S. Winstein

Neighboring methoxyl participation in solvolytic nucleophilic substitution

Abstract In the present paper are summarized some of the results of the investigation of MeO-3,4,5,6 and 7 participation in solvolytic substitution. Among the ω-methoxy-1-alkyl bromobenzene-sulfonates, solvolysis of the 4-methoxy-1-butyl and 5-methoxy-1-pentyl esters is rapid and dominated by anchimerically assisted ionization in the common solvents. While MeO-5 and 6 participation is important, MeO-3,4 and 7 is not, the corresponding esters solvolyzing with a rate constant essentially equal to the k s value estimated from a ϱ*ν* correlation of solvolysis rates for primary bromobenzenesulfonates solvolyzing without anchimeric assistance. On the basis of the calculated k s values, ratios of anchimerically assisted and unassisted solvolysis rates ( k Δ/ k s ) are derived for the MeO-5 and 6 cases. In the case of MeO-5 assisted ionization of 4-methoxy-1-butyl bromobenzenesulfonate in acetic acid at 25°, an α- or a δ-methyl group is rate-enhancing by a factor of 6. Both the 5-methoxy-2-pentyl and 4-methoxy-1-pentyl esters give rise to an identical mixture of 5-methoxy-2-pentyl and 4-methoxy-1-pentyl acetates from methylene-O cleavage of the common cyclic oxonium ion intermediate. The participating methoxyl group may be an o -methoxyl substituent in a phenyl group, as in o -methoxyneophyl, 3- o -anisyl-1-propyl, 3-methyl-3- o -anisyl-1-butyl and trans -2- o -anisyl cyclo pentyl arenesulfonates. With the compounds investigated, o -MeO-5, o -MeO-6, but not o -MeO-7 participation has proved to be important. Me-O cleavage of the intermediate oxonium ions formed by o -MeO-5 and 6 participation is important, so that benzodihydrofurans or benzodihydropyrans are produced, sometimes as the major product. The behavior of the cyclic oxonium ion intermediates as ion pairs determines some interesting kinetic features of the acetolysis of the methoxyl-substituted alkyl arenesulfonates solvolyzing with methoxyl participation.

Neighboring hydrogen, isotope effect, and conformation in solvolysis of 3-methyl-2-butyl p-toluenesulfonate☆☆☆

Abstract In connection with hydrogen participation in solvolysis, 3-methyl-2-butyl p-toluenesulfonate is a marginal system. Comparison of the behavior of 3-D-3-methyl-2-butyl toluenesulfonate with that of the undeuterated analog discloses a substantial isotope effect on solvolysis rate, but only a small effect on product composition. The evidence points to a relatively high value of (kΔ/ks), the ratio of rate constants for anchimerically assited and unassisted solvolysis processes. Conformational considerations make understandable the relatively low degree of rate acceleration displayed by 3-methyl-2-butyl toluenesulfonate.

Nuclear magnetic resonance and 203Hg exchange studies of allylmercury systems: A comment on allylmercuric perchlorate

Abstract Nuclear magnetic resonance data for allyl-, β-methallyl-, crotyl- and cinnamylmercuric halides and acetates are presented and the dramatic effect of added mercuric salts on some of these spectra is discussed. The exchange of mercury between allyl- and β-methallylmercuric halides (chloride or bromide) and the appropriate mercury-(II) halide, utilising 203Hg as a tracer, is very rapid (statistical exchange in less than a minute at 20°) and appears to be one of the fastest organic-inorganic HgII exchanges reported. Mechanisms of the SEi′ variety are suggested, and the significance of the exchange in relation to the effect of HgII halides on the PMR spectra of allylmercury systems is pointed out. The preparation and some properties of a compound argued to be allylmercuric perchlorate are presented.

Homoconjugation and homoaromaticity—XVII: The nature and behavior of the unsubstituted trishomocyclopropenyl cation

Abstract A more extensive investigation of the solvolysis of the 3-bicyclo-[3.1.0]hexyl tosylates has been carried out using kinetic, NMR and VPC techniques not employed previously. For labeled starting materials, 6,6-dideuterated species have been used. The trishomocyclopropenyl cation is here discussed on the basis of the new information. In line with the presence of a special salt effect in acetolysis of the cis -tosylate, the latter exhibits equilibration of the deuterium label, originally at C 6 , over positions 2, 4 and 6 during acetolysis. The sum of the first order equilibration rate constant, k eq , and the titrimetric rate constant, k t , represents a minimum value for the ionization rate constant, k 1 . Lithium perchlorate reduces considerably the gap between k t and ( k eq + k t ), but it does not eliminate it entirely. Thus, the special salt effect eliminates only part of the ion pair return during the acetolysis. With respect to salt effects, ion pair behavior and lack of common ion rate depression, the cis -3-bicyclo[3.1.0]hexyl system is analogous to the 3-anisyl-2-butyl analog. As regards mechanism of solvolysis of cis -tosylate, the essentially exclusive formation of cis -acetate with retention of configuration and complete equilibration of the label, the absence of elimination, the lack of rearrangement to 2-bicyclo[3.1.0]hexyl and monocyclic structures, and the special salt effect and ion pair return phenomena provide a complete contrast with the behavior of the trans -tosylate. All of the results are explicable on the basis of the trishomocyclopropenyl intermediate formed by anchimerically assisted ionization ( k Δ ) of cis -tosylate. Rate constant k Δ evidently exceeds the rate constant for anchimerically unassisted solvolysis, k s , by a factor of ca. 50 in acetolysis and by a somewhat smaller factor in aqueous acetone. The most striking aspect of the solvolysis of trans -tosylate is the complete absence of equilibration of the deuterium label in the cis -acetate part of the acetolysis product. Leakage from the classical cation to the nonclassical trishomocyclopropenyl structure competes unusually poorly with its other reactions. Probably the most important reason for this very inefficient leakage is that the geometry of the classical ion is unfavorable for the change. The reason for this goes back to the preferred boat conformation of 3-bicyclo[3.1.0]hexyl derivatives. This preferred boat conformation also provides part of the reason for the relatively low ( k Δ / k s ) ratios in solvolysis of the cis -tosylate. Coreys alternative suggestion of three rapidly equilibrating “almost classical” nonclassical ions instead of the trishomocyclopropenyl species is discussed. None of the data require this modification, so the application of “Ockhams razor” to the three species and the retention of the simpler interpretation are recommended.

Homoconjugation in 7-norbornadienyl cations

Abstract The electronic structure of 7-norbornadienyl and its 2-methyl substituted cations have been investigated using CNDO/2 method. The structure of the carbonium ions are discussed with their molecular energy curves and electronic population analyses. These carbonium ions are shown to be homoconjugated and stable by their overlap populations.

Aromatic hydrocarbon-carbonium ion molecular complexes

Molecular complexes of aromatic hydrocarbons and carbonium ions are described according to Mullikens donor — acceptor theory. Estimations of electron affinities of several cations are deduced from charge-transfer transition energies and the indepently measured electron affinity of the tropylium ion.

Reaction of the trityl cation with dimethylketene dimethylacetal

Abstract : The reaction of trityl bromide with dimethylketene dimethylacetal yields a quinoidal ester derivative, resulting from reaction of the trityl cation at a para position. Treatment of this ester with methanolic sodium methoxide converts it to an aromatized isomer. The formation of the quinoidal ester makes it clear that the trityl ion reacts at a para position with dimethylketene acetal as a relatively highly hindered nucleophile to give a quinoidal cation which then leads to the quinoidal ester. The results emphasize the possibility of reactions of the trityl cation with nucleophiles at a benzene ring position, giving rise to new final products or possible transient intermediates with the nucleophile at a ring position instead of the central carbon atom. (Extracted)