Radical addition reactions alkynes

One of the most general and useful reactions of alkenes and alkynes for synthetic purposes is the addition of electrophilic reagents. This chapter is restricted to reactions which proceed through polar intermediates or transition states. Several other classes of addition reactions are also of importance, and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes were covered in Chapter 1, and cycloadditions involving concerted mechanisms will be encountered in Chapter 6. Free-radical addition reactions are considered in Chapter 10. 

ALKENES AND ALKYNES I. IONIC AND RADICAL ADDITION REACTIONS... 

Alkenes and Alkynes I. Ionic and Radical Addition Reactions... 

Alkenes and Alkynes I. Ionic and Radical Addition Reactions more stable, the 1-bromo-2-propyl radical, 5, or the 2-bromo-1 -propyl radical, 6 ... 

The principles of radical addition reactions of alkenes appear to apply equally to alkynes, although there are fewer documented examples of radical additions to triple bonds. Two molecules of hydrogen bromide can add to propyne first to give cis-1 -bromopropene (by antarafacial addition) and then 1,2-dibromopropane ... 

Et3B as a simple radical initiator was first discovered in hydrostannylation of alkynes [5], The reactions were performed at room temperature or below in the presence of a trace amount of oxygen (Scheme 2). Hydrostannylation was applied to the synthesis of dehydroiridodiol (1) and a-methylene-y-butyrolactone (2). Et3B-induced radical addition reactions of triphenylgermane [7], tris(trimethylsilyl)silane... 

Atom transfer reactions encompass a broad range of radical addition reactions in which C-heteroatom or heteroatom-heteroatom bonds are added across alkenes, alkynes, or other multiply bonded functionality. Atom transfer processes were first proposed [1] to account for the low degree of polymerization of polystyrene in CCI4... 

Other activated systems, specifically activated alkynes and hydrazones, have been employed as radical acceptors for alkyl radicals generated from alkyl halides and Sml2 [11]. Radical addition reactions to activated alkynes appear to be more capricious than the reactions with activated alkenes, and thus are perhaps more limited in scope. Additionally, mixtures of diastereomers inevitably result from such systems (Eq. 11) [12]. Diphenylhydrazones were shown to react approximately 200 times faster than the analogous alkenes in 5-exo additions as determined in a series of studies by Fallis and Sturino [13]. Reasonable diastereoselectivities were exhibited in these processes (Eq. 12). 

BusSnH-mediated intramolecular arylations of various heteroarenes such as substituted pyrroles, indoles, pyridones and imidazoles have also been reported [51]. In addition, aryl bromides, chlorides and iodides have been used as substrates in electrochemically induced radical biaryl synthesis [52]. Curran introduced [4-1-1] annulations incorporating aromatic substitution reactions with vinyl radicals for the synthesis of the core structure of various camptothecin derivatives [53]. The vinyl radicals have been generated from alkynes by radical addition reactions [53, 54]. For example, aryl radical 27, generated from the corresponding iodide or bromide, was allowed to react with phenyl isonitrile to afford imidoyl radical 28, which further reacts in a 5-exo-dig process to vinyl radical 29 (Scheme 8) [53a,b]. The vinyl radical 29 then reacts in a 1,6-cyclization followed by oxidation to the tetracycle 30. There is some evidence [55] that the homolytic aromatic substitution can also occur via initial ipso attack to afford spiro radical 31, followed by opening of this cyclo-... 

During the last decade the application of radical addition reactions for solving problems in organic synthesis has grown, as documented by numerous review articles . In this survey we will deal mainly with the synthetical aspects of the radical-based addition of a molecule to alkynes, with Section IV dedicated to radical cyclizations and to the preparation of biologically active compounds. In Section II, in order to familiarize the reader with the properties of the vinyl radical, we will briefly examine the structural characteristics of vinyl radicals and make some considerations about the stereochemical outcome of their reactions. 

Alkynes are usually reluctant to undergo addition by alkyl radicals however, the strained alkyne (131) was expected to be a good substrate for radical addition reactions. Thus, when a mixture of (131) and AIBN in benzene was heated, the adducts (134) (39%) and (135) (51%) were obtained. Compound (134) can be converted to (135) on heating (Scheme 21) <86T1693>. 

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