Alkynes radical reactions

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The (TMS)3Si radical addition to terminal alkenes or alkynes, followed by radical cyclization to oxime ethers, were also studied (Reaction 50). The radical reactions proceeded effectively by the use of triethylborane as a radical initiator to provide the functionalized pyrrolidines via a carbon-carbon bond-forming process. Yields of 79 and 63% are obtained for oxime ethers connected with an olefin or propargyl group, respectively. 

Only a few radical reactions have been applied to the functionalization of 1,3-dioxanes or 1,3-dithianes bearing one exo- or mdo-Ao ih c bond. In all cases, ring formation was the goal. The common reagent system, BusSnH/AIBN, was used to achieve a stereoselective ring closing hydrostannylation of an alkenyl alkyne subunit (AIBN = 2,2 -azobisiso-butyronitrile Equation 36) <1998JOC9626>. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

However, through quartz at 95% conversion, a mixture of I (62%) and l-carboxymethyl-2-phenylcyclooctatetraene (IT) (38%) was obtained. Compound n could be obtained exclusively (92%) from triplet-sensitized irradiation of I with high-energy sensitizers or from the sensitized reaction of methyl phenylpropiolate with benzene. From their experiments with sensitizers, the authors concluded that the primary adduct is formed from triplet alkyne (ET < 69 kcal/mol 1) and ground-state benzene and that the formation of I also proceeds via a triplet state in a two-step radical reaction. Hanzawa and Paquette [64] have also used the photochemical addition of an alkyne to benzene to produce a derivative of tetra-cyclo[3.3.0.02,4.03 6]oct-7-ene. 

Primary alkyl halides can also be alkynylated by silver acetylides. Isabelle and coworkers reported the reaction of methyl iodide, ethyl iodide and <7rmethyl iodide with several silver acetylides to give disubstituted alkynes.116 The authors preferred a non-radical-mediated mechanism for this reaction, as neither methane nor ethane, expected byproducts of a radical reaction, were observed. 

Carbon Monoxide Laser Resonance Absorption Studies of 0(3P) + 1-Alkynes and Methylene Radical Reactions... 

The CO laser resonance absorption technique is a useful tool for studying the dynamics of chemical reactions that involve the initial production of vibrationally excited CO molecules. We have recently applied this technique to study various atomic and free radical reactions related to combustion and electronic-to-vibrational energy transfer processes U—6). In this brief account, we discuss mainly the dynamics of 0(3P) + 1-alkynes and associated free radical reactions. 

Hydrostannation of alkynes is one of the simplest and direct routes to vinylstannanes which have great versatility as building blocks in syntheses. However, a variety of regio and stereo isomers are usually formed.17-22 Until now, the selectivities are controlled by using different types of tin hydrides described above.26,92 For aryl-subsutituted alkynes, radical stannylation leads to the regioselective formation of the /3-adducts but with poor stereoselectivity. The addition of bulky tris[(phenyl dimethylsilyl)methyl]tin hydride indicates that these reactions take place with complete kinetic or thermodynamic stereoselectivity (Equation (30)).93... 

In the Bu3SnH-promoted radical reactions to aliphatic alkynes, using initiators such as AIBN, Et3B, and ultrasound104 furnishes /3-adducts as a mixture of (E)- and (Z)-isomers. Lewis acid catalysts give /3-(Z) isomers,96 whereas transition metal catalysts furnish the predominant formation of (3-(E) isomers.105 The a-stannylation of simple aliphatic alkynes, however, is particularly difficult because of the absence of anchor substituents such as ethers. In the general hydrostannations of aliphatic alkynes, a-adducts are obtained only as minor adducts in the Pd-catalyzed reaction (Equation (35)). 

Although phenylsenenyl radicals can be generated easily by irradiation of diphenyl diselenide, their low reactivity toward unactivated alkenes and alkynes has prevented their extensive use in synthesis.267,268 But diphenyl diselenide can be used as an efficient trap for alkyl radicals. The products are alkyl phenyl selenides. This trapping reaction has found application to different radical reactions and the selenides obtained by this reaction can be used in subsequent reactions. 

Examples of non-platinum metal hydrogenation catalysts include (arene)-chromiumtricarbonyls which will hydrogenate dienes, alkynes, and so on, while ReH7(PCy3)2 will selectively hydrogenate acenaphthalene. Lanthanides and early transition metals are discussed later. Those catalysts operate via non-radical processes, but a few systems are known to involve radical reactions. The complex [CoH(CN)5]3 is a water-soluble catalyst that is selective for the hydrogenation of ajS-unsaturated compounds. 

Since the computational studies support the general perception of good or poor leaving groups in self-terminating radical reactions, but clearly contradict the experimental findings, radical cyclization cascades initiated by N-centered radical addition to alkynes are not terminated by homolytic p-fragmentation. 

Of the higher main group (V) elements, only P-centered radicals have been used in intermolecular radical additions to alkynes, whereas radical reactions with the unpaired electron located at the higher metallic elements arsenic, antimony, and bismuth have not been reported. 

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