Retro-allylation and Deallylation

Cleavage of Carbon-Carbon Single Bonds by Transition Metals, First Edition. 

In 1996, when transition-metal-catalyzed carbon-carbon bond cleavage was just emerging as a current topic in organic chemistry, Kondo and Mitsudo reported the first transition-metal-catalyzed retro-allylation of homoallylic alcohol to simply cleave off the allylic moiety with a ruthenium complex [7]. After 10 years of silence, transition-metal-catalyzed retro-allylation has been rapidly developing since 2006. Nowadays, catalytic retro-allylation can be regio- and stereoselective, hence being a useful tool for modern organic synthesis [8]. 

The configurations of o-allylic transition metal complexes are generally fluxional, and the initially formed o-allylic metal can isomerize into other isomers, including Jt-allylic metal, resulting in the loss of the original regio- and stereochemistry. 

It is necessary to suppress this isomerization for selective reactions. The allylic transition metal complexes generated in situ can undergo protonation, fi-hydride elimination, nucleophilic addition, or reductive elimination. The fate of the initially formed o-allylic metal species depends on the reaction conditions, mainly on the character of the transition metal used. With proper conditions, retro-allylation is a useful tool in organic synthesis. This section categorizes the chemistry of retro-allylation according to the transition metals used. 

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Kondo and Mitsudo are the pioneers of transition-metal-catalyzed retro-allylation [7]. They described this phenomenon as deallylation and fi-allyl elimination . Ruthenium-catalyzed reaction of tertiary homoallylic alcohols proceeds in the presence of allyl acetate under an atmosphere of carbon monoxide in tetrahydro-furan (THF) at a relatively high temperature of 180 C to afford the corresponding ketones (Scheme 5.3). Although the exact roles of allyl acetate and carbon monoxide are not clear, they propose a mechanism involving oxidative addition of the... 

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