Orientation complex

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

The results of the above-mentioned Langmuir analysis of the SHG responses may be interpreted in terms of a tightly packed monolayer of the SHG active cation complexes at the membrane surface. The tight layer may, however, also be a layer thicker than a monolayer in which the potential aligns the complexes to the electric field. As a consequence of the increase of the potential near the surface, the oriented complexes would on the average be nearer to the surface than the average of all complexes. 

If one examines the minimal sequences of reaction steps for [2+2] cycloadditions, Eqs. 12—18, 32—35, one concludes that stereochemistry of addition, and perhaps relative reactivities might be calculable at several points. Oriented complexes could control regiospecificity, or the transition state leading to a biradical could be the important stage. Relative rates of product formation would be derived from relative perturbation stabilization energies for different configurations of the two reactants. 

Methacrylic acid also polymerizes in bulk under precipitating conditions. It forms molecular associations very similar to those of acrylic acid. However, the conversion curves were found to be linear under a variety of experimental conditions temperatures of 16.5 to 60°C and broad ranges of initiation rates and monomer concentration in numerous solvents (7). It was assumed that structures of type III do arise but owing to steric hindrance and to the rigidity of the poly(methacrylic acid) molecule the monomer cannot align to form a "pre-oriented" complex as in the case of acrylic acid and propagation is not favored. 

A geometric isomer of the vinylallene mentioned above also undergoes [4 + 21-cycloaddition with butadiene to furnish a cydohexene derivative in 90% yield with excellent diastereomeric purity (>99 1). The preferred formation of a jt-allylpalla-dium complex from the axially oriented complex accounts for the trans selectivity (Scheme 16.79) [89]. 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

Prediction of the regioselectivity in the intermolecular photocycloaddition of enones to alkenes following this method provides similar results to those rationalized by the oriented -complex. However, it is in contrast with Weedon s previously discussed trapping results which indicate no selectivity in the first bond formation at the a- or /J-carbon positions in cyclic enones. 

Spectroscopic analysis revealed that the thermally initiated [3 + 2] polycycloaddition produced 1,4- and 1,5-substituted triazole isomers in an approximately 1 1 ratio. This ratio appears to be statistic and dependant on the bulkiness of the organic moieties. For example, hfr-r-P[30(4)-20] with butyl spacers contained slightly more 1,4-triazole isomers than did hb-r-P[30(6)-20] with hexyl spacers. This becomes clearer if we look at the proposed transition states a and b of the [3 + 2]-dipolar cycloaddition (Scheme 16). Because of their molecular orbital symmetry, the acetylene and azide functional groups arrange in two parallel planes, a so-called two-plane orientation complex [48], which facilitates a concerted ring formation. If the monomer fragment or the polymer branch ( ) attached to the functional groups are bulky, steric repulsion will come into play and transition state a will be... 

SO 2 seems to be a very specific catalyst. It is not excluded that the reaction goes via the same orientated complex which results in a Diels-Alder adduct under normal conditions. In this connection, it should be noted that the normal addition of acrylic ester and acrylonitrile to 3,4-dimethoxyfuran, according to Eugster and Hofmann,170 proceeds without added acid. 

Four basic structural arrangements as shown in Scheme 16 have so far proven to be of broad utility in synthesis a parent type (I), with a minimal but very variable set of functionalities, and three more target-oriented complexer units (II)-(IV). The bold lines in the formulae depicted emphasize the functionality pattern characteristic of each representative, constituting in sum a fully flexible and complementary set of possible synthetic modifications at the diquinane core. 

Fig. 9.10. Oriented complexes and nitrocyclohexadienylium intermediate in the nitration of benzene. Adapted from J. Am. Chem. Soc., 125, 4836 (2003), by permission of the American Chemical Society.

The intermediate 6.11 cannot be planar. Interaction of the ethene n molecular orbital of styrene can take place with the MO of the sydnone 1,3-dipole only if the o-bond systems of these reagents are located in two approximately parallel planes. Therefore, Huisgen (1963b) suggested the two-plane orientation complex 6.13 for the transition state of this cycloaddition. This transition state is feasible not only for... 

The photocycloadditions show all the characteristics of concerted reactions, including stereospecificity and regioselectivity. Concerted 1,3-dipolar additions are known to proceed via a two-plane orientation complex. For the case of diphenylazirine and methyl acrylate, there are two possible orientation complexes (26 or 27). The interaction of substituent groups in the syn complex 26 can be of an attractive (tc overlap, dipole-dipole interaction) or of a repulsive nature (van der Waals strain). Both effects are probably negligible in the anti complex 27. The ratio of the two steric courses syn and anti) functions... 

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