Alkynes radical additions

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

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. 

In certain cases, Michael reactions can take place under acidic conditions. Michael-type addition of radicals to conjugated carbonyl compounds is also known.Radical addition can be catalyzed by Yb(OTf)3, but radicals add under standard conditions as well, even intramolecularly. Electrochemical-initiated Michael additions are known, and aryl halides add in the presence of NiBr2. Michael reactions are sometimes applied to substrates of the type C=C—Z, where the co-products are conjugated systems of the type C=C—Indeed, because of the greater susceptibility of triple bonds to nucleophilic attack, it is even possible for nonactivated alkynes (e.g., acetylene), to be substrates in this... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

Scheme 1 5-exo Cyclization initiated by phosphinyl radical addition onto an alkyne... 

There are several procedures for synthesis of terminal alkenyl stannanes that involve addition to aldehydes. A well-established three-step sequence culminates in a radical addition to a terminal alkyne.150... 

Alkane free radical addition to aklenes and alkynes generates products with more carbon atoms than the reactants—that is, the result is alkylation. Generally,... 

Warsinsky R, Steckhan E (1995) Oxidative free radical additions of a-nitro amides to alkenes and alkynes mediated by electrochemically regenerable manganew (HI) acetate. In Torii S (ed) Novel trends in electroorganic synthesis, Kodansha, Tokyo, pg 135 J. Chem. Soc. Perkin Trans 1 (1994) 1994 2027... 

A range of addition reactions of (TMS)3GeH with alkynes, alkenes, ketones, azines, and quinones has been studied using EPR. In addition, synthetic studies of hydrogermylation of alkynes have shown that the reaction proceeds regio- and stereo-selectively, whereas reactions with alkenes do not take place (presumably owing to the reversibility of the germyl radical addition) (Scheme 29). 

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. 

Free radical addition of HBr to alkynes peroxide effect. Preparation of bromoalkenes... 

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

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

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