I. Rearrangement of Cyclopropyldiphenylmethyllithium and 4,4-Diphenyl-3-Buten-1-Yllithium. II. Deuterium Isotopic Perturbation of the Cyclopropylmethyl-Cyclobutyl Carbocation

Citation

Brittain, William Joseph (1982) I. Rearrangement of Cyclopropyldiphenylmethyllithium and 4,4-Diphenyl-3-Buten-1-Yllithium. II. Deuterium Isotopic Perturbation of the Cyclopropylmethyl-Cyclobutyl Carbocation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1c58-f235. https://resolver.caltech.edu/CaltechTHESIS:05102018-165616024

Abstract

PART I

Cyclopropyldiphenylrnethyllithium was prepared directly from cyclopropyldiphenylmethane by trimethylsilylmethyllithium metalation or indirectly by transmetalation of cyclopropyldiphenylmethylpotassium. The organolithium is stable in tetrahydrofuran, dimethyl ether, or 2-methyltetrahydrofuran and exists predominately as loose ion pairs. In diethyl ether or isopropyl methyl ether, cyclopropyldiphenylmethyllithium isomerizes to 4,4-diphenyl-3-buten-1-yllithium; a process that can be reversed by addition of tetrahydrofuran. 4,4-Diphenyl-3-buten-1-yllithium is unstable in most solvents; in cyclopentane, β-elimination of lithium hydride and ortho-cyclisation were major reaction pathways. The solvent dependence of the organolithium rearrangement can be adequately explained by three principle equilibria; the rearrangement occurring in contact ion pairs.

PART II

Ionization of (2E,2Z,3Z-d₃-cyclopropyl)methanol. 9, in SbF₅-SO₂ClF-SO₂F₂ afforded primarily one stereoisomer (endo-C₄H₄D₃+) of the trideuterated cyclopropylmethyl-cyclobutyl carbocation. Ionization of (2,2-d₂-cyclopropyl)-1-d-methanol, 10, in SbF₅-SO₂ClF-SO₂F₂ produces a 1:1 mixture of endo- and exo-C₄H₄D₃+. The low-temperature ¹H, ²H, and ¹³C NMR spectra of both C₄H₄D₃+ stereoisomers are reported. The methine resonance (¹³CH) of exo-C₄H₄D₃+ is shifted 0.4 ppm downfield from unlabeled cation, C₄H₇+, while the endo-C₄H₄D₃+ resonance is coincident with the corresponding carbon in C₄H₇+. This can be taken as evidence for the deuterium isotopic perturbation of the equilibrium between bicyclobutonium, 2, and bisected cyclopropylmethyl, 4, structural isomers. The deuterium perturbation of equilibria involving structurally similar, nondegenerate isomers is a larger effect and induces dramatic changes in the methylene carbon resonances. The ¹³C resonance corresponding to the mondeuterated carbon (¹³CH₂) is shifted in opposite directions relative to C₄H₇+ in endo- and exo-C₄H₄D₃+. Endo-C₄H₄D₃+ slowly isomerizes to exo-C₄H₄D₃+; a lower limit for the rotation barrier in C₄H₄D₃+ is 14 ± 1 kcal/mol. The endo-hydrogens in endo-C₄H₄D₃+ were assigned to the high-field methylene resonance in the ¹H spectrum of this ion in contrast to previous assignments by other workers. The results of this NMR study of C₄H₄D₃+ carbocations are consistent with two rapidly equilibrating structures, 2 and 4, of C₄H₄D₃+ under stable-ion conditions.

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