Alkenes electrophilic cyclization

The next transformation is an electrophilic cyclization. Protonation of the terminal alkene produces the more favourable tertiary carbocation, then ring formation occurs by electrophilic addition to the neighbouring alkene. 

The seleniranium ion intermediates in the cyclization reactions can be either generated from the corresponding /3-hydroxyselenides as shown in Scheme 6 or from suitably substituted alkenes. Depending on the alkene and on the selenium electrophile, cyclizations can be performed with high selectivities. The size of the electrophilic reagent has... 

Samarium(ir) iodide in the presence of HMPA effectively promotes the intramolecular coupling of unactivated alkenic ketones by a reductive ketyl-alkene radical cyclization process (eq 25). This protocol provides a means to generate rather elaborate carbocycles through a sequencing process in which the resulting organosamarium species is trapped with various electrophiles to afford the cyclized product in high yield. ... 

Inverse electron demand Dihydroxylation of alkenes Electrophilic aromatic substitution Enyne cyclization Epoxidation... 

Intramolecular alkene-acetylene cyclization of terf-butyl enynoate proceeds to afford the titanacycle, which in turn reacts regioselectively with electrophiles at... 

The synthetic scope of radical cyclizations can be further extended by tandem trapping by electrophilic alkene. 

Electronic factors also influenced the outcomes of these cyclization reactions cyclization of pyrrole 84 to bicyclic amine 85 is catalyzed by the sterically open complex 79a. In this reaction, initial insertion into the Y - H bond occurred in a Markovnikov fashion at the more hindered olefin (Scheme 19) [48]. The authors proposed that the Lewis basic aromatic ring stabilizes the electrophilic catalyst during the hydrometallation step, overriding steric factors. In the case of pyrroles and indenes, the less Lewis basic nitrogen contained in the aromatic systems allowed for the cyclization of 1,1-disubstituted alkenes. 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

In many of these cases, the nucleophile is a C=C double bond (usually an alkenic group and less frequently an aromatic group). Alkenic oxime mesylates enable intramolecular cyclization by an electrophihc addition of the double bond to the electrophilic intermediate. These reactions are terminated by a proton loss. 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride. The net result is the addition of hydrogen and the nucleophile to the alkene. The regioselectivity is excellent and is in the same sense as is observed for proton-initiated additions.16 Scheme 4.1 includes examples of these reactions. Electrophilic attack by mercuric ion can affect cyclization by intramolecular capture of a nucleophilic functional group, as illustrated by entries 9-11. Inclusion of triethylboron in the reduction has been found to improve yields (entry 9).17... 

ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS A. Chlorinating agents Sodium hypochlorite solution 7V-Chloro succi n i m i de Antimony pentachloride Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Controlled chlorination of acetylenes... 

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