Couplings ortho, meta, para

In separations limited by azeotrope formation under nonreactive conditions, the addition of a reaction (that is, usually adding catalyst) constantly changes the concentrations such that the separation continues beyond the azeotrope. In processes with coupled products (enantiomers, ortho-/meta-/para-substitution), the in-situ removal of the desired product will favor the reaction towards this product, and will therefore strongly increase the selectivity. 

Porphyrin Pyrrole ortho meta para -CH2- Herr0 K gef 0 K Nature of (CDjClj) (solid) coupling (Up) (up) in solution Ref. 

There are several stereoisomers in disubstituted [2.2]paracyclophane, such as ortho-, meta-, para-, pstudo-ortho-, pseudo-m fa-, pseudo-para-, and pseudo-g mma/-disubstituted [2.2]paracyclophanes. Iron-catalyzed dibromination of commercially available [2.2]paracyclophane yields the mixtures of the dibromo[2.2]paracyclo-phane isomers, and pseudo-para-dibromo[2.2]paracyclophane 9 can be obtained by recrystallization as it has the poorest solubility in common organic solvents [76]. Thermal isomerization of 9 in triglyme afforded the psowdo-ortho isomer 38, as shown in Scheme 10. This transformation is an equilibrium reaction when the reaction was allowed to cool to room temperature, pseudo-para isomer 9 was precipitated as a result of its low solubility and was readily separated by simple filtration to obtain pseudo-arf/za isomer 38 in moderate isolated yield [76]. V ondo-ortho-dibromo[2.2]paracyclophane 38 was converted into pseudo-arf/ a-diethynyl[2.2] paracyclophane 40 via pseudo-arf/ a-diformyl[2.2]paracyclophane 39 (Scheme 10) [77]. Sonogashira-Hagihara coupling polymerization of 40 with 12 afforded the corresponding jc-stacked polymer 41 in 23 % isolated yield with the of 3,800 [78]. 

A fluorine substituent on benzene has a characteristic effect upon the 13C spectrum of benzene, and it couples in a distinctive and highly consistent manner with the ipso, ortho,meta, and para carbons (Scheme 3.55). 

The ortho, meta, and para carbons (hydrogen bound) are clearly seen with diminishing coupling to the CF3 fluorines, whereas both the CF3 carbon and C-l of the benzene can be detected, but are much weaker signals. If you were not looking for them, you would not have noticed the four peaks of the CF3 quartet (with coupling constant of 272Hz), at about 119.1,122.7,126.4, and 129.8 ppm, and the middle two peaks of the C-l quartet, which are barely seen at about 130.7 and 131.1 ppm. 

Hz for the ortho, meta and/or para coupling constants of p-nitroanisole, two dinitrotoluenes and 2,4-dinitrochlorobenzene. Both reports indicate that other aromatic compounds investigated showed even less solvent dependence. 

In aromatic systems, the coupling constant between protons attached to an aromatic ring is characteristic of the relative position of the coupled protons i.e. whether they are ortho, meta or para. 

A recent development in the synthesis of 3//-3-benzazepin-2-ones has been the photocyc-lization of A-(chloroacetyl)phenethylamines (Scheme 25). Ring closure is by homolysis of the alkyl halide followed by intramolecular coupling of the alkyl radical with an aromatic radical cation. Yields are good, especially with a stabilizing electron-donating group (MeO, NMe2) at the position meta to the ethylamino function (i.e. ortho or para to the site of cyclization). Isomeric benzazepinones are normally obtained (Scheme 25) with meta-substituted phenethylamines (80H(14)ll). 

In the carbon spectrum, the ortho, meta, and para carbons (hydrogen bound) are clearly seen with diminishing coupling to the CF3 fluorines,... 

With the development of the cross coupling methodology, many 6-C-substituted purines have been prepared in the past decade. Thus, 6 halopurine derivatives react with arylmagnesium halides,25 alkyl(aryl)zinc or tin reagents,26 trialkylaluminum,27 or alkylcuprates28 to give the 6-alkylpurine derivatives. Also a reverse approach based on the reaction of purine-6-zinc iodide with aryl or vinyl halides has recently been described.29 For the synthesis of 6-arylpurines, an alternative approach makes use of radical photochemical reactions of adenine derivatives with aromatic compounds,30 but this method is very unselective and for substituted benzenes, mixtures of ortho-, meta-, and para substituted derivatives were obtained. 

An example of an aromatic fluorine-containing compound can be found in Figure 6.7, where we have recorded the l9F spectra (both proton-coupled and decoupled) of fluorobenzene along with the H and l3C spectra. Once again we find a singlet for the fluorine atom in the ptoton-decoupled spectrum and a complex multiplet for the fluorine atom in the proton-coupled spectrum. The fluorine atom couples differently to the ortho-, meta-, and para-protons in this mono-substituted compound. Coupling constants for proton-fluorine can be found in Appendix F of Chapter 3. 

Substituent Meta coupling (in Hz) ortho-ortho ortho-para meta-meta Para coupling (in Hz)... 

They give rise to the overall CBO matrix elements reflecting the 7r-electron population on orbital Xi = z yM = Z, Yu = 1/ and the chemical coupling between x< and its counterparts on carbon atoms in the relative ortho-, meta-, and para-positions, respectively, yiii+1 = 2/3, yIiI+2 = 0, yu+3 = -1/3. The resultant scattering of AO probabilities for the it electrons in benzene is shown in... 

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