Addition reactions of mononuclear η 3-allenyl/propargyl transition metal complexes: a new class of potent organometallic carbon electrophiles

Abstract

The cationic mononuclear η 3-allenyl/propargyl complexes exhibit new non-classical organometallic bonding of σ,π-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°. 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 η 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 η 3-allyl, η 3-oxa- and η 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 η 3-allenyl/propargyl complexes react with carbanions to form η 3-trimethylenemethane complexes. The electrophilicity of η 3-C 3H 3 allows us to conduct regioselective carboncarbon coupling with the electron-rich aromatics to achieve aromatic electrophilic substitution. Although both η 3-allenyl/propargyl and η 1-allenyl complexes are essentially subject to similar nucleophilic addition, the former are distinctly more reactive and versatile. From the mechanistic viewpoint, the cationic η 3-allenyl/propargyl complexes prefer to undergo external nucleophilic attack at the central carbon. In contrast, nucleophilic addition to the η 1-allenyl complexes requires preceding coordination of the nucleophile to a metal.