Alkyne coupling reactions mediated by iron (II) complexes: Highly chemo- And regioselective formation of η6-coordinated arene and pyridine complexes

Article abstract of DOI:10.1021/om0201967

Reaction of the bis(acetonitrile) complex 1 with HC≡CPh affords the (allyl)carbene complex 3, which results from a head-to-tail coupling of two alkynes. The tris(acetonitrile) complex [Fe(C₅Me₅)(CH₃CN)₃][PF₆] (2) mediates highly chemo- And regioselective [2 + 2 + 2] cycloaddition reactions of C≡C and C≡N triple bonds, providing access to π-arene and -pyridine complexes. The pyridine complex 5a has been characterized by an X-ray crystal structure.

Full text of DOI:10.1021/om0201967

2578
Organometallics 2002, 21, 2578-2580
Alk yn e Cou p lin g Rea ction s Med ia ted by Ir on (II)
Com p lexes: High ly Ch em o- a n d Regioselective
F or m a tion of η6-Coor d in a ted Ar en e a n d P yr id in e
Com p lexes
Karine Ferre´,† Lo¨ıc Toupet,‡ and Ve´ronique Guerchais*,†
Institut de Chimie de Rennes, UMR CNRS-Universite´ de Rennes 1 6509, “Organome´talliques
et Catalyse”, Campus de Beaulieu, 35042 Rennes Cedex, France, and Groupe Matie`re
Condense´e, UMR CNRS-Universite´ de Rennes 1 6626, Campus de Beaulieu,
35042 Rennes Cedex, France
Received March 8, 2002
Sch em e 1
Summary: Reaction of the bis(acetonitrile) complex 1
with HCtCPh affords the (allyl)carbene complex 3,
which results from a head-to-tail coupling of two alkynes.
The tris(acetonitrile) complex [Fe(C5Me5)(CH3CN)3][PF6]
(2) mediates highly chemo- and regioselective [2 + 2 +
2] cycloaddition reactions of CtC and CtN triple bonds,
providing access to π-arene and -pyridine complexes. The
pyridine complex 5a has been characterized by an X-ray
crystal structure.
Activation of alkynes by transition-metal complexes
has attracted considerable attention during the past few
decades, allowing the discovery of new C-C bond-
forming reactions. In this context, Ru-catalyzed reac-
tions of alkynes and related stoichiometric reactions
have been extensively developed.1 In contrast, corre-
sponding studies on related iron complexes have been
less explored and the use of iron(II) complexes contain-
ing a cyclopentadienyl-based ligand has not been re-
ported.2 This led us to investigate the chemistry of the
readily accessible iron bis(acetonitrile) and tris(aceto-
nitrile) complexes [Fe(C5Me5)(CH3CN)2(PMe3)][PF6] (1)3
and [Fe(C5Me5)(CH3CN)3][PF6] (2)3 in order to deter-
mine their potential utility in synthesis. We report that
compounds 1 and 2 are able to promote C-C bond
alkyne coupling reactions such as dimerization and
[2 + 2 + 2] cycloaddition4 reactions. The first iron(II)-
mediated cycloaddition reactions of CtC and CtN
triple bonds is presented. High chemoselectivity is
observed, depending on the nature of the alkyne. These
reactions selectively provide access to π-arene and the
hitherto unknown π-pyridine complexes of iron. The
convenient synthesis of iron sandwich complexes is very
attractive, since they constitute an important class of
molecules as organometallic “electron reservoirs” 5 and,
when these complexes are attached to dendrimers,
applications in anionic recognition were found.6
Oxidative coupling of two alkynes can be mediated
by the bis(acetonitrile) complex 1. Treatment of 1 with
phenylacetylene in CH2Cl2 affords the allylcarbene
complex 3, in which the phenyl substituents are exclu-
sively located in the 1- and 3-positions (Scheme 1). As
previously proposed for the related Ru complex,7 the
mechanism may involve a ferracyclopentadiene inter-
mediate which results from a head-to-tail coupling of
two molecules of alkynes; then PMe3 migrates to the
less hindered electrophilic carbon atom.
† Institut de Chimie de Rennes.
‡ Groupe Matie`re Condense´e.
(1) For recent reviews, see: (a) Grotjahn, D. B. In Comprehensive
Organometallic Chemistry II; Hegedus, L. S., Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 12. (b)
Naota, T.; Takaya, H.; Murahashi, S.-I. Chem. Rev. 1998, 98, 2599.
(c) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311. (d) Trost,
B. M.; Toste, D.; Pinkerton, A. B. Chem. Rev. 2001, 101, 2067.
(2) Knoch, F.; Kremer, F.; Schmidt, U.; Zenneck, U.; Le Floch, P.;
Mathey, F. Organometallics 1996, 15, 2713. Schmidt, U.; Zenneck, U.
J . Organomet. Chem. 1992, 440, 187.
The presence of two labile ligands allows a dimeriza-
tion reaction to occur, whereas cycloaddition takes place
(3) Catheline, D.; Astruc, D. Organometallics 1984, 3, 1094.
(4) For [2 + 2 + 2] cycloadditions mediated by Ru complexes, see:
(a) Lindner, E.; J ansen, R.-M.; Mayer, H. A.; Hiller, W.; Fawzi, R.
Organometallics 1989, 8, 2355. (b) Yamamoto, Y.; Ogawa, R.; Itoh, K.
Chem. Commun. 2000, 549. (c) Yamamoto, Y.; Kitahara, H.; Ogawa,
R.; Kawaguchi, H.; Tatsumi, K.; Itoh, K. J . Am. Chem. Soc. 2000, 122,
4310. (d) Yamamoto, Y.; Ogawa, R.; Itoh, K. J . Am. Chem. Soc. 2001,
123, 6189. (e) Yamamoto, Y.; Takagishi, H.; Itoh, K. J . Am. Chem. Soc.
2002, 124, 28. (f) Pertici, P.; Verrazzani, A.; Vitulli, G.; Baldwin, R.;
Bennett, M. A. J . Organomet. Chem. 1998, 551, 37. (g) Ernst, C.;
Walter, O.; Dinjus, E.; Arzberger, S.; Go¨rls, H. J . Prakt. Chem. 1999,
341, 801.
(5) For leading references, see: (a) Hamon, J . R.; Astruc, D.;
Michaud, P. J . Am. Chem. Soc. 1981, 103, 758. (b) Astruc, D.; Blais,
J .-C.; Cloutet, E.; Djakovitch, L.; Rigaut, S.; Ruiz, J .; Sartor, V.; Vale´rio,
C. Top. Curr. Chem. 2000, 210, 229.
(6) Vale´rio, C.; Alonso, E.; Ruiz, J .; Blais, J .-C.; Astruc, D. Angew.
Chem., Int. Ed. 1999, 38, 1747.
(7) Such behavior has been recently reported for the related Ru
complex; see: (a) Mauthner, K.; Soldouzi, K. M.; Mereiter, K.; Schmid,
R.; Kirchner, K. Organometallics 1999, 18, 4681. (b) Becker, E.; Ru¨ba,
E.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics 2001, 20,
3851.
10.1021/om0201967 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/25/2002

Communications
Organometallics, Vol. 21, No. 13, 2002 2579
Sch em e 2
F igu r e 1. ORTEP drawing of 5a . Selected bond distances
(Å) and angles (deg): Fe-(C5Me5) centroid ) 1.688, Fe-
(pyridine ring) centroid ) 1.542, Fe-C(pyridine ring) )
2.082 (range 2.044(3)-2.109(3)), Fe-N ) 2.069(3), N-C(11)
) 1.368(4), N-C(15) ) 1.374(4); N-C(11)-C(17) ) 114.8-
(3), C(12)-C(11)-C(17) ) 122.1(3), C(14)-C(15)-C(16) )
122.5(3), C(16)-C(15)-N ) 116.6(3), C(14)-C(13)-C(20)
) 118.1(3), C(12)-C(13)-C(20) ) 123.1(3).
in the case of the tris(acetonitrile) derivative 2. Complex
2 reacts with 3 equiv of HCtCR (R ) Ph, Hex) in CH2-
Cl2 (room temperature) to give the arene complexes [Fe-
(C5Me5)(1,2,4-C6H3R3)][PF6] (R ) Ph (4a ), Hex (4b)) in
65% yield (Scheme 2). Despite the presence of acetoni-
trile in 2, homocyclotrimerization of terminal alkynes
occurs. NMR analyses of the crude reaction mixtures
show that compounds 4 are formed with high selectivity
(ca. 95%, based on Fe-arene protons by NMR). No
reaction occurs with the disubstituted alkynes PhCt
CPh and EtCtCEt, probably due to the steric bulk of
the C5Me5 ligand.8
Sch em e 3
Complexes [Fe(C5Me5)(arene)]+ are accessible from
the bromo precursor [Fe(C5Me5)(CO)2(Br)] under drastic
reaction conditions, the presence of AlCl3 being re-
quired.5a We found that complex 2 is inert toward
ligand-exchange reactions involving hexamethylbenzene
and 2,6-dimethylpyridine. Therefore, the above ap-
proach constitutes an alternative method to prepare
mixed-sandwich iron(II) derivatives under mild condi-
tions.
By using CH3CN as solvent instead of CH2Cl2, no
reaction occurs except with ethyl propiolate. The coor-
dinating solvent acetonitrile inhibits alkyne complex-
ation, but the presence of a better ligand such as a
carbonyl group in HCtCCO2Et allows the reaction to
proceed. Remarkably, the reaction of 2 in the presence
of HCtCCO2Et (CH3CN) leads to the formation of the
free pyridine derivative [2-Me-3,6-(CO2Et)2-C5H2N] (6)
in 73% yield vs 2 (Scheme 3). However, we were not
able to idenfy any organometallic products from the
reaction mixture. The nature of the solvent has a
dramatic effect on the reaction product, and it is
noteworthy that a different regioisomer of the pyridine
ring is formed. The aromatic protons of 6 appear in the
1H NMR spectrum (CDCl3) as low-field doublets at δ
8.30 and 8.00.
In sharp contrast, by using alkynes having a hetero-
atom bonded to the propargyl position such as ethyl
propiolate and 1-(dimethylamino)-2-propyne, the reac-
tion affords π-pyridine complexes instead of arene
complexes. Treatment of 2 with HCtCR (R ) CO2Et,
CH2NMe2) in CH2Cl2 gives the π-2-Me-4,6-R2-pyridine
complex 5 (R ) CO2Et (5a ), CH2NMe2 (5b)) (Scheme
2). Heterocyclotrimerization of two alkynes and one
acetonitrile occurs; the latter comes from 2. In both
cases, the coordination of the heterocycle is character-
1
ized in the H NMR spectra by an upfield shift of the
1
aromatic protons. In the H NMR spectrum (CDCl3) of
5b, the aromatic protons give rise to singlets at δ 6.00
and 5.89 in agreement with an asymmetrically 2,4,6-
trisubstituted structure. The structure of the first
π-pyridine of iron, 5a , has been confirmed by an X-ray
diffraction analysis (Figure 1).9 The Fe-(C5Me5) cen-
troid and Fe-(pyridine) centroid distances (1.688 and
1.542 Å, respectively) are very comparable to that of the
arene complex [Fe(C5Me5)(η6-C6Et5H)][PF6].8
(8) (a) Hamon, J .-R.; Saillard, J .-Y.; Toupet, L.; Astruc, D. J . Chem.
Soc., Chem. Commun. 1989, 1662. (b) Hamon, J .-R.; Astruc, D.
Organometallics 1989, 8, 2243.
(9) Crystal data for 5a : yellow needles were grown by slow diffusion
of Et2O in an acetone solution of 5a , FeC22H30NO4PF6, Mr ) 573.29,
monoclinic, space group P21/n, Z ) 4, a ) 7.9282(2) Å, b ) 20.8095(4)
Å, c ) 15.3784(4) Å, â ) 97.940(1)°, V ) 2512.83 Å3, T ) 293 K,
Fcalcd ) 1.515 g cm-3, F(000) ) 1184. A total of 5732 reflections were
measured. The agreement factors for the 3376 observations with I >
2σ(I) are R ) 0.056 and Rw ) 0.145. A Nonius Kappa CCD instrument
was used, with graphite-monochromated Mo KR radiation (λ ) 0.710 73
Å). The whole structure was refined by full-matrix least-squares
techniques on F 2, with hydrogens refined using the Riding mode.
The above results show that the formation of pyri-
dines, as complexes or not, is controlled by the presence
of a heteroatom. As suggested by the dimerization
reaction, we assume that a metallacyclopentadiene
intermediate, resulting from coupling of the two coor-
dinated alkyne molecules, is initially formed.10 However,

2580 Organometallics, Vol. 21, No. 13, 2002
Communications
coordination of the CtC bond of the third molecule of
alkyne could be inhibited by the presence of the het-
eroatom. Only a molecule of acetonitrile could interact
and then insert into the Fe-C bond to afford the
observed product.
dition should be similar to that of 5 and 3, since all these
reactions are performed in CH2Cl2.
In conclusion, we developed alkyne coupling reactions
at an iron(II) center including highly chemo- and
regioselective [2 + 2 + 2] cycloadditions, the formation
of pyridine derivatives being heteroatom-assisted. These
results show that labile complexes of iron, 1 and 2, are
promising candidates for other C-C bond-forming reac-
tions, and this clearly opens up new prospectives for the
development of iron chemistry.
The formation of the π-pyridine complexes 5 (CH2-
Cl2) is deduced to arise from an initial head-to-tail
oxidative coupling step, whereas that of the iron-free
pyridine 6 (CH3CN) results from a tail-to-tail coupling,
showing the crucial role of the solvent. For the π-arene
complexes 4, the regiochemistry of the metallacycload-
Su p p or tin g In for m a tion Ava ila ble: This material is
(10) (a) Vollhardt, K. P. C.; Angew. Chem., Int. Ed. Engl. 1984, 23,
539. (b) Bo¨nnemann, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 248.
(b) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49.
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