Catalysis radical cyclization

Low-Valent Iron Catalysis Radical Cyclizations and Tandem Processes... 

Co(I)-Co(II)-Co(III) Catalysis Radical Cyclizations Catalyzed by Cobaloxime and Related Complexes... 

The selection of the thirty procedures clearly reflects the current interest of synthetic organic chemistry. Thus seven of them illustrate uses of T1(I), T1 (III), Cu(I), and Li(I), and three examples elaborate on the process now termed phase-transfer catalysis. In addition, newly developed methods involving fragmentation, sulfide contraction, and synthetically useful free radical cyclization arc covered in five procedures. Inclusion of preparations and uses of five theoretically interesting compounds demonstrates the rapid expansion of this particular area in recent years and will render these compounds more readily and consistently available. 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

Whereas the previous methods for the synthesis of pyrrolo-oxazines were based on traditional ionic cyclization, the preparation of 7-rncthyltctrahydro-l //-pyrrolo[2,1 -c][l,4]oxazin K3//)-one 176 was reported using a new radical cyclization induced by photoelectronic transfer catalysis. This reaction is proceeding under very mild conditions and in neutral medium (Scheme 26). 

Intramolecular radical cyclization of an aryl bromide and an alkyne can be used to produce dihydroquinolines (Equation 57) <1998TL2965>. An analogous reaction setup utilizes a Lewis acid-catalyzed novel one-pot domino pathway using silver catalysis in high regioselectivity (Scheme 26) <2005OL2675>. Three mole equivalents of the alkyne are used with the final cyclization step arising from alkynic addition. 

Radical A-exo cyclizations can also be catalyzed without recourse to the Thorpe-Ingold effect by applying the concept of template catalysis (Fig. 24) [157]. Here, 20 mol% of a cationic titanocene(IV) precatalyst 82 is applied, which contains a coordinating neutral tether. After reductive opening of epoxide 81, a cationic titanocene(III) complexed radical 83 is formed, in which both the epoxide and the xfi-un saturated carboxamide radical acceptor are coordinated. This provides the template to accelerate the slow A-exo radical cyclization step considerably. Cyclobutanes 84 were isolated in 46-84% yield with mostly good / / [Pg.145]

Ru(II)-Ru(III) Catalysis Atom Transfer Radical Cyclizations (ATRC)... 

The extension of this method to heterocyclic products took advantage of nickel catalysis for the iodine-zinc exchange, for example, of iodoacetals 22, and the organozinc intermediates 23 formed by radical cyclization were again transmetallated to the corresponding copper species which could be reacted with the usual Michael acceptors to afford the products 24 with high diastereoselectivity (Scheme 6).33>33a... 

A radical cyclization has been used by Hart, at The Ohio State University, to prepare the manzamine tricyclic core (139). Utilizing tri-w-butyltin hydride to mediate the cyclization of selenide 171, the desired stereochemistry of the octahydroisoquinoline ring (172) was established (Scheme 13) electrophilic catalysis generated the tricyclic intermediate 174. An anionic cyclization leading to a tricyclic intermediate is being developed by Marko, at the University of Sheffield, toward which he has recently reported a model study (140). 

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