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Scheme 4 Selective hydration of aromatic nitriles into carboxamides using the PtIJPPh 3 ) 4 /1 and PtIJCOD)Cl 2 /AgNO 3 /1 catalytic systems.
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Platinum complexes modified with a chiral non-racemizing SPO preligand 1 have been applied in the hydration of aromatic nitriles. [Pt(1)3Cl]Cl formed readily from Pt(COD)Cl2. The chiral secondary phosphine oxide complex showed moderate activity in the hydration of para- and meta-substituted benzonitriles, but failed in converting the ortho-substitu...
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Citations
... Interestingly, van Leeuwen showcased the possibility of kinetic resolution in the hydration of the racemic 1,1′binaphthalene-2,2′-dicarbonitrile (61) in tert-amyl alcohol, illustrated in Scheme 15, bottom. 285 The cationic platinum Another interesting class of nitrile substrates to hydrolyze is the cyanohydrins, as it affords an atom-economical route to high value α-hydroxyamides, α-hydroxycarboxylic acids, and αhydroxycarboxylic esters. An acid-free catalytic process is desirable as it reduces the number of side reactions and eliminates the stoichiometric formation of salts or alkyl chlorides. ...
Transition metal catalysis is of utmost importance for the development of sustainable processes in academia and industry. The activity and selectivity of metal complexes are typically the result of the interplay between ligand and metal properties. As the ligand can be chemically altered, a large research focus has been on ligand development. More recently, it has been recognized that further control over activity and selectivity can be achieved by using the "second coordination sphere", which can be seen as the region beyond the direct coordination sphere of the metal center. Hydrogen bonds appear to be very useful interactions in this context as they typically have sufficient strength and directionality to exert control of the second coordination sphere, yet hydrogen bonds are typically very dynamic, allowing fast turnover. In this review we have highlighted several key features of hydrogen bonding interactions and have summarized the use of hydrogen bonding to program the second coordination sphere. Such control can be achieved by bridging two ligands that are coordinated to a metal center to effectively lead to supramolecular bidentate ligands. In addition, hydrogen bonding can be used to preorganize a substrate that is coordinated to the metal center. Both strategies lead to catalysts with superior properties in a variety of metal catalyzed transformations, including (asymmetric) hydrogenation, hydroformylation, C-H activation, oxidation, radical-type transformations, and photochemical reactions.
... Although the structure of 2-bromobenzamide is reported twice (Izumi & Okamoto, 1972;Gulyá s et al., 2015) in the current version of the Cambridge Structural Database (version 5.42, Nov 2020;Groom et al., 2016), no 2,6-dibromo-or 4-carboxylbenzamides were found. The title compound (I) is an example of both classes, and was accidentally prepared in an attempt to selectively hydrolyze the ester group of a cyano ester (II) (Fig. 1). ...
In the title solvated crystal, C 8 H 5 Br 2 NO 3 ·C 3 H 8 O, the acid molecules form inversion dimers by pairwise N—H...O hydrogen bonds between carbamoyl groups and the carboxyl and carbamoyl groups link to form head-to-tail inversion dimers. The 2-propanol hydroxyl group interposes between adjacent head-tail pairs, resulting in C 3 ³ (10) chains of hydrogen bonds propagating along [100]. The molecules of 2-propanol are disordered over two sets of sites in a 0.598 (8):0.402 (8) ratio. The best-fit planes of the carbamoyl group and benzene ring are inclined by 88.26 (11)°. This is a greater inclination than was previously reported with CH 3 , Cl, F or H in place of the Br atoms, although those analogues did not have a para carboxyl group.
... [79] Catalysts based on dppf were active even at room temperature and also cyanohydrins could now be used. Gulyàs and co-workers developed an enantiomeric version of Parkins' Pt catalyst using ligand 22. [80] For simple substrates no resolution could be obtained, as mentioned above, probably due to racemization under the conditions. Instead, binaphthyl dinitriles (Scheme 14) and the derived products did not racemize and they gave an approximate rate difference of 4-5 between the two enantiomers for the conversion of the first nitrile to amide. ...
Metal complexes of secondary phosphine oxides (SPOs) were introduced as homogeneous catalysts in the 1980s for hydroformylation and hydrogenation with platinum as the metal. As neutral species, the ligand properties resemble those of the corresponding tertiary phosphines as was shown in the coordination chemistry developed in the 1970s, but the participation of the OH in bonding and reaction mechanisms provides them with a peculiar additional function. While this was already proposed and recognized in the first publications, it took a while before this found wider appreciation. Meanwhile, SPOs have become popular ligands for homogeneous catalysts, and more recently also for catalysis based on metal nanoparticles. Here we review the relatively small number of publications that pay attention to SPOs as bifunctional ligands.
... Among the well-established synthetic methods for amide synthesis, nitrile hydration reactions are considered one of the most straightforward and economic. For this purpose, some elegant synthetic methods involving the hydration of nitriles and employing transition metal catalysts (e.g., Ru [30][31][32][33][34][35], Rh [36,37], Pd [38,39], Os [40], Ir [41], Pt [42], Cu [43][44][45], Ag [46,47], Au [48,49], Fe [50], Co [51], Ni [52,53], Mn [54,55], etc.) have been well documented. Meanwhile, some alternative methods for the nitriles' hydration reactions with transition-metal-free catalysts such as acids [56][57][58], bases [59][60][61][62][63][64][65], and others [66,67] have also been developed. ...
The hydration of nitriles to amides in a water extract of pomelo peel ash (WEPPA) was realized with moderate to excellent yields without using external transition metals, bases or organic solvents. This reaction features a broad substrate scope, wide functional group tolerance, prominent chemoselectivity, and good reusability. Notably, a magnification experiment in this bio-based solvent at 100 mmol further demonstrated its practicability.
... Hence, in the presence of [PtCl 2 IJCOD)] and an excess of the ligand, coordination of two units and formation of the intramolecular H-bridge were expected. 180 The trans-labilising effect of the P moieties allows the coordination of another SPO. The final release of the remaining ancillary Cl forms the catalytically active species, which can be alternatively achieved upon treatment of [PtIJPPh 3 ) 4 ] with an excess of the SPO. ...
... 205 PtIJII) complexes bearing a chiral SPO ligand (Scheme 68) have been applied to the hydration of aromatic nitriles by van Leeuwen and coworkers (Scheme 90). 180 The [PtIJSPO) 3 Cl]Cl complex (Scheme 68) showed moderate activity in the hydration of mand p-substituted benzonitriles and was totally inactive in the hydration of o-substituted derivatives. In contrast, the hydride complex formed by the reaction of the SPO and [PtIJPPh 3 ) 4 ] and the cationic complex derived from [PtIJSPO) 3 Cl]Cl by chloride extraction were found to be much more active for many benzonitriles, including the ortho-substituted ones. ...
Secondary phosphine oxides present a tautomeric equilibrium between the pentavalent oxide form (SPO) and the trivalent phosphinous acid (PA). This dichotomy is the origin of the rich coordination chemistry of this class of compounds. As the pentavalent oxide form usually predominates, SPOs are air-stable but at the same time metal coordination can shift the tautomerism towards the PA form, making the ligand act as an ordinary trivalent phosphine. For this reason this class of ligands have found application in numerous homogeneously catalysed reactions, including some enantioselective transformations. This review aims to give an up to date account on the synthesis, coordination chemistry and homogeneous catalysis with SPOs.
... Remarkably, contrary to other metal catalysts reported in the literature, complex 1 was able to operate under neutral conditions. The studies by Parkins and Ghaffar also showed that the size of the substituents on the phosphorus ligand is key to achieving good catalytic activity, the performances of the related complex [PtH{(PPh2O)2H}(PPh2OH)] containing a bulkier phosphinito ligand being significantly lower in comparison to those of 1 (an additional example has been recently described [20]). Further evidence of the synthetic potential of complex [PtH{(PMe2O)2H}(PMe2OH)] (1) was given by de Vries and co-workers, who reported excellent yields and chemoselectivities in the hydration of a variety of sterically hindered tertiary nitriles and nitriles containing acid-or base-sensitive functional groups (some representative examples are shown in Figure 2) [21]. ...
The air-stable hydride-platinum(II) complex [PtH{(PMe2O)(2)H}(PMe2OH)], reported by Parkins and co-workers in 1995, is the most versatile catalyst currently available for the hydration of C N bonds. It features remarkable activity under relatively mild conditions and exceptionally high functional group compatibility, facts that have allowed the implementation of this complex in the synthesis of a large number of structurally complex, biologically active molecules and natural products. In this contribution, synthetic applications of the Parkins catalyst are reviewed.
Hydration of nitriles is catalyzed by the enzyme nitrile hydratase, with iron or cobalt active sites, and by a variety of synthetic metal complexes. This Perspective focuses on parallels between the reaction mechanism of the enzyme and a class of particularly active catalysts bearing secondary phosphine oxide (SPO) ligands. In both cases, the key catalytic step was proposed to be intramolecular attack on a coordinated nitrile, with either an S-OH or S-O- (enzyme) or a P-OH (synthetic) nucleophile. Attack of water on the heteroatom (S or P) in the resulting metallacycle and proton transfer yields the amide and regenerates the catalyst. Evidence for this mechanism, its relevance to the formation of related metallacycles, and its potential for design of more active catalysts for nitrile hydration is summarized.
We achieved chemoselective synthesis of α-alkylated arylacetonitriles and acetamides by combining an Ir complex-catalysed direct coupling of alcohols and nitriles by a simple adjustment of the base. Methanol and ethanol...
