Stoichiometric homo-aldol coupling of acetone, cyclohexanone, and acetophenone by the [(η5-Cp(R))Co] (R = Me5, 1,2,4-tri-tert-butyl) moiety

Article abstract of DOI:10.1002/(SICI)1099-0682(200004)2000:4<713::AID-EJIC713>3.0.CO;2-A

Acetone, cyclohexanone, and acetophenone react with the toluene-bridged triple-decker complexes [{(η⁵-Cp(R))Co}₂-μ-(η⁴:η⁴-toluene)] (R = Me₅ 3; 1,2,4-tri-tert-butyl 4) by toluene substitution and homo-aldol coupling of two ketone molecules to form the first oxadiene cobalt complexes. These reactions may involve a C(sp3)-H activation step of the ketones by 14 e^- [(η⁵-Cp(R))Co] fragments generated from the triple-decker complexes 3 and 4. If no acidic protons are present in the position α to the carbonyl carbon position in the ketone, no C(sp3)-H activation occurs. In agreement with this, benzophenone does not react with complex 3.

Full text of DOI:10.1002/(SICI)1099-0682(200004)2000:4<713::AID-EJIC713>3.0.CO;2-A

FULL PAPER
Stoichiometric Homo-Aldol Coupling of Acetone, Cyclohexanone, and
Acetophenone by the [(η5-CpR)Co] (R ؍ Me5, 1,2,4-tri-tert-butyl) Moiety
Jörg J. Schneider,*[a] Dirk Wolf,[a] Dieter Bläser,[a] and Roland Boese[a]
Keywords: Cobalt / Sandwich complexes / Aldol reactions / Ketones / Csp3ϪH activation
Acetone, cyclohexanone, and acetophenone react with the
toluene-bridged triple-decker complexes [{(η5-CpR)Co}2-µ-
(η4:η4-toluene)] (R = Me5 3; 1,2,4-tri-tert-butyl 4) by toluene
14 e– [(η5-CpR)Co] fragments generated from the triple-
decker complexes 3 and 4. If no acidic protons are present in
the position α to the carbonyl carbon position in the ketone,
substitution and homo-aldol coupling of two ketone molec- no Csp3–H activation occurs. In agreement with this, benzo-
ules to form the first oxadiene cobalt complexes. These reac- phenone does not react with complex 3.
tions may involve a Csp3–H activation step of the ketones by
Introduction
During the last decade or so it has been well established,
both spectroscopically and crystallographically, that car-
bonyl compounds are able to coordinate to transition metal
fragments and, consequently, metal-mediated reactions,
which are stoichiometric or even catalytic in nature, have
been studied extensively.[1] The aldol reaction is generally
regarded as one of the most powerful carbon–carbon bond
forming reactions. The primary metal coordination site is
at the CϭO group, resulting in three different bonding situ-
ations A–C towards the transition metal center (Figure 1).[2]
Recent results by Brookhart, who observed selective coor-
dination of phthalaldehyde over acetone with the
Scheme 1
Herein we report on our studies on the synthesis and
structure of cobalt-oxadiene complexes which are formed
via a stoichiometric homo-aldol coupling reaction of acet-
one, cyclohexanone, and acetophenone mediated by 14-e
[(η5-CpR)Co] fragments.
bis({trimethylsilyl}alkene)(η5-Me5C5)Co complex
1 are
striking.[3a] The bis-aldehyde complex 2, which simultan-
eously displays coordination modes A and B is formed as
the exclusive product from the activation of phthalaldehyde
even though acetone was used as the reaction solvent
(Scheme 1).[3a] Other examples of similar aldehyde over ke-
tone reactivity are documented for the cobalt complex 1 by
the same authors.[3b][3c]
Results and Discussion
Reaction of [{(η5-Me5C5)Co}2-µ-(η4:η4-toluene)] (3) and
[{(η5–1,2,4-tri-tert-butyl-C5H2)Co}2-µ-(η4:η4-toluene)] (4) with
Ketones
Reaction of the triple-decker compound [{(η5-Me5C5)Co}2-
µ-(η4:η4-toluene)] (3) either in neat acetone as solvent or with
a tenfold excess of acetone in ether results in the formation of
the s-cis-oxadiene cobalt complex 5 (Scheme 2). No s-trans
regioisomer was observed. The question of whether the prod-
uct selectively adopts one of the regioisomers (Figure 2) is of
interest since the two arrangements expose different faces of
the α, β-unsaturated carbonyl moiety which may lead to dif-
ferent enantioselective additions to this complex.
The reaction of 3 with acetone proceeds stoichiometrically
and no free 2-oxo-4-methylpent-3-ene was observed as the or-
ganic aldol coupling product resulting from a catalytic reac-
tion sequence. The η2-keto coordination is characterized by a
significant 13C high-field coordination shift of the carbonyl C-
atom resonance of ∆δ ϭ –72 ppm relative to free acetone.
Figure 1. σ- and π-coordination modes of a transition metal frag-
ment M to a ketone; R1,2 ϭ H or alkyl
[a]
Institut für Anorganische Chemie der Universität/GH-Essen,
Universitätsstrasse 5–7, 45117 Essen
Fax: (internat.) ϩ49 (0)201/1834195
E-mail: joerg.schneider@uni-essen.de
Eur. J. Inorg. Chem. 2000, 713Ϫ718
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ1948/00/0404Ϫ0713 $ 17.50ϩ.50/0
713

J. J. Schneider, D. Wolf, D. Bläser, R. Boese
FULL PAPER
Scheme 2. General reaction scheme for the formation of Co-oxadiene complexes 5–8 via homo-aldol coupling of acetone, cyclohexanone
and acetophenone. The characteristic η4-oxadiene unit of 5–8 is depicted in bold.
within 12 hours to give the oxadiene complex [{(η5-
tBu3C5H2)Co(methyl-2-oxo-pent-3-ene)}] (8) whose struc-
1
ture is confirmed by its mass, IR and H- and 13C-NMR
spectra (see Experimental Section) (Scheme 2). In 8, as well
as in the other oxadiene Co complexes 5–7, the ν(CϭO)
stretching frequencies of the complexed η2-carbonyl moiet-
ies are unfortunately obscured by the strong intensity of the
Figure 2. s-cis versus s-trans arrangement of a metal-oxadiene unit
stretching frequencies of the Me and 1,2,4-tert-butyl res-
idues of the Cp ring ligands.
Molecular Structure of 5
An X-ray crystal structure determination[6] (Figure 3) of
5 reveals the s-cis-arrangement of the oxadiene moiety in
agreement with the solution NMR results. The C1–O1 dis-
˚
tance [1.310(5) A] of the π-coordinated carbonyl group is
in the same range as for the η2-coordinated CϭO group of
the dialdehyde complex 1[3a] as well as for other η2 bound
ketones.[1,7] This suggests a significant amount of π-back-
bonding from the CoI d-orbitals into the π* orbitals of the
A tenfold excess of acetophenone in diethyl ether reacts
˚
carbonyl C atom. The Co–O [1.985(3) A] and the Co1–C2
with 3 within 30 minutes to give the oxadiene cobalt com-
˚
plexes
[{(η5-Me5C5)Co(η4-(E,Z)-3-methyl-1-oxo-1,3-di-
[1.994(4) A] distances are in agreement with a symmetrical
phenylbut-2-ene)} (6) as a mixture of cis and trans isomers
(ratio 2:3) (Scheme 2). As in the case of 5, a significant
high-field coordination shift of ∆δ ϭ –49.2 ppm for the
resonance of C1 characterizes the η2-keto coordination of
the [(η5-Me5C5)Co] fragment of 6. The complete set of 13C-
NMR signals for both isomers is not observed probably just
due to a low concentration of the sample studied.
When 3 is dissolved in neat cyclohexanone a reaction oc-
curred within two days as judged from the color change
from brown red to green black. The oxadiene cobalt com-
plex
[(η5-Me5C5)Co(η4-2-cyclohexylidenecyclohexane-1-
1
one)] 7 is formed in moderate yields. (Scheme 2). H- and
13C-NMR spectroscopy shows the formation of only the s-
cis isomer (assignment via 1D- and 2D-NMR techniques).
In the 13C-NMR spectrum the η2-keto coordination is char-
acterized by a coordination shift of ∆δ ϭ –75 ppm for the
resonance of C1 relative to free cyclohexanone.
To study the influence of the steric bulk of the [(CpR)Co]
fragment on the homo-aldol coupling reaction we studied
the reaction of acetone with the triple-decker complex 4.
The 1,2,4 tri-tert-butyl Cp ligand has already proven its ver-
satility in kinetically stabilizing unusual metal coordination
environments.[4,5] When 4 is dissolved together with a ten-
Figure 3. Molecular structure of 5 in the solid state; selected bond
˚
length [A] and angles [°]: C(2)–O(1) 1.310(5),C(2)–C(3) 1.396(6),
C(3)–C(4) 1.437(6),C(1)–C(2) 1.497(6), C(4)–C(5) 1.503(7), C(4)–
C(6) 1529(6), Co(1)–C(2) 1.994(4), Co(1)–O(1) 1.985(3), Co(1)–
C(3) 1.985(4), Co(1)–C(4) 2.041(5) O(1)–C(2)–C(3) 116.9(4), C(2)–
fold excess of acetone in diethyl ether the reaction proceeds C(3)–C(4) 119.0(4)
714
Eur. J. Inorg. Chem. 2000, 713Ϫ718

Stoichiometric Homo-Aldol Coupling
FULL PAPER
η2-bonding of the [Co(η5-C5Me5)] fragment to the keto
group. The atoms C2, C3, C4, and O1 define an almost
˚
ideal plane. The bond length deviation Co–C2 [1.994(4) A],
˚
˚
Co1–C3 [1.985(4) A], Co1–C4 [2.041(5) A], and Co1–O
˚
[1.985(3) A] is within experimental error, thus the bonding
can be regarded as symmetrical η4. However, the distance
˚
C3–C4 [1.437(6) A] within the planar oxadiene unit is signi-
ficantly longer than C2–C3 [1.396(6) A]. This indicates an
˚
intermediate bonding situation between a pure π-coordina-
tion mode A and a metallacyclic type coordination B for 5
in the solid state (Figure 4). A and B represent the two ex-
tremes of coordination for an M(L)x fragment to a oxadi-
ene ligand system.
Scheme 3. Solid state versus solution equilibrium of the triple-
decker complexes 3 and 4[10]
hydroacylation of olefins to form ketones at RhI centers
studied by Bosnich.[12] However, such reactivity is usually
not in the realm of first row transition metal chemistry and
only a few cases are reported for organo cobalt complexes
but to the best of our knowledge none so far for a Csp3–H
bond activation mediated by Co fragments. Extensive stud-
ies by Bergman[13], Jones,[14] and Graham[15] have estab-
lished that 16 e species of the type [(η5-R5C5)ML] (M ϭ
Rh, Ir) generated in situ, undergo facile oxidative addition
reactions with the C–H bonds of saturated hydrocarbons
and arenes. For Co, Brookhart[3a,16] demonstrated facile ox-
idative addition of aldehydes via the 16 e [(η5-Me5C5)CoL]
fragment. However, it is noteworthy with respect to our
work that no activation of the less reactive acetone used as
solvent is reported by these authors for the [(η5-CpR)CoL]
fragment which is derived from 1. Wadepohl[17] observed
facile Csp2–H activation processes of cycloalkenes in the
course of the formation of polynuclear [(η5-CpR)Co]-based
clusters which are derived from the Jonas complex [(η5-
Cp)Co(ethene)2]. Crucial for the ability of Co centers to
participate in C–H-bond activation processes is the genera-
tion of reactive sites, especially the formation of either coor-
Figure 4. π-Metal coordination A versus σ-metallacycle formation
B in an oxadiene complex
Discussion
Compounds 5–8 belong to a group of organometallic ox-
adiene complexes of which [(4-phenylbut-3-ene-2-one)-
Fe(CO)3] (9) represents the first example.[8] This and other
enone complexes of a similar bonding type are accessible
by standard complexation procedures.[9] In contrast, 5–8 are
formed via a Co-mediated aldol-type coupling initiated by
reactive 14 e [(η5-CpR)Co] (R ϭ Me5, 1,2,4-tri-tert-butyl)
fragments. Their formation from the triple-decker com-
plexes 3 and 4 was verified by detailed mechanistic stud-
ies.[5,10] Complexes 3 and 4 are valuable sources of either
one or two [(η5-CpR)Co] fragments. The decay of 3 and 4
into [(η5-CpR)Co] (R ϭ Me5, 1,2,4-tri-tert-butyl) fragments
(Scheme 3) proceeds in any common donor solvent and is
especially fast in the presence of ketones (e.g. acetone).
dinatively unsaturated, electron deficient 16
e
[(η5-
CpR)Co(Lx)] or even 14 e [(η5-CpR)Co] fragments.[18] C–H
activation is assumed to be a significant step in the homo-
aldol coupling reaction of the ketones under study herein.
This is based on results obtained from the reaction of
benzophenone with 3. Benzophenone contains no acidic
protons at the α-carbon position, thus no homo-aldol coup-
ling can be observed with benzophenone which is recovered
unchanged from a reaction with 3. A possible reaction
pathway invoking Csp3–H bond activation of acetone, cyclo-
hexanone and acetophenone as crucial reaction step, lead-
ing to the oxadiene cobalt complex 5–8 is outlined in
Scheme 4 exemplified for the reaction of 3 and acetone.
Reactive 14 e [(η5-Me5C5)Co]solv fragments are generated
There has been quite intense research on the C–H bond
activation by various transition metal complexes especially from 3. These can first coordinate the keto group of acetone
of the iron group triad which may be followed by func- followed by an activation of the hydrogen atoms of the
tionalization of the activated substrate.[11] A process which neighboring α-methyl group giving intermediate A. Reac-
combines C–H and C–C coupling reactions is the catalytic tion of intermediate A with the ‘‘acidic’’ carbonyl carbon
Eur. J. Inorg. Chem. 2000, 713Ϫ718
715

J. J. Schneider, D. Wolf, D. Bläser, R. Boese
FULL PAPER
Scheme 4. Proposed reaction scheme for the Co-mediated homo-aldol coupling of acetone
atom of another acetone molecule forms (ii), B results in a
C–C coupling, leading to intermediate C (iii). Finally C can
eliminate water (v), D to yield complex 5. This elimination
of water parallels the classical base catalyzed aldol type
coupling reaction.[19]
Conclusions
Experimental Section
Our findings presented here indicate a pronounced react-
ivity enhancement of the arene triple-decker type complexes Compounds 3[21] and 4[5] were prepared according to published
procedures. All ketones were commercial products (purity 99%),
they were distilled prior to use and stored under nitrogen in the
dark. – The NMR spectra were recorded on a Bruker AC 300 spec-
trometer (300 MHz for 1H, 75 MHz for 13C) and referenced against
the residual protons of the deuterated solvent. – MS spectra were
recorded on a MAT 8200 instrument under standard conditions
(EI, 70 eV) and the fractional sublimation technique for compound
inlet. – Tvap gives the individual vaporization temperature of the
sublimation process during the MS experiment.
3 and 4 over half sandwich complexes of type 10, which in
contrast to 3 and 4 do not show similar carbonyl coupling
reactions and not even a reactivity towards acetone under
similar conditions. This points to the fact that 14 e [(η5-
CpR)Co] fragments derived from the triple-deckers 3 and 4
in contrast to 16 e [(η5-CpR)CoL] derived from 10 are able
to activate ketones at their Csp3-α-carbon atom under mild
conditions. This is to the best of our knowledge the first
Csp3–H bond activation of ketones by an organocobalt
compound leading to a C–C coupling under neutral and
ambient conditions. It remains to be studied if this coupling
proceeds regio- and/or stereospecifically with other ketones
and if it can be tuned from stoichiometric to catalytic react-
ivity. Such investigations may open up new synthetic routes
with respect to cobalt mediated cycloaddition reactions of-
fering access to complex organic products as in the case of
oxadiene complexes of Mo and W.[20]
[(η5-Me5C5)Co(η4-4-methyl-2-oxopent-3-ene)] (5): Complex
3
(0.41 g, 0.85 mmol) was dissolved together with acetone (0.5 g,
8.6 mmol) in 30 mL of diethyl ether. Continued stirring for 24
hours resulted in a color change from red brown to greenish brown.
Removal of all volatiles under vacuum gave a semi-solid residue
which was dissolved in a minimum amount of pentane. Cooling
to –78 °C afforded 5 (0.38 g, 1.31 mmol, 77%) as brown material.
The same material resulted when the reaction was performed in
acetone only. – MS (EI, 70 eV, Tvap 250 °C); m/z (%): 292 (100)
716
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Stoichiometric Homo-Aldol Coupling
FULL PAPER
[Mϩ], 277 (3) [Mϩ – CH3], 249 (20) [Mϩ – CH3CO], 209 (13) [Mϩ
–
(CH3)2CϭCHCO], 192 (70) [(Me5C5)Co]. – 1H NMR ([D6]ben-
zene, 300 MHz, 27 °C): δ ϭ 0.95 (s, 3 H), 1.17 (s, 3 H), 1.50 (s, 15
H), 2.13 (s, 3 H), 4.03 (s, 1 H). – 13C{1H} NMR ([D6]benzene,
75.1 MHz, 27 °C): δ ϭ 9.4 (C7), 20.9 [C6(5)], 21.8 [C5(6)], 28.2
(C1), 77.9 (C3), 89.1 (C8), 127.2 (C4), 134.9 (C2). – IR (KBr): ν˜ ϭ
1473, 1448, 1431, 1385, 1351, 1364 (all vs, Me5C5), 1236, 1155,
1140, 1065, 1027, 911, 862, 602 (all vs) cm–1. – C16H25CoO
(292.29): calcd. C 65.74, H 8.62, Co 20.16; found C 65.85, H 8.69,
Co 20.22.
[{(η5-tBu3H2C5)Co(4-methyl-2-oxopent-3-ene)}] (8): Synthetic pro-
cedure fas described for 5; crystallization from pentane at –78 °C
affords 8 as a brown solid. – MS (EI, 70 eV, Tvap ϭ 120 °C); m/z
(%): 390 (32) [Mϩ], 333 (100) [Mϩ – tBu], 57 (33) [tBu]. – 1H NMR
([D6]benzene, 300 MHz, 27 °C): δ ϭ 0.99 [s, 9 H, C11], 1.08 [s, 3
H, C5(6)], 1.18 [s, 3 H, C6(5)], 1.44 [s, 9 H, C7(9)], 1.77 [s, 9 H,
C9(7)], 2.29 [s, 3 H, C1], 3.39 [s, 1 H, C13(14)], 4.14 [s, 1 H,
C14(13)], 4.69 [s, 1 H, C3], in addition signals due to [{(η5-
tBu3H2C5)2Co][22] were present in a 1:2 ratio. – 13C{1H} NMR
([D6]benzene, 75.1 MHz, 27 °C): δ ϭ 22.2 [C5(6)], 22.7 [C6(5)], 31.3
[C7(9,11)], 31.4 (C1), 32.7 [C9(7,11)], 33.2, 33.4, 35.1 [C11(7,9)],
76.3, 78.1, 79.21, 148.50 (CO). – IR (KBr): ν˜ ϭ 1486, 1458, 1391,
1242 (all vs, all tBu), 1199, 1163, 1020, 990, 824, 668 (all m)
cm–1. – No satisfactory elemental analysis could be obtained due
to trace amounts of [{(η5-tBu3C5)2Co][22] already contained in the
starting material 4.
[{(η5-Me5C5)Co(η4-(E,Z)-3-methyl-1-oxo-1,3-diphenylbut-2-ene)}]
(6): Synthetic procedure as described for 5; crystallization from di-
ethyl ether/pentane (2:1) gives 6 (0.28 g, 0.68 mmol, 63%) as a mix-
ture of its (E)- (63%) and (Z)-isomers (37%). – MS (EI, 70 eV, Tvap
:
100 °C); m/z (%): 416 (100) [Mϩ], 311 (3) [Mϩ – C6H5CO], 281
(55) [Mϩ – Me5C5], 192 10 [(Me5C5)Co]. – 1H NMR ([D6]benzene,
300 MHz, 27 °C): δ ϭ 1.23 [s, 15 H, (Z)], 1.42 [s, 15 H, (E)], 1.54
[s, 3 H, (E)], 1.57 [s, 3 H, C4 (Z)], 4.99 [s, 1 H, (E)], 6.11 [s, 1 H,
(Z)], 7.17 [m, arenes (E ϩ Z)], 7.19 [m, arenes (E ϩ Z)], 7.31 [m,
arenes (E ϩ Z)], 7.39 [m, 3 H, C12–C14 (E)], 7.41 [m, 3 H, C12–
C14 (Z)], 8.16 [m, 2 H, C11, C15 (E)], 8.27 [m, 2 H, C11, C15
(Z)]. – 13C{1H} NMR ([D6]benzene, 75.1 MHz, 27 °C): δ ϭ 8.6
[C17 (Z)], 9.0 [C17 (E)], 17.1 [C4 (Z)], 27.40 [C4 (E)], 71.23 [C2
(Z)], 71.8 [C2 (E)], 89.6 [C18 (E)], 89.7 [C18 (Z)], 124.5, 124.5,
126.0, 126.7, 127.0, 127.3, 128.6, 129.2, 146.7 (C1). – IR (KBr):
ν˜ ϭ 3055, 3034, 1540, 1488, 762, 695 (all strong, all phenyl), 1488,
1449, 1072, 1022 (all vs, all Me5C5) cm–1. – C26H29CoO (416.42):
calcd. C 74.99, H 7.02; found C 74.36, H 7.02.
Reaction of 3 with Benzophenone: Complex 3 (0.31 g, 0.64 mmol)
and benzophenone (0.46 g, 2.52 mmol) were dissolved in 30 mL of
diethyl ether and stirred for 48 h. Although the color of the solu-
tion changed during this time from red-brown to orange-brown,
removal of all volatiles gave mainly unchanged benzophenone.
However, 3 was no longer present.
Acknowledgments
This work was supported by the DFG (Heisenberg fellowship to J.
J. S. and grant SCHN 375/6-1) and the Fonds der Chemischen In-
dustrie. We thank Dr. D. Stöckigt and Dr. K. Seevogel and co-
workers (MPI für Kohlenforschung, Mülheim) for recording MS
and IR spectra.
[{(η5-Me5C5)Co(η4-2-cyclohexylidenecyclohexan-1-one)}] (7): Syn-
thetic procedure as described for 5; crystallization from acetonitrile/
ether mixture gives 7 (0.1 g, 0.27 mmol, 22%) as cube-shaped black
crystals. – MS (EI, 70 eV, Tvap 150 °C); m/z (%): 372 (100) [Mϩ],
354 (8) [Mϩ – H2O], 233 (17) [Mϩ – C9H15], 194 (17) [Me5Co]. –
1H NMR ([D6]benzene, 300 MHz, 27 °C): δ ϭ 1.09 (m, 1 H), 1.52
(m, 2 H), 1.58 (m, 2 H), 1.61 [s, 15 H, C13], 1.77 (m, 2 H), 1.90
(m, 2 H), 1.94 (m, 2 H), 2.0 (m, 1 H), 2.14 (m, 1 H), 2.20 (m, 1
H), 2.32 [t (dd), 1 H], 2.45 (m, 1 H), 2.65 (ddd, 1 H). – 13C{1H}
NMR ([D6]benzene, 75.1 MHz, 27 °C): δ ϭ 9.4 (C13), 24.2 (sec.),
24.4 (sec.), 26.3 (sec.), 28.1 (sec.), 28.4 (sec.), 28.6 (sec.), 31.0 (sec.),
31.6 (sec.), 32.1 (sec.), 35.7 (sec.), 63.0 (q, C7), 85.5 (q, C2), 88.7
(q, C14), 134.5 (q, C1). – C22H33CoO (372.41): calcd. C 70.95, H
8.93, Co 20.16; found C 71.03, H 9.05.
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