Nickel-Catalyzed Cycloadditions of Unsaturated Hydrocarbons, Aldehydes, and Ketones

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).