Vinylidene carbocyclization

In this chapter, we focus on catalytic carbocyclization involving ruthenium vinylidene as a reaction intermediate. The carbocyclization reactions involving allenylidene intermediates are described in Chapter 7. The reactions are categorized according to the types of substrates. Several stoichiometric carbocyclizations are also included in this chapter because of their mechanistic significance. 

Ruthenium vinylidene species can be transformed into small carbocyclic rings via carbocyclization reactions. Ruthenium vinylidene complex 2, generated from the electrophilic reaction of alkyne complex 1 with haloalkanes, was deprotonated with "BU4NOH to give the unprecedented neutral cyclopropenyl complex 3 (Scheme 6.2) [5]. Gimeno and Bassetti prepared ruthenium vinylidene species 4a and 4b bearing a pendent vinyl group when these complexes were heated in chloroform for a brief period, cyclobutylidene products 5a and Sb formed via a [2 + 2] cycloaddition between the vinylidene Ca=Cp bond and olefin (Scheme 6.3) [6]. 

Catalytic Carbocyclization via Electrocyclization of Ruthenium-Vinylidene Intermediates 195... 

Catalytic Carbocyclization via Cycloaddition of Ruthenium Vinylidene Intermediates... 

Catalytic Carbocyclization via Cycloadclition of Ruthenium Vinylidene Intermediates 209... 

Carbocyclization Initiated by Addition of C-Nucleophile to Ruthenium Vinylidene 213... 

As terminal alkynes and ethynyl alcohols are the convenient sources to generate ruthenium vinylidene and allenylidene intermediates, many carbocyclizations have been achieved via nucleophilic addition and other activations at the two intermediates. Most reported carbocyclizations appear to be synthetically useful, not only because of their chemoselectivities but also because of their tolerance toward organic functional groups. Additional examples of catalytic carbocyclization based on ruthenium vinylidenes are still growing, and on the basis of the concepts developed here one can expect to see many new applications in the near future. 

Catalytic transformations of alkynes have recently led to tremendous developments of synthetic methods with useful applications in the synthesis of natural products and molecular materials. Among them, the selective activations of terminal alkynes and propargylic alcohols via vinylidene- and allenylidene-metal intermediates play an important role, and have opened new catalytic routes toward anti-Markovnikov additions to terminal alkynes, carbocyclizations or propargylations, in parallel to the production of new types of molecular catalysts. 

The most important discoveries in ruthenium catalysis are highlighted and innovative activation processes, some of which are still controversial, are presented in this volume. They illustrate the usefulness in organic synthesis of specific reactions including carbocyclization, cyclopropanation, olefin metathesis, carbonylation, oxidation, transformation of silicon containing substrates, and show novel reactions operating via vinylidene intermediates, radical processes, inert bonds activation as well as catalysis in water. 

Abstract Several routes to access ruthenium (Ru)-vinylidene complexes are described, the majority of which employ alkynes and a Ru source as the starting materials. The successful application of Ru-vinylidenes as efficient pre-catalysts for the synthesis of carbocyclic and heterocyclic compoimds by ring-closing metathesis (RCM) of a, -dienes, and in the synthesis of polymers by ring-opening metathesis polymerization (ROMP) of cyclooctene, norbomene, 5-substituted norbomene, and dicyclopentadiene is fully illustrated. Relevant aspects concerning the activity and selectivity of this type of Ru complexes in metathesis reactions are highlighted. 

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