Ferrocenyl-substituted (μ5-hydroxyalkylcyclohexadienyl) tricarbonylmanganese complexes: Synthesis, structural determinations, and formation of carbenium ions

Article abstract of DOI:10.1021/om200905p

Ferrocenyl-substituted (μ⁵-hydroxyalkylcyclohexadienyl) -tricarbonylmanganese complexes have been synthesized as well as the corresponding carbenium dinuclear complexes after dehydroxylation. Studies in solution and in the solid state of these zwitterionic species show not only the electronic influence of the positive charge on the μ⁵-Mn complex but also the participation of the two metal atoms in the stabilization of this charge. Catalytic tests show a high activity of these cations as Lewis acids in a typical Diels-Alder reaction.

Full text of DOI:10.1021/om200905p

Communication
pubs.acs.org/Organometallics
Ferrocenyl-Substituted (η5-Hydroxyalkylcyclohexadienyl)
tricarbonylmanganese Complexes: Synthesis, Structural
Determinations, and Formation of Carbenium Ions
Antoine Eloi, Mael Poizat, Albin Hautecoeur, Armen Panossian, Franco̧ ise Rose-Munch,* and Eric Rose
UPMC Univ Paris 6, Institut Parisien de Chimie Molec
Bat. F, porte 239, Case 181, 4, Place Jussieu, F-75252 Paris Cedex 05, France
̂
́ ́
ulaire, CNRS UMR 7201, Equipe Chimie Organique et Organometallique,
S
* Supporting Information
ABSTRACT: Ferrocenyl-substituted (η 5-hydroxyalkylcyclohexadienyl)-
tricarbonylmanganese complexes have been synthesized as well as the
corresponding carbenium dinuclear complexes after dehydroxylation. Studies in
solution and in the solid state of these zwitterionic species show not only the
electronic influence of the positive charge on the η5-Mn complex but also the
participation of the two metal atoms in the stabilization of this charge. Catalytic tests show a high activity of these cations as
Lewis acids in a typical Diels−Alder reaction.
he reactivity of an arene ring is dramatically modified by
coordination to a metal tricarbonyl tripod such as
Furthermore, the first catalytic tests are described using these
bimetallic carbocations in a typical Diels−Alder reaction.
Hydroxyalkyl-substituted (η 5-cyclohexadienyl)Mn(CO)3
complexes have been described in the literature and can be
obtained using two methodologies: either by reduction of the
corresponding keto or aldehyde derivative complexes13a,b or by
lithiation of the corresponding (η 5-cyclohexadienyl)Mn(CO)3
complex followed by quenching with an aldehyde or a
ketone.13c
In the present study, we chose two synthetic strategies for
the preparation of ferrocenyl-substituted (η 5 -
hydroxyalkylcyclohexadienyl)Mn(CO)3 complexes at the
three different positions of the π-system. Both are based on
the lithiation−electrophilic quenching sequence of a η5-Mn
complex but differ in the lithiation first step: one occurs by
deprotonation,7b,13 whereas the other involves metal−halogen
exchange starting from bromo η5-Mn complexes.14
T
Cr(CO)3 or Mn(CO)3, allowing a variety of transformations
which are impossible to achieve on a free arene.1 The reactivity
is altered of not only ring carbons but also that of groups in
benzylic positions. In terms of the side-chain reactivity of π-
complexed arenes, (η6-arene)Cr(CO)3 compounds have been
extensively studied, showing interesting reactivities, stereo-
selectivities, and stereospecificities.2 They have been described
as “hermaphroditic”,3 and their reactivity toward anions,
cations, and radicals gave rise to remarkable computational
and experimental studies.4 In contrast, very few studies have
appeared on the isoelectronic cationic (η 6-arene)Mn(CO)3
+
complexes.5 For example, deprotonation at a benzylic position
in the [(η6-alkylarene)Mn(CO)3]+ series using a large excess of
strong base occurred with formation of (η 5-alkylbenzyl)Mn-
(CO)3 complexes presenting a highly activated exocyclic double
bound.6 As for the neutral (η 5-cyclohexadienyl)Mn(CO)3
derivatives whose chemical properties have been shown to be
very close to those of (η 6-arene)Cr(CO)3,7 the first
functionalization at a “benzylic” position (i.e., α to the η5
system) by a lithiation/electrophilic quench sequence was very
recently described involving the formation of an intermediate
“benzylic” carbanion.8 In the course of the studies of (η5-
hydroxyalkylcyclohexadienyl)Mn(CO)3 complexes, the forma-
tion of “benzylic” carbocations was postulated to explain the
reactivity of such complexes but, in all the examples studied, the
intermediate carbocation was too unstable to be isolated.9,10
Indeed, only products due to the rearrangement of these elusive
species were obtained. We wondered whether it would be
possible to stabilize these cationic species by an electron-donor
substituent such as a ferrocenyl group.11,12 This paper is
devoted to the synthesis and structural determination of
ferrocenyl-substituted (η 5-hydroxyalkylcyclohexadienyl)Mn-
(CO)3 complexes and their transformation into the corre-
sponding carbocations by elimination of the hydroxy group.
The C1 regioisomers were synthesized as depicted in Scheme
1. The starting bromo-substituted η5-Mn complexes 2,14 3,14
Scheme 1. Synthesis of C1 Hydroxyalkyl Complexes
Received: September 28, 2011
Published: October 20, 2011
© 2011 American Chemical Society
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Communication
and 4 were readily formed by addition of LiAlH4, PhMgCl, and
MeMgCl, respectively, to the (p-bromoanisole)Mn(CO)3
complex 114 regioselectively at the C6 carbon atom. After
reaction with nBuLi at −78 °C in THF and then with
ferrocenecarboxaldehyde at the same temperature, complexes
5−7 were isolated in 68, 90, and 52% yields, respectively.
It is worth noting the formation of a single diastereoisomer in
the case of 6 and 7, whereas two diastereoisomers in a 2.5/1
ratio were obtained in the case of 5. This selectivity is likely due
to the differences of steric hindrance at the C6 carbon atoms.
We were lucky enough to obtain single crystals of 6 suitable
for X-ray diffraction analysis by slow evaporation of a
dichloromethane/pentane solution of the purified diaster-
eoisomer. The ORTEP view of the molecular structure is
presented in Figure 1. The η5-cyclohexadienyl moiety exhibits
Scheme 2. Synthesis of C2 and C3 Hydroxyalkyl Complexes
After the preparation of these different regioisomers of
ferrocenyl-substituted (η 5-hydroxyalkylcyclohexadienyl)Mn-
(CO)3 complexes, the next goal was to achieve the synthesis
of the corresponding carbocations by a dehydroxylation
reaction, achieved in the presence of an acid. In a typical
example, treatment of the alcohol 5 with 3 equiv of the ether
complex HBF4·OMe2 in CH2Cl2 at room temperature
furnished within 10 min a deep blue solution from which
complex 14 could be isolated in 85% yield as the BF4− salt after
precipitation by addition of diethyl ether. Under the same
experimental conditions, the cationic complexes 15 and 16
were obtained as dark blue powders from 6 and 7 in >80%
yields (Scheme 3).
Figure 1. ORTEP views of complexes 6 (left) and 10 (right) with
thermal ellipsoids at the 30% probability level.
the classical five-coplanar geometry (C1C2C3C4C5), with the
C6 carbon lying out of this plane with a dihedral angle of 39°.
The 1.422(4) Å value for the C7−O8 bond length of the
alcohol group is in agreement with a single C−O bond. The
cyclohexadienyl plane and that of the substituted cyclo-
pentadienyls are almost perpendicular, with the Fe atom far
from the sterically demanding Mn(CO)3 group. The main
features of this structure are very close to those of two other
structures of (η5-cyclohexadienyl)Mn(CO)3 complexes sub-
stituted by an hydroxyalkyl group at the C1 position, with the
thienyl group instead of the ferrocenyl group, and with the
phenyl13b or the thienyl group10 at the C6 carbon.
Scheme 3. Synthesis of C1 Carbenium Ions
1H and 13C NMR spectra of complex 14 unfortunately could
not be recorded, likely because of the presence of some
paramagnetic impurities due probably to the oxidation of the
ferrocenyl unit. As for 15 and 16, the signals for the C+−H
protons in acetone-d6 both lie downfield, as expected for
protons on a positively charged center: 8.16 ppm for 15 and
7.92 ppm for 16, in good agreement with data for various
ferrocenyl carbenium ions previously reported.16 An overall
deshielding effect of the cyclohexadienyl protons and a
spectacular deshielding effect of H2 protons (6.98 and 6.66
ppm for 15 and 16, respectively) with respect to the H3
protons (6.66 and 6.61 ppm) have to be pointed out as being
certainly due to the strong electronic effect of the C7
carbocations, whereas in the starting neutral alcohol complexes
6 and 7, the H3 protons are, as expected for η5-Mn complexes,
the most deshielded.17 These data reveal a significant electronic
perturbation of the η5-Mn moiety which participates in the
distribution of the positive charge. The IR spectra show two
absorption bands in the region of the carbonyl ligands which
are compared to those of the parent alcohol complexes. As
evidenced by results gathered in Table 1, in the salts the
Starting from the η5-Mn complex 9, easily obtained by
regioselective addition of PhMgCl to the cationic (m-
bromoanisole)Mn(CO)3 complex 8,14 the same experimental
conditions delivered not only the corresponding alcohol 10 as a
single diastereoisomer in 54% yield but also the unexpected
regioisomer 11 functionalized at the C3 carbon atom. Its
formation is likely due to the reaction of the initial carbanion
from 9 with n-BuBr that is generated from the n-BuLi, to afford
small amounts of complex 13, which is then deprotonated to
form the rearranged anion, giving 11 after quenching with the
ferrocenecarboxaldehyde (Scheme 2). The X-ray structure of
10 reported in Figure 1 shows clearly the position of the
ferrocenyl moiety above the η5 plane in order to avoid the
sterically demanding Mn(CO)3 group.
An alternative for the formation of the C3 regioisomer 11
was proposed starting from the η5-Mn complex 13 readily
obtained by regioselective nucleophilic addition of PhMgCl to
the cationic (η6-anisole)Mn(CO)3 complex 1215 (Scheme 2).
Regioselective deprotonation ortho to the methoxy group
followed by electrophilic quenching with ferrocenecarboxalde-
hyde delivered complex 11 in a more satisfying 50% yield.
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Communication
Table 1. ν(CO) Stretching Bands (cm−1)
Table 2. Catalytic Tests in a Diels−Alder Reaction: Results
1
Followed by H NMR Spectroscopy
5
6
7
14
15
16
1898
1996
1908
2005
1910
2009
1937
2026
1949
2027
1966
2032
carbonyl absorptions are displaced to higher frequencies due to
the strong electron-withdrawing effect of the cations.18
The molecular structure of 15 is displayed in Figure 2. In
contrast to the case of alcohol 6, the two metal atoms now lie
entry no.
cat.
1
2
3
15
3
17
18
3
a
quantity
0.1
2
b
time
24
30
85/15
24
71
85/15
c
conversion
100
90/10
exo/endo
a
b
c
In percent mol equiv. Reaction time in hours. In percent.
C3 carbenium regioisomers proved to be active, affording the
same exo/endo ratio (85/15) but with a better conversion for
18 (71%) than for 15 (30%) (Table 2, entries 1 and 3). A
noticeable progress was observed in the case of the C2
regioisomer, for which the reaction is very fast (2 h) and
quantitative, with the best selectivity being a 90/10 exo/endo
ratio (entry 2).20 Furthermore, it is worth noting that the
quantity of catalyst used is very low, 0.1 mol % in comparison
with monometallic ferrocene carbenium complexes described in
the literature,16 and that the catalyst can be recovered by
precipitation in ether and reused without loss of its efficiency.
In conclusion, we have reported the synthesis of ferrocenyl-
substituted (η 5 -hydroxyalkylcyclohexadienyl)-
tricarbonylmanganese complexes which can be easily converted
to unique stable carbenium dinuclear complexes by dehydrox-
ylation. The preliminary results of the studies in solution
(NMR, IR) and in the solid state (crystallography) of these
ionic species show not only the electronic influence of the
positive charge on the η 5-Mn complex but also the
participation of the two metal atoms to the stabilization of
the positive charge through significant deviation angles toward
Fe and Mn in the structure of the corresponding cation. This is
the first evidence of stabilization of “benzylic” cations in (η5-
cyclohexadienyl)tricarbonylmanganese series. Work is in
progress to develop the reactivity of such species toward
nucleophiles. As the stabilization of “benzylic” anions of η5-Mn
complexes has been recently described,8 it seems that the term
“hermaphroditic” could be applied to η5-Mn complexes in
analogy with what is well precedented in (η 6-arene)-
tricarbonylchromium series.3,4 Although the catalytic tests are
in their infancy, the C2 dinuclear carbocation shows a high
activity as a Lewis acid catalyzing a typical Diels−Alder
reaction. These results shed new light on the applications of η5-
Mn complexes in organic and organometallic syntheses.
Figure 2. ORTEP view of complex 15 with thermal ellipsoids at the
30% probability level. The BF4 anion is omitted for clarity.
−
on opposite faces of the η5 system, with the ferrocenyl moiety
axis almost perpendicular to the carbenium plane. First, this
avoids the hindrance by sterically demanding Mn(CO)3 tripod,
and second, this permits a better overlap between iron and
carbenium orbitals. Furthermore, C1−C7 and C7−C16 bond
lengths move from 1.512(4) and 1.497(4) Å in 6 to 1.432(4)
and 1.414(4) Å in 15, thus exhibiting a partial double-bond
character. The C7−Mn and C7−Fe distances vary from
3.399(3) and 3.144(3) Å in the alcohol 6 to 3.035(2) and
2.818(3) Å, respectively, in the carbenium 15. The differences
correspond to deviation angles of 17.419 and 16.8° (bending of
the C7−C1 and C7−C16 bonds toward Mn and Fe,
respectively).
These significant bendings of the exocylic carbenium atom,
not only out of the Cp ring plane toward the iron atom but also
out of the η5-cyclohexadienyl ring plane toward the manganese
atom, are in good agreement with a direct participation of both
metals in the stabilization of the positive charge.
The same experimental conditions as for 14−16 were used
to generalize the formation of carbenium to C2 and C3
regioisomers. Thus zwitterionic complexes 17 and 18 were
isolated in 68 and 54% yields, respectively, starting from parent
alcohols 10 and 11 (Figure 3).
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving crystallographic data for complexes 6, 10, and
15 (CCDC 845687, 845149, and 845688, respectively), tables
giving crystal data and refinement details, and text giving details
of the syntheses. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 3. C2 and C3 carbenium complexes 17 and 18.
The catalytic activity of the novel binuclear Lewis acids was
then tested in a Diels−Alder reaction. The regioisomers 15, 17,
and 18 were chosen as potential catalysts in the reaction of
cyclopentadiene with methacrolein. Selected results are
gathered in Table 2. First of all, under the conditions used, −
20 °C in CH2Cl2 for 24 h, no reaction was observed in the
presence of parent alcohols 6, 10, and 11 as catalysts. C1 and
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: francoise.rose@upmc.fr.
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Communication
(17) The same phenomenon was observed to a smaller extent for
complexes substituted at the C1 position by electron-acceptor keto
groups; see: Eloi, A.; Rose-Munch, F.; Rose, E.; Chavarot-Kerlidou,
ACKNOWLEDGMENTS
■
We thank P. Herson and G. Gontard, Centre de Res
Structures, IPCM, UMR 7201, Paris, for X-ray structure
analyses.
́
olution de
́
M.; Gerard, H. Organometallics 2009, 28, 925.
(18) Very few examples of heteronuclear cationic ferrocenyl
carbenium species are known; see for example: (a) Cordier, C.;
Gruselle, M.; Vaissermann, J.; Troistskaya, L. L.; Bakhmutov, V. I.;
Sokolov, V. I.; Jaouen, G. Organometallics 1992, 11, 3825. (b) Capon,
J. F.; Le Berre-Cosquer, N.; Leblanc, B.; Kergoat, R. J. Organomet.
Chem. 1996, 508, 31.
REFERENCES
■
(1) Recent reviews: (a) Rosillo, M.; Dominguez, G.; Perez-Castells,
J. Chem. Soc. Rev. 2007, 36, 1589. (b) Rose-Munch, F.; Rose, E.; Eloi,
A. In Patai’s Chemistry of Functional Groups, The Chemistry of
Organomanganese Compounds; Rapoport, Z., Marek, I., Eds.; J. Wiley
Ltd.: Chichester, U. K., 2011; pp 489−558. (c) Rose-Munch, F.; Rose,
E. Org. Biomol. Chem. 2011, 9, 4725.
(19) This value is noticeably larger than that of 7° found in the
cation of the (η5-fulvene)Mn(CO)3 complex: Volland, M. A. O.;
Kudis, S.; Helmchen, G.; Hyla-Kryspin, I.; Rominger, F.; Gleiter, R.
Organometallics 2001, 20, 227.
(2) See for example the following reviews: (a) Davies, S.; Donohoe,
(20) Catalysis by HBF4 (potential impurity from hydrolysis of the
cation) was ruled out by a control experiment: 2% mol equiv of HBF4
gave 22% conversion instead of 100% with 0.1% mol equiv of 17.
T. Synlett 1993, 323. (b) Kundig, E. P.; Pache, S. H. In Science of
̈
Synthesis: Arene Complexes of Chromium, Molybdenum and Tungsten in
Methods of Synthesis; Noyori, R., Imamoto, T., Eds.; Thieme Verlag:
Stuttgart, Germany, 2002; Vol. 2, pp 155−228.
(3) Jaouen, G.; Top, S.; Mc Glinchey, M. J. Organomet. Chem. 1980,
195, C5 and references therein.
(4) (a) Merlic, C. A.; Walsh, J. C.; Tantillo, D. J.; Houk, K. N. J. Am.
Chem. Soc. 1999, 121, 3596. (b) Merlic, C. A.; Miller, M. M.;
Hietbrink, B. N.; Houk, K. N. J. Am. Chem. Soc. 2001, 123, 4904.
(c) Merlic, C. A.; Hietbrink, B. N.; Houk, K. N. J. Org. Chem. 2001, 66,
6738. (d) Pfletschinger, A.; Dargel, T. K.; Bats, J. W.; Schmalz, H. G.;
Koch, W. Chem. Eur. J. 1999, 5, 537.
(5) Unlike (η6-arene)Mn(CO)3 complexes, cationic iron sandwich
+
complexes where the [(η5-C5H5)]Fe+ fragment plays a similar role
toward the coordinated arene as the [Mn(CO)3]+ moiety can be easily
deprotonated at a benzylic position. See for example: (a) Hamon, J.
R.; Astruc, D.; Roman, E.; Batail, P.; Mayerlee, J. J. J. Am. Chem. Soc.
1981, 103, 2431. (b) Astruc, D. Tetrahedron 1983, 4027. (c) Abd-El-
Aziz, A. S.; Bernardin, S. Coord. Chem. Rev. 2000, 203, 219. (d) Astruc,
D. Acc. Chem. Res. 2000, 287.
(6) (a) LaBrush, D. M.; Eyman, D. P.; Baenziger, N. C.; Mallis, L. M.
Organometallics 1991, 10, 1026. (b) Hull, J. W.; Roesselet, K. J.;
Gladfelter, W. L. Organometallics 1992, 11, 3630.
(7) (a) Prim, D.; Andrioletti, B.; Rose-Munch, F.; Rose, E.; Couty, F.
Tetrahedron 2004, 60, 3325. (b) Jacques, B.; Chavarot, M.; Rose-
Munch, F.; Rose, E. Angew. Chem. 2006, 118, 3561.
(8) (a) Dubarle-Offner, J.; Rose-Munch, F.; Rose, E.; Elgrishi, N.;
Rousselier
Dubarle-Offner, J.; Rose-Munch, F.; Ger
35, 2375.
̀
es, H. Organometallics 2010, 29, 4643. (b) Rose, E.;
́
ard, H. New J. Chem. 2011,
(9) Cetiner, D.; Tranchier, J.-P.; Rose-Munch, F.; Rose, E.; Herson,
P. Organometallics 2008, 27, 784.
(10) Cetiner, D.; Norel, L.; Tranchier, J. P.; Rose-Munch, F.; Rose,
E.; Herson, P. Organometallics 2010, 29, 1778.
(11) (a) Lopez, J. L.; Tarraga, A.; Espinosa, A.; Desamparados
Velasco, M.; Molina, P.; Lloveras, V.; Vidal-Gancedo, J.; Rovira, C.;
Veciana, J.; Evans, D. J.; Wurst, K. Chem. Eur. J. 2004, 10, 1815.
(b) Garabatos Perera, J. R.; Wartchow, R.; Butenschon, H. J.
̈
Organomet. Chem. 2004, 689, 3541.
(12) Dyker, G.; Hagel, M.; Muth, O.; Schirrmacher, C. Eur. J. Org.
Chem. 2006, 2134.
(13) (a) Jacques, B.; Eloi, A.; Chavarot-Kerlidou, M.; Rose-Munch,
F.; Rose, E.; Gerard, H.; Herson, P. Organometallics 2008, 27, 2505.
́
(b) Eloi, A.; Rose-Munch, F.; Jonathan, D.; Tranchier, J. P.; Rose, E.
Organometallics 2006, 27, 4554. (c) Jacques, B.; Chanaewa, A.;
Chavarot-Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ger
́
ard, H.
Organometallics 2008, 27, 626.
(14) Eloi, A.; Rose-Munch, F.; Rose, E.; Lennartz, P. Organometallics
2009, 28, 5757.
(15) Chung, Y. K.; Williard, P. G.; Sweigart, D. A. Organometallics
1982, 1, 1053.
(16) (a) Taudien, S.; Riant, O.; Kagan, H. Tetrahedron Lett. 1995, 36,
3513. (b) Brunner, A.; Taudien, S.; Riant, O.; Kagan, H. B. Chirality
1997, 9, 478.
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