Selective generation of free radicals from epoxides using a transition-metal radical. A powerful new tool for organic synthesis

Article abstract of DOI:10.1021/ja00082a021

Bis(cyclopentadienyl)titanium(III) chloride reacts with epoxides by initial C-O homolysis. The regiochemistry of the opening is determined by the relative stabilities of the radicals. Depending on the reaction partners, these radicals undergo intramolecular (hex-5-enyl cyclization) or intermolecular additions to olefins. The resultant radicals are efficiently scavenged by a second equivalent of Ti(III) to afford the corresponding Ti(IV) derivative. Treatment of this intermediate with electrophiles such as H⁺ or halogens provides a route to functionalized cyclopentanes and other useful products. The radical initially formed from an epoxide can also be trapped by H-atom donors such as 1,4-cyclohexadiene or tert-butyl thiol, resulting in an overall reduction of the epoxide. In the absence of a H-atom donor or an olefin, this radical is trapped by Ti(III), resulting in a β-oxido-Ti organometallic species which undergoes facile elimination to give an olefin. The reaction conditions are remarkably mild and are applicable to very sensitive substrates.