Ni-H bonds

The influence of the Ni atoms becomes clear from a comparison of the actual reaction path, which consists of physical adsorption and subsequent dissociative chemisorption, with the theoretical alternative reaction path, consisting of dissociation of H2 followed by the formation of two Ni-H bonds. H2 is a very stable molecule and, as a consequence, the potential energy of the dissociated H-atoms is very high. In moving to the adsorbed state, Ni-... 

While the reductive elimination is a major pathway for the deactivation of catalytically active NHC complexes [127, 128], it can also be utilized for selective transformations. Cavell et al. [135] described an interesting combination of oxidative addition and reductive elimination for the preparation of C2-alkylated imida-zohum salts. The in situ generated nickel catalyst [Ni(PPh3)2] oxidatively added the C2-H bond of an imidazolium salt to form a Ni hydrido complex. This complex reacts under alkene insertion into the Ni-H bond followed by reductive elimination of the 2-alkylimidazolium salt 39 (Fig. 14). Treatment of N-alkenyl functionalized azolium salts with [NiL2] (L = carbene or phosphine) resulted in the formation of five- and six-membered ring-fused azolium (type 40) and thiazolium salts [136, 137]. 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

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 initiation step in polymerisation with a catalyst derived from the [(MeAll)(Cod)Ni]+[PF6] complex and P(Chx)3 involves the secondary insertion of styrene into a Ni H bond in the cationic nickel hydride species formed in situ ... 

The dimerization of propylene carried out by IFP is called the DIMEROSOL process and involves the use of nickel catalysts. This is shown in Fig. 7.7. Complexes 7.20 and 7.21 are the anti-Markovnikov and Markovnikov insertion products into the Ni-H bond. Structures 7.23(A) and (B) are intermediates derived from 7.21 by inserting the second propylene molecule in a Markovnikov and anti-Markovnikov manner, respectively. Similarly 7.22(A) and (B) are intermediates from 7.20 by the insertion of the second propylene molecule. These lour nickel-alkyl intermediates by /3-elimination give six alkenes. Under the process conditions these alkenes may undergo further isomerization. 

Oxidative addition of HCN onto 9.54 with the elimination of COD leads to the formation of 9.56. Oxidative addition by HCN and coordination by the substrate onto 9.55 and 9.56, respectively, lead to the formation of 9.57. Insertion of the alkene functionality into the Ni-H bond leads to the formation of the p3-allyl intermediate 9.58. Substrate addition or oxidative addition of... 

The BDEs of Ni-H bonds in HNiCp, and H NiCp are approximately 300 kJ/mol, their difference being less than 20 kJ/mol. Although the Ni-Cp BDE is weaker in H NiCp relatively to NiCp (230 instead of 310 kJ/mol), these values suggest that both the Ni-H and the NiCp bonds are quite strong. Consequently, if hydrogen is available in the vicinity of NiCp, the formation of HNiCp and H NiCp, can not be excluded. 

The insertion reaction of NiH[P(OEt)3]4 with a series of dienes has shown that the anti isomer is commonly the kinetically preferred one, and an interpretation has been advanced that the diene rotates into the cisoid configuration as the Ni—H bond is added across 284) [Eqs. (84), (85)]. This addition to the cisoid diene appeared to be the preferred mechanism however, this did not occur when there was a cis substituent on one of the double bonds since cisoid m-pentadiene-1,3 did not give anti-anti-dimethyl-l,3-7T-allyl. The syn isomer appeared to be thermodynamically favored to a small extent for highly substituted zr-allyls. The addition of the Ni—H bond across a double bond to form the zr-allyl moiety was... 

The isomerization reaction of butene-1 has been carried out with D2SO4 in CH3OD [282). The product contained both deuterated and nondeuterated olefins in a ratio consistent with a random scrambling model. The initial step in the reaction involves an insertion of the olefin into the Ni—H bond to form an alkyl. Elimination from the alkyl obtained by Markownikov addition to the olefin can lead to isomerization, whereas elimination from the anti-Markownikov addition product leads to butene-1 being re-formed. The rate of isomerization to deuteration of the olefin is of the order of 170. 

Physisorption involves only a weak attraction between the substrate and the adsorbent but in chemisorption a chemical reaction takes place between the adsorbent and atoms on the catalyst surface. As a result, chemisorbed species are attached to the surface with chemical bonds and are more difficult to remove. If the adsorption of hydrogen on nickel is considered as an example, the reaction involves the breaking of an H-H bond and the formation of two Ni-H bonds on the surface. As shown in Fig. 2.3, this adsorption occurs by way of an initially adsorbed dihydrogen molecule. It proceeds via a electron donation and back bonding to the a orbitals of the hydrogen molecule with the final formation of the two surface M-H species. 

The minimum of the chemisorption potential energy curve (C) corresponds to the sum of the Ni and H atomic radii (y), a result of the formation of the Ni-H bonds. Fig. 2.7 also shows the atomic arrangements at the various... 

Scheme 9.15 shows the cycles that are assumed to represent conversion of 1,3-butadiene to either 68 or 69, which are quite similar to that for ethene hydro-cyanation.100 Intermediates in these cycles, however, consist of T)2-buladicnc complexes or rf-allyl complexes that form after insertion of one of the C=C bonds of the diene into the Ni-H bond. 

In this case, a reaction pathway similar to that used to explain the formation of Diels-Alder products (see Section 2.2.2.1.) can be assumed to be operating. A butadiene molecule which is generated from MCP in the coordination sphere of the nickel, formally by proximal addition of the Ni —H bond to the three-membered ring and subsequent 6-hydrogen elimination, then reacts with a second MCP molecule in a [3 + 2] cycloaddition. 

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