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

Bis(oxazolinylmethyl)pyrrole derivatives (LNH)-L-R (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 (LNH)-L-R (4a-d) with tBuLi and subsequent reaction with [RhCl(CO)(2)](2) gave the corresponding, rhodium(I) complexes [Rh(L-R(N))(CO)] (R = iPr: 5a, Ph: 5b, Bn: 5c, Ind: 5d), which were also prepared by reaction of (LNH)-L-R 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-L-R(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)l in dichloromethane at ambient temperature(14) gave the octahedrally coordinated product of an oxidative addition [Rh(L-iPr(N))(CH3)l(CO)] (7), while reaction of complex 5a with one molar equiv of CsBr3 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-L-iPr(N))Br-2(CO)] (8).