ORGANOMETALLIC CHEMISTRY OF ALKENES AND ALKYNES

During the last decade, the chemistry of transition-metal complexes containing unsaturated carbenes such as :C=CRR' (vinylidenes) and :C=C=RR' (allenylidenes) as ligands has attracted a great deal of attention. Using rhodium as an example of an electron-rich metal centre, this paper describes preparative routes for the conversion of terminal alkynes to vinylidenes and allenylidenes and briefly discusses the mechanism of these reactions. Rhodium complexes which contain a linear RC=C-Rh=C=CHR' chain have been prepared from [(eta3-C3H5)RhL2] or [(eta3-CH2C6H5)RhL2] (L = P(i)Pr3) and terminal alkynes and have been used as starting materials for the coupling of C2 units to either enynes or butatrienes. In the course of attempts to obtain square-planar carbene rhodium derivatives regarded as the missing link in the series of double bond systems [Rh]=C=C)(n)RR', where [Rh] is [RhCl(PiPr3)2] and n is 0, 1 and 2, a new catalytic reaction of ethene and diazoalkanes to give trisubstituted olefins has been discovered. Studies aimed at elucidating the mechanism of this process have led to a whole series of novel carbene rhodium complexes which not only react with ethene to. give substituted olefins but also have a great potential as precursors for organic and other organometallic compounds.