Bioinspired catalytic nitrile hydration by dithiolato, sulfinato/thiolato, and sulfenato/sulfinato ruthenium complexes

Abstract

A series of Ru(II) catalysts inspired by the metalloenzyme nitrile hydratase catalyze the hydration of benzonitrile with up to 242 turnovers under neutral conditions with very low catalysts loading. Catalysts with an oxidized sulfur environment are less susceptible to product inhibition increasing the catalytic efficiency at low nitrile : water ratios.

Metal-catalyzed nitrile hydration reactions: The specific contribution of ruthenium

“…Quite recently, in an attempt of mimicking the active sites of the iron-containing NHase enzymes [5], C. A. Grapperhaus and co-workers have also described the catalytic hydration of benzonitrile using the octahedral ruthenium(II) derivatives 39 ( Fig. 9) [52]. These complexes were able to operate in neat substrate/water mixtures, without the requirement of added base or buffer, generating benzamide in a selective manner albeit in very low yields (< 10%; TON and TOF values up to 242 and 13 h -1 , respectively, at 124 ºC using ca.…”

Metal-centered oxidation decreases nitrile hydration activity of bioinspired (N2S3)Ru-PPh3 precatalysts

“…[1][2][3][4][5][6] The active site of NHase contains a mononuclear low-spin non-heme iron(III) or non-corrin cobalt(III) center in an unusual N 2 S 3 X type donor motif that contains two carboxamido nitrogens and three cysteine derived sulfur donors in distinct oxidation states; thiolate (RS -), sulfenate/sulfenic acid (RSO¯/RSOH) and sulfinate (RSO 2¯) as shown in Scheme 1. [7,8] Although asymmetric sulfur-oxidation is crucial for the catalytic activity of NHase, [9] the Co(III) peptide maquettes of Shearer [10] and our Ru(II) bioinspired catalysts [11,12] are the only two systems to have successfully implemented a mixed sulfenato/sulfinato donor set into a catalytically active synthetic system. In fact, the total number of functional NHase complexes remains small and limited to low-spin d 6 Co(III) complexes, with the exception of our low-spin d 6 Ru(II) catalysts.…”

“…In fact, the total number of functional NHase complexes remains small and limited to low-spin d 6 Co(III) complexes, with the exception of our low-spin d 6 Ru(II) catalysts. [11][12][13][14][15][16] To date, no functional NHase models with low-spin d 5 iron(III) centers are known despite its natural occurrence in the enzyme. Previously, we noted that aerobic conditions decreased the hydration activity of our Ru(II) catalyst 1, which was attributed to metal-centered oxidation since ferrocenium hexafluorophosphate (FcPF 6 ) had the same effect.…”

“…Previously, we noted that aerobic conditions decreased the hydration activity of our Ru(II) catalyst 1, which was attributed to metal-centered oxidation since ferrocenium hexafluorophosphate (FcPF 6 ) had the same effect. [11] Herein we compare the effect of metal-versus ligand-centered oxidation on hydration activity through the oxidation of 1 -3 to their low-spin d 5 Ru(III) derivatives 4 -6 with FcPF 6 (Scheme 1). The Ru(III) complex 4 is also isolated and characterized by a variety of methods including x-ray crystallographic studies.…”

Ruthenium-Catalyzed Amide-Bond Formation

“…Secondary coordination sphere activation of water, via hydrogen bonding with the OH group of the ligand, also explains the outstanding performances of the related phosphiniteruthenium(II) complex [RuCl 2 ( 6 -p-cymene){PMe 2 (OH)}] (12), which was also able to hydrate cyanohydrins [50,51]. Although other catalytic systems for the selective hydration of organonitriles in water have been described [52][53][54][55][56][57], none of them presented an activity and scope comparable to those of the bifunctional catalysts 4-12. Additional examples of nitrile hydration reactions catalyzed by homogeneous ruthenium catalysts in organic media are (Scheme 8): (i) The hydration of benzoxazolylacetonitrile by the arene-ruthenium(II) dimer [{RuCl(-Cl)( 6 -p-cymene)} 2 ], which led to benzoxazolylacetamide in high yield [58], and (ii) the asymmetric hydration of -benzyl--methylmalononitrile by the chiral catalysts 13 [59].…”