Stcric Factors

Evidence for the importance of stcric factors comes from a consideration of the effect of a-alkyl substituents. It is found that the presence of an a-melhyl substituent raises A//p by at least 20 kJ mol1 (Table 4.10, compare entries for AA and MAA, MA and MMA, AN and MAN, S and AMS). The higher AHp probably... 

This reaction occurs with an extremely low stcric factor P = 1.6 x 10-11. Therefore, the rate of reactant orientation is very low, and the reaction is limited by reactant orientation in the polymer matrix. This reaction occurs according to the following kinetic scheme ... 

Steroid side chain.1 The key step in a method for stereocontrolled addition of the side chain to 17-kelo steroids is hydroboration of a 17(20)-(Z)-ethylidene steroid (I), which proceeds selectively to give 2, with the desired natural configuration at C,- and C2ft. The product reacts with most alkylating reagents in rather low yield, possibly because of stcric factors however alkylation with the anion of chloroacetonitrile (potassium 2,6-di-r-butyl-4-methylphenoxide) in T1IF gives the nitrile 3 in 60 70% yield. One added attraction of this route is that 9-BBN reacts preferentially with a 17(20)-double bond in the presence of a 5(6 )-double bond. 

Table XII.2 lists some stcric factors predicted by the detailed and transition-state theories for different structures of A, B, and For pur-...

In this double catalytic cycle the olefin is first transformed into aldehyde and then into alcohol. Owing to the increased sieric crowding of the bulky tertiary phosphine Hgand, all the stcric factors discussed in the previous section, which favour the production of normal aldehydes, are much favoured here. 

Conjugate addition to conjugated dienones. Marshall and co-workers have studied the reaction of the reagent wdth cyclohexadienones and conclude that stcric factors account for the ratio of 1,4- to 1,6-addition. The last two examples illustrate the virtual absence of angular methylation. 

Derivatives of durene and mesitylene— In Chapter 9 it was pointed out that in derivatives of durene and mesitylene stcric factors due to the methyl groups may hinder the resonance of the substituent group with the ring system. In Tables Cl and C//the dipole moments of these compounds are compared with the corresponding aromatic and aliphatic derivatives. The dipole moments of the acetyl derivatives of durene and mesitylene are very similar to the value for acetone but lower than that for acetophenone, where resonance with the ring occurs. In aldehydes, on the other hand, where apparently there is no steric hindrance, the moment is almost identical with that of benzaldehyde. 

Figure 14.2. Stcric factor in the Sn2 reaction. Crowding raises enc rgy of transition siuic and slows down reaction.

The nature of the substituent groups (X, Y, and Z) determines hydrolysis rates of the three ester linkages (1,2, and 3). Substituents effects arise from both electronic (inductive, field, and resonance) and stcric factors (March, 1985). Interaction with... 

This selectivity is attenuated by conformational and stcric factors for example, modest chirality transfer (63%) is reported for the rearrangement of tertiary ether 4845. 

Sec Meek. D. W. Nicpon. P. E. Meek, V. I. J. Am. Cltem. Sue. 1970. 92. S3SI-S359. Huhccy. J. E. Grim, S. O. tnorg. Nucl. Cliem. Lett. 1974.10.973-975. These papers analyze the electronic and stcric factors in compounds of this type. 

Demethylation of quaternary pyridinium salts. The rate of demethylation of these salts by DMF (4, 184) is accelerated by addition of triphenylphosphine. The combination of the phosphine and DMF was used in a study of electronic and stcric factors in demethylation of quaternary pyridinium salts. 

The role of stcric factors is illustrated by the association of the di-ter/-biilyl derivative of dioxastannolanc as dimer only. 76, both in solution and in the solid state 230J. In the dimer the intcrmolecular tin oxygen distances suggest strong (perhaps dative) bonding. 

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