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r.t., monitoring the disappearance of the substrate by
GC and TLC. Acetic acid was removed under vac-
uum without heating above 30°C, and the residue was
washed with water to remove unreacted mercuric ac-
etate and dried.
2.11. Ferrocenylphenylethyne
FcCꢀCPh, 1.57–2.79×10−4 M; Hg(OAc)2, 8.38×
10−4–8.38×10−2 M (275, 280, 330 nm).
2.12. Ferrocenyl(4-fluorophenyl)ethyne
The acetoxymercurio derivatives were insoluble in
organic solvents and, when heated, decomposed with-
out melting. Any attempt to obtain EI or FAB mass
spectra failed.
p-FC6H4CꢀCFc,
6.58×10−5–8.22×10−2
M;
Hg(OAc)2, 8.38×10−4–8.38×10−2 M (290, 315, 320
nm).
Reductive demercuration was performed on a por-
tion of the acetoxymercurio compound, suspended in
water. An aqueous basic (NaOH) solution of NaBH4
was added dropwise, under stirring, at 0°C. Metallic
mercury separated almost immediately. The organic
layer was washed with water to neutrality, dried over
anhydrous Na2SO4, and immediately examined by
GC–MS. Only one compound was always observed.
All mass spectra presented a base peak with m/z cor-
responding to M+ of the vinyl ester (see Scheme 2).
Significant fragmentations were observed with clusters
around M+ −Ac (Fe isotopes) and, more impor-
tantly, around m/z 213 (Fe isotopes). The latter is
relative to FcCO+ and indicates that the regioisomer
is FcC(OAc)ꢁCHAr. Finally, a peak with m/z 121
(CpFe+) was always present.
2.13. (4-Chlorophenyl)ferrocenylethyne
p-ClC6H4CꢀCFc, 6.24–9.36×10−5 M; Hg(OAc)2,
8.38×10−3–8.38×10−2 M (280, 285, 320 nm).
2.14. (4-Bromophenyl)ferrocenylethyne
p-BrC6H4CꢀCFc, 8.22–9.36×10−5 M; Hg(OAc)2,
8.38×10−3–8.38×10−2 M (270, 280, 330, 340 nm).
2.15. Ferrocenyl(4-nitrophenyl)ethyne
p-NO2C6H4CꢀCFc, 7.55×10−5 M; Hg(OAc)2,
1.68–8.38×10−2 M (260, 280, 315 nm).
2.9. Kinetic measurements
2.16. Determination of HOMO energy for
arylferrocenylethynes
Preliminary experiments, carried out by recording
spectra at subsequent times, in the range 280–320
nm, showed in most cases the presence of an isos-
bestic point around 290 nm, with absorbance values
decreasing at wavelengths lower and increasing at
wavelengths higher than this point. Kinetic experi-
ments have been carried out under pseudo-first-order
conditions. Solutions containing known concentra-
tions of alkyne and mercuric acetate in acetic acid
were separately put into the two parts of a silica cell
with septum and allowed to reach 25°C in the ther-
mostatted cell compartment of the spectrophotometer.
After mixing, the absorbance variation with time was
read at the appropriate wavelength. The same kobs
values were obtained for kinetics run at wavelengths
before and after the isosbestic point. Pseudo-first-or-
der rate constants were calculated using SigmaPlot
[29].
The structures of substituted arylferrocenylethynes
were first optimised starting from a structure where
ferrocenyl and aryl rings were coplanar. The ZINDO/
1 method was used, parametrized for iron-containing
compounds, as supplied by the HyperChem program
[30]. The following conditions were used: total
charge=0; spin multiplicity=1; SCF controls: con-
vergence limit=0.001, iteration limit=400; accelerate
convergence; overlap weighting factors: s–s=1 (de-
fault), p–p=1 (default); state: lowest; spin pairing:
RHF; configuration interaction: none. Options for op-
timisation were as follows. Algorithm: Polak–Ribiere
(conjugate gradient); termination condition: RMS gra-
−1
dient of 0.01 kcal A
mol−1 or 2000 max. cycles; in
,
vacuo. HOMO energies were obtained, after optimisa-
tion, by a single point calculation.
The data are reported in Table 1, as overall sec-
ond-order constants. They are mean values of several
runs, carried out under conditions specified as follows
(wavelengths in parentheses).
Acknowledgements
We thank the Ministero dell’Universita` e della
Ricerca Scientifica e Tecnologica (MURST) and the
Consiglio Nazionale delle Ricerche (CNR, Roma) for
financial support, and Mr Fabio Bertocchi for 13C-
NMR spectra.
2.10. Ferrocenyl(4-methoxyphenyl)ethyne
p-MeOC6H4CꢀCFc, 7.91×10−5 M; Hg(OAc)2,
8.38×10−3–8.38×10−2 M (275, 330 nm).