A unique and novel cyclopropylmethyl cation intermediate: a DFT study

Stereospecific Construction of Quaternary Carbon Stereocenters from Quaternary Carbon Stereocenters

“… 9 Actually, cyclopropyl carbinol derivatives could undergo a regio- and stereoselective nucleophilic substitution at the quaternary carbon center C 1 , with pure inversion of configuration, to provide the acyclic products as a single diastereoisomer. The selectivity of the substitution was attributed to the existence of a bicyclobutonium species, 10 reacting at the most substituted carbon center. Spurred by this remarkable transformation, we implemented this discovery in a newly devised strategy to address the highly challenging and selective preparation of three-dimensional hydrocarbon chains possessing vicinal and congested sp 3 centers.…”

“… 11 Among all tested leaving groups, phosphates 3 displayed the right compromise between stability and reactivity. 10 …”

Asymmetric Synthesis of Azepine-Fused Cyclobutanes from Yne-Methylenecyclopropanes Involving Cyclopropanation/C–C Cleavage/Wagner–Meerwein Rearrangement and Reaction Mechanism

“…(Here, we mean that the chiral center at C1 is kept from INT2-Ph to INT5-Ph , even though this carbon temporarily becomes planar without chirality in the intermediate between INT2-Ph and INT5-Ph .) The computed activation free energy for the carbocation rearrangement from INT4-Ph to INT5-Ph , is only 1.0 kcal/mol . There is another possibility that intermediate INT4-Ph could form the C2–C3 bond and break the C2–C4 bond to form another homoallylic cation at C4, but this homoallylic cation rearrangement is prohibited sterically by the phenyl group adjacent to C2 and can be excluded for consideration (see more discussion in the Supporting Information).…”

Nucleophilic Substitution at Quaternary Carbon Stereocenters

“…To rationalize the observed selectivity for the nucleophilic substitution at the most substituted carbon center, one has first to consider that the reaction is independent of the stereochemistry at the carbinol center (Scheme a), suggesting that the reaction proceeds through the formation of a cyclopropyl carbocation A (Scheme b), best represented as the hybrid form B . As the transition state has a positive charge spread all over the molecule, the reaction of B with nucleophiles should then occur at the site bearing the most stable positive charge leading to the homoallyl products 2 with inversion of configuration at the quaternary stereocenter.…”