Methyl group, model structure

In order to confirm the appropriateness of the models for the artificial DNA, we first verify the model dependence of the inter-base pair interaction. We adopt three models, (a) a hydrogen atom model (dP=H) and (b) a methyl group model (dP=CH3) (c) real model (dP=5 -deoxyribose) (Fig. 25.2). These models are often used in the analyses of the inter-base pair interaction between the bases in the natural B-DNA, where it is known that the backbone molecules do not contribute much to the stability of the duplex structure. We here also make the same assumption for the artificial DNA. 

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

Scott et al. [45] prepared diimine derivatives of 2,2 -diamino-6,6 -dimethyl-biphenyl (as structure 37 in Scheme 19) as copper chelates for the catalyzed cyclopropanation reaction. All catalysts were active in this reaction but enan-tioselectivities varied importantly according to the substitution pattern of the imine aryl group only ortho-substituted ligands (by chloride or methyl groups) led to products with measurable enantioselectivity for the model test reaction (up to 57% ee with 37). 

In contrast with the role of cofactor B12 in methionine synthase (methyl group transfer to a thiol), functional Bi2 model complexes have provided a formidable challenge. Several oxime alkyl-cobalt (structural) B12 models when reacted with arene- and alkanethiolates lead only to... 

Fig. 24 Calculated structures of cationic phenoxy-amine complexes in the absence of 1-hexene (a), in the presence of 1-hexene (1-hexene-coordinated state) (b), and for transition state (c) (polymer chain model methyl group). Reproduced with permission from Saito et al. [27]. Copyright 2006, American Chemical Society...

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

Polyisobutene has a regular head-to-tail structure, and the crowding of the methyl groups is so great that the molecule can only be built from conventional atomic models with great... 

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