RESUMO
Herein, we report an unusual α-methylene oxidation of N-substituted tetrahydroquinoline to lactams using the cyclometalated Ru(II)-complex as a catalyst. Cyclic-α-methylene C-H bonds are selectively oxidized under the reaction conditions even in the presence of α-methyl or reactive α-methylene C-H bonds. This methodology is also useful in the late-stage functionalization of pharmaceuticals. Mechanistic study demonstrates that the high-valent Ru(VI)-cis-dioxo species plays an important role in controlling selectivity.
RESUMO
The multifunctional ligand HL [(E)-1-(((di(pyridin-yl)methylene)hydrazono)methyl)naphthalen-2-ol] has been used in the present work. The mononuclear complex 1, ([Re(HL)(CO)3Cl]), has been synthesised in excellent yield by reacting rhenium(i) pentacarbonyl chloride [Re(CO)5Cl] with HL in a 1 : 1 ratio in dry toluene under an argon atmosphere. The complex 2, ([Re(L)(CO)3]), has also been prepared in a similar way in the presence of triethylamine. X-ray crystallographic study reveals that in complex 1, the ligand binds as an N^N coordinating bidentate ligand while in 2 it behaves as a monoanionic N^N^O coordinating tridentate ligand. Complexes 1 and 2 were characterised by different physicochemical techniques and the observed properties have been interpreted with the help of DFT (density functional theory) and TDDFT (time dependent density functional theory) calculations. Interestingly, complex 1 interacts with Zn(OAC)2 in methanol to yield amido binding highly reactive α-amino ether containing rhenium(i) complex 3, ([Re(HL-OMe)(CO)3]). The X-ray structure and spectral properties of the generated complex 3 have also been studied. Complex 1 selectively reacts with a cyanide ion and a high detection limit (5.0 × 10-8 M) was observed. The properties of intermediates and the final product upon addition of cyanide were also matched with the DFT, TDDFT and NTO (natural transition orbital) analysis.
