Effect of bulky groups

Functionalities such as ether and acetals have been tolerated in an organo-yttrium-catalyzed cyclization of 1,5- and 1,6-dienes [100]. The precatalyst 4(Y Me)/THF could be easily prepared by a one-pot reaction. The enormous effect of bulky groups R in the 3-position of an 1,5-diene on the regio- and diastereos-electivites is in agreement with the debated mechanism (eq. (7)). Under the prevailing reaction conditions (H2 ) cyclization of 1,6-dienes is complicated by hydrogenation reactions. 

Effect of Bulky Groups at Reaction Center Favors SN1 as crowding decreased in carbocation Disfavors SN2 as transition state is more crowded (pentavalent)... 

We wish to follow the same technique of low temperature polycondensation for synthesis of polyamideimide from rosin. Accordingly, we have tried to react RMID with m-phenylene diamine in the presence of excess thionyl chloride at low temperature (0 -5 C). We failed to get any polymer. This may be due to low reactivity of carboxylic acid groups of RMID because of steric hindrance. The effect of bulky group on the reactivity of -COOH groups of RMID has also been observed earlier in the synthesis of polyesterimide . 

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

Ogimachi et al. (1955) also investigated in more detail the steric effect on the equilibrium constants of the complex formation of ICl with various benzene derivatives. The basicity decreases greatly on substitution of bulky groups, e.g. the t-butyl group. This effect is particularly pronounced for 1,3,5-tri-t-butylbenzene. 

Effect of Bulkiness of Ester Group (R) of Acetoacetate on the Hydrogenation Rate and the Optical Yield (OY) ... 

The measurement of surface pressure/area isotherms provides a method for studying the influence of parameters such as temperature, head group charge and size, alkyl chain length, and pH on membrane properties. The effect of head group bulkiness on surface pressure/area isotherms of polymerizable lysophospholipid analogs is illustrated in Fig. 7 36). 

Because the steric effect contributes to the complex formation between guest and host, the chiral resolution on these CSPs is affected by the structures of the analytes. Amino acids, amino alcohols, and derivatives of amines are the best classes for studying the effect of analyte structures on the chiral resolution. The effect of analyte structures on the chiral resolution may be obtained from the work of Hyun et al. [47,48]. The authors studied the chiral resolution of amino alcohols, amides, amino esters, and amino carbonyls. The effects of the substituents on the chiral resolution of some racemic compounds are shown in Table 6. A perusal of this table indicates the dominant effect of steric interactions on chiral resolution. Furthermore, an improved resolution of the racemic compounds, having phenyl moieties as the substituents, may be observed from this Table 6. ft may be the result of the presence of n—n interactions between the CCE and racemates. Generally, the resolution decreases with the addition of bulky groups, which may be caused by the steric effects. In addition, some anions have been used as the mobile phase additives for the improvement of the chiral resolution of amino acids [76]. Recently, Machida et al. [69] reported the use of some mobile phase additives for the improvement of chiral resolution. They observed an improvement in the chiral resolution of some hydrophobic amino compound using cyclodextrins and cations as mobile phase additives. 

---