Steric effects nucleophilic substitution

Equation 4 can be classified as S, , ie, substitution nucleophilic bimolecular (221). The rate of the reaction is influenced by several parameters basicity of the amine, steric effects, reactivity of the alkylating agent, and solvent polarity. The reaction is often carried out in a polar solvent, eg, isopropanol, which may increase the rate of reaction and make handling of the product easier. 

Alkyl groups under nonacidic conditions sterically deflect nucleophiles from C, but under acidic conditions this steric effect is to some extent offset by an electronic one the protonated oxirane opens by transition states (Scheme 40) which are even more 5Nl-like than the borderline Sn2 one of the unprotonated oxirane. Thus electronic factors favor cleavage at the more substituted carbon, which can better support a partial positive charge the steric factor is still operative, however, and even under acidic conditions the major product usually results from Cp attack. 

Examples of effects of reactant stmcture on the rate of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. The general trends of reactivity of primaiy, secondary, and tertiaiy systems and the special reactivity of allylic and benzylic systems have been discussed in other contexts. This section will emphasize the role that steric effects can pl in nucleophilic substitution reactions. 

In addition to steric effects, there are other important substituent effects which determine both the rate and mechanism of nucleophilic substitution reactions. It was... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

The selective reaction of anionic 3,6-dichloro-4-sulfanilamidopy-ridazine with excess methanolic methoxide at the 3-position is another indication of the absence of major steric effects in most nucleophilic substitutions, as a result of the direction of nucleophilic attack (cf. Section II, A, 1). The selectivity at the 3-position is an example of the interaction of substituent effects. The sulfonamide anion deactivates both the 3-chloro (ortho direct deactivation) and... 

When Z is SOR or SO2R (e.g., a-halo sulfoxides and sulfones), nucleophilic substitution is retarded. The SnI mechanism is slowed by the electron-withdrawing effect of the SOR or SO2R group,and the Sn2 mechanism presumably by the steric effect. 

Oae and Khim measured the rates of hydrolysis of chlorophenyl phenyl sulfoxides and sulfones with hydroxide ion in aqueous DMSO at 158 °C. Both SOPh and S02Ph were found to activate the nucleophilic substitution from ortho- and para-positions, but the effect of SOjPh was considerably larger than that of SOPh. The results were interpreted in terms of 7t(pd) conjugation in the intermediate complexes. In a later paper it was shown that the introduction of a methyl group ortho to SOPh or S02Ph slightly retards the above and related reactions but this was attributed to the inductive effect of Me rather than steric inhibition of 7t(pd) conjugation (Section III.A.l). 

If the substituents are nonpolar, such as an alkyl or aryl group, the control is exerted mainly by steric effects. In particular, for a-substituted aldehydes, the Felkin TS model can be taken as the starting point for analysis, in combination with the cyclic TS. (See Section 2.4.1.3, Part A to review the Felkin model.) The analysis and prediction of the direction of the preferred reaction depends on the same principles as for simple diastereoselectivity and are done by consideration of the attractive and repulsive interactions in the presumed TS. In the Felkin model for nucleophilic addition to carbonyl centers the larger a-substituent is aligned anti to the approaching enolate and yields the 3,4-syn product. If reaction occurs by an alternative approach, the stereochemistry is reversed, and this is called an anti-Felkin approach. 

For the methyl-substituted ethylenes, i.e. in the absence of any steric effects, there is a roughly linear relationship between the chemoselectivity and the 13C nmr chemical shift of the most substituted carbon atom of the bromonium ions (Dubois and Chretien, 1978). This selectivity is therefore discussed in terms of the magnitude of the charge on the carbon atom and the relative hardness of the competing nucleophiles, according to Pearson s theory (Ho, 1977). However, this interpretation does not take into account the substituent dependence of the nucleophilic solvent assistance, which must play a role in determining this chemoselectivity. 

I. Dostrovsky, E. D. Hughes, and C. K. Ingold, XXXII. The role of steric hindrance, magnitued of steric effect, range of occurrence of steric and polar effects, and place of the Wagner rearrangement in nucleophilic substitution and elimination, Chem. Soc. 173 (1946). 

Steric effects on the nucleophile, aniline, were clearly evident. Rate constants for bimolecular attack of 2,6-dimethyl- 70a, 2,6-diethyl- 70b, and 3,5-dimethylaniline 70c at 308 K indicate that the ort/zo-substituted anilines react more than an order of magnitude slower at the same temperature (Table 7). Structure 70c must be able approach the reactive nitrogen more closely.42,43 A comparison of the rate constants for reaction of aniline 72c, /V-methyl- 71a and /V-phenylaniline 71b provides further evidence of steric effects although the very small rate constant for the diphenylamine could also be accounted for by reduced nucleophilicity on account of lone pair resonance into the additional phenyl ring. 

Steric effects, of A-acyloxy-/V-alkoxyamides mutagenic activity and, 105-106, 109-115 in nucleophilic substitution (SN2) reactions, 79-81... 

The Role of Steric Hindrance (Section G) Magnitude of Steric Effects. Range of Occurrence of Steric and Polar Effects, and Place of the Wagner Rearrangement in Nucleophilic Substitution and Elimination," JCS 149 (1946) 173194. 

Similar results were then found for piperidino-debromination of various nitro-activated five-membered ring heterocycles103. The existence of such linear Hammett plots for ortho-substituted substrates was interpreted as a peculiar feature of five-membered ring heterocycles, where steric effects of substituents ortho to the site of the nucleophilic attack are minimized13. 

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