Ring-Opening Mechanism of Lithium Bromosilacyclopropylidenoids to Silaallenes

Diverse reactions of a fluorostannylenoid towards ethynes

Fluoro(dialkyl)stannylenoid 2 exhibits unique reactivity towards ethynes with acetylenic hydrogen and those with trimethylsilyl groups, though the corresponding free dialkylstannylene 1 is inactive against those ethynes. Stannylenoid 2 reacts smoothly with gaseous ethyne and phenylethyne at room temperature, giving the corresponding diethynylstannanes, di(phenylethynyl)stannane 3 and diethynylstannane 6, respectively, in good yields with the concomitant evolution of H₂. Trimethylsilyl-substituted ethynes such as 1-trimethylsilyl-(2-phenyl)ethyne and 1,2-bis(trimethylsilyl)ethyne react similarly to give 3 and bis(trimethylsilylethynyl)stannane 8, respectively. Rather unexpectedly, the reaction of 2 with (trimethylsilyl)ethyne affords 1,2-bis(ethenylstannyl)ethyne 7 in a good yield. The reactions of 2 with methyl and ethyl propynoates give the same products 4 and 5 as those obtained during the reaction of dialkylstannylene 1 without CsF. Pathways involving the nucleophilic attack of cesium acetylide to an ethyne-complexed stannylene were proposed, while the detailed mechanisms remain unknown. The structure of 7 was studied by single crystal X-ray diffraction analysis.

Reversible Stannylenoid Formation from the Corresponding Stannylene and Cesium Fluoride

A fluorostannylenoid (Cs⁺ [R₂ SnF]^- (9), R₂ =(TMS)₂ CCH₂ CH₂ C(TMS)₂ ) was prepared by reacting a stable dialkylstannylene (R₂ Sn (8), R₂ =(TMS)₂ CCH₂ CH₂ C(TMS)₂ ) with cesium fluoride at room temperature in THF. While 9 is stable in THF and DME, removal of the solvent leads to the regeneration of stannylene 8. No reaction occurred when 8 was treated with CsF in a hydrocarbon solvent. Addition of dibenzo-21-crown-7 ether to the THF solution of stannylenoid 9 followed by usual workup affords the corresponding crystalline stannylenoid crown ether complex, the X-ray structural analysis of which revealed a fluorine-bridged contact ion-pair structure. The reaction of 9 with excess phenylacetylene gives the corresponding di(phenylethynyl)stannane.

A mechanistic investigation on the formation and rearrangement of silaspiropentane: A theoretical study

The formation of silaspiropentane from addition of singlet silacyclopropylidene 1 and silacyclopropylidenoid 8 to ethylene has been investigated separately at the B3LYP, X3LYP, WB97XD, and M05-2X theories using the 6-31+G(d,p) basis set. The silacycloproylidenoid addition follows a stepwise route. In contrast, a concerted mechanism occurs for silacyclopropylidene addition. Moreover, the intramolecular rearrangements of silaspiropentane 9 to methylenesilacyclobutane 11 and 2-silaallene + ethylene 12 have been studied extensively. The required energy barrier for the isomerization of 9 to 10 was determined to be 44.0 kcal mol(-1) at the B3LYP/6-31+G(d,p) level. After formation of 10, the rearrangement to methylenesilacyclobutane 12 is highly exergonic by -15.9 kcal mol(-1), which makes this reaction promising. However, the conversion of 9 to 11 is calculated to be quite endergonic, by 26.5 kcal mol(-1).