Allenylidene reactivity

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established [26 0]. They are governed by the electron deficient character of the and Cy carbon atoms in the cumulenic chain, the Cp being a nucleophilic center. This can also be rationalized by considering the mesomeric forms depicted in Fig. 14. However, it should be noted that, as commented on previously, only contribution from the alkynyl resonance form is supported by X-ray diffraction. 

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established, being governed by the electron-deficient... 

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

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. 

Cp=Cy bond of indenyl-allenylidene complexes 30 which leads to the stereoselective formation of cationic amino-allenylidenes 31. When R = Ph, complexes 31 can be transformed into the secondary derivatives 32 via treatment with LiBHEts and subsequent purification on sUica-gel column. Further insertions of MeC=CNEt2 into 32 allow the preparation of polyunsaturated cumulene chains (related insertion reactions will be discussed in the reactivity section). 

In accord with the expected trans influence of the 71-acceptor aUenylidene unit [212], substitution of the chloride ligand by different anionic nucleophiles in complexes frans-[MCl(=C=C=CR R )(Pi-Pr3)2] (M = Rh, Ir) is favored, affording new aUenylidene derivatives frans-[MX(=C=C=CR R )(P/-Pr3)2] (X = I, F, OH, N3, etc.) (see reactivity studies below). Of particular interest is the behavior of the Rh(I) species frans-[RhCl(=C=C=CPh2)L2] (L = Pf-Pr3, f-Pr2AsCH2CH20Me) towards NaCsHs since the reactions lead to the clean formation of complexes 40 (Scheme 13), the only half-sandwich-type Group 9 allenylidenes presently known [206, 209]. 

The regioselectivity of the nucleophilic additions on allenylidene complexes (C vs Cy) is subtly controlled by the electronic and steric properties both of the substituents on the unsaturated hydrocarbon chain and the ancillary ligands on the metal atom, as well as by the nucleophile employed. In this section we will summarize the nucleophilic reactivity of mononuclear Group 6-9 allenylidenes. 

The reactivity of Group 6 allenylidenes [M(=C=C=CR R )(CO)5] (M = Cr, W R and R = alkyl, aryl or H) towards nucleophiles is clearly dominated by the additions at the electrophilic a-carbon. In this sense, the most common reaction of these complexes (usually generated in sim) is the addition of alcohols R OH across the C =Cp bond to afford Fischer-type a,p-unsaturated alkoxycarbene... 

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

Concerning the M=Co, bond, most of the reported examples result from inter- or intramolecular additions of anionic nucleophiles containing at least two reactive heteroatoms. Thus, sodium dimethyldithiocarbamate was found to react with the cationic allenylidene [RuTp(=C=C=CPh2)(PPh3)2] [PFg] (76) to generate the alle-nyl-metallacycle 77 (Scheme 26) as the result of the nucleophilic addition of one of the sulfur atoms at the Cq, carbon and subsequent coordination of the second sulfur to the ruthenium center, with concomitant release of a triphenylphosphine ligand [282]. Complex 77 could also be synthesized by treatment of the neutral derivative... 

In this chapter an updated state of the art of the chemistry of allenylidene and higher cumulenylidene complexes has been presented. The spectacular growth of this chemistry seen in recent years stems mainly from the versatile reactivity of... 

Bmneau C, Dixneuf PH (eds) (2008) Metal vmylidenes and allenylidenes in catalysis from reactivity to applications in synthesis. Wiley, Weinheim... 

This unusual reactivity was subsequently extended to a broad variety of allylic and propargylic amines, in combination with other ruthenium(II) metal fragments, allowing the isolation of aminoaUenylidene complexes 33-35 [47 9] (Fig. 6). In addition, the S3mthesis of the thio-allenylidene [44] and seleno-aUenylidene [50] derivatives 36 from the in situ generated butatrienylidene tra s-[RuCl (=C=C=C=CH2)(L2)2] (L2 = dppm, dppe) and the corresponding aUyl sulfides... 

Scheme 13 Reactivity of chromium allenylidene complexes 5 towards amines...

As commented previously, alkenyl(amino)allenylidene ruthenium(II) complexes 41 are easily accessible through the reaction of indenyl-Ru(ll) precursors with ynamines (Scheme 10) [52-54]. Based on this reactivity, an original synthetic route to polyunsaturated allenylidene species could be developed (Scheme 19) [52, 53]. Thus, after the first ynamine insertion, complex 41 could be transformed into the secondary derivative 62 by treatment with LiBHEts and subsequent purification on silica-gel column. Complex 62 is able to insert a second ynamine molecule, via the cyclization/cycloreversion pathway discussed above, to generate the corresponding dienyl(amino)allenylidene species. Further transformations of this intermediate in the presence of LiBHEts and Si02... 

Cadiemo V, Gamasa MP, Gimeno J (2001) Eur J Inorg Chem 571-591 Bmneau C, Dixneuf PH (eds) (2008) Metal vinylidenes and allenylidenes in catalysis from reactivity to applications in synthesis. Wiley, Weinheim Cadiemo V, Gimeno J (2009) Chem Rev 109 3512-3560... 

Preparation and Stoichiometric Reactivity of Metal Allenylidene Complexes... 

In this chapter we have tried to give a general presentation of the most efficient synthetic routes, the main characteristic structural features and a discussion of the reactivity patterns. Special attention is devoted to the chemistry of mononuclear derivatives containing linear allenylidene groups as they are most frequently the active species in catalytic processes. Illustrative examples of the synthetic applications via stoichiometric reactions will also be covered. For more comprehensive information we refer the reader to the reviews and accounts mentioned above [3, 4, 7]. 

The use of methanol or ethanol as solvent (or sometimes the molecule of water resulting from the spontaneous dehydration) often leads to the isolation of a Fischer-type alkoxy- or hydroxy-carbene [M]=C(OR)CH=CR R instead of the desired allenylidene. Addition of nucleophiles to allenylidenes dominates the reactivity of these electrophilic groups (see below). Nevertheless, in some cases, the use of silver (I) salts Ag[X] (X = PFg, TfO, BF4 ) results in a more practical and flexible synthetic method since the use of nucleophilic polar solvents can be avoided. 

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