Manganese porphyrin catalyzed cycloisomerization of enynes

Article abstract of DOI:10.1021/ol301416f

Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III) catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordina

Full text of DOI:10.1021/ol301416f

ORGANIC
LETTERS
2012
Vol. 14, No. 12
3008–3011
Manganese Porphyrin Catalyzed
Cycloisomerization of Enynes
Takuya Ozawa, Takuya Kurahashi,* and Seijiro Matsubara*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 615-8510, Japan
tkuraha@orgrxn.mbox.media.kyoto-u.ac.jp; matsubar@orgrxn.mbox.media.kyoto-u.ac.jp
Received April 21, 2012
ABSTRACT
Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III)
catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordinating axial ligand is the key to bringing
out the catalytic reactivity of manganese for the reaction. The axial ligand of the catalyst has a marked effect on the product and selectively aids the
formation of five- or six-membered cyclic products.
In the past few decades, many transition-metal (Ru, Rh,
Ir Pt, and Au) complexes have emerged as powerful
catalysts for the cycloisomerization of enynes, a reaction
that gives easy access to highly functionalized carbocyclic
and heterocyclic compounds in an essentially atom-
economical manner.1 This cycloisomerization is consid-
ered to be triggered by activation of the alkyne moiety
through coordination to the electrophilic metal complex to
afford key intermediates such as vinyl metal and metal
alkylidene species. Therefore, taking into account the
Lewis acidity of the metal complexes toward π-bonds, we
state that higher-valent redox-stable complexes that do not
oxidize the substrate are more favorable catalysts for the
cycloisomerization than are low-valent metal complexes.
In this context, we supposed that a cationic high-valent
metalloporphyrin would be an ideal catalyst because the
structurally rigid tetradentate porphyrin ligand with a
large π-conjugated planar aromatic structure can help
in maintaining the high oxidation state of the metal
throughout the catalytic process. This characteristic fea-
ture of porphyrin ligands can be exploited to reveal the
potential catalytic ability of some high-valent transition
metals that have not been unexplored for use as catalysts
in cycloisomerization.2 Herein, we report that the
cationic manganese(III) porphyrin complex behaves like
a precious-metal complex such as a Pt complex and
catalyzes the cycloisomerization of enynes to afford cyclic
compounds.
[Mn(TPP)]SbF6 was synthesized by treating [Mn(TPP)]Cl
with AgSbF6 in CH2Cl2 at ambient temperature for 5 h
(Scheme 1).3 Recrystallization of the resulting [Mn(TPP)]SbF6
from toluene afforded the [Mn(TPP)]SbF6(toluene) com-
plex. The X-ray crystal structure analysis at ambient
(2) For examples of the use of metalloporphyrins in nonoxidative
bond formation, see: (a) Suda, K.; Baba, K.; Nakajima, S.-I.;
Takanami, T. Chem. Commun. 2002, 2570. (b) Suda, K.; Kikkawa, T.;
Nakajima, S.-I.; Takanami, T. J. Am. Chem. Soc. 2004, 126, 9554. (c)
Suda, K.; Baba, K.; Nakajima, S.-I.; Takanami, T. Tetrahedron Lett.
1999, 40, 7243. (d) Chen, J.; Che, C.-M. Angew. Chem., Int. Ed. 2004, 43,
4950. (e) Li, Y.; Chan, P. W. H.; Zhu, N.-Y.; Che, C.-M.; Kwong, H.-L.
Organometallics 2004, 23, 54. (f) Schmidt, J. A. R.; Lobkovsky, E. B.;
Coates, G. W. J. Am. Chem. Soc. 2005, 127, 11426. (g) Zhou, C.-H.;
Chan, P. W. H.; Che, C.-M. Org. Lett. 2006, 8, 325. (h) Fujiwara, K.;
Kurahashi, T.; Matsubara, S. J. Am. Chem. Soc. 2012, 134, 5512. (i)
Morandi, B.; Dolva, A.; Carreira, E. M. Org. Lett. 2012, 14, 2162.
(3) (a) Williamson, M. M.; Hill, C. L. Inorg. Chem. 1987, 26, 4155. (b)
Munro, O. Q.; Camp., G. L. Acta Crystallogr., Sect. C 2003, 59, 132. (c)
Williamson, M. M.; Hill, C. L. Inorg. Chem. 1986, 25, 4668. (d) Hill,
H. A. O.; Macfarlane, A. J.; Williams, R. J. P. J. Chem. Soc. A 1969,
1704. (e) Reed, C. A.; Kouba, J. K.; Grimes, C. J.; Cheung, S. K. Inorg.
Chem. 1978, 17, 2666. (f) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. J. Am.
Chem. Soc. 1977, 99, 1093.
(1) For reviews, see: (a) Belmont, P.; Parker, E. Eur. J. Org. Chem.
2009, 6075. (b) Lee, S. I.; Chatani, N. Chem. Commun. 2009, 371. (c)
€
Furstner, A. Chem. Soc. Rev. 2009, 38, 3208. (d) Gorin, D. J.; Sherry,
ꢀ
ꢀ~
B. D.; Toste, F. D Chem. Rev. 2008, 108, 3351. (e) Jimenez-Nunez, E.;
Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (f) Michelet, V.; Toullec,
ꢀ
ꢀ
P. Y.; Genet, J.-P. Angew. Chem., Int. Ed. 2008, 47, 4268. (g) Jimenez-
Nunez, E.; Echavarren, A. M. Chem. Commun. 2007, 333. (h) Zhang, L.;
ꢀ~
Sun, J.; Kozmin, A. S. Adv. Synth. Catal. 2006, 348, 2271. (i) Nevado, C.;
Echavarren, A. M. Synthesis 2005, 167. (j) Echavarren, A. M.; Nevado,
C. Chem. Soc. Rev. 2004, 33, 431. (k) Fairlamb, I. J. S. Angew. Chem.,
Int. Ed. 2004, 43, 1048. (l) Aubert, C.; Buisine, O.; Malacria, M. Chem.
Rev. 2002, 102, 813. (m) Trost, B. M. Science 1991, 254, 1471.
r
10.1021/ol301416f
Published on Web 06/06/2012
2012 American Chemical Society

temperature revealed the short contact distances between
the manganese(III) complex and toluene: the Mn C1T,
out using [Mn(TPP)]TFPB afforded 2a in 75% isolated
yield and 3a in 21% yield (entry 5).4 Another manganese
catalyst, [Mn(salen)]SbF6, however, showed lower cataly-
tic activity for the cycloisomerization than did the cationic
manganese(III) porphyrin complexes (entry 6). The use of
AgSbF6 or AgOTf in place of the cationic manganese
complex afforded trace amounts of products (entries 7
and 8). The reaction with the [Fe(TPP)]SbF6 catalyst did
not afford 2a or 3a (entry 9).2h
3 3 3
Mn C2T, and Mn C3T distances were 3.65(6),
3 3 3
3 3 3
˚
3.28(1), and 3.44(4) A, respectively (Figure 1). These
contact distances were significantly shorter than the
sum of the van der Waals radii of the manganese and
˚
carbon atoms (3.86 A), indicating the electrophilic inter-
action between the cationic manganese(III) porphyrin
complex and toluene. Thus, we presumed that this type
of cationic manganese complex would activate the un-
saturated carbonꢀcarbon bonds of the enyne for
cycloisomerization.
Table 1. Manganese-Catalyzed Cycloisomerization of 1aa
yield (%)b
entry
catalyst
2a
3a
1
2
3
4
5
6
7
8
9
[Mn(TPP)]SbF6
[Mn(TPP)]BF4
[Mn(TPP)]OTf
[Mn(TPP)]Cl
61
5
30
86
95
<1
21
17
<1
14
<1
<1
<1
75
34
7
Figure 1. ORTEP drawing of [Mn(TPP)]SbF6(toluene).
Scheme 1. Synthesis of [Mn(TPP)]SbF6 Catalyst
[Mn(TPP)]TFPBc
d
[Mn(salen)]SbF6
AgSbF6
AgOTf
13
<1
[Fe(TPP)]SbF6
a Reactions were carried out using the catalyst (10 mol %) and enyne
1a (0.2 mmol) in 1.6 mL of xylene at 160 °C for 24 h. b Isolated yields
based on enyne 1a. c TFPB: tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate.
butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)
hexafluoroantimonate.
d [Mn(salen)]SbF6:
(R,R)-(ꢀ)-N,N0-bis(3,5-di-tert-
In order to demonstrate the scope of this counteranion-
dependent manganese-catalyzed cycloisomerization, we
carried out the reaction using various enynes 1 (Table 2).
The reaction of 1b with [Mn(TPP)]TFPB afforded 2b in
67% yield along with 3b in 21% yield, while the reaction of
1b with the [Mn(TPP)]OTf catalyst furnished 3b as the sole
product in 96% yield (entry 2). Enyne 1c, which had a
cyclopropyl moiety, reacted with [Mn(TPP)]SbF6 to yield
the correspondingly substituted product 2c in moderate
yield (entry 3); the cyclopropyl substituent was well tolerated
under the reaction conditions. On the other hand, very low or
trace amounts of the cyclic product were obtained in the
reaction of 1c with [Mn(TPP)]TFPB and [Mn(TPP)]OTf.
The [Mn(TPP)]OTf-catalyzed reaction of 1d, 1e, and 1f
afforded the corresponding five-membered products 3
selectively in high yields, irrespective of the tether groups.
In sharp contrast, the product selectivity in the [Mn(TPP)]-
TFPB- and [Mn(TPP)]SbF6-catalyzed reactions was lar-
gely affected by the tether groups (entries 4ꢀ6). For
example, 1d, which had a 2-nitrobenzenesulfonamide
(NsN) tether group, underwent cycloisomerization in the
presence of [Mn(TPP)]TFPB to give 2d and 3d in 71% and
19% yields, respectively (entry 4), but afforded2dand 3din
Cycloisomerization of enyne 1a with [Mn(TPP)]SbF6
(Table 1, entry 1) afforded the cyclopropane-annulated
six-membered product 2a in 61% yield, along with the five-
membered product 3a in 30% yield. Encouraged by this
result, we screened the reaction parameters and found that
the counteranion had a significant effect on the product
selectivity of 2a and 3a. In the presence of [Mn(TPP)]BF4
catalyst, 3a was obtained in 86% yield along with trace
amounts of 2a (5% yield) (entry 2). Further, 3a was
obtained as the sole product in 95% yield when
[Mn(TPP)]OTf was employed as the catalyst (entry 3).
However, [Mn(TPP)]Cl was inefficient for the reaction,
probably because strong coordination with the chloride
ion made the manganese center less cationic and less
electrophilic (entry 4). Further, the effects of tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate (TFPB), which is a
ꢀ
weaker coordinating counteranion than are SbF6
,
BF4ꢀ, TfOꢀ, and Clꢀ, were examined. The reaction carried
€
(4) Beck, W.; Sunkel, K. Chem. Rev. 1988, 88, 1405.
Org. Lett., Vol. 14, No. 12, 2012
3009

Table 2. Manganese-Catalyzed Cycloisomerization of 1a
Table 3. Manganese-Catalyzed Cycloisomerization of 1a
a Reactions were carried out using a Mn catalyst (10 mol %) and
enyne 1 (0.2 mmol) in 1.6 mL of xylene at 160 °C for 24 h. b Isolated
yields.
in the presence of [Mn(TPP)]TFPB and [Mn(TPP)]SbF6,
and the product yields were 44% and 60%, respectively
(entry 5). The reaction of aniline-tethered enyne 1f selec-
tively furnished 3f, irrespective of the counteranion of the
catalyst (entry 6). These results indicated that the sulfonyl
moiety played an important role in the formation of the six-
membered cyclic product 2.5
As can be seen in Table 3, the aryl groups on the alkene
facilitate the cycloisomerization with [Mn(TPP)]SbF6 to
afford the six-membered products 2 selectively (entries
1ꢀ3). The structure of 2g was unambiguously confirmed
by X-ray analysis (Supporitng Information). On the other
hand, the reaction of the same enynes, 1g, 1h, and 1i, with
[Mn(TPP)]OTf afforded neither 2 nor any other cyclic
compound, and the enynes were recovered. However, in
the presence of [Mn(TPP)]TFPB, the enynes decomposed.
The effect of the double bond geometry was evaluated by
carrying out the reaction using cis-1g (entry 4). The reaction
of cis-1g with[Mn(TPP)]SbF6 afforded 2g as the sole product
in 61% yield. The same isomer was predominantly formed in
the reaction of trans-1g with [Mn(TPP)]SbF6, indicating that
the reaction is independent of the double bond geometry of
the enyne (entries 1 and 4). That is, the [Mn(TPP)]SbF6-
catalyzed cycloisomerization probably involved a stepwise
reaction pathway via the formation of a cationic intermedi-
ate, which induced loss of the double bond geometry.
Taking into account the aforementioned data, we proposed
the following reaction pathway for the cycloisomerization
a Reactions were carried out using a Mn catalyst (10 mol %) and
enyne 1 (0.2 mmol) in 1.6 mL of xylene at 160 °C for 24 h. b Isolated
yields. c NMR yields. d NsN: 2-nitrobenzenesulfonamide. e MesSO2N:
2,4,6-trimethylbenzenesulfonamide.
almost equal amounts in the presence of [Mn(TPP)]SbF6.
Further, the cycloisomerization of 1e, which had a rela-
tively bulkier amide tether group (2,4,6-trimethylphenyl-
sulfonamide (MesSO2N)), afforded 3e as the major product
€
(5) Furstner et al. reported Pt catalyzed cycloisomerization of the
same enyne 1 to form cyclopropane-annulated six-membered product 2.
They observed that the formation of 2 takes place only if enynes were
tethered with heteroatoms such as nitrogen and oxigen but not with
carbon. The manganese-catalyzed cycloisomerization of ether-tethered
enyne also resulted in complete consumption of starting enyne; however,
we could not obtain any cyclic products probably due to decomposition
€
during the reaction. (a) Furstner, A.; Stelzer, F.; Szillat, H. J. Am. Chem.
Soc. 2001, 123, 11863. (b) Chatani, N.; Furukawa, N.; Sakurai, H.;
Murai, S. Organometallics 1996, 15, 901 (in ref 21).
3010
Org. Lett., Vol. 14, No. 12, 2012

of 1 to 2 in the presence of [Mn(TPP)]TFPB or
[Mn(TPP)]SbF6 (Scheme 2, pathway A). The reaction
would be triggered by coordination of the cationic
manganese(III) to the alkyne to afford intermediate 4,
which undergoes 6-endo cyclization via nucleophilic ad-
dition of the internal olefin to the alkyne to give inter-
mediate 5. Isomerization of 5 yields the manganese
alkylidene complex 6, and subsequent 1,2-hydride shift
yields the expected product 2. A plausible reaction path-
way (pathway B) for the formation of the five-membered
product 3 in the presence of [Mn(TPP)]OTf is also out-
lined in Scheme 2. This mechanism involves the formation
of vinyl metal spices 8 via 5-exo cyclization initiated by
coordination of the cationic manganese(III) to the alkyne
moiety. Further reaction with proton shift affords 3, and
the cationic manganese catalyst is regenerated.
the C-5 position of 2k could be explained by the competing
reaction of the manganese alkylidene intermediate 6 with
trace amounts of H2O that might have been present in the
reaction mixture. To test the effect of residual H2O on
deuteration, we performed an experiment involving the
reaction of 1a in the presence of D2O (Scheme 4). The
reaction gave 2a with 40% deuteration at the same C-5
position of the six-membered ring. The observed data
indicate that the conversion of 1 to 2 proceeded via the
formation of 6, which underwent a 1,2-hydogen shift in
competition with protonation. On the other hand, the
conversion of 1 to 3 did not proceed via the formation of
intermediate 6.
Scheme 4. Cycloisomerization of 1a in the Presence of D2O
Scheme 2. Plausible Reaction Pathways
In summary, we have developed manganese-catalyzed
cycloisomerization of enynes for the first time. The key to
the success of this reaction is the use of a porphyrin ligand
along with a weakly coordinating axial counteranion
ligand, which makes the manganese center cationic and
sufficiently electrophilic to activate a triple bond. More-
over, the counteranion has a considerable effect on the
reaction pathway, so that either six- or five-membered ring
systems are obtained. Thus, when using the porphyrin
template, the Lewis acidity of the cationic manganese,
which affects the mode of substrate activation, can be
easily controlled by simply changing the axial counter-
anion ligand.
Scheme 3. Reaction of Deuterium-Labeled Enynes
Acknowledgment. This work was supported by Grants-
in-Aid from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. T.K. acknowledges
Tokuyama Science Foundation, Kurata Memorial
Hitachi Science and Technology Foundation, and Toyota
Physical and Chemical Research Institute. We thank
Dr. Kenji Yoza (Bruker AXS) for the valuable help in
X-ray crystal structural analysis.
The reaction of deuterium-labeled enynes was per-
formed to elucidate the reaction pathways (Scheme 3).
The reaction of 1j with [Mn(TPP)]SbF6 afforded 2j with
99% deuterium labeling at the six-membered ring and 3j
with 99% deuterium labeling at the olefin. Under the same
reaction conditions, the gem-dideuterated enyne 1k af-
forded vic-dideuterated 2k with 60% deuteration at the
C-5 position of the six-membered ring and 3k with 99%
gem-dideuterium labeling. The incomplete deuteration at
Supporting Information Available. Experimental pro-
cedures including spectroscopic and analytical data of
new compounds (PDF). CIF files of [Mn(TPP)] SbF6-
(toluene) and 2g. This material is available free of charge
via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 12, 2012
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