Acetylene-dicobaltcarbonyl complexes with chiral phosphinooxazoline ligands: Synthesis, structural characterization, and application to enantioselective intermolecular Pauson-Khand reactions

The reaction of the phenylacetylene-dicobalthexacarbonyl complex (2) with t he 4-R-2-(2-diphenylphosphinophenyl)oxazolines 1 (R = Ph) and 4 (R = CH2CH2 SCH3) leads to the selective formation of the chelated complexes 3 and 5, r espectively. On the other hand, the tert-butyl-substituted phosphinooxazoli ne 6 acts as a monodentate ligand, and its reaction with several 1-alkyne-d erived complexes (2,7-10) affords readily separable mixtures of the diaster eomer nonchelated complexes 11a,b-15a,b. The interconversion rate between d iastereomeric pairs is dependent on the steric bulk of the alkyne substitue nt, and neither 3 nor 5 epimerize at room temperature. The structures of bo th kinds of complexes have been ascertained by a combination of spectroscop ical (IR, NMR), X-ray diffraction, and chiroptical methods; this has allowe d the development of a practical procedure for the establishment of the abs olute configuration of the chiral alkyne-dicobaltcarbonyl complexes obtaine d by the selective substitution of a carbon monoxlde on one of the diastere otopic cobalt atoms. The intermolecular Pauson-Khand reaction of the chelat ed complexes 3 and 5 with norbornadiene respectively affords the (+) and (- ) enantiomers of expected enone adduct 25, but in low enantiomeric excesses . Contrary to that, the tertiary amine N-oxide-promoted intermolecular Paus on-Khand reactions of nonchelated complexes 11a,b-13a,b give the correspond ing norbornadiene- or norbornene-derived adducts both in high yields (85-99 %) and enantioselectivities (93-97% enantiomeric excess), in what constitut es a substantial improvement over preexisting procedures for this reaction. The possibility of achieving chiral induction in the Pauson-Khand reaction of symmetrical alkynes (via the corresponding dicobaltpentacarbonyl comple xes with ligand 6) has been demonstrated for the first time. An enantiosele ctivity mnemonic rule and a mechanistic model that explains the observed as ymmetric sense of induction have been developed, and have been found to be in agreement with the results of model semiempirical molecular orbital calc ulations.