Homotopic

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic ... 

Homotopic (Section 13.8) Hydrogens that give the identical structure on replacement by X and thus show identical NMR absorptions. 

The problem of tr-facial differentiation, i.e. diastereomer formation, encountered in the metal complexation of the above mentioned annulated cyclopentadienyl ligands is avoided when C2-symmetrical ligands [153] are utilized. Since in such ligands both sides of the five-membered rings are homotopic, only one isomer is... 

So, bis(2-butyl)dimethyltin shows three CH3Sn signals, one for the homotopic methyls of RRSnMe2 and of its enantiomer SSSnMe2, plus two for the diastereo-topic CH3Sn groups, of the meso-compound RSSn(Me)A(Me)B 14,21). 

Both homotopic fluorines such as those in difluoromethane and 2,2-difluoropropane and 1,1-difluoroethene, and enantiotopic fluorines such as those in chlorodifluoromethane and 2,2-difluorobutane (Scheme 2.8) would be chemically equivalent. 

These principles are nicely illustrated by the contrast between the serine-threonine and serine-allothreonine (allothr) systems. The relative orientation of molecular serine vis-a-vis its various crystal faces suggests that allothr can be adsorbed on the homotopic 100 faces as well as on the enantiotopic 011 faces (Figure 21). 

CYSTATHIONINE y-LYASE HOMOSERINE DEHYDROGENASE ASPARTATE KINASE HOMOSERINE SUCCINYLTRANSFERASE HomotopIc,... 

Figure II. Some synthetic strategies leading to chiral crown compounds with homotopic faces stalling from chiral piecuisois with C2 symmetry.

C2-symetric initiators have a pair of equivalent homotopic sites, both of which prefer the same monomer enantioface, that is, both sites prefer the re enantioface or both prefer the si face. Isoselective propagation proceeds with or without migratory insertion since coordination and insertion of monomer at either site give the same stereochemical result. 

Both bridged and unbridged C2v-symmetric metallocenes, mostly the unsubstituted biscyclopentadienyl initiators, but also others such as (CH3)2SiFlu2ZrCl2, have been studied. These initiators are achiral, and their two coordination (active) sites are both achiral and homotopic. The result is that atactic polymer is formed via chain end control. Modest tendencies toward slight isotactic or syndiotactic placement are observed for some initiators, depending on the temperature and other reaction conditions. 

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

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