Sulfur and Selenium Containing Compounds

7 Sulfur- and Selenium-containing Compounds - a-Sulfenylation and a-selenylation of 1-phosphoryl sulfoxides 243 has been used to prepare the a-methylsulfenyl-a-phosphorylsulfoxides 244 and the analogous a-phenylselenides with low levels of diastereoselectivity. Similar sulfenylation of (+)-(5)-dimethoxyphosphorylmethyl /7-tolyl sulfoxide (244, R = Me, R =/7-tolyl) followed by Wadsworth-Emmons olefination provided an enantioselective 

Lawesson s reagent has been used to convert a,a-difluoromethylphosphonates into the corresponding sulfides 249 in good to excellent yields. The sulfides 249 can be converted back into the phosphonates by treatment with either dimethyl-dioxirane or perfluoro-m-2-butyl-3-propyloxaziridine. The naphthalene deriva- 

JO Sulfur and Selenium Containing Compounds. - The interaction of 0,0-dialkyl methylphosphonothioates acid with POCI3 to give the chloridates (286) and alkyl phosphorodichloridates parallels reactions of thiophosphoric esters already encountered. The use of Lawesson s reagent to replace the phosphoryl bond in (l,l-difluoroalkyl)phosphonic esters by thiophosphoryl, has been noted (see also reference 258). -  

The (5 p)-phosphinothioate (287) can be separated in pure, crystalline, form from the oily racemic mixture the addition to it of selenium yields the phosphonoselenothioic ester, oxidation of which, by iodine in water, affords the (i p/ p)-diselenide (288). ° Reactions between ( Sp)-(287) and carbon tetrahalides CX4 (X = Cl or Br)-Et3N yield the thiophosphonic halides (289), evidently with retention of configuration at phosphorus, since the anilide derived from them has a configuration known to be (S p).  

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In the above-described displacement reactions (Sh2) of sulfur- and selenium-containing compounds, the mechanism could either be a synchronous or a stepwise process (Scheme 4.2). Thus, in the Sh2 stepwise mechanism an intermediate sulfuranyl or selenyl radical is formed, followed by a-cleavage [56]. Based on competitive studies, a stepwise process was suggested to occur in the reaction of (TMS)3Si radical with -decylphenylselenide [43]. On the other hand, ab initio calculations supported the synchronous process and the same observations were explained in terms of an overall reversible reaction [57]. 

Anodic Oxidation of Sulfur- and Selenium-Containing Compounds... 

Sulfur- and Selenium-containing Compounds. A new route to a-thioalkyl-phosphonates (292) and -phosphinates (294) has been reported from the reaction of selenothioic acid S-esters (291) with phosphites and phosphinites, respectively. By-products (293), derived from the P(III) ester alkylating the Se atom, are formed in many cases. a-Phosphono-p-substituted a,P-unsaturated dithioesters (296), prepared from diethyl phosphonodithioacetate (295), undergo Diels-Alder reactions with enol and thioenol ethers to give excellent yields of novel 5-... 

Nitrogen-, Sulfur- and Selenium-containing Compounds.- 2,3.4-Tri-O-acetyl-P-D-arabinopyranosyl azide, 5 the azido orthoester 15, the diazirine 16, episulfide 17, selenourea 18, a-D-ribopyranosylamine l,3-(cyclic carbonate), lactams 19a/b and 20, the 6-nitro-7-methylthioheptitol 21,2-acetamido-2-deoxy-P-D-glycopyranosyl nitromethane (3-... 

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