Nickelacycles

Scheme 7 Fomation of chain-end functionalized nickelacycle initiators...

Deming TJ (1998) Amino acid derived nickelacycles intermediates in nickel-mediated polypeptide synthesis. J Am Chem Soc 120 4240-4241... 

Diyne 66, which possesses a three carbon linkage between the alkynes, also reacts with aldehyde 67 in presence of [Ni(COD)J/SIPr catalytic system (Scheme 5.19). However, the connectivity of the dienone 69 obtained from this diyne was different from those obtained from diyne 60. In the former case, adduct 69 was obtained from a )5-hydride elimination of the nickelacycle 68 instead of a reductive elimination. 

A further development is the reaction of enones with ene-ynes in the presence of ZnCl2 [290]. Reaction of 6/4-50a with 2equiv. of 6/4-51a in the presence of equimolar amounts of Ni(cod)2 and 1.5 mol% of ZnCl2 in acetonitrile at room temperature for 6 h led to 6/4-52aa with an (E)-configuration in 50% yield and 96% selectivity. In this transformation, 6/4-53aa is formed as a minor isomer. Nickelacycles 6/4-54 or 6/4-55 can be assumed as a first intermediate (Scheme 6/4.14). 

In relation to the above mechanisms, direct observation of oxidative cyclizations of nickel-enal complexes such as 191 with Ni(COD)2/R3P (1 equiv.) (R = Cy or Ph) to nickelacycles 192 was reported (Scheme 89).432... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

The advantage of using a sacrificial anode has been clearly pointed out. Magnesium was found to be the most convenient, the oxidation of which produces Mg ions which can enter the catalytic cycle to cleave the nickela-cycle intermediate and liberate Ni for further catalytic cycles (Scheme 7). Such a mechanism has been substantiated on the basis of the formation of the nickelacycle and its characterization by cyclic voltammetry. In the absence of Mg (reactions conducted in a divided cell in the presence of ammonium ions) the nickelacycle does not transform and the reaction stops when all the starting nickel compound has been reacted. Upon addition of MgBr2 to an electrochemically prepared solution of the nickelacycle, Ni(II) is recovered [114]. 

Head-tail linkage of PhNCO occurs in the presence of Ni(L) to give five- or six-membered nickelacycles, depending on whether L is Me4en or 2Et3P. Similar reactions carried out in the presence of aldehydes afford the condensation of PhNCO with PhCHO to give oxazanickelacyclopentanones (equations 93 and 94).283... 

Cyclobutanones can act as l-oxobutane-l,4-diyl units they undergo intermolecu-lar alkyne (RC=CR) insertion to give cyclohexanones (68), catalysed by Ni(cod)2, apparently via a seven-membered nickelacycle.197... 

Analogous to the cycloadditions described above, the first step of these ring expansion reactions is believed to involve the initial oxidative coupling between the carbonyl and the alkyne to afford a nickelapentenacycle (12, Scheme 2) [37,38]. Subsequent (3-carbon elimination relieves ring strain and affords a seven-membered nickelacycle 13a that reductively eliminates the... 

Eive-membered nickelacycles have also been investigated by Ceder et al. (Equation 37) <19950M5544>. 

The reaction of (methyl acrylate)bis(triphenylphosphane)nickel (34) with 3,3-dimethylcyclo-propene provides the orange, crystalline nickelacycle, methyl 6,6-dimethyl-2-nickela-2-(triphenylphosphane)bicyclo[3.1.0]hexane-3-e c/o-carboxylate (35), in 65% yield.The same complex 35 can also be prepared in 74% yield from the reaction of the cyclopropene with (cyclododeca-l,5,9-triene)nickel and triphenylphosphane. Treatment of 35 with 1,2-bis(dimethylphosphino)ethane affords derivative 36 in 88% yield as yellow-orange crystals. 

A nickelacycle 10, obtained from cyclopropabenzene 9 and Ni(cod)(PMc3)2, upon carbonyl-ative decomplexation gives a pentacyclic cyclopentanone 11 with two cyclopropane subunits. ... 

The formation of the dimerization products can be envisioned as resulting from reductive elimination from nickelacycle 7. Besides 2, which directly results from this process, the... 

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