Badreldin Ibrahim

1,3-Dipolar character of six-membered aromatic rings. Part XXI. Thermal cycloadditions of 1-(5-nitro-2-pyridyl)- and 1-(4,6-dimethylpyrimidin-2-yl)-3-oxidopyridinium with 2, 4, and 6 π-electron components

The regio- and stereo-selectivities of reactions of the title compounds at the 2- and 6-positions with mono- and disubstituted ethylenes are elucidated and rationalised by FMO theory including secondary orbital overlap and taking into account steric and dipolar effects. For monosubstituted ethylenes, FMO theory correctly predicts that electron donor, electron acceptor, and conjugated olefins all add in the same regioselective sense. Stereoselectivity is shown to depend rationally on the structures of the betaine and the addend.Buta-1,3-diene, its 2,3-dimethyl and 1-monomethyl derivatives, and cyclopentadiene add as 4π-electron addends at the 2- and 4-positions of the betaines. Such reactions are favoured kinetically, but under thermodynamic control reaction also occurs with the diene acting as a 2π-addend at the 2- and 6-positions of the betaine. The peri-selectivities and stereoselectivities are discussed in terms of FMO theory: the reversal of electron demand leading to easier reaction with electron-rich dienes is explained by PNIFLS.Fulvenes add at the betaine 2- and 6-positions. Peri-, regio-, and stereo-selectivity are successfully rationalised in terms of FMO theory.

Carbon-13 nuclear magnetic resonance spectroscopy of polymers. Part I. High resolution carbon-13 nuclear magnetic resonance spectroscopy: tacticity studies on poly(vinyl acetate) and monomer distribution analysis in ethylene–vinyl acetate copolymers

Noise-decoupled pulsed Fourier transform 13C n.m.r. spectra reveal the tacticity in poly(vinyl acetate) and the monomer sequence distribution (MSD) in ethylene (E)–vinyl acetate (V) copolymers. Assignments of carbon absorptions were made from 13C ‘un-decoupled’ spectra and the methylene and methine carbon atoms were found to be especially sensitive to environment. The signal due to the methine carbon atoms in poly(vinyl acetate) is resolved into two peaks, showing a preponderance of heterotactic triads. In E–VA five MSD triad environments are predicted for the five methylene peaks observed and four MSD triad environments predicted for the four methine peaks observed. In addition, multiplicity in the four methine MSD triad peaks reveals a partially resolved MSD pentad and a tactic triad for block vinyl acetate VVV. Evaluation of corrected methine and methylene absorption intensities allows direct comparison of MSD triad intensities and shows the self-consistency in assignments. The compositions of the three samples (67, 38, and 14 mole % vinyl acetate) were determined in terms of the % population of all eight MSD triads and plotted to show triad composition for a whole range of ethylene–vinyl acetate copolymers. In these commercial samples both ethylene and (especially) vinyl acetate units show a tendency to block.

1,3-Dipolar character of six-membered aromatic rings. Part IX. Rearrangement and additions of 1-(2,4-dinitrophenyl)-3-oxidopyridinium

1-(2,4-Dinitrophenyl)-3-oxidopyridinium rearranges easily to 3-(2,4-dinitrophenoxy)pyridine. The betaine forms adducts with dipolarophiles by addition across the 2- and 6-positions.

1,3-Dipolar character of six-membered aromatic rings. Part XX. Preparation and dimerisation of 1-(5-nitro-2-pyridyl)- and 1-(4,6-dimethylpyrimidin-2-yl)-3-oxidopyridinium

The title betaines form dimers, the structures and stereochemistry of which are elucidated. The regio- and stereoselectivity of dimer formation are rationalised and the variation of reactivity of 3-oxidopyridinium betaines with their substitution patterns is discussed in MO terms.

Rationalisation of the variation of reactivity towards cycloaddition with structure in 3-oxido-pyridinium and -azinium betaines and a study of 1-(4,6-dimethyl-2-pyrimidinyl)-3-oxidopyridinium

Frontier M.O. theory rationalises the effects of aza-substitution on the dimerisation tendency in 3-oxidopyridiniums, the 1-(2-pyrimidinyl) betaine displaying the predicted high reactivity; fulvenes add as 6 π-electron systems across the 2,6-positions of 3-oxido-pyridiniums.

The dimer of 1-(5-nitro-2-pyridyl)-3-oxidopyridinium and its reactions

The betaine 1-(5-nitro-2-pyridyl)-3-oxidopyridinium reversibly forms a dimer which provides a convenient source of the reactive betaine monomer; olefinic dipolarophiles add readily at the 2,6-positions, 2,3-dimethylbutadiene reacts across the 2,4-positions and stereochemical integrity is preserved during the addition of maleic and fumaric esters.

1-(2,4-Dinitrophenyl)-3-oxidopyridinium: rearrangement and dipolar additions

1-(2,4-Dinitrophenyl)-3-oxidopyridinium rearranges spontaneously to the isomeric ether on attempted isolation, but can be trapped with olefins and behaves as a highly reactive 1,3-dipole.