Iridium(I)-catalyzed coupling of (Z)-2-En-4-yn-1-ols with activated alkynes: A new synthetic route to 7-oxanorbornadienes

Article abstract of DOI:10.1002/adsc.201000408

Taking advantage of the ability shown by the iridium(I) dimer [{Ir(μ-Cl)(COD)}₂] to promote the cycloisomerization of (Z)-enynols into furans, an unprecedented synthetic route to 7-oxanorbornadienes has been developed just by performing the cat

Full text of DOI:10.1002/adsc.201000408

COMMUNICATIONS
DOI: 10.1002/adsc.201000408
Iridium(I)-Catalyzed Coupling of (Z)-2-En-4-yn-1-ols
with Activated Alkynes: A New Synthetic Route to
7-Oxanorbornadienes
Alba E. Dꢀaz-ꢁlvarez,a Pascale Crochet,a,* and Victorio Cadiernoa,*
a
Departamento de Quꢀmica Orgꢂnica e Inorgꢂnica, Instituto Universitario de Quꢀmica Organometꢂlica “Enrique Moles”
(Unidad Asociada al CSIC), Universidad de Oviedo, Juliꢂn Claverꢀa 8, 33006 Oviedo, Spain
Fax : (+34)-98-510-3446; e-mail: crochetpascale@uniovi.es or vcm@uniovi.es
Received: May 24, 2010; Revised: July 27, 2010; Published online: September 23, 2010
In memory of Prof. Josꢃ Manuel Concellꢄn who passed away in March 2010.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000408.
Abstract: Taking advantage of the ability shown by
the iridium(I) dimer [{Ir(m-Cl)(COD)}2] to promote
construction of these oxabicyclic alkenes involves the
Diels–Alder reaction of furans with activated al-
kynes.[5–7] However, the accessibility of furan skele-
tons with the desired substitution pattern is one of the
most critical factors determining the applicability of
the process in synthesis. In fact, the development of
synthetic routes allowing the facile assembly of poly-
substituted furans still remains an important objective
for organic chemists.[8] Transition metal-catalyzed het-
eroannulation reactions of suitable acyclic precursors
are probably the most appealing and innovative strat-
egies to access furans since they usually proceed with
complete atom-economy. Among the different acyclic
substrates described to date in the literature,[8] (Z)-2-
en-4-yn-1-ols are particularly attractive since they are
readily available starting materials. In this context,
G
ACHTUNGTRENNUNG
the cycloisomerization of (Z)-enynols into furans,
an unprecedented synthetic route to 7-oxanorborna-
dienes has been developed just by performing the
catalytic reactions in the presence of activated al-
kynes. The process, which proceeds under solvent-
free conditions, furnishes the bicyclic alkenes in
good yields with complete atom-economy.
Keywords: bicyclic alkenes; cycloisomerization;
Diels–Alder reaction; furans; iridium
The development of new methodologies aimed at im- several group 8–11 metal complexes have been suc-
proving efficiency is an important goal of contempo- cessfully employed to promote the cycloisomerization
rary organic synthesis. In particular, the replacement of (Z)-2-en-4-yn-1-ols into the corresponding furans,[9]
of multistep transformations by more straightforward with ruthenium-, palladium- and gold-based catalysts
“one-pot” processes is attracting considerable atten- showing the best performances.
tion due to the obvious advantages of the latter, i.e.,
they minimize the overall reaction times, the genera-
Continuing with our interest in this type of cyclo-
AHCTUNGTRENNUNG
tion of chemical waste and energy consumption, limit- communicate that the commercially available iridi-
ing therefore the global cost of the synthetic path-
ACHTUNGRTENuUGNN m(I) dimer [{IrACHUTNRTGEG(NNUN m-Cl)ACHTGUNTREN(NUGN COD)}2] (COD=1,5-cyclooc-
way.[1] In this context, transition metal catalysts pro- tadiene) is also a highly efficient and selective catalyst
vide an excellent tool for generating complex organic for the atom-economic conversion of (Z)-enynols into
molecules, with high atom-economy,[2] from readily furans, representing a competitive alternative to the
available starting materials via “one-pot” tandem pro- previously described Ru, Pd and Au systems. In addi-
cesses.[3,4]
7-Oxabicyclo
oxanorbornadienes) represent a relevant class of com- been developed just by performing the catalytic reac-
pounds used as versatile synthetic intermediates in a tions in the presence of activated alkynes (Scheme 1).
tion, taking advantage of this, an unprecedented
ACHTUNGTRENNUNG[2.2.1]hept-2,5-diene derivatives (7- “one-pot” route to 7-oxanorbornadienes has also
ACHTUNGTRENNUNG
huge number of cyclization, cycloaddition and cou-
Despite the prominent role that rhodium and iridi-
pling reactions,[5] as well as serving as monomers for um species play in homogeneous catalysis, their utility
functional polymers.[6] The traditional method for the in the heteroannulation of (Z)-enynols still remains
Adv. Synth. Catal. 2010, 352, 2427 – 2431
ꢅ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2427

Alba E. Dꢀaz-ꢁlvarez et al.
COMMUNICATIONS
ing; entries 7 and 8). Interestingly, both compounds
were also able to promote the heteroannulation of 1a
at room temperature (entries 9 and 10). In particular,
using [{IrACHTUNTRGENN(UG m-Cl)ACHTUGNTERN(NUGN COD)}2] as catalyst, complete forma-
tion of 2a could be reached after 3 h (entry 9). We
note that, as far as we are aware, such a remarkable
activity under mild conditions has only been previous-
ly described with palladium[9e,n] and gold cata-
lysts,[9f,l,p,r] but never under solvent-free conditions.
Complex [{IrACTHNUGRTNENUG(m-Cl)CAHUTNGTREN(NUGN COD)}2] was also effective in
the selective cycloisomerization of a variety of other
(Z)-enynols, thus confirming its potential for practical
applications. As shown in Table 2, replacement of the
hydrogen atom by alkyl (1b, c), aryl (1d, e) or alkenyl
Scheme 1. One-pot synthesis of 7-oxanorbornadienes from
(Z)-enynols and activated alkynes.
almost unexplored.[9h,m] That is why, using the cyclo- (1f, g) groups at C-1 of the 3-methyl-2-penten-4-yn-1-
ACHTUNGTRENNUNGisomerization of (Z)-3-methyl-2-penten-4-yn-1-ol (1a) ol skeleton was compatible with the cyclization condi-
into 2,3-dimethylfuran (2a) as a model reaction, we tions, furans 2b–g being selectively generated in excel-
decided to evaluate the efficiency and selectivity of lent GC yields (ꢀ97%) after 0.2–10 h of stirring at
several commercially available or readily accessible room temperature (entries 1–7). Alkynyl substituents
Rh and Ir precursors in this transformation. Initial ex- at C-1 were also tolerated, i.e., (Z)-enynols 1h, i (en-
periments were performed at 808C, under solvent- tries 8 and 9). However, in these cases the cycloisome-
free conditions, employing 5 mmol of 1a and a metal rization process only takes place at 808C, the starting
loading of 1 mol% (see Table 1). Under these condi- materials being recovered unchanged when the reac-
tions (entries 1–8), all of the compounds tested led to tions were performed at room temperature for 24 h.
the selective and almost quantitative formation of the Remarkably, the heteroannulation of (Z)-enynols 1j–l
ꢁ
desired furan 2a in less than 1 h (ꢀ96% GC yield). bearing an internal C C bond was conveniently ac-
From this general catalyst screening, iridium clearly complished by [{Ir(m-Cl)(COD)}2] performing the cat-
emerged as the metal of choice, with dimers [{Ir(m- alytic reactions at 808C (entries 10–12), a reactivity to
Cl)(COD)}2] and [{Ir(m-Cl)(COE)2}2] showing the best
N
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
performances (>99% yield after only 10 min of heat-
Table 2. Cycloisomerization of different (Z)-enynols cata-
lyzed by [{Ir(m-Cl)
(COD)}2].[a]
N
ACHTUNGTRENNUNG
Table 1. Comparison of different Rh and Ir catalysts in the
cycloisomerization of (Z)-3-methyl-2-penten-4-yn-1-ol (1a)
into 2,3-dimethylfuran (2a).[a]
Entry Substrate (R1/R2)
t [h] Product Yield [%][c]
1
2
3
4
5
6
7
H/H (1a)
Et/H (1b)
n-Bu/H (1c)
Ph/H (1d)
3
2a
>99 (87)
>99 (89)
>99 (90)
97 (81)
Entry Catalyst
T [8C] t [min] Yield [%][b]
0.2 2b
0.2 2c
6
10
2
1
6
1
5
1
2
3
4
5
6
7
8
9
10
[{Rh
[{Rh
[{Rh
[{Rh
[{Rh
N
R
E
50
20
20
60
30
60
10
10
180
300
>99
>99
>99
97
2d
2e
2f
2g
2h
2i
2j
2k
2l
A
80
4-C6H4OMe/H (1e)
CH2CH=CH2/H (1f)
CH2C(Me)=CH2/H (1g)
98 (83)
(COE)2}2] 80
>99 (85)
>99 (87)
92 (79)
>99 (86)
83 (71)
80
80
80
80
r.t.
r.t.
98
96
>99
>99
>99
98
ꢁ
RhCl3·nH2O
8[b]
9[b]
10[b]
11[b]
12[b]
C CSiMe3/H (1h)
ꢁ
[{Ir
[{Ir
[{Ir
[{Ir
N
E
E
N
G
C CPh/H (1i)
H/Ph (1j)
H/CH=CMe2 (1k)
Et/Ph (1l)
72
4
93 (80)
95 (81)
[a]
[a]
Reactions performed under an N2 atmosphere using
5 mmol of (Z)-methyl-2-penten-4-yn-1-ol (1a). [enynol]:
[metal] ratio=100:1. Abbreviations used: COD=1,5-cy-
clooctadiene; NBD=norbornadiene; TFB=tetrafluoro-
benzobarralene; COE=cyclooctene.
Reactions performed at room temperature under N2 at-
mosphere using 5 mmol of the corresponding (Z)-enynol
1a–l. [enynol]:[Ir] ratio=100:1.
Reaction performed at 808C.
Determined by GC (isolated yield after appropriate
work-up is given in brackets).
[b]
[c]
[b]
Determined by GC.
2428
asc.wiley-vch.de
ꢅ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 2427 – 2431

Iridium(I)-Catalyzed Coupling of (Z)-2-En-4-yn-1-ols with Activated Alkynes
date restricted to palladium and gold catalysts, ruthe- 88%) after appropriate chromatographic work-up. It
nium and rhodium species being completely inopera- is important to note that the two steps involved in
tive with internal alkynes.[9b,h] Purification of the this tandem process, i.e., the cycloisomerization reac-
crude reaction mixtures by bulb-to-bulb distillation tion and the Diels–Alder cycloaddition, are promoted
(2a–c) or column chromatography on silica gel (2d–l) by the iridium catalyst. Effectively, when isolated 2,3-
provided analytically pure samples of the furans dimethylfuran (2a) was treated with dimethyl acety-
which were isolated in 71–90% yield.
N
ACHTUNGTRENNUNG(m-Cl)-
Interestingly, when these cycloisomerization pro-
ACHTUNGTRENNUNG
cesses were performed in the presence of a slight temperature. Only on heating the mixture at 808C
excess (1.3 equivalents) of the activated alkynes di- does the cycloaddition process occur.[10] We note that,
methyl acetylenedicarboxylate (3a) and diethyl acety- to the best of our knowledge, participation of Ir(I)
lenedicarboxylate (3b) the clean formation of the cor- catalysts in Diels–Alder-type cycloadditions has not
responding 7-oxanorbornadiene derivatives was ob- been yet quoted in the literature, the only examples
served, via Diels–Alder cycloaddition between the in reported involving Ir
situ formed furan and the alkyne. We must note that, In summary, a straightforward, operationally simple
although the two individual transformations have and highly efficient procedure for the preparation of
been extensively studied, such a tandem cycloisomeri- 7-oxabicyclo[2.2.1]hept-2,5-diene derivatives from
zation/cycloaddition process for the synthesis of 7- readily accessible (Z)-enynols and activated alkynes
oxanorbornadienes is unprecedented. As shown in has been developed. The “one-pot” catalytic transfor-
(III) derivatives.[11]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Table 3, with the exception of the alkynyl-substituted mation presented herein, which represents a new ex-
derivatives 4ia, ib (entries 11 and 12), all reactions ample of the utility of iridium complexes in synthe-
proceeded efficiently at room temperature furnishing sis,[12] is based on an unprecedented tandem process
the oxabicyclic alkenes in good isolated yields (60– consisting of the initial cycloisomerization of the (Z)-
enynol into the corresponding furan and subsequent
Diels–Alder cycloaddition of the latter with the
alkyne. The overall process, which proceeds under sol-
vent-free and remarkable mild conditions, represents
Table 3. Synthesis of 7-oxanorbornadienes by Ir-catalyzed
coupling of (Z)-enynols with activated alkynes.[a]
an appealing synthetic route to this class of relevant
oxabicyclic alkenes. We believe that the feasibility
and scope of this atom-economical process (no by-
products are generated) represent a competitive and
very appealing methodology which could find applica-
tions in different synthetic programs in the near
future. Further studies aimed at broadening the scope
of this sequential catalytic process are currently in
progress in our laboratory.
Entry (Z)-Enynol Alkyne t [h] Product Yield [%]c)
1
2
3
4
5
6
7
8
1a
1a
1b
1b
1d
1d
1f
1f
1g
1g
1i
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
4
4
7
7
7
9
18
18
7
4aa
4ab
4ba
4bb
4da
4db
4fa
4fb
4ga
4gb
4ia
83
81
88
85
88
87
71
77
69
75
60
63
Experimental Section
General Procedure for the Catalytic
Cycloisomerization of (Z)-Enynols into Furans
The corresponding (Z)-enynol 1 (5 mmol) and [{IrACHTUNGTRENNUNG(m-Cl)-
AHCTUNGTREN(NGUN COD)}2] (17 mg, 0.025 mmol) were introduced into a
sealed tube under a nitrogen atmosphere. The resulting solu-
tion was then stirred at room temperature (1a–g), or heated
at 808C in an oil bath (1h–l), for the indicated time (see
Table 2; the course of the reaction was monitored by regular
sampling and analysis by GC). The residue was then purified
by bulb-to-bulb distillation (1a–c), or by column chromatog-
raphy on silica gel (1d–l) using hexanes as eluent, to give
furans 2; yields: 71–90%. The identity of these compounds
9
10
11[b]
12[b]
9
2
2
1i
4ib
[a]
Reactions performed at room temperature under an N2
atmosphere using 5 mmol of the corresponding (Z)-
enynol 1a–i and 6.5 mmol of the appropriate alkyne 3a,
b. [enynol]:
Reaction performed at 808C.
1
[alkyne]:[Ir] ratio=100:130:1.
was assessed by comparison of their H and 13C{1H} NMR
spectral data with those previously described in the litera-
ture and by their fragmentation in GC/MSD analysis.
[b]
[c]
Isolated yield after chromatographic work-up.
Adv. Synth. Catal. 2010, 352, 2427 – 2431
ꢅ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2429

Alba E. Dꢀaz-ꢁlvarez et al.
COMMUNICATIONS
971–983; d) W. Tam, N. Cockburn, Synlett 2010, 1170–
1189.
General Procedure for the Catalytic Synthesis of 7-
Oxanorbornadienes from (Z)-Enynols and Alkynes
[6] See, for example: a) G. C. Bazan, J. H. Oskam, H.-N.
Cho, L. Y. Park, R. R. Schrock, J. Am. Chem. Soc.
1991, 113, 6899–6907; b) J. G. Hamilton, J. J. Rooney,
D. G. Snowden, Makromol. Chem. Phys. 1995, 196,
1031–1042; c) B. M. Novak, A. L. Safir, Polym. Prepr.
1996, 37, 335–336; d) M. R. Buchmeiser, G. K. Bonn,
PCT Int. Appl. WO9827423, 1998; e) L. Delaude, A.
Demonceau, A. F. Noels, Macromolecules 1999, 32,
2091–2103; f) V. Amir-Ebrahimi, D. A. Corry, J. G.
Hamilton, J. M. Thompson, J. J. Rooney, Macromole-
cules 2000, 33, 717–724; g) V. Amir-Ebrahimi, J. J.
Rooney, J. Mol. Catal. A: Chem. 2004, 212, 107–113.
[7] See, for example: a) B. H. Lipshutz, Chem. Rev. 1986,
86, 795–819; b) F. Fringuelli, A. Taticchi, in: The
Diels–Alder Reaction: Selected Practical Methods, John
Wiley & Sons, West Sussex, 2002; c) D. Heber, P.
Rçsner, W. Tochtermann, Eur. J. Org. Chem. 2005,
4231–4247.
The corresponding (Z)-enynol 1 (5 mmol), the appropriate
alkyne
3
(6.5 mmol) and [{IrACHTGUNTRENNU(G m-Cl)ACHTUGNRTEN(NUNG COD)}2] (17 mg,
0.025 mmol) were introduced into a sealed tube under a ni-
trogen atmosphere. The resulting solution was then stirred
at room temperature (1a, b, d, f, g), or heated at 808C in an
oil bath (1i), for the indicated time (see Table 3; the course
of the reaction was monitored by regular sampling and anal-
ysis by TLC). The residue was then purified by column
chromatography on silica gel, using a mixture of hexanes/di-
ethyl ether (90:10) as eluent, to give 7-oxanorbornadienes 4;
yields: 60–88%. Characterization data, as well as copies of
1
the H and 13C{1H} NMR spectra, of all the bicycles synthe-
sized can be found in the Supporting Information.
Acknowledgements
[8] For recent reviews, accounts, and highlights dealing
with the synthesis of furans, see: a) X. L. Hou, H. Y.
Cheung, T. Y. Hon, P. L. Kwan, T. H. Lo, S. Y. Tong,
H. N. C. Wong, Tetrahedron 1998, 54, 1955–2020;
b) B. A. Keay, Chem. Soc. Rev. 1999, 28, 209–215;
c) A. Jeevanandam, A. Ghule, Y.-C. Ling, Curr. Org.
Chem. 2002, 6, 841–864; d) R. C. D. Brown, Angew.
Chem. 2005, 117, 872–874; Angew. Chem. Int. Ed.
2005, 44, 850–852; e) S. F. Kirsch, Org. Biomol. Chem.
2006, 4, 2076–2080; f) D. M. DꢈSouza, T. J. J. Mꢇller,
Chem. Soc. Rev. 2007, 36, 1095–1108; g) N. T. Patil, Y.
Yamamoto, ARKIVOC 2007, 10, 121–141; h) G.
Balme, D. Bouyssi, N. Monteiro, Heterocycles 2007, 73,
87–124; i) V. Cadierno, P. Crochet, Curr. Org. Synth.
2008, 5, 343–364; j) X.-L. Hou, Z. Yang, K.-S. Yeung,
H. N. C. Wong, Prog. Heterocycl. Chem. 2009, 21, 179–
223.
Financial support from the Spanish MICINN (Projects
CTQ2006-08485/BQU and Consolider Ingenio 2010
CSD2007-00006) and the Gobierno del Principado de Astu-
rias (FICYT Project IB08-036) is acknowledged.
References
[1] See, for example: a) L. F. Tietze, Chem. Rev. 1996, 96,
115–136; b) S. F. Mayer, W. Kroutil, K. Faber, Chem.
Soc. Rev. 2001, 30, 332–339; c) J. M. Lee, Y. Na, H.
Han, S. Chang, Chem. Soc. Rev. 2004, 33, 302–312.
[2] The concept of atom-economy has emerged as one of
the major driving forces of modern chemistry: a) B. M.
Trost, Science 1991, 254, 1471–1477; b) B. M. Trost,
Angew. Chem. 1995, 107, 285–307; Angew. Chem. Int.
Ed. Engl. 1995, 34, 259–281; c) R. A. Sheldon, Pure
Appl. Chem. 2000, 72, 1233–1246; d) B. M. Trost, Acc.
Chem. Res. 2002, 35, 695–705; e) B. M. Trost, M. U.
Frederiksen, M. T. Rudd, Angew. Chem. 2005, 117,
6788–6825; Angew. Chem. Int. Ed. 2005, 44, 6630–
6666.
[3] See, for example: a) G. A. Molander, C. R. Harris,
Chem. Rev. 1996, 96, 307–338; b) P. Eilbracht, L. Bꢆr-
facker, C. Buss, C. Hollmann, E. Kitsos-Rzychon, C. L.
Kranemann, T. Rische, R. Roggenbuck, A. Schmidt,
Chem. Rev. 1999, 99, 3329–3336; c) G. Poli, G. Giam-
bastiani, A. Heumann, Tetrahedron 2000, 56, 5959–
5989; d) S.-I. Ikeda, Acc. Chem. Res. 2000, 33, 511–
519; e) J.-C. Wasilke, S. J. Obrey, R. T. Baker, G. C.
Bazan, Chem. Rev. 2005, 105, 1001–1020; f) Metal Cat-
alyzed Cascade Reactions, (Ed.: T. J. J. Mꢇller), Spring-
er Verlag, Berlin, 2006.
[9] For representative examples, see: a) D. Vꢃgh, P. Zalup-
ˇ
sky, J. Kovꢂc, Synth. Commun. 1990, 20, 1113–1123;
b) B. Seiller, C. Bruneau, P. H. Dixneuf, Tetrahedron
1995, 51, 13089–13102; c) J. A. Marshall, C. A. Sehon,
J. Org. Chem. 1995, 60, 5966–5968; d) B. C¸ etinkaya, I.
ꢉzdemir, C. Bruneau, P. H. Dixneuf, J. Mol. Catal. A:
Chem. 1997, 118, L1–L4; e) B. Gabriele, G. Salerno, E.
Lauria, J. Org. Chem. 1999, 64, 7687–7692; f) A. S. K.
Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost, Angew.
Chem. 2000, 112, 2382–2385; Angew. Chem. Int. Ed.
2000, 39, 2285–2288; g) B. C¸ etinkaya, I. ꢉzdemir, C.
Bruneau, P. H. Dixneuf, Eur. J. Inorg. Chem. 2000, 29–
32; h) S. Elgafi, L. D. Field, B. A. Messerle, J. Organo-
met. Chem. 2000, 607, 97–104; i) P. Pale, J. Chuche,
Eur. J. Org. Chem. 2000, 1019–1025; j) I. ꢉzdemir, B.
˘
Yigit, B. C¸ etinkaya, D. ꢊlkꢇ, M. N. Tahir, C. Arıcı, J.
Organomet. Chem. 2001, 633, 27–32; k) B. C¸ etinkaya,
N. Gꢇrbꢇz, T. SeÅkin, I. ꢉzdemir, J. Mol. Catal. A:
Chem. 2002, 184, 31–38; l) Y. Liu, F. Song, Z. Song, M.
Liu, B. Yan, Org. Lett. 2005, 7, 5409–5412; m) A. E.
Dꢀaz-ꢁlvarez, P. Crochet, M. Zablocka, C. Duhayon, V.
Cadierno, J. Gimeno, J. P. Majoral, Adv. Synth. Catal.
2006, 348, 1671–1679; n) V. Cadierno, J. Dꢀez, J.
Garcꢀa-ꢁlvarez, J. Gimeno, N. Nebra, J. Rubio-Garcꢀa,
Dalton Trans. 2006, 5593–5604; o) J. Albers, V. Cadier-
[4] For a highly informative review on tandem catalysis,
see: D. E. Fogg, E. N. dos Santos, Coord. Chem. Rev.
2004, 248, 2365–2379.
[5] For reviews, see: a) M. Lautens, W. Tam, Adv. Met.-
Org. Chem. 1998, 6, 49–101; b) M. Lautens, K. Fagnou,
S. Hiebert, Acc. Chem. Res. 2003, 36, 48–58; c) D. K.
Rayabarapu, C.-H. Cheng, Acc. Chem. Res. 2007, 40,
2430
asc.wiley-vch.de
ꢅ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 2427 – 2431

Iridium(I)-Catalyzed Coupling of (Z)-2-En-4-yn-1-ols with Activated Alkynes
no, P. Crochet, S. E. Garcꢀa-Garrido, J. Gimeno, J. Or-
ganomet. Chem. 2007, 692, 5234–5244; p) X. Du, F.
Song, Y. Lu, H. Chen, Y. Liu, Tetrahedron 2009, 65,
1839–1845; q) C. C. Schneider, H. Caldeira, B. M. Gay,
D. F. Back, G. Zeni, Org. Lett. 2010, 12, 936–939;
r) C. H. Oh, H. J. Yi, K. H. Lee, Bull. Korean Chem.
Soc. 2010, 31, 683–688.
lution of [{IrACTHNGUTERNNU(G m-Cl)ACHTNUGTREN(NUNG COD)}2] was treated at room tem-
perature with a mixture of (Z)-3-methyl-2-penten-4-yn-
1-ol (1a) and dimethyl acetylenedicarboxylate (3a), the
clean formation of 2a was initially observed by
1H NMR, evolving over time into 4aa. Apparently, the
active species of the Diels–Alder process is generated
in situ from [{IrACHTNUGTRENNUG(m-Cl)HCATUNGTNER(NUGN COD)}2] and the (Z)-enynol
during the initial cycloisomerization step.
[10] In order to gain some insight on the role of iridium in
the Diels–Alder cycloaddition step, the behaviour of
[{IrACHTUNGTRENNUNG(m-Cl)ACHTUNGTRENNUNG(COD)}2] towards 2,3-dimethylfuran (2a) and
[11] For recent examples, see: a) A. C. Atesin, J. Zhang, T.
Vaidya, W. W. Brennsel, A. J. Frontier, R. Eisenberg,
Inorg. Chem. 2010, 49, 4331–4342; b) D. Carmona,
M. P. Lamata, F. Viguri, R. Rodrꢀguez, C. Barba, F. J.
Lahoz, M. L. Martꢀn, L. A. Oro, L. Salvatella, Organo-
metallics 2007, 26, 6493–6496; c) D. Carmona, M. P.
Lamata, F. Viguri, R. Rodrꢀguez, F. J. Lahoz, I. T. Do-
brinovitch, L. A. Oro, Dalton Trans. 2007, 1911–1921;
d) D. Carmona, R. Medrano, I. T. Dobrinovitch, F. J.
Lahoz, J. Ferrer, L. A. Oro, J. Organomet. Chem. 2006,
691, 5560–5566.
dimethyl acetylenedicarboxylate (3a) was explored by
1
means of H NMR spectroscopy. Thus, we have found
that, while [{IrACHTUNGTRENNUNG(m-Cl)CAHTUNGTRNE(NUGN COD)}2] remains unchanged upon
addition of an excess of 2a in CDCl3 at room tempera-
ture, it readily reacts, under the same conditions, with
3a to afford a complicated mixture of products. Forma-
tion of only minor amounts of 7-oxanorbornadiene 4aa
1
was detected by H NMR spectroscopy upon addition
of 2a to this mixture. This fact indicates that [{Ir
(COD)}2] is not the real active species in the final cy-
cloaddition step. On the other hand, when a CDCl3 so-
ACHTUNGTRNE(NUNG m-Cl)-
G
[12] Iridium Complexes in Organic Synthesis, (Eds.: L. A.
Oro, C. Claver), Wiley-VCH, Weinheim, 2009.
Adv. Synth. Catal. 2010, 352, 2427 – 2431
ꢅ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2431