Regiospecific Acylation of Cycloplatinated Complexes: Scope, Limitations, and Mechanistic Implications

A series of platinum complexes based on the tridentate cyclometalating ligand derivatives 6-arylamino-2,2′-bipyridine, 6-phenoxy-2,2′-bipyridine, 6-phenylthio-2,2′-bipyridine, 6-benzyl-2,2′-bipyridine, and 6-benzoyl-2,2′-bipyridine were synthesized, and their acylation reactions were studied. Acylation of platinum complexes based on 6-(4-R-phenylamino)-2,2′-bipyridine derivatives (R = CH₃O, CH₃, Cl, COOEt) tolerates both electron-donating and electron-withdrawing substituents on the aryl ring that are para to the amino group. However, platinum complexes based on 6-(3-R′-phenylamino)-2,2′-bipyridine (R′ = CH₃, Cl, Br) did not undergo the acylation reaction under the same conditions. Interestingly, the acylation of the platinum complexes based on 6-(3-fluorophenylamino)-2,2′-bipyridine proceeded smoothly, and the results indicate that the acylation is regiospecific and occurs at the metalated carbon. Complexes based on 6-phenoxy-2,2′-bipyridine, 6-phenylthio-2,2′-bipyridine, and 6-benzyl-2,2′-bipyridine are also regioselectively acylated. A cyclometalated platinum complex based on 6-benzoyl-2,2′-bipyridine, where the benzene is more electron deficient than those in other cyclometalated platinum complexes, failed to undergo the acylation reaction. The acylation can be carried out in acetic acid, 1,2-dichloroethane, benzonitrile, and acetonitrile. Other acyl halides such as benzoyl chloride and crotonyl chloride are also effective acylating reagents. On the basis of the fact that the reaction is discouraged by the electron deficiency of the phenyl ring and contrasting results of the acylation of platinum complexes based on 6-(3-R′-phenylamino)-2,2′-bipyridine (R′ = CH₃, F, Cl, Br), an unprecedented electrophilic addition–platinum migration–rearomatization cascade mechanism is proposed for the regiospecific acylation reaction