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

• Published in: Organometallics 2012-01, Vol.31 (2), p.687-699
• Main Authors: Kritchenkov, Andreii S, Bokach, Nadezhda A, Kuznetsov, Maxim L, Dolgushin, Fedor M, Tung, Tran Q, Molchanov, Alexander P, Kukushkin, Vadim Yu
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om201026n

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)­{N a C­(R)­ON­(C6H4R′-p)­C b Ph2}](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{N a C­(R)­ON­(C6H4R′-p)­C b Ph2}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.