Selenium atoms, reaction + olefins

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

The role of SnCLt as catalyst in [2+2] cycloaddition reactions of olefines which are activated by selenophenyl and silyl groups in the 1,1 position with vinyl ketones has been examined in a combined experimental/theoretical study by Yamazaki et al.166. Calculations at the HF level showed that the formation of a chelate complex where the selenium atom of the olefin and the oxygen atom of the keto group are bonded as ligands to the... 

The presence of the soft selenium atom and the hard oxygen however, make, the reaction of p-hydroxy selenides site selective. These have in fact been transformed selectively to vinylselenides 7) or olefins4-9,11 12), by selective activation of the hydroxy group, inter alia, with thionyl chloride alone or with thionyl, mesyl and phosphoryl chloride, trifluoracetic anhydride, phosphorus triiodide or diphosphorus tetraiodide in the presence of triethyl amine (Schemes 8Ab 8Bb). The formation of olefins from p-hydroxyselenides is regio- and stereoselective and occurs by formal removal of the hydroxyl and selenenyl moiety in an anti fashion. 

The presence of the soft selenium atom and the hard oxygen however, make, the reaction of p-hydroxy selenides site selective. These have in fact been transformed selectively to vinylselenidesor olefins by selective activation of the... 

Ley reported that selenium promoted carbocyclization reactions can also be effected by the enolic olefinic bonds of -dicarbonyl compounds [111]. These reactions occur with N-PSP in the presence of zinc iodide, tin tetrachloride or aluminium trichloride. An example is reported in Scheme 33. In the intermediate 219, derived from the -ketoester 218, cyclization through the oxygen atom to afford 220 is kinetically favoured. This reaction, however, is reversible, and upon prolonged reaction times and in the presence of strong acids the carbocyclization product 221 is formed. This procedure has been recently employed by Ley to effect the conversion of the alkenyl p-keto lactone 222 into the tricyclic selenide 223 (Scheme 33) which is a key intermediate in the preparation of model compounds with antifeedant activity [112]. 

The increasing reactivity with alkyl substitution in the acetylenes is very pronounced something in excess of a factor of 100 in relative rates at 298°K. This, it may be observed, is substantially greater than the factor of 20 between the reactivities of ethylene and 2-butene. Such large relative reactivity differences are rare in free radical reactions, and one realizes that the triplet sulphur atom must be very selective in its reaction with different olefins. A comparison of the behaviour of the triplet state atoms of oxygen, sulphur and selenium will be deferred until the end of the discussion of this whole group of atom additions. 

Clearly, more research needs to be done. For instance, there is a real need for more detailed product studies. The initial products with oxygen and sulphur atoms are the epoxides and episulphides. With selenium and tellurium the cyclic adducts are apparently only transient, so there is no accumulation of initial product to serve as a measure of the reaction rate. Callear and Tyerman [123] measure the selenium—olefin reaction rate by photometric methods, in which the appearance of bands attributed to an alkyl selenide are observed. These products are not stable, and decay according to... 

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