Bond Stretch Isomerism in Bicyclo[1.1.0]tetrasilanes. Contrasts Between Strained Silicon and Carbon Ring Systems

Abstract

group in 14c has been introduced for two reasons: firstly, for the later conversion of the photolactones 4 and 6 into the stereoisomeric dienone lactones and, secondly, to achieve regioselective Wessely acetoxylation. The oxidation of 14c with Pb(OAc),, however, must then be catalyzed by Lewis acid: with BF3.ether in a mixture of methanol and ethyl acetate["I the o-quinol acetates 1 and 3 are isolated in 85% yield.To sum up: the synthesis of 8 shows, for the first time, how useful photolactonization for the synthesis of macrolides really is and, once again, that in a cyclic system the information of a remote center of chirality can be transmitted to a high degree to a newly developing stereogenic center.

MC‐SCF and CI calculations on the Si4H4 system

“…These structural properties are met best by isomer 4 of Ref. 6, where this bond stretch isomerism of the Si4H6 system has been described for the first time. …”

Isolable silylene, disilenes, trisilaallene, and related compounds

“…We noticed immediately the importance of the structural issue of 1,3-disilabicyclo[1.1.0]butane 13b because we were aware of the theoretical papers on metallabicyclo[1.1.0]butanes reported by Schleyer et al [29] during our study of Si 4 R 6 isomers. The theoretical papers showed that two isomers, long-bond (LB) and shortbond isomers (SB), exsisted in metallabicyclo[1.1.0]butanes.…”

“…Whereas the theoretical prediction [11] was not confirmed by the X-ray analysis of these disilenes because the geometry around Si@Si bond was very flexible and strongly influenced by the steric bulkiness of the substituents and crystal packing force. The 29 Si resonances for the unsaturated silicons in 2a-2e appeared at 140-150 ppm. These unusually low-field resonaces were understood by the significantly large deshielding along one principal axis related to the r !…”

“…The facile intramolecular isomerization between (E)-2d and (Z)-2d was observed by temperature-dependent 29 Si NMR spectroscopy to occur with the free energy of activation less than 15 kcal/mol at 300 K [12a]. The activation energy was ca.…”

“…The 29 Si resonance of the divalent silicon in 28 appeared at the extraordinarily low magnetic field (+567.4 ppm) and the absorption maximum due to n ! p transition was observed at 440 nm (e 500), indicating that 28 was well-protected sterically from dimerization but least perturbed electronically.…”