Clearing terminal substituents

If one compares the 1,2,3-triphenylcyclopropyl anion as well as the cyclopropyl anion 320" with the many more cyclopropyl anions not showing the ring-opening reaction such as all cyclopropyl anions with H or alkyl groups at C(l) like the parent 323 or Walborsky s 324" it is immediately clear that substituents which stabilize a negative charge at both carbon atoms which become terminal centers of the allyl anion facilitate the ring scission. If the cyclopropyl anion is prepared by deprotonation of a cyclopropane it is... 

Which groups on the termini are in or out is less clear for 1,3-dipoles than it is for 1,3-dienes. The three atoms of the dipole form an arc. The terminal substituents pointing to the concave side of the arc are the in groups, and the substituents pointing to the convex side are the out groups. 

As indicated already, the optical transition energy are an extremely sensitive probe for the electronic and steric properties of the three-electron-bonded species and their respective relative contributions. However, the effect of substituents on the optical transitions becomes of much lesser importance in intramolecular radical cations derived from open-chain dithianes (type 7-9). Changing the terminal substituents in R-S-(CH2)3-S-R from methyl to isopropyl results in a just 15 nm change (440 vs. 455 nm), i.e., structure clearly appears to be the dominating parameter. This is fully corroborated by the pulse radiolysis results on 2-substituted-l,3-dithiacyclopentanes.l23 As mentioned already, the radical cation (11), derived from 1,3-dithiacyclopentane (12), is very unstable if formed at all ( niax > 650 nm). The analogous radical cation generated upon oxidation of l,3-dithia-2,2-dimethylcyclopentane (13), on the other hand, exhibits a pronounced and blue-shifted absorption at 610 nm as well as a considerable kinetic and thermodynamic stability. 

Similarly, the effects of a range of terminal substituents on the clearing temperatures of dimers also mirror those observed for conventional low molar mass mesogens [29-32, 34, 46, 67]. For example, Jin et al. [29] established the efficiency of the terminal group in enhancing nematic behaviour for the dimer series... 

The clearing temperatures of some homologous series are displayed in Figure 3. The influence of the linking groups is clearly visible in the different levels of the clearing temperatures however, there is also a decisive influence of the terminal substituents, which will be discussed later. 

There are many compounds with branched terminal substituents, particularly chiral materials. The effect of a branch depends substantially on its position in a chain [ 144-147]. This is demonstrated in Table 31. The reduction in the clearing temperature... 

Fluoroalkenes, RCF=CF2 add H at the terminal position and the resulting dimer is the 2,2 isomer. The terminal H substituents are clearly identifiable in the H NMR spectrum of the product, confirming the mechanistic pathway. 

The polymers obtained from unsymmetrically terminally disubstituted butadiene such as 4-methyl-1,3-pentadiene are made up of 1,2 monomeric units only, irrespective of the catalyst used this is due to the presence of two methyl substituents at the C4 atom in the monomer. Two stereoregular polymers have so far been obtained from 4-methyl-l,3-pentadiene, one with a 1,2-isotactic structure and one with a 1,2-syndiotactic structure. The isotactic polymer has been yielded by heterogeneous Ziegler-Natta catalysts, e.g. TiCU—AlEt3 and a — TiCl3— [AlEt3 [182]. The factors that determine the orientation of the coordinating monomer in this case are not, however, completely clear [41]. 

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