Solvent, effects leaving group ability

General base and nucleophile catalysis. Solvent isotope effects. Leaving group ability. 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

Kinetic measurements also show that the solvolysis is of the SnI-type small solvent polarity effects were found in the correlation with the ionizing power parameter qts, with a small m value of 0.12, characteristic of a reaction of a cationic substrate to give a cationic product. Furthermore, the rate data show that the leaving group ability of the phenyliodonio group is about 10 times as great as triflate or 10 -fold higher than iodide. ... 

Such a reaction is termed El(anion) or Elcb(anion)- For a deuterium-labeled reactant (D—C -C -L), this t5 e of reaction is characterized by rapid exchange of deuterium with protons from protic solvents and by a j8-hydro-gen 1° kinetic isotope effect ( h/ d) of 1.0. There is also an appreciable element effect, meaning that the rate constant for the reaction depends on the leaving group ability of L. An example of an El(anion) mechanism is the elimination of methanol from 2-phenyl-f ra s-2-methoxy-l-nitrocyclopentane (4) by the mechanism shown in Figure 10.13. ... 

In Summary We have seen further evidence supporting the SnI mechanism for the reaction of tertiary (and secondary) haloalkanes with certain nucleophiles. The stereochemistry of the process, the effects of the solvent and the leaving-group ability on the rate, and the absence of such effects when the strength of the nucleophile is varied, are consistent with the unimolecular route. 

Because of their crucial role in the ionization step, solvents have a profound effect on the rates of El reactions. These rates for a number of tertiary halides have been determined in a variety of solvents. For r-butyl chloride there are huge differences in the rates in water (log k = -. 54), ethanol (log k = -7.07), and diethyl ether (log k = — 2.1A)P Similarly, the rates of the El reaction of 1-methylcyclopentyl bromide range from 1 x 10 s in methanol to 2 x 10 s in hexane. Polar aprotic solvents such as DMSO (k = 2x lO s ) and acetonitrile (k = 9x 10 s ) are also conducive for ionization. The solvent properties that are most important are polarity and the ability to assist leaving group ionization. These, of course, are the same features that favor reactions, as we discussed in Section 3.8. 

In several of the reactions that you learned in introductory organic chemistry, and many of those that are discussed in Chapters 10 and 11, nucleophilic attack on an electron deficient center such as a carbocation or a polarized cr or -ir bond occurs. The ability of nucleophiles to participate in these reactions depends upon their molecular structure. A structural dependence suggests that there should be scales of relative nucleophilicity that depend upon the sterics, polarizability, and inductive/resonance properties of the nucleophile. Hence, changing the nucleophile becomes one of the tools that we have to study reactions. As with solvent and electronic substituent effects, LFERs provide insight into how the change in nucleophile structure affects the reaction. Similarly, the ability of a leaving group to depart... 

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