Metalative Reppe Reaction. Organized Assembly of Acetylene Molecules on Titanium Template Leading to a New Style of Acetylene Cyclotrimerization

Ruthenium-catalyzed [2 + 2+ 2] cycloaddition of three different alkynes

“…Although intensive studies have been done to date, there are quite few reports for the selective [2 + 2 + 2] cycloaddition of three different alkynes, due to difficult control of the chemo-and regio selectivity [4]. Early-transition metals such as zirconium [8][9][10] and titanium [11,12] are known to mediate the selective cycloaddition, in which the chemoselectivity is well regulated by stepwise addition of the different alkynes. As for catalytic reactions using late-transition metals, a palladium-catalyzed unique arene formation from two different alkynes and a diyne has been reported by Yamamoto and co-workers, which mechanism is actually not simple [2 + 2 + 2] cycloaddition but dimerization of the alkynes and successive [4 + 2] benzannulation with the diyne [13].…”

5.34 Early Transition Metal Mediated Reductive Coupling Reactions

“…Addition of the ethyl sulfone then results in the formation of a metalated benzene by a sequence that has been proposed to proceed through regioselective [4 þ 2] cycloaddition, isomerization, and then elimination. 149 In a related report, it was also demonstrated that addition of propargyl bromide to a metallacyclopentadiene intermediate results in regioselective [4 þ 2] cycloaddition and elimination to deliver a benzylic organometallic. 150 Ti-mediated [2 þ 2 þ 2] annulation has emerged as a powerful approach to the synthesis of substituted pyridines.…”

Metal‐Catalyzed Multicomponent Reactions

“…This approach is nevertheless limited by the fact that at least one of the alkynes must be symmetrical, and by the necessity of using stoichiometric amounts of metal. A major improvement was introduced by Sato and co-workers, who have developed a titanium route to polysubstituted benzene rings [155]. Dialkoxytitanacyclopentadiene 163 was first prepared from two different and unsymmetrical alkynes.…”