Charge polarization isomer

Azacyclohexatriene-2-ylidene (3), the 2-isomer of pyridine, has been generated by one-electron oxidation of the corresponding radical cation in neutralization-reionization mass spectrometry.17 It was determined by ab initio H-F calculations that the charge polarization of the radical fonned by H-abstraction from pyrazine can be... 

The structure of [FeCo(CO)g] , with idealized Cg symmetry, is closely related to that of the lowest energy M2(CO)g isomer (which has Z)2d symmetry) but does not correspond to a minimum of the PES computed from steric forces alone (Lauher, to force the calculation toward the experimental stereogeometry, had to constrain the Fe-Co-C angle of the semibridging carbonyl group to its experimental value). As a matter of fact, the sterically favored D2d structure, when the metals contribute a different number of valence electrons, is highly polarized and distortion must occur to quench charge polarization partially (Fig. 2). 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

Polar-substituted alkenes where the functionality is not attached to a strained ring are considerably more discriminating in their compatibility with metathesis catalysts and as a rule require relatively high catalyst charges. In the aliphatic series, unsaturated esters have received the most attention. Boelhouwer reported in 1972 the metathesis of the ester methyl oleate and its trans isomer, methyl elaidate, with a homogeneous catalyst based on a 1/1.4 molar combination of WCl6/(CH3)4Sn (23). At 70°C and an ester/W molar ratio of 33, near-thermodynamic equilibrium was attained, and 49 and 52% of the respective esters were converted to equal amounts of 9-octadecene and the dimethyl ester of 9-octadecene-1,18-dioic acid. 

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

The stabilisation is particularly marked in that not only is an extra (fourth) canonical state involved in the stabilisation of the o- and p-positive charge is located on oxygen, are inherently more stable than their other three complementary forms, in which the positive charge is located on carbon (cf. 48a— 48c, and 49a— 49c). This effect is sufficiently pronounced to outweigh by far the electron-withdrawing inductive (polar) effect also operating in these two cr complexes, substitution is thus almost completely o-lp- ( 1% of the m-isomer is obtained in the nitration of PhOMe), and much more rapid than on benzene itself (kcsH50Me/fcc 6H = 9-7x 10 for chlorination). 

The trans isomer turned out to be favored over the cis isomer by 41.8 kJmol at B3LYP/aug-cc-pvTZ level of theory. In both isomers the C—N—O moiety is strongly polarized. The charges are 4-0.24 e for both nitrogens and —0.50 and 0.51 e for both... 

Under these conditions only one mixed [6,6]-triazoline-azahomofullerene isomer 25 was formed, which is explained by the fact that 24 behaves as a strained electron-poor vinylamine. The significantly highest Mulliken charge of 0.06 (AMI) is located at C-1 and C-6, and the lowest of-0.07 at C-2 and C-5 (Figure 10.10). The most negatively polarized N atom of the azide (AMI) is that bearing R. A kinetically controlled attack of the azide, therefore, leads predominantely to 25. 

The nitrone 127 (96% ee) containing an additional carbon atom was synthesized in a similar manner as 124. However, for nitrone 127, no spontaneous cyclization took place (Scheme 12.43). Upon heating of 127 in toluene, an inseparable mixmre of product isomers was formed. It was also found that in the presence of a stoichiometric amount of ZnCl2, the reaction proceeded to give the bicyclo[4.2.1] product 128 as the only observed product with close to complete retention of enantiopurity (ee=94%). The authors propose a more polarized transition state of the Zn-mediated reaction with a well-developed positive charge on one of the carbon atoms of the alkene moiety to account for the inverted direction of the regioselectivity of the reaction (230). A subsequent reduction gave the functionalized cycloheptane 129. 

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