Why ketonization is favored over enolization in 5-membered ring H-transfers by CH3C(OH+)CH2CH2 and in 6-membered ring H-transfers by CH3(COH+)CH2CH2CH2

摘要: Abstract The reactions leading to dissociation of the metastable enol isomers 2-butanone have a puzzling feature: why does CH3C( OH+)CH2C H2 (2) isomerize less stable cation (3) and dissociate exclusion conversion CH2 C(OH+ )CH2CH3 (1) CH3C(OH+ ) CHCH3 (4)? To answer this, stationary points on pathways established previously are located using ab initio hybrid density functional theory their geometries energies determined. Rate curves for some also obtained by RRKM theory. It is found that 2 isomerizes 3 rather than its more 1 4 because ketonization has lowest barrier, despite giving highest energy product. For 2 → 1, this barrier arises from strained, twisted transition state geometry being required in transfer hydrogen π-bonded methylene group whereas TS ( ⇌ completely planar. elevated different phenomenon, need an electron go into antibonding orbital 1,2-H-shift, raising above ). critical 5-membered ring isomerizations 1–2 2–3 significantly higher those 6-membered processes ketonize enolize OH+)CH2CH2C (6). However, similarly 2, isomerization 6 O+ )CH2CH2CH3 lower preferred )CH2CH2CH3. This attributable twisting at C1 as transferred it.