Synthesis of η 4 :π 2 -Exocyclic-Diene Iridium(I) Complexes Derived from 1,3-Oxazolidin-2-ones and Their Transformation into Iridium(III) Derivatives by Reaction with a Phosphine and with Aldehydes Vero ´nica Salazar,* ,² Oscar R. Sua ´rez-Castillo, ² Rosa Padilla, ² J. Carlos Macı ´as P., ² Miguel A Ä ngel Me ´ndez-Rojas, ²,‡ Joaquı ´n Tamariz, § and Adriana Benavides § Centro de InVestigaciones Quı ´micas, UniVersidad Auto ´ noma del Estado de Hidalgo, Ciudad UniVersitaria, Km. 4.5 Carretera Pachuca-Tulancingo, Pachuca Hgo., Me ´ xico, and Departamento de Quı ´mica Orga ´ nica, Escuela Nacional de Ciencias Biolo ´ gicas, IPN, Prol. Carpio y Plan de Ayala, 11340 Me ´ xico D.F., Me ´ xico ReceiVed July 12, 2005 The reaction of the dimer [Ir(µ-Cl)(coe) 2 ] 2 with two exocyclic dienes like N-substituted 4,5-dimethylene- 2-oxazolidinones a and b, and KTp Me2 yields derivatives of the composition Tp Me2 Ir(a or b)(1a,b). Derivative 1a reacts with Ph 2 PCtCPPh 2 , with the formal oxidative addition of the diene moiety to the metal center and coordination of the phosphine in the κ 1 mode. Derivatives 1a,b react with aromatic aldehydes to form the new compounds 3 (with benzaldehyde) and 4 (anisaldehyde), which contain an elaborated bidentate ligand formed by coupling of the two organic fragments (diene and aldehyde). The structures of compounds 3a,b have been determined by X-ray diffraction analysis. Introduction Ir butadienes and their methyl derivatives of composition Tp Me2 Ir(CH 2 dC(R)C(R)dCH 2 ) (R ) H, Me; Tp Me2 ) hydrotris- (3,5-dimethylpyrazolyl)borate 1 ) have shown to exhibit a reactiv- ity quite different from that of monomer olefin compounds in C-H activation processes. 2 The Ir(I) complexes Tp Me2 Ir(CH 2 d CHR) 2 (R ) H, Me) (which contain ethylene or propene) easily react thermally by C-H activation, either intramolecularly 3 or toward hard bases 4 or other hydrocarbons, 5 and experience olefin substitution when reacted with soft bases. 6 In contrast, com- pounds of the composition Tp Me2 Ir(CH 2 dC(R)C(R)dCH 2 ) 7 (R ) H, Me) are very reluctant to experience intramolecular thermal C-H activation, which requires photochemical irradiation to take place. These compounds react with either soft or hard Lewis bases to yield Ir(III) adducts in which the diene has experienced a formal oxidative addition process to the metal center. 8 This process is on occasion followed by subsequent steps, including C-C or C-X bond formation involving the diene moiety and the Lewis base. 9 For aldehydes, this reaction ultimately yields products resulting from the formal decarbonylation of this substrate, but the reaction has been shown to proceed through a series of isolable kinetic intermediates with a bicyclic structure. 10 In this contribution we report the preparation of novel diene Ir complexes derived from the N-substituted exocyclic dienes 4,5-dimethylene-2-oxazolidinone (a, b). These dienes 11 have proved to be highly reactive, both regio- and stereoselec- tively, in Diels-Alder cycloadditions. 12 The observed reactivity and selectivity were mainly due to the strong activation of the diene moiety by the heteroatoms of the heterocyclic ring. 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