Enantioselective rhodium-catalyzed cycloisomerization of (E)-1,6-enynes

Article abstract of DOI:10.1002/anie.201601061

An enantioselective rhodium(I)-catalyzed cycloisomerization reaction of challenging (E)-1,6-enynes is reported. This novel process enables (E)-1,6-enynes with a wide range of functionalities, including nitrogen-, oxygen-, and carbon-tethered (E)-1,6-enynes, to undergo cycloisomerization with excellent enantioselectivity, in a high-yielding and operationally simple manner. Moreover, this Rh^I-diphosphane catalytic system also exhibited superior reactivity and enantioselectivity for (Z)-1,6-enynes. A rationale for the striking reactivity difference between (E)- and (Z)-1,6-enynes using Rh^I-BINAP and Rh^I-TangPhos is outlined using DFT studies to provide the necessary insight for the design of new catalyst systems and the application to synthesis. A solution after 15 years: The first rhodium(I)-catalyzed asymmetric cycloisomerization reaction of challenging (E)-configured 1,6-enyes is reported. The reaction provides access to five-membered carbo- and heterocycles with excellent enantioselectivity. DFT studies provide a rationale for the striking reactivity difference between (E)- and (Z)-1,6-enynes using Rh^I-BINAP and Rh^I-TangPhos.

Full text of DOI:10.1002/anie.201601061

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201601061
German Edition: DOI: 10.1002/ange.201601061
Asymmetric Synthesis
Enantioselective Rhodium-Catalyzed Cycloisomerization of
(E)-1,6-Enynes
Xu Deng, Shao-Fei Ni, Zheng-Yu Han, Yu-Qing Guan, Hui Lv, Li Dang,* and Xu-Mu Zhang*
Abstract: An enantioselective rhodium(I)-catalyzed cycloiso-
merization reaction of challenging (E)-1,6-enynes is reported.
This novel process enables (E)-1,6-enynes with a wide range of
functionalities, including nitrogen-, oxygen-, and carbon-
tethered (E)-1,6-enynes, to undergo cycloisomerization with
excellent enantioselectivity, in a high-yielding and operation-
ally simple manner. Moreover, this RhI-diphosphane catalytic
system also exhibited superior reactivity and enantioselectivity
for (Z)-1,6-enynes. A rationale for the striking reactivity
difference between (E)- and (Z)-1,6-enynes using RhI-
BINAP and RhI-TangPhos is outlined using DFT studies to
provide the necessary insight for the design of new catalyst
systems and the application to synthesis.
lacyclopentene.[8a–f,i,j,l] Additional studies resulted in the
development of a novel rhodium(I)-catalyzed enyne cyclo-
isomerization with an intramolecular halogen shift, which
proceeds through a p-allyl rhodium intermediate.[8g,h,j,m] In
related studies, Mikami and Bergens reported a different
rhodium-complexes to facilitate a similar process.[8n] More
recently, Nicolaou and co-workers also reported a cationic
RhI-BINAP catalyzed asymmetric cycloisomerization of
terminal 1,6-enynes with superior results.[8k] Additionally,
Krische also reported a related enantioselective hydrogen-
mediated reductive cyclization of 1,6-enynes using a cationic
RhI-BINAP system.[10] However, most of these rhodium(I)-
based catalytic systems require (Z)-configured 1,6-enynes to
achieve optimal efficiency and selectivity. In contrast, (E)-
configured 1,6-enynes are either inert or inefficient in these
systems. Since the (E)-configured alkene is more readily
available and more thermodynamically stable than corre-
sponding (Z)-isomer, there are compelling reasons to develop
an efficient rhodium(I)-catalyzed cycloisomerization with an
(E)-1,6-enyne. The inherent challenges with (E)-1,6-enyne
substrates, which can primarily be ascribed to the increased
steric hindrance of (E)-1,6-enynes, make it significantly more
challenging compared to the corresponding (Z)-isomer.
Herein, we report the first enantioselective rhodium(I)-
catalyzed cycloisomerization of (E)-1,6-enynes to access
highly functionalized five-membered cycles (Figure 1).
T
he development of new strategies for the efficient and
stereoselective construction of highly functionalized polycyc-
lic compounds remains a long-standing challenge for synthetic
organic chemistry. To this end, the transition metal-catalyzed
cycloisomerization of 1,n-enynes, 1,n-dienes and 1,n-diynes
has emerged as powerful tools for the synthesis of various
types of cyclic compounds.[1] Hence, the direct transformation
of a linear substrate to a cyclic product without additional
reactants that results in the rapid increase inmolecular
complexity in an atom economic manner provides an
attractive strategy. Among these methods, transition metal-
catalyzed cycloisomerization of 1,6-enynes, leading to stereo-
defined cyclic 1,3-dienes and 1,4-dienes, as exemplified by g-
butyrolactones, lactams, tetrahydrofurans, pyrrolidines and
cyclopentanes is of particular interest. A number of transition
metal complexes, such as palladium,[2] ruthenium,[3] tita-
nium,[4] gold,[5] platinum,[5] cobalt,[6] rhodium[7,8] and iron,[9]
have been proven effective for this type of carbon-carbon
bond forming cyclization under mild reaction conditions.
Previously, Zhang et al. developed a series of cationic
rhodium(I)-catalyzed cycloisomerization reations of internal
1,6-enynes to access a variety of functionalized five-mem-
bered cycles through a intermediacy of a rhodium(III)-metal-
[*] Dr. X. Deng, Z. Y. Han, Y. Q. Guan, Prof. Dr. H. Lv,
Prof. Dr. X. M. Zhang
Key Laboratory of Biomedical Polymers of Ministry of Education,
College of Chemistry and Molecular Sciences, Wuhan University
430072, Wuhan (P.R. China)
Figure 1. Significance of the present study.
E-mail: xumu@whu.edu.cn
Preliminary studies focused on the identification of the
optimal chiral ligand to facilitate the enantioselective
rhodium(I)-catalyzed cycloisomerization of challenging (E)-
1,6-enyne 1a (see the Supporting Information (SI) for details
for the preparation of 1a). Treatment of 1a with the chiral
complex generated in situ from [{Rh(COD)Cl}2] (5 mol%)
with various chiral ligands L1–L5 (12 mol%) and AgSbF6
S. F. Ni, Prof. Dr. L. Dang
Department of Chemistry
South University of Science and Technology of China
518055, Shenzhen (P.R. China)
E-mail: dangl@sustc.edu.cn
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601061.
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(20 mol%) in degassed 1,2-dicholoroethane (DCE) at room
temperature furnished the cycloisomerization product 2a[11]
in good to excellent yields, albeit with significant variation in
enantiocontrol (Table 1, entries 1–5). For instance, (S)-
BINAP (L1), which was highly effective for various (Z)-1,6-
provided 2a in excellent yields and excellent enantioselectiv-
ities (entries 8 and 9). Additional studies to probe the effect of
solvent demonstrated that 1,4-dioxane and 1,2-dicholoethane
(DCE) were optimal with [{Rh(L5)(COD)}BF4] (entries 9
and 13), whereas dichloromethane, tetrahydrofuran and
toluene were inferior (entries 10–12). Interestingly, reducing
the catalyst loading maintains the overall efficiency, albeit the
enantioselectivity is significantly reduced (entry 14).
Table 1: Reaction development and optimization.[a]
Table 2 summarizes the application of the optimized
conditions (Table 1 entries 8 and 9) to a range of (E)-1,6-
enynes 1 to demonstrate the scope of the reaction. For
nitrogen-tethered (E)-1,6-enynes, an examination of different
N-protecting groups demonstrated that N-benzoyl group gave
comparable yield and enantioselectivity to that of p-toluene-
sulfonyl group, whereas N-(4-methoxyphenylsulfonyl) group
Entry RhI precatalyst
Ligand Solvent
Additive Yield ee
[%][b] [%][c]
1
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[{Rh(COD)Cl}2]
[Rh(L4)(NBD)]BF4
[Rh(L5)(COD)]BF4
[Rh(L5)(COD)]BF4
[Rh(L5)(COD)]BF4
[Rh(L5)(COD)]BF4
[Rh(L5)(COD)]BF4
[Rh(L5)(COD)]BF4[f]
L1
L2
L3
L4
L5
L5
L5
–
–
–
–
–
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
THF
toluene
AgSbF6 85
AgSbF6 70
AgSbF6 77
AgSbF6 81
AgSbF6 65
25
35
35
2
3
4
5
6
7
8
9
resulted in
a slight decrease of the enantioselectivity
(entries 1–2). Furthermore, electron-donating protecting
group (i.e. N-benzyl) was significantly less efficient (not
shown), which illustrates the necessity for an electron-with-
drawing protecting group. Additional variation at the alkene
moiety revealed that increasing the length of alkyl substituent
À75
74
AgBF4
none
none
none
none
none
none
73
81
trace ND[d]
69
85
46
11
85
88
88
À91
91
10
11
12
13
14
89
Table 2: Scope of asymmetric cycloisomerization reactions.[a]
ND[d]
64
–
dioxane[e] none
92
67
–
dioxane[e] none
[a] All the reactions were conducted with 1a (0.1 mmol) in the presence of
an in situ generated ligand (12 mol%)-RhI (5 mol%) complex in degassed
1,2-dicholoroethane at 258C. [b] Isolated yields were reported. [c] Deter-
mined by HPLC using a chiral stationary phase, see details in the
Supporting Information. [d] Not determined. [e] 1,4-Dioxane. [f] 1 mol%
catalyst was used. COD=1,5-cyclooctadiene, NBD=norbornadiene,
DCM=dichloromethane, DCE=1,2-dicholoroethane, THF=tetrahydro-
furan.
enynes in our previous studies,[8c–f, k] provided poor enantio-
selective control (entry 1). Interestingly, the related C2-
symmetric biarylphosphane ligands (S)-DTBM-SegPhos
(L2), (S)-DTBM-BIPHEP (L3)) also displayed modest
enantioselectivities (entries 2 and 3). In contrast, the elec-
tron-donating phosphocyclic ligands (Sc,Rp)-DuanPhos
(L4)[12] and (S,S,R,R)-TangPhos (L5)[13] (entries 4 and 5)
furnished 2a in promising yields and enantiocontrol. More-
over, the neutral metal complex was significantly less
effective (entry 7) and BF4 is preferred to SbF6 (entries 5
and 6) as the counter-ion, indicating that the formation of
highly coordinatively unsaturated rhodium-species with
weakly coordinating counter-ion is important for catalytic
turnover. Gratifyingly, changing the catalyst to the preformed
cationic catalyst (such as [{Rh(L4)(NBD)}BF4], [{Rh(L5)-
(COD)}BF4]),[8k] which avoids the silver salt metathesis,[14]
[a Reactions were run with substrates (0.1 mmol) in the presence of
5 mol% [{Rh(L5)(COD)}BF4] in degassed 1,2-dicholoroethane at the
specific temperature for 0.25–24 h unless otherwise specified. [b] Iso-
lated yields. [c] Determined by HPLC using a chiral stationary phase, see
details in the Supporting Information. [d] Not determined. [e] [{Rh(L4)-
(NBD)}BF4] was used as the catalyst. Ph=phenyl, Ts =p-toluenesul-
fonyl, Bz=benzoyl.
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leads to a decrease of enantioselectivities (entries 3 and 14).
were carried out based on the proposed mechanism.[8e] In
Interestingly, both alkyl (entry 4) and aryl substitution
(entries 5–12) on the alkyne afford good yields and excellent
enantioselectivities. Moreover, the stereoelectronics of the
aryl substitution pattern has an interesting impact on the yield
and enantioselectivity. For example, the electron-donating
groups at the para position of aryl group were favorable for
both reactivity and enantioselectivity (entries 6 and 7),
whereas electron-withdrawing groups (such as COOMe) are
completely unreactive even at high temperature and with
prolonged reaction time (entry 8). Nevertheless, electron-
withdrawing groups at the meta position of aryl group provide
high yields and maintain the excellent enantioselectivities
(entries 9–11), whereas ortho substitution and disubstitution
was not tolerated and led to unexpected olefin isomerization
product (not shown) and lower enantioselectivity (entry 12),
respectively. Interestingly, RhI-DuanPhos performs better
than RhI-TangPhos for 2d, 2l, 2m and 2o (entries 3, 11, 12,
14), which suggest that the latter system was sensitive to the
stereoelectronic nature of the substrates. In addition, all the
carbon, oxygen and nitrogen-tethered functionalized alkyne
substrates provided excellent enantiocontrol (entries 13, 16
and 17), which might partially attribute to chelation control
through the coordination of rhodium to the carbonyl group.[15]
Whereas, the use of oxygen-bridged 1,6-enyne that has
a phenyl group in alkyne position leads to significantly
lower enantioselectivity (entry 15). Overall, substituted
alkynes significantly decrease the reactivity but improve
enantioselectivity, which contrasts the impact of substituents
on the alkene moiety that exert favorable effect on reactivity
but decrease enantioselectivity.
these theoretical results, the reaction pathway starts with
tetra-coordinated square planar RhI-diphosphine enyne com-
plexes 1B1 and 1T1 (S-BINAP in 1B1, S,S,R,R-TangPhos in
1T1, Scheme 1). From these square planar RhI complexes, the
Scheme 1. Reaction pathways for RhI-catalyzed cycloisomerization.
oxidative cyclization of 1,6-enynes occurs through transition
states, TS1-2B1 and TS1-2T1 to give the key intermediates, RhIII-
metallacyclopentene 2B1 and 2T1. During this process, the
methyl group on the terminal carbon of olefin gets closer to
metal center and the steric effect between this methyl group
and the substitutes on phosphine ligand increases accordingly,
thus the transition states are distorted to be semi-tetrahedron
coordinated. From 2B1 and 2T1, a b-H elimination and
reductive elimination occur fast with small reaction barriers.
At last, the ligand transfer happens to yield the product and
regenerate 1B1 and 1T1. It is obvious that the oxidative
cyclization process in the reaction pathways in Scheme 1 is the
rate determining step because this step has higher barrier than
the other steps from the energy analysis. However TS1-2T1 is
more relatively stable than TS1-2B1, showing that the cyclo-
isomerization of (E)-1,6-enyne can be catalyzed by RhI-
Tangphos rather than RhI-BINAP. At the same time, this
reaction step defines the enantioselectivity of the product
because other oxidative cyclization modes which give R-
isomer have higher barriers than the oxidative cyclization
steps in Scheme 1 (Figures S30 and S33 in SI). It is aso
revealed that TS1-2B1 is relatively more unstable than TS1-2B3
where (Z)-1,6-enyne is involved (Figure S31 in SI), while TS1-
Encouraged by these interesting results among (E)-1,6-
enyne, we then tried to testify whether (Z)-1,6-enynes are also
applicable under the optimized reaction condition. Interest-
ingly, both RhI-TangPhos and RhI-DuanPhos exhibited com-
parable reactivity and enantioselectivity to that of RhI-
BINAP[8] for the (Z)-1,6-enyne 3 (Table 3, entries A and B).
Additionally, RhI-TangPhos was also proved effective for the
mixture of (E)-1,6-enyne 1a and (Z)-1,6-enyne 3, albeit with
a slight decreace of enantioselectivity (entry C).
To gain more insights into the essence that why RhI-
TangPhos performs better than RhI-BINAP for the cyclo-
isomerization reaction of (E)-1,6-enynes, DFT calculations
Table 3: RhI-catalyzed cycloisomerization of (Z)-1,6-enynes.[a]
Entry
Ratio
Yield [%][b]
ee [%][c]
has comparable relative energy to TS1-2T3 where cis-1,6-
1a:3
2T1
enyne is involved (Figure S34 in SI). These results are
consistent with the experimental observation that RhI-
BINAP can catalyze the cycloisomerization reaction of (Z)-
1,6-enynes, but not (E)-1,6-enynes. However, RhI-TangPhos
works both for (E)- and (Z)-1,6-enynes.
A
0:1
0:1
1:1
61
65
81
99
À98
86
B[d]
C
[a] Reactions were run with substrates (0.1 mmol) in the presence of
5 mol% [{Rh(L5)(COD)}BF4] in degassed 1,2-dicholoroethane at 258C
for 0.25–1 h. [b] Isolated yields. [c] Determined by HPLC using a chiral
stationary phase. [d] [{Rh(L4)(NBD)}BF4] was used as the catalyst.
In order to gain insight into the catalytic ability of these
two catalysts towards (E)-1,6-enynes, we analyze the struc-
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tural parameters of TS1-2B1 and TS1-2T1 and find that the
distortion angle of metal center coordination is smaller in TS1-
[1] For reviews on enyne cycloisomerizations see: a) I. Ojima, M.
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than in TS1-2B1, showing that the steric effect in TS1-2T1 is
2T1
smaller than in TS1-2B1. Here, the square planar transition
states TS1-2B’ and TS1-2T’ are not located due to very large
repulsion between Me and Ph(tBu) as shown in Scheme 1B.
This finding well explains the reactivity difference between
RhI-TangPhos and RhI-BINAP for cycloisomerization of (E)-
1,6-enynes.
In conclusion, we have developed the first rhodium(I)-
catalyzed asymmetric cycloisomerization of (E)-1,6-enyes
that afford highly functionalized five-membered cycles. This
new process features not only high enantiomeric excess
(> 99% ee), but also broad substrate scope and functionality
tolerance, wherein both terminal and internal alkynes with
a variety of functional groups are tolerated, which provides
a complementary approach for undertaking rhodium(I)-
catalyzed cycloisomerization of 1,6-enynes. Moreover, this
RhI-diphosphane catalytic system also exhibited superior
reactivity and enantioselectivity for (Z)-1,6-enynes. Finally,
a DFT study provides the necessary insight to understand the
reactivity difference between RhI-TangPhos and RhI-BINAP
for cycloisomerization of 1,6-enynes. We anticipate that this
process will find significant utility in target directed synthesis
given the ubiquity of (E)-configured olefins.
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Experimental Section
Precedure for the rhodium(I)-catalyzed cycloisomerization of 1a to
form (À)-2a: In a flame-dried Schlenk tube, [{Rh(S,S,R,R-TangPhos)-
(COD)}BF4] (3 mg, 5 mol%) was dissolved in freshly distilled 1,2-
dichloroethane (1 mL), then 1a (26.3 mg, 0.1 mmol) was introduced
into the solution at RT under argon. The reaction was completed in
10 min. The reaction mixture was directly subjected to column
chromatography, from which (À)-2a was obtained. The ee value was
determined by chiral HPLC with an OJ-H column (n-hexane/
isopropanol 95:5, 1 mLminÀ1, 254 nm), [a]2D0 = À6.59 (c = 0.44 in
CHCl3); 1H NMR (400 MHz, CDCl3) d = 7.71 (d, J = 8.2 Hz, 2H),
7.34 (d, J = 8.0 Hz, 2H), 5.51 (ddd, J = 16.7, 10.5, 8.1 Hz, 1H), 5.13 (s,
1H), 5.12–5.07 (m, 1H), 5.01–4.92 (m, 1H), 4.91–4.78 (m, 1H), 3.99
(d, J = 14.1 Hz, 1H), 3.77–3.67 (m, 1H), 3.67–3.55 (m, 1H), 3.34–3.09
(m, 1H), 2.87 (t, J = 9.1 Hz, 1H), 2.44 ppm (s, 3H); 13C NMR
(101 MHz, CDCl3) d = 146.6, 143.7, 135.5, 132.6, 129.7, 127.8, 118.1,
108.3, 53.1, 51.8, 47.7, 21.5 ppm; HR-ESI-MS (m/z): calcd for
C14H17NO2S [M + H]+, 264.1053, found 264.1053.
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Acknowledgements
We are grateful for financial support by Wuhan University
(grant 203273463), the “111” Project of the Ministry of
Education of China, the China Postdoctoral Science Founda-
tion (2015M572189), and the National Natural Science
Foundation of China (grants 21302193, 21372179, 21432007,
21402145).
Keywords: (E)-1,6-enyne · asymmetric synthesis ·
cycloisomerization · rhodium catalysis · TangPhos
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6295–6299
Angew. Chem. 2016, 128, 6403–6407
6298 www.angewandte.org
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Angewandte
Communications
Chemie
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[11] Since the product obtained by our RhI-TangPhos system was the
same as that afforded by Nicolaouꢀs RhI-(S)-BINAP system (see
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Received: January 29, 2016
Revised: February 27, 2016
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Angew. Chem. Int. Ed. 2016, 55, 6295 –6299
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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