Bis(oxazolinylmethyl)pyrrole derivatives (R)L(N)H (4a-e), which were designed as protioligands for meridionally coordinating "pincer" ligands, were synthesized by cyclization of pyrrole-2,5-diethylacetate with a series of chiral amino alcohols. Deprotonation of (R)L(N)H (4a-d) with tBuLi and subsequent reaction with [RhCl(CO)(2)](2) gave the corresponding rhodium(I) complexes [Rh((R)L(N))(CO)] (R = iPr: 5a, Ph: 5b, Bn: 5c, Ind: 5d), which were also prepared by reaction of (R)L(N)H with one molar equivalent of [Rh(acac)(CO)(2)]. Upon heating the compounds at 100 degrees C in toluene over a period of 2-5 h, complete rearrangement via a 1,3-H shift between the pyrrole ring and the bridging methylene groups took place to yield the corresponding isomeric complexes [Rh(iso-(R)L(N))(CO)] (6a-d). The transformation induced a planarization of the tridentate ligand system, resulting from the formation of a series of conjugated double bonds. Stirring the rhodium(I) complex 5a with an excess of CH(3)I in dichloromethane at ambient temperature(14) gave the octahedrally coordinated product of an oxidative addition [Rh((iPr)L(N))(CH(3))I(CO)] (7), while reaction of complex 5a with one molar equiv of CsBr(3) as a mild brominating reagent in toluene at 80 degrees C led to complete conversion of the rhodium(I) species to the dibromorhodium(III) complex [Rh(iso-(iPr)L(N))Br(2)(CO)] (8).
The synthesis of the three ligands employed in this study is based on the condensation of two molar equivalents of (S)-valinol with the diester precursors pyrrole-2,5-bis(ethyl)acetate (2a), furan-2,5-bis(ethyl)acetate (2b) and thiophene-2,5-bis(ethyl)acetate (2c). This gave the corresponding bis(oxazolinylmethyl)pyrrole (iPrL N H, 4a), bis(oxazolinylmethyl)-furan (iPrL O , 4b) and bis(oxazolinylmethyl)thiophene (iPrL S , 4c) ligands. Stirring 4a in MeOD at ambient temperature in the presence of a catalytic amount of acetic acid (1 mol-%) led to complete hydrogen/deuterium exchange in the two bridging methylene groups of the ligand. This behaviour is explained by an acetate-mediated reversible proton transfer between the oxazoline N atom and the methylene bridge, a conjecture which was supported by a DFT study of the process. Deprotonation of iPrL N H (4a) with tBuLi at -78°C and subsequent stirring with NiCl 2 yielded the square planar nickel(II) complex [Ni(iPrL N )Cl] (5). However, on stirring
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