TY  - UNPB
ID  - pittir35155
UR  - http://d-scholarship-dev.library.pitt.edu/35155/
A1  - Burrows, Lauren
Y1  - 2018/09/26/
N2  - The Rh(I)-catalyzed allenic Pauson?Khand reaction (APKR) is an efficient, redox-neutral method of synthesizing ?-acyloxy cyclopentenones. An enantioselective APKR would provide access to chiral, nonracemic ?-acyloxy and ?-hydroxy cyclopentenones and their corresponding redox derivatives. Rapid scrambling of axial chirality of allenyl acetates in the presence of Rh(I) catalysts enables the conversion of racemic allene to enantiopure cyclopentenone product in a Type I dynamic kinetic asymmetric transformation (DyKAT). A combined experimental and computational approach was taken to develop an effective catalytic system to achieve the asymmetric transformation. The optimization of the denticity, and steric and electronic properties of the ancillary ligand (initially (S)-MonoPhos, 16% ee), afforded a hemilabile bidentate (S)- MonoPhos-alkene-Rh(I) catalyst that provided ?-acyloxy cyclopentenone product in up to 72% ee. Upon discovery of the reactive and enantioselective (S)-MonoPhos-alkene catalyst, the remaining reaction conditions were optimized using a statistical design of experiments (DOE) strategy. Reaction temperature, CO atmosphere, catalyst counteranion, concentration, solvent identity, additive equivalents, and ligand/Rh stoichiometry were evaluated simultaneously in two iterations of DOE experimentation. Discovery of optimal conditions enabled an increase in enantioselectivities and an expansion of the APKR scope to include methyl-substituted alkynes and a three-carbon-tethered allene-yne. The good yields and enantioselectivities effected by the phosphoramidite class of ligands in the APKR inspired an exploration of electron-deficient phosphoramidite ligands in the enantioselective Pauson?Khand reaction (PKR) of 1,6-enynes. Lowest-energy reaction profiles of the cationic Rh(I)-(R)-BINAP, Rh(I)-(S)-MonoPhos, and Rh(I)-?CO-only? catalysts were calculated, and agreement between experimental reaction rates and the activation energies of the oxidative cyclization step was found. We observed that the PKR of cationic Rh(I)- catalyst is accelerated 3000-fold in the presence of (R)-BINAP, and 180-fold in the presence of either one or two (S)-MonoPhos ligands (ligand to Rh ratio of 1.1 or 2.2). The absolute configuration of the PKR product was confirmed by VCD spectroscopy and matches that predicted by calculations. We anticipate that these mechanistic studies will enable the application of phosphoramidite ligands in the PKR of new enyne substrates.
KW  - allene
KW  -  carbonylation
KW  -  rhodium catalysis
KW  -  DyKAT
KW  -  design of experiments
KW  -  Pauson-Khand reaction
TI  - The Rhodium-Catalyzed Dynamic Kinetic Asymmetric Pauson?Khand Reaction of Allenyl Carboxy Esters
EP  - 405
AV  - restricted
ER  -