Titanocene-Catalysed Electron Transfer-Mediated Opening of Epoxides

Abstract

The use of epoxides as substrates for radical reactions via electron transfer is described. The discussion of the evolution of stoichiometric reagents focuses on recent developments in the field of titanocene(III) reagents. Special attention is devoted to the emergence of catalytic conditions for this useful procedure that has led to the discovery of reagent‐controlled radical reactions by variations of the cyclopentadienyl ligands.

Cobalt Catalysis Enables Regioselective Ring-opening of Epoxides with Aryl Halides

“…In contrast, the more hindered halide -2-iodotoluene (6c)undergo coupling with styrene oxide (5a) in reduced reaction yield (compare 7aa, 7ab, and 7ac). Though vitamin B12 exhibits exquisite reactivity in dehalogenation reactions, 11 which often precludes the use of halogenated substrates, in our conditions product 7ad forms in 44% yield. Importantly from the standpoint of possible further functionalizations, other functional groups: hydroxyl, carbonyl, protected amine remain unaffected.…”

“…On the other hand, in the presence of a titanocene co-catalyst secondary alkyl radicals are generated facilitating the formation of branched products. 10,11 Aryl epoxides, however, react predominantly at the benzylic position, regardless of the conditions employed. A similar reactivity pattern has been recently reported by the Doyle group who used organic iodides and the Ti/Ni/photoredox catalytic system in ring-opening reactions of three major classes of epoxides, namely: aryl, aliphatic, and bicyclic.…”

Total synthesis and antiviral activity of indolosesquiterpenoids from the xiamycin and oridamycin families

“…These functionalized decalin systems are expected to arise from a cascade cyclization of dienylepoxide precursors, such as 9 and 10 . This type of radical cyclization may be initiated by reduction of the epoxide by a Ti(III) species 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 , although, to our knowledge, α,β-epoxy esters/ketones have not been fully explored as the substrates for such cyclization 14 48 . Notably, Cuerva and coworkers 49 disclosed that enol radicals derived from α,β-unsaturated ketones inhibit the cyclization reaction.…”

New Advances in Titanium-Mediated Free Radical Reactions

“…Titanocene-catalyzed procedure has been immediately extended by Gansäuer et al to the enantioselective opening of meso -epoxides. Titanocene complexes with chiral ligands, such as those reported in Figure 2 , were employed in combination with manganese dust and collidine hydrochloride, the former as a stoichiometric reductant and the latter having the role of regenerating the alcohol and titanocenes [ 95 , 96 , 97 , 98 , 99 , 100 , 101 , 102 ] ( Scheme 34 ).…”