✍ Scribed by Robert A. Moss; Dina C. Merrer
No coin nor oath required. For personal study only.
Intramolecular 1,2-H shifts of alkyl or alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example, k H in hydrocarbon solvents at 25 °C for Me2CHCCI, PhCH(Me)CC1, or EtCC1 exceeds l0 s s-~, 1' 2 while k, for PhCH2CCI is 5-7 x 107 s-l. ~' 3
Recently, however, we found that the electron-donating, carbene-stabilizing effect of an acetoxy "spectator" substituent 4 slows the H-shift of Me2CHCOAc by >12 times, relative to Me2CHCC1, while the H-shift of cyclobutylacetoxycarbene decreases >900 times, relative to the analogous chlorocarbene) This suggests that a family of alkylacetoxycarbenes would be well-suited to an examination of H-shift kinetics as a function of structure.
Here, we describe the rearrangements in a family of these carbenes, RCOAc, where R is varied from ethyl (1) to isopropyl (2), and from benzyl (3) to c~-methylbenzyl (4). Competitive 1,2-H and 1,2-acetyl shifts 6 occur with each carbene; e.g., Me2CHCOAc (2) yields both enol acetate 5 and dione 6) Determination of the product distributions, absolute kinetics, and activation parameters of these rearrangements affords a satisfying structure-reactivity correlation for the carbenes, and helps quantitate the relative abilities of methyl and phenyl substituents to stimulate 1,2-H carbenic migrations ("bystander" effects). 7' 8 Carbenes 1-4 were generated photochemically from diazirines 7a-d. These were prepared by "modified" Graham reactions, 5"9 in which the appropriate amidine hydrochloride 1° in DMSO-pentane was oxidized with excess 12% aqueous HOCI, saturated with NaOAc.
Mixtures of 7a-d, accompanied by the analogous alkylchlorodiazirines, were obtained and separated by silica gel chromatography (2:1 pentane/CH2Cl2) of the water-washed, dried, and concentrated pentane extracts of the reaction solutions) ~ This procedure was described in detail for diazirine 7b) The new diazirines exhibited appropriate NMR and UV spectra.
📜 SIMILAR VOLUMES
acid chain was carried out in about 90% yield by catalytic hydrogenolysis of the 2-iodoethyl derivative l c which was prepared from hexamethyl dicyanocobyrinate [2] via the primary alcohol l a and the corresponding mesylate l b (cf. Exper. Part). When oxidized with O2 in the presence of ascorbic aci