Alkene carbon ligands

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

The extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl ligands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of Comprehensive Organometallic Chemistry .1 In the present section, chromium complexes... 

Alkenes bonded to platinum(II) can be displaced by strongly coordinating ligands such as cyanide ion or tertiary phosphines. The displacement of ethylene from Zeise s salt by phosphines is a useful method of preparation of complexes trawa-PtCl2(PR3)2.711 Amines will also displace alkenes from coordination to platinum(II), but this reaction can compete with nucleophilic attack at the coordinated alkenic carbon. The stability of platinum(II) alkene complexes follows the sequence C2H4 > PhCH=CH2 > Ph2C=CH2 555 Ph(Me)C=CH2.712... 

Conjugate Additions of Carbon Ligands to Activated Alkenes and Alkynes Mediated by Lewis Acids... 

Alkenes (unsaturated hydrocarbons with double bonds) are the oldest and most studied carbon ligands, with interest in their study dating from around 1827 [3,185-189]. An important aspect is the structure of 71-alkene and similar n-alkyne complexes. Their structural data is summarized in reviews [186-189] and presented in... 

All the above Mn111 compounds of (196) are stable as solids under dry nitrogen although they slowly decompose in air.535 They also decompose in aqueous base but in acid undergo reversible protonation. The protonation site is probably the alkenic carbon of the /1-diketone fragment equation (20). Ni" complexes of related macrocyclic ligands have been reported to undergo similar protonation reactions.716,717... 

The mechanism proposed by Albertin et al. involves ptotonation of 62 to give the vinylidene complex 63, as confirmed by the highly deshielded signal at 6 382.8 ppm in the spectrum (Scheme 5). The vinyl phospho-nium complex 64 was formed by intramolecular attack of one phosphonite on of the vinylidene ligand. The proton-coupled and -decoupled NMR spectra of 17 allowed clear assignment of the characteristic carbon atoms in the molecule the carbonyl appeared as a multiplet at 8 207.7 ppm, the metallate C-Co at 8 168.4 ppm, and the two alkene carbons at 8 63.8 and 32.1 ppm. 

Lee, V. J. Conjugate Additions of Carbon Ligands to Activated Alkenes and Alkynes Mediated by Lewis Acids, in Comp. Org. Synth, (eds. Trost, B. M.,Fleming, I.), 4, 139-168 (Pergamon, Oxford, 1991). 

Pd(O) species to an allyl chloride, transmetalation of TMSCN with the resulting tz-allylpalladium chloride, conjugate addition of the cyanide ligand of the rr-alIylpalladium cyanide to an activated alkene, and reductive elimination of the carbon ligands. 

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