290
VORONKOV et al.
To establish the mechanism of reaction (1), we stud-
The possibility for dimethylsilane formed to cleave the
ied the reaction of disiloxane I with a twofold excess of ≡Si–O–Si≡ group was confirmed by the model reaction of
triethylsilane in the presence of colloidal nickel. As the equimolar amounts of disiloxane I with diethylsilane in
products of this reaction, hexaethyldisiloxane (II) and the presence of colloidal nickel. Along with α,ω-dihydro-
1,1-dimethyl-3,3,3-triethyldisiloxane
(III)
were oligopermethylsiloxanes (H(Me2SiO)nSiMe2H (n =
isolated and characterized. In addition, 1,1,3,3-tetra-
methyl-5,5,5-triethyltrisiloxane (IV), 3,3-dimethyl-
1,1,1,5,5,5-hexaethyltrisiloxane (V), 3,3,5,5-tetramethyl-
1,1,1,7,7,7-hexaethyltetrasiloxane (VI), dimethylsi-
lane, and hydrogen were identified using spectroscopy
and chromatography–mass spectrometry.
The formation of disiloxane II can be explained by
a two-step reaction of triethylsilane with disiloxane I to
give disiloxane III at the first step,
2−9), we identified 3,3-dimethyl-1,1,5,5-tetraethyltrisi-
loxane (VIII), formed in about 5% yield according to
the scheme
Et2SiH2 + HMe2SiOSiMe2H
HEt2SiOSiMe2H + HMe2SiOSiMe2H
–Me2SiH2
(15)
HEt2SiOSiMe2OSiMe2H + H2SiEt2
Et2HSiOSiMe2OSiEt2H + Me2SiH2.
–Me2SiH2
Et3SiH + HMe2SiOSiMe2H
(8)
(VIII)
Et3SiOSiMe2H + Me2SiH2,
The formation of trisiloxane VIII could also be
explained by the insertion of dimethylsilanone into
intermediate 1,1,3,3-tetraethyldisiloxane (IX) accord-
ing to the scheme
(III)
followed by its cleavage by triethylsilane,
Et3SiOSiMe2H + HSiEt3
(9)
Et2HSiOSiMe2H + Et2SiH2
(16)
Et3SiOSiEt3 + Me2SiH2.
(II)
Et2HSiOSiEt2H + Me2SiH2.
Trisiloxane IV results from the reaction of disiloxane
III with siloxane I according to the following scheme:
(IX)
However, we failed to identify the latter in the reaction
mixture.
Et3SiOSiMe2H + HMe2SiOSiMe2H
(10)
Et3SiOSiMe2OSiMe2H + Me2SiH2.
In the reaction of disiloxane I with an equimolar
mixture of di- and triethylsilane (a 1 : 1 : 1 molar ratio)
in the presence of colloidal nickel, we identified penta-
ethyldisiloxane (X), 3,3-dimethyl-1,1,5,5,5-pentaeth-
yltrisilane (XI), and 3,3,5,5-tetramethyl-1,1,7,7,7-pen-
taethyltetrasiloxane (XII), along with α,ω-dihydrooli-
gopermethylsiloxanes (H(Me2SiO)nSiMe2H (n = 2–4)
and siloxanes (II–VI, VIII). The formation of silox-
anes II–VI, VIII, and X–XII and the absence of higher
oligomers with terminal Et2Si groups indicates that
these groups prevent further elongation of the siloxane
chain.
(IV)
The reaction of triethylsilane with trisiloxane IV
yields trisiloxane V:
Et3SiH + HMe2SiOSiMe2OSiEt3
(11)
Et3SiOSiMe2OSiEt3 + Me2SiH2.
(V)
The formation of tetrasiloxane VI might be
explained by the reaction of trisiloxane IV with disilox-
ane I,
Et3SiOSiMe2OSiMe2H + HMe2SiOSiMe2H
(12)
EXPERIMENTAL
Et3SiOSiMe2OSiMe2OSiMe2H + Me2SiH2,
(VII)
Reaction of 1,1,3,3-tetramethyldisiloxane with col-
loidal nickel. 1,1,3,3-Tetramethyldisiloxane (13.4 g,
0.1 mol) and colloidal nickel (0.6 g, 0.01 mol) were
placed into a three-necked flat-bottom flask purged
with argon and equipped with a reflux condenser and a
thermometer and connected to a trap cooled with liquid
nitrogen. The reaction mixture was magnetically stirred
at ambient temperature for 7 h. Colloidal nickel was fil-
tered off, and the filtrate was distilled at atmospheric
pressure and in vacuum (3 mmHg) to give 9.1 g of a
product whose composition determined by chromatog-
raphy–mass spectrometry corresponded to previously
described α,ω-dihydrooligopermethylsiloxanes [2].
Dimethylsilane condensed in the cold trap was purged
with a dry argon flow and bubbled through a solution of
bromine in xylene. The reaction mixture after bromina-
tion was distilled over copper metal to give 7.3 g dimeth-
followed by the reaction of nascent tetrasiloxane VII
with triethylsilane,
Et3SiOSiMe2OSiMe2OSiMe2H + HSiEt3
(13)
Et3SiOSiMe2OSiMe2OSiEt3 + Me2SiH2.
(VI)
However, the absence of even traces of tetrasiloxane
VII in the products of reaction of disiloxane I with tri-
ethylsilane indicates another mechanism of formation
of tetrasiloxane VI, which is based on the insertion of
dimethylsilanone into a molecule of trisiloxane V:
Et3SiOSiMe2OSiEt3 + [Me2Si=O]
(14)
Et3SiOSiMe2OSiMe2OSiEt3.
(VI)
DOKLADY CHEMISTRY Vol. 393 Nos. 4–6 2003