Reactivity of Cyclic Silenolates Revisited

Abstract

The stable exocyclic silenolates 2a–c (2a, R = Mes; 2b, R = o-Tol; 2c, R = 1-Ad) were fully characterized by NMR and UV–vis spectroscopy. According to spectroscopic and structural features, 2a–c are best described as acyl silyl anions (tautomeric structure I) in solution. This behavior is also reflected by the reaction of 2a,c with MeI. Both alkylation reactions take place at the corresponding silicon atom and lead to the formation of the methylated structures 4a,b in nearly quantitative yields. Furthermore, the thermal stability of exocyclic silenolates 2a,c was investigated. In the case of 2a, 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 5 as a structurally complex, bicyclic silicon framework. In contrast to that, heating of 2c, 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 8, which is structurally closely related to 5, 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 8. 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 of Stable Dianionic Cyclic Silenolates and Germenolates

“…We selected MeI as a carbon-centered electrophile, because it represents a benchmark reagent with numerous examples found in the literature. ,, In the reaction of 2a , b and 4a , b with MeI, the same reactivities in terms of reaction sites were observed. In all cases, alkylation of the negatively charged silicon as well as germanium atoms were found in nearly quantitative yields (Scheme ).…”

“…26 Noteworthy, CAM-B3LYP achieved a better consistency for dianions 2a , b and 4a , b in calculated vertical excitations in comparison to B3LYP, which was previously applied to UV–vis calculations on silenolates S 1 a , b and germenolates S 2 a , b . 24 , 27 The silenolates 2a , b exhibit intense absorption maxima in the range between 433 and 450 nm, which are red-shifted in the order 2b → 2a . The same bathochromic trend 4b → 4a also applies to the germenolates 4a , b with absorption maxima between 420 and 447 nm.…”

“…Analytical and spectroscopic data (see the Experimental Section) clearly supported the bicyclic structure of 11 (see Scheme ). Moreover, this compound was obtained by a previous study by our group …”

Synthesis and Characterization of New Counterion-Substituted Triacylgermenolates and Investigation of Selected Metal–Metal Exchange Reactions

“…The direct approach circumvents the usage of alkali metals and results in higher yields of the target compounds (Scheme ). In general, the synthesis of HG 14 enolates was focused exclusively on lithium and potassium derivatives. − ,− Other counterions were not investigated so far, although they can significantly influence the reactivity and the structural properties of these enolates. Consequently, the aim of this study was to introduce new counterions.…”