Breaking the regioselectivity rule for acrylate insertion in the Mizoroki–Heck reaction

Abstract

In modern methods for the preparation of small molecules and polymers, the insertion of substrate carbon-carbon double bonds into metal-carbon bonds is a fundamental step of paramount importance. This issue is illustrated by Mizoroki-Heck coupling as the most prominent example in organic synthesis and also by catalytic insertion polymerization. For unsymmetric substrates H 2 C ¼ CHX the regioselectivity of insertion is decisive for the nature of the product formed. Electron-deficient olefins insert selectively in a 2,1-fashion for electronic reasons. A means for controlling this regioselectivity is lacking to date. In a combined experimental and theoretical study, we now report that, by destabilizing the transition state of 2,1-insertion via steric interactions, the regioselectivity of methyl acrylate insertion into palladiummethyl and phenyl bonds can be inverted entirely to yield the opposite "regioirregular" products in stoichiometric reactions. Insights from these experiments will aid the rational design of complexes which enable a catalytic and regioirregular MizorokiHeck reaction of electron-deficient olefins.density functional theory calculation | homogeneous catalysis | organometallic | regiochemistry W hereas the palladium-catalyzed Mizoroki-Heck coupling is an established powerful strategy for the formation of carbon-carbon bonds from electron-deficient and electron-rich olefins (1-5), insertion (co)polymerization (6-8) of acceptor or donor substituted olefins has only been demonstrated since the mid-1990s, and only a few catalyst motifs are known to promote such polymerizations (9, 10), which are based on palladium. The regioselectivity of insertion follows the same pattern for both reactions. Electron-deficient olefins [e.g., methyl acrylate (MA)] selectively insert in a 2,1-fashion (6, 9-11), whereas electron-rich olefins (e.g., vinyl ethers) insert in a 1,2-fashion (3, 6, 12-14, †) (Fig. 1). Finally, apolar olefins (e.g., α-olefins) commonly afford mixtures of both insertion modes in palladium-catalyzed Mizoroki-Heck (15) and polymerization reactions (16), whereas closely related nickel-catalyzed polymerizations of α-olefins can proceed with high selectivity by 1,2-insertion (17)-e.g., under kinetically controlled low-temperature conditions when sterically demanding ligands coordinate to nickel (16,18).The accepted rationale for these reactivity patterns is that electronic effects govern the regiochemistry of insertion for polarized carbon-carbon double bond substrates: In the Cossée-Arlman-type insertion step, the metal-bound, nucleophilic carbon atom migrates to the lower electron-density carbon atom of the double bond, while the electrophilic palladium atom migrates to the higher electron-density carbon atom of the double bond. In contrast, the insertion regiochemistry of apolar carbon-carbon double bonds is rather determined by steric effects (given that there is little electronic discrimination of the two olefinic carbon atoms), and under strict kinetic control the 1,2-insertion mode may preva...

Computational Study of Rh-Catalyzed Carboacylation of Olefins: Ligand-Promoted Rhodacycle Isomerization Enables Regioselective C–C Bond Functionalization of Benzocyclobutenones

“…21 The previously reported procedures to generate the steric contour maps were used and are illustrated in Scheme 5. The ligand geometries are taken from the optimized structures of the intermediate 7 and transition state 19-TS ( A ).…”

“…18 The effects of ligands on the rate-determining rhodacycle isomerization step were analyzed using different ligand steric parameters, including ligand bite angle, 19 buried volume, 20 and steric contour map. 21 In addition, the origin of enantioselectivity in the asymmetric version of the reaction was investigated computationally (Scheme 1b). …”

Ligand–Substrate Dispersion Facilitates the Copper-Catalyzed Hydroamination of Unactivated Olefins

“…6–11 Classical transition state models usually describe these through-space interactions as steric repulsion. 12–16 London dispersion, an essential type of van der Waals forces arising from the attraction between instantaneous dipoles, also contributes to the ligand-substrate interactions. Although dispersion has been recognized as an important stabilizing component in inter- and intramolecular non-covalent interactions, 17–19 the effects of dispersion have rarely been elucidated in transition metal catalysis.…”

Synthesis and Applications in Catalysis of Metal Complexes with Chelating Phosphinosulfonate Ligands

“…37,41 Under atmospheric pressure of CO, insertion occurred on the anionic palladium to give the corresponding acetyl anionic palladium [PdCl(3a)Ac] À complex. 23,24,45,46 Changing the palladium precursor [PdMeCl 44 The use of disulfonated prochelates such as 6i and 6k led to novel mononuclear or polymeric anionic palladium complexes.…”

“…diazaphospholidinesulfonate ligand could force the reverse 1,2-insertion of methyl acrylate into a palladium-methyl bond (Scheme 3.52) 23,24. diazaphospholidinesulfonate ligand could force the reverse 1,2-insertion of methyl acrylate into a palladium-methyl bond (Scheme 3.52) 23,24.…”