Mechanism of hydrocyanations

The mechanism of hydrocyanation by nickel catalysts should proceed through a nickel hydride addition on the double bonds. The nickel hydrides should result from the oxidative HCN addition to the metal, or from the above Lewis acid-assisted dissociation of HCN. The oxidative HCN addition to low-valent metal complexes has been demonstrated, particularly by NMK spectroscopy with Ni(0)(P(OF.t)3 4. 

The mechanism of hydrocyanation turns out to be another classic example of a set of catalytic cycles that use many of the fundamental types of organometallic reactions that we have already encountered. In fact, the investigation of the details of this mechanism went hand in hand with the advancement of important general... 

The mechanism of hydrocyanation of alkenes catalyzed by soluble complexes is closely related to the mechanism of hydrogenation and hydrosilation. Hydrocyanation occurs by a sequence consisting of oxidative addition of HCN, olefin insertion into the M-H bond, and reductive elimination to form the new C-C bond. The mechanism of the original hydrocyanation catalyzed by cobalt carbonyl has not been studied in depth, but the mechanism of the reactions catalyzed by nickel complexes has been studied in depth and is better defined. 

The mechanism of hydrocyanation of ethylene catalyzed by the combination of Ni(0) and P(0-o-To1)3, as deduced by Tolman, is shown in Scheme 16.27 The L2Ni(ethylene) complex has been isolated and shown to add HCN. The Ni(0) complexes of higher olefins are less stable. In this mechanism, oxidative addition of HCN to a Ni(0) olefin complex forms a cyanometal-hydride complex. In the presence of ethylene, this complex contains olefin, but in the presence of higher olefins this complex has the composition L3Ni(H)(CN). Insertion of an olefin into the metal hydride occurs by a migratory insertion mechanism initiated by coordination of olefin to the cyanometal hydride. Reductive elimination of the alkyl cyanide completes the cycle, and this step is accelerated by Lewis acids, as presented later in this chapter. 

The mechanism of the simplest reaction HCNO+ + HCN —> cvc/o-HCCHN+ + NO has been explored at the MP2/6-31G(d) level of theory. The most favorable reaction profile involves the formation of a C—N bond between the positively charged carbon atom of HCNO+ and the nitrogen atom of hydrocyanic acid giving an HCNO+/HCN intermediate which isomerizes into an ionized nitrosoazirine before losing NO. 

Fig. 9. Mechanism of ethylene hydrocyanation. Dashed arrows imply irreversible reactions.

Fig. 16. The mechanism of PN isomerization/hydrocyanation in the absence of a Lewis acid. R"Ni = NCCH2(CH3CH2)CHNi, R" Ni = NC(CH3CH2CH2)CHNi.

In 2000, Kagan and Holmes reported that the mono-lithium salt 10 of (R)- or (S)-BINOL catalyzes the addition of TMS-CN to aldehydes (Scheme 6.8) [52]. The mechanism of this reaction is believed to involve addition of the BI NO Late anion to TMS-CN to yield an activated hypervalent silicon intermediate. With aromatic aldehydes the corresponding cyanohydrin-TMS ethers were obtained with up to 59% ee at a loading of only 1 mol% of the remarkably simple and readily available catalyst. Among the aliphatic aldehydes tested cyclohexane carbaldehyde gave the best ee (30%). In a subsequent publication the same authors reported that the salen mono-lithium salt 11 catalyzes the same transformation with even higher enantioselectivity (up to 97% Scheme 6.8) [53], The only disadvantage of this remarkably simple and efficient system for asymmetric hydrocyanation of aromatic aldehydes seems to be the very pronounced (and hardly predictable) dependence of enantioselectivity on substitution pattern. Furthermore, aliphatic aldehydes seem not to be favorable substrates. 

Figure 1 Proposed mechanism of butadiene hydrocyanation with Ni catalysts showing the pathway that introduces the first CN group...

The active nickel catalyst contains one bidentate phos-phinite ligand and the overall mechanism of the reaction is believed to be similar to butadiene hydrocyanation except that the final reductive elimination step is irreversible under the conditions of the reaction. jr-Allyl intermediates (7) are believed to play an important role in the exclusive formation of the branched nitrile product observed. Formation of the C-CN bond in the final reductive ehmination from the r-allyl intermediate occurs at C(2) and not C(4), because the aromaticity of the naphthalene ring is preserved only when the bond forms with C(2). A a-alkyl complex see a-Bond) with the Ni bound to C(l), which could give the linear (anti-Markovnikov) nitrile product, does not contribute because of the much greater stability of intermediate (7), accounting for the high regioselectivity observed. 

Cobalt B12 Enzymes Coenzymes Copper Hemocyanin/Tyrosinase Models Heterogeneous Catalysis by Metals Hydride Complexes of the Transition Metals Hydrocyanation by Homogeneous Catalysis Hydrogen Inorganic Chemistry Mechanisms of Reaction of Organometalhc Complexes Nickel OrganometaUic Chemistry Ohgomerization Polymerization by... 

Carbonylation Processes by Homogeneous Catalysis Hydrocyanation by Homogeneous Catalysis Mechanisms of Reaction of OrganometaUic Complexes Ohgomeriza-tion Polymerization by Homogeneous Catalysis Osmium Inorganic Coordination Chemistry. 

A complete quantitative mechanism cannot be constructed with confidence on the basis of the products since not all of them were determined. A variety of secondary processes is likely. Hydrogen abstraction is expected to involve mainly the N-bound H atom. The activation energy for abstraction by methyl is given as 4.8 kcal/mol (277). Reaction 85 has been found (277) to yield ethylene and nitrogen in the radical-sensitized decomposition of ethyleneimine. Reaction 70 (see above) could explain the production of hydrocyanic acid. 

Basically the mechanism of homogeneous hydrocyanation can be separated into four principle steps which are demonstrated in eqs. (2)-(5), in which ligands are omitted for the sake of simplicity [16]. 

The mechanism of NiL4-catalyzed hydrocyanation (L=P(0-o-tolyl)3) of ethylene has been studied in detail, offering the advantage that olefin isomerization is avoided (cf. Scheme 1 [10]). Scheme 1 contains the main features of the process, such as oxidative addition, n- and (r-complexes, reductive elimination, and catalyst deactivation by Ni(CN)2 formation. 

Taillades, J., Commeyras, A. Strecker and related systems. II. Mechanism of formation in aqueous solution of a-alkylaminoisobutyronitriles from acetone, hydrocyanic acid, ammonia, and methyl- or dimethylamine. Tetrahedron 1974, 30, 2493-2501. 

The mechanism and regioselectivity of hydrocyanation is discussed in several reviews13 and a short recent survey on the current status of knowledge about stereochemistry and asymmetric induction is available9. In particular, synthetic and mechanistic aspects of the nickel-catalyzed hydrocyanatioii of butadiene is examined10 in view of the importance of the DuPont adiponi-trile process. 

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