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).

本文报道了镍催化不饱和烃与羰基化合物的环加成反应。反应以二炔和烯炔作为偶联底物，羰基类底物涵盖醛与酮两类。二炔与醛的反应可高产率生成[3,3]电开环互变异构体，而非吡喃类产物。烯炔与醛的环加成反应可得到两种不同产物：在镍烷氧基中间体发生竞争性β-氢消除之前，羰基碳可分别与烯烃或炔烃的任一碳原子形成新的碳-碳键。烯炔的空间位阻对其与醛的环加成反应的化学选择性具有显著影响，部分情况下也可生成二氢吡喃产物。本研究将环加成反应的底物范围拓展至烯炔与酮的偶联体系：烯炔与酮的环加成反应中未观察到β-氢消除过程，仅发生碳-氧键形成的还原消除，以优异产率生成二氢吡喃产物。所有反应均表现出完全的化学选择性，仅得到羰基碳与烯烃（而非炔烃）碳原子形成碳-碳键的二氢吡喃产物。所有环加成反应均在室温下进行，所使用的镍催化剂负载有位阻型配体1,3-双(2,6-二异丙基苯基)咪唑-2-卡宾(IPr)及其饱和类似物1,3-双(2,6-二异丙基苯基)-4,5-二氢咪唑啉-2-卡宾(SIPr)。