A simple route to keto-substituted (η5-cyclohexadienyl) Mn(CO)3 complexes using organomanganese transmetalation: Structural and theoretical characterizations

Article abstract of DOI:10.1021/om800863t

Efficient synthesis of keto-substituted (η³-cyclohexa- dienyl)Mn(CO)₃ complexes is achieved by organomanganese transmetalation catalyzed by Fe(acac)₃. Density functional theory (DFT) calculations highlight the influence of

Full text of DOI:10.1021/om800863t

Organometallics 2009, 28, 925–928
925
A Simple Route to Keto-Substituted (η5-Cyclohexadienyl)Mn(CO)3
Complexes Using Organomanganese Transmetalation: Structural
and Theoretical Characterizations
Antoine Eloi,† Franc¸oise Rose-Munch,*,† Eric Rose,*,† Murielle Chavarot-Kerlidou,† and
He´le`ne Ge´rard‡
UPMC UniV Paris 06, CNRS UMR 7611, Laboratoire de Chimie Organique, Tour 44, 1er Etage,
Case 181, 4 place Jussieu, 75252 Paris, France, and UPMC UniV Paris 06, CNRS UMR 7616,
Laboratoire de Chimie The´orique, 4 place Jussieu, 75252 Paris, France
ReceiVed September 16, 2008
Scheme 1. Reactivity Features of (η5-Cyclohexadienyl)Mn(Co)3
Summary: Efficient synthesis of keto-substituted (η5-cyclohexa-
dienyl)Mn(CO)3 complexes is achieVed by organomanganese
transmetalation catalyzed by Fe(acac)3. Density functional
theory (DFT) calculations highlight the influence of the keto
group position on the strength of conjugation between the RCO
function and the cyclohexadienyl ring and shed light on the
regioselectiVity of nucleophilic attacks on such complexes.
Complexes
During these past five years, tremendous strides have been
made in the study of the reactivity of (η5-cyclohexadienyl)Mn-
(CO)3 complexes with the development of efficient synthetic
procedures such as Pd cross-coupling reactions5 and lithiation/
electrophilic quench sequence,6 thus giving rise to the formation
of unprecedented functionalized complexes. Among them the
η5 complexes substituted by synthetically useful keto groups
appeared to be of crucial importance in the development of
applications of such complexes.7 Starting from halogeno
substrates, we have synthesized several keto-substituted η5
complexes using Stille Pd coupling under carbonylative
conditions,5a but limitations concerning the presence of the
halide on the arene ring of the starting material, the experimental
conditions requiring a CO atmosphere, and the use of stannous
derivatives narrow the convenience of the method. As for the
lithiation/electrophilic quench sequence, only sterically demand-
ing electrophiles such as 2,2-dimethylpropionic acid chloride
were shown to react efficiently with the lithiated anion of η5
Mn complexes to give rise to the formation of the corresponding
keto-substituted (η5-cyclohexadienyl)Mn(CO)3 complexes in
high yield.6b All the other attempts using less bulky acid
chlorides (thienyl or phenyl acid chlorides, for example)8 led
to the formation of a mixture of unidentifiable products and a
significant quantity of unreacted starting material. We therefore
modified the nature of the electrophile and chose a Weinreb
amide.9 Indeed, to prevent further addition of the anion to the
Introduction
Transition metal complexes containing η6-arene ligands
constitute an important class of organometallic compounds
whose properties have been investigated for many years. Among
them, (η6-arene)tricarbonylchromium and isoelectronic cationic
(η6-arene)tricarbonylmanganese complexes1 present a decreased
electron density of the arene ring coordinated to the M(CO)3
entity and, consequently, a very high electrophilicity, which
found widespread applications in organic as well as in orga-
nometallic synthesis.2 In this context, cationic (η6-arenetricar-
bonyl)Mn(CO)3+ complexes and, in particular, the neutral (η5-
cyclohexadienyl)Mn(CO)3 derivatives formed by nucleophilic
addition to the arene ring have received considerable attention
because of their strategic importance in both fields.3 For
example, treatment of an η5 complex with a nucleophile Nu-
followed by oxidation results in the formation of highly
functionalized cis-disubstituted cyclohexadienes (Scheme 1, path
a),4a whereas its rearomatization occurs upon hydride abstraction
4b-d
by [CPh3][BF4] (Scheme 1, path b).
* To whom correspondence should be addressed. E-mail: francoise.rose@
upmc.fr; eric.rose@upmc.fr.
† Laboratoire de Chimie Organique.
‡ Laboratoire de Chimie The´orique.
(1) (a) McGlinchey, M. J.; Ortin, Y. Seward, C. M. ComprehensiVe
Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.;
Elsevier Science, Ltd.: Oxford, 2006; Vol. 5, p 201. (b) Sweigart, D. A.;
Reingold, J. A.; Son, S. U. ComprehensiVe Organometallic Chemistry III;
Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier Science, Ltd.: Oxford,
2006; Vol. 5, p 761.
(2) (a) McQuillin, F. J.; Parker, D. G.; Stephenson, G. R. Transition
Metal Organometallics for Organic Synthesis; Cambridge University Press:
Cambridge, U.K., 1991. (b) Djukic, J. P.; Rose-Munch, F.; Rose, E.
Organometallics 1995, 14, 2027. (c) Rose-Munch, F.; Gagliardini, V.;
Renard, C.; Rose, E Coord. Chem. ReV. 1998, 249, 178–180. (d) Pape,
A. R.; Kaliappan, K. P.; Ku¨ndig, E. P. Chem. ReV. 2000, 100, 2917. (e)
Giner Planas, J.; Prim, D.; Rose-Munch, F.; Rose, E.; Monchaud, D.; Lacour,
J. Organometallics 2001, 20, 4107. (f) Ku¨ndig, E. P. Topics in Organo-
metallic Chemistry; Springer: Berlin, 2004; Vol. 7. (g) Rosillo, M.;
Dom´ınguez, G.; Pe´rez-Castells, J. Chem. Soc. ReV. 2007, 36, 1589.
(3) For recent reviews, see (a) McDaniel, K. F. ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 6, p 93. (b) Pike, R. D.;
Sweigart, D. A. Coord. Chem. ReV. 1999, 187, 183. (c) Rose-Munch, F.;
Rose, E. Eur. J. Inorg. Chem. 2002, 1269.
(4) See, for example, (a) Roell, B. C.; M; Daniel, K. F.; Vaughan, W. S.;
Macy, T. S Organometallics 1993, 12, 224. (b) Pearson, A. J.; Bruhn, P. R.
J. Org. Chem. 1991, 56, 7092. (c) Pearson, A. J.; Shin, H. Tetrahedron
1992, 48 (36), 7527. (d) Pearson, A. J.; Vickerman, R. J. Tetrahedron Lett.
1998, 39, 5931.
(5) (a) Auffrant, A.; Prim, D.; Rose-Munch, F.; Rose, E.; Schouteeten,
S.; Vaisserman, J. Organometallics 2003, 22, 1898. (b) Prim, D.; Andrioletti,
B.; Rose-Munch, F.; Rose, E.; Couty, F. Tetrahedron 2004, 60, 3325.
(6) (a) Jacques, B.; Chavarot, M.; Rose-Munch, F.; Rose, E. Angew.
Chem., Int. Ed. 2006, 45, 3481. (b) Jacques, B.; Chanaewa, A.; Chavarot-
Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ge´rard, H. Organometallics 2008,
27, 626. (c) Jacques, B.; Eloi, A.; Chavarot-Kerlidou, M.; Rose-Munch,
F.; Rose, E.; Ge´rard, H.; Herson, P. Organometallics 2008, 27, 2505.
(7) Eloi, A.; Rose-Munch, F.; Rose, E.; Herson, P. Organometallics
2006, 25, 4554.
(8) Eloi, A.; Rose-Munch, F.; Rose, E. Unpublished results.
(9) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22 (39), 3815.
10.1021/om800863t CCC: $40.75
2009 American Chemical Society
Publication on Web 01/08/2009

926 Organometallics, Vol. 28, No. 3, 2009
Notes
reported in the literature5a and those of complex 12 whose
synthesis has been already published.5a Indeed it was interesting
to compare, first of all, the proton chemical shifts of the three
regioisomers 2a, 2b, and 2c with those of the unsubstituted
complex 1, and then, the proton chemical shifts of the
regioisomers 4 and 12 with those of the monosubstituted
complex 3.
Scheme 2. Transmetallation Procedure
It is clear that the deshielding effects ∆δ ) δHi(2)-δHi(1)
indicated in brackets can reach impressive values, and the
highest ones correspond to the keto function at the C1 and C3
carbon atoms: 1.75 and 1.51 ppm for 2c and 2a, respectively
(entries 2 and 4), and the lowest one to the keto group at the C2
carbon atom: 1.00 ppm for each proton ꢀ to the ketone of
complex 2b (entries 1 and 3). Again, the same trend is observed
for the second set of complexes (4, 12 and 3): the highest value
is observed for the keto group linked to the C1 carbon atom
(1.06 ppm for 12 compared to 0.50 for 4, entry 4). Thus, the
electronic effects of the keto group on the ꢀ protons depend on
the position of this functionality with respect to the η5 system.
An alternative approach for the synthesis of complexes 4 or
5 is based on the ,Pd. methodology that we applied
successfully to prepare complex 4 (Scheme 3) under the
experimental conditions described for the Stille reaction of (η5-
cyclohexadienyl)Mn(CO)3 complexes.5a The reaction of 10 with
2-thienyltributyltin in the presence of Pd2dba3, AsPh3 as catalyst,
under CO atmosphere delivered complex 4 in 86% yield. This
last yield can be favorably compared with the 91% yield
obtained with the transmetalation method (Scheme 2). Never-
theless, the higher yield of the preparation of complex 3 (90%)13
in comparison with that of 10 (46% yield) and the regioisomer
11 (39% yield) after addition of Grignard reagent to cationic
complex 9, increases the importance of the much more
environmentally friendly transmetalation method to give access
to 4. The procedure described here is particularly valuable to
afford the chloro complexes 7 and 8 because they cannot be
synthesized via the Stille reaction. Indeed, to our knowledge,
no synthesis of the starting dichloro (η5-cyclohexadienyl)Mn-
(CO)3 complex has been reported up to now. Furthermore, the
chlorine atom in these two complexes remains available for
subsequent functionalization.5a Complex 4 was crystallized from
diethyl ether by a two-well diffusion procedure (petroleum ether
in the outer well).14 The ORTEP view of the structure (Figure
1) confirms that the keto function is indeed located at the C3
position adjacent to the methoxy group.
keto group of the final product, this electrophile has been shown
to be efficient for the synthesis of benzoyl-substituted (η6-
arene)Cr(CO)3 complexes.10 Unfortunately, only very low yields
of the expected Mn complexes and low conversion were
observed when using the 2-thienyl Weinreb amide ThCON-
(Me)OMe (Th ) thienyl), for example.8 One of the problems
we had to face was the very high instability of the anions formed
on the η5 system, thus it was impossible to reach a suitable
temperature to react them with acid chlorides without degrada-
tion of these anions or without recovering a significant quantity
of the starting material. Thus, we decided to use an organoman-
ganese route involving the formation of a manganese chloride
intermediate that could react with the acid chloride in the
presence of Fe(acac)3 as the catalyst,11,12 and our results are
reported herein.
Results and Discussion
The lithiation of the three (η5-cyclohexadienyl)Mn(CO)3
complexes 1, 3, and 6 was achieved by reaction with an excess
of n-butyllithium at -78 °C in the presence of tetramethyleth-
ylenediamine (TMEDA). After stirring with MnCl4Li2 for 30
min at the same temperature, the acid chloride (phenyl or thienyl
acid chlorides) and the iron-based catalyst were added. These
reactions give efficient access to the corresponding ketones with
yields ranging from 45 to 91% (Scheme 2).
Without the presence of Fe(acac)3, the yields of keto-
substituted complexes dramatically decreased (for example, only
25% yield in the case of complex 3), and a significant quantity
of the starting material was recovered. This is in a good
agreement with the catalytic role of the iron (III) complex, which
increased the reactivity of the η5 complex intermediate obtained
after lithium-manganese exchange. The regioselectivity is
governed by the control of the formation of the lithiated anion:
The η5 system is represented by five coplanar sp2 carbon
atoms, while the remaining sp3 carbon atom is located 35° above
this plane. The C7-O2 bond of the thienyl group (COTh) is not
in the cyclohexadienyl plane but is deviated by 39° toward the
Mn(CO)3 entity, and the dihedral angle between the [C3, C7,
O2] and [C8, C9, C10, C11] planes is 12°. Whereas the Mn-C3
bond is usually the shortest Mn-cyclohexadienyl bond length,
in the example reported here, the shortest distance is the Mn-C4
bond. The same observation can be made in the case of two
X-ray structures of other η5 complexes substituted by a keto7
or a thio-ester group5a where the shortest distance is also the
one between the Mn atom and the carbon ꢀ to the keto group.
With these two methods in hand (Pd catalysis and transmeta-
6b
in the case of the unsubstituted η5 system (complex 1) two
regioisomers were formed in a 2/1 ratio in favor of the C2
position. When an activating group such as a methoxy (complex
3) or a chloride (complex 6) group is present at C2, only the
regioisomer at the C3 position was isolated.
1
Selected H NMR data of complexes 1, 2a-b, 3, and 4 are
gathered in Table 1 as well as those of complex 2c obtained
via a Pd cross-coupling reaction following the procedure
(10) Eloi, A.; Rose-Munch, F.; Rose, E.; Herson, P. J. Organomet. Chem.
2007, 26, 5727.
(11) (a) Cahiez, G.; Chavant, P. Y.; Metais, E. Tetrahedron Lett. 1992,
33, 36–5245. (b) Cahiez, G.; Alami, M.; Tetrahedron, 1989, 45, 4163. (c)
Cahiez, G. Encyclopedia of Reagents for Organic Synthesis; Paquette, L.,
Ed.; Wiley: Chichester, U.K., 1995; p 3227. (d) Cahiez, G. Encyclopedia
of Reagents for Organic Synthesis; Paquette, L., Ed.; Wiley: Chichester,
U.K., 1995; p 925.
(13) Chung, Y. K.; Williard, P. G.; Sweigart, D. A. Organometallics
1982, 1, 1053.
(14) Crystal data for 4. C21H15MnO5S, M ) 434.34, monoclinic, P21, a
) 9.8801 (13) Å, b ) 15.8666 (15) Å, c ) 12.4541 (9) Å, R (deg) ) 90,
ꢀ (deg)) 98.608 (8), γ (deg) ) 90, V (Å3) ) 1930,.4 (3), Z ) 4, density
F (g cm3) ) 1.494, θ limits (°) ) 2-32, nb of data collected ) 19737, nb
of unique data collected ) 6658, nb of unique data used for refinement
5603(F0)2 > 3σ(F0)2, R(F) ) 0.0334, Rw(F2) ) 0.0294.
(12) Cardellichio, C.; Fiandanese, V.; Marchese, G.; Ronzini, L.
Tetrahedron Lett. 1987, 28, 2053.

Notes
Organometallics, Vol. 28, No. 3, 2009 927
1
Table 1. Selected H NMR Data (δ in ppm) for Complexes 1, 2a-c, 3, 4, and 12 ([Mn] ) Mn(CO)3)
c
d
e
f
g
a In C6D6. b In CDCl3. δHi(2a)-δHi(1). δHi(2b)-δHi(1). δHi(2c)-δHi (1). δHi(4)-δHi(3). δHi(12)-δHi(3).
Scheme 3. Alternative Synthesis of 4
Figure 2. Energies relative to the most stable regioisomer and νCO
lation), it is possible now to introduce a keto function at any of
the carbons of the η5 system.
computed vibrational frequencies (in parenthesis, reported unscaled)
for the three aldehyde (left) or ketone (right) derivatives. The most
stable conformer is used and represented for each regioisomer.
A theoretical study of the influence of the position of the
keto group on the stability and on the reactivity of the
corresponding complexes was undertaken for the three keto-
substituted regioisomers 2a-c. As no effect of the Ph substituent
at C6 was observed in the course of previous theoretical studies
dealing with either lithiation6b or aldehyde formation6c in the
η5 Mn complex series, computations were carried out on the
H-substituted complexes. First, the preferred conformations and
the relative stabilities of the three regioisomers were examined
and compared to those obtained in the case of the analogous
aldehydes.6c Two conformations of the CdO/C-S bonds can
be proposed, s-cis or s-trans, as well as two conformations, noted
Scheme 4. Relative Conformations of CdO/C-S Bonds and
CdO/C6 Carbon Atom
as syn or anti, of the CdO group in the plane of the η5 cycle,
depending on the direction of the CdO bond with respect to
the C6 sp3 carbon (Scheme 4). Whatever the complex, the S-cis
conformer is slightly more stable. This is consistent with the
conformation obtained for the X-ray structures of compound 4
and its regioisomer with the keto function at the C1 carbon atom7
showing that, despite the small energy difference between the
S-cis and S-trans isomers, no crystal effect seems to alter the
gas phase preference. In contrast, the Syn/Anti conformational
preference is regiodependent, and opposite conformation is
obtained compared to the most stable isomer in the case of the
aldehyde analogues, as shown in Figure 2, most probably due
to steric factors.
The most stable conformers are then used to study the
regiopreference of the functionalization. Substitution in position
2 (to form a keto-complex referred to as b) leads to the less
stable regioisomer (by 4 kcal · mol-1), whereas a (keto-complex
substituted in position 3) and c (position 1) are isoenergetic.
The stability of a and c can be related to the participation of the
CdO group within the metal-ligand coordination. Indeed, the
highest occupied molecular orbital (HOMO) exhibits a strong
binding interaction between the metal center and atoms C1, C3,
and C5. As a consequence, substitution by a CdO group on
one of these carbon leads to further delocalization of the HOMO
to the carbonyl, and thus larger stabilization. Despite their
Figure 1. ORTEP view (ellipsoids at 30% probability) of
complex 4. Selected interatomic distances: Mn-C1: 2.231(3);
Mn-C2: 2.211(3); Mn-C3: 2.156(3); Mn-C4: 2.143(4); Mn-C5:
2.240(4).

928 Organometallics, Vol. 28, No. 3, 2009
Notes
is very close to zero (-0.05e with respect to -0.16e in a and
-0.25e in b), and the LUMO exhibits a significant weight on
this atom, in opposition to what is observed for a. As a
consequence, nucleophilic addition should occur cleanly with
full regioselectivity on the C(O) carbon for compounds b and
a. Some 1,4-addition may be observed in the case of c. This in
a good agreement with a relevant experimental result obtained
by reacting NaBH4 (or NaBD4) with (η5-cyclohexadienyl)Mn-
(CO)3 complexes substituted at the C1 carbon atom by the
thienone group:7 addition of the nucleophile mainly occurred
to the keto group yielding the corresponding alcohol derivatives,
but also to the C2 carbon atom, giving rise to the formation of
cyclohexadienes in yields ranging from 13 to 20%.
Figure 3. LUMO isodensity representation for the three regioisomers.
In conclusion, these results have shown that tools used for
functionalization of arenes in organic synthesis can also be
applied in organometallic chemistry for the π system of η5 Mn
complexes. Thus, organomanganese transmetalation is an easy
and complementary method to prepare keto η5 Mn complexes
that cannot be made by Pd cross-coupling reactions. Density
functional theory (DFT) calculations highlighted that, despite
the influence of the RCO position on the strength of conjugation
between the keto group and the cyclohexadienyl ring (evidenced
from computed vibrational frequencies of the CdO bond), the
carbonyl atom remains the favored site for nucleophilic attack,
whatever the position it is branched on. These two strategies
will be of interest for the synthesis of new η5 Mn complexes,
as building blocks for highly functionalized cyclohexadiene
targets.
energetic similarity, a exhibits a νCO vibrational frequency 20
cm-1 higher than c. This is consistent with the proposal that
substitution in position 3 allows less conjugation of the CdO
bond with the π-system than in position 1. This is in a good
agreement with the 1H NMR data, which show a strong
electronic effect of the ketone on the ꢀ proton when the function
is linked to the C1 carbon atom. At the same time, the CdO
bond in c exhibits a smaller tilt angle with respect to the
cyclohexadienyl plane (16°) than in a (23°). This can be linked
to a larger steric hindrance in position C1 than in C3. Competition
between conjugation and steric hindrance results in the small
energy difference between a and c. This is consistent with the
fact that a is 1.5 kcal · mol-1 above c in the case of the aldehyde,
where less steric hindrance is at stake. As a consequence, the
relative stability of keto-complexes substituted in C1 and C3
positions is most probably highly sensitive to the exact nature
of the substituent at the keto-group.
Experimental Section
Complexes 1, 3, and 6 were prepared according to literature
procedures.13 Complex 2c was prepared using Pd coupling
reaction.5a Typical procedure for the transmetalation process
(preparation of 2a, 2b, 4, 5, 7, and 8): A solution of (η5-
cyclohexadienyl)Mn(CO)3 complex (0.80 mmol) and freshly dis-
tilled TMEDA (2.65 mmol, 3.3 equiv) in 8 mL of tetrahydrofuran
(THF) was cooled to -78 °C. A solution of n-BuLi (2.40 mmol, 3
Insight within the reactive properties toward nucleophilic
attack on these ketones is now searched by examining various
reactivity indexes. Charge control is probed by computing the
charges within the framework of the natural population analysis
(NPA)15 or atoms in molecules (AIM)16 approaches, whereas
orbital control is probed by plotting the lowest unoccupied
molecular orbital (LUMO)17 and the nucleophilic Fukui indexes
(F+)18 of the system (Figure 3). Whatever the regioisomer, only
the C(O) carbon bears a positive charge (averaging to 0.55e
for NPA and 1.01e for AIM) and exhibits a significant
nucleophilic Fukui index (0.19 in average). In addition, the
LUMO of all three regioisomers presents the largest coefficient
on the carbonyl carbon. The F+ indexes in positions 1 and 5
are very small (below 0.05) compared to that of the nonsub-
stituted complex (0.17), so that nucleophilic attack on these
positions, as shown in Scheme 1 (path a), is expected to be
deactivated in these species. No clear-cut evidence concerning
the reactivity in the ꢀ position of the keto carbon can be found,
except for compound c. In that case, the negative charge at C2
equiv) was slowly added. After stirring for 1 h at -78 °C,
11b
MnCl4Li2
(2.41 mmol, 3 equiv) was added. The mixture was
stirred for 30 min at -78 °C. A solution of Fe(acac)3 (0.15 mmol,
0.2 equiv) in 2 mL of THF was then added followed by the addition
of acid chloride (3.28 mmol, 4.1 equiv). After stirring for 1 h at
the same temperature, before warming at room tempetrature, the
reaction was hydrolyzed with an aqueous HCl solution (2 mol.L-1).
Usual work up and purification by flash chromatography on silica
gel led to the isolation of the pure keto-substituted η5 complexes.
Acknowledgment. This work was supported by the
CNRS. We thank P. Herson (CIM2, UMR 7071, UPMC Univ
Paris 06) for the X-Ray structure analysis and the Ecole
Normale Supe´rieure (Paris) for financial support to A.E. The
calculations have been performed at the CCRE of the
University Paris 06 and at the CRIHAN (76800 Saint-
Etienne-du-Rouvray, France) regional supercomputing centre.
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735.
(16) Popelier, P. L. A. Atoms in Molecules: An Introduction; Prentice-
Hall: Harlow, U.K., 2000.
Supporting Information Available: Spectroscopic data, 1H
NMR and 13C NMR spectra for all new complexes, computational
details as well as text giving X-ray crystallographic data, and the
CIF files for complex 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) (a) Semmelhack, M. F.; Garcia, J. L.; Cortes, D.; Farina, R.; Hong,
R.; Carpenter, B. K. Organometallics 1983, 2, 467. (b) Eisenstein, O.; Butler,
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OM800863T