Journal article

Migratory insertion reactions of norbornene, norbornadiene and 7-oxanorbornene derivatives into the palladium-carbon bond of heterodinuclear Fe-Pd-Acyl complexes. Substrate influence on the Pd coordination sphere and crystal structure of [(OC)(3)Fe{mu-Si(OMe)(2)(OMe)}(mu-dppa)Pd{C9H9NO3C(=O)Me}]

Norbornene and norbornadiene insert across the exo-face of their C=C bond into the Pd-acyl bond of [(OC)(3)Fe{mu-Si(OMe)(2)(OMe)}(mu-dppm)Pd{C(=O)Me}] and (1a) (dppm = Ph2PCH2PPh2) to afford the heterodinuclear complexes [(OC)(3){(MeO)(3)Si}Fe(mu-dppm)Pd{C7H10C(=O)Me}] (2) and C(OC)(3){(MeO)(3)Si}-Fe(mu-dppm)Pd{C7H8CO)Me}] (3), respectively. In these insertion products, the alkoxysilyl ligand has become terminally bound whereas a dative bond between the acetyl oxygen and the metal allows the square-planar Pd centre to achieve a 16e configuration, thus resulting in a five-membered Pd-C-C-C-O ring. This chemistry was extended to chiral olefins and when the dppm and dppa complexes 1a,b were reacted with a 9:1 mixture of (+/-)-(2-cyano-7-oxabicyclo[2.2.1]hept-5-ene-2-endo-yl and -2-exo-yl acetate (4,4'), the stable insertion products [(OC)(3)Fe{mu-Si(OMe)(2)(OMe)}(mu-Ph2PXPPh2)Pd{C9H9NO3C(=O)Me}] (5a,5a' X = CH2;5b,5b' X = NH) were isolated in high yield. A high stereo- and regioselectivity was found for these metallacarbonylations. Spectroscopic data as well as X-ray diffraction studies show that in contrast to 2 and 3, the mu(2)-eta(2)-Si-O interaction present in the precursors la,b is retained in 5a,b. The Pd atom adopts a pseudo square-pyramidal coordination resulting from an additional weak ketonic interaction with the 5-exo-acetyl group of the 2-endo-acetoxy-2-cyano-7-oxabicyclo [2.2.1] hept-5-exo-yl moiety, which leads to the formation of a five-membered chelate ring. Electronic effects are invoked to account for these different structural features. Under a carbon monoxide atmosphere, CO migratory insertion into the Pd-oxanorbornyl bond of pure 5a,b occurs and leads to the acyl complexes [(OC)(3)Fe{mu-Si(OMe)(2)(OMe)}(mu-Ph2PXPPh2)Pd{C(=O)C9H9NO3C(=O)Me}] (6a X = CH2; 6b X = NH). These successive insertion reactions constitute the first elementary steps on the way to polyketone chain growth mediated by a bimetallic system.


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