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2.6 Metal-Catalyzed (2 + 2 + 2) Cycloadditions

DOI: 10.1055/sos-SD-222-00161

Tanaka, K.; Shibata, Y.Science of Synthesis: Metal-Catalyzed Cyclization Reactions, (20162265.

General Introduction

The transition-metal-catalyzed (2 + 2 + 2) cycloaddition is a useful and atom-economical method for the synthesis of substituted six-membered-ring molecules. The reaction has been extensively investigated and this research topic has been thoroughly reviewed.[‌1‌‌13‌] Compared with the well-established transition-metal-catalyzed cross-coupling approach, the transition-metal-catalyzed (2 + 2 + 2)-cycloaddition approach is advantageous for the synthesis of densely substituted six-membered carbocycles and heterocycles. In the (2 + 2 + 2) cycloaddition, many different substituents can be introduced through the formation of the six-membered ring. This chapter discusses the metal-catalyzed (2 + 2 + 2) cycloaddition of not only alkynes but also heterocumulenes, alkenes, and carbonyl compounds for the synthesis of six-membered carbocycles and heterocycles. Asymmetric variants of these reactions will also be discussed. Although this chapter focuses on catalytic reactions, (2 + 2 + 2) cycloadditions mediated by stoichiometric amounts of metal complexes have also been reported.[‌1‌‌13‌]

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References