Allenylidenes: their multifaceted chemistry at rhodium

Abstract

Rhodium allenylidenes, which represent a class of ‘flat’ metallabutatrienes, are useful tools for the generation of metal-bound species that are hardly accessible (sometimes not accessible at all) by other synthetic routes. In contrast to various octahedral allenylidene metal complexes, e.g. of Cr 0 , Mn I or Ru II , the square-planar rhodium compounds trans- [RhX(CCCRR′)(PPr i 3 ) 2 ] are attacked by both nucleophiles and electrophiles. Moreover, non-polar substrates such as H 2 or Cl 2 react cleanly with the title complexes to give products in which the allenylidene unit is preserved as part of a newly formed ligand. Attempts to prepare a square-planar cationic metallaheptahexaene, trans-[Rh(CCCRR′) 2 (PP r i 3 ) 2 ] + , unexpectedly led to the formation of two isomeric hexapentaenerhodium(I) complexes of which the isomer having the polyene coordinated in an unsymmetrical fashion is the thermodynamically more stable. C–C coupling reactions also occur on treatment of trans-[RhCl(CCCPh 2 )(PPr i 3 ) 2 ] with vinyl Grignard reagents or phenylacetylene leading to highly unsaturated RhC 5 and RhC 5 P frameworks from which vinylallenes and phosphacumuleneylides are obtained.