Peri strain

Substituted derivatives with bulky groups, however, are reduced at higher rates and with opposite selectivity on palladium [Eq. (11.18)]. This was explained by the partial release of peri strain in the transition state during hydrogenation. 

Anthracenium253,255 271 272 and phenanthrenium ions271-274 have also been well-studied. A detailed investigation of 9-alkylanthracenes showed that protonation always occurs at C(10).275 An additional methyl group at C(l) or C(8) results in the C(9) protonation as well (relief of peri strain). 

Summarizing the results related to the barriers to rotation of a secondary alkyl group in the 9-position of a 1-substituted triptycene, we note that the maximum barrier is realized when the peri substituent is chlorine or bromine. Evidently the size of the substituent that gives the maximum barrier to rotation is shifted from that in the 9-rm-alkyl systems. This is considered to be a reflection of the strain in the ground state, which is usually larger in the tertiary alkyl systems than in the secondary, if the same substituent is present in the peri position. 

Formation of 247 may be favored by steric factors, as the 3-and 4-positions of benzo[6]thiophene are analogous to the peri positions of naphthalene. Removal of the 4-substituent from the plane of the ring would be expected to relieve steric strain. 

Coplanarity is made impossible by the close approach of the neighboring peri hydrogen atoms (H-l, H-l and H-8, H-87H-9, H-9 ), and these compounds have two principal routes to minimize the steric strain based on the mechanisms mentioned in Section I. One way, twisting, is to rotate the two halves of the molecule about the formal double bond... 

Lammertsma and Cerfontain269 performed a detailed study on the ions generated from isomeric dimethylnaphthalenes. An interesting observation is the transformation upon increasing the temperature of cation 97 through an intermediate to cations 98 and 99. The driving force is the relief of steric strain in the peri position [Eq. (3.38)]. For the... 

The increased stabilization upon thienoannulation as compared to benzo-annulation may be caused by the more effective stabilization of the positive charge, by diminished angular ring strain, and by limited peri interactions between hydrogen atoms (due to the heteroatoms) and hence by nearly perfect planarity. 

If the released electrophile HSOs+ is not intercepted during the protodesulfonylation as in Figure 5.7, it reacts with the defunctionalized aromatic compound again. In this way an isomer of the original sulfonic acid may be obtained. The best-known example of such an isomerization is the conversion of naphthalene-1-sulfonic acid into naphthalene-2-sulfonic acid (Figure 5.8). Naphthalene-1-sulfonic acid is destabilized by the so-called peri-interaction, that is, the steric interaction between the Cs—H bond of the naphthalene and the substituent on Cl. The peri-interaction is thus a cis-alkene strain. Because naphthalene-2-sulfonic acid does not... 

Four years after Grundemann et al. s original observation [49], Peris and coworkers [40] proved that steric strain is not the decisive factor in the abnormal coordination of the carbene in these particular iridium(ni) hydride complexes. We may well suspect that differences in... 

Strained peri-bridged naphthalenes with monoatomic bridge (N, O, S), structure, properties, synthesis 81YGK682. 

The conformational profile of c/.v.1,2,3,4-tetrach lorotetrahydro-naphthalene, 7 (R, R = -CH=CH-CH=CH-), is influenced by A(1,2) strain. Of the two conformers 7a and 7b, 7a constitutes the nearly exclusive conformer at equilibrium [8]. It is important to note that both the chlorine atoms on the allylic positions in 7a are (pseudo)axial and, thus, significantly away from the hydrogen atoms on C5 and C8 peri positions to avoid the otherwise eminent A(1 2) strain. 

When the carbonyl group is exo to a ring as in cyclohexanone, both the equatorial and the axial approach can lead to a chair intermediate (cf. 7 and 8) with stereoelectronic control because, in each case, the oxygen atom has an electron pair anti peri planar to the C-Y bond. So, in this case, one process is favored over the other solely on the basis of steric effects and torsional strain due to the presence of the ring (8). ... 

It is a non-steroidal antiinflammatory drug (NSAID) and used mainly as its sodium salt for the relief of pain and inflammation in various conditions, such as musculoskeletal and joint disorders viz., rheumatoid, arthritis, osteoarthritis and ankylosing spondolytis peri-articular disorders, for instance bursitis and tendenitis soft-tissue disorders, such as sprains and strains and other painful conditions, namely renal colic, acute gout, dysmenorrhoea, and following certain surgical procedures. 

A sigmatropic rearrangement involving a three-membered ring. The o-bonds in three-membered rings are strained and more reactive than normal o-bonds and take part readily in peri cyclic reactions. 

The higher thermodynamic stability of the ion with the proton at the 4-position is due to a decrease in the steric-strain on movement of one peri-CHj-group from the ring plane. 

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