Palladium-catalyzed hydrocyanation

The stereochemistry of palladium-catalyzed hydrocyanation has been studied further using [Pd(DIOP)2] (133) as catalyst.607 It was shown that the addition of HCN to both cyclic and acyclic alkenes is cis. The mechanism is believed to be the same as for the nickel-catalyzed reaction (Scheme 58). 

Palladium-catalyzed hydrocyanation of olefins has been reported [31]. However, the corresponding reactions with conjugated dienes have not been reported explicitly. The analogous nickel-catalyzed hydrocyanation of conjugated dienes has been described [32] and is the basis for the commercial adiponitrile process. In this case, it has been shown [33] that the overall addition of HCN to the 1,3-diene occurs with cis stereochemistry consistent with path B in Scheme 8-1. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

Mechanistic studies on the reductive elimination of square-planar type aryl(j7 -allyl)palladium complexes demonstrated occurrence of bond formation between the aryl carbon and one of the allyl termini that are located cis to each other (Scheme 8.53) [91]. The allyl ligand remained 17 -coordinated during the coupling. Similar reductive elimination between 17 -allyl and cyano ligands may be a key step in the industrially important nickel catalyzed hydrocyanation of dienes (Scheme 8.54) [92]. 

The mechanism of the related palladium-catalyzed asymmetric hydrocyanation of alkenes has also been examined in detail using [Pd(RR-diop)(C2H4)] and related chiral species as precursor complexes. These additions gave the exo nitrile product with up to 40% enantiomeric excess, indicating stereoselective complexa-tion of norbornene to the Pd(0) via the exo face. The plausible reaction intermediates [Pd(diop)(norbornene)] and the hydrido cyanide (60) were characterized by and P NMR spectroscopy. Oxidative addition of HCN to Pd(0) is believed to precede rate-determining alkene binding. 

This chapter focuses on the subset of these reactions that have been studied most intensively and that draw from the stoichiometric reactions presented earlier in this text. Thus, the first sections of this chapter highlight certain aspects of hydrocyanation, hydrosilylation, disilylation, hydroboration, diboration, silylborations, and hydroami-nation. The last section presents aspects of palladium-catalyzed oxidation and metal-catalyzed oxidative amination of olefins. 

The use of oxidative addition of a C-CN bond in a more elaborate synthetic process was first demonstrated in 1994 by Nozaki and Takaya. The palladium-catalyzed reaction of acyl cyanide 64 with terminal alkyne 65 yields nitrile 66 in the reaction, alkyne 65 is formally inserted into a C(=0)-CN bond in 64 (Scheme 6.12) [37]. It is proposed that the reaction is initiated by oxidative addition of 64, which acylates 65 to form alkyne 68, along with hydride-palladium 69. Subsequent hydrocyanation affords nitrile 70, which finally isomerizes to 66 under these catalytic conditions. 

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

Some metal complexes of metals other than nickel are known to catalyze the hydrocyanation of alkenes, among the best being those of palladium and cobalt. ... 

Finally, carbohydrate ligands of enantioselective catalysts have been described for a limited number of reactions. Bis-phosphites of carbohydrates have been reported as ligands of efficient catalysts in enantioselective hydrogenations [182] and hydrocyanations [183], and a bifunctional dihydroglucal-based catalyst was recently found to effect asymmetric cyanosilylations of ketones [184]. Carbohydrate-derived titanocenes have been used in the enantioselective catalysis of reactions of diethyl zinc with carbonyl compounds [113]. Oxazolinones of amino sugars have been shown to be efficient catalysts in enantioselective palladium(0)-catalyzed allylation reactions of C-nucleophiles [185]. 

Alkynes are readily hydrocyanated in the presence of a homogeneous catalyst, especially a nickel-based catalyst system. However, zerovalent palladium compounds are reported to catalyze the reaction as well, but are less efficient [60], The reaction gives an easy access to the synthetically valuable a,P-un-saturated nitriles. The use of acetone cyanohydrin as a synthetic equivalent for the difficult-to-handle HCN provides an efficient alternative, but the substrate/ catalyst ratio has to be increased in comparison with the reaction with HCN. The regioselectivity of the reaction is controlled by steric, electronic, and chelative effects. Investigations were predominantly performed by changing the substituent pattern on the acetylenic substrate [61]. 

Table 1. Typical Hydrocyanation Reactions of Olehns Catalyzed by Palladium...

Hydrocyanation is the formal addition of hydrogen cyanide to alkenes. alkynes, and dienes to yield nitriles. These reactions can be catalyzed by various copper, cobalt, nickel and palladium catalysts modified with phosphanes and phosphites and/or Lewis acids. Hydrocyanation of carbonyl groups in aldehydes and ketones is covered in Section D.l.3.7. 

Other notable addition reactions catalyzed by sol-gel entrapped catalysts are the (enantioselective) hydrocyanation of benzaldehyde (Scheme 24-17) (Shvo, 1994) hydrocar-boxylation of amines by a sol-gel bound ruthenium catalyst (Scheme 24-18) (Krocher, 1996), the hydrosilylation of olefins by an immobilized rhodium catalyst (Scheme 24-19) (Capka, 1992b), and the hydration ofalkynes by sol-gel entrapped palladium or platinum compounds (Scheme 24-20) (Blum, 2000 Israelsohn, 2002). 

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