Phosphinito- and phosphonito-oxazoline Pd(ii) complexes as CO/ethylene insertion intermediates: synthesis and structural characterization

Abstract

The phosphinito-oxazoline ligand 4,4-dimethyl-2-[methoxy(diphenylphosphine)]-4,5-dihydrooxazole (2a) and the phosphonite-oxazoline ligand 4,4-dimethyl-2-[methoxy(6H-dibenz[c,e][1,2]oxaphosphorin)]-4,5-dihydrooxazole (8a) were prepared by deprotonation of (4,5-dihydro-4,4-dimethyloxazol-2-yl)methanol (1a) and reaction with the corresponding P–Cl function, similar to the ligands 2b (4,4-dimethyl-2-[1-oxy(diphenylphosphine)-1-methylethyl]-4,5-dihydrooxazole) and 8b (4,4-dimethyl-2-[1-oxy(6H-dibenz[c,e][1,2]oxaphosphorin)-1-methylethyl]-4,5-dihydrooxazole) reported previously. These ligands react with [PdClX(COD)] to give complexes of the type [PdClX(P,N)] (3a P,N = 2a, X = Cl; 4a P,N = 2a, X = Me; 4b P,N = 2b, X = Me; 9a P,N = 8a, X = Cl; 9b P,N = 8b, X = Cl; 10a P,N = 8a, X = Me; 10b P,N = 8b, X = Me). Complexes 4a,b and 10a,b reacted with AgCF₃SO₃ to yield [PdMe(P,N)OSO₂CF₃] 5a,b and 11a,b, respectively. From the stepwise insertion reaction of CO and ethylene into the Pd–C bond of 5a and 11a,b, the alkyl ketone chelate complexes [Pd{CH₂CH₂C(O)Me}(P,N)]CF₃SO₃7a and 14a,b respectively, have been isolated and spectroscopically characterized. Complexes 3a·CH₂Cl₂, 5a, 9b, 10a,b, [PdMe(H₂O)(P,N)]CF₃SO₃12b, (P,N = 8b) and 14a,b have also been characterized by X-ray crystallography and the structures of 14a,b represent still rare examples of structurally characterized CO/ethylene coupling products.