Addition Reactions

This Chapter Contains Sections titled: 2 + 2-Cycloaddition 2 + 3-Cycloaddition 2 + 4-Cycloaddition Miscellaneous References Tetrafluorothiophene S,S-dioxide is a potent cycloaddend that reacts as a Diels–Alder diene, a dienophile, and a 2+2-addend. Both Diels–Alder and 2+2-cycloadditions can occur with terminal alkynes. Formal 4+1-/4+3-cycloaddition reactions of diazo esters with hexahydro-1,3,4-triazines have been accomplished using a gold complex. The reactions provide evidence of the involvement of a gold metalloenolcarbene intermediate. Mechanistic studies indicate that the triazines react directly with the metal carbene. An extensive review of 2+2-cycloaddition reactions of allenes, allenenes, allenamides, ketenes, alkynes, siloxy alkynes, phenylthioacetylene, and ynamides promoted by metalbased catalysts has been published. A further review covers all aspects of the 2+2-photocycloaddition chemistry of synthetically relevant regio- and stereo-selective reactions. Copper(I) and photoinduced electron transfer catalysis together with direct excitation or sensitization are discussed.