Investigation of Steric and Electronic Features of 3-Iminophosphine-Based Palladium Catalysts for Intermolecular Hydroamination

Abstract

A series of (3-iminophosphine)allylpalladium triflate complexes with systematic variation of both steric and electronic features was isolated and characterized. The ability of the complexes in this series to catalyze the regioselective hydroamination of 3-methyl-1,2-butadiene with aryl amines to form solely the kinetic product was probed by observing conversion to products via NMR spectroscopy. The previously unstudied 3-iminophosphine ligand composed of a di-tert-butyl phosphine, cyclobutene backbone, and tert-butyl imine provided the most active palladium hydroamination catalyst for this transformation known to date.

Cationic [(Iminophosphine)Nickel(Allyl)]+ Complexes as the First Example of Nickel Catalysts for Direct Hydroamination of Allenes

“…Overall, the synthesis of (allyl)nickel complexes proceeded similarly to our reported palladium analogues, except for the fact that usage of a non‐polar solvent is crucial in some steps to obtain the desired nickel complexes (Table ). Attempts to use other solvents in the complexation reaction of the (allyl)nickel bromide dimer with the 3IP ligand, such as diethyl ether, tetrahydrofuran, dichloromethane, or acetonitrile, led to intractable mixtures.…”

“…The production of linear products was not surprising, as this is the thermodynamically favored product in these reactions . Primary amines of various types were invariably found to be unreactive, as observed in many cases with our less reactive Pd analogues …”

“…Ac omparison of the catalytic activity of 1a-d using the reactionofcyclohexylallene with morpholine as abenchmark reactionshowedthat 1b was the best catalyst, whereas 1c had somewhat reduced reactivity (see Supporting Information forf urther details). [11] Although cyclo-oligomerization andp olymerization of allenes is well-known, [13] very little of such byproducts was detected after 48 hours.D uring reaction optimization, it was found that using coordinating solvents (tetrahydrofuran, acetonitrile, or dioxane) resulted in lower conversions, possibly due to coordinativec ompetition with the substrate amine. Thus, 1b was chosen as the catalyst for utilization in as tudy of substrate scope in this reaction.…”

“…[2a] Primary amines of various typesw ere invariably found to be unreactive, as observed in many cases with our less reactive Pd analogues. [11] Although cyclo-oligomerization andp olymerization of allenes is well-known, [13] very little of such byproducts was detected after 48 hours.D uring reaction optimization, it was found that using coordinating solvents (tetrahydrofuran, acetonitrile, or dioxane) resulted in lower conversions, possibly due to coordinativec ompetition with the substrate amine. This was also examined through an NMR scale catalytic reaction in which cyclohexylallene, thiomorpholine, and triethylamine (1, 1, and 2equiv,r espectively)w ere allowed to react in the presence of 2.5 mol %o f1b.A so bservedb yf requent 1 HNMR spectra of the mixture, only 10 %c onversion after 30 hw as detected,w hereas 25 %c onversionw as obtained in only 12 hours without triethylamine.Thus, it was concluded that coordination of the substrate amine plays ak ey role in this catalytic process,w hich is especiallye vident when comparing entries 8and 9, with methylindolinehaving less reactivity than indoline due to steric hinderance.…”

Electronic Role of 3-Iminophosphine Ligands in Palladium-Catalyzed Intermolecular Hydroamination

“…56 This parallel study helped elucidate the catalytic effect of σ donation from the phosphorus and ring strain due to backbone composition. In a summary of these results, tert-butyl groups on the phosphine resulted in higher yields for hydroamination than phenyl groups.…”