Emergence of a Novel Catalytic Radical Reaction:  Titanocene-Catalyzed Reductive Opening of Epoxides

Abstract

The preparatively important catalytic opening of epoxides to -titanoxy radicals via single-electron transfer (SET) is described. These radicals can be reduced to alcohols or participate in C-C bond-forming reactions. A key step in the catalytic cycle is the conceptually novel protonation of titanium-oxygen and -carbon bonds. Our method combines the advantages of radical reactions, e.g., high functional group tolerance and stability of radicals under protic conditions, with the ability of organometallic complexes to determine the course of transformations in reagent-controlled reactions.

Efficient Propargylation of Aldehydes and Ketones Catalyzed by Titanocene(III)

“…Bearing all this in mind it occurred to us that titanoceneA C H T U N G T R E N N U N G (III) (Cp 2 TiCl), a mild single-electron transfer reagent extensively studied by RajanBabu and Nugent, [10] Gansäuer [11] and our group, [12] might achieve this interesting propargylation reaction with a better chemical profile. This working hypothesis was supported by our previous work on a general and efficient Barbier-type allylation and prenylation reaction of carbonyl compounds mediated/catalyzed by titanoceneA C H T U N G T R E N N U N G (III) complexes.…”

A Radical Roundabout for an Unprecedented Tandem Reaction Including a Homolytic Substitution with a Titanium‐Oxygen Bond

“…When the reaction was run under conditions typical of reductive epoxide opening [29,31] yields of 55Ϫ57% were obtained depending on the reductant employed (Entries 5 and 6). As expected, two equivalents of the reductant and 2.5 equivalents of Coll·HCl are not needed for complete consumption of the starting material.…”

“…of [Cp 2 TiCl]. [31] In the case of 48 this reduction is retarded due to the ϩM-substituent and thus the product 49 was obtained in a much higher yield. Thus, the stability of the benzylic radical formed seems to be less important than its persistence towards reduction to the benzylic anion.…”

Transannular Cyclization of Epoxycaryophyllenes Catalyzed by TiIII: An Efficient Synthesis of Tricyclo[6.3.0.02,5]undecanes

“…Titanocene chloride has been widely used in the radical cyclization of epoxypolyenes. [15][16][17][18][19][20][21] This reaction takes place under mild conditions and is tolerated by numerous functional groups (alcohols, amides, ketones, acids, esters). [21] Continuing with our work on new applications of radical cyclizations of epoxyterpenoids, [19,20] we present here a study on transannular cyclizations of epoxycaryophyllenes (2-7) catalyzed by Ti III .…”