A carbon-13 NMR study of some alkyl-substituted indenes. An experimental study of the baker-nathan order of hyperconjugation in the neutral ground state

Unexpected Pyrolytic Behaviour of Substituted Benzo[c]thiopyran and Thieno[2,3-c]thiopyran S,S-dioxides

Flash vacuum pyrolysis (FVP) of benzo[c]thiopyran S,S-dioxide (1) results in formation of indene and 2-vinylbenzaldehyde as previously described. A range of eight analogues with various substitution patterns are found to behave differently. In general, there is no extrusion of SO2 to give products analogous to indene, but unsaturated carbonyl products analogous to 2-vinylbenzaldehyde are formed in most cases by way of ring expansion to a 7-membered ring sultine, extrusion of SO, and intramolecular hydrogen atom transfer. Other processes observed include formation of anthracene via an isomeric 7-membered sultine with loss of SO, CO and methane or butane, and formation of 4-ethylidene-4,5-dihydrocyclobuta[b]thiophenes by way of SO loss, a radical rearrangement, and extrusion of acetone. The analogues with a halogen substituent at position 8 on the benzene ring require a higher temperature to react and give naphthalene resulting from net elimination of HX and SO2. The X-ray crystal structure of 1 is also reported.

A general one-pot synthesis for elusive 2-substituted indenes: does bis[2-(tert-butyl)indenyl]zirconium(IV) dichloride/MAO polymerise ethene?

Among the 2-substituted indenes, 2-trifluoroindene and 2-tert-butylindene are poorly or incompletely described in the open literature. We herein describe an efficient one-pot synthesis of these compounds as a variation of the Perkin reaction which allows us to refute an earlier claim that bis(2-tert-butylindenyl)zirconium(IV) dichloride (2a) will not polymerise ethene. In fact, 2a/MAO polymerises ethene to extremely high molecular weights. Extensive DFT calculations on the polymerisation mechanism revealed an unprecedented suppression of the otherwise predominant chain termination by beta-hydride transfer due to steric congestion of the active site.

Parabolic Relationship between the Basicity of the Nucleophile and pi-Face Selection in Addition of the Substituted Acetylide Ions to Cyclohexanone and Cyclohexanethione

Energy changes, structural variation, and electron density shifts during nucleophilic addition to cyclohexanone and cyclohexanethione were examined by means of the ab initio calculations at the HF/6-31G level. The atomic charge on the carbonyl C was found to become more positive upon approach of the nucleophile; the density deformation maps suggest that the charge polarization occurs to a large extent in the pi bond. Since this effect is not compensated for by charge transfer until in the late stage of addition, the reaction site is considerably electron deficient (more so than the carbonyl C in the substrate) for most of the reaction path, and its interactions with the ligands are dominated by hyperconjugation with the vicinal C-H and C-C bonds. Relative stabilities of the axial and equatorial transition-state structures were examined in the series HN=C, ON&tbd1;C(-), N&tbd1;C(-), FC&tbd1;C(-), HC&tbd1;C(-), SBeC&tbd1;C(2-), O(3)SC&tbd1;C(2-), H(3)AlC&tbd1;C(2-), SC&tbd1;C(2-), H(3)BC&tbd1;C(2-), and C&tbd1;C(2-) and found to depend on polarization of the electrophiles, C=O vs C=S, and basicity of the nucleophiles. The latter dependence is parabolic: the axial preference reaches a maximum for the moderately basic anions, and it is diminished or even reversed for the most and least basic nucleophiles, i.e., in the case of reactions proceeding through the very early or very late transition states. Thus, the stereoelectronic effect is largest in the region of the reaction coordinate where the electron deficiency at the reaction site reaches a maximum. These findings corroborate the premises of the hypothesis of hyperconjugative assistance to bond formation and are consistent with major trends in the experimental data if the stereochemistry of alkylation of 4-tert-butylcyclohexanone is correlated with Pauling electronegativity of metals in the case of methylmetals, methylmetal ate complexes, allylmetals and arylmetals, and with the pK(a) of conjugated acids in the case of sulfur-, carbonyl-, and nitrile-stabilized carbanions.