Rigid compounds hindrance

Fig. 3 Important 19F-labelled amino acids, (a) Compounds that are wo-steric to native amino acids can be incorporated into proteins biosynthetically, but they possess too many degrees of torsional freedom to be useful for ssNMR structure analysis, (b) In these artificial amino acids the 19F-reporter group is rigidly attached to the peptide backbone. They can be incorporated by solid-phase peptide synthesis, but some problems can arise due to racemisation (4F-Phg, 4CF3-Phg), steric hindrance of coupling (F3-Aib) or HF elimination (fluoro-Ala, F3-Ala). 4F-Phg is additionally problematic due to an ambiguity of the side-chain rotamer. The preferred 19F-labels for ssNMR structure analysis are CF3-Bpg and CF3-Phg (as suitable substitutes for Leu, lie, Met, Val and Ala), as well as F3-Aib and CF3-MePro...

It was not a perfect correlation but Hammett had removed the examples where steric hindrance was important. Aliphatic compounds can adopt a variety of conformations (Chapter 18) and the substituent in some of them will interfere with the reaction. Similarly, in ortho-substituted aromatic compounds the nearby substituent might exert steric hindrance on the reaction. Only with meta-and para-substituted compounds was the substituent held out of the way, on a rigid framework, and in electronic communication with the reaction site through the flat but conjugated benzene ring. The diagrams show the para substituent. 

A sesquiterpene of furanodiene structure 60 is chiral because of its conformational rigidity as confirmed by a doubling of resonances in the C NMR spectrum in the presence of Yb(hfc)3/Ag(fod) . The methyl groups at positions 4 and 5 in 3,4,5,6-tetramethylphenanthrene (61) distort out of the plane to minimize steric hindrance, causing the compound to be chiral. Enantiomeric discrimination was observed in the H spectrum in the presence of Ln(hfc)3/Ag(fod) reagents . 

Figure 14(c)." " Interestingly, while the hexadenate sex-ipyridine can form a double helicate with the Jahn-Teller distorted copper(II), quaterpyridine does not because the preference for a distorted octahedral geometry of the metal ion is inconsistent with the helix-forming requirements of the ligand. As a result, reduction of the mononuclear cop-per(n) quaterpyridine complex results in redox-reversible helicate formation. One-electron oxidation of the compound gives a mixed-valence Cu(I)-Cu(ll) helicate, which on further oxidation decomposes to give the mononuclear Cu(II) species (Scheme 2). The Cu(II)/Cu(I) redox interconversion in helicates with various podands has been extensively studied and different electrochemical behavior is observed according to the structural feamres such as denticity, rigidity, and steric hindrance of the helicand. In one case, the mixed-valence Cu(II)/Cu(I) helicate complex is stabilized by specific metal-metal interactions and can be isolated and structurally characterized by X-ray crystallography. ...

It has also been demonstrated [52] that interaction of NO with radiation-produced macroradicals XFV at 150-200 Cleads to their decay without the production of ARs. The absence of ARs in irradiated PTFE was explained by the hindrance of spatial migration of the free valence within the rigid matrix of the fluorinated polymer in which the nitroso compounds formed by recombination of XIV with NO cannot serve as spin traps. It has been noted [52] that PTFE samples irradiated in air and exposed to NO at room temperature exhibit an ESR spectrum tentatively assigned to aminoxyl macroradicals, but the conditions under which these radicals are formed have not been clearly defined. Furthermore, to reliably identify the radicals by their solid-phase spectra, one should assess isotropic values with the appropriate parameters of low-molecular perfluoroalkylaminoxyl radicals studied thoroughly in the liquid phase [53, 54]. 

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