Solvolysis of Cyclohexenyliodonium Salt, a New Precursor for the Vinyl Cation: Remarkable Nucleofugality of the Phenyliodonio Group and Evidence for Internal Return from an Intimate Ion-Molecule Pair
摘要:
Solvolysis of (4-tert-butyl-1-cyclohexenyl)aryliodonium tetrafluoroborates was examined in alcohol and aqueous solutions at 25-70 degrees C. Phenyliodonio group was found to be a remarkably good nucleofuge with a leaving ability about 10(6) times greater than triflate. In addition to the expected solvolysis products, 1-alkoxycyclohexene and/or cyclohexanone and iodobenzene, the internal return products, (4-tert-butyl-1-cyclohexenyl)iodobenzenes, were obtained in yields of 13-15% in methanol and aqueous solutions and of 30-40% in trifluoroethanol. The ortho derivative is the predominant isomer of these products, and this isomeric distribution is taken as indicating that the return is occurring from the intimate ion-molecule pair. The formation of a free-radical product, 4-tert-butylcyclohexene (1%), was detected during the solvolysis in the presence of 1,4-cyclohexadiene or in 2-propanol, while a considerable amount of the cationic rearrangement product, cyclopentyl methyl ketone, was obtained in the solvolysis of (2-methyl-1-cyclohexenyl)phenyliodonium tetrafluoroborate. A proposed mechanism involves mainly heterolysis to give the cyclohexenyl cation-iodobenzene pair with accompanying single electron transfer (or with competing homolysis) to lead to a free radical intermediate.
Alkenyl(phenyl)iodonium tetrafluoroborates dissolved in chloroform, or in the solid state, decompose thermally at 60 degrees C to yield fluoroalkenes and iodobenzene as major products via an S(N)1- or S(N)2-type reaction within the ion pair of the substrates. (C) 2000 Elsevier Science Ltd. All rights reserved.
Effects of Complexation with 18-Crown-6 on the Hypernucleofugality of Phenyl-λ<sup>3</sup>-iodanyl Groups. Synthesis of Vinyl-λ<sup>3</sup>-iodane·18-Crown-6 Complex
4-tert-Butyl-1-cyclohexenyl(phenyl)(tetrafluoroborato)-lambda(3)-iodane forms a discrete supramolecular complex by the reaction with 18-crown-6. Solvolysis of the cyclohexenyl-lambda(3)-iodane in the presence of 18-crown-6 indicates that the complexation with 18-crown-6 tends to decrease the leaving group ability of hypervalent phenyl-lambda(3)-iodanyl groups.