Nickel catalysis cycloaddition reactions

Low-valent transition metal catalyzed versions of [2 + 2] cycloadditions. especially with nickel catalysts, were recognized early as useful alternatives to thermal and photochemical methods12-15. The observation of transition metal catalysis, active in [2 + 2]-cycloaddition reactions, originally caused considerable discussion of the mechanism as an inversion of symmetry rules, effected by the transition metal, may be assumed. Thus, it was suggested that, in the presence of the metal catalyst, a forbidden reaction becomes allowed 16,17. This interpretation, however, could not be verified for the overall process, since experimental investigations revealed a stepwise mechanism with metallacycle intermediates18-23. 

Despite the tremendous range of thermal and Lewis acid-promoted [4+2] cycloadditions that can be routinely accomplished, the nickel-catalyzed counterpart to these reactions provides unique opportunities for efficiency and selectivity. The [4+2] cycloaddition of 1,3-dienes with alkynes is a subclass of reactions that enjoys particular benefits from nickel catalysis. In illustrations involving assembly of the all-carbon cyclohexadiene units, reports from Wender described the efficient intramolecular cycloaddition of 1,3-dienes with alkynes (Scheme 3-20). Notably, the corresponding thermal processes either failed to proceed or required harsher conditions, providing substantially lower yields than the corresponding nickel-catalyzed processes. The stereochemistry of the diene is conserved in the cycloadditions, as a comparison in Scheme 3-20 of E,E- and ,Z-dienes illustrates. 

Even cyclopropanes lacking a methylene or vinyl group can be involved in formal cycloadditions. Cyclopropyl ketones 11.137 undergo cycloaddition to enones under nickel catalysis with an NHC ligand (Scheme 11.47) in the absence of the enone, their dimerization is observed. The reaction is proposed to proceed via a metallacyclic enolate complex 11.140, perhaps after initial oxidative addition to one of the cyclopropyl C-C bonds. 

Transition metal catalysts can exert an important influence on the regio-selectivity of formal cycloaddition reactions. Cycloaddition of butadiene and XLIV in the absence of metal catalysts takes place at 135° giving a 20 1 mixture of XLV and XLVI. Catalysis by the Ni(acac)2-triphenylphosphine-triethylaluminum system affords XLVI, XLVII as the major product, and only a trace of XLV (Garratt and Wyatt, 1974). A bis(7r-allyl)nickel(0) complex (XLVIII) has been suggested as a reaction intermediate. 

Scheme 3.5 Domino carbonyl ylide formation-l,3-dipolar cycloaddition reaction catalysed by a combination of rhodium catalysis and chiral nickel catalysis.

Bicyclopropylidene (1) also reacts with activated alkenes under transition-metal catalysis. With electron-deficient alkenes under nickel(O) catalysis, the [2-1-2] cycloadduct 263 was the main component in the reaction mixture [2b, 150]. Under palladium(O) catalysis, formal [3-1-2] cycloaddition of electron-deficient (Scheme 60) as well as strained alkenes can be achieved exclusively... 

In contrast to the purely thermal cycloaddition which occurs in a nonstereospecific manner with respect to alkenes in low yield, nickel(0) catalysis results in a highly stereospecific cycloaddition, as demonstrated by the reactions between (11 equation 4) and dimethyl fumarate or dimethyl maleate. 

A suitable approach to the synthesis of spiro[2.3]hexanes is the [2-1-2] cycloaddition of alkenes to the double bond of methylenecyclopropanes. This reaction is often described as a codimerization and usually requires catalysis by a nickel(O) complex such as bis(cycloocta-l,5-diene)nickel. l,l-Dimethyl-2-methylenecyclopropane reacts with alkyl acrylates to give a mixture of alkyl cis- and tranj-l,l-dimethylspiro[2.3]hexane-5-carboxylates 1 (19-40%) and alkyl 3,3-dimethyl-4-methylenecyclopentanecarboxylate 2 (60-81 %). The proportion of spiro [2.3]hexane derivative was highest when rer/-butyl acrylate was used as the activated alkene. 

Only a limited number of vinyl sulfones, e.g. phenyl ( )-2-phenylvinyl sulfone (14), undergo codimerization with MCR Homocyclodimerization of MCP is the most efficient side reaetion. Interestingly, yields and product distributions are solvent dependent. No reaction takes place with catalytic amounts of bis(t -cycloocta-l,5-diene)nickel(0)/triphenylphosphane. In this case the vinyl sulfones are strongly coordinated to the catalyst metal, thus preventing interaction with MCP. When the sulfones bear alkyl-substituted vinyl groups, isomerization to yield allyl sulfones usually proceeds faster than cycloaddition, at least in the case of palladium(O) catalysis. 

Alkynes can also serve as substrates in [3 -I- 2] cycloadditions to MCP, as exemplified by the reaction of but-2-yne under nickel/phosphite catalysis to provide l,2-dimethyl-4-methylene-cyclopent-l-ene (2). 2 However, alkyne oligomerization, specifically cyclotrimerization, cannot be avoided with alkyl-substituted alkynes. When alkynes with electron-withdrawing substituents are employed, cyclotrimerization becomes the exclusive reaction. 

As shown in Section 2.2.2.3.1., bicyclopropylidene (1) is capable of undergoing [2-1-2] cycloadditions with electron-deficient alkenes such as diethyl fumarate under nickel(O) catalysis. The [3 -I- 2] cyclodimer and a cyclotrimer are obtained only as minor products from this reaction. In contrast, exclusive [3 -I- 2] cycloaddition can be achieved with many other substrates when palladium(O) catalysts are employed. These cycloaddition products are also produced with phosphite-modified nickel(O) catalysts, but both yields and selectivities are markedly lowered. The reactions of 1 with norbornadiene and norbornene serve as examples for the reaction with strained hydrocarbons, providing the cyclodimers 2 and 3 in 61% and 66% yield, respectively. ... 

A representative of the hetero-Diels-Alder reaction of inverse electron demand is the cycloaddition of A-sulfonyl-l-azadienes with vinyl ethers. It is amenable to asymmetric catalysis, for example, by a nickel(II) complex of 101. ... 

Butadiene dimerisation is very sensitive to catalysis by zero-valent nickel complexes, which can direct the reaction towards 1,2-cycloaddition or (4-1-4) cycloaddition, through a bis-7r-allyl intermediate, with small amounts of Diels-Alder product . Larger quantities of the latter were obtained in other cases . This and other examples of activation of olefins by transition metal complexes have been associated with excitation of the coordinated 7r-system . 

The nickel-iminophosphine-catalysed 4- -2-cycloaddition of enones with allenes formed highly substituted dihydropyrans. The enantioselective amine-catalysed 4-I-2-cycloaddition of allenoates with oxo-dienes produced polysubstituted dihydropyrans in high yields and with high enantioselectivities. Novel enam-ine/metal Lewis acid bifunctional catalysis has been used in the asymmetric inverse-electron-demand hetero-Diels—Alder reactions of cyclic ketones with Q ,j9-unsaturated a-ketoesters. The 4- -2-cycloaddition of acylketenes (80) with 2-unsubstituted and 2-monosubstituted 3-aryl-2//-azirines (81) produced 1 1 (82) or 2 1 (83) adducts, being derivatives of 5-oxa-l-azabicyclo[4.1.0]hept-3-ene or 5,7-dioxa-l-azabicyclo[4.4.1]undeca-3,8-diene. The formation of the monoadducts proceeds via a stepwise non-pericyclic mechanism (Scheme 25). A-heterocyclic carbene-catalysed 4- -2-cycloaddition of ketenes with 1-azadienes yielded optically active 3,4-dihydropyrimidin-2-ones (93% ee) ... 

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