Cyanide, catalyst precursors

In order to evaluate the distribution of the precipitated cyanide complexes over the support, samples of the dried cyanide catalyst precursors were investigated within a Philips EM 4 transmission electron microscope. 

NaBH4 and zinc chloride (Eq. 7) [16]. These additives are necessary for reduction of the catalyst precursor and for prevention of deactivation of the catalyst by excess cyanide anion in the aqueous phase, respectively. As the use of zinc chloride in a Zn/CN molar ratio of more than 0.25 1 is required, the active cyanide source may be tetracyanozincate or zinc cyanide [26], The efficiency of the counter-PTC in the heptane-water system exceeds that of the mixed catalyst system of lipophilic catalyst and normal PTC, though the cyanide anion is easily extractable by the normal PTCs (Table 6). 

The transition metal-catalyzed carbonylation of allylic compounds has been developed as a simple extension of carbonylations of benzylic derivatives. In this respect nickel cyanide has been used as a catalyst precursor for carbonylation of allyl... 

In a more detailed study, the structure of the catalyst precursor was determined and found to be Pd(Diop),32. Other L2Pd and L2Ni complexes [L = Diop, BPPM, BINAP, etc.] were prepared [e.g., by in situ reduction of Pd(Il)Cl,L with sodium borohydride or as isolated palladium(O) complexes] and used as catalysts for the asymmetric addition of hydrogen cyanide to norbornene. norbornadiene, benzonorbornadiene, and cyclopentadiene dimer. In the presence of excess ( + )-Diop and L,Pd, norbornene gives 91 -95% of exo-2-cyanonorbornane with 24% cc of the ( + )-(15.25,4/ )-isomer. Similarly, use of the ( —)-Diop complex leads to the (-)-(l/ ,2f ,4S)-isomer with 24% ee (95% yield). Lower reaction temperatures, instead of the 120 "C used above, give better ee values (80 =C 32% ee with 94% yield 35 °C 35 % ee with 6% yield)32. 

To assess the suitability of metal cyanide complexes as active precursors for supported catalysts, a series of homo- and heteronuclear cyanide complexes has been precipitated in the presence of alumina, titania, and silica supports. To establish the distribution of the insoluble cyanide complexes on the support, the catalyst precursors were investigated by transmission electron microscopy. Conversion of the cyanide precursors into oxidic or metallic catalysts can be performed by thermal treatments in oxygen, argon, and hydrogen, respectively. Detailed results of the thermal treatment of a copper-iron cyanide precursor on alumina are presented. Oxidation of the cyanide precursors to highly dispersed oxides calls for treatment at relatively low temperatures, viz., about 573 K. The resulting oxide can subsequently be reduced smoothly to the corresponding (bi)metallic supported catalyst. 

Besides by electron microscopy, the catalyst (precursor) systems were characterized by Mossbauer spectroscopy, XRD, and thermal an ysis. It was demonstrated that the proportion of the metals within the stoichiometric cyanide precursors was retained in the reduced bimetallic catalysts. Heteronuclear cyanide complexes are therefore very well appropriate to produce bimetallic catalysts of a uniform chemic composition of the individual suppmted alloy particles. 

The catalyst precursors were prepared by deposition-precipitation of complex cyanides onto oxidic supports. Precipitation was effected by addition of a solution of a soluble complex cyanide to a suspension of the powdered support in a solution of a simple metal salt. 

In an effort to move away from precious metal catalysts, various reports in recent years have focused on the use of first-row metal catalysts for direct arylations [57-60]. As a representative example of these new developments, we illustrate in Scheme 23.15 the chelate-assisted ortho-C-H arylation of arenes with Fe catalysts [61]. With iron being cheap, nontoxic, and ubiquitous, this protocol is highly attractive for pharmaceutical syntheses. Using the catalyst precursor Fe(acac)j in conjunction with bidentate pyridine ligands, Zn-aryl reagents as aryl transfer reagents and 1,2-dichloroisobutane as the oxidant, excellent yields of the arylated product were obtained. An interesting feature of this reaction is the hydrolysis of the imine moiety after work-up. The reaction conditions tolerate additional functionalities such as cyanides, chlorides, triflates, tosylates, and thiophenes. 

Impact A continuous process for making long-chain polyether polyols as precursors for polyurethanes. Uses a double metal cyanide catalyst. 

Acyloxy-l-cyanoalkanes [45, 46], which can be used as precursors for ketones [47], a-hydroxy ketones [48] and 1,4-dicarbonyl compounds [47], are prepared in one pot from the appropriate aldehyde, sodium or potassium cyanide, and the acylating agent under phase-transfer catalytic conditions [47-49]. Attempts to synthesize chiral cyanhydrins using chiral phase-transfer catalysts have been unsuccessful (see Section 12.3). 

The phase-transfer catalysed reaction of nickel tetracarbonyl with sodium hydroxide under carbon monoxide produces the nickel carbonyl dianions, Ni,(CO) 2- and Ni6(CO)162, which convert allyl chloride into a mixture of but-3-enoic and but-2-enoic acids [18]. However, in view of the high toxicity of the volatile nickel tetracarbonyl, the use of the nickel cyanide as a precursor for the carbonyl complexes is preferred. Pretreatment of the cyanide with carbon monoxide under basic conditions is thought to produce the tricarbonylnickel cyanide anion [19], as the active metal catalyst. Reaction with allyl halides, in a manner analogous to that outlined for the preparation of the arylacetic acids, produces the butenoic acids (Table 8.7). 

With DIOP-Pd(0) or -Ni(0) complexes as catalysts, moderate optical yields of up to 35% have been observed (126). Norbomene is convertible to the exo nitrile with up to 40% ee when a BINAP-Pd(0) complex is used (Scheme 57) (127). Ni(0) complexes of sugar-derived 1,2-diol phosphinites catalyze highly selective asymmetric addition of hydrogen cyanide to vinylarenes (128). This method gives the 2-naphthalene-2-propionitrile precursors of nonsteroid anti-inflammatory agents in up to 85% ee and in high yield. 

Very recently Geus and co-workers [44, 45] have applied another method based on chemical complexes. This is the complex cyanide method to prepare both monocomponent (Fe or Co) and multicomponent Fischer-Tropsch catalysts. A large range of insoluble complex cyanides are known in which many metals can be combined, e.g. iron(n) hexacyanide and iron(m) hexacyanide can be combined with iron ions, but also with nickel, cobalt, copper, and zinc ions. Soluble complex ions of molybdenum(iv) which can produce insoluble complexes with metal cations are also known. Deposition precipitation (Section A.2.2.1.5) can be performed by injection of a solution of a soluble cyanide complex of one of the desired metals into a suspension of a suitable support in a solution of a simple salt of the other desired metal. By adjusting the cation composition of the simple salt solution, with a same cyanide, it is possible to adjust the composition of the precursor from a monometallic oxide (the case when the metallic cation is identical to that contained in the complex) to oxides containing one or several foreign elements. 

The protected dipeptide Z-His-Phe-NH2 is a precursor for cyclo[-His-Phe-], which is known as a catalyst for addition of cyanide to aldehydes, with high enantiomeric excess.f l Such optically active cyanohydrins are valuable starting materials for the synthesis of a variety of chiral, bioactive compounds.The easy scale-up (to mole scale) of the suspension-to-sus-pension conversion approach has been demonstrated by the thermolysin-catalyzed synthesis of Z-His-Phe-NH2.[ ]... 

Another important class of reactions involves the introduction of a cyano group by substitution in an Ar-Z precursor. In fact, novel pathways leading to aromatic nitriles-for example, photosubstitution reactions-are desirable in view of the many applications of aryl cyanides as agrochemicals and pharmaceuticals. Today, the classical copper(l)-mediated Rosenmund-von Braun and Sandmeyer reactions, from aryl halides and aryldiazonium salts respectively, have been supplanted by reactions which employ palladium- or copper-catalysis [57]. The rather common use of excess cyanide anion may lead to a deactivation of the catalyst, and affect to a remarkable extent each of the key steps of the catalytic cycle [58aj. Although the use of complex iron cyanide has been shown to offer an effective solution to this limitation [58b,c], photocyanation provides an equally useful alternative [10],... 

A new acid catalyst of potential use in the Bischler-Napieralski cyclization is P2O5 in methanesulfonic acid. The phenethylamines used in isoquinoline syntheses are usually prepared from the reduction of the corresponding /S-nitrostyrenes. A more versatile procedure starts with a substituted benzyl chloride which is converted to the nitrile using sodium cyanide in DMSO (dimethylsulfoxide). Reduction of the nitrile with LiAlH4 in the presence of AICI3 gives the desired amine in excellent yield. Benzylamines or their quaternary salts may also be utilized in appropriate solvents in place of benzylic chlorides, so that they too may act as nitrile precursors. ... 

Reduction of the oxidic precursors resulting from the oxidation of the supported complex cyanides leads to metal or alloy particles. Figure 4 shows temperature-programmed reduction (IPR) profiles measured with the thermal conductivity detector for the oxidic precursors of iron, copper-iron and nickel-iron catalysts. With the pure iron catalyst profiles for the alumina and for the titania supported ones are presented. The reduction profiles of the iron-copper and... 

Homo- and heteronuclear cyanide complexes are attractive precursors for the preparation of supported catalysts. First of all since the reaction with the support to an inactive compound does not proceed appreciably, and secondly, since the fixed stoichiometry of the complex cyanides results in active particles of a uniform chemical composition. 

The attractiveness regarding this proposal is that HOSCN is the perfect precursor for the formation of hydrogen cyanide and sulfate ions after sequential oxidation. Thus, according to all the information gathered and discussed above, the proposed reaction mechaiusm for thiocyanate oxidation in pH 4 using CoPc as catalyst should look like the following ... 

Hydrogen cyanide can also be converted to cyanoacetylene and hydrogen cyanate, both precursors of pyrimidines. These reactions were reproduced in the laboratory. In fact, in 1828, F. Wohler made urea from hydrogen cyanate and ammonia, the first synthesis of an animal substance from inorganic materials. Very likely, all these processes occurred primarily in an aqueous environment where and OH" ions acted as specific-acid or specific-base catalysts. It is particularly impressive that the three major classes of nitrogen containing biomolecules, purines, pyrimidines, and amino acids are formed by the hydrolysis of the oligomers formed directly from... 

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