Alkylidene... s. a. Methylene

Alkylene oxides s. Oxido compds. Alkyliden... s.a. Methylen... Alkylideneamino... s.a. Azomethines y-(Alkylideneamino)acetals... 

A similar synthesis of (Z)-a-alkylidene-y-lactones from alkenyl iodides was reported by Luo et al. In a related procedure,the presence of an acetyl group a to the hydroxyl group of the iodide led to the formation of a,/S-unsaturated butenolides. The presumed intermediate a-methylene lactone probably undergoes isomerization to the more stable butenolide under the reaction conditions, as shown in Scheme 8. 

Scheme 4.17 2-Alkylidene-5-oxoalkanoates 66 as precursors of a-methylene-S-lactones 65.

An interesting route to a-carboxy-8-lactones (81) and a-methylenelactones (80), based on hydrolysis of Knoevenagel products (79) of Meldrum s acid with cyclic aliphatic ketones (78), has been developed (Scheme 14). Reduction of 5-methylene derivatives of Meldrum s acid has been performed catalyti-caiiyi30 or by use of LAH. Imidoylation reaction of Meldrum s acid and subsequent solvolysis of the resulting (82) yields -enamino esters (83) in good yields.Flash vacuum pyrolysis of alkylidene derivatives of Meldrum s acid can be used to prepare methylene ketenes (84), a class of compounds difficult to prepare by conventional methods. By this procedure, methylene ketenes are obtained from aromatic aldehydes and ketones and from aliphatic ketones in only two steps. Intramolecular trapping of the methylene ketene obtained from the ketone (85) has been used successfully in the synthesis of the naphthol (86). ... 

It is Schrock s methylene complex that was isolated and characterized first, this complex being more stable partly because of a weaker polarity, which allowed the determination of its X-ray crystal structure that confirmed the Ta=CH2 double bond. This double bond is also apparent in H NMR, and the two methylene protons remain non-equivalent up to at least lOO C, the decomposition temperature AG > 22 kcal mol" or 92 kJ mol" ). When the methylene bears one or two alkyl substituents, the carbene complexes are alkylidene complexes. - The electronreleasing property of these substituents strengthens the triplet structure of the carbene and the nucleophilicity of the metal-alkylidene complex, although it does not turn a Pettit-type carbene complex nucleophilic. 

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