Stable Germenolates and Germenes with Exocyclic Structures

The first germenolates with exocyclic structures [(Me₃Si)₂Ge­(Si₂Me₄)₂GeC­(R)­O]⁻K⁺ (<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>–<b>c</b> could be isolated by crystallization. The remarkable thermal stability of <b>5a</b>–<b>c</b> even at room temperature allowed full characterization by NMR and UV–vis spectroscopy and by single-crystal X-ray crystallography. Spectroscopic and structural features in combination with DFT quantum mechanical calculations indicated that <b>5a</b>–<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>–<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 ClSiMe₃ to give the exocyclic germenes (Me₃Si)₂Ge­(Si₂Me₄)₂GeC­(OSiMe₃)­R (<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 UV–vis absorption spectra and to gain more insight into the electronic nature of <b>5a</b>–<b>c</b> and <b>6a</b>

Reactivity of Cyclic Silenolates Revisited

The stable exocyclic silenolates <b>2a</b>–<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 UV–vis spectroscopy. According to spectroscopic and structural features, <b>2a</b>–<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 °C. This transformation allowed straightforward access to 2-oxahexasilabicyclo[3.2.1]­octan-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-bis­(trimethylsilyl)­cyclohexasilane 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]­octasilanes, Me₃Si-Si₈Me₁₂-X, X-Si₈Me₁₂-X, and X-Si₈Me₁₂-Y [X, Y = −SiMe_<i>n</i>Ph_3–<i>n</i> (<i>n</i> = 1, 2) (<b>2</b>, <b>3</b>, <b>10</b>), −SiMe₂Fc (Fc = ferrocenyl) (<b>4</b>, <b>11</b>, <b>13</b>, <b>14</b>), −COR (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 Me₃Si-Si₈Me₁₂^–K⁺ or K^+–Si₈Me₁₂^–K⁺ 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 σ­(SiSi)/π­(aryl) conjugation, a 20–30 nm bathochromic shift of the longest wavelength UV absorption band relative to Me₃Si-Si₈Me₁₂-SiMe₃ (<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/CO interaction via the σ­(SiSi) bicyclooctasilane framework

Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes

Previously unknown 1,1,4-tris­(trimethylsilyl)-4-acyldodecamethylcyclohexasilanes (Me₃Si)₂Si₆Me₁₂(Me₃Si)­COR (<b>16a</b>, R = <i>tert</i>-butyl; <b>16b</b>, R = 1-adamantyl) have been synthesized by the reaction of the potassium silanides (Me₃Si)₂Si₆Me₁₂(Me₃Si)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 λ >300 nm radiation, they underwent Brook type 1,3-Si → O migration reactions to generate the cyclohexasilanes <b>17a</b>,<b>b</b> with exocyclic SiC bonds along with smaller amounts of the ring-enlarged species <b>19a</b>,<b>b</b> with endocyclic Si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 [(Me₃Si)₂Si­(Si₂Me₄)₂SiC­(R)­O]⁻K⁺ (<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>–<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⁺ counterion to the SiC­(R)O moiety. <b>2b</b>,<b>c</b>, thus, reacted with ClSi<i>i</i>Pr₃ to give the exocyclic silenes (Me₃Si)₂Si­(Si₂Me₄)₂SiC­(OSi<i>i</i>Pr₃)­R (<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 sp² silicon atom incorporated into a cyclopolysilane framework, could be fully characterized structurally and spectroscopically