Allenylidene complex

Since the vinylcarbenes la-c and the aryl substituted carbene (pre)catalyst Id, in the first turn of the catalytic cycle, both afford methylidene complex 3 as the propagating species in solution, their application profiles are essentially identical. Differences in the rate of initiation are relevant in polymerization reactions, but are of minor importance for RCM to which this chapter is confined. Moreover, the close relationship between 1 and the ruthenium allenylidene complexes 2 mentioned above suggests that the scope and limitations of these latter catalysts will also be quite similar. Although this aspect merits further investigations, the data compiled in Table 1 clearly support this view. 

Although the ruthenium allenylidene complexes 2 have not yet been as comprehensively studied as their carbene counterparts, they also seem to exhibit a closely related application profile [6]. So far, they have proven to tolerate ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and free secondary hydroxyl groups in the substrates (Table 1). 

In the case of macrocyclic rings, the situation is better understood. In contrast to earlier statements in the literature [3a], even diene substrates devoid of any conformational pre-disposition towards ring closure turned out to be excellent substrates for macrocylization reactions catalyzed by ruthenium-carbene or -allenylidene complexes. From these investigations [30], however, a set of parameters has been deduced which turned out to be decisive ... 

The allenylidene complex formation is indicated by a color change from yellow to purple and can be monitored by the disappearance of the vinylidene p-H NMR resonance. The reaction is completed by heating under reflux for some hours. The neutral allenylidene complexes are rather stable towards oxygen and water. According to the H, and NMR spectra, two isomers of the... 

Abstract Allenylidene complexes have gained considerable significance in the context of transition-metal carbene chemistry due to their potential applications in organic synthesis. The aim of this chapter is to draw together a general presentation of the most efficient synthetic routes, the main structural features and reactivity patterns, as well as current applications in homogeneous catalysis, of aU-carbon-substituted allenylidenes and related cumulenylidene complexes containing an odd number of carbon atoms. 

Fig. 1 Major mesomeric forms of allenylidene complexes of types A and B...

As commented in the introduction of this chapter, the most general synthetic approach to allenylidene complexes employs propargylic alcohols HC=CCR R (OH) as source of the unsaturated C3 skeleton. In 1982, Selegue introduced for the first time this synthetic strategy for the high yield preparation of the cationic Ru(II)... 

Scheme 1 Synthesis of the allenylidene complex 1 from l,l-diphenyl-2-propyn-l-ol...

Selegue s route has been widely used during the last two decades for the preparation of transition-metal allenylidene complexes, its efficiency allowing the access to a huge number of representatives. Although other synthetic alternatives of aUenylidenes are presently known, their applications have been comparatively less developed. In the following subsections updated syntheses of allenylidene complexes are presented by Periodic Group,... 

Fig. 5 Typical half-sandwich ruthenium fragments used in the preparation of allenylidene complexes. Ancillary ligands include CO, mono- and bidentate phosphines or N-heterocyclic carbenes...

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