First characterisation of zirconium enolate radical cations in solution and their mesolytic bond cleavage to zirconocene cations

Article abstract of DOI:10.1039/a708495a

Zirconocene enolate radical cations are generated and characterised in solution for the first time; the sterically congested radical cations undergo a mesolytic Zr-O bond cleavage process yielding zirconocene cations, the kinetics of which are determined.

Full text of DOI:10.1039/a708495a

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First characterisation of zirconium enolate radical cations in solution and their
mesolytic bond cleavage to zirconocene cations
Michael Schmittel* and Rolf So¨llner
Institut fu¨r Organische Chemie, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Table 1 Oxidation potentials of zirconocene enolates determined by CV
Zirconocene enolate radical cations are generated and
characterised in solution for the first time; the sterically
Zirconocene
enolate
Yield of
benzofuran
congested radical cations undergo a mesolytic Zr–O bond
cleavage process yielding zirconocene cations, the kinetics of
which are determined.
Epaa,b/V
E
Oxa,c/V
1
2
1
2
3
4
5
6
+0.45
+0.32
+0.41
+0.51
+0.30
+0.35
+0.57
+0.43
+0.44
+0.46
+0.35
+0.36
95% of 7
90% of 8
88% of 9
82% of 7
86% of 8
80% of 9
In light of the importance of transition metal enolates for
organic synthesis1 we have recently become interested in the
redox umpolung of these nucleophilic species.2 We were able to
show that titanium enolate radical cations can undergo carbon–
carbon bond formation to 1,4-diketones, a reaction which can
even be conducted in a diastereoselective fashion.3 In the
present paper we describe for the first time the properties and
reactions of zirconium enolate radical cations and their clean
mesolytic† bond cleavage to zirconocene cations. The latter are
well known as the catalytic active species in the Ziegler–Natta
polymerization of a-alkenes.4
a vs. Fc–Fc†. b In acetonitrile at n = 100 mV s21
n = 1000 V s21
.
c In dichloromethane at
.
Table 1) in acetonitrile at 100 mV s21. By changing the solvent
to dichloromethane and increasing the scan rates up to 1000 V
s21 reversible waves could be obtained. We observed partially
reversible waves for 1–3 and 5 already at scan rates n ! 100 V
s21, for 4 at n ! 200 V s21, and for 6 at n ! 50 V s21. This
represents the first direct observation of zirconium enolate
radical cations in solution, allowing for the determination of
Since zirconium enolates constitute highly reactive com-
pounds with a high sensitivity towards hydrolysis, only a small
number of zirconium enolates5 has been isolated and investi-
gated. The kinetic stability of zirconium enolates should be
largely increased by starting from sterically bulky b,b-
dimesitylenols.6 These were deprotonated quantitatively with
sodium hydride in THF at room temperature and subsequen-
tially reacted with stoichiometric amounts of zirconocene
dichloride. All chloro zirconocene enolates could be obtained as
bright yellow crystals by crystallisation from n-hexane or
n-hexane–toluene.
A route leading to the methyl zirconocene enolates was
realised by reacting dimethyl zirconocene with the corre-
sponding b,b-dimesitylenols accompanied by elimination of
methane. The zirconocene enolates 4–6 could be obtained in
analytically pure form as pale yellow solids by crystallization
from acetonitrile–dichloromethane mixtures (Scheme 1).
In standard cyclic voltammetry (CV) experiments all zirco-
nocene enolates showed irreversible oxidation waves (Epa;
their thermodynamically relevant redox potentials E1Ox (Table
2
1). The oxidation potentials fall in the range +0.35 to +0.57 V
vs. ferrocene–ferrocenium† and are ca. 100 mV more anodic
than those of structurally analogous titanocene enolates,2 an
effect which is already known from simple titanocene and
zirconocene compounds.7
The chemical one-electron oxidations of the zirconium
enolates 1–6 which were performed on a preparative scale using
tris(1,10-phenanthroline)iron(iii)
hexafluorophosphate
1
(E = +0.69 V), as a well-defined outer-sphere one-electron
2
1
2
oxidant, or nitrosonium hexafluorophosphate (E = +0.89 V) in
acetonitrile–dichloromethane afforded the benzofuran deriva-
tives 7–9 in good yields (Scheme 2, Table 1).
[Fe(phen)3][PF6]3
or
O
R
X
NOSbF6
Zr
Mes
Mes
O
200 mol%
Mes
R
Cl
i, NaH
ii, Cp2ZrCl2
Zr
1–6
R = H, But, Ph
X = Cl. Me
7 R = H
Mes
Mes
O
8 R = But
9 R = Ph
THF, 25 °C
R
1 43%, R = H
2 65%, R = But
3 23%, R = Ph
Scheme 2
Mes
Mes
OH
R
Formation of benzofurans by one-electron oxidation of stable
enols8 and various enol derivatives9 (including titanium eno-
lates2) is known to be initiated by a mesolytic bond cleavage of
the O–X bond followed by several reaction steps that are well
established (Scheme 3).
For the zirconocenes the Zr–O bond must have been broken
at the stage of the enolate radical cation since the direct
cyclization is highly unfavourable because of steric reasons.
Equally, a dissociative electron transfer process can be ruled out
since all the radical cations could be characterised at high scan
rates in the CV experiment. Hence, we conclude that mesolytic
Zr–O bond cleavage is the primary follow-up reaction of the
Me
Zr
Mes
Mes
O
Cp2ZrMe2
THF, 25 °C
R
4 69%, R = H
5 74%, R = But
6 88%, R = Ph
Scheme 1
Chem. Commun., 1998
565

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Mes
Mes
O
R
in progress to use this mesolytic cleavage reaction in photo-
induced electron transfer reactions for triggering poly-
merisation.
We gratefully acknowledge the financial support by the
Deutsche Forschungsgemeinschaft (SFB 347: ‘Selective Reac-
tions of Metal Activated Molecules’). In addition, we are most
indebted to the Fonds der Chemischen Industrie for the ongoing
support of our research as well as to Degussa for a generous gift
of electrode materials.
Mech. A
Mech. B
Cp2ZrIVX+
–e–
•+
+
+
Mes
Mes
OZrCp2X
R
–e–
O
Mes
+
Cp2ZrIIIX
Mes
R
Notes and References
O
R
O
R
* E-mail: mjls@chemie.uni-wuerzburg.de
+
[1,2]-CH3
–H+
† 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., Int. Ed. Engl., 1990, 29, 283.
‡ All potentials are referenced to the ferrocene–ferrocenium (Fc) redox
couple unless otherwise noted. To obtain values vs. SCE, simply add +0.39
V.
Mes
Mes
Scheme 3
§ We determined the reduction potential of the corresponding cation
[Cp2ZrIVMe(MeCN)2]BPh4 to Epc = 21.94 V (in acetonitrile at 100 mV
s21; supporting electrolyte: NaBPh4). The preparation of this cation is
described in: R. F. Jordan, C. S. Bajgur, W. E. Dasher and A. L. Rheingold,
Organometallics, 1987, 6, 1041.
radical cations 1·+–6·+ which can follow two distinct mecha-
nistic pathways:9d the heterolytic scission (Mech. A) would
result in the formation of an a-carbonyl radical and a
zirconocene cation, the homolytic variant (Mech. B) would lead
to an a-carbonyl cation and a zirconocene(iii) species. Both
cleavage selectivities are known for enol radical cations:
homolytic cleavage can be observed for radical cations of enol
esters,9a enol carbonates and enol carbamates9d whereas the
radical cations of enols, silyl enol ethers,9b enol phosphites and
enol phosphinates9c and of the structurally very closely related
titanium enolates2 follow the heterolytic variant.
When the oxidation potentials of the homolytic cleavage
fragments (a-carbonyl radicals and Cp2ZrIIIX) are known then
the selectivity of such fragmentation processes can be deter-
mined by using a simple thermochemical cycle calculation. The
fragments which are split off in a cationic form must have the
lower redox potential. We hence measured the oxidation
potential of Cp2ZrIIIMe to Epa = 21.9 V§ and compared it with
those of the relevant a-carbonyl radicals. Since the latter are
much more positive (0.15–0.36 V)8a it is clear that mesolytic
bond cleavage of the methyl zirconocene enolates 4–6 takes
place according to the heterolytic pathway (Mech. A). This
result is in analogy to Jordan’s findings, who performed one-
electron oxidations of dimethyl zirconocene and other dialkyl
zirconocenes with equimolar amounts of ferrocenium and
isolated the cationic complexes [Cp2ZrR]+ after Zr–C bond
cleavage.10 Owing to the much larger differences of the
oxidation potentials between oxidant and substrate the desired
reaction time of this conversion is much longer (10 h) than in
our experiments (1 min). In case of the chlorine substituted
model compounds 1–3 simple considerations¶ propose that
their fragmentation should follow the same pathway.
¶ The difference between the oxidation potentials of Cp2ZrIIIMe and
Cp2ZrIIICl can be approximated by the difference between the half-wave
1
2
1
2
potentials of Cp2ZrMe2 (E = 23.06 V) and Cp2ZrCl2 (E = 22.04 V). As
the potential of Cp2ZrIIIMe is Epa ≈ 21.9 V we assume that that of
Cp2ZrIIICl is Epa ≈ 20.9 VFc being distinctly lower than those of the
a-carbonyl radicals.
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The knowledge of the exact mechanism of this process
enabled us to determine the lifetime of zirconocene enolate
radical cations by following the kinetics of the mesolytic bond
cleavage through fast scan cyclic voltammetry at ultramicro
electrodes in dichloromethane. From the ratio of cathodic to
anodic peak current Ipc/Ipa the kinetic parameter kt was
evaluated according to the method of Nicholson and Shain11
applying a working curve for an ECirrE mechanism (electron
transfer–chemical reaction–electron transfer). The working
curve had been obtained from digital simulation of cyclic
voltammograms using the implicit Crank–Nicholson tech-
nique.12 This kinetic analysis at room temperature provided a
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7 P. G. Gassman, D. W. Macomber and J. W. Hershberger, Organome-
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A. Burghart, Chem. Commun., 1996, 2349; (d) M. Schmittel and
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first order-rate constant of kf = 3.3 3 102 s21 (t = 2.1 3 10
23
10 S. L. Borkowsky, R. F. Jordan and G. D. Hinch, Organometallics, 1991,
10, 1268.
1
2
s) for the mesolytic bond cleavage of 2·+, kf = 3.1 3 102 s21 (t
1
2
11 R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706.
12 A. Lasia, J. Electroanal. Chem., Interfacial Electrochem., 1983, 146,
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= 2.2 3 1023 s) for that of 3·+, kf = 8.3 3 102 s21 (t = 8.4 3
1
2
1024 s) for that of 4·+ and kf = 5.0 3 102 s21 (t = 1.4 3 10
23
1
2
s) for that of 5·+.
In conclusion, we have characterised for the first time
zirconium enolate radical cations in solution and identified them
as rapidly cleaving precursors for zirconocene cations. Work is
Received in Cambridge, UK, 25th November 1997; 7/08495A
566
Chem. Commun., 1998