Solvolysis reactions steric effects

Correlations with o in carboxylic acid derivative reactions have been most successful for variations in the acyl portion, R in RCOX. Variation in the alkyl portion of esters, R in RCOOR, has not led to many good correlations, although use of relative rates of alkaline and acidic reactions, as in the defining relation, can generate linear correlations. The failure to achieve satisfactory correlations with cr for such substrates may be a consequence of the different steric effects of substituents in the acyl and alkyl locations. It has been shown that solvolysis rates of some acetates are related to the pA", of the leaving group, that is, of the parent alcohol. The pK of alcohols has been correlated with but this relationship... 

Quite a contrasting effect was found for the solvolysis of sterically hindered palla-dium(II) complexes of the ligand Et2NCH2CH2NHCH2CH2NEt2 (= Et4dien).16 The reactions, with X = halide or pseudohalide, are... 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

The limiting nature of the solvolysis reactions of 2-adamantyl solvolyses apparently arises as a result of steric inhibition of rearside nucleophilic solvent participation by the axial hydrogens shown in 95. Such steric effects are absent... 

Tertiary amines greatly accelerate the reactions. The effect is complex since the amines will bind iron as ligands and also decrease proton activity in the medium. That the latter effect must be of major significance is demonstrated by comparing the effects of pyridine and 2,6-lutidine. The latter exerts a more powerful catalysis effect despite the fact that steric hindrance weakens its ligand effect. Consequently, we infer that solvolysis to produce species such as FeCl(OCH3) and Fe( 00113)2 is extensive in the presence of bases and that the alkoxides must be more reactive than FeCb as substrates. The rate in the presence of 0.4M piperidine was immeasurably fast. The powerful acceleration is attributed to the high basicity of piperidine (pK = 2.80 in 50% ethanol) compared with pyridine (pK = 9.62) and 2,6-lutidine (pK, = 8.23). 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

Steric Effects.—The consequences upon chemical reaction of non-bonded interactions between enantiomeric pairs of molecules have been discussed an antipodal interaction effect was observed in a reductive camphor dimerization and in a camphor reduction. The full paper on the correlation of the rates of chromic acid oxidation of secondary alcohols to ketones with the strain change in going from the alcohol to the carbonyl product has now appeared. It is concluded that the properties of the product are reflected in the transition state for the oxidation. High yields of hindered carbonyls are available from the corresponding alcohols by reaction with DMSO and trifluoroacetic anhydride (TFAA) indeed, the more hindered the alcohol, the higher the yield of carbonyl compound reported Since the DMSO-TFAA reaction occurs instantaneously at low temperatures (<—50°C), it is possible to oxidize alcohols that form stable sulphonium salts only at low temperature. Thus, ( )-isoborneol reacts at room temperature to give camphene, the product of solvolysis of the sulphonium salt the oxidation product, ( + )-camphor, was obtained by the addition of base at low temperature. 

Steric effects are important in solvolysis reactions. Problems 7.75 and 7.76 illustrate this point. 

---