I. Studies on Cycloproply-Stabilized Vinyl Cation Intermediates. II. Molecular Orbital Calculations on Cationic Intermediates and Displacement Reactions. III. On the Complexation of NMR Lanthanide Shift Reagents with Organic Substrates

Citation

Kelsey, Donald Ross (1973) I. Studies on Cycloproply-Stabilized Vinyl Cation Intermediates. II. Molecular Orbital Calculations on Cationic Intermediates and Displacement Reactions. III. On the Complexation of NMR Lanthanide Shift Reagents with Organic Substrates. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QV6K-J118. https://resolver.caltech.edu/CaltechTHESIS:09072018-101527022

Abstract

I. The heterogeneous and homogeneous silver-catalyzed acetolyses of E- and Z-1-cyclopropyl-1-iodopropene (1E and 1Z, R = methyl), E- and Z-1,2-dicyclopropylethylene (1E and 1Z, R = cyclopropyl), E- and Z-1-cyclopropyl-3-ethyl-1-iodopentene (1E and 1Z, R = 3-pentyl), and 1-iodo-3,4-hexadiene (4) are examined in detail. The acetate product distributions reveal small, but important, contributions of heterogeneous catalysis, clearly show overall net inversion of geometry in the formation of the corresponding vinyl acetates under homogeneous conditions, and reveal selectivity in the cyclopropyl ring-opening rearrangements of the intermediate vinyl cations. The results are consistent with the intervention of non-equilibrated ion-pair intermediates which undergo both some solvent trapping and ring-opening rearrangement with net inversion and dissociation to linear, sp-hybridized vinyl cations. It is also found that the steric effects of the β-alkyl substituents R are important in determining the extent of ion-pair contribution. The product distributions are also highly dependent upon the R substituent, with the ratio of vinyl acetates to rearranged cyclobutyl-type acetates decreasing as R changes from hydrogen, to 3-pentyl, to methyl, to cyclopropyl. It is shown that this is consistent with the conjugative electron-releasing ability of R in stabilizing the transition state for rearrangement of the vinyl cations, whereas the initial ionizations of the vinyl iodides (Z isomers) qualitatively depend upon the inductive electronic properties of R. Other electronic and steric effects of substituent R are discussed.

It is also shown that 1E and 1Z, R = cyclopropyl, with deuterium label in the β-cyclopropyl ring, undergo reaction with no isotopic scrambling, indicating the absence (< 0.2%) of 1,2-hydride shift across the double bond in the 1,2 dicyclopropylvinyl cation. A similar result (< 1% rearrangement) is found for the 1,2-dicyclopropylethyl cation. These results are discussed in terms of σ-delocalization of the cyclopropyl rings adjacent to the cationic centers.

II. This part has been published in full, see D.R. Kelsey and R.G. Bergman, Journal of the American Chemical Society, 93, 1953 (1971).

III. An explicit treatment of the complexation equilibrium between organic substrates and the lanthanide nmr shift reagents is developed. It is shown that for a single equilibrium, plots of the observed shifts of two substrate protons, i and j, against one another must be linear with slopes that reflect the ratios (Δ_maxⁱ/Δ_max^j) of the chemical shifts in the bound complex. This "internal standard proton" method is applied to sixteen vinyl and eleven allyl acetates using the acetoxy methyl protons as standard and Eu(fod)₃ shift reagent. Characteristic slopes are obtained which depend upon the stereochemical relationships to the reference group. The method is shown to be a valuable and reliable means of establishing substrate structures even for isomeric mixtures, since impurities and concentrations have little effect.

It is shown that at large ratios of substrate to reagent concentrations, S_t/E_t, a plot of reciprocal observed shift, 1/Δ_obsd, against the total substrate concentration, S_t, should be linear if a 1:1 stoichiometry obtains. The plots of the data for representative substrates (allyl acetate, isopropenyl acetate, 2-butanone, tetrahydrofuran, 2-propanol, and dimethyl sulfoxide) with Eu(fod)₃ show excellent linearity and estimated bound chemical shifts, Δ_max, and equilibrium constants, K, are obtained. The results for dimethyl sulfoxide suggest that the major complex stoichiometry is 1:1 at room temperature, even though the literature data suggest formation of a 2:1 (DMSO: Eu(fod)₃) complex at low temperatures. It is observed that the limiting observed shifts, Δ_lim, obtained at high E_t/S_t ratios are generally less than the Δ_max values. This and other observations are discussed in terms of non-ideal solution behavior at high reagent concentrations. A bibliography of 240 references is included.

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