Zirconium enolate radical cations in solution - Characterisation and kinetics of their mesolytic Zr-O bond cleavage

Article abstract of DOI:10.1039/a808400i

Six zirconocene enolate radical cations and one hydroquinone biszirconocene radical cation have been generated and characterised in solution by standard and fast scan cyclic voltammetry. The electroanalytical results along with those from product analysis of preparative one-electron oxidations indicate that the sterically congested radical cations undergo a mesolytic Zr-O bond cleavage process. The kinetics of the mesolytic Zr-O bond cleavage yielding zirconocene cations and the lifetime (t_1/2) of the zirconium enolate radical cations in dichloromethane at room temperature are determined.

Full text of DOI:10.1039/a808400i

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Zirconium enolate radical cations in solution—characterisation and
kinetics of their mesolytic Zr–O bond cleavage†
Michael Schmittel* and Rolf Söllner
Institut für Organische Chemie der Universität Würzburg, Am Hubland, D-97074 Würzburg,
Germany
Received (in Cambridge) 29th October 1998, Accepted 16th December 1998
Six zirconocene enolate radical cations and one hydroquinone biszirconocene radical cation have been generated
and characterised in solution by standard and fast scan cyclic voltammetry. The electroanalytical results along with
those from product analysis of preparative one-electron oxidations indicate that the sterically congested radical
cations undergo a mesolytic Zr–O bond cleavage process. The kinetics of the mesolytic Zr–O bond cleavage yielding
₁
zirconocene cations and the lifetime (t ) of the zirconium enolate radical cations in dichloromethane at room
₂
temperature are determined.
tatively by sodium hydride in THF at room temperature.
Introduction
The generated sodium enolates were subsequently converted
Over the last 15 years the use of early transition metals
into the corresponding zirconocene enolates by transmetal-
(titanium, zirconium) has become an established method1 in
lation with stoichiometric amounts of zirconocene dichloride
modern organic synthesis to control enolate reactivity, allow-
(Scheme 1). All chlorine substituted zirconocene enolates
ing for example the improvement of the regioselectivity and
diastereoselectivity of aldol additions.2 Furthermore, highly
enantioselective aldol reactions (with chiral ligands at the metal
atom) have become part of the synthetic repertoire.3 Despite
the importance of these metal enolates little is known
about structure–reactivity relationships. In general, these highly
reactive compounds exhibit a pronounced sensitivity towards
hydrolysis which is reflected in the fact that in synthetic appli-
cations they are generated and converted without isolation.
Therefore, only a small number of titanium4,5 and zirconium
enolates6 have been isolated and investigated with regard to
spectroscopic and/or structural data. To some extent, the hydro-
lytic stability of these compounds can be increased by using a
kinetically non-labile and coordinatively saturated metallocene
Scheme 1
fragment as a stabilising motif. More importantly, however,
Z1–Z3 were obtained in an analytically pure form as bright
yellow crystals after recrystallisation from n-hexane and a
n-hexane–toluene mixture, respectively.
A second route leading to methyl substituted zirconocene
by choosing sterically shielded enolate systems the hydrolytic
stability of the corresponding titanium enolates5 can be
increased immensely, thus facilitating investigations on struc-
ture and reactivity.
Although early transition metal enolates are rather electron-
enolates was realised by treating the corresponding β,β-
dimesitylenols with dimethyl zirconocene9 that was prepared
rich compounds their one-electron oxidation chemistry has
from zirconocene dichloride with methylmagnesium chloride
been overlooked for a long time. Actually, the first reports on
in diethyl ether at room temperature (Scheme 2). The methyl-
ated zirconocene enolates ZM1–ZM3 could be obtained in
analytically pure form as pale yellow solids. The analogous
reaction was used for the preparation of the hydroquinone
based methyl substituted biszirconocene ZM4.
The preparative scale one-electron oxidations of the zir-
conium enolates were performed in an acetonitrile–dichloro-
titanium enolate radical cations in solution generated by one-
electron oxidation from their neutral precursors have become
available just recently from our laboratory.7 In the present
investigation8 we make use again of sterically shielded enolates
in order to prepare isolable zirconium enolates and to examine
for the first time the properties and reactions of zirconium eno-
late radical cations. In addition, a hydroquinone biszirconocene
was synthesised and subjected to one-electron oxidation.
methane mixture by using the one-electron oxidant tris(1,10-
₁
phenanthroline)iron() hexafluorophosphate (Fephen, E =
₂
ϩ 0.69 VFc) in the case of the chlorine substituted zirconocene
enolates Z1–Z3 and nitrosonium hexafluoroantimonate
Results
₁
(NOSbF6, E = ϩ0.89 VFc) in the case of the methyl zirconocene
₂
We started our investigation with the preparation of sterically
shielded zirconium enolates. The first strategy leading to the
chlorine substituted zirconocene enolates started from simple
stable β,β-dimesitylenols which were deprotonated quanti-
enolates ZM1–ZM3. For all compounds the corresponding
benzofuran derivatives B1–B3 were formed in good to excellent
yields (cf. Scheme 3).
We started the electroanalytical investigation with the
determination of the oxidation potentials of these compounds.
All zirconocene enolates exhibit irreversible oxidation waves
† Electroactive Protecting Groups and Reaction Units. Part 8. For Part
7 see ref. 7c.
(Epa; cf. Table 1) in acetonitrile at 100 mV sϪ1 in cyclic voltam-
J. Chem. Soc., Perkin Trans. 2, 1999, 515–520
515

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Fig. 1 a) Multisweep CV of Z3 in CH3CN at ν = 500 mV sϪ1; b) multisweep CV of ZM2 in CH2Cl2 at ν = 500 mV sϪ1
.
Table 1 Oxidation potentials of zirconocene enolates determined by
CV
Zirconocene
enolate
ox/VFc
₁
Epa/VFc
E
₂
Z1
ϩ0.45a
ϩ0.57b
ϩ0.43b
ϩ0.44b
ϩ0.46b
ϩ0.35b
ϩ0.36b
ϩ0.30b
Z2
ϩ0.32a
ϩ0.41a
ϩ0.51a
ϩ0.30a
ϩ0.35a
ϩ0.30c
Z3
ZM1
ZM2
ZM3
ZM4
a In acetonitrile at ν = 100 mV sϪ1
.
b In dichloromethane at ν = 1000 V
sϪ1
.
c In dichloromethane at ν = 100 mV sϪ1
.
Table 2 Kinetics of the mesolytic Zr–O bond cleavage (kf) and the
₁
lifetime (t ) of the zirconium enolate radical cations in dichloromethane
₂
at room temperature
Scheme 2
Z2
Z3
ZM1
ZM2
ZM4
kf/sϪ1 3.3 × 102
t /s
₂
3.1 × 102
8.3 × 102
5.0 × 102
3.2 × 102
2.1 × 10Ϫ3 2.2 × 10Ϫ3 8.4 × 10Ϫ4 1.4 × 10Ϫ3 2.2 × 10Ϫ3
₁
chemical and/or anodic oxidation is possible in the presence of
many other common functional groups.
At higher anodic potentials all zirconium enolates exhibit
two additional oxidation waves in standard CV investigations in
acetonitrile as well as in dichloromethane (Fig. 1). These waves
can readily be ascribed to the oxidation of the corresponding
enols (wave II) and benzofurans (wave III) by comparison of
the observed oxidation potentials with data obtained from
authentic samples.26b In multisweep CV experiments the current
for wave I decreases rapidly with an increasing number of cycles
whereas the currents for waves II and III increase relative to
that for wave I. These findings reveal that on the time scale of
the CV experiment zirconium enolates Z1–Z3 and ZM1–ZM3
are converted into the corresponding enols and into the benzo-
furan derivatives B1–B3.
As already mentioned above, partially reversible waves of all
compounds could be recorded by using fast scan cyclic vol-
tammetry, which allowed determination of the kinetics of the
follow-up reaction (mesolytic12 Zr–O bond scission) of some of
the investigated zirconocene enolate radical cations. The kinetic
analysis was undertaken by relating the ratio of the cathodic to
anodic peak current Ipc/Ipa to the kinetic parameter kt accord-
ing to the method of Nicholson and Shain13 by applying a
working curve14 for an ECirrE mechanism (electron transfer/
irreversible chemical reaction/electron transfer). The rate
constants for the Zr–O bond cleavage process and the related
lifetimes of the corresponding radical cations are depicted in
Table 2.
Scheme 3
metry (CV) experiments. Likewise in dichloromethane, irrevers-
ible waves were obtained at the usual scan rates. However, upon
increase of the scan rates by using fast scan cyclic voltammetry
at ultramicroelectrodes, fully or at least partially reversible
waves were recorded: partially reversible waves showed up for
Z1–Z3 at scan rates ν ≥ 100 V sϪ1, for ZM1 at ν ≥ 200 V sϪ1, for
ZM2 at ν ≥ 100 V sϪ1, for ZM3 at ν ≥ 20 V sϪ1 and for ZM4 at
ν ≥ 100 V sϪ1. As a consequence, the above electroanalytical
investigations constitute the first evidence for the existence of
zirconium enolate and zirconium phenolate radical cations in
solution. These measurements allowed us to determine the
ox
₁
thermodynamically relevant redox potentials E (cf. Table
₂
1), which range between ϩ0.30 to ϩ0.57 V vs. ferrocene/
ox
₁
ferrocenium (VFc). The E values are about 100 mV more
₂
anodic than those of the structurally analogous titanium enol-
ates,7 an effect which is already known for other titanocene and
zirconocene compounds.11 Depite this fact the redox potentials
of Z1–Z3, ZM1–ZM4 are still sufficiently low so that their
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J. Chem. Soc., Perkin Trans. 2, 1999, 515–520

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C1–C3 ϩ eϪ
Cp2Zr()Yϩ ϩ eϪ
R1–R3
Discussion
The key intermediate in oxidative benzofuran formation start-
ing from simple stable enols15 and various enol derivatives16
(including titanium enolates7) is the α-carbonyl cation C1–C3
which undergoes a Nazarov type cyclisation, [1,2]-methyl shift
and deprotonation to B1–B3. This species is generated from the
Cp2Zr()Y Y = Me, Cl
The potentials of the corresponding α-carbonyl radicals R1–
R3 are known to be in the range Epa = ϩ0.15 to ϩ0.36 VFc.15a
Unfortunately, the oxidation potentials of the zirconocene()
species Cp2Zr()Me and Cp2Zr()Cl have not been deter-
mined yet. Therefore, we prepared the zirconocene cation
[Cp2Zr()Me(MeCN)2][BPh4]17 which reveals
a reduction
potential of Epc = Ϫ1.94 VFc (at ν = 100 mV sϪ1 in CH3CN, sup-
porting electrolyte: NaBPh4). Although this is an irreversible
peak potential this value should be a sufficiently good approxi-
mation for the oxidation potential of Cp2Zr()Me. As the
anodic peak potentials of the α-carbonyl radicals are about 2 V
more anodic it is a safe statement that methyl zirconocene eno-
late radical cations cleave according to the heterolytic pathway
since that pathway is by about 110 kJ molϪ1 (derived from the
difference of the oxidation potentials between the concerned
redox active species) more favorable than the homolytic variant.
This finding is in agreement with results by Jordan, who carried
out one-electron oxidations of dimethyl zirconocene and other
dialkyl zirconocenes with stoichiometric amounts of ferro-
cenium and isolated the cationic alkyl zirconocene complexes
[Cp2ZrR]ϩ.18 The oxidation potential of Cp2Zr()Cl, which is
not known yet, was estimated as follows: the difference between
the thermodynamic E of Cp2Zr()Meϩ and Cp2Zr()Clϩ
₁
₂
should be smaller19 than the difference between the half-wave
₁
potentials of Cp2Zr()Me2 (E = Ϫ3.06 VFc) and Cp2Zr()-
₂
₁
Cl2 (E = Ϫ2.04 VFc). As we could determine the cathodic
₂
peak potential of Cp2Zr()Meϩ to Epc = Ϫ1.94 VFc a rough
approximation for the reduction potential Epc of Cp2Zr()Clϩ
provides a value more cathodic than Ϫ0.9 VFc. As a con-
sequence, oxidation of Cp2Zr()Cl occurs at a significantly
more cathodic potential (>1 V) than that of the α-carbonyl
radicals. Therefore, we suppose that equally the mesolytic bond
cleavage of the chloro zirconocene enolate radical cations
follows the heterolytic fragmentation pathway (according to
Mechanism A).
If the exact mechanism of the mesolytic bond cleavage is
known, the kinetics of this process can be determined using CV
investigations. However, the mechanistic situation is much more
complicated than displayed in Scheme 4. The proton split off
during benzofuran formation is able to hydrolyse the neutral
zirconium enolate on the time scale of the CV experiment.
Accordingly, the oxidation waves of the corresponding enols
can always be observed besides the waves of the zirconium enol-
ates in cyclic voltammograms (Fig. 1) except for completely
reversible oxidation waves of the zirconium enolates. As a con-
sequence, an additional pathway may open up for the zirconium
enolate radical cation, which is endergonic electron transfer
reduction of the zirconium enolate radical cation by the neutral
enol E1–E3 (Epa = 0.61 Ϫ 0.67 VFc).15a As enol radical cations
are known to deprotonate fairly rapidly, however, the ender-
gonic equilbrium can be driven to the right hand side (see
Scheme 5).
For this reason we investigated the effect of the electron
transfer equilibrium on the kinetics obtained from cyclic
voltammetry experiments. We chose zirconium enolate Z1 as
a model compound because the difference of the oxidation
potentials between Z1 and E1 is the smallest of all the com-
pounds examined. Thus, the effect of the electron transfer equi-
librium should be the largest of all the zirconium enolates Z1–
Z3, ZM1–ZM3. For a detailed understanding, partially revers-
ible oxidation waves for Z1 were digitally simulated20 based on
the mechanistic scenario as shown in Scheme 5. In addition,
oxidation waves for Z1 were simulated according to a simple
ECirrE mechanism (no effect of the electron transfer equi-
librium). The waves thus obtained were compared directly with
each other. Notably, the differences in reversibility of the waves
Scheme 4
ϩ
above mentioned enol radical cations C᎐C–OX by meso-
᎐
᝽
lytic12 bond cleavage of the O–X bond.
In the case of the investigated zirconocene enolates the Zr–O
bond must have been broken somewhere during this reaction. A
dissociative electron transfer process can be ruled out because
reversible (or at least partially reversible) waves were obtained
for all compounds in cyclic voltammetric experiments. As direct
cyclisation at the stage of the radical cation is highly unlikely
because of steric reasons for all investigated β,β-dimesityl zir-
conium enolates Z1–Z3, ZM1–ZM3, this mechanistic pathway
can be excluded as well. Hence, mechanistic considerations
suggest that in fact the Zr–O bond is cleaved in a mesolytic
fragmentation process which is the primary follow-up reaction
of the radical cations Z1–Z3 ϩ and ZM1–ZM3
.
ϩ
᝽
᝽
As a matter of fact there are two possibilities for how the
mesolytic bond cleavage may take place: on the one hand a
Zr–O bond scission of the radical cation providing an α-carbonyl
radical R1–R3 and a zirconocene() cation according to
Mechanism A (heterolytic variant12) whereas on the other
hand a bond cleavage leading to an α-carbonyl cation C1–C3
and a zirconocene() species (Mechanism B, homolytic vari-
ant12) is conceivable. Both cleavage selectivities are realised in
radical cations of structural analogous enols and their deriva-
tives: homolytic cleavage can be observed for radical cations of
enol esters,16a enol carbonates and enol carbamates16d whereas
the radical cations of enols,15b silyl enol ethers,16b enol phos-
phites and enol phosphinates16c and of the structurally very
closely related titanocene enolates7 follow the heterolytic
variant.
The thermodynamic preference for either Mechanism A or B
could be predicted if the oxidation potentials of the fragments
concerned [α-carbonyl radicals and zirconocene() species] or
alternatively the reduction potentials of the α-carbonyl cations
and Cp2Zr()Y (with Y = Me, Cl) were known.
J. Chem. Soc., Perkin Trans. 2, 1999, 515–520
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Scheme 5 Electron transfer equilibrium between zirconium enolate radical cations and neutral enols.
are extremely small indicating that the influence of the electron
transfer equilibrium on the shape of the oxidation waves of the
zirconium enolate and thus the kinetics of the mesolytic Zr–O
bond cleavage is negligible. Thus, the kinetics of the mesolytic
bond cleavage can be evaluated according to an ECirrE-
basic alumina (ICN); CH2Cl2 was purchased in HPLC quality
from Riedel-de Haën, distilled from P4O10 and filtered through
basic alumina (ICN). Zirconocene dichloride,22 2,2-dimesityl-
ethenol,23 2,2-dimesityl-1-phenylethenol24 and 1,1-dimesityl-
3,3-dimethylbuten-2-ol25 were prepared according to liter-
ature procedures. The supporting electrolyte tetra(n-butyl)-
ammonium hexafluorophosphate (Fluka) was of electro-
mechanism (Table 2).
The kinetics of the follow-up reaction of ZM4 were ana-
ϩ
᝽
1
logously determined according to an ECirrE-mechanism. The
analysis provides a first-order rate constant (kf) for this process
similar to those of the zirconocene enolate radical cations
(Table 2), suggesting a reaction as depicted in Scheme 6
chemical grade and used without further purification. H and
13C NMR spectra were recorded on a Bruker AM 200 spec-
trometer; chemical shifts (in ppm) refer to tetramethylsilane;
owing to the nature of the β,β-dimesityl moiety signals in the
13C NMR spectra are often superimposed. IR spectra were
recorded on a Perkin-Elmer FT-IR 1605. Elemental analyses
were carried out on a Carlo Erba Elemental Analyzer 1106.
Melting points were determined by using a Dupont 910 differ-
ential scanning calorimeter.
Preparation of (2,2-dimesitylethenoxy)zirconocene chloride (Z1)
2,2-Dimesitylethenol (841 mg, 3.00 mmol) was added in small
portions at room temperature to a suspension of sodium
hydride (636 mg, 26.5 mmol) in THF (50 cm3). After stirring for
1 h the excess NaH was filtered off and the filtrate was added
slowly to a solution of zirconocene dichloride (877 mg, 3.00
mmol) in THF (50 cm3) at room temperature. The mixture was
stirred for 12 h then the solvent was evaporated in vacuo. The
residue was extracted with Et2O (100 cm3) and then the extract
was filtered. The filtrate was concentrated to about 10 cm3 and
n-hexane (90 cm3) was added. The mixture was kept for 3 days
at Ϫ40 ЊC affording the crude product which was recrystallised
from n-hexane. Z1 could be obtained as bright yellow crystals
(953 mg, 1.78 mmol, 59%). Mp 118 ЊC (Found: C, 67.03; H,
6.48. C30H33ClOZr requires: C, 67.19; H, 6.20%); νmax(KBr)/
cmϪ1 3092, 2955, 2917, 2849, 1609, 1586, 1555, 1472, 1438,
1374, 1228, 1177, 1092, 1017, 904, 867, 857, 808, 674, 629, 568,
487; δH (200 MHz; CDCl3) 1.6–2.7 (coalescence, 18 H, Mes-
Scheme 6
initiated by mesolytic Zr–O bond cleavage. We are currently
validating the generality of such fragmention reactions as an
entry to the facile generation of zirconocene cations using
other hydroquinone biszirconocenes.
In summary, we have synthesised six zirconium enolates Z1–
Z3, ZM1–ZM3 and one hydroquinone biszirconocene ZM4.
The radical cations generated thereof by one-electron oxidation
were characterised by cyclic voltammetry and fast scan cyclic
voltammetry, which represents the first characterisation of
zirconium enolate and phenolate radical cations in solution.
The sterically congested zirconocene enolate radical cations
undergo a clean mesolytic Zr–O bond scission according to
the heterolytic pathway (Mechanism A) yielding α-carbonyl
radicals and zirconocene() cations. The kinetics of this process
could be determined. Investigations are under way to probe
whether such electron transfer induced bond cleavage reactions
resulting in the formation of zirconocene() cations can be
used to trigger polymerisation of propene and other alkenes.21
CH ), 6.14 (s, 10 H, C H ), 6.73 (s, 1 H, C᎐CH), 6.83 (br s, 4 H,
᎐
3
5
5
Mes-H); δC (50 MHz; CDCl3) 20.5 (Mes-CH3), 20.7 (Mes-CH3),
20.8 (Mes-CH3), 21.0 (Mes-CH3), 112.8 (C2), 114.1 (C5H5),
129.9 (Mes), 128.7 (Mes), 129.5 (Mes), 130.9 (Mes), 132.3
(Mes), 134.4 (Mes), 136.8 (Mes), 137.7 (Mes), 154.0 (C1).
Preparation of (1,1-dimesityl-3,3-dimethylbut-1-en-2-oxy)-
zirconocene chloride (Z2)
To a suspension of sodium hydride (427 mg, 17.8 mmol) in
THF (30 cm3), solid 1,1-dimesityl-3,3-dimethylbut-1-en-2-ol
(1.01 g, 3.00 mmol) was added in small portions at room tem-
perature. The mixture was stirred for 1 h and the excess NaH
was filtered off. The filtrate was added slowly to a solution of
zirconocene dichloride (877 mg, 3.00 mmol) in THF (30 cm3)
and stirred for 1 h. The solvent was removed under reduced
pressure and the residue extracted twice with boiling n-hexane
(200 cm3). After storing the filtrate at Ϫ40 ЊC Z2 was obtained
as bright yellow crystals (1.15 g, 1.94 mmol, 65%). Mp 127 ЊC
(Found: C, 68.68; H, 7.25. C34H41ClOZr requires C, 68.94; H,
6.98%); νmax(KBr)/cmϪ1 2990, 2950, 2920, 2861, 1608, 1546,
1477, 1442, 1393, 1374, 1265, 1244, 1233, 1197, 1168, 1154,
1099, 1072, 1023, 1010, 968, 937, 914, 853, 832, 814, 801, 733,
675; δH (200 MHz; CDCl3) 1.02 (s, 9 H, -C(CH3)3), 2.1–2.3
Experimental
All reactions were carried out under an atmosphere of dry
argon by using standard Schlenk tube techniques. Solvents were
purified by standard literature methods and distilled directly
from their drying agents under nitrogen: THF–potassium,
Et2O–sodium, n-hexane–potassium, toluene–sodium, CH2Cl2–
P4O10. Solvents for CV measurements and one-electron oxid-
ation experiments: CH3CN was purchased in HPLC quality
from Riedel-de Haën, distilled from CaH2 and filtered through
518
J. Chem. Soc., Perkin Trans. 2, 1999, 515–520

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(coalescence, 18 H, Mes-CH3), 6.16 (s, 10 H, C5H5), 6.73 (br s,
2 H, Mes-H), 6.83 (br s, 2 H, Mes-H); δC (50 MHz; CDCl3) 20.7
(Mes-CH3), 20.8 (Mes-CH3), 22.5 (Mes-CH3), 29.3 (CMe3),
40.9 (C3) 111.4 (C1), 114.2 (C5H5), 128.2 (Mes), 128.6 (Mes),
129.6 (Mes), 135.0 (Mes), 135.1 (Mes), 137.2 (Mes), 137.8
(Mes), 138.6 (Mes), 167.6 (C2).
then the precipitated magnesium chloride was removed by
filtration. The filtrate was concentrated to a volume of about 10
cm3 and n-hexane (40 cm3) was added. After storing this solu-
tion overnight at Ϫ40 ЊC a colorless solid could be removed by
filtration. The filtrate was evaporated to dryness affording ZM2
as a pale yellow solid (1.16 g, 2.07 mmol, 68%). Mp 222 ЊC
(Found: C, 73.41; H, 7.83. C35H44OZr requires C, 73.50; H,
7.75%); νmax(KBr)/cmϪ1 3000, 2952, 2924, 2873, 2855, 1608,
1542, 1474, 1459, 1442, 1392, 1370, 1352, 1266, 1235, 1198,
1169, 1158, 1104, 1015, 967, 940, 916, 850, 800, 788, 734, 669,
568; δH (200 MHz; CDCl3) Ϫ0.22 (s, 3 H, Zr-CH3), 0.96 (s, 9H,
-C(CH3)3), 2.0–2.3 (coalescence, 18 H, Mes-CH3), 5.94 (s, 10 H,
C5H5), 6.71 (br s, 2 H, Mes-H), 6.76 (br s, 2 H, Mes-H); δC (50
MHz, CDCl3) 20.7 (Mes-CH3), 22.0 (Mes-CH3), 22.2 (Mes-
CH3), 25.2 (Zr-CH3), 29.4 (CMe3), 39.9 (C3), 110.5 (C1), 110.8
(C5H5), 128.3 (Mes), 129.5 (Mes), 134.3 (Mes), 134.8 (Mes),
138.2 (Mes), 138.6 (Mes), 138.7 (Mes), 169.1 (C2).
Preparation of (2,2-dimesityl-1-phenylethenoxy)zirconocene
chloride (Z3)
To a suspension of sodium hydride (436 mg, 18.2 mmol) in
THF (50 cm3), solid 2,2-dimesityl-1-phenylethenol (713 mg,
2.00 mmol) was added in small portions at room temperature.
The mixture was stirred for 1 h and the surplus sodium hydride
was removed by filtration. The filtrate was added slowly to a
solution of zirconocene dichloride (585 mg, 2.00 mmol) in THF
(50 cm3) at room temperature. After stirring the mixture for 12
h the solvent was removed under reduced pressure. The residue
was extracted with boiling n-hexane (100 cm3) and after fil-
tration the solution was stored at Ϫ40 ЊC for 3 days affording
a yellow solid. The crude product was recrystallised from
n-hexane–toluene. Z3 was obtained as bright yellow crystals
(284 mg, 464 µmol, 23%). Mp 119 ЊC (Found: C, 70.59; H, 6.34.
C36H37ClOZr requires C, 70.61; H, 6.09%); νmax(KBr)/cmϪ1
2990, 2955, 2915, 2855, 1610, 1587, 1560, 1491, 1438, 1370,
1292, 1278, 1245, 1204, 1158, 1082, 1012, 969, 922, 850, 807,
779, 748, 702, 670, 638; δH (200 MHz; CDCl3) 1.9–2.3 (coales-
cence, 18 H, Mes-CH3), 6.04 (s, 10 H, C5H5), 6.59 (s, 2 H,
Mes-H), 6.86 (br s, 2 H, Mes-H), 7.10–7.22 (m, 3 H, Ph-H),
7.24–7.35 (m, 2 H, Ph-H); δC (50 MHz; CDCl3) 20.7 (Mes-
CH3), 20.9 (Mes-CH3), 21.2 (Mes-CH3), 21.6 (Mes-CH3), 114.4
(C5H5), 115.2 (C2), 127.2 (Ar), 127.6 (Ar), 129.0 (Ar), 129.2
(Ar), 129.8 (Ar), 135.2 (Ar), 135.5 (Ar), 135.9 (Ar), 137.9 (Ar),
138.2 (Ar), 160.1 (C1).
Preparation of (2,2-dimesityl-1-phenylethenoxy)methyl-
zirconocene (ZM3)
To a suspension of zirconocene chloride (877 mg, 3.00 mmol)
in Et2O (50 cm3) a solution of methylmagnesium chloride (2.0
cm3, 3.0 M solution in THF, 6.0 mmol) was added at room
temperature. The mixture was stirred for 3 h then a solution of
2,2-dimesityl-1-phenylethenol (1.07 g, 3.00 mmol) in Et2O (50
cm3) was added dropwise. After stirring at room temperature
for an additional 3 h the precipitate was removed by filtration.
The solution was concentrated to a volume of about 10 cm3 and
n-hexane (40 cm3) was added. After storing the mixture at
Ϫ40 ЊC for 24 h the colorless precipitate was removed and the
filtrate was evaporated to dryness affording ZM3 as a pale
yellow solid (1.56 g, 2.64 mmol, 88%). The product could
be obtained in crystalline form by recrystallisation from an
CH3CN–CH2Cl2 mixture. Mp 218 ЊC; νmax(KBr)/cmϪ1 2989,
2953, 2930, 2917, 2876, 2854, 1609, 1586, 1558, 1490, 1475,
1443, 1371, 1299, 1280, 1248, 1205, 1160, 1083, 1018, 969, 938,
922, 852, 801, 778, 749, 740, 701, 669, 638, 594, 558; δH (200
MHz; CDCl3) Ϫ0.10 (s, 3 H, Zr-CH3), 1.9–2.3 (coalescence,
18 H, Mes-CH3), 5.89 (s, 10 H, C5H5), 6.62 (br s, 2 H, Mes-H),
6.81 (br s, 2 H, Mes-H), 7.0–7.2 (m, 5 H, Ph-H); δC (50 MHz,
CDCl3) 20.7 (Mes-CH3), 20.9 (Mes-CH3), 21.1 (Mes-CH3), 21.7
(Mes-CH3), 24.9 (Zr-CH3), 111.0 (C5H5), 113.9 (C2), 127.1
(Ar), 127.3 (Ar), 127.6 (Ar), 128.7 (Ar), 129.0 (Ar), 129.1 (Ar),
129.2 (Ar), 134.8 (Ar), 135.0 (Ar), 137.0 (Ar) 137.9 (Ar), 140.4
(Ar), 159.0 (C1); m/z (EI) 575.1896. C37H40OZr requires
575.1891.
Preparation of (2,2-dimesitylethenoxy)methylzirconocene (ZM1)
A suspension of zirconocene dichloride (877 mg, 3.00 mmol) in
Et2O (40 cm3) was treated with a solution of methylmagnesium
chloride (2.0 cm3, 3.0 M solution in THF, 6.0 mmol) at room
temperature. The mixture from which a white solid precipitated
was stirred at this temperature for 3 h. Now a solution of 2,2-
dimesitylethenol (841 mg, 3.00 mmol) in Et2O (60 cm3) was
added slowly over a period of 2 h. The mixture was stirred
for an additional 3 h, then the precipitate was removed by
filtration. The filtrate was concentrated to a volume of about 10
cm3 and n-hexane (40 cm3) was added. After storing the solu-
tion overnight at Ϫ40 ЊC a colorless solid could be removed
by filtration. The filtrate was evaporated to dryness affording
ZM1 as a pale yellow solid (1.07 g, 2.07 mmol, 69%). Mp 45 ЊC
(Found: C, 71.98; H, 6.70. C31H36OZr requires C, 72.18; H,
7.03%); νmax(KBr)/cmϪ1 2982, 2951, 2918, 2870, 2855, 1591,
1560, 1476, 1458, 1439, 1373, 1248, 1230, 1175, 1092, 1014, 905,
851, 800, 741, 676, 629, 485; δH (200 MHz; CDCl3) 0.10 (s, 3 H,
Zr-CH3), 1.7–3.0 (coalescence, 18 H, Mes-CH3), 5.99 (s, 10 H,
Preparation of 1,4-bis(methylzirconocenoxy)benzene (ZM4)
A solution of dimethylzirconocene (251 mg, 1.00 mmol) in
CH3CN (4 cm3) was treated dropwise with a solution of hydro-
quinone (55.0 mg, 500 µmol) in CH3CN (3 cm3) at room tem-
perature. After stirring for 3 h the precipitate was collected and
dried under reduced pressure affording ZM4 as a colorless
powder (197 mg, 339 µmol, 68%). Mp 147 ЊC (Found: C, 57.66;
H, 5.34. C28H30O2Zr2 requires C, 57.89; H, 5.20%); νmax(KBr)/
cmϪ1 3101, 3019, 2923, 2870, 1489, 1441, 1245, 1082, 1016, 862,
802; δH (200 MHz, CDCl3) 0.20 (s, 6 H, Zr-CH3), 6.10 (s, 20 H,
C5H5), 6.30 (s, 4 H, Ph-H); δC (50 MHz, CDCl3) 21.2 (Zr-CH3),
110.9 (C5H5), 117.5 (o-Ph), 158.1 (quart.-Ph).
C H ), 6.68 (s, 1 H, C᎐CH), 6.89 (br s, 4 H, Mes-H); δ (50
᎐
5
5
C
MHz; CDCl3) 20.7 (Mes-CH3), 20.8 (Mes-CH3), 20.9 (Mes-
CH3), 21.1 (Mes-CH3), 23.1 (Zr-CH3), 110.8 (C5H5), 112.7 (C2),
128.5 (Mes), 129.4 (Mes), 130.4 (Mes), 134.8 (Mes), 134.9
(Mes), 136.0 (Mes), 137.5 (Mes), 153.5 (C1).
Preparation of (1,1-dimesityl-3,3-dimethylbut-1-en-2-oxy)-
methylzirconocene (ZM2)
General procedure for one-electron oxidations
In an argon filled glovebox the desired amounts of the one-
electron oxidant (Fephen or NOSbF6) and the zirconium enol-
ates were placed into two separate test tubes equipped with
stirring rods. At a high purity argon line, 3 cm3 of the appropri-
ate solvent (CH3CN or CH2Cl2) were added to each test tube
to dissolve the reactants. When the blue solution of the one-
electron oxidant Fephen was added through a syringe to the
solution of the zirconium enolate, the color of the mixture
A suspension of zirconocene dichloride (877 mg, 3.00 mmol) in
Et2O (50 cm3) was treated with a solution of methylmagnesium
chloride (2.0 cm3, 3.0 M solution in THF, 6.0 mmol) at room
temperature. The mixture was stirred at this temperature for
3 h. Now a solution of 1,1-dimesityl-3,3-dimethylbut-1-en-2-ol
(1.01 g, 3.00 mmol) in Et2O (50 cm3) was added slowly over a
period of 2 h. The mixture was stirred for an additional 3 h,
J. Chem. Soc., Perkin Trans. 2, 1999, 515–520
519

View Article Online
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immediately turned to a bright red. The resulting mixture was
stirred at room temperature for 1 min, quenched with saturated
aqueous NaHCO3 (10 cm3) and diluted with CH2Cl2 (10 cm3).
The aqueous layer was extracted three times with CH2Cl2, the
combined organic layers were washed with water and dried with
Na2SO4. The solution was filtered through silica gel in order to
remove the red Fe(phen)3(PF6)2 as well as zirconium fragments
split off in the course of the reaction. Removal of the solvent
afforded the crude product. Product analysis was performed by
1H NMR spectroscopy. All products were identified by com-
parison with data of authentic samples. For 3-mesityl-4,6,7-
trimethylbenzofuran (B1) see ref. 26a, for 2-tert-butyl-3-mesityl-
4,6,7-trimethylbenzofuran (B2) see ref. 26a and for 3-mesityl-2-
phenyl-4,6,7-trimethylbenzofuran (B3) see ref. 26b. The yields
were determined by adding m-nitroacetophenone as an internal
1H NMR standard.
5 M. Schmittel, H. Werner, O. Gevert and R. Söllner, Chem. Ber./
Recueil, 1997, 130, 195.
6 (a) J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E.
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D. A. Straus, J. Armantrout, B. D. Santasiero, R. H. Grubbs and
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521; ( f ) see also ref. 4a and ref. 4e.
7 (a) M. Schmittel and R. Söllner, Angew. Chem., 1996, 108, 2248;
Angew. Chem., Int. Ed. Engl., 1996, 35, 2107; (b) M. Schmittel and
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A. Burghart, W. Malisch, J. Reising and R. Söllner, J. Org. Chem.,
1998, 63, 396.
8 For a preliminary communication, see M. Schmittel and R. Söllner,
Chem. Commun., 1998, 565.
9 E. Samuel and M. D. Rausch, J. Am. Chem. Soc., 1973, 95, 6263.
10 All potentials are referenced to the ferrocene/ferrocenium (Fc) redox
couple unless otherwise noted. To obtain values vs. SCE, simply add
ϩ0.39 V.
Cyclic voltammetry
In a glove box tetra(n-butyl)ammonium hexafluorophosphate
(232 mg, 600 µmol) and the electroactive species (6 µmol) were
placed into a thoroughly dried CV cell. At a high purity argon
line CH3CN or CH2Cl2 (6.0 cm3) was added through a gastight
syringe. Then a platinum disc working electrode (id = 1 mm),
a platinum wire counter electrode and a silver wire as pseudo
reference electrode were placed into the solution. The cyclic
voltammograms were recorded at various scan rates using dif-
ferent starting and switching potentials. For determination of
₁
the oxidation potentials, ferrocene (E = ϩ 0.39 V vs. SCE) was
₂
added as the internal standard. Cyclic voltammograms were
recorded using a Princeton Applied Research Model 362 poten-
tiostat with a Philips model PM 8271 XYt-recorder for scan
rates ν < 1 V sϪ1. For fast scan cyclic voltammetry, a Hewlett
Packard model 3314A function generator was used connected
to a three-electrode potentiostat developed by C. Amatore.27
The employed working electrodes were laboratory-made gold
(diameter: 25 µm) and platinum (diameter: 10 µm) ultramicro-
electrodes. The ratios Ipc/Ipa were determined according to the
procedure of Nicholson.28
11 (a) P. G. Gassman, D. W. Macomber and J. W. Hershberger,
Organometallics, 1983, 2, 1470; (b) M. J. Burk, W. Tumas, M. D.
Ward and R. D. Wheeler, J. Am. Chem. Soc., 1990, 112, 6133.
12 The expression mesolytic was coined by Maslak to describe
bond cleavage of radical ions to yield radical and ionic products:
P. Maslak and J. N. Narvaez, Angew. Chem., 1990, 102, 302; Angew.
Chem., Int. Ed. Engl., 1990, 29, 283.
13 R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706.
14 H. Trenkle, PhD Thesis, Würzburg, 1997.
Digital simulation of the cyclic voltammograms
15 (a) M. Röck and M. Schmittel, J. Chem. Soc., Chem. Commun.,
1993, 1739; (b) M. Schmittel, G. Gescheidt and M. Röck, Angew.
Chem., 1994, 106, 2056; Angew. Chem., Int. Ed. Engl., 1994, 33, 1961.
16 (a) M. Schmittel, J. Heinze and H. Trenkle, J. Org. Chem., 1995, 60,
2726; (b) M. Schmittel, M. Keller and A. Burghart, J. Chem. Soc.,
Perkin Trans. 2, 1995, 2327; (c) M. Schmittel, J.-P. Steffen and
A. Burghart, Chem. Commun., 1996, 2349; (d) M. Schmittel and
H. Trenkle, Chem. Lett., 1997, 299.
The computer simulation of the redox chemistry concerning
the two mechanistic scenarios was carried out on a P166ϩ
Computer using the Crank–Nicholson technique20 and Digi-
Sim 2.0 for Windows.29 All chemical reactions were assumed to
be irreversible first-order processes except the ET-equilibrium.
The cyclic voltammograms were simulated using a standard
heterogeneous electron transfer rate constant k0hetero = 0.5 cmϪ1
17 R. F. Jordan, C. S. Bajgur, W. E. Dasher and A. L. Rheingold,
Organometallics, 1987, 6, 1041.
and standard diffusion coefficients of D = 5 × 10Ϫ5 cm2 sϪ1
.
18 S. L. Borkowsky, R. F. Jordan and G. D. Hinch, Organometallics,
1991, 10, 1268.
Acknowledgements
19 One methyl group is replaced by a chlorine in Cp2Zr()Yϩ but two
methyl groups by two chlorines in Cp2Zr()Y2 (with Y = Me, Cl).
20 (a) A. Lasia, J. Electroanal. Chem., Interfacial Electrochem., 1983,
146, 397; (b) J. Heinze, M. Störzbach and M. Mortensen,
J. Electroanal. Chem., Interfacial Electrochem., 1984, 165, 61; (c)
J. Heinze and M. Störzbach, J. Electroanal. Chem., 1993, 346, 1.
21 W. Kaminsky, Macromol. Chem. Phys., 1996, 109, 3907 and
literature cited therein.
On the occasion of his 65th birthday we would like to ack-
nowledge the important contributions that Professor H. Werner
has made to this field of research and to our understanding of
organometallic chemistry. We gratefully acknowledge financial
support by the Deutsche Forschungsgemeinschaft (SFB 347:
“Selective Reactions of Metal Activated Molecules”). In
addition, we are most indebted to the Fonds der Chemischen
Industrie for the ongoing support of our research and to
Degussa AG for a generous gift of electrode materials.
22 E. Samuel, Bull. Soc. Chim. Fr., 1966, 3548.
23 S. E. Biali and Z. Rappoport, J. Am. Chem. Soc., 1984, 106, 5641.
24 E. B. Nadler and Z. Rappoport, J. Am. Chem. Soc., 1987, 109, 2112.
25 S. E. Biali and Z. Rappoport, J. Am. Chem. Soc., 1985, 107, 3669.
26 (a) M. Schmittel and U. Baumann, Angew. Chem., 1990, 102,
571; Angew. Chem., Int. Ed. Engl., 1990, 29, 541; (b) M. Röck and
M. Schmittel, J. Prakt. Chem., 1994, 336, 325.
27 C. Amatore, C. Lefrou and F. Pflügler, J. Electroanal. Chem.,
Interfacial Electrochem., 1989, 270, 43.
28 R. S. Nicholson, Anal. Chem., 1966, 38, 1406.
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