Iridium catalyzed carbocyclizations: Efficient (5 + 2) cycloadditions of vinylcyclopropanes and alkynes

Article abstract of DOI:10.1002/chem.201405729

Third-row transition metal catalysts remain a largely untapped resource in cycloaddition reactions for the formation of medium-sized rings. Herein, we report the first examples of iridium-catalyzed inter- and intramolecular vinylcyclopropane (VCP)-alkyne (5 + 2) cycloadditions. DFT modeling suggests that catalysis by iridium(I) proceeds through a mechanism similar to that previously reported for rhodium(I)-catalyzed VCP-alkyne cycloadditions, but a smaller free energy span for iridium enables substantially faster catalysis under favorable conditions. The system is characterized by up to quantitative yields and is amenable to an array of disubstituted alkynes and vinylcyclopropanes.

Full text of DOI:10.1002/chem.201405729

DOI: 10.1002/chem.201405729
Communication
&
Cycloadditions
Iridium Catalyzed Carbocyclizations: Efficient (5+2)
Cycloadditions of Vinylcyclopropanes and Alkynes
Michaela-Christina Melcher, Henrik von Wachenfeldt, Anders Sundin, and Daniel Strand*[a]
To this end, we envisioned that comparatively inexpensive
Abstract: Third-row transition metal catalysts remain
iridium would be a suitable candidate.[10] Strong interactions
a largely untapped resource in cycloaddition reactions for
between carbon and iridium suggest that appropriate iridium-
the formation of medium-sized rings. Herein, we report
based systems would combine a propensity for facile oxidative
the first examples of iridium-catalyzed inter- and intramo-
cyclometalation of VCPs with a favored insertion of p-compo-
lecular vinylcyclopropane (VCP)–alkyne (5+2) cycloaddi-
nents into the resulting metallacycles;[11a] both likely prerequi-
tions. DFT modeling suggests that catalysis by iridium(I)
sites for efficient catalysis with less activated substrates.[11b,c]
proceeds through a mechanism similar to that previously
Herein, we report that cationic iridium complexes with cyclooc-
reported for rhodium(I)-catalyzed VCP–alkyne cycloaddi-
tadiene (cod)[12a,b] or dibenzo[a,e]cyclooctatetraene (dbcot)[12c]
tions, but a smaller free energy span for iridium enables
ligands are exceptionally efficient catalysts for inter- and intra-
substantially faster catalysis under favorable conditions.
molecular (5+2) cycloadditions between VCPs and disubstitut-
The system is characterized by up to quantitative yields
ed alkynes. Comparative studies show rates of catalysis
and is amenable to an array of disubstituted alkynes and
50 times or higher compared to those of analogous rhodium
vinylcyclopropanes.
complexes in intermolecular reactions. Density functional
theory (DFT) calculations suggest that a small free energy span
in the catalytic cycle for iridium accounts for this difference. Iri-
Metal-catalyzed cycloadditions constitute a unique platform for
assembling high-value cyclic products from simple compo-
nents. Formation of seven-membered carbocycles from vinylcy-
clopropanes (VCPs) and alkynes catalyzed by rhodium serves
as an illustration. First reported by Wender in 1995,[1] this pro-
cess has enabled streamlining complex molecule syntheses[2]
and provided a mechanistic basis for developing new reactions
including higher order cycloadditions[3] and (5+2) reactions
with other 2p components.[4] Intramolecular versions catalyzed
by ruthenium,[5] nickel,[6] and iron[7] have been reported, but
the intermolecular reaction that benefits from simple and
often commercially available substrates has remained exclusive
to rhodium catalysis.[8] A remaining challenge towards improv-
ing the efficiency and cost, as well as providing new mechanis-
tic opportunities for interception of intermediates, is the intro-
duction of third-row transition metals as catalysts in this con-
text.[9]
dium catalysis is moreover shown to be compatible with a vari-
ety of functional groups with up to quantitative yields.
At the outset, we explored the use of a cationic iridium(I)
catalyst formed in situ from commercially available [{Ir(cod)Cl}2]
and AgPF6 for the reaction between 4-octyne[13] and commer-
cially available VCP 1a (Table 1).[14]
In a mixture of 1,2-dichloroethane (DCE)/2,2,2-trifluoroetha-
nol (TFE)[15] as solvent (9:1 v/v), the reaction worked remarkably
well to give cycloadduct 3a in an essentially quantitative yield
in under two minutes at ambient temperature (Table 1,
entry 1). Varying the reaction conditions revealed that
[{Ir(cod)Cl}2] did not catalyze the reaction without removal of
the chloride (Table 1, entry 3). A slower reaction with tetra-
ꢀ
kis(3,5-trifluoromethyl)phenyl borate (BArF ) as the counter ion
is likely a reflection of a less efficient counter ion metathesis
due to a limited solubility of NaBArF in DCE (Table 1, entry 4).
With [{Ir(cod)Cl}2] as the precatalyst, 1.25 mol% was necessary
to preserve a high turnover rate throughout the reaction
(Table 1, entry 5).[16] Exchange of the cod ligand in precatalyst
2a for a more tightly coordinated dbcot ligand[17] in 2b ena-
bled a preserved turnover rate using only 0.7 mol% of precata-
lyst (Table 1, entries 6 and 7). A simplified protocol using
a
well-defined cationic h6-arene iridium complex, [(cod)-
Ir(C10H8)]SbF6 (2c),[12a,b,18] was also investigated. This complex
gave a complete conversion of the starting material within
15 min and a 98% yield of 3a after hydrolysis (Table 1, entry 8).
With some effort, single crystals of 2c were also obtained from
which the solid phase structure was solved (Table 1; see the
Supporting Information for details).[19] To our knowledge, this
represents the first XRD structure of a cationic h6-arene iridi-
um–cod complex. Although air stable in the crystalline form,
[a] M.-C. Melcher, H. von Wachenfeldt, A. Sundin, Prof. D. Strand
Centre for Analysis and Synthesis, Lund University
Box 124, 22100 Lund (Sweden)
E-mail: daniel.strand@chem.lu.se
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405729.
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energy profile for the analogous reaction catalyzed by
[Rh(cod)]+ was also calculated for comparison. Despite an ap-
parent similarity between the two pathways, several distin-
guishing features can be highlighted. Formation of both I2
and I3 in this system is exergonic for iridium whereas the corre-
sponding processes are marginally endergonic for rhodium.
The migratory insertion barrier is more favorable with iridium,
whereas reductive elimination is more facile for rhodium. The
largest activation barrier between a transition state and a pre-
ceding intermediate is approximately 16 kcalmolꢀ1 for both
metals. The energy surface for iridium catalysis is however no-
ticeably more level and the turnover frequency determining
transition state and intermediate are not the same in the two
cycles.[25] As a consequence, the free energy span (dE), suggest-
ed to be proportional to the rate of catalysis,[26] is only
16.03 kcalmolꢀ1 for iridium (I4Ir!TS3Ir) compared to 28.22 kcal
molꢀ1 for rhodium (I5Rh!TS2Rh), which implies that catalysis
by cationic iridium(I) would be comparatively fast under favor-
able conditions.
Table 1. Optimization of the intermolecular (5+2) reaction.[a]
Entry VCP (Pre-)Catalyst
(mol%)
Additive
(mol%)
Time
Solvent Yield
[%][c]
1
2
1a 2a (2.5)
1a 2a (2.5)
AgPF6 (8)
AgPF6 (8)
2 min DCE/TFE 99[d]
5 min DCE/
HFIP[e]
95
3
4
5
6
7
8
9
10
11
1a 2a (2.5)
1a 2a (2.5)
1a 2a (1.25)
1a 2b (1.25)
1a 2b (0.7)
1a 2c (5.0)
1b 2a(2.5)
1c 2a (2.5)
1a 2a (2.5)
–
24 h
DCE/TFE
–
NaBArF (8)
AgPF6 (4)
AgPF6 (4)
AgPF6 (2)
–
AgPF6 (8)
AgPF6 (8)
AgPF6 (8)
90 min DCE/TFE 99
5 min DCE/TFE 95
5 min DCE/TFE 98
30 min DCE/TFE 96
<15 min DCE/TFE 98
Motivated by this prediction, the relative rates of catalysis
with a less-reactive 5C component, VCP 1d,[27] were investigat-
ed in a comparative study between [Ir(cod)]+, [Ir(dbcot)]+, and
their analogous rhodium complexes (Figure 2). [Ir(dbcot)]+
4 h
5 h
DCE/TFE 31
DCETFE 53[d]
30 min Me-THF/ 96
TFE
12
13
1a 2a (2.5)
1a 2a (2.5)
AgPF6 (8)
AgPF6 (8)
180 min Me-THF 77
20 min PC/TFE
83[d]
Table 2. Intermolecular (5+2) reactions; substrate scope.[a]
[a] Procedure: To a solution of 2a/b/c (up to 16 mm) in the solvent or sol-
vent mixture indicated (9:1 v/v co-solvent) was added VCP (1.0 equiv)
and alkyne (1.2 equiv) followed by the additive (if applicable). After con-
sumption of the VCP (TLC control), or no observable change in reaction
progress, the mixture was subjected to acidic hydrolysis; [b] thermal ellip-
ꢀ
soids drawn at 30% probability. Hydrogen atoms and the SbF6 counter
1
ion are omitted for clarity; [c] determined by H NMR spectroscopy of the
crude reaction mixture using 1-methoxynaphthalene as internal standard;
[d] yields of isolated product; [e] 1,1,1,3,3,3-hexafluoro-isopropanol.
amorphous 2c was susceptible to aging, explaining why in situ
preparation of the active species from [{Ir(L)Cl}2] precursors
generally gave superior results in catalysis. When using DCE as
the solvent, addition of fluorinated co-solvents was necessary
to retain high yields and reaction rates (not shown).[20] Impor-
tantly, nonchlorinated solvents, such as 2-Me-THF (with or
without TFE added) and propylene carbonate (PC),[21] also al-
lowed for efficient catalysis (Table 1, entries 11–13).
The mechanism and selectivity of rhodium-[22] and rutheni-
um[23]-catalyzed (5+2) reactions have been extensively stud-
ied. For insight into the iridium(I)-catalyzed process, the reac-
tion between 2-butyne and 1-methoxy-1-vinyl cyclopropane
catalyzed by [Ir(cod)]+ was explored by DFT using the m06-2x
functional and the LACVP** basis set (Figure 1).[24] The lowest
energy pathway found originated with an oxidative cyclometa-
lation of the VCP–Ir complex I1Ir to form metallacyclohexene
I2Ir. An alternative mechanism initiated with an ene–yne cyclo-
metalation of I1’Ir was discarded based on the comparatively
high activation barrier for this process (I1’Ir-TS1’Ir =26.95 kcal
molꢀ1). As catalysis by iridium(I) shares the principal elements
of the mechanism previously invoked for rhodium(I), the
[a] Yields of isolated product are given unless otherwise stated;
[b] [{Ir(dbcot)Cl}2] was used; [c] 5.0 mol% precatalyst used; d] 1-trimethyl-
silylpropyne was used as the alkyne component; [e] yield determined by
1H NMR spectroscopy of the crude reaction mixture using 1-methoxy-
naphtalene as internal standard; [f] HFIP was used; [g] ratio of regioisom-
ers measured by 1H NMR spectroscopy of the crude reaction mixture.
Major regioisomer is shown.
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Figure 1. Free energy surfaces for the [Ir(cod)]+- and [Rh(cod)]+-catalyzed (5+2) cycloadditions between 1-methoxy-1-vinyl cyclopropane and 2-butyne. Ener-
gies are given in kcalmolꢀ1
.
gave the fastest conversion in the series and reached comple-
tion within four minutes under the conditions studied. The
turnover frequency (observed) of 10.0 minꢀ1 for this catalyst
was seven times higher than for [Ir(cod)]+ (1.3 minꢀ1) and 50
times higher than for [Rh(dbcot)]+ (0.2 minꢀ1).[28]
of terminal alkynes via protodesilylation of the cycloadduct
during the workup, or to give vinylsilane products (Table 2, en-
tries 6–7). Of utility for synthetic applications is also the partici-
pation of aryl halide-substituted alkynes, as exemplified by for-
mation of aryl chloride 3m (Table 2, entry 12). Sulfonamide
and ether substituents are well tolerated in the reaction. How-
ever, the precise nature and position of basic groups on the
alkyne component will in some cases give reduced efficiency
(Table 2, entries 8–11). For more challenging substrate combi-
nations, the use of [{Ir(dbcot)Cl}2] as the precatalyst and/or
a higher catalyst load generally provided superior results. Re-
placing TFE by HFIP as co-solvent avoided the formation of
stable ketals, which were formed from the intermediate enol
ethers with TFE in some reactions. In reactions between VCP
1d and unsymmetrical aryl-methyl substituted alkynes, a slight
preference for insertion of the alkyne with the aryl group distal
to the metal was seen (Table 2, entries 13–17). Varying the elec-
tronic properties of the aryl group had only marginal influence
on this selectivity. In contrast, insertion of an isopropyl-methyl-
substituted alkyne showed a modest preference for insertion
of the larger isopropyl moiety proximal to the metal (Table 2,
entry 18).
With respect to substrate tolerance, a broad range of alkynes
competently participates in the (5+2) reaction with VCPs 1a
and 1d under iridium catalysis (Table 2). Noteworthy examples
include 1-silylalkynes that can function as synthetic equivalents
The conditions developed for intermolecular (5+2) reactions
with iridium extended also to efficient intramolecular reactions
with tethered VCPs and alkynes (Table 3).
The best results were again obtained using [{Ir(dbcot)Cl}2] as
the precatalyst. Substrates in which the VCP is not pre-organ-
Figure 2. Kinetic comparison of iridium and rhodium catalysis.
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[5] For leading references see: a) B. M. Trost, F. D. Toste, H. Shen, J. Am.
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d) P. A. Wender, D. N. Fournogerakis, M. S. Jeffreys, R. V. Quiroz, F. Inaga-
ki, M. Pfaffenbach, Nat. Chem. 2014, 6, 448–452.
[9] For a review on iridium catalysis see: a) R. Takeuchi, S. Kezuka, Synthesis
2006, 20, 3349–3366; For cycloadditions see: b) carbonylative ring-ex-
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Tsuji, Y. Ito, J. Org. Chem. 1998, 63, 4–5; c) trimerization of alkynes: J. P.
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Takagi, Synlett 2002, 10, 1681–1682; g) Pauson–Khand: T. Shibata, K.
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[a] Yields of isolated product are given; [b] 5 mol% precatalyst used.
ized by substitution at the C1 position of the cyclopropane[21c]
reacted efficiently at ambient temperature to give a series of
diversely functionalized fused bicyclic products in good yields
(Table 3, entries 1–4).
[10] The current cost of iridium metal is 25.3 USD per gram.
[11] For leading references see: a) L. Vaska, Acc. Chem. Res. 1968, 1, 335–
344; b) Z.-X. Yu, P. H.-Y. Cheong, P. Liu, C. Legault, P. A. Wender, K. N.
Houk, J. Am. Chem. Soc. 2008, 130, 2378–2379; c) P. Liu, P. H. Cheong,
Z.-X. Yu, P. A. Wender, K. N. Houk, Angew. Chem. Int. Ed. 2008, 47, 3939–
3941; Angew. Chem. 2008, 120, 4003–4005.
[12] For analogous rhodium complexes in (5+2) reactions, see: a) P. A.
Wender, T. J. Williams, Angew. Chem. Int. Ed. 2002, 41, 4550–4553;
Angew. Chem. 2002, 114, 4732–4735; b) ref. 8c; (dinaphtho[a,e]cyclooc-
tatetraene) c) P. A. Wender, A. B. Lesser, L. E. Sirois, Angew. Chem. Int. Ed.
2012, 51, 2736–2740; Angew. Chem. 2012, 124, 2790–2794.
[13] Internal alkynes were explored as 2C components to avoid the likely
formation of unproductive iridium-species with terminal alkynes. See:
M. Martꢂn, E. Sola, O. Torres, P. Plou, L. A. Oro, Organometallics 2003, 22,
5406–5417.
[14] P. A. Wender, A. J. Dyckman, C. O. Husfeld, M. J. C. Scanio, Org. Lett.
2000, 2, 1609–1611.
[15] Terminal alkynes, such as phenyl acetylene and methyl propiolate that
readily participate in rhodium catalysis under otherwise identical condi-
tions, gave <5% cycloadduct, which corroborates that trace rhodium is
not a significant contributor to catalysis.
[16] Catalyst decomposition may release free cod that can act as a catalyst
poison. In agreement, [Ir(cod)2]BArF did not show activity in catalysis.
[17] a) D. R. Anton, R. H. Crabtree, Organometallics 1983, 2, 621–627; b) A.
Singh, P. R. Sharp, Inorg. Chim. Acta 2008, 361, 3159–3164.
[18] For leading references on cationic iridium–arene-complexes see: a) R. R.
Schrock, J. A. Osborn, J. Am. Chem. Soc. 1971, 93, 3089–3091; b) A. C.
Sievert, E. L. Mutterties, Inorg. Chem. 1981, 20, 489–501.
In summary, the first examples of iridium-catalyzed (5+2)
cycloadditions are presented. The system is characterized by
rapid catalysis and up to quantitative yields. In a comparative
study of intermolecular (5+2) reactions, previously restricted
to rhodium catalysis, iridium showed rates of catalysis 50 times
higher or more compared to those of structurally analogous
rhodium salts. DFT calculations suggested that this difference
originates from a surprisingly level free-energy surface in iridi-
um catalysis, which is a reflection of facile cyclometalation and
insertion steps in the catalytic cycle. The conditions presented
are amenable to both inter- and intramolecular reactions and
compatible with a range of functional groups. Further synthet-
ic and mechanistic studies based on this system are currently
under way.
Acknowledgements
The Swedish Research Council and the Royal Physiographical
Society are gratefully acknowledged for funding. We thank M.
Winkler for initial experiments.
Keywords: cycloadditions · density functional calculations ·
homogeneous catalysis · iridium · seven-membered rings
[19] Single crystals were grown from petroleum ether at +48C over 3
months. CCDC-1020150 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. For related examples see: Rh: ref. [11a]; Ru: M. Crock-
er, M. Green, J. A. Howard, N. C. Norman, D. M. Thomas, J. Chem. Soc.
Dalton Trans. 1990, 2299–2301.
[20] A. Saito, T. Ono, Y. Hanzawa, J. Org. Chem. 2006, 71, 6437–6443.
[21] P. Lenden, P. M. Ylioja, C. Gonzꢃlez-Rodrꢂguez, D. A. Entwistle, M. C.
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H. Rieck, P. A. Wender, K. N. Houk, J. Am. Chem. Soc. 2010, 132, 10127–
10135; d) X. Xu, P. Liu, A. Lesser, L. E. Sirois, P. A. Wender, K. N. Houk, J.
Am. Chem. Soc. 2012, 134, 11012–11025.
[1] a) P. A. Wender, H. Takahashi, B. Witulski, J. Am. Chem. Soc. 1995, 117,
4720–4721; b) P. A. Wender, D. Sperandio, J. Org. Chem. 1998, 63,
4164–4165; c) P. A. Wender, A. J. Dyckman, C. O. Husfeld, D. Kadereit,
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[2] For recent examples see: a) B. L. Ashfeld, S. F. Martin, Org. Lett. 2005, 7,
4535–4537; b) B. M. Trost, Y. Hu, D. B. Horne, J. Am. Chem. Soc. 2007,
129, 11781–11790.
[3] For examples see: a) P. A. Wender, G. G. Gamber, R. D. Hubbard, L.
Zhang, J. Am. Chem. Soc. 2002, 124, 2876–2877; b) P. A. Wender, G. G.
Gamber, R. D. Hubbard, S. M. Pham, L. Zhang, J. Am. Chem. Soc. 2005,
127, 2836–2837.
[4] Alkenes: P. A. Wender, C. O. Husfeld, E. Langkopf, J. A. Love, N. Pleuss,
Tetrahedron 1998, 54, 7203–7220. Allenes: P. A. Wender, M. Fuji, C. O.
Husfeld, J. A. Love, Org. Lett. 1999, 1, 137–139.
[23] X. Hong, B. M. Trost, K. N. Houk, J. Am. Chem. Soc. 2013, 135, 6588–
6600.
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[24] Computational details and citations are given in the Supporting Infor-
mation.
[25] a) S. Kozuch, S. Shaik, J. Am. Chem. Soc. 2006, 128, 3355; b) S. Kozuch, S.
Shaik, Acc. Chem. Res. 2011, 44, 101–110.
[28] The conversion of VCP was quantified by GC using naphthalene as in-
ternal standard. Each data point is the average of three independent
experiments. See the Supporting Information for details.
[26] C. Amatore, A. Jutand, J. Organomet. Chem. 1999, 576, 254–278.
[27] In previous reports, 48–24 h was required to reach completion with
VCP 1d using 0.5 or 2 mol% active catalyst. See ref. [8c].
Received: October 20, 2014
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COMMUNICATION
Third row’s a charm: Efficient inter- and
intramolecular vinylcyclopropane–
alkyne (5+2) cycloadditions catalyzed
by iridium(I) are presented. Density
functional calculations suggest that cat-
alysis by iridium(I) proceeds through
a mechanism similar to the established
rhodium(I)-catalyzed method, but
a smaller free energy span for iridium
enables substantially faster catalysis
under favorable conditions (see
scheme).
&
Cycloadditions
M.-C. Melcher, H. von Wachenfeldt,
A. Sundin, D. Strand*
&& – &&
Iridium Catalyzed Carbocyclizations:
Efficient (5+2) Cycloadditions of
Vinylcyclopropanes and Alkynes
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