Contributions to the Enantioselective Heck Reaction Using MeO−Biphep Ligands. The Case Against Dibenzylidene Acetone

Abstract

It is shown that the Pd-catalyzed enantioselective Heck reaction of p-XC6H4OTf, X = OMe, H, CO2Me, with dihydrofuran gives higher enantioselectivities when the chelating diphosphine MeO−Biphep, 1a, is replaced with its disubstituted analogue 3,5-di-tert-butyl MeO−Biphep, 1b. The phenylation of 5-methyl-2,3-dihydrofuran produces a new dihydrofuran containing a quaternary stereogenic center (ee, >98% with 1b, ca. 20% with 1a). Catalytic results for the reaction of phenyl triflate with dhf, together with stoichiometric oxidative addition reactions of aryl halides on Pd complexes of 1, show that the use of Pd(dba)(1), dba = dibenzylidene acetone, slows the oxidative addition relative to the reaction in which the Pd(0) precursor is generated from PdCl2(1) + NaBH4. The solid-state structures for two PdI(aryl)(1a), 3, derivatives, aryl = p-MeOOC−C6H4 (3a) and C6F5 (3b) are reported.

Platinum-Catalysed Allylic Alkylation: Reactivity, Enantioselectivity, and Regioselectivity

“…It has recently been reported that palladium-dba complexes are less active catalysts than the combination of palladium(ii) and a reducing agent. [18] In order for allylic alkylation reactions to proceed, the dba has to be replaced by the allylic acetate, and studies on diphosphine-platinum complexes, [(P^P)Pt(dba)], have revealed that dba is not easily displaced, even by phosphine ligands. [19] After further studies, it became clear that we needed to determine which complex types would catalyse the reaction with maximum efficiency.…”

Catalyzed Asymmetric Arylation Reactions

“…Thus, this palladium(0) catalyst is almost unreactive towards oxidative addition to phenyl triflate (56) under the standard reaction conditions. [71] A mechanistically related aryl ± olefin coupling, which involves the activation of an arene C À H bond, is the Fujiwara ± Moritani reaction. [12, 13a] Recently, Mikami et al described the first example of an asymmetric version of this transformation.…”

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…Therefore, any investigation on particular aspects of the characteristics and of the reactions promoted by palladium complexes needs to be specifically addressed. Among all the palladium catalysts, the zero-valent derivatives bearing coordinated olefins are particularly important since they are often used or identified as the active catalytic species in a variety of cross-coupling reactions [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Moreover, the bonding attitude of olefins toward palladium(0) was extensively studied from theoretical and structural point of view [17] and were developed on the basis of the original work of Dewar [18] and Chatt and Duncanson [19].…”