Synthesis of boracyclobutenes by transmetallation; insertion and demetallation reactions of boracyclobutenes and titanacyclobutene complexes

• Main Author: Bauer, Richard
• Other Authors: Stryker, Jeffrey (Chemistry)
• Format: Others
• Language: en
• Published: 2009
• Subjects: boracyclobutene; titanacyclobutene; transmetallation
• Online Access: http://hdl.handle.net/10048/395

A series of new titanacyclobutene complexes were prepared by samarium diiodide-mediated central carbon alkylation of η³-propargyltitanium complexes. The titanacyclobutenes were synthesized in high yield and completely characterized using NMR spectroscopy and, where applicable, single crystal X-ray analysis. For sterically crowded η³-(tert-butyl)propargyltitanocene, attack at the titanium centre by benzyl radical is also observed. Migratory insertion of isonitriles to titanacyclobutene complexes was investigated. Treatment with one equivalent of isonitrile affords iminoacyl titanacyclopentene complexes by insertion on the alkyl side. Subsequent treatment with another equivalent of isonitrile produces titanium-diamidate complexes by vinyl-side insertion and reductive cyclization. Demetallation of iminoacyl titanacyclopentene complexes with mCPBA yields homoallylic amides. Boracyclobutenes were prepared in moderate to excellent yield by the novel transmetallation of titanacyclobutene complexes with dihaloorganoboranes. The transmetallation is efficient with 2-phenyltitanacyclobutenes, however two equivalents of titanacycle are required for clean conversion of 2-methyltitanacyclobutenes. Characterization was achieved using NMR spectroscopy; based on ¹³C NMR data, there appears to be a 1,3 bonding interaction between boron and the β-ring carbon. A cyclic six membered borinic ester was synthesized by transmetallation of a 2-oxatitanocenecyclohex-5-ene complex with dichlorophenylborane. Exposure of boracyclobutenes to protic sources yields products arising from protodeboronation. Lewis acidity, as measured by the Gutmann-Beckett method, is relatively low for a trialkylborane; additionally, stable tetra-coordinate adducts could not be formed upon treatment of boracyclobutenes with anions and amines. Oxidation of boracyclobutenes with dry DABCO·2H₂O₂ produces the expected β-hydroxy ketones, however wet oxidation reagents produce mixtures of β-hydroxy ketone and protodeboronation products. Boracyclobutenes react with aldehydes in a non-selective manner, producing butadienes from an intermediate formed by allylboration, as well as 1,2-oxaborins by an uncommon insertion into the sp³-hybridized carbon-boron bond.