An experimental and computational investigation of the site selective and lattice driven cycloaddition reactions of SNS + salts with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and 1,1,2,2-tetracyanoethylene: Accounting for experimental findings for SNS + cycloaddition reactions with other related multi cyano-containing compounds using DFT and VBT approaches

Abstract

Reaction of SNSSbF 6 and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone ( 1)) in SO 2 solution in a 2:1 ratio afforded the dicycloaddition product 1(SNS) 2(SbF 6) 2 · SO 2 in 85% isolated yield. The dicycloaddition product 1(SNS) 2(SbF 6) 2 · SO 2 was fully characterized by IR, Raman, NMR, elemental analysis, and single crystal X-ray crystallography. The product of the site selective dicycloaddition of SNS + at the nitrile functionalities of 1 represents the first structurally characterized ortho-bis-1,3,2,4-dithiadiazolylium salt which adopts a conformation that utilizes the intramolecular S δ+ –O δ− electrostatic interactions. The reaction of 1 and SNS[Al(OC(CF 3) 4] in a 1:1 ratio yielded only the monocycloaddition product 1(SNS)[Al(OC(CF 3) 3) 4] indentified by multinuclear 13C, 14N, 19F, 27Al NMR in SO 2 solution. The assignment of the 13C NMR of 1(SNS) + in SO 2 solution, were supported by comparison of the calculated (MPW1PW91/6-31G ∗) 13C NMR and experimental chemical shifts, and by comparison of the observed chemical shifts of the related 2(SNS) + produced on reacting the mono-nitrile CDMQ (2-cyano-5,6-dichloro-3-methoxy-1,4-benzoquinone ( 2)) with SNS + (as the SbF 6 - salt). The reaction of SNSSbF 6 with TCNE (1,1,2,2-tetracyanoethylene ( 3)) gave the tri-cycloaddition product 3(SNS) 3(SbF 6) 3 · SO 2 (IR, Raman microscopy and X-ray crystallography) in 40% yield. The energetics of the cycloaddition reactions of SNS + with 1– 3 and other multifunctional unsaturated centers (e.g. HC C–CN, NC–CN, C ( CN ) 3 - , o, m, p-(CN) 2C 6H 4, 1,3,5-(NC) 3C 6H 3, 4– 10, previously reported, see Chart 1), were estimated in the gas phase (MPW1PW91/6-31G ∗), solution (PBE0/6-311G ∗) and in the solid state by the ‘volume based thermodynamics’ (VBT) approach. The general thermodynamic trends associated with the stepwise cycloaddition of SNS + with multifunctional nitriles were established. The 1:1 cycloaddition products were calculated to be stable in the gas phase and solution, while in the solid state the larger lattice enthalpy of the 1:2 salt, relative to twice that of the 1:1 salt, favored the 1:2 cycloaddition product.