Synthesis, Structure, and Bonding of 1-Oxa-6,6aλ(4)- chalcogenopentalenes and Related Derivatives; The Role of Intramolecular Coordination.

The synthesis and characterization of a series of alicyclic organochalcogen compounds derived from 2-chloro-1-formyl-3-hydroxymethylenecyclohexene (16) are described. The reaction of 16 with disodium disulfide afforded an unexpected 1-oxa-6,6aλ(4)-dithiapentalene 33, whereas the reaction of disodium diselenide afforded 1-oxa-6,6aλ(4)-diselenapentalene 34, along with 1,6-dioxa-6a-selenapentalene 35. In contrast, when 16 was treated with Na2Te2, it resulted in the formation of 1-oxa-6,6aλ(4)-ditellurapentalene 36 along with 1,6-dioxa-6a-tellurapentalene 37 and cyclic monotelluride 38. The oxidation of 1-oxa-6,6aλ(4)-diselenapentalene 34 with m-CPBA provided the corresponding 1-oxa-6,6aλ(4)-diselenapentalene-6,6aλ(4)-dioxide 39 in which both of the selenium atoms were found to be oxidized. The 1-oxa-6,6aλ(4)-dichalcogenopentalenes 33, 34, and 36 are stabilized by 3c-4e bond resulting from intramolecular E···O interaction. The donor oxygen donates a lone pair of electrons to the σ* orbital of acceptor E-E bond (E = S, Se, and Te) whose σ bond already has its bond pair electrons. The existence of intramolecular E···O interactions was established by multinuclear NMR spectroscopy, single crystal X-ray analysis, and computational studies. The single crystal X-ray structures of compounds 33, 34, and 36 reveal that the molecules are almost planar. Nucleus-independent chemical shifts were calculated to compare the aromaticity in both the five-membered rings of 33, 34, 36 and 1,6-dioxa-6a-chalcopentalenes (35 and 37). The isotropic shift values of the covalently fused five-membered heterocyclic rings are more negative than the five-membered heterocyclic rings formed by intramolecular coordination.