Transmetallation to nickel

Bis(triethylphosphine)platinum[l,8]naphthalene 56 undergoes diphenylacetylene insertion and reductive elimination to yield 1,2-diphenylacenaphthalene (Equation 18) <2005JA13494>. As noted in Equation (11), an analogous zirconacyclobutene complex inserts alkyne only upon prior transmetallation to nickel. 

Transition-metal-mediated cycloaddition of aUcenes and alkynes meets a major challenge to achieve both high reactivity and predictable selectivity between different unsaturated substrates in the formation of complex molecules. The employment of zirconacyclopentanes incorporating a carboranyl unit to reaction with alkynes after transmetalation to nickel can realize the three-component [2 + 2 + 2] cycloaddition of carboryne, unactivated alkenes, and alkynes. 

Scheme 37 Transmetalation of chromium to nickel in a metal carbene-mediated cyclisation reaction (L=cod, MeCN, alkyne)...

The aforementioned dienyl dicopper derivatives show the characteristic reactivity of orga-nocopper compounds. However, one limitation to the use of copper is that an electron-withdrawing group is usually required for reaction with alkynes. In order to develop an insertion protocol for alkynes bearing electron-donating groups, transmetalation of zirco-nacyclopentadienes to nickel was investigated. 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

The cadmium secocorrinoid carboxylic acid (102 M = Cd) also undergoes photocyclization to the acid (103 M = Cd), which on transmetallation to the nickel(II) complex (103 M = Ni) and treatment with triethylamine and acetic acid yields the parent corrin complex (100 M = Ni).268 The decarboxylation process is extremely facile. A related base-catalyzed cyclization of the secocorrinoid aldehyde (104) gives the corrin complex (105), which can be decarbonylated to the parent complex (100 M = Ni) by treatment with potassium hydroxide (Scheme 66).268... 

Transmetallation from an early to a late transition metal is kinetically accessible and, most often, thermodynamically favorable. Treatment of 1,9-anthracendiyl zirconocene 36 with bis(triphenylphosphine)nickel(ii)bromide in the presence of diphenylacetylene gives 1,2-diphenylaceanthrylene in good yield (Equation 11), suggesting that the transmetallation of zirconium to nickel proceeds efficiently <2000JA9880>. 

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... 

The extension of this method to heterocyclic products took advantage of nickel catalysis for the iodine-zinc exchange, for example, of iodoacetals 22, and the organozinc intermediates 23 formed by radical cyclization were again transmetallated to the corresponding copper species which could be reacted with the usual Michael acceptors to afford the products 24 with high diastereoselectivity (Scheme 6).33>33a... 

A commonly proposed mechanism of the standard NHK protocol involves reduction of Ni(II) to Ni(0) by CrC, then oxidative addition of Ni(0) to the alkenyl iodide to generate an alkenyl Ni(II) species (Scheme 3-69). Transmetallation to CrCls (from the initial nickel reduction step) produces a Cr(III) alkenyl species, which undergoes direct addition to the aldehyde to generate product. Alternatively, alkenyl transfer from a vinyl nickel(II) species to CrCb may instead occur with release of the Cr(III) alkenyl species and a Ni(I) species. 

Due to their widespread application in the Suzuki-Miyaura reaction, arylboronic acids are attractive aryl transfer precursors. However, in the zinc-promoted reaction, an excess of Et2Zn (up to 7equiv.) must be added in order for efficient transmetallation to occur. The application of a reactive aryl metal species necessitating only a catalytic amount of metal would thus be advantageous. In recent years, several rhodium-catalyzed enantioselective protocols have been described, as well as processes involving palladium and nickel. 

The same group reported an extension of the direct alkylation of (benz)oxazoles with various alkyl bromides and chlorides by using the stronger base lithium tert-butoxide (Scheme 19.23) [38]. 5-Aryloxazoles containing electronically diverse substituents such as CFj and OMe were also alkylated successfully. Various linear alkyl chains were introduced, such as phenylpropyl, citronellyl, or octyl, affording interesting lipophilic molecules. The optimized reaction conditions failed to apply to benzothiazoles, and the authors had to turn their attention to nickel catalysis to achieve the corresponding alkylation (Section 19.2.3). Experiments were run to understand the reaction mechanism that presumably involves Sj 2-type oxidative addition of the alkyl halide to palladium(O) followed by transmetallation by the in situ-lithiated (benz)oxazole (Scheme 19.24). 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

Oxidative addition of C-CN bonds to nickel(0) can be followed by transmetalation with various main-group organometaUic reagents, and subsequent reductive elimination can result in the functionalization of C-CN bonds of nitriles (Scheme 5). As the simplest case, C-CN bonds can be transformed to C-H bonds via transmetalation with metal hydrides. Indeed, nickel-catalyzed hydrodecyanation of various aromatic and aliphatic nitriles proceeds with tetramethyldisUoxane as a hydride donor (Scheme 6) [44]. While a wide range of nitriles can be decyanated by this protocol, a relatively high amount of catalyst is required in this process, presumably because of the formation of catalyticaUy inactive (PCy3)2Ni(CN)2. The use of AlMe3 as a Lewis acid is effective in some cases to promote the C-CN bond activation. Under these reaction conditions, the relative reactivity order of different aryl electrophiles is estimated Ar-SMe>Ar-CN>Ar-OAr>Ar-OMe. 

A transmetalation of the styrylcarbene chromium complex 62 in the presence of stoichiometric amounts of [Ni(cod)2] to give the nickel carbene intermediate 63 was applied to the synthesis of Cr(CO)3-coordinated cycloheptatriene 64 upon reaction with terminal alkynes [57] (Scheme 37). The formation of pen-tacarbonyl(acetonitrile)chromium is expected to facilitate the metal exchange. 

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