Mechanism, cyclopropane ring electrophilic addition

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

The cyclopropanation is initiated by the interaction of the electrophilic metal-carbene species with the jr-system of the olefin (Scheme 4). Two different mechanisms have been proposed for the formation of the cyclopropane ring a concerted pathway (a) or a two-step process via a metallacyclobutane (b). The first pathway (a) resembles the mode of addition of free carbenes to (C=C) double bonds [33] and has been proposed for reactions of metal carbenoids by various authors [7,11]. The principal bonding interaction in this case initially develops between the electrophilic carbenoid C-atom and the Ti-system of the C-C double... 

An alternative cyclization of the radical-cation intermediate has been observed in the irradiation of 40a that affords, in addition to the cyclopropane 42a, the dihydroquinoline 50. The formation of 50 is in agreement with a mechanism involving the generation of the radical-cation centered in the CC double bond 51. Intramolecular cyclization in 51, by electrophilic attack on the phenyl ring attached to the nitrogen atom, generates the intermediate 52, which by aromatization and hydrogen abstraction yields 50 (Scheme 12). 

The mechanism proposed for the cyclopropanation differs with the electronic balance. Electron-poor alkenes generally require higher temperatures and are involved in a mechanism characterized by primary decarbonylation followed by coordination of the alkene, generation of a metallacydobutane 1 and reductive elimination to give cyclopropane derivatives 3 (Scheme 11.1) [11]. Cydopropana-tion of electron-rich alkenes starts with their addition to the electrophilic carbene carbon atom forming a zwitterionic intermediate 2 which undergoes ring dosure... 

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