Radical Addition to Alkynes

Among the many variants that add two heteroatoms, those that add phenylsulfonyl groups are particularly useful because they proceed in high yield under mild conditions and provide functionality for subsequent synthetic transformations. The examples contained in Scheme 88 illustrate some recent applications.230-232 In contrast to most radical additions to alkynes, the additions of ArS X are often highly stereoselective. 

Samarium(H) diiodide mediated ketyl radical addition to alkyne results in the formation of an alkene after radical cyclization [95SL277]. Compound 65, a product from one such reaction, has been converted to the carbohydrate portion of miharamycin A, a nucleoside antibiotic. 

INTERMOLECULAR RADICAL ADDITIONS TO ALKYNES CASCADE-TYPE RADICAL CYCLIZATIONS... 

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. 

The apparent chemoselectivity for the addition of the electrophilic S-centered radicals to the less electron-rich alkyne moiety in enyne 138 can be rationalized by the fact that addition of S radicals to both aUcenes and alkynes proceeds smoothly (the rate constants for addition of S radicals to alkenes are about three orders of magnitude larger than those for the addition to alkynes), but is also reversible. However, the reversibility is less pronounced for the radical addition to alkynes, due to the high reactivity of the vinyl radicals formed (compared to alkyl radicals), which undergo subsequent reactions at faster rates than undergoing fragmentation back to the S... 

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. 

Selenosulfonate resin 44 has also been used in radical additions to alkynes in an approach to alkynyl sulfones (Scheme 11) [9]. Radical selenosulfonation... 

There is an inherent competition between simple reduction of the ketone and the reductive cyclization process with unsaturated carbonyl substrates. Cyclization processes that are slower than that of the ketyl-alkene cyclization forming a five-membered ring, suffer frtxn lower yields owing to this competition. For example, ketyl-alkyne coupling can also be achieved when mediated by Smb, but yields are lower than those achieved with analogous keto-alkenes (equation 68). This might have been expected on the basis that radical additions to alkynes are slower than corresponding additions to alkenes. Similarly, the rate... 

Radical addition to alkynes. Thiolacetic acid reacts vigorously with alkynes to give mono and di adducts. In some cases the yield is improved by irradiation or addition of a peroxide, but the main value of initiation hes in improved reproducibility of the experiments. The reaction provides a means of converting a terminal acetylene into an aldehyde. ... 

Stereoselective radical addition to alkynes and reduction of iodoalkynes Et jB has been used as a catalyst for the reactions with (MejSifjSiH to yield (Z)-alkenes. 

Free-radical additions to alkynes generate vinyl radicals (equation 1) and information about the structure of these intermediates has been obtained either by spectroscopic or chemical means. Vinylic intermediates are generally cr-type radicals (1), in which the unpaired electron is in an orbital with substantial s character. The degree of bending and the inversion barrier depends on the a-substituent. That is, for vinyl (R=H) the rate constant for the inversion lies between 3 x 10 and 3 x 10 s at -180 °C, whereas 1-methylvinyl inverts somewhat more slowly. Electronegative substituents, such as alkoxy, increase the barrier of inversion. ... 

However, radical addition to alkynes presents two problems compared to alkenes. Firstly, comparable cyclisations are one order of... 

Figure 7.47 Regio- and stereoselectivity observed for sulfonyl radical additions to alkynes. NBO energies calculated at the B3LYP/6-31C(d,p) level of theory, from ref. 59a. The a-spin corresponds to the larger electron subset in the radicals (NBO analysis analyzes the Lewis structures for the a- and fi-spins separately).

Using ort/zo-alkynylated Ar,A/ -dialkylamines as substrates, Zhou and coworkers demonstrated radical addition to alkynes followed by C-O bond formation [20]... 

WiUe U. Radical cascades initiated by intermolecular radical addition to alkynes and related triple bond systems. Chem Rev. 2013 113 813-853. 

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