Rhodium and iridium complexes bearing an alkyloxazoline-substituted indenyl ligand (Indox ligand) and their stereoselective construction of metal-centered chirality

Abstract

The Indox ligands, [{( S)-( i Pr)Indox} n ]H ( 1) [ n=2 ( a), 3 ( b)] and [{(H)Indox} n=3 ]H ( 2), in which an indenyl group and an oxazoline ring are connected by an ethylene or propylene spacer, have been prepared. Reaction of [Ir(coe) 2Cl] 2 or [RhCl(C 2H 4) 2] 2 with the potassium salt of 1 afforded η 5-[{( S)-( i Pr)Indox} n ]Ir(coe) 2 ( 3) or η 5-[{( S)-( i Pr)Indox} n ]Rh(C 2H 4) 2 ( 6) as a 1:1 mixture of two diastereomers. The oxazoline ring in 3 and 6 did not coordinate to the metal center. When the complexes 3 or 6 reacted with iodine in diethyl ether, oxidative addition proceeded and the oxazoline ring coordinated to the metal center to give diiodoiridium(III) or rhodium(III) complexes, η 5:η 1-[{( S)-( i Pr)Indox} n ]M(I) 2 [M=Ir ( 4), Rh ( 7)]. The corresponding diiodoiridium(III) complex bearing the Indox ligand 2, η 5:η 1-[{(H)Indox} n=2 ]Ir(I) 2 ( 5), was also prepared by a similar method. Reaction of 4 or 7 with PPh 3 in THF afforded diiodo-phosphine complexes, η 5-[{( S)-( i Pr)Indox} n ]M(PPh 3)(I) 2 [M=Ir ( 8), Rh ( 9)] as a 1:1 mixture of two diastereomers in which the oxazoline ring dissociated from the metal center. The related reaction of 8 or 9 with more than 2 equiv. of AgOTf afforded the cationic complexes, [η 5:η 1-[{( S)-( i Pr)Indox} n ]M(PPh 3)(OTf)]OTf [M=Ir ( 10), Rh ( 11)], having a stereogenic center at the metal center as a mixture of only two diastereomers. From 1H and 31P NMR analyses, each diastereomer of 8 or 9 afforded only a single isomer of 10 or 11. The corresponding iridium(III) complex bearing the Indox ligand 2, [η 5:η 1-[{(H)Indox} n=3 ]Ir(PPh 3)(OTf)]OTf ( 12) was also prepared. The coordinated triflate ligand of 12 was slowly replaced by water in CDCl 3 to afford the dicationic aquo complex, ( S * pl, R * Ir)-[η 5:η 1-[{(H)Indox} n=3 ]Ir(PPh 3)(H 2O)](OTf) 2 ( 13). The monocationic complex, [η 5:η 1-[{( S)-( i Pr)Indox} n=2 ]Ir(PPh 3)(I)]OTf ( 14a), having metal-centered chirality, was observed as a mixture of only two diastereomers in the reaction of 10a (a mixture of two diastereomers) with 1 equiv. of AgOTf. These observations indicated that the ligand exchange reaction of 8 or 9 with AgOTf contained the following three steps: (i) abstraction of one of the two prochiral iododes by AgOTf, (ii) recoordination of the oxazoline ring, and (iii) exchange of the remaining iodide for the triflate by AgOTf. The stereochemistry around the metal center was determined at the second step. All complexes have been characterized by usual spectroscopic methods as well as elemental analyses, and 4 and 13 have been characterized by X-ray analyses.