Nonstereospecificity in the cycloadditions of keteneiminium salts to olefins. Evidence for a stepwise mechanism.

Theoretical Study of the Electronic Structure of Ketene, Ketenimine, Keteniminium Ion and Related Cumulenes. Evaluation of a 1,2 Dipolar Model

“…also W n exporimantally ahown that the C1UC are illuatrated in PIG. 4. about 111' in both ketenimine 3 and vinylketenimine 5 to 180' in the correaponding carbenium ion 8. anglo in N-alkyl aubstitued keteniminea bearing strongly electron-withdrawing aubatituents (mulfonyl group.)…”

Revisiting Keteniminium Salts: More than the Nitrogen Analogs of Ketenes

“…[18,29] This moiety, by its truly ancillary character, belongs to a rare class of "traceless" chiral auxiliaries. [18,29] This moiety, by its truly ancillary character, belongs to a rare class of "traceless" chiral auxiliaries.…”

“…[16] Later, Houge and Ghosez and co-workers [18] studied experimentally the intermolecular asymmetric [2+2]-cycloaddition reaction between a keteniminium salt and cyclopentene. In 1983, Saimoto et al [29] showed that these formal cycloadditions are not concerted, but instead proceed in a stepwise manner through cationic intermediates. The mecha-nism they proposed for the cycloaddition of a chirally substituted keteniminium salt to cis-2-butene is depicted in Scheme 24.…”

A Convergent Strategy for the Asymmetric Synthesis of Enantiomerically Pure Bicyclic Compounds by Using a Silicon-Directed Cycloaddition Reaction: The Synthesis of Enantiomerically Pure Bicyclo[3.2.0]hept-2-en-6-one

“…We anticipated that the silyl substituent would not only activate the double bond towards electrophilic attack but would also control the facial selectivity. [4] However, since the rate and selectivity determining step of the cycloaddition involved the formation of a seven membered ring intermediate [5] (such as 3 a and 3 b, Scheme 3) we assessed our hypothesis by computational studies. Investigations of this reaction with ab initio and density functional theory methods [6] have led to a mechanistic scenario involving initial electrophilic attack of the keteniminium group on the alkene to form a bridged enamine cation (a distorted cyclopropylcarbinyl cation) which cyclizes with a very low activation barrier to form the cyclobutaneiminium ion (Scheme 3).…”

“…[4] Moreover, the problems of fragility, acidity, and thermal stability of the host lattice have not been overcome. [5] Hence, the design and preparation of robust, air-stable, porous coordination zeolites provides a considerable challenge in materials chemistry.…”