Anion radical intermediates alkynes

The first alkyne cyclisations, from 377, 379 and 381, predate the early alkene cyclisations by a couple of years these three date from 1966173 and 1967,174 and illustrate the favourability of both exo and endo-dig cyclisation. All three generate benzylic vinyllithiums (378, 380 and 382), and both aryl (377, 379) and alkyl halides (381) are successful starting materials. Similar organomagnesium cyclisations were described at about the same time.175 However, it is not clear in these reactions how much of the product is due to participation of radicals in the mechanism - alkylbromides undergo halogen-metal exchange with alkyllithiums via radical intermediates (chapter 3).176 If it really is an anionic cyclisation, cyclisation to 378 is remarkable in being endo. Endo-dig anionic cyclisations are discussed below. 

Sml2-mediated radical cyclisations involving alkyl, alkenyl and aryl radical intermediates can be used to construct efficiently five-membered and, in certain cases, six-membered ring systems. This approach provides a useful alternative to trialkyltin hydride-mediated methods as toxic reagents and problematic tin byproducts are avoided. In addition, the use of Sml2 to induce radical cyclisations has led to the development of a number of powerful, radical/anionic sequential processes for the construction of complex systems. Sequential reactions involving radical-alkene/alkyne cyclisations are discussed in Chapter 6. 

When an alkyne is then added to the solution, an electron adds to the triple bond to yield an intermediate anion radical—a species that is both an anion has a negative charge) and a radical (ha.s an odd number of electrons). This anion radical is a strong base, which removes from ammonia to give a vinylic radical. Addition of a second electron to the vinylic radical gives a vinylic anion, which abstracts a second from ammonia to give trans alkene product. The mechanism is shown in Figure 8.4. 

The stereoelectronic effects of alkyne additions are not limited to anions, as radicals react similarly. The sulfonyl radical adds regio- and stereoselectively to the terminal carbon, giving the E-bromo vinylsulfone. NBO analysis shows that the radical center can act as both a donor and acceptor in interactions with the antiperiplanar C-S bond. In the radical intermediate formed by addition of tosyl radical to 1-hexyne, the donor character dominates. The energy of the n- CT was larger than the (Tj, g n (-27 vs. 7 kcal/mol, for the sum of a and p-spins. Figure 7.47). ... 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

The rate-determining step in the Na/NH3 reduction of alkynes is the protonation of the radical anion A. The next step, the reaction of the alkenyl radical C to the alkenyl-sodium intermediate B, determines the stereochemistry. The formation of B occurs such that the substituents of the C=C double bond are in trims positions. This trans-selectivity can be explained by product-development control in the formation of B or perhaps also by the preferred geometry of radical C provided it is nonlinear at the radical carbon. The alkenylsodium compound B is protonated with retention of configuration, since alkenylsodium compounds are configurationally stable (cf. Section 1.1.1). The Na/NH3 reduction of alkynes therefore represents a synthesis of fnms-alkencs. 

Going beyond equations (1) and (2), a nucleophilic attack on an alkyne may be one step in a coupled sequence. The first intermediates, anion (V ), zwitterion ( V") or radical anion (A ) are valuable synthons which may continue on in cyclization. 

The mechanism of dissolving metal reductions depends on the nature of the solvent and the nature of the substrate. The proposed mechanism for the reduction of dialkylacetylenes by sodium in HMPA in the presence of a proton donor is illustrated in equation (18). The addition of an electron to the triple bond of (45) is proposed to produce the rran -sodiovinyl radical (46), or the corresponding radical anion (47), which undergoes protonation by the added alcohol to produce the radical (48). Further reduction of (48) by sodium produces the rrans-sodiovinyl compound (49), which on protonation produces the trans-a -kene (50). In the absence of a proton donor, the reduction of (45) with sodium in HMPA results in the formation of a mixture of cis- and trans-2- and 3-hexenes. Control studies showed that the isomerization products 2- and 3-hexene are not formed by rearrangement of the cis- or frans-3-hexenes. It was concluded that the starting alkyne (45) acts as a reversible proton donor reacting with an intermediate anion or radical anion to produce the delocalized anion (51) which is then protonated to produce the al-lene (52). Reduction of the allene (52), or further rearrangement to the alkyne (53) followed by reduction, then leads to the formation of the mixture of the cis- and trans-2- and 3-hexenes (equation 19). ... 

A variety of substituted radicals may be generated by the electro-oxidation of salts of 1,3-dicarbonyl compounds, aliphatic nitrocompounds, phenols, oximes, alkynes, and organometallic compounds. In all of these reactions, an anionic (or partially anionic) species is formed from which the intermediate radical is generated, e.g., in the anodic oxidation of diethylmalonate... 

The alkyne-bridged radical anions [Co2(CO)6(li-RC2R)] are not only of interest because of their electronic structure but also for their chemical reactivity. First, they are intermediates in the ETC catalysed carbonyl substitution reactions26.30 of [Co2(CO)6( i-F3CC2CF3)]... 

A single electron is transferred from the sodium atom to the alkyne, generating a radical anion intermediate... 

In the first step of the mechanism, a single electron is transferred to the alkyne, generating an intermediate that is called a radical anion. It is an anion because of the charge associated with the lone pair, and it is a radical because of the unpaired electron ... 

A dissolving metal reduction will convert an alkyne into a trans alkene. The reaction involves an intermediate radical anion and employs fishhook arrows, which indicate the movement of only one electron. 

This complexed electron can add to an alkyne to give an intermediate called a radical anion (Fig. 10.80). A radical anion is a negatively charged molecule that has an unpaired electron. 

The mechanism for this reduction, shown in the preceding box, involves successive electron transfers from lithium (or sodium) atoms and proton transfers from amines (or ammonia). In the first step, a lithium atom transfers an electron to the alkyne to produce an intermediate that bears a negative charge and has an unpaired electron, called a radical anion. In the second step, an amine transfers a proton to produce a vinylic radical. Then, transfer of another electron gives a vinylic anion. It is this step that determines the stereochemistry of the reaction. The trawi-vinylic anion is formed preferentially because it is more stable the bulky alkyl groups are farther apart. Protonation of the trani-vinylic anion leads to the trans-alkene. 

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