Rhodium-catalyzed asymmetric hydrogenation of olefins with PhthalaPhos, a new class of chiral supramolecular ligands.
A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (PhthalaPhos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2-acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1-yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3-phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect.
Università degli Studi di Milano, Dipartimento di Chimica Organica e Industriale, Centro Interdipartimentale CISI, Istituto di Scienze e Tecnologie Molecolari del CNR, Milano, Italy. luca.pignataro@unimi.it
Luca
L
Pignataro
Michele
M
Boghi
Monica
M
Civera
Stefano
S
Carboni
Umberto
U
Piarulli
Cesare
C
Gennari