Cumulenes metallation

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

Since cumulenes and alkynes are often easily interconvertible, many syntheses discussed above have allenic counterparts, especially base-catalyzed cyclizations of allenic alcohols.77 And, of course, several of the alkyne-based syntheses may well have allenic intermediates. There are, however, a few syntheses based specifically upon allene chemistry. In an important one, due to Stirling and his collaborators,78 an allenic sulfonium salt reacts with an enolate anion. Scheme 12 sketches the main features yields as high as 86% are recorded. Methoxyallene is easily metallated by butyllithium and so converted into an allenic epoxide that can be isomerized by fe/T-butoxide into a furan (Scheme 13) or an exocyclic equivalent similar to 15 clearly this method is particularly suited to the preparation of 3-methoxyfuran... 

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

In principle, three basically different types of reaction modes are applied for cross-coupling reactions of allenes. First, cross-couplings of allenes with suitable halogen or metal substituents at one of the sp2-hybridized carbons furnish products still bearing the intact cumulene it-system. On this basis, numerous reactions for conversions of precursor 1 or 3 into substituted allenes 2 have been developed (Schemes 14.1 and 14.2). 

In a search of jr-donor systems for the preparation of compounds having a metallic conductivity, the bis-thioxanthene cumulene 56 was obtained. It was oxidized by cone. H2SO4 to the acetylenic dication 57 rather than undergoing the expected protonation of the multiple bonds (equation 22)36. 

As already mentioned, CAAC ligands can stabilize electron-deficient metal centers such as in cationic gold complexes. Complex 29 catalyzes a very unique reaction of enamines with acetylene, which produces a cumulene and an imine... 

Besides the classical additions of carbon-centered nucleophiles to the electrophilic sites of the cumulenic chain, transition-metal allenyhdenes are able to promote... 

Transition-metal allenylidenes are prone to undergo cycloaddition and related cyclization reactions involving both M=Co-, Co,=Cp, and Cp=Cy bonds of the cumulenic chain. In some cases, cyclization/cycloreversion pathways have been observed leading to the final isolation of acyclic products. 

Metal-catalyzed substitution reactions involving propargylic derivatives have not been studied in much detail until recently [311, 312]. In this context, the ability shown by transition-metal allenylidenes to undergo nucleophilic additions at the Cy atom of the cumulenic chain has allowed the development of efficient catalytic processes for the direct substitution of the hydroxyl group in propargylic alcohols [313]. These transformations represent an appealing alternative to the well-known and extensively investigated Nicholas reaction, in which stoichiometric amounts of [Co2(CO)g] are employed [314-317]. 

Aspects related to the chemistry of the heteroatom-terminated -carbon ligands R3P C=C and C3O have also been discussed. Thus, upon coordination, the former seem to present a partial cumulenic character [M]=C=C=PR3, but little is known about the chemical behavior of this coordinated unit. In the case of the tricarbon monoxide ligand, recent theoretical calculations have shown that coordination chemistry could be an alternative to stabilize this highly unstable heterocumulene. However, the access to metal complexes containing the C3O unit represents an exciting experimental challenge for the near future. 

Preparation and Reactivity of Higher Metal Cumulenes Longer than Allenylidenes... 

Cyclodimerization of cumulenes can take place either by thermal activation (see Section 1.3.1.1.2.) or by metal catalysis. Nickel catalysts promote the cyclodimerization of buta-1,2,3-trienes (e.g., 8) to 4-radialenes (e.g., 9).21,22... 

Metal-catalyzed cyclodimerizations of cumulenes can often result in different regiochemistry compared to their thermal counterpart. For example the nickel-catalyzed dimerization of the cumulene 22 gives exclusively the 4-radialene 23 whereas the thermal reaction gives the symmetrical 4-radialene 24.29 Metal-catalyzed cyclodimerizations of alkenes as seen in this and other examples often proceed under mild conditions. However, as the result of the intervention of metal-complexcd intermediates, regio- and stereochemistry may differ from their thermal counterparts. 

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