Base‐Free Asymmetric Transfer Hydrogenation of 1,2‐Di‐ and Monoketones Catalyzed by a (NH)₂P₂‐Macrocyclic Iron(II) Hydride

Abstract

The hydride isonitrile complex [FeH(CNCEt )(1 a)]BF (2) containing a chiral P (NH) macrocycle (1 a), in the presence of 2-propanol as hydrogen donor, catalyzes the base-free asymmetric transfer hydrogenation (ATH) of prostereogenic ketones to alcohols and the hemihydrogenation of benzils to benzoins, which contain a base-labile stereocenter. Benzoins are formed in up to 83 % isolated yield with enantioselectivity reaching 95 % ee. Ketones give the same enantioselectivity observed with the parent catalytic system [Fe(CNCEt ) (1 a)] (3 a) that operates with added NaO Bu.

Self‐Assembly of Chiral 1,8‐Diaza‐3,6,10,13‐tetraphosphacyclotetradecanes via Dynamic Transformation of 7‐ and 14‐Membered Aminomethylphosphines

“…Some chiral macrocycles containing O,N,S donor centres have been successfully applied as pre‐catalysts in different asymmetric organic reactions . However, chiral macrocycles with P,N donors have rarely been employed in asymmetric reactions, due to the difficult synthesis of phosphorus‐based macrocycles and the formation of diastereomeric mixtures when two or more phosphorus atoms are present . A synthetic strategy based on the dynamic covalent chemistry of macrocyclic aminomethylphosphines was developed for numerous 14‐, 16‐, 18‐ and 20‐membered P 4 N 2 macrocycles .…”

Benzoin condensation of aromatic aldehydes catalyzed by N-heterocyclic carbenes under mild conditions

“…For comparison, we used 2-(perfluorophenyl)-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-2-ium tetrafluoroborate, an active NHC for several reactions [45][46][47][48][49][50][51][52], but its performance is similar to the leading catalyst in this report, Table 2, entries 5 vs. 6. However, the fact that the leading catalyst is slightly superior in performance and easier to prepare, by alkylation of benzoimidazole, encouraged us to continue this study using it.…”

Recent developments in enantioselective iron-catalyzed transformations

“…For example, a-hydroxy ketones whose base-labile stereocenter easily racemizes in basic media, render challenging the asymmetric hemihydrogenation of 1,2-diketones. In 2017, catalyst 6a was successfully investigated by Mezzetti and De Luca to promote the base-free asymmetric transfer hydrogenation of this type of challenging substrates (Scheme 5) [19]. Indeed, the first asymmetric transfer hydrogenation of a range of benzyls 9 with isopropanol as a hydrogen source was performed in the presence of 1 mol% of catalyst 6a at 50°C, leading in the absence of base to the corresponding chiral benzoins 10 in both moderate to high yields (39-83%) and enantioselectivities (41-95% ee).…”