Nickel-mediated Reactions

Adducts were obtained as exclusive adducts or as by-products in the nickel mediated reactions of some substituted norbomadienes with various dienophiles. The formation of these products was considered to result from an intermediate metallocy-clopentane species built up of the metal center, the dienophilic double bond and one of the double bonds of the norbomadiene moiety. 

Palladium-catalyzed reactions and cross-couplings are vital and widely used methods in the synthesis of organic compounds. Many examples have emerged utilizing pyran-2-ones and coumarins in palladium- and nickel-mediated reactions, and several of these are presented in this section. 

In 1975, three different protocols were available in the literature, each describing the synthesis of internal alkynes. Cassar described palladium- or nickel-mediated reactions between aryl or vinyl halides and alkynes complexes with phosphine as ligands in the presence of NaOMe [1]. As a second protocol, Heck pubhshed a variation of the Mizoroki-Heck couplings, in which the olefins were replaced by alkynes and coupled with (hetero)aryl, as weU as alkenyl bromides or iodides at 100 °C in the presence of a basic amine [2]. More than a decade earUer, Stephens and Castro had described the details of a palladium-free coupling of aryl iodides with cuprous acetylides in refluxing pyridine [3]. 

Preparation of 3-Arylpropanenitriles by Nickel-Mediated Reaction of Benzylic Halides with Haloacetonitriles... 

Abstract A literature overview, up to the end of 2004, of the most important microwave-assisted transition-metal-mediated processes used for the decoration and construction of heterocycles is presented. The emphasis of the chapter lies in the use of palladium-assisted reactions but examples of copper- and nickel-mediated processes are also incorporated. 

Rieke Ni-promoted (Scheme 11) and cat. NiCk/Mg/MesSiCl-mediated pinacol coupling have been investigated [38]. The similar nickel-catalyzed reaction was also reported later [39]. 

The formation of l,3,4-triarylpyrroline-2,5-diones from the reaction of diphenylcyclopropenone and N-sulfinylarylamines with nickel carbonyl has been described earlier (see Scheme 36 in Section IV,A,2).64 In contrast with this result, 2,4,5-triphenyl-3-isothiazolone 1-oxide is produced in this type of process using iron pentacarbonyl, and the analogous cyclohexyl derivative is formed in a nickel carbonyl-mediated reaction (Scheme 115).64 It is probable that metallocyclic species (cf. 93) are intermediates in these transformations. 

Novel transition metal-mediated strategies were also well represented this past year. Takahashi and co-workers reported a s nickel-catalyzed reaction between azaziconacyclopentadienes (9) and alkynes to form pyridines (10) of varying substitution patterns <00JA4994>. This methodology, a formal cyclotrimerization, is also noteworthy since two different alkynes can be used. In similar fashion, Eaton reported an aqueous, cobalt(II) catalyzed cyclotrimerization between two identical acetylenes and one nitrile to afford substituted pyridines . 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Barluenga and co-workers reported a novel [2 + 2 + 2 + 1 (-reaction based on the use of two metals, the nickel-mediated [2 + 2 + 2 + l]-reaction of Fischer carbenes with alkynes (Scheme 73).142 While, at present, this process requires the stoichiometric use of both metals, it is highly regioselective, affords good yields, and is a novel route to seven-membered rings. 

Beyond the numerous applications of palladium in transition metal-catalyzed domino reactions there are a lot of other metals inducing domino processes. Ihara et al. found a strategy for the enan-tioselective synthesis of (+)-equilenin catalyzed by mangan and palladium complexes,1841 Whitby et al.1851 initiated domino cyclizations on a zirconocene template and furthermore Scherf et al. generated phenantrones by a nickel-mediated one-pot domino reaction.1861... 

Prenylaniline (945) required for the synthesis of (+ )-carquinostatin A [( )-278] (see retrosynthesis in Scheme 5.126) was obtained by a nickel-mediated cross-coupling reaction of the N-protected 4-bromoaniline 952 with bis[n-bromo(r -prenyl)nickel] (825) (646) (Scheme 5.128). 

Formally copper catalyzed couplings are analogous to palladium and nickel catalyzed reactions. Carbon-carbon and carbon-heteroatom bonds can be formed in such transformations alike. From the mechanistic point of view there is a significant difference between nickel, palladium and copper catalyzed processes however. While in the former cases the catalyst usually oscillates between the 0 and +2 oxidation states, in copper mediated transformations the common oxidation numbers are +1, +2 and +3. 

Figure 8-7. A nickel-mediated pbosphanation of a bromoarene. The precise mechanistic details of this reaction are not known.

Synthetic applications of organic germanium, tin and lead compounds 1367 3. Nickel-mediated coupling reactions... 

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