H. G. Viehe

Alpha-cyclization of Tertiary-amines .3. Captodative Or Push-pull Enamines Form Pyrrolines, Pyrrolizidines and Their Ring Homologs With Dimethyl Acetylenedicarboxylate in a Highly Diastereoselective Reaction

The title one-pot reaction occurs through a sequence of [2+2] cycloaddition ring opening to dienamines and formation of a 1,5-dipole by [1,6] hydrogen-shift. This intermediate may cyclize or in sterically suitable cases and at lower temperatures, lead to isolable N-vinyl enamines by proton transfer.

Captodative Substituent Effects .31. Olefins With Captodative Substitution in [2+2] Cyclo-additions

Abstract Olefins with captodative substitution are excellent partners in [2+2] cycloadditions leading to cyclobutane derivatives. The reaction rates increase with the radical stabilising power of the substituents. Thio- and selenoalkyl(aryl) substituted gemdifluoroolefins allow the synthesis of new cyclobutane derivatives.

Captodative substituent effects XXI. : Synthesis of selenenylated captodative olefins via selenenyl halide addition to olefins bearing electron-withdrawing substituents

Abstract Addition of methane- and benzeneselenenyl bromide or chloride and benzene sulfenyl chloride to carbon-carbon double bonds substituted by electronwithdrawing groups is achieved in solvents of different polarity. Two regioisomeric adducts 6 and 7 or 8 and 9 are generally formed, which can be interconverted by equilibration in refluxing acetonitrile. It is of mechanistic interest that the regioisomers may also derive from selenenyltrihalide adducts. In comparison to acrylic esters, the propiolic ester reacts more slowly, producing mainly the α-selenenyl adduct. Dehydrohalogenation of adducts provides a general and valuable method for the preparation of olefins carrying methyl or phenylselenenyl groups in α-position to electron-withdrawing substituents.

Capto-dative Substituent Effect On the Cis-trans Isomerization of Cyclopropanes .11.

Abstract Very low activation energies were found for the cis-trans isomerisation of capto-datively substituted cyclopropanes.

Capto-dative Substituent Effects .14. Kinetics of the Cis-trans Isomerization of Cyclopropanes - Comparison Between Capto-dative and Di-capto Radical Stabilization

Abstract Capto-dative (cd) (c = CN, d = OR, SR, NR2) compared to di-capto substitution (c = CN, c′ = COOR) was found to be more efficient in lowering the activation energy of cis-trans isomerisation of cyclopropanes.

Alpha-cyclization of Tertiary-amines .1.

general, diastereoselective synthesis of functionalised pyrrolidine, indolizidine, pyrrolizidine and its seven-, eight-membered homologues is reported, starring from readily available enamine eaters and acetylenedicarboxlate (DMAD).

Capto-dative substituent effects—VIII

Abstract Dehydrodimerization is suppressed completely in the presence of radicophiles such as cd-substituted olefins. Particularly α-t-butylmercaptoacrylonitrile (TBMA) has been examined as a trapping agent for carbon centered radicals. The adducts dimerize thus building C4-bridges between the original radicals. This general principle is synthetically useful as demonstrated by C4-bridging of alkanes, ethers, amines, amides, aldehydes and ketones.

Alpha-cyclization of Tertiary-amines .4. Cyano-imidoyl Substituted Furans From Captodative Enamines and Dibenzoylacetylene

alpha-Cyano-enamines and dibenzoylacetylene lead to dienes (via [2+2] cycloaddition followed by ring-opening). Their a-Cyclisation to oxygen produces the tide compounds.

Synthesis of Fluorinated Vinyl Sulfides and Selenides

Abstract The reaction between fluoroolefins 1 and 10e and benzenesulfenyl or selenenyl halides 6 is found to be solvent-dependent and gives in most cases predominantly the regioisomer 8 .The structure of adducts 8 and 9 are ascertained by 1 H, 19 F and 77 Se NMR spectroscopy. Compounds 8 are easily halogenated and treatment of the products with magnesium or zinc leads to the desired polyfluorovinyl Sulfides and selenides 10 . A second route of synthesis of these reagents results from the reaction (of fluorovinyllithio derivatives 11 with benzenesulfenyl or selenenyl halides. Olefin 10e is also obtained from the selenoacetal 8t of trifluoroacetaldehyde.

Cycloadditions of Methylenecyclopropanes and Strained Bicyclo[n.1.0]alkanes To Radicophilic Olefins

Abstract The methylenecyclopropanes 1, 2, 11 upon reaction with α-donorsubstituted acrylonitriles 3a, 3c gave two types of products, regular [2+2] cycloadducts 6, 8, 10, 13, 14 and thiacyclopentane derivatives 7, 9. The capto-dative substituted methylenecyclopropane 12 preferentially homodimerized to 15 even in the presence of 2 . The capto-dative olefin 3a also added across the central single bonds of a bicyclo[1.1.0.]butane derivative 4b and bicyclo[2.1.0]pentane 5a to yield 16 and 19 via diradical intermediates.