Electrophilic selenium

Thiem J,see Werschkun B (2001) 2/5 293-325 Thutewohl M,see Waldmann H (2000) 2ii 117-130 Tichkowsky I, see IdeeJ-M (2002) 222 151-171 Tiecco M (2000) Electrophilic Selenium, Selenocychzations. 208 7-54 Tohma H, Kita Y (2003) Synthetic Applications (Total Synthesis and Natural Product Synthesis). 224 209-248... 

Various electrophilic selenium reagents such as those described in Scheme 4.3 can be used. V-Phenylselenylphthalimide is an excellent reagent for this process and permits the formation of large ring lactones." The advantage of the reagent in this particular application is the low nucleophilicity of phthalimide, which does not compete with the remote internal nucleophile. The reaction of phenylselenenyl chloride or V-phenylselenenylphthalimide with unsaturated alcohols leads to formation of (3-phenylselenenyl ethers. 

Alkynes react with electrophilic selenium reagents such as phenylselenenyl tosylate.155 The reaction occurs with anti stereoselectivity. Aryl-substituted alkynes are regioselective, but alkyl-substituted alkynes are not. 

This reaction depends upon the facile solvolysis of (J-haloselenides and the facile oxidative elimination of a selenoxide, which was discussed in Section 6.6.3. An alternative method, which is experimentally simpler, involves reaction of alkenes with a mixture of diphenyl diselenide and phenylseleninic acid.189 The two selenium reagents generate an electrophilic selenium species, phenylselenenic acid, PhSeOH. 

Recently, the semi-synthesis of Vancomycin (48) on solid supports was accomplished using an allylic linker (Scheme 3.2) [123, 124]. Polymer-bound chiral electrophilic selenium reagents have been developed and applied to stereoselective se-lenylation reactions of various alkenes (Tab. 3.9) [125]. 

A new cyclization strategy via PET-generated electrophilic selenium species starting from diphenyldiselenide was introduced by Pandey et al. They succeeded in the cyclization of enyne compounds in good yields in the range 50-60% as well as in synthesizing a,a -trans dialkyl cyclic ethers via diastereoselective oxyselenation of l,n-diolefins (Scheme 31) [42,43],... 

In many synthetic applications, the sulfur or selenium substituent is subsequently removed by elimination, as will be discussed in Chapter 6. Arenesulfenyl halides, ArSCl, are the most commonly used of the sulfur reagents. A variety of electrophilic selenium reagents have been employed, and several examples are given in Scheme 4.3. 

Electrophilic selenium reagents are very effective in promoting cyclization of unsaturated molecules containing potentially nucleophilic substituents.72 Unsaturated carboxylic acids, for example, give selenolactones, and this reaction has been termed selenolactonization73 ... 

This lack of reactivity could be circumvented to some extent by the use of an analogous selenolactonization/reduction procedure.76 N-(phenylseleno)phthalimide was used as the electrophilic selenium source in the presence of a catalytic quantity of camphorsulfonic acid. Analysis of the crude product mixture obtained using acids 124 and 126 suggested... 

Scheme 6.1.36. Electrophilic selenium anchors as multifunctional linkers.

Phenyselenyl chloride reacts with 2-alkyl-l,3-dioxolanes having at least one a-hydrogen atom to give a-phenylseleno products accompanied by considerable amounts of disubstituted products. It is probable that this reaction proceeds as shown in Scheme 14, with acid catalyzed ring opening to an enol followed by attack of the electrophilic selenium reagent (79S982). 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

It is possible to perform selenenylation-deselenenylation sequences with only catalytic amounts of selenium species. This reaction sequence provides double bond transpositioned allylic ethers, allylic esters, or allylic alcohols 240 from the corresponding alkenes (Scheme 71). This sequence can be performed electrochemically, and the selenium electrophile is generated from catalytic amounts of diphenyl diselenide.467,468 It has been shown that the electrophilic selenium species can also be generated using diselenides and peroxosulfates together with copper (ii)... 

An inorganic selenium electrophile, selenium oxychloride (SeOCl2), has been utilized to prepare fused selenophenes <2002JOC2453>. Deprotonation of bis(cyanide) 104 with LDA followed by treatment with selenium oxychloride gave the benzo[c]selenophene 105 (Equation 15). This sequence also allowed for the preparation of a thieno[3,4-f]selenophene. 

Several types of electrophilic selenium species have been immobilized on solid supports, for more convenient application to organic synthesis. These include selenenyl halides, " selenocyanates, and selenosulfonates. ... 

---