Hydration of Cyanohydrins by Highly Active Cationic Pt Catalysts: Mechanism and Scope

Abstract

Cyanohydrins (α-hydroxy nitriles) are a special type of nitriles that readily decompose into hydrogen cyanide (HCN) and the corresponding carbonyl compounds. Hydration of cyanohydrins that are readily available through cyanation of aldehydes and ketones provides the most straightforward route to valuable α-hydroxyamides. However, due to low stability of cyanohydrins and deactivation of the catalysts by the released HCN, catalytic direct hydration of cyanohydrins still remains largely unsolved. As a general trend, cyanohydrins containing bulkier substituents, such as α,α-diaryl cyanohydrins, degrade more quickly and thus are more difficult to be hydrated. Here, we report development of cationic platinum catalysts that exhibit high reactivity for hydration of various cyanohydrins. Detailed mechanistic investigations for hydration of nitriles by (P∼P)­Pt­(PR2OH)­X­(OTf) reveal a catalytic cycle involving the formation of a five-membered metallacyclic intermediate and subsequent hydrolysis via attacking on the phosphorus of the secondary phosphine oxide (PR2OH) ligand by H2O. We discovered that Pt catalyst A bearing the electron-rich, appropriately small-bite-angle bisphosphine ligand provides super reactivity for hydration of cyanohydrins. The hydration reactions catalyzed by A proceed at ambient temperatures and occur with a wide variety of cyanohydrins, including the most difficult α,α-diaryl cyanohydrins, with good turnover numbers.

Diastereoselective and Reversible Metallacycle Formation by Attack of a Pt-PR2OH Group on a Coordinated Nitrile in Pt-Catalyzed Hydration

“…The crystal structure of PEt 2 OH complex 2 (Figure ) was similar to that of the PMe 2 OH complex 1 and several related derivatives with different diphos ligands, , with a large Me-FerroLANE bite angle of 100.78(7)° …”

“…Treatment of Pt–Cl cations 1–4 and 5 with AgOTf gave “three-coordinate” dications 9–12 and 14 (Scheme ), for which the A 2 X doublet/triplet patterns in the 31 P{ 1 H} NMR spectra were consistent with Fe → Pt dative interactions, as observed in related complexes. ,, Despite the structural similarity of Et-FerroTANE to Me-FerroLANE and dppf ligands, the 31 P{ 1 H} NMR spectrum of 13 , formed from 6 and AgOTf, showed that it was four-coordinate, with inequivalent FerroTANE 31 P nuclei and, presumably, a coordinated triflate.…”

“…To develop chiral metal complexes as catalyst precursors for kinetic resolution of racemic nitriles, we considered the proposed mechanism of a popular class of Pt catalysts bearing PMe 2 OH (phosphinous acid) ligands, formed by tautomerization of secondary phosphine oxide (SPO) precursors (Scheme ). As first proposed by Ghaffar and Parkins, and recently observed directly, the key step is nucleophilic attack of the pendant PMe 2 OH oxygen on coordinated nitrile to yield a metallacycle . After attack of water at phosphorus and a series of proton transfers, the catalyst forms the amide and rebinds a nitrile for the next cycle.…”

Oxidative Addition of Water to Ir(I) Complexes Bearing a Pincer-Type Silyl Ligand

“…Water is an abundant, ubiquitous, cheap, and ecologically benign substrate that acts as a source of oxy-functionalities for organic transformation. Transition-metal complexes act as catalysts for the transformation of water in organic synthesis, such as hydration of alkenes, alkynes, and nitriles, C–H oxidation of organic compounds, and Wacker–Tsuji reaction . Artificial water splitting into H 2 and O 2 is a big challenge for the transition to renewal energy and transition-metal catalysis plays an important role .…”

TBPEH-TBPB Initiate the Radical Addition of Benzaldehyde and Allyl Esters

“…For instance, the free radical addition reaction in the synthesis of glufosinate-ammonium by Hoechst company [ 8 , 9 ], which involved the alkene radical phosphorylation of the key intermediate 1-cyanoallyl acetate (ACA), was found to have the advantages of high rate, simple conditions and high atom economy. Moreover, the cyanohydrin [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ] and carbon-carbon double bond [ 18 , 19 , 20 , 21 , 22 , 23 ] structure possessed by this compound have a wide range of applications in the chemical field and pharmaceutical synthesis, which determines its excellent practicability and ductility. However, few studies were conducted on 1-cyanoallyl acetate-related responses.…”