American Chemical Society
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Ligand-Induced and Thermally-Induced Orthometalation of the Bis(ylide) Ligand [Ph3PC(H)]2CO. Generation of the C,C-Chelating Group C6H4-2-PPh2C((H)COCH2PPh3

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journal contribution
posted on 1998-12-03, 00:00 authored by Larry R. Falvello, Susana Fernández, Rafael Navarro, Angel Rueda, Esteban P. Urriolabeitia
The dinuclear complex [Pd(μ-Cl){[C(H)PPh3]2CO}]2(ClO4)2 (2c) undergoes thermal rearrangement in refluxing NCMe, giving the dinuclear orthometalated derivative [Pd(μ-Cl)(C6H4-2-PPh2C(H)COCH2PPh3)]2(ClO4)2 (4c) as a mixture of two diastereoisomers (RR/SS and RS/SR). The orthometalation proceeds through an electrophilic substitution pathway, and the formation of the C,C-chelating ligand (C6H4-2-PPh2C(H)COCH2PPh3) results from an intramolecular acid−base reaction in which the proton generated in the orthometalation reaction is captured by an ylide group. A decrease in the cone angle of the phosphonium group dramatically reduces the conversion of the bis(ylide) ligand into the orthometalated ligand. The orthometalation reaction can also be induced by ligand addition to the dimer [Pd(μ-Cl){[C(H)PPh3]2CO}]2(ClO4)2 (2c) under very mild conditions. For instance, complex 2c reacts with PPh3 or PPhMe2 in CH2Cl2 at room temperature to give [PdCl(C6H4-2-PPh2C(H)COCH2PPh3)(PR3)](ClO4) (PR3 = PPh3 8, PPhMe2 9). Less sterically hindered ligands such as pyridine or 3,5-lutidine react with 2c to give in a first step the bis(ylide) complexes [PdCl{[C(H)PPh3]2CO}(L)](ClO4) (L = py; 3,5-lut), which are transformed into the corresponding orthometalated derivatives [PdCl(C6H4-2-PPh2C(H)COCH2PPh3)(L)](ClO4) (L = py 6, 3,5-lut 7) by thermal treatment in refluxing NCMe. This different behavior is explained on the grounds of the different steric requirements of the incoming ligand (phosphine/pyridine). Similar behavior has been observed for the complex [Pd{[C(H)PPh3]2CO}(NCMe)2](ClO4)2 (3c). 3c reacts with py or dppm giving [Pd{[C(H)PPh3]2CO}(L)2](ClO4)2 (L = py 10, L2 = dppm 11), which is transformed into [Pd(C6H4-2-PPh2C(H)COCH2PPh3)(L2)](ClO4) (L = py 12, L2 = dppm 13a + dppm-O 13b) by refluxing in NCMe. However, complex 3c reacts with PPh3, dppe, or phen in CH2Cl2 at room temperature giving [Pd(C6H4-2-PPh2C(H)COCH2PPh3)(L2)](ClO4) (L2 = PPh3, NCMe 14, dppe 15, phen 16). Complex 3c is not transformed into its corresponding orthometalated derivative [Pd(C6H4-2-PPh2C(H)COCH2PPh3)(NCMe)2](ClO4)2 (17) by refluxing in NCMe, but 17 can be obtained by treatment of 4c with TlClO4 in NCMe. The orthometalation reaction of the bis(ylide) ligand can even occur spontaneously. The acetate-bridged dimer [Pd(μ-OOCCH3){[C(H)PPh3]2CO}]2(ClO4)2 (18) transforms spontaneously at room temperature into the mixed orthometalated bis(ylide)complex [(C6H4-2-PPh2C(H)COCH2PPh3)Pd(μ-OOCCH3)2Pd{[C(H)PPh3]2CO}](ClO4)2 (19). The crystal structure of [Pd(C6H4-2-PPh2C(H)COCH2PPh3)(PPh3)(NCMe)](ClO4)2 (14) has been determined and reveals the presence of an orthometalated C6H4-2-PPh2 unit, a C-linked ylide Pd−C(H), and a phosphonium fragment CH2PPh3. The phosphine group is coordinated cis to the orthometalated carbon atom.