Chemoselective synthesis of (allyloxy)carbene chelate complexes of iron

Article abstract of DOI:10.1016/s0020-1693(99)00040-7

Chelate (allyloxy)carbene complexes [Fe(C₅H₅)(CO)(η²-C(OCH ₂CH=CHPh)C₆H₄-o-X)][OTf] (4a-b) (a, X=OMe; b, X=Cl) are readily accessible from methoxycarbene complexes 2 and 3 containing a hemilabile or labile ligand. The facile coordination of the alkenyl group, due to the presence of a potential vacant coordination site, allows the selective formation of 4.

Full text of DOI:10.1016/s0020-1693(99)00040-7

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Inorganica Chimica Acta 291 (1999) 449–451
Note
Chemoselective synthesis of (allyloxy)carbene chelate
complexes of iron
Claudia Schulz, Morgan Tohier, Sourisak Sinbandhit, V e´ ronique Guerchais *
UMR 6509 CNRS, Uni6ersit e´ de Rennes 1, ‘Organom e´ talliques et Catalyse’, Uni6ersit e´ de Rennes 1, Campus de Beaulieu,
35042 Rennes Cedex, France
Received 24 September 1998
Abstract
¸
¹¹¹¹¹¹¹¹¹¹¹º
2
Chelate (allyloxy)carbene complexes [Fe(C H )(CO){h -C(OCH CHꢀCHPh)C H -o-X}][OTf] (4a–b) (a, X=OMe; b, X=Cl)
5
5
2
6
4
are readily accessible from methoxycarbene complexes 2 and 3 containing a hemilabile or labile ligand. The facile coordination
of the alkenyl group, due to the presence of a potential vacant coordination site, allows the selective formation of 4. © 1999
Elsevier Science S.A. All rights reserved.
Keywords: Iron complexes; Chelate complexes; Alkene–carbene complexes
2
with the appropriate alcohol is ruled out [6,7] , due to
the thermal unstability of these electrophilic species. We
Alkoxycarbene complexes are well-known species ex-
tensively used in synthesis, in particular of unsaturated
report here the synthesis of chelate (allyloxy)carbene
carbene derivatives [1]. For instance, alkene–carbene
complexes [8], their chemoselective formation being
complexes of Group 6 transition-metals have been
promoted by the presence of a labile or hemilabile
shown to undergo intramolecular carbene transfer reac-
ligand, allowing the facile complexation of the double
tions allowing the formation of cyclopropanes [2,3].
bond.
Moreover, Trost and Kulawiec published the ruthe-
Treatment of the dicarbonyl carbene complex
nium-catalyzed condensation of allylic alcohols to ter-
[
Fp{C(OMe)C H -o-X}][OTf] (1a–b) (a, X=OMe; b,
6
4
minal alkynes yielding b,g-unsaturated ketones; this
3
X=Cl) [9] with a THF solution of NaOCH CHꢀ
CHPh (3 equiv., −80°C) gives a thermally unstable
2
process may involve the intermediacy of chelate (allyl-
1
oxy)carbene species [4] . This led us to investigate a
addition product; subsequent protonation by HBF4–
convenient route to iron–(allyloxy)carbene complexes,
which can be considered as useful models of catalytic
OEt provides a mixture of two carbene species: the
2
starting methoxycarbene complex 1a–b and the new che-
+
¸¹¹¹¹¹¹¹¹¹¹¹¹º
transformations. [Fp(alkoxycarbene)] complexes (Fp=
2
late complex [Fe(C H )(CO){h -C(OCH CHꢀCHPh)-
5
5
2
Fe(C H )(CO) ) are generally prepared from the corre-
5
5
2
C H -o-X}][OTf] (4 a–b) (vide infra).
6
4
sponding acyl FpC(O)R derivatives; however, strong
[
[
Fp{C(OMe)C H -o-X}][OTf]+NaOCH CHꢀCHPh“
alkylating reagents are required [5]. The alternative
6
4
2
+
4 2
HBF -OEt
access from [Fp(vinylidene)] complexes by reaction
Fp{C(OCH CHꢀCHPh)(OMe)C H -o-X}]ꢁꢁꢁꢁꢁꢁꢂ1+4
2
6
4
2
*
Corresponding author. Tel.: +33-2-9928 6729; fax: +33-2-9928
646.
E-mail address: veronique.guerchais@univ-rennes1.fr (V. Guer-
chais)
See Esteruelas et al. [7] for an example of an (allyloxy)-
1
alkenylcarbene complex from a Ru–allenylidene derivative.
3
The precursor complexes are prepared following the classical
procedure, as described by Brookhart et al. [9]. We found that
methylation of the benzoyl complex (X=H) was not complete;
therefore, its reactivity was not developed.
1
Thermolysis of a tungsten–(allyloxy)carbene complex has been
previously shown to give a b,g-unsaturated ketone (see Ref. [2]).
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00040-7

450
C. Schulz et al. / Inorganica Chimica Acta 291 (1999) 449–451
We assume that nucleophilic addition of the alkoxide
gives rise to the dissymmetrical acetal adduct
Fp{C(OMe)(OCH CHꢀCHPh)C H -o-X}], but at-
[
2
6
4
tempts to isolate it were unsuccessful due to decomposi-
tion. This species was thus extracted with pentane at
−
30°C, and chemically trapped as stable cationic car-
bene derivatives, protonation occurring at both alkoxy
substituents. It is noteworthy that the formation of 4
arises from spontaneous decarbonylation, a process
probably favored by the coordination of the alkenyl
group. In this context, complexes containing a potential
vacant coordination site, i.e. a labile or hemilabile
ligand, are expected to be good precursors of 4.
Scheme 2.
carbon resonances, shifted upfield upon coordination,
are comparable to those of [Fp(alkene)] complexes
+
[11], and are located at l 85.8 (ꢀCHPh) and 69.1
3
(ꢀCH). The coupling constant
JH–H=13 Hz is in
agreement with an E configuration of the double bond.
¸
¹¹¹¹¹¹¹¹¹
2
2
The chelate complex (h -C,O)[Fe(C H )(CO){h -
The methylene protons are magnetically unequivalent,
5
5
¹
¹¹¹¹¹¹¹¹¹º
1
the H NMR spectrum exhibits two doublets of dou-
C(OMe)C H -o-OMe}][OTf] (2) is readily obtained as
6
4
2
3
blets at l 6.12 and 4.42 ( J
=11 Hz, JH–H=7 Hz).
red microcrystals in 71% yield by irradiation of the
H–H
2
The carbene carbon atom resonates typically at l 322.6.
Analogously, the related carbene complex 4b is ob-
tained from 3, under the same conditions (Scheme 1).
The NMR data [l( C) 90.4 (ꢀCHPh) and 71.1 (ꢀCH)]
are comparable to those of 4a. In both cases, the
methoxycarbene complex 1a. By contrast, the (h -C,Cl)
2
chelatecomplex[Fe(C H )(CO){h -C(OMe)C H -o-Cl}]-
5
5
6
4
[
OTf] is not accessible from 1b, decomposition
is observed upon irradiation in CH Cl [10] . If the
13
4
2
2
photolysis of 1b is performed in pure acetonitrile,
the reaction gives the acetonitrile complex
(
allyloxy)carbene complex 4 is formed as a single
product as supported by the spectroscopic analysis of
the crude residue, no methoxycarbene derivative being
detected.
These results show that (allyloxy)carbene complexes
4, in which the alkenyl group is coordinated to the iron
center, are formed whatever precursor is used. The
chelation is favored in the case of complexes containing
a labile or hemilabile ligand (Scheme 2), and therefore
allows the selective formation of the desired carbene
complex.
[
Fe(C H )(CO)(CH CN){C(OMe)C H -o-Cl}][OTf] (3)
5
5
3
6
4
which is stable in various organic solvents.
Reaction of 2 with NaOCH CHꢀCHPh, carried out
2
under the conditions described for 1, gives, after proto-
¸
¹¹¹¹¹¹¹¹¹
2
nation, a yellow–brown solution of [Fe(C H )(CO){h -
5
5
¹
¹¹¹¹¹¹¹º
C(OCH CHꢀCHPh)C H -o-OMe}][OTf] (4a) (Scheme
2
6
4
1
). Red crystals are obtained in 60% yield after crystal-
13
lization in a CH Cl /ether mixture. The C NMR
2
2
(
CDCl ) spectrum clearly indicates that the CꢀC double
3
bond is coordinated to the iron center. The olefinic
1
. Supplementary material
Spectroscopic data of compounds 1, 2, 3 and 4a–b
are available from the author (V.G.) upon request.
References
[
1] For a recent review, see: W.D. Wulff, Organometallics 17 (1998)
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3
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[
[
[
5
Scheme 1. Reagents and conditions: (i) PhCHꢀCHCH ONa, THF,
2
−
80°C (1 h); (ii) HBF4–OEt , pentane.
2
4
4
This result shows the dramatic influence of the permethylation of
the Cp ring on the coordination of the chlorine atom to the iron
2
center, since the C Me5 parent (h -C,Cl) chelate carbene complex is
[7] M.A. Esteruelas, A.V. Gomez, A.M. Lopez, E. Onate, N. Ruiz,
Organometallics 17 (1998) 2297–2306.
5
stable and isolable.

C. Schulz et al. / Inorganica Chimica Acta 291 (1999) 449–451
451
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