Selenation/Thionation of α-Amino Acids: Formation and X-ray Structures of Diselenopiperazine and Dithiopiperazine and Related Compounds

N-Phenylglycine reacts with an equivalent of 2,4-bis-( phenyl)-1,3-diselenadiphosphetane 2,4-diselenide (Woollins' reagent, WR) to generate as the major product 1,4-diphenylpiperazine-2,5-diselenone and as the minor product 1,4-diphenyl-5-selenoxopiperazin-2-one. Whereas, reacting N-phenylglycine with an equivalent of 2,4-bis(4-methoxyphenyl)- 1,3-dithia-2,4-diphosphetane 2,4-disulfide (Lawesson's reagent, LR) affords 1,4-diphenylpiperazine-2,5-dithione and N-phenyl-2-(phenylamino) ethanethioamide. 2-Phenylglycine treated with an equivalent of Woollins' reagent under identical reaction conditions leads to only the formation of 2,5-diphenylpyrazine. Six X-ray structures are reported. DFT-B3LYP calculations suggest that the planar conformation is around 2 kcal/mol less stable than the puckered conformation for the bis-seleno derivative.