Novel Metallacycle Complexes from Bis(oxazolinyl)pyridine−Rhodium(I) Species and Diynes

Abstract

The reaction of [Rh(coe) 2 Cl] 2 (coe ) ciscyclooctene), bis(oxazolinyl)pyridine (pybox), and diynes (O(CH 2 CtCCO 2 Me) 2 and PhCtCCH 2 OCH 2 CtCCO 2 -Me) gave new types of metallacycle complexes, which were analyzed by X-ray crystallography and showed catalytic activity for cycloaddition of diynes and several alkynes.Metallacycle complexes have been intensively studied as important intermediates for the cyclotrimerization of alkynes and/or diynes in terms of mechanistic view and discovery of new efficient catalysts. [1][2][3] In the course of our research on nitrogen-based ligands and catalytic reactions with their transition-metal complexes, 4 we found that the oxidative addition reaction of alkyl chlorides (e.g. CH 2 Cl 2 and ClCH 2 CO 2 Me) to pybox-Rh I -Cl species generated in situ from pybox and [Rh-(coe) 2 Cl] 2 proceeds to form stable pybox-Rh III -alkyl complexes. 5a In this context, the existence of such rhodium(I) species with certain tridentate nitrogen ligands (κ 3 -N,N,N type) has recently been proved by isolation and X-ray analysis by other chemists. 3b,c,h On the basis of these findings, we thought that diynes could add to the in situ pybox-Rh I Cl species to oxidatively cyclize and to give new types of metallacycles, pyboxrhodacyclopentadienes.The reaction of pybox-dh and [Rh(coe) 2 Cl] 2 (2:1 mol ratio) with the diyne 1 was carried out in an absolute tetrahydrofuran (THF) solution at 25-30 °C for 17 h. Chromatographic separation of the reaction residue gave thermally stable orange solids, which proved to be the corresponding rhodacycle complex 2, in 68% yield (Scheme 1). 6 Structural elucidation of 2 was performed by NMR spectroscopy and X-ray crystallographic analysis (Figure 1). The complex 2 has an octahedral configuration with the rhodacyclopentadiene skeleton, where the two carbon atoms (C1 and C4) are located at an equatorial position and an axial position, respectively. The rhodacycle moiety also proved to be a typical metallacyclopentadiene, 1b-d not a metallacyclopentatriene, as postulated for some ruthenium complexes: 7 2.0 Å, Rh-C1; 1.34 Å, C1-C2; 1.44 Å, C2-C3; 1.35 Å, C3-C4; 2.04 Å, C4-Rh; 80.5°, C1-Rh-C4. 1b-d The two methoxycarbonyl groups of 2 are located just filling the spaces over the pybox plane and by the side of the chloride atom, respectively. Accordingly,

Allenes, versatile unsaturated motifs in transition-metal-catalysed [2+2+2] cycloaddition reactions

“…Finally, the arene is formed by the reductive elimination of the metal and the catalyst is recovered after decoordination. The mechanistic cycle depicted in Scheme 2 has been validated by theoretical calculations [3][4][5] and by the isolation of metallacyclopentadiene IIa 6 and metallacyclopentatriene IIb 7 intermediates. More advanced intermediates in the catalytic cycle have also been detected.…”

2-(1,3-Oxazolin-2-yl)pyridine and 2,6-bis(1,3-oxazolin-2-yl) pyridine

“…The presented compounds, 2-(1,3-oxazolin-2-yl)pyridine (Py-ox) and 2,6-bis(1,3-oxazolin-2-yl)pyridine (Py-box), are commonly applied as ligands for complexes with transition metals: cobalt [2] , rhenium [3] , platinum and palladium [4] , [5] for Py-ox, as well as copper [6] , [7] , ruthenium [8] , [9] , [10] , [11] , rhodium [12] , manganese [13] , silver [14] , nickel [15] , cobalt [16] , terbium [17] , and iron [18] , in the case of Py-box. Some of them reveal catalytic properties.…”

Oxidative addition reactions and stereochemistry on rhodium/4,5-bis(2-oxazolinyl)xanthene complexes

“…We previously found metallacycle formation by the reaction of diynes and an in situ generated Rh(I)-nitrogen ligand adduct [8]. In order to study stereochemistry around the facial circumstance of Xabox in the metallacycle formation, we examined the reaction of in situ generated Rh(I)-Xabox complex and the symmetric diyne dimethyl ester 6 (Scheme 4).…”