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Half-Wave Reduction Potentials of 2 5-bis(trimethylsilyl)- and 2 5-Di-t-butyl-1 4-benzoquinone.

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The I R spectrum measured in the NaCl and KBr regions
shows strong absorptions at 1208, 1298, and 1972 cm-1 and
bands of medium strength at 511, 843, 930, 956, 2043, 2395,
2500, 2633, and 2930 cm-1.
The 19F-NMR spectrum at 30 "C contains one very broad
signal, which becomes narrower with decrease in the temperature. At -60°C the chemical shift amounts to -55.1 ppm
referred to CC13F as internal standard.
The mass spectrum of (3) contains the typical fragments
SC-NCS ( m / e = 102), CNCS (70), SCF (63), CS (44), and
F C (31), as well as the molecule ion at m / e = 121.
An equimolar mixture of (3) and chlorine reacts at -70 "C
in a bomb tube in an extremely short time, the color becoming
paler and the chlorine being consumed quantitatively. Elemental analysis and the IR, 19F-NMR, and mass spectra of
the yellow liquid product showed that FCI(SCN)C-SCI ( 4 )
is formed quantitatively in this reaction; this compound boils
at 53 "CjlO mm and has the typical odor of sulfenyl chlorides.
The IR spectrum contains a strong absorption band at 1958
cm-1 and bands of medium strength at 530, 575, 811, 1040,
and 1165cm-1. The 19F-NMR spectrum of ( 4 ) at 3OoC
contains a single signal with chemical shift +44.7 ppm referred to CC13F as internal standard. The mass spectrum shows,
inter aliu, fragments FCl(S)C-NCS ( m / e = 156), FClC-NCS
(124), F(S)C-NCS (121), and SCClF (98).
Received: May 26th and June 5th, 1967
[Z 540b IE]
German version: Angew. Chem. 79,978 (1967)
Publication delayed at author's request
*! Dozent Dr. A. Haas and Dip1.-Chem. W. Klug
Anorganisch-Chemisches Institut der Universitat
Hospitalstr. 8-9
34 Gottingen (Germany)
[l] A. Huas and H. Reinke, Angew. Chem. 79, 687 (1967);
Angew. Chem. internat. Edit. 6,705 (1967).
Half-wave Reduction Potentials of
2,5-Bis(trimethylsilyl)- and
2,5-Di-t-butyl-l,Cbenzoquinone[**I
By H. Bock and H. A f t [*I
The interpretation of d-orbital effects
is made difficult in
the case of phosphorus-nitrogen compounds according to the
oxidation state and coordination number of phosphorus by
the presence of free electron pairs, positive charges, or partial
double bonds. However, the d-x and d-x* interactions and
their differences can be investigated by comparing siliconsubstituted x-electron systems with the corresponding alkyl
derivatives using suitable methods of measurement [21. Thus
the half-wave reduction potentials Ell, are related to the
eigenvalues of the lowest unoccupied levels of n-electron
systems into which the electron is inserted on reduction [31.
The prerequisite of a reversible redox equilibrium is fulfilled
for alkylated p-benzoquinones 141.
The 2,5-bis(trimethylsilyl)-l,4-benzoquinone (5) needed for
comparison with the 2,Sdi-t-butyl derivative was obtained
as follows: ( I ) is converted with trimethylchlorosilane/
qBr
- &rBr RQSiO
Br
OH
(1)
R,SiO
R3si@siR3
OSiR3
R
H
C(CH,),
Si(CH3),
+0.04
-0.15
+0.04
-0.47
-0.71
-0.51
The results show that the silicon-substituted quinone and
semiquinone have higher oxidation potentials than the alkyl
derivatives, i.e. they are stronger electron-acceptors. This is
inexplicable if one assumes exclusively an inductive effect
(+ISiR3 > + I c R ~ ) [ ~ Jand indicates an additional d-x*
interaction between the vacant 3d levels of the silicon and
the lowest unoccupied or singly occupied molecular orbitals
of the p-benzoquinone x-system.
This type of model, already postulated several times171, for
the Si-C,,z
bond, which requires Sid c C, back-donation
of electrons alongside the 8% + Cs- inductively polarized
5 skeleton, is in accord with the results of further polarographic measurements on silyl- and alkyl-butadienes, -styrenes, -naphthalenes, and -phenyl ketones 121.
Received: July 24th. 1967
[Z 591a IE]
German version: Angew. Chem. 79, 932 (1967)
[*I Priv.-Doz. Dr. H. Bock and Dip1.-Chem. H. Alt
Institut fur Anorganische Chemie der Universitat
Meiserstr. 1
8 Miinchen 2 (Germany)
[**I Part I of d-Orbital Effects in Silicon-substituted x-Electron
Systems.
[I] Cf. H. Bock, Chimia 21, 35 (1967).
[2] H. Bock, H. Alt, [J. Krjnitz, and H. Xidl, unpublished.
131 Cf. A. Sfreifwieserj r . : Molecular Orbital Theory for Organic
Chemists. Wiiey, New York 1961, p. 173f.
[4] M. E. Peover, J. chem. SOC.(London) 1962, 4540.
[ 5 ] P. H. Given and M. E. Peover, J. chem. SOC.(London) 1959,
1602.
[6] H. Bock and H . AIt, Angew. Chem., in press.
I71 Cf.,e.g., M . D . Curtis and A. C. Alfred, 3. Amer. chem. SOC.
87, 2554 (1965).
OSiRS
131
(2)
pyridine in boiling toluene into the ether (2) (m.p. 58-59 " C )
which reacts with sodiumjtrimethylchlorosilane in boiling
toluene to form (3) (m.p. 121 "C). Cleavage of the O-SiR3
bonds with 4 M sodium methoxide in methanol at 20 "C leads
after neutralization with diluted acetic acid to ( 4 ) (m.p. 183
to 186'C), which can be oxidized with silverli) oxide in
benzene at 20 "C to (5).
The 2,5-bis(trimethylsilyl)-l,4-benzoquinone (n + x * : vm =
19999 cm-1; E~ = 25 1 mole-1 cm-1) is orange in contrast
t o the yellow 2,Sdi-t-butyl compound (n +x* : vn, = 21 900
cm-1;
= 36 1 mole-1 cm-1); it melts a 188 OC and gives
two N M R singlets at T = 3.24 and 9.77 in the required ratio
2: 18. The half-wave reduction potentials for the two steps (I)
and (11) in acetonitrile were measured with tetra-n-butylammonium iodide as conducting electrolyte against the
constant 151 mercury anode.
U V and ESR Studies of p-Bis(trimethyIsily1)- and
p-Dialkylbenzenes[**I
By H. A h , H. Bock, F. Gerson, and J. Heinzer[*]
A notable feature of the U V spectrum of p-bis(trimethy1sily1)benzene ( I n ) , when compared with the UV spectrum
of p-di-t-butylbenzene, is that the lLa band (lBlu -+ lAlg)
Angew. Chem. internat. Edit.
Vol. 6 (1967)
1 No. I 1
94 1
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potential, trimethylsilyl, butyl, reduction, wave, benzoquinone, bis, half
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