Nickel-catalyzed olefin isomerization

The regiospecificity of DCN addition using ZnCl2, and the fact that very little deuterium is found in recovered 3PN, strongly suggest that, at least in this system, the back reactions in steps 3, 6, 14, and 16 are slow and olefin isomerization is catalyzed by cationic nickel hydrides, as shown in Fig. 6. This may also occur with other Lewis acids. 

Among early reported Pd-catalyzed reactions, the Mori-Ban indole synthesis has proven to be very useful for pyrrole annulation. In 1977, based on their success with the nickel-catalyzed synthesis of indole from 2-chloro-A -allylaniline, the group led by Mori and Ban disclosed Pd-catalyzed intramolecular reactions of aryl halides with pendant olefins [111]. Compound 117, easily prepared from 2-bromo-A-acetylaniline and methyl bromocrotonate, was adopted as a cyclization precursor. Treatment of 117 with PdiOAc), (2 mol%), PhjP (4 mol%), and NaHCOj in DMF provided indole 118 via an intramolecular Heck reaction followed by olefin isomerization to afford the fully aromatic product. Although yields fr om the initial report were moderate, they have been greatly improved over the last two decades [112]. 

Furthermore, a vast number of organometallic catalyzed reactions can be performed in a biphasic manner thus proving that also uncommon reactions may be worth to be investigated in liquid/liquid systems. For instance, Braddock describes the atom economic nitration of aromatics in a two-phase process [192], Nitration of aromatics leads usually to excessive acid waste streams and the classical Lewis acid catalysts such as boron trifluoride are destroyed in the aqueous quench after the reaction thus making any recycle impossible. In the method of Braddock the ytterbium triflate catalyst is solved in the aqueous phase and can be recycled by a simple evaporative process. Monflier and Mortreux [193] investigated the nickel catalyzed isomerization of olefins, for instance allylbenzene, in a two phase system yielding good yields of cis- and trans-methylstyrene. 

Nickel-catalyzed isomerizations are thought to proceed by this mechanism. In general, in these reactions, isomerization occurs in a stepwise manner and the final isomer concentrations approach the thermodynamic equilibrium values. The ease of isomerization seems to follow the order of the complexing ability of olefins terminal > disubstituted >... 

Besides various iron and ruthenium complexes [43, 44], nickel-based catalysts have recently been shown to be highly reactive in this respect as well [45]. Thus, a nickel catalyst prepared in situ from equimolar amounts of NiCl2(dppe) (dppe, l,2-bis(diphenylphosphino)ethane) and LiBHEtj (5 mol% each), which presumably resulted in the formation of NiHCl(dppe) as the active catalyst, efficiently catalyzed the isomerization/aldol event of allyl alcohols 82 and aldehydes 83 in combination with the Lewis acid MgBr2 (5 mol%) to furnish aldol products 84 in typically excellent yields and variable isomeric ratios (Table 8.11). Allyl alcohols with a terminal alkene reacted much faster than those with an internal olefin, and the aldol reaction occurred exclusively on the side of the former allyl alcohol. 

Nickel-catalyzed hydrocyanation of a-olefins t)q)ically produces the terminal nitrile as the more abundant, but not exclusive isomeric product. In contrast, nickel-catalyzed hydrocyanation of vinylarenes typically generates the branched product. This branched selectivity arises fijom the stability of -phenethyl complexes, as is shown in more detail in Section 16.2.5 on as)unmetric hydrocyanation. The relative rates for hydrocyanation follow the trend ethylene > styrene > propene 1-hexene > disubstituted olefins. Examples of these reactions and selectivities for formation of the linear and branched products are shown in Scheme 16.1. ... 

Depending on the nature of olefins, hydrosilanes, and catalysts, side reactions can also occur, such as H-Cl exchange in silanes as well as hydrogenation and olefin isomerization (144,145). In some cases, in the reaction catalyzed by some complexes of ruthenium, rhodium, iron, iridium, platinum, and nickel, alkenylsilanes have been obtained as a major product of the dehydrogenative... 

Before discussing hydrocyanation chemistry we will explore the interaction of zero-valent nickel phosphite complexes with various independent components of the catalytic system. Then, in turn, we will examine the catalyzed addition of HCN to butadiene, the isomerization of olefins, and the addition of HCN to monoolefins. Finally, a summary of the mechanism as it is now understood will be presented. 

In the case of isomerization which proceeds according to the 7c-allyl mechanism, 1,3-hydrogen transfer takes place. These reactions are catalyzed by palladium(II) complexes which easily form 7r-allyl complexes from 7r-olefin compounds. Also, compounds of nickel, rhodium, iron, etc., are utilized as catalysts. Effective isomerization is possible if the hydrogen addition to both terminal carbon atoms of the 7r-allyl asymmetric grouping takes place. 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

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