Electrophilic Sulfur and Selenium Reagents

Sulfur and selenium compounds in which the sulfur and selenium atoms are bound to more electronegative elements can react with alkenes to give addition products. The mechanism is similar to that in halogenation with a cyclic cationic intermediate being involved. In many synthetic applications the sulfur and selenium 

Mechanistic studies have been most complete with the sulfenyl halides.The reaction shows moderate sensitivity to alkene structure with electron-releasing groups accelerating the reaction. The addition can occur in the Markownikoff or anti-Markownikoff sense depending upon the structure of the alkene and the electrophile. These results can be understood by focusing attention on the addition intermediate, which may range from a sulfonium intermediate to a less electrophilic cyclic chlorosulfurane  

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

The stereospecific trans addition to norbornene is a particularly interesting reaction. Neither carbonium ion rearrangements nor the syn addition seen with many other electrophilic reagents is observed. These results, which are indicative 

The reaction of phenylselenenyl chloride or / -phenylselenophthalimide with unsaturated alcohols leads to cyclization and formation of /3-phenylselenenyl ethers. 

ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS A. Chlorinating agents Sodium hypochlorite solution A-Chlorosuccinimide Antimony pentachloride Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Controlled chlorination of acetylenes 

Zefirov, I. V Bodrikov, and M. Z. Krimer, Acc. Chem. Res. 12 282 (1979) G. H. Schmid and D. G. Garratt, The Chemistry of Double-Bonded Functional Groups, S. Patai, ed., John Wiley Sons, New York, 1977, Chapter 9 G. A. Jones, C. J. M. Stirling, andN. G. Bromby, J. Chem. Soc.. Perkin Trans. 2 1983 385. 

Pyridinium hydrotribromide (pyridinium Substitute for bromine when increased selectivity or 

Dioxane-bromine complex Same as for pyridinium hydrotribromide 

TV-Bromosuccinimide Substitute for bromine when low Br concentration is required 

In many synthetic applications, the sulfur or selenium substituent is subsequently removed by elimination as will be discussed in Chapter 6. Arenesulfenyl halides. 

Compared to the C-Br bonds in the bromonium ion, the C—S bonds will be stronger so that steric interactions which dictate access by the nucleophile become a more important factor in determining the direction of addition. For reactions involving phenylsulfenyl chloride or methylsulfenyl chloride, the intermediate is a fairly stable species, and ease of approach by the nucleophile is the major factor in determining the direction of ring opening. In these cases, the product has the anti-Markownikoff 

The stereospecific anti addition of phenylsulfenyl chloride to norbomene is a particularly interesting example of the stability of the intermediate. Neither rearrangement nor syn addition products, which are observed with many electrophilic reagents, are formed. This result indicates that the intermediate must be quite stable and reacts only by nucleophilic attack.  

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Addition Reactions with Electrophilic Sulfur and Selenium Reagents... 

Cyclizations induced by electrophilic sulfur and selenium reagents... 

Scheme 4.3. Sulfur and Selenium Reagents for Electrophilic Addition Reactions...

Electrophilic derivatives of both sulfur and selenium can add to alkenes. A variety of such reagents have been developed and some are listed in Scheme 5.1. They are characterized by the formulas RS-X and RSe—X, where X is a group that is more electronegative than sulfur or selenium. The reactivity of these reagents is sensitive to the nature of both the R and the X group. 

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. 

Several recent reviews have included specific types of electrophilic cyclofunctionalization reactions.1 Important areas covered in these reviews are halolactonization u cyclofunctionalization of unsaturated hydroxy compounds to form tetrahydrofurans and tetrahydropyrans lb cyclofunctionalization of unsaturated amino compounds lc cyclofunctionalization of unsaturated sulfur and phosphorus compounds ld lf electrophilic heterocyclization of unconjugated dienes 1 synthesis of y-butyrolactones 1 h synthesis of functionalized dihydro- and tetrahydro-furans lj cyclofunctionalization using selenium reagents lk lm stereocontrol in synthesis of acyclic systems 1" stereoselectivity in cyclofunctionalizations lP and cyclofunctionalizations in the synthesis of a-methylenelactones.lq Previous reference works have also addressed this topic.2... 

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