Regioselective [3+2] Cycloaddition Reactions of the Phosphorus and Arsenic Analogues of the Thiocyanate Anion

Abstract

Salt metathesis reactions of tris-amide zirconium iodide complex ([NRR']3ZrI (1-I) (with NRR' = 3,5-Xylyl-tert-butylamide, N(Xyl)( t Bu)) and sodium 2-phosphaethynthiolate (Na(diox)3SCP) or 2-arsaethynthiolate (Na(diox)3SCAs) in THF result in a [3+2] cycloaddition of two SCE (E = P, As) units forming five membered 2-thio-1,3,4-thiadiphosphole and 2-thio-1,3,4-thiadiarsole heterocycles with an exocyclic sulfur atom, bridging two zirconium fragments (general formula (N(RR')3Zr(κ-C,S-(SCE)2-Zr(NRR')3 with E = P (2-PP) or As (3-AsAs)). The reactions are regioselective and only the P,P / As,As isomers are formed in THF and quantum chemical investigations suggest a concerted ring formation in line with a [3+2] cycloaddition reaction. Switching the solvent to toluene, salt metathesis with NaSCP results in the selective formation of the second possible [3+2] cycloaddition regioisomer 2-SP, with a 3-thio-1,2,4-thiadiphosphole bridge, while for NaSCAs both regioisomers with an As-As bond (3-AsAs) and an S-As bond (3-SAs; 3-thio-1,2,4-thiadiarsole bridge) are observed. Attempts to obtain the free diphosphole/diarsole rings using methyl triflate resulted In the cleavae of only one zirconium center, giving access to the trilfate complex 1-OTf and the mono-metallated diphospholes/diarsoles 4-PP, 4-PS and 5-AsAs respectively.