Cyclization homolytic substitution

The C-Se and C-Te bonds are formed by an internal homolytic substitution of vinyl or aryl radicals at selenium or tellurium with the preparation of selenophenes and tellurophenes, respectively. An example is shown below, where (TMSIsSiH was used in the cyclization of vinyl iodide 65 that affords... 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

It was proposed that an initially generated silyl radical 3, by reaction of i-BuO radical and polysilane 2, attacks another silicon atom in the same backbone to give a cyclic polysilane that contains an acyclic chain and another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize or abstract a hydrogen atom from another macromolecule, thus propagating the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular homolytic substitution (ShO, a class of reactions discussed in Chapter 6. 

Redox and homolytic substitution reactions almost never directly form C—C, C—N and C—O bonds. Such bonds are generated in radical addition reactions (Scheme 14). Intermolecular addition reactions are presented in this chapter. Cyclization reactions have important similarities with, and differences from, bimolecular additions, and they are presented in Chapter 4.2 of this volume. Falling under the umbrella of addition reactions are radical eliminations (the reverse of addition) and radical migrations (which are usually, but not always, comprised of an addition and an elimination). 

Exposure of the diazonium salt 1107 to iodide ion generates an aryl radical, which undergoes intramolecular homolytic substitution at sulfur to liberate the acyl radical intermediate 1108. An exo-mode cyclization with concomitant incorporation of iodide then occurs to afford the 3-disubstituted-chroman-4-one 1109 in excellent yield (Scheme 275) <1997JOC5982>. 

Co-cyclization of 1,6-diene (113) with a thiyl radical can be carried out involving the addition of thiyl radical to an olefinic group, cyclization via 5-exo-trig manner with a chair-like transition state, and termination by homolytic substitution at the sulfur (SHi). Preparation of cis-fused thiabicyclo[3.3.0]octane (114) is shown in eq. 4.39. 

Cyclizations to sulfones which lead to a homolytic substitution at sulfur have been employed for the preparation of cyclic sulfinamides [59]. 

Fig. 25 Titanium-catalyzed radical cyclization with catalyst regeneration by homolytic substitution (only major diastereomer shown)...

A radical approach to cyclization is offered by the intramolecular homolytic substitution (ShO reaction at a silicon center. Reaction of phenyl bromoacetate with a stannylated silyl homoallyl ether under atom transfer conditions provides cyclic alkoxysilanes (Equation (119)).2... 

There are very few examples of sulfur-carbon bond formation. Cyclic sulfinamides 204 (R = R = H) can be obtained through intramolecular homolytic substitution from 442 using classical tributyltin hydride conditions. An alkylsulfinamide which demonstrates the ability to cyclize in such conditions affording 443 is PhSe(CH2)3NHCOCMe3 <2006AGE633>. 

Tin. Bu3SnH has recently been demonstrated to effect the intramolecular pinacol coupling of 1,5- and 1,6-dialdehydes and keto aldehydes (Eqs. 3.24 and 3.25) [41]. 1,5-Dicarbonyl compounds undergo cyclization with consistently excellent (> 20 1) cis selectivity, whereas 1,6-dicarbonyl compounds provide variable stereoselection. Mechanistic studies furnish evidence for an interesting new pathway for pinacol coupling, involving homolytic substitution as a key step (Fig. 3-2). 

Cyclization on to aromatic rings has been investigated by a number of workers. The use of reductive conditions (BusSnH-AIBN) facilitates an intramolecular homolytic substitution of imidazoles and benzimidazoles (Scheme 15). This chemistry allowed for a range of ring sizes to be produced. Another oxidative cyclization, this time mediated by the addition of the p-toluenesulfinate radical to an alkene, has been reported. Oxidative addition occurs in the presence of Cu (OAc)2 which oxidizes any intermediate cyclohexadienyl radicals back to the aromatic ring (Scheme 16)." ... 

Beckwith and Storey have developed a tandem translocation and homolytic aromatic substitution sequence en route to spiro-oxindoles [95CC977]. Treatment of the bromoaniline derivative 122 with tin hydride at 160 °C generated the aryl radical 123 which underwent a 1,5-hydrogen atom transfer to give intermediate 124. Intramolecular homolytic aromatic substitution and aromatization gave the spiro-oxindole 125. Intramolecular aryl radical cyclization on to a pyrrole nucleus has been used to prepare spirocyclic heterocycles [95TL6743]. 

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