Allenylidene enyne metathesis

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 ... 

Allenylidene-Ruthenium Complexes in RCM, Enyne Metathesis and ROMP... 

The allenylidene-mthenium complexes I also catalyze the enyne metathesis to alkenylcydoalkenes with a 1,3-diene stmcture. Initial studies showed the transformation of simple enynes with ether function [37] (Scheme 8.5). This reaction was significantly accelerated by initial catalyst photodiemical activation, which is now understood to favor the rearrangement of the allenylidene- into the active indenylidene-ruthenium moiety and arene displacement. 

Scheme 8.5 Enyne metathesis catalyzed by the photochemically activated ruthenium allenylidene precursor la.

In parallel, since the first preparation of allenylidene-metal complexes in 1976, the formation of these carbon-rich complexes developed rapidly after the discovery, in 1982, that allenylidene-metal intermediates could be easily formed directly from terminal propargylic alcohols via vinylidene-metal intermediates. This decisive step has led to regioselective catalytic transformations of propargylic derivatives via carbon(l)-atom bond formation or alternately to propargylation. Due to their rearrangement into indenylidene complexes, metal-allenylidene complexes were also found to be catalyst precursors for olefin and enyne metathesis. 

The allenylidene complex [RuCl(=C=C=CPh2)(PCy3)(p-cymene)] PFg la was observed to be a very active enyne metathesis catalyst, particularly under reaction conditions utilizing UV irradiation (300 nm) at 80 °C, where reactions were found to be complete in only 0.5 h [24, 25]. The allenylidene complex la was also shown to be an active catalyst for the ROMP of unstrained, cycUc olefins after its exposure to either heat (80 °C) or UV irradiation [26]. Heat or light transforms the allenylidene complex into the catalyst, which was later... 

The allenylidene-ruthenium system also appeared as effieient catalyst precursors for enyne metathesis and they were applied to the preparation of fluorinated cyclic amino esters with a 1,3-diene structure allowing Diels-Alder reactions (Scheme 20) [43],... 

Scheme 20. Amino esters by eonsecutive enyne metathesis/Diels—Alder reaction. 4.2 Allenylidene-ruthenium and polymerization...

Actually, applications of indenylidene-ruthenium complexes for alkene metathesis were reported before, at a time when the action mode of their ruthenium allenylidene precursors was not known. These complexes catalyzed a variety of RCM reactions of dienes and enynes [31, 32, 47] (see Section 8.2.2). 

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