20 research outputs found
Stable Germenolates and Germenes with Exocyclic Structures
The
first germenolates with exocyclic structures [(Me<sub>3</sub>Si)<sub>2</sub>GeĀ(Si<sub>2</sub>Me<sub>4</sub>)<sub>2</sub>GeCĀ(R)ĀO]<sup>ā</sup>K<sup>+</sup> (<b>5a</b>: R = Mes; <b>5b</b>: <i>o</i>-Tol; <b>5c</b>: 1-Ad) were synthesized
by the reaction of the corresponding cyclic acylgermanes with KO<i>t</i>Bu. <b>5a</b>ā<b>c</b> could be isolated
by crystallization. The remarkable thermal stability of <b>5a</b>ā<b>c</b> even at room temperature allowed full characterization
by NMR and UVāvis spectroscopy and by single-crystal X-ray
crystallography. Spectroscopic and structural features in combination
with DFT quantum mechanical calculations indicated that <b>5a</b>ā<b>c</b> are best described as acyl germyl anions in
solution and in the solid state as well. The reactivity of <b>5a</b>ā<b>c</b> versus chlorosilanes parallels that observed
for the structurally related silenolates. The aryl-substituted compounds <b>5a</b>,<b>b</b>, thus, reacted with ClSiMe<sub>3</sub> to
give the exocyclic germenes (Me<sub>3</sub>Si)<sub>2</sub>GeĀ(Si<sub>2</sub>Me<sub>4</sub>)<sub>2</sub>Geī»CĀ(OSiMe<sub>3</sub>)ĀR
(<b>6a</b>: R = Mes; <b>6b</b>: <i>o</i>-Tol),
while the alkyl-substituted species <b>5c</b> afforded the Ge-silylated
cyclic acylgermane. TDDFT calculations were used to assign the UVāvis
absorption spectra and to gain more insight into the electronic nature
of <b>5a</b>ā<b>c</b> and <b>6a</b>
Reactivity of Cyclic Silenolates Revisited
The stable exocyclic
silenolates <b>2a</b>ā<b>c</b> (<b>2a</b>, R = Mes; <b>2b</b>, R = <i>o</i>-Tol; <b>2c</b>, R = 1-Ad) were fully characterized by NMR
and UVāvis spectroscopy. According to spectroscopic and structural
features, <b>2a</b>ā<b>c</b> are best described
as acyl silyl anions (tautomeric structure I) in solution. This behavior
is also reflected by the reaction of <b>2a</b>,<b>c</b> with MeI. Both alkylation reactions take place at the corresponding
silicon atom and lead to the formation of the methylated structures <b>4a</b>,<b>b</b> in nearly quantitative yields. Furthermore,
the thermal stability of exocyclic silenolates <b>2a</b>,<b>c</b> was investigated. In the case of <b>2a</b>, a thermally
induced intramolecular sila-Peterson alkenation was observed at 60
Ā°C. This transformation allowed straightforward access to 2-oxahexasilabicyclo[3.2.1]Āoctan-8-ide <b>5</b> as a structurally complex, bicyclic silicon framework. In
contrast to that, heating of <b>2c</b>, as an example of an
alkyl-substituted silenolate, led to an unexpected degradation to
uncharacterized polymers. However, we were able to isolate the 1-adamantyl-substituted,
bicyclic compound <b>8</b>, which is structurally closely related
to <b>5</b>, by the treatment of 1,4-dipotassium-1,4-bisĀ(trimethylsilyl)Ācyclohexasilane
with 1 equiv of 1-adamantoyl chloride. Again an intramolecular sila-Peterson
alkenation is responsible for the formation of <b>8</b>. The
mechanism for this highly selective reaction sequence is outlined
and supported by density functional theory (DFT) calculations, which
highlight the thermodynamic driving force and the low activation barriers
of this multistep transformation
Synthesis and Properties of Bridgehead-Functionalized Permethylbicyclo[2.2.2]octasilanes
A series
of previously unknown bridgehead-functionalized bicyclo[2.2.2]Āoctasilanes,
Me<sub>3</sub>Si-Si<sub>8</sub>Me<sub>12</sub>-X, X-Si<sub>8</sub>Me<sub>12</sub>-X, and X-Si<sub>8</sub>Me<sub>12</sub>-Y [X, Y =
āSiMe<sub><i>n</i></sub>Ph<sub>3ā<i>n</i></sub> (<i>n</i> = 1, 2) (<b>2</b>, <b>3</b>, <b>10</b>), āSiMe<sub>2</sub>Fc (Fc = ferrocenyl)
(<b>4</b>, <b>11</b>, <b>13</b>, <b>14</b>), āCOR (R = Me, <i>t</i>Bu) (<b>6</b>, <b>7</b>, <b>12</b>), COOMe (<b>8</b>), COOH (<b>9</b>)], have been prepared by the reaction of the silanides Me<sub>3</sub>Si-Si<sub>8</sub>Me<sub>12</sub><sup>ā</sup>K<sup>+</sup> or
K<sup>+ā</sup>Si<sub>8</sub>Me<sub>12</sub><sup>ā</sup>K<sup>+</sup> with proper electrophiles and fully characterized.
The molecular structures of <b>2</b>, <b>3</b>, <b>4</b>, <b>6</b>, <b>8</b>, <b>9</b>, <b>10</b>, and <b>13</b> as determined by single-crystal X-ray diffraction
analysis exhibit a slightly twisted structure of the bicyclooctasilane
cage. Endocyclic bond lengths, bond angles, and dihedral angles are
not influenced considerably by the substituents attached to the bridgehead
silicon atoms. Due to ĻĀ(SiSi)/ĻĀ(aryl) conjugation, a 20ā30
nm bathochromic shift of the longest wavelength UV absorption band
relative to Me<sub>3</sub>Si-Si<sub>8</sub>Me<sub>12</sub>-SiMe<sub>3</sub> (<b>1</b>) is evident in the UV absorption spectra
of the phenyl and ferrocenyl derivatives. Otherwise, UV absorption
data do not support the assumption of aryl/aryl or aryl/Cī»O
interaction via the ĻĀ(SiSi) bicyclooctasilane framework
Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes
Previously unknown 1,1,4-trisĀ(trimethylsilyl)-4-acyldodecamethylcyclohexasilanes
(Me<sub>3</sub>Si)<sub>2</sub>Si<sub>6</sub>Me<sub>12</sub>(Me<sub>3</sub>Si)ĀCOR (<b>16a</b>, R = <i>tert</i>-butyl; <b>16b</b>, R = 1-adamantyl) have been synthesized by the reaction
of the potassium silanides (Me<sub>3</sub>Si)<sub>2</sub>Si<sub>6</sub>Me<sub>12</sub>(Me<sub>3</sub>Si)K with acid chlorides ClCOR, and
their photochemical rearrangement reactions have been studied. The
molecular structures of <b>16a</b>,<b>b</b> as determined
by single-crystal X-ray diffraction analysis exhibit an unusual twist-boat
conformation of the cyclohexasilane ring. When <b>16a</b>,<b>b</b> were photolyzed with Ī» >300 nm radiation, they
underwent
Brook type 1,3-Si ā O migration reactions to generate the cyclohexasilanes <b>17a</b>,<b>b</b> with exocyclic Siī»C bonds along
with smaller amounts of the ring-enlarged species <b>19a</b>,<b>b</b> with endocyclic Siī»C double bonds. While <b>17a</b>,<b>b</b> were stable enough to allow characterization
by NMR and UV absorption spectroscopy, the less stable products <b>19a</b>,<b>b</b> could only be observed in the form of their
methanol adducts
Stable Silenolates and Brook-Type Silenes with Exocyclic Structures
The
first silenolates with exocyclic structures [(Me<sub>3</sub>Si)<sub>2</sub>SiĀ(Si<sub>2</sub>Me<sub>4</sub>)<sub>2</sub>SiCĀ(R)ĀO]<sup>ā</sup>K<sup>+</sup> (<b>2a</b>: R = 1-adamantyl; <b>2b</b>:
mesityl; <b>2c</b>: <i>o</i>-tolyl) were
synthesized by the reaction of the corresponding acylcyclohexasilanes <b>1a</b>ā<b>c</b> with KO<i>t</i>Bu. NMR
spectroscopy and single-crystal X-ray diffraction analysis suggest
that the aryl-substituted silenolates <b>2b</b>,<b>c</b> exhibit increased character of functionalized silenes as compared
to the alkyl-substituted derivative <b>2a</b> due to the different
coordination of the K<sup>+</sup> counterion to the SiCĀ(R)O moiety. <b>2b</b>,<b>c</b>, thus, reacted with ClSi<i>i</i>Pr<sub>3</sub> to give the exocyclic silenes (Me<sub>3</sub>Si)<sub>2</sub>SiĀ(Si<sub>2</sub>Me<sub>4</sub>)<sub>2</sub>Siī»CĀ(OSi<i>i</i>Pr<sub>3</sub>)ĀR (<b>3b</b>: R = Mes; <b>3c</b>: <i>o</i>-Tol), while <b>2a</b> afforded the Si-silylated
acylcyclohexasilane <b>1d</b>. The thermally remarkably stable
compound <b>3b</b>, which is the first isolated silene with
the sp<sup>2</sup> silicon atom incorporated into a cyclopolysilane
framework, could be fully characterized structurally and spectroscopically