Bis(oxazolinylmethyl)pyrrole Derivatives and Their Coordination as Chiral “Pincer” Ligands to Rhodium

Bis(oxazolinylmethyl)pyrrole derivatives RLNH (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 RLNH (4a−d) with tBuLi and subsequent reaction with [RhCl(CO)2]2 gave the corresponding rhodium(I) complexes [Rh(RLN)(CO)] (R = iPr: 5a, Ph: 5b, Bn: 5c, Ind: 5d), which were also prepared by reaction of RLNH with one molar equivalent of [Rh(acac)(CO)2]. Upon heating the compounds at 100 °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-RLN)(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 CH3I in dichloromethane at ambient temperature14 gave the octahedrally coordinated product of an oxidative addition [Rh(iPrLN)(CH3)I(CO)] (7), while reaction of complex 5a with one molar equiv of CsBr3 as a mild brominating reagent in toluene at 80 °C led to complete conversion of the rhodium(I) species to the dibromorhodium(III) complex [Rh(iso-iPrLN)Br2(CO)] (8).