Reduction reactions nickel metallacycles

In general, nickel metallacycles react readily with carbon monoxide. In most cases, this reaction is almost immediately followed by metal reduction (Scheme 66) and formation of a cyclic carbonylated organic compound (ketone,250.314,329,346 76,127,128,130,307,320,338 thiolactone, or related heterocycles25 )... 

As in the case of carbonylation, the reaction of alkynes with nickel metallacycles is often followed by reductive elimination. In some cases, the insertion product is stable and can be isolated (e.g., alkyne insertion in nickelalac-tones or fluorinated metallacycles, Equation (99)), but more frequently it is unstable and decomposes rapidly to afford substituted benzenes, dihydronaphthalenes,naphthalenes, or phenanthrenes. " The reaction of Ni(0) dippe complexes with biphenylene and alkynes in the presence of traces of O2 catalytically produces phenanthrenes, in a process that involves the intermediacy of a carbonickelacycle. " ... 

In order to liberate hydrocarbon fragments from metallacycles such as a,a -bipyridyl-5-nickela-3,3,7,7-tetramethyl-cxo-tricyclo[4.1.0.0 ]heptane(l), there have been various attempts to react them with cosubstrates to achieve reductive elimination. When bis(> -cycloocta-l,5-diene)nickel(O) is treated with 2,2 -bipyridyl and 3,3-dimethylcyclopropene, complex 1 is initially formed. Reaction of 1 with dibromomethane results in a 47% yield of 3,3,7,7-tetramethyl-tricyclo[4.1.0.0 ]heptane (2). ... 

Similar to the nickel-catalyzed reactions, metallacycles of palladium are assumed to be intermediates of these conversions. They were isolable in some cases. Thus, 3,3-dimethylcyclopropene with -allyl( ) -cyclopentadienyl)palladium, on cyclizing oxidative addition, formed metallacyclopentanes 20 and metallacyclononanes 21, depending on the reaction conditions. Thermal decomplexation via reductive elimination gave cyclopropane systems and products thereof. ... 

Aryne-nickel complexes, which were carefully studied by Bennett [6, 7], show a different reactivity, since following the insertion of a first unsaturated species, the metallacycles so formed usually undergo a second insertion and subsequent reductive elimination (Scheme 9). Thus, complex 44 undergoes the insertion of two molecules of 3-hexyne to afford 43 in good yield, and double insertion of the asymmetric alkyne t-butylacetylene into complex 44 yields naphthalene 45 with a high regioselectivity attributed to steric factors. Interestingly, the reaction of 44 with the more electron-deficient alkyne hexafluoro-... 

The dearest empirical evidence for the productive involvement of an 1] , O-bound nickel enolate comes from the intermolecular reductive coupling of alkynes and enals (Scheme 8.9) [27]. The extremely high levels of Z-isomer stereoselectivity (>98 2) can best be rationalized via the metallacycle intermediate 5 which undergoes o-bond metathesis to afford nickel hydride 6, followed by reductive eUmina-tion to yield the Z-selective enol silane product 7. A mechanism consisting of a nickel Jt-allyl species would not be expected to lead to high selectivities of Z-enol silanes, and has been imphcated in reactions leading to the selective production of T-enol silanes [28],... 

Whereas the preceding processes involving ring openings of cyclopropanes provide useful [3+2] entries to cyclopentanes, the use of simpler substrates in [3+2] cycloadditions is made possible by the development of reductive cycloaddition pathways. Such reactions were initially developed in the context of stoichiometric processes, where metallacycles were prepared by oxidative cyclizations of enones and alkynes, followed by either protonation or alkylation of the nickel 0-enolate functionality. Catalytic protocols involving various intramolecular combinations of 7t-systems include formal [3+2] reductive cycloadditions of bis-enones to form bicyclooctanols (Scheme 3-33), of enones with unsaturated acyl oxazolidinones to form triquinane derivatives, and of enals with alkynes to form bicyclooctenols. ... 

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