Alkyne based radicals

A recent theoretical study by Takeuchi et al. [140] has examined the mechanism for the reaction of both alkenes and alkynes with the H-terminated silicon surface using periodic DFT calculations, and the results are in good agreement with the proposed radical-based mechanism [137]. In particular, the calculations show that the reaction occurs through a carbon-based radical intermediate which must be sufficiently stabilized to proceed by abstraction of a surface hydrogen (as in the case of styrene) if the intermediate is not stable enough, it will preferentially desorb (as in the case of ethylene). The calculations also show that reaction with terminal alkynes should proceed faster and lead to more stable products than with terminal alkenes [140]. 

When thiols are added to substrates susceptible to nucleophilic attack, bases catalyze the reaction and the mechanism is nucleophilic. These substrates may be of the Michael type or may be polyhalo alkenes or alkynes. As with the free-radical mechanism, alkynes can give either vinylic thioethers or dithioacetals ... 

A lithium atom donates an electron to The radical anion acts as the 7i bond of the alkyne. An electron a base and removes a... 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

Most of the recent synthetic developments in the field of radical cyclization have involved the reactions of carbon-centered radicals with alkenes and alkynes. Other useful acceptors include allenes,31 dienes30 and vinyl epoxides.32 The same methods are used for cyclizations to these acceptors as for radical additions, and the preceding chapter should be consulted for specific details on an individual method (the organization of this section parallels that of Section 4.1.6). Selection of a particular method to conduct a proposed cyclization is based on a variety of criteria, including the availability of the requisite pre-... 

To predict which of the two alkyne carbons, C1 or C2, HNC will preferentially attack, one now invokes the local hard-soft acid-base (HSAB) principle (cf. [157]), which says that interaction is favored between electrophile/nucleophile (or radical/radical) of most nearly equal softness. The HNC carbon softness of 1.215 is closer to the softness of C1 (1.102) than that of C2 (0.453) of the alkyne, so this method predicts that in the reaction scheme above the HNC attacks C1 in preference to C2, i.e. that reaction should occur mainly by the zwitterion A. This kind of analysis worked for -CH3 and -NH2 substituents on the alkyne, but not for -F. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

An interesting three-component reaction occurred when a solution of diphenyl diselenide (SI), an electron-deficient alkyne (e.g., ethyl propiolate), and an excess of an isocyanide (SO) was irradiated through Pyrex with a tungsten lamp (hv > 300 nm). The reaction is based on a selective sequentially radical addition, as illustrated in Scheme 3.33, and the coupling products were formed in 58-85% yield [83], As for the example reported in Scheme 3.33, the elaboration of compound 52 yielded a precursor for the construction of the carbapenem framework. 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

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