Allenylidene heterocyclizations

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

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...

The central Co,=Cp double bond of an allenylidene backbone can also react with a variety of dipolar organic substrates to yield cyclic adducts. Most of the cychza-tion processes reported occur in a stepwise manner via an initial nucleophilic attack at the Coi atom and further rearrangement of the molecule involving a coupling with the Cp carbon. Representative examples are the reactions of the electron-poor ruthenium-allenylidene complex 46 with ethyl diazoacetate and 1,1-diethylpropar-gylamine to yield the five- and six-membered heterocyclic compounds 82 and 83, respectively (Scheme 29) [260, 284]. 

Going one step beyond, the reaction of these n-donor-substituted Group 6 allenylidenes with bifunctional N,N- or W, 5-dinucleophiles opened up a fruitful route for the synthesis of an extensive family of N- or 5-heterocyclic carbenes. Thus, treatment of complex [Cr =C=C=C(NMe2)Ph (CO)5] with benzamidine, guanidine or thioacetamide has been reported to yield the a,(3-unsaturated carbenes 54 (Scheme 16) [62], arising from nitrogen attack at Cy, subsequent HNMe2... 

The addition of other bifunctional nucleophiles also allowed the preparation, in moderate to high yields, of the five-, seven-, and eight-membered heterocyclic carbene species 55-58 (Fig. 9) which, in some cases, were formed along with minor amounts of other derivatives [63]. Related 1,2,3-diheterocyclizations involving (ethoxy)allenylidene complexes have been described, ethanol instead of HNMe2 being released in this case [62],... 

Otherwise, treatment of the chromium complex 10 with an excess of [W (C0)5(THF)] afforded the tungsten allenylidene 61 by transmetallation of the cumulenic ligand and further addition of W(CO)s to the A-atom of the heterocyclic substituent (Scheme 18) [30]. The related chromium complexes [Cr... 

Azetidines are an interesting class of four-membered heterocyclic compounds, with various biological activities. These derivatives are difficult to synthetize due to ring strain. The allenylidene complex 43 has shown to be a useful substrate to generate this skeleton and to study its evolution into hexahydroquinoline [41]. Treatment of dichloromethane solutions of 43 with 1 equiv of dicyclohexylcarbodiimide at room temperature affords the iminiumazetidinylidenemethyl complex 111, which is isolated as a 4 1 mixture of the isomers Z and E shown in Eq. 5. 

Metal vinylidenes and allenylidenes in catalysis of anti-Markovnikov additions to terminal alkynes and alkene metathesis, particularly with participation and/or formation of heterocycles 06AG(E)2176. 

The classical synthetic methodologies for alkenyl complexes based on the nucleophilic additions to 7r-alkyne complexes and the insertion reactions of alkynes into [M]-H and [M]-R bonds have been employed to prepare a number of alkenyl, butadienyl, and analogous derivatives. Nevertheless, the chemistry of vinylidene and allenylidene complexes has disclosed new routes to alkenyl derivatives, most of them incorporating heterocyclic systems. 

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