Addition reactions of mononuclear η3-allenyl/propargyl transition metal complexes:: a new class of potent organometallic carbon electrophiles

The cationic mononuclear eta(3)-allenyl/propargyl complexes exhibit new non-classical organometallic bonding of sigma,pi-mode and remarkable activity of potent carbon electrophiles. The organic moieties of the title species, general formula R'R " CCCR, use all three adjacent unsaturated carbons to bond with metal in a unique coplanar manner. The C-3 skeletons thus are severely distorted from the normal linear configuration to 150 +/- 5 degrees. The R'R " C-C distances are in the range of long double bonds; whereas C-CR are between double and triple bonds, indicating the resonance structures of allenyl and propargyl. The cationic complexes with eta(3)-allenyl/propargyl ligands can undergo feasible reactions of regioselective nucleophilic addition which provide a pragmatic synthetic access to new organometallic species of central-carbon-substituted eta(3)-allyl, eta(3)-oxa- and eta(3)-aza-trimethylenemethane. The broad spectrum of C-Nu bond formation encompasses hydroxylation, alkoxylation, phenoxylation, acyloxylation, thioxylation, thio-and seleno-phenoxylation, amination, amidation, phosphination, etc. As to carbation, the eta(3)-allenyl/propargyl complexes react with carbanions to form eta(3)-trimethylenemethane complexes; The electrophilicity of eta(3)-C3H3 allows us to conduct regioselective carbon-carbon coupling with the electron-rich aromatics to achieve aromatic electrophilic substitution. Although both eta(3)-allenyl/propargyl and eta(1)-allenyl complexes are essentially subject to similar nucleophilic addition, the former are distinctly more reactive and versatile. From the mechanistic viewpoint, the cationic eta(3)-allenyl/propargyl complexes prefer to undergo external nucleophilic attack at the central carbon. In contrast, nucleophilic addition to the eta(1)-allenyl complexes requires preceding coordination of the nucleophile to a metal. (C) 1999 Elsevier Science S.A. All rights reserved.