Synthesis and Unusual Reactivity of Acyl-Substituted 1,4-Disilacyclohexa-2,5-dienes.

In continuation of our recent studies on group 14 rings with exocyclic silicon-carbon double bonds, we report here on the synthesis and reactivity of previously unknown acyl-substituted 1,4-disilacyclohexa-2,5-dienes. 1,1,4,4-Tetrakistrimethylsilyl-1,4-disilacyclohexa-2,5-diene 1 cleanly afforded the silyl anion 1-K after addition of 1 equiv of KO t Bu. 1-K subsequently could be reacted with various electrophiles to the expected substitution products including compounds 4 and 5. When photolyzed with λ > 300 nm radiation, 4 and 5 undergo Brook-type 1,3-Si → O migration reactions to generate the corresponding 1,4-disilacyclohexadienes with exocyclic Si=C bonds as the primary products. These metastable silenes only could be characterized in form of appropriate quenching products. The reaction of compound 4 with KO t Bu followed by the addition of 1 equiv of PhMe2SiCl surprisingly gave the silylated 1,4-disilanorbornadiene cages 8 and 9 instead of the expected exocyclic silene. The responsible sila-Peterson-type mechanism could be elucidated by density functional theory calculations at the conductor-like polarizable continuum model (THF) B3LYP-GD3/6-31 + G(d) level and by the isolation and characterization of unstable intermediate products after proper derivatization.

Synthesis of d-Galactose-Substituted Acylsilanes and Acylgermanes. Model Compounds for Visible Light Photoinitiators with Intriguing High Solubility.

A convenient synthetic method to obtain d-galactose-substituted acylsilanes and acylgermanes is described. These acyl group 14 compounds are easily accessible in good yields. Their structural properties were analyzed by a combination of NMR, single crystal X-ray crystallography, and UV/vis spectroscopy. A d-galactose-substituted tetraacylgermane represents a new interesting visible light photoinitiator based on its absorption properties as well as its high solubility.

Sila-Peterson Reaction of Cyclic Silanides.

Sila-Peterson type reactions of the 1,4,4-tris(trimethylsilyl)-1-metallooctamethylcyclohexasilanes (Me3Si)2Si6Me8(SiMe3)M (2a, M = Li; 2b, M = K) with various ketones were investigated. The obtained products strongly depend on the nature of the ketone component. With 2-adamantanone 2a,b afforded the moderately stable silene 3. 3 is the first example of an Apeloig-Ishikawa-Oehme-type silene with the tricoordinate silicon atom incorporated into a cyclopolysilane framework and could be characterized by NMR and UV spectroscopy as well as by trapping reactions with water, methanol, and MeLi. The reaction of 2b with aromatic ketones also follows a sila-Peterson type mechanism with formation of carbanionic species. With 1,2-diphenylcyclopropenone 2b reacted by conjugate 1,4-addition to give a spirocyclic carbanion. In most cases the underlying reaction mechanism could be elucidated by the isolation and characterization of unstable intermediates and final products after proper derivatization.

Branched Hydrosilane Oligomers as Ideal Precursors for Liquid-Based Silicon-Film Deposition.

Herein a convenient synthetic method to obtain 2,2,3,3-tetrasilyltetrasilane 3 and 2,2,3,3,4,4-hexasilylpentasilane 4 on a multigram scale is presented. Proton-coupled 29 Si NMR spectroscopy and single-crystal X-ray crystallography enabled unequivocal structural assignment. Owing to their unique properties, which are reflected in their nonpyrophoric character on contact with air and their enhanced light absorption above 250 nm, 3 and 4 are valuable precursors for liquid-phase deposition (LPD) and the processing of thin silicon films. Amorphous silicon (a-Si:H) films of excellent quality were deposited starting from 3 and characterized by conductivity measurements, ellipsometry, optical microscopy, and Raman spectroscopy.

Synthesis of Structurally Complex Silicon Frameworks through the First Sila-Aldol Reaction.

Herein, we report on the first sila-aldol reaction, which emphasizes the tight connection between silicon and carbon chemistry. This novel synthetic method provides straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide, a structurally complex silicon framework, in quantitative yield. Its structure was confirmed by NMR spectroscopy and X-ray crystallography, and it displays a distinctive charge-transfer transition. The complete mechanism of this highly selective rearrangement cascade is outlined and supported by density functional theory (DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this powerful silicon-carbon bond-forming strategy.

Tetraacylgermanes: Highly Efficient Photoinitiators for Visible-Light-Induced Free-Radical Polymerization.

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R]4 (R=mesityl (1 a), phenyl (1 b)), a previously unknown class of highly efficient Ge-based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one-pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The efficiency of 1 a,b as photoinitiators is demonstrated in photobleaching (UV/Vis), time-resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo-DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long-wavelength visible-light photoinitiators for free-radical polymerization.