Facile and Reversible 1,3-Dipolar Cycloaddition of Aryl Ketonitrones to Platinum(II)-Bound Nitriles: Synthetic, Structural, and Theoretical Studies

The reaction between trans-[PtCl2(NCR)2] (R = Et 1, NMe2 2, NEt2 3, NC5H10 4) and the acyclic triaryl ketonitrones Ph2CN­(O)­C6H4R′-p (R′ = H 5, Me 6, Cl 7, OMe 8) proceeds as a facile and consecutive two-step
intermolecular cycloaddition to give the mono-cycloaddition products trans-[PtCl2(NCR)­{NaC­(R)­ON­(C6H4R′-p)­CbPh2}](a−b) (R/R′ = Et/H 9, Et/Me 10, Et/Cl 11, Et/OMe 12, NMe2/H 13, NMe2/Me 14, NMe2/Cl 15, NMe2/OMe 16, NEt2/H 17, NEt2/Me 18, NEt2/Cl 19, NEt2/OMe 20, NC5H10/H 21, NC5H10/Me 22, NC5H10/Cl 23, NC5H10/OMe 24) and
then the bis-2,3-dihydro-1,2,4-oxadiazole complexes trans-[PtCl2{NaC­(R)­ON­(C6H4R′-p)­CbPh2}2](a−b) (R/R′ = Et/H 25, Et/Me 26, Et/Cl 27, Et/OMe 28, NMe2/H 29, NMe2/Me 30, NMe2/Cl 31, NMe2/OMe 32,
NEt2/H 33, NEt2/Me 34, NEt2/Cl 35, NEt2/OMe 36, NC5H10/H 37, NC5H10/Me 38, NC5H10/Cl 39, NC5H10/OMe 40). The
ketonitrones Ph2CN­(O)­C6H4R′-p were found to be unexpectedly much more
reactive toward the platinum­(II)-bound nitriles than the related aldonitrones p-R‴C6H4CHN­(O)­R″
(R′′ = Me, Ph; R‴ = H, Me), and the difference
in the reactivity in 1,3-dipolar cycloaddition (DCA) of the keto-
and aldonitrones was interpreted by theoretical calculations and was
explained in terms of the orbital arguments as a result of the increase
of the HOMOnitrone energy from aldo- to ketonitrones. The
first example of the reversibility in metal-mediated DCA of nitrones
to nitriles was observed, and this phenomenon, as follows from the
performed theoretical study, is justified by the thermodynamic instability
of the PtII-bound 3,3-diaryl-2,3-dihydro-1,2,4-oxadiazoles.
Metal-free C5-diphenyl-2,3-dihydro-1,2,4-oxadiazoles 42 and 43 were liberated from corresponding (oxadiazole)2PtII complexes 26 and 30 by treatment with excess NaCN, and these heterocycles were characterized
by high-resolution ESI+-MS and 1H and 13C­{1H} NMR spectroscopies.