Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.

A mild and general route for preparing 2-pyrones from CO2 and diynes is described. Under only 1 atm of CO2, excellent yields of pyrone are obtained using catalytic amounts of Ni(COD)2 and imidazolylidene ligand, IPr. In addition, stoichiometric reactions between the isolated complex Ni(IPr)2, diynes, and CO2 suggest that the pathway involves initial reaction with CO2 as the key step.

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The nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyls is reported. Diynes and enynes were used as coupling partners. Carbonyl substrates include both aldehdyes and ketones. Reactions of diynes and aldehydes afforded the [3, 3] electrocyclic ring-opened tautomers, rather than pyrans, in high yields. The cycloaddition reaction of enynes and aldehydes afforded two distinct products. A new carbon-carbon bond is formed, prior to a competitive β-hydrogen elimination of a nickel alkoxide, between the carbonyl carbon and either one of the carbons of the olefin or the alkyne. The steric hindrance of the enyne greatly affected the chemoselectivity of the cycloaddition of enynes and aldehydes. In some cases, dihydropyran was also formed. The scope of the cycloaddition reaction was expanded to include the coupling of enynes and ketones. No β-hydrogen elimination was observed in cycloaddition reaction of enynes and ketones. Instead, C-O bond-forming reductive elimination occurred exclusively to afford dihydropyrans in excellent yields. In all cases, complete chemoselectivity was observed; only dihydropyrans where the carbonyl carbon forms a carbon-carbon bond with a carbon of the olefin, rather than of the alkyne, were observed. All cycloaddition reactions occur at room temperature and employ nickel catalysts bearing the hindered 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or its saturated analogue, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolin-2-ylidene (SIPr).

A convenient method for preparing pyridines from air-stable, commercially available catalyst precursors is described. The addition of n-BuLi to Ni(acac)2 and an NHC salt (such as IPr.HCl or SIPr.HCl) rapidly generates an active Ni0/NHC catalyst for the cycloaddition of diynes and nitriles that affords pyridines without a decrease in observed yields. The in situ method also converts diynes and carbon dioxide to the corresponding pyrones.

A mild and general route for preparing pyrans and dienones from carbonyls and diynes is described. Ni imidazolylidene complexes were used to mediate cyclizations between diynes and aldehydes. The reaction of an enyne and an aldehyde afforded a mixture of cyclized products. In addition, a spiropyran was prepared from the cycloaddition of a diyne and cyclohexanone. [reaction: see text]

We incorrectly assigned the regiochemistry of pyrone 25. X-ray analysis revealed that the TMS group is at the 3-position rather than the 6-position. The correct structure is shown here: JA046854R