Ethylidene complexes

Reactions with Other Substrate Complexes. The ruthenium analog of 45, coordinatively unsaturated RuCl(NO)(PPh3)2, also yields methylene and ethylidene complexes on treatment with diazomethane and diazoethane (39,85). 

Transition organometallic hydride complexes CpFe(C0)PPh3(H), CpFe(Ph2PCH2CH2PPh2)H, and HFeCCO) " also reacted with a-alkoxy-ethylidene complexes 10,22 (64), but two pathways were observed. 

The chemistry of the 1 1 and 1 2 complexes differs with respect to hydrogenation (84,89). The 1 2 derivatives are inert to hydrogenation, while the 1 1 compounds are smoothly transformed into an ethylidene complex (see Scheme 1). This difference in behavior may well reflect the cause of differences in behavior of olefins on metal surfaces toward hydrogenation. The ethylidene complex may be converted back to the olefin adduct by reaction with trityl ion. The ethylidene adduct was first obtained for ruthenium by interaction of ethylene with H RujfCO) (89), and is structurally related to the corresponding cobalt derivatives, Co3(CO)9RC. As discussed above, the structure has been established in detail and involves a capping of the metal triangle... 

Stereoselectivity) is observed however, for ethylidene complexes of Fe(CO)(PR3)Cp (69) the products reflect trans selectivity. This difference in stereoselectivity has been suggested to be dependent upon which conformer is more reactive. The reaction of a chiral-at-iron cationic carbene complex (70) with styrene or vinyl acetate affords optically active cyclopropane products with high enantioselectivity (Scheme 24). h >3 intramolecular cyclopropanation, as in the case of (71), proceeds moderately well for the formation of norcarane-type ring systems however, intramolecular C-H insertion is a competing pathway when the alkene is highly... 

Flash-vacuum pyrolysis of spiro[2.5]octa-4,6-diene-tricarbonyliron complex 57 gave the free ligand 58 in 53% yield accompanied by ring-opened products, while the analogous 7-methylene and 7-ethylidene complexes of 57 did not give the unchanged free ligand under the same conditions. ... 

In a similar way, the complexes 10 and 11 react with simple alkenes to produce new alkylidenes. For example, the reaction with 2-butene results in the formation of the stable ethylidene complex 16. 

The smoothly descending distribution in deuteroethanes (obtained for solutions in 50% acetic acid) may be explained by both mechanisms, the one involving the ethylidene complex and the other consistent with only ji-complex formation, hi effect, if we accept that the conversion... 

The term alkylidene refers to carbenes, CR2 with alkyl substituents for example, MeCH=ML is an ethylidene complex but alkylidene is sometimes used as a synonym for Schrock carbene in the older literature because the first alkylidenes were of the Schrock type. There are electrophilic Fischer alkylidenes as well as nucleophilic Schrock ones, however, so the terms should be kept separate. For example, [Cp2W(=CH2)Me], and Cp2Ta(=CH2)Me are isoelectronic, but the former is electrophilic and the latter nucleophilic at the carbene carbon the net positive charge on the tungsten complex must stabilize the M(d,) levels and is therefore probably the main reason for the difference. 

The addition of base reverses the first step by a nucleophilic abstraction. The ethylidene complex readily gives a 1,2 shift of the p proton to give the thermodynamically more stable alkene complex. Even carbene.s that lack p hydrogens can be unstable [Cp(CO)2Fe=CH—CMesI and Cp(CO)2Fe=CH-CMe2Phl ... 

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