinylcyclopropane 6 was isolated
under the reaction conditions used for the unsubsti-
tuted alkenes, although with good enantioselectivity
(
entry 1, Table 2). Since the reaction in this case was
quite clean, we decided to optimize the reaction con-
ditions. To begin with, the effect of an added base was
evaluated (entry 2, Table 2) but on adding three
equivalents of potassium carbonate to the reaction
mixture only traces of the cyclized compound were
formed. The reaction solvent was then switched to
1
,2-dichloropropane (1,2-DCP) to increase the tem-
[
[
a]
b]
The reaction was performed with 0.10 mmol of substrate perature but this resulted in a decrease in both the
1
in dichloroethane (1.5 mL, 0.067M). The mixture of
yield and the enantioselectivity (entry 3, Table 2).
In these reactions, the catalytically active species
are generated in situ by hydrogenation of a mixture
of the cationic diolefin complex [Rh(cod) ]BF and
the biphosphine in dichloromethane. Heller et al.
have shown that the solvent in which this hydrogena-
tion is carried out is crucial in determining the cata-
[
Rh(cod) ]BF and (S)-BINAP was treated with hydro-
2
4
gen in DCM solution for catalyst activation prior to the
reaction.
The enantiomeric excess could not be determined due to
the low solubility and absorptivity.
2
4
tether was also introduced between the hydrazone lytically active species that are formed and their reac-
[
22]
and the alkyne and the cyclized product 2e was ob- tivity.
Given this, we decided to test the reaction
tained with moderate yield but high ee. By the reac- with the catalytically active species generated in
tion of substrates that feature a phenyl ring between methanol. To our delight, vinylcyclopropane 6 was ob-
the alkyne and the toluenesulfonylhydrazone, inden- tained in a much improved 63% yield and with a 74%
yl-substituted bicyclo[3.1.0] derivatives were also ee (entry 4, Table 2). Other biphosphine ligands were
available. The cyclized product 2f was efficiently ob- then evaluated. Whereas DTBM-SegPhos provided
tained when an NTs tether was placed in the enyne the product with a lower yield and lower enantiose-
moiety. When this moiety was linked through a cyclic lectivity (entry 5, Table 2), H -BINAP was selective
8
acetal (2,2-dimethyl-1,3-dioxane), the reaction to pro- but less active in the cascade transformation devel-
vide product 2g was only moderately efficient. Finally, oped (entry 6, Table 2). Finally, a reaction was run in
[
21]
and as has been found in other studies, no reaction which methanol was used as the solvent both in the
was observed when the tether length of the enyne activation of the catalyst and the cyclization. This re-
moiety was increased (2h). The absolute stereochem- action gave vinylcyclopropane 6 in a 73% yield in just
istry of each of the products shown in Table 1 was as- 1.5 h reaction time, but the enantioselectivity was con-
signed by analogy with product 2a.
siderably reduced (entry 7, Table 2). Best results are
The substituents on the alkene group markedly thus obtained by activating the catalyst in methanol
affect the outcome of the carbene/alkyne metathesis– whilst performing the reaction in DCE (entry 4,
cyclopropanation sequence. When substrates 3, which Table 2). These optimized conditions were tested in
feature a trans-1,2-disubstituted alkene, were subject- the reaction of selected substrates 1, but the reactions
ed to the reaction conditions, the efficiency of the re- were not improved.
action drastically dropped and only low quantities
4a) or traces (4b) of the corresponding vinylcyclopro- formation of the catalytic species from the cationic
panes were detected by NMR in the crude reaction rhodium complex and the (S)-BINAP mixture pre-
Scheme 4 outlines the proposed catalytic cycle. The
(
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[a]
Table 2. Optimization of the cyclization of 5.
Entry
1
Solvent I
Phosphine
Solvent II
Yield [%]
ee [%]
DCM
DCM
DCM
(S)-BINAP
(S)-BINAP
(S)-BINAP
(S)-BINAP
DCE
DCE
1,2-DCP
DCE
DCE
29
<5
14
63
37
49
73
71
n.d.
53
74
23
73
46
[
b]
2
3
4
5
6
7
CH OH
3
CH OH
(R)-DTBM-SegPhos
3
CH OH
(R)-H -BINAP
DCE
3
8
[
c]
CH OH
(S)-BINAP
CH OH
3
3
[
a]
The reaction was performed with 0.10 mmol of substrate 5 in solvent II (1.5 mL, 0.067M). The mixture of [Rh(cod) ]BF4
2
and (S)-BINAP was treated with hydrogen in solvent I solution for catalyst activation prior to the reaction.
[
[
b]
c]
Reaction run with K CO (3 equiv.).
2
3
The reaction was run for 1 h 30 min.
Experimental Section
Cyclization Reaction of 1a; Typical Procedure
[
Rh(cod) ]BF (0.0112 g, 0.028 mmol) and (S)-BINAP
2 4
(
0.0172 g, 0.028 mmol) were dissolved in CH Cl (4 mL)
2 2
under N . Hydrogen gas was bubbled to the stirred catalyst
2
solution for 30 minutes and the resulting mixture was con-
centrated to dryness. The mixture was then dissolved in 1,2-
dichloroethane (1.5 mL) and a solution of 1a (0.1092 g,
0
.28 mmol) in 1,2-dichloroethane (1.5 mL) was added. The
reaction mixture was heated at reflux for 1.5 hours (TLC
monitoring). The solvent was evaporated and the residue
was purified by column chromatography on silica gel
(
hexane/ethyl acetate, 10:0 to 9:1) to afford 2a as a colour-
less solid; yield: 0.0599 g (71%), 78% ee.
Scheme 4. Postulated mechanism.
Acknowledgements
[
23]
We are grateful for the financial support by the Spanish Min-
istry of Education and Science (MINECO) (Project
CTQ2014-54306-P) and the DIUE of the Generalitat de Cat-
alunya (Project 2014-SGR-931 and FI predoctoral grant to
cedes the formation of the rhodium carbene A. The
rhodium carbene then reacts with the alkyne in a car-
bene/alkyne metathesis process to afford the rhodium
vinyl carbene B that intramolecularly cyclopropanates
the tethered alkene in an enantioselective manner to
furnish the chiral vinylcyclopropane and recover the
catalytic species.
ꢀ.T.).
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An Enantioselective Cascade Cyclopropanation Reaction
Catalyzed by Rhodium(I): Asymmetric Synthesis of
Vinylcyclopropanes
Adv. Synth. Catal. 2016, 358, 1 – 6
ꢀscar Torres, Anna Roglans, Anna Pla-Quintana*
Adv. Synth. Catal. 0000, 000, 0 – 0
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