Aqueous‐Phase Nitrile Hydration Catalyzed by an In Situ Generated Air‐Stable Ruthenium Catalyst

Abstract

RuCl 2 (PTA) 4 (PTA = 1,3,5-triaza-7-phosphaadamantane) is an active, recyclable, air-stable,a queous-phase nitrile hydration catalyst. The development of an in situ generated aqueous-phase nitrileh ydration catalyst (RuCl 3 ·3 H 2 O + 6equivalents PTA) is reported.T he activity of the in situ catalyst is comparable to RuCl 2 (PTA) 4 .T he effects of [PTA] on the activity of the reaction were investigated:t he catalytic activity, in general, increases as the pH goes up, which shows ap ositive correlation with [PTA].T he pH effectsw ere furthere xplored for both the in situ and RuCl 2 (PTA) 4 cata-lyzed reactioni np hosphate buffer solutionsw ith particular attention given to pH 6.8 buffer.I ncreased catalytic activity was observed at pH 6.8 versus waterf or both systems with turnover frequency (TOF) up to 135 h À1 observed for RuCl 2 (PTA) 4 and 64 h À1 for the in situ catalyst. Catalystl oading down to 0.001 mol %w as examinedw ith turnover numbers as high as 22 000 reported. Similar to the preformed catalyst, RuCl 2 (PTA) 4 ,t he in situ catalyst could be recycled more than five times withouts ignificant loss of activity from eitherw ater or pH 6.8 buffer. Results and DiscussionThe relatively simple synthesis [34,35] and high activity of RuCl 2 (PTA) 4 as ac atalystf or the hydration of nitrilesp rompted us to investigate utilizingRuCl 3 and PTAasaninsitu generated [a] Dr.

Selective Hydration of Nitriles to Corresponding Amides in Air with Rh(I)-N-Heterocyclic Complex Catalysts

“…However, the resulting complex species are coordinatively saturated and coordination of the nitrile substrate and/or H O or OHto the metal ion in a Rh(I)-complex seems unlikely. In the case of [RuCl 2 (PTA) 4 ]-catalyzed nitrile hydration, Frost suggested that the increased catalytic activity in the presence of a large excess of PTA was due to the pH shift into the alkaline region in concentrated PTA solutions caused by the protonation of PTA [34]. This may happen in our reactions with added PTA, too, however, it is certainly not the case with mtppms which is protonated only in concentrated aqueous acid solutions.…”

Hydration of Nitriles Catalyzed by Ruthenium Complexes: Role of Dihydrogen Bonding Interactions in Promoting Base-Free Catalysis

“…Amides belong to an essential and versatile category of organic compounds constituting a variety of biological molecules, polymers, and synthetic compounds and intermediates. − Various synthetic methods have been evolved for the preparation of amides. − Hydration of nitriles is one of the most convenient and desirable methods for the synthesis of amides as there is no generation of any toxic chemical or byproduct. − However, traditional methods for the hydration of nitriles involve harsh reaction conditions including highly alkaline or acidic media, which are not only environmentally and practically unsustainable but often lead to overhydrolysis of amides to the corresponding free carboxylic acids or ammonium carboxylates. − To circumvent these problems and to stop hydration of a cyanide substrate at the amide stage, several catalysts have been developed. ,− Nitrile hydratase enzymes, a family of biocatalysts that selectively promote hydration of nitriles to amides, also suffer from a limited substrate scope, thus restricting their widespread utilization. , In that context, transition metal-based catalysts have been found quite successful not only for their high activity but also for the better substrate scope. − As a result, various catalysts have been developed for promoting the direct nucleophilic addition of a water molecule to a nitrile bond, thus producing an amide. ,, The ruthenium complexes have also been explored as the efficient catalysts for the hydration of nitriles, including promotion of the metal–ligand cooperative catalysis. ,− …”

Synthesis of New Ruthenium‐CAP Complexes and Use as Catalysts for Benzonitrile Hydration to Benzamide

“…[12] Among octahedral Ru-PTA complexes, [Ru(PTA) 4 Cl 2 ], a well-studied complex with peculiar isomerism properties, found use as catalyst in several reactions in aqueous or biphasic media, including hydrogenation of aldehydes, [13a] olefins, [13b] carbon dioxide, [14] and more recently hydration of nitriles to amides. [15] Amide synthesis by metal-catalyzed organonitrile hydration [16] can bring several advantages over traditional protocols based on the use of carboxylic acids with amines and strong acids or bases, [17] as it can allow milder reaction conditions, wide functional group compatibility, high atom efficiency and high selectivities by minimizing the formation of kinetically favoured carboxylic acids.…”