Three-component cyclo-coupling

In contrast to these linear couplings of propargyl alcohol derivatives, a three-component cyclo-coupling of 72 proceeded in the presence of 17 and acetic acid to give rise to an alkylidenecyclobutene 73 in 66% yield (Scheme 4.27) [61]. The four-mem-bered ring skeleton vas considered to be derived from the ruthenium(II) cyclobutadiene complex, vhich might be formed via the corresponding ruthenacyclopenta-diene. 

The selective synthesis of the 2-allyltetrazoles 55 by the three-component coupling reaction of the cyano compounds 54, allyl methyl carbonate 5b, and trimethylsilyl azide 42 was accomplished in the presence of Pd2(dba)3.CHCl3 and P(2-furyl)3 (Scheme 19) [55,56]. Most probably, the formation of rj -allyl)(77 -tetrazoyl)-palladium complex 56 took place through [3 + 2] dipolar cyclo addition of 7r-allylpalladium azide 44 with the nitrile 54. The complex 56 thus formed would undergo reductive elimination to form the products 55. 

The reactions initiated by the addition of a carbon-carbon multiple bond to the cation pool bring in unique one-pot transformations otherwise difficult to realize, because the reactive carbocations are existing in the solution in relatively high concentration. For example, a sequential one-pot three-component coupling reactions have been developed (Scheme 5) [9, 10]. [4 -f 2] cyclo addition reactions in which an N-acyliminium ion was used as a hetero diene [11], and cationic carbohydroxylation of alkenes and alkynes using the cation pool method [12] were also accomplished. 

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