Seleniranium intermediate

Diphenyl diselenide is an especially useful co-reagent with [bis(acetoxy)-iodo]benzene. For example, the BAIB/PhSeSePh (2 1) combination has been employed for trans, Markovnikov additions of PhSeOAc and PhSeOH to alkenes [35]. Such formal additions appear to be regulated by seleniranium intermediates, and were extended to intramolecular cyclizations of olefinic alcohols, carboxylic acids, and / -dicarbonyl compounds (Scheme 12). 

Compounds of type 77, passing through the seleniranium intermediates 78, give rise to the four-membered cyclic compounds 79 which, according to Baldwin s rules [61], are the result of a 4-exo-trig cyclization, or to the five-membered derivatives 80 as the result of a 5-endo-trig cyclization. Only very few cases are known in which the cyclization affords a four-membered heterocyclic... 

The imino nitrogen atom is sufficiently nucleophilic to attack the carbon atom of the seleniranium intermediate. An interesting example of these reactions is the conversion of imidates into y-lactams reported by Toshimitsu and Uemura [88-90] and by other authors [91,92] which is illustrated in Scheme 24. The cyclization of 155 occurs through the formation of a carbon-nitrogen bond with the generation of the iminium salt 156. The success of this reaction is due to the use of PhSeBr as the selenenylating agent. In this way, in fact, the bromine... 

In all the examples reported so far, only the nitrogen atom can act as the nucleophile in the intramolecular capture of the seleniranium intermediates. We will... 

The first class of compounds in which such a competition was observed is that of alkenyl amides. Toshimitsu [90] investigated the cyclization of the alkenyl amides promoted by PhSeCl or PhSeBr. Some of the results obtained are reported in Scheme 26. Compound 169 gave rise to a 5-exo-trig ring closure reaction and afforded the imidate 170 derived from the trapping of the seleniranium intermediate by the oxygen atom. No evidences of the formation of the y-lactam... 

Carbocyclic compounds can be formed by the nucleophilic intramolecular capture of a seleniranium intermediate of an olefinic bond. The carbonium ion which is formed as intermediate can react with another nucleophile or with the solvent. The first examples of these carbocyclization reactions were observed with dienes. Clive [105] reported that the reaction of the diene 203 with phenyl-selenyl chloride in acetic acid afforded the intermediate 204 which reacted with the solvent to give the bicyclic compound 205 (Scheme 31). Carbocyclization reactions were efficiently promoted by phenylselenyl iodide produced by diphenyl diselenide and iodine. As indicated in Scheme 31,Toshimitsu reported that the reaction of 1,5-hexadiene 206, in acetonitrile and water, afforded the acetamido cyclohexane derivative 209, derived from the cyclization of the seleniranium intermediate 207 followed by the reaction of the carbocation 208 with acetonitrile [106]. In several cases, carbocyclization reactions can be more conveniently effected by independently generating the seleniranium intermediates. A simple procedure consists of the reaction of trifluoromethane-sulfonic acid with j9-hydroxyselenides, which can be easily obtained from the... 

An aromatic carbon atom of an electron rich benzene derivative can also trap seleniranium intermediates. Deziel has reported recently, that the reaction of... 

The absence of further products, particularly those resulting from -attack on the seleno moiety and those arising from Wagner-Meerwein rearrangements, points to a mechanism involving a non-configurationally selective attack by Cl or methanol upon the seleniranium intermediate, as demonstrated below for Cl . 

This regioselectivity is consistent with an unsymmetrically bridged seleniranium intermediate in which the more positive charge is remote from the EWG substituent. The directive effect is contrary to regiochemistry being dominated by the chloride ion approach, since chloride addition should be facilitated by the dipole of an EWG. 

According to the calculations at the B3LYP/6-311-l-G(d,p) level, the addition of selenium dihalides to vinyl ethers proceeds in two steps via the seleniranium intermediate. The high selectivity stems from the consistent effects of the charge and orbital factors." ... 

---