Allenylidene

A -Methylpyrrole oxidatively adds to [Ru(C=C=C=CH2)(PPli3)2(Cp)][PF6] via its C2 center, the product being the allenylidene species 38 [98JCS(D)467]. Alkyne mesomeric form 39 was postulated to make a significant contribution, which explains well the nature of the deprotonated product 40, obtained from 38 and n-butyllithium. 

Transition metal complexes containing allenylidene, cumulenylidene, and related ligands with heterocyclic fragments 98CRV2797. 

Scheme 6/3.31. Cyclization/reconstitutive addition process involving allenylidene ruthenium complexes as reactive intermediates.

Keywords Ruthenium-carbenes, Ruthenium-allenylidenes, Ring closing metathesis, Natural product synthesis, Fine chemicals. 

The direct comparison of 1 and 2 in a variety of RCM reactions also indicates a presumably close relationship between these catalysts (Table 1) [6]. Both of them give ready access to cycloalkenes of almost any ring size > 5, including medium sized and macrocyclic products. Only in the case of the 10-membered jasmine ketolactone 16 was the yield obtained with 2a lower than that with lc this result may be due to a somewhat shorter lifetime of the cationic species in solution. However, the examples summarized in Table 1 demonstrate that the allenylidene species 2 exhibit a remarkable compatibility with polar functional groups in the substrates, including ethers, esters, amides, sulfonamides, ketones, acetals, glycosides and even free hydroxyl groups. 

Table 1.RCM employing Ru-allenylidene catalyst 2aa comparison with literature data using Ru-carbene lc as the catalyst [6]...

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 ... 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

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... 

Bruneau C (2004) Ruthenium Vinylidenes and Allenylidenes in Catalysis. 11 125-153 Bruneau C, D4rien S, Dixneuf PH (2006) Cascade and Sequential Catalytic Transformations Initiated by Ruthenium Catalysts. 19 295-326 Brutchey RL, see Fujdala KL (2005) 16 69-115... 

Addition of HC CCsCSiMes to FeCl(dppe)Cp in MeOH in the presence of NaBPh4 gave allenylidene [Fe =C=C=CMe(OMe) (dppe)Cp ], possibly via the unobserved intermediate [Fe(=C=C=C=CH2)(dppe)Cp ] which adds solvent MeOH across the Cy=Ca bond. " ... 

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