150 research outputs found
Competing reactions of hypercoordinate silicon dichelates
No full textNeutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino-donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol-type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four-membered ring. In the absence of et-protons necessary for the condensation, the four-membered ring residue adds directly to one of the imino-carbon atoms forming a new C-C bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 alpha-protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t-butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright (C) 2008 John Wiley & Sons, Ltd.Israel Science Foundation [ISF-139/05]; INTAS [03-51-4164
Donor-stabilized silyl cations. 11. Bis-zwitterionic penta- and hexacoordinate silicon dichelate complexes derived from (ClCH2)(2)SiCl2 through double internal displacement of chloride
No full textThe reaction of (ClCH2)(2)SiCl2 (4) with O-Me3Si-hydrazides (3) yields three new types of products: a 1: 2 reagent ratio leads to a neutral hexacoordinate dichelate with two six-membered rings each containing an N+-N- ylide structure, as well as to the corresponding dissociated pentacoordinate dichelate siliconium chloride salt. These products readily undergo partial hydrolysis to form a dinuclear complex: dicationic disiloxane dichloride. All three product types have been characterized by means of single-crystal structure analysis. Nonempirical ab initio calculations were carried out to determine the atomic charges. Natural bond order (NBO) calculations show that there are no significant charge accumulations on either the ammonium or the imidate nitrogen in the ylide moieties and that the net charge on silicon (which could formally be -2) is between +1.8 and +2.1 e in all species, regardless of overall molecular charge
Simon Dubnow (standing, center) at the 1st international YIVO conference, Vilna, Poland
No full textOther historians and notables on the stage and dais include (seated, from left) Joseph Tshernikhov, Elias Tcherikower, Yankev Botoshansky; (behind Botoshansky’s left shoulder) Zelig Kalmanovitch; (to Dubnow’s right) Ignacy Schiper; (behind Dubnow’s right shoulder) Rafail Abramovich; (seated, first to fourth from right) Yudl Mark, Yankev Shatzky, Max Weinreich, Zalmen Reyzen; (standing behind Weinreich), Nakhman Meisel. The portrait of Tsemaḥ Szabad (right) is draped in black in commemoration of his death, which had occurred a few months earlier.Digital imagedigitize
Competitive molecular rearrangements in hexacoordinate cyano-silicon dichelates
No full textPentacoordinate siliconium chloride or neutral hexacoordinate silicon complexes with imino-nitrogen donor groups react with cyanotrimethylsilane in two competing reactions, leading either to addition of the cyano group to the imino carbon or to hexacoordinate cyano-silicon complexes. The latter may further transform to a rearranged tricyclic pentacoordinate complex. The common driving force for these reactions seems to be the conversion of one of the two initial N -> Si dative bonds, present in the starting complexes, to a shorter formal covalent bond
Combined Carbometalation–Zinc Homologation–Allylation Reactions as a New Approach for Alkoxyallylation of Aldehydes
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