Transition metals metal enyne structures

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

The transition metal-catalyzed enyne cycloisomerization is one of the most important strategies for the synthesis of functionalized cyclic stractures [90]. The reaction has been applied widely in the total synthesis, since polycyclic structures could be nicely assembled from acyclic precursors in a single step. A number of transition metal complexes are capable of catalyzing enyne cycloisomerizations. Among th. ... 

Conjugated enynes possess several of the same transition metal binding modes exhibited by their diyne counterparts, including Jt-coordination, a-carbenoid formation, and a/jt metal complexation (Scheme 35). In some cases, these constructs are very stable and can be isolated, but they are frequently transient structures in which the metal ion acts as a catalyst for enyne cycloisomerisation and ligand transformation reactions. 

Transition metals mainly form n complexes with enynes. Nearly all the metal-mediated enyne cycloisomerization reactions occur via this structural pathway (11). In some cases, it has been shown (Scheme 36) that transition... 

Several reviews of the chemistry of strained cyclic alkynes and arynes and otiho-ztene cyclynes, " which include synthesis, structure, and reactivity of platinum complexes, have also appeared. The behavior of 1,4-enynes in the presence of transition metals has been reviewed. ... 

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