Mechanistic investigations of bipyrimidine-promoted palladium-catalyzed allylic acetoxylation of olefins

Abstract

Several pyridine-like ligands were found to improve Pd(OAc) 2 -catalyzed allylic oxidation of allylbenzene to cinnamyl acetate by p-benzoquinone in acetic acid. The best ligand examined, bipyrimidine, was used to identify the catalyst precursor for this system, (bipyrimidine)Pd(OAc) 2 , which was fully characterized. Mechanistic studies suggest the reaction takes place through disproportionation of (bipyrimidine)Pd(OAc) 2 to form a bipyrimidine-bridged dimer, which reacts with olefin to form a Pd II -olefin adduct, followed by allylic C-H activation to produce (η 3 -allyl)Pd II species. The (η 3 -allyl)Pd II intermediate undergoes a reversible acetate attack to generate a Pd 0 -(allyl acetate) adduct, which subsequently reacts with p-benzoquinone to release allyl acetate and regenerate (bipyrimidine)Pd(OAc) 2 . No KIE is observed for the competition experiment between allylbenzene-d 0 and allylbenzene-d 5 (CD 2 =CDCD 2 C 6 H 5 ), suggesting that allylic C-H activation is not rate-determining. Catalytic allylic acetoxylations of other terminal olefins as well as cyclohexene were also effected by (bipyrimidine)Pd(OAc) 2 .Key words: olefin, palladium catalysis, allylic C-H oxidation, p-benzoquinone, bipyrimidine.Résumé : On a observé que plusieurs ligands apparentés à la pyridine améliorent l'oxydation allylique de l'allylbenzène en acétate de cinnamyle par la p-benzoquinone dans l'acide acétique et catalysée par le Pd(OAc) 2 . On a utilisé le meilleur ligand étudié, la bipyrimidine, pour identifier le précurseur du catalyseur pour ce système, le (bipyrimidine)Pd(OAc) 2 , qui a été complètement caractérisé. Des études mécanistiques suggèrent que la réaction se produit par le biais d'une réaction de dismutation du (bipyrimidine)Pd(OAc) 2 qui conduit à la formation d'un dimère à pont bipyrimidine qui réagit avec l'oléfine pour conduire à un adduit Pd(II)-oléfine, le tout suivi de l'activation du C-H allylique conduisant à la formation de l'espèce (η 3 -allyl)Pd(II). L'intermédiaire (η 3 -allyl)Pd(II) subit une attaque réversible par l'ion acétate qui génère un adduit Pd°-(acétate d'allyle) qui réagit subséquemment avec la p-benzoquinone pour libérer l'acétate d'allyle et régénérer le (bipyrimidine)Pd(OAc) 2 . On n'observe aucun effet isotopique cinétique pour l'expérience de compétition entre l'allylbenzène-d 0 et l'allylbenzène-d 5 (CD 2 =CDCD 2 C 6 H 5 ); ce résultat suggère que l'activation allylique n'est pas l'étape cinétiquement déterminante. Les acétoxylations allyliques catalytiques du cyclohexène et d'autres oléfines terminales sont aussi affectées par le (bipyrimidine)Pd(OAc) 2 .

Crystal structure of diacetato(2,2′-bipyridine-k2N,N′)palladium(II)pentahydrate, Pd(CH3CO2)2(C10H8N2) · 5H2O

“…In the complex, the central Pd(II) ion has adistorted squareplanar coordination defined by two Natoms of the chelating 2,2'-bipyridine ligand and two Oatoms of the two distinct acetate anions. The acetate ions are located on opposite sides of the PdN 2O2 plane, whereas the acetate ions in the related Pd(II) complex [Pd(CH 3 CO 2 ) 2 (C 8 H 6 N 4 )] · H 2 O(where C 8 H 6 N 4 is 2,2'-bipyrimidine) lie on the same side of the plane [1]. The tight N1-Pd1-N2 chelate angle (81.6(2)°)r esults in non-linear trans axes (∠N1-Pd1-O1 =175.6(2)°and ∠N2-Pd1-O3 =174.9(2)°).…”

Direct Allylic Functionalization Through Pd‐Catalyzed C–H Activation

“…[33,35] However, other studies on related systems point to a reversible acyloxy anion addition on the η 3 -allylpalladium complex not coordinated to BQ, [36] and a subsequent exothermic olefinic product/BQ ligand exchange [37] that displaces the equilibrium (Scheme 20). [33,35] However, other studies on related systems point to a reversible acyloxy anion addition on the η 3 -allylpalladium complex not coordinated to BQ, [36] and a subsequent exothermic olefinic product/BQ ligand exchange [37] that displaces the equilibrium (Scheme 20).…”

“…We saw (Scheme 19, case C) that a postulated extra role of p-benzophenone, besides its redox action, is coordination of the η 3 -allylpalladium complex, which, thanks to π-backdonation, electrophilically activates the complex. [33,35] However, other studies on related systems point to a reversible acyloxy anion addition on the η 3 -allylpalladium complex not coordinated to BQ, [36] and a subsequent exothermic olefinic product/BQ ligand exchange [37] that displaces the equilibrium (Scheme 20).…”

Oxidation of Hindered Allylic C–H Bonds with Applications to the Functionalization of Complex Molecules

“…9 Many groups have investigated the mechanisms of these transformations in depth. 6d,g,i,k,l,10 Recently, our group reported a system for the palladium-catalyzed oxidation of α -olefins to linear allylic benzoates with high selectivity for the linear ester under neutral conditions in combination with iridium-catalyzed allylic substitution. 11 This process was used to prepare synthetically valuable enantioenriched products containing new C–O, C–N, and C–C bonds in one pot.…”

“…However, based on numerous precedents, we suggest that BQ could serve as a ligand that promotes the reductive elimination of an allyl palladium intermediate (see the Supporting Information). 6j,k,10c,18 Alternatively, BQ could prevent catalyst decomposition.…”