Cobalt‐Catalyzed Intramolecular Reactions between a Vinylcyclopropane and an Alkyne: Switchable [5+2] Cycloaddition and Homo‐Ene Pathways

Cobalt–diphosphine catalysts have been found to promote intramolecular reactions between a vinylcyclopropane and an alkyne to selectively afford either the [5+2] cycloaddition product or the homo‐ene reaction product under solvent control. The former product is exclusively formed in noncoordinating 1,2‐dichloroethane, whereas the latter is dominant in coordinating solvents, such as acetonitrile and dimethylacetamide. Furthermore, a highly enantioselective variant of the homo‐ene reaction afforded chiral tetrahydrofuran, pyrrolidine, and cyclopentane derivatives bearing 1,3‐diene and alkylidene substituents.     

Three‐Component Gallium(III)‐Promoted Addition of Halide Anions and Acetylenes to Donor–Acceptor Cyclopropanes

A new strategy for the three‐component addition of halide anions and acetylenes to donor–acceptor cyclopropanes (DACs) is presented. This reaction, which occurs with high E selectivity, is promoted by gallium(III) salts and based on the 1,2‐zwitterionic reactivity of DACs. It opens up a new group of processes involving DACs. The reaction occurs readily with a broad range of substrates and is tolerant of various functional groups. This methodology makes it possible to assemble highly functionalized vinyl halides, which are very convenient building blocks in organic synthesis. A possible mechanism of this reaction and its stereochemical aspects are discussed in detail.     

Structure-dependent transformations of the tetrachlorocyclopentadienone dimer and the product of its substitutive rearrangement in reactions with NaBH4, CrCl2, LiAlH4, and Zn

Dechlorination of the tetrachlorocyclopentadienone dimer and perchloro-7-oxotricyclo[6.1.0.0^4,8]nona-2,5-diene-9-carboxylic acid N,N-diethylamide with hydride ion donors (NaBH₄, LiAlH₄, etc.) and electron donors (CrCl₂ or Zn) were studied. The possible pathways of unusual transformations of both sterically hindered molecules are considered. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1001–1007, June, 2006.     

Divergent synthesis of 2-C-branched pyranosides and oxepines from 1,2-gem-dibromocyclopropyl carbohydrates

The ring opening of 1,2-(gem-dibromo)cyclopropyl carbohydrates by two different modes leads to either 2-C-(bromomethylene)pyranosides (using base) or 2-bromooxepines (using silver salts), as shown previously by us for a d-glucal-derived cyclopropane. The base-promoted ring opening is extended to encompass additional alcohol, thiol and amine nucleophiles, and diastereoisomeric cyclopropane precursors. Cross-coupling of the 2-C-(bromomethylene)pyranosides leads to extended 2-C-branched pyranosides. Silver-promoted ring expansion of the cyclopropyl carbohydrates in the presence of various alcohols is described. Cross-coupling of the resulting benzyl 2-bromooxepines affords 2-C-substituted oxepines.     

Calculation of the polymerization process of some 1,1-disubstituted cyclopropanes: the first propagation step

A theoretical study of the first steps of the polymerization propagation process of 1,1 cyano, fluoro and methyl disubstituted cyclopropanes and 1-cyano 1-amino cycloprane is presented. The thermodynamic and kinetic feasibility of the reactions are discussed through 6-31+G(d,p) Hartree–Fock calculations. The anionic initiation and propagation process follows an S_N2 like reaction path mainly governed by the differences in electron affinities between the incoming and outgoing anions, further modulated by the nature of the substituents. Main conclusions are that the π electron-withdrawing groups maintain the barrier heights low, making the process feasible from a kinetic viewpoint. The σ electron-withdrawing groups give more exothermic reactions but their reaction barriers are very high.     

Synthesis and transformations of metallacycles

A regioselective method for the synthesis of 1,1-dialkylcyclopropanes was developed. The method is based on the reaction of 2,3-dialkyl-1-ethylalumacyclopent-2-enes with an excess of dialkyl sulfates (Me₂SO₄ or Et₂SO₄). A plausible reaction mechanism was suggested. For Part 20, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1092–1095, June, 2000.     

Copper-Catalyzed Enantioselective Hydrosilylation of gem-Difluorocyclopropenes Leading to a Stereochemical Study of the Silylated gem-Difluorocyclopropanes

Optically active cyclopropanes have been widely investigated especially from the views of pharmaceutical and agrochemical industries, and substituting one of the methylenes with the difluoromethylene unit should be promising for developing novel biologically relevant compounds and functional materials. In this paper, the copper-catalyzed enantioselective hydrosilylation of gem-difluorocyclopropenes to provide the corresponding chiral gem-difluorocyclopropanes is presented. The use of copper(I) chloride, chiral ligands including bidentate BINAPs and monodentate phosphoramidites, and silylborane Me₂PhSi-Bpin accompanying sodium tert-butoxide in methanol was appropriate for the enantioselective hydrosilylation of the strained C=C double bond, and the resultant chiral difluorinated three-membered ring was unambiguously characterized. Subsequent activation of the silyl groups in enantio-enriched gem-difluorocyclopropanes showed substantial reduction of the enantiopurity, indicating cleavage of the distal C−C bond leading to the transient acyclic intermediates.