Expanding the Scope of Indium in Frustrated Lewis Pair Chemistry

The "soft" pyridine stabilized FLP ^tBu₂In(py)CH₂P^tBu₂ (1·py) was reacted with SO₂, PhNCO, PhNSO, azobenzene, pyridazine, 1,2,4,5-tetrazine, tosylaziridine, and hydrazine, respectively. The reactivity can be roughly classified into three types of FLP activation. (a) Typical FLP-type reactivity is observed for the reaction with azobenzene and tosylaziridine, with the formation of 1,2-addition or ring-opening products, respectively. (b) "Soft" FLP-type reactivity occurs toward substrates containing multiple sites for FLP attack-in this case 1·py prefers coordination to the "softer" binding site, resulting in the formation of "soft" isomers of possible FLP adducts, as was demonstrated for PhNCO and PhNSO. (c) An unexpected reactivity results that can be described as a "masked" In/C-FLP; when 1·py was reacted with SO₂, an insertion dimer formed, with the methylene bridge connected to the sulfur atom; the reaction of 1·py with hydrazine afforded a four-membered hydrazide heterocycle, under loss of the methylene bridge as MeP^tBu₂, instead of the reaction with the P^tBu₂ fragment, pyridine, or excess hydrazine. The products were characterized by a variety of NMR spectroscopy methods supported by X-ray crystallography as well as quantum-chemical calculations for insights into the thermodynamics and selectivity of product formation.