Abstract
Metal-catalyzed [2+2+2] cycloaddition is a powerful tool that allows rapid construction of functionalized 6-membered carbo- and heterocycles in a single step through an atom-economical process with high functional group tolerance. The reaction is usually regio- and chemoselective although selectivity issues can still be challenging for intermolecular reactions involving the cross-[2+2+2] cycloaddition of two or three different alkynes and various strategies have been developed to attain high selectivities. Furthermore, enantioselective [2+2+2] cycloaddition is an efficient means to create central, axial, and planar chirality and a variety of chiral organometallic complexes can be used for asymmetric transition-metal-catalyzed inter- and intramolecular reactions. This review summarizes the recent advances in the field of [2+2+2] cycloaddition.
1 Introduction
2 Formation of Carbocycles
2.1 Intermolecular Reactions
2.1.1 Cyclotrimerization of Alkynes
2.1.2 [2+2+2] Cycloaddition of Two Different Alkynes
2.1.3 [2+2+2] Cycloaddition of Alkynes/Alkenes with Alkenes/Enamides
2.2 Partially Intramolecular [2+2+2] Cycloaddition Reactions
2.2.1 Rhodium-Catalyzed [2+2+2] Cycloaddition
2.2.2 Molybdenum-Catalyzed [2+2+2] Cycloaddition
2.2.3 Cobalt-Catalyzed [2+2+2] Cycloaddition
2.2.4 Ruthenium-Catalyzed [2+2+2] Cycloaddition
2.2.5 Other Metal-Catalyzed [2+2+2] Cycloaddition
2.3 Totally Intramolecular [2+2+2] Cycloaddition Reactions
3 Formation of Heterocycles
3.1 Cycloaddition of Alkynes with Nitriles
3.2 Cycloaddition of 1,6-Diynes with Cyanamides
3.3 Cycloaddition of 1,6-Diynes with Selenocyanates
3.4 Cycloaddition of Imines with Allenes or Alkenes
3.5 Cycloaddition of (Thio)Cyanates and Isocyanates
3.6 Cycloaddition of 1,3,5-Triazines with Allenes
3.7 Cycloaddition of Aldehydes with Enynes or Allenes/Alkenes
3.8 Totally Intramolecular [2+2+2] Cycloaddition Reactions
4 Conclusion
Key words
cycloaddition - cyclotrimerization - transition metal - arenes - heterocycles - asymmetric catalysis - alkynes - homogeneous catalysis