Alkynes alkyne radical anions

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)]... 

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. 

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... 

Radical anions have also been obtained recently from the tin equivalents of the alkynes, the distannynes (see Section 3.14.24.6). Reduction of 2,6-di(2,4,6-triisopropylphenyl)phenyltin(ll) chloride (ArSnCl) with potassium in THF gave [ArSnSnAr] K+, which showed g= 2.0069, (117Sn) = 0.83 mT, (119Sn) = 0.85 mT, and in the crystal, the radical anion adopts a tram-bent structure. The 2,6-di(2,6-diisopropylphenyl)phenyltin analog behaves similarly.533 534... 

Reactions with Protic, ionic, Poiar Reagents. The reactions of radical anions with proton donors include the reduction of arenes, the well-known Birch reduction, as well as alkynes by alkali metals in liquid ammonia. Both reactions have synthetic utility and belong to the few radical ion reactions included in elementary textbooks. 

Another interesting example of an insertion reaction is found through the addition of benzophenone to complex 59 (Scheme 15). In this case hydrogen is abstracted from an amine group with addition of an alkyne unit across the carbonyl to produce a radical anion. 

The radical anion of /3-trimethylsilylstyrene also undergoes dimerization but coupling takes place at the carbons a to silicon 33). The kinetics of the alkyne dimerization, followed by ESR, showed the reaction to be second order in radical anion 43). With Li+, Na+, K+, or Rb+ as the counterions, the rate increases in the order Si < C < Ge 45). Consistent with the increased stability of the trimethylsilyl-substituted radical anion, the radical anion of 1,4-bis(trimethylsilyl)butadiyne, produced by reduction with Li, Na, K, Rb, or Cs in THF is stable at room temperature even on exposure to air, whereas the carbon analog, 1,4-di-r-butyl-1,3-butadiyne radical anion, dimerizes by second-order kinetics at -40° (42). The enhanced stability of the trimethylsilylalkynyl radical anions has been attributed to p-drr interactions (42). 

With cyclic alkynes, the sulfate anion radical acts as an oxygen-transfer reagent, transforming the alkyns into a,(3-epoxy compounds (Wille 2000). In this sense, the sulfate anion radical can also be considered a donor of atomic oxygen in solution. The reaction leads to the release of S()3, which is significantly less reactive than SOT (Muller et al. 

In this reaction, the alkali metal donates its valence electron to the alkyne to produce a radical anion (Following fig.). This removes a proton from ammonia to produce a vinylic radical that receives an electron from a second alkali metal to produce a trans-... 

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 inter-... 

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. 

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. 

The metal-ammonia reduction proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation to give a neutral radical. Protons are provided by the ammonia solvent or by an alcohol added as a cosolvent. Addition of another electron, followed by another proton, gives the product. 

Step 1 An electron adds to the alkyne, forming a radical anion. 

The mechanism of the Birch reduction (shown next) is similar to the sodium/liquid ammonia reduction of alkynes to fnmy-alkencs (Section 9-9C). A solution of sodium in liquid ammonia contains solvated electrons that can add to benzene, forming a radical anion. The strongly basic radical anion abstracts a proton from the alcohol in the solvent, giving a cyclohexadienyl radical. The radical quickly adds another solvated electron to form a cyclohexadienyl anion. Protonation of this anion gives the reduced product. 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

For reduction of PhC=C(CH2)4X (X = Cl, Br entries 13, 14) the relatively low half-wave potentials relate, for the bromide, to cathodic cleavage of the carbon-bromine bond, but for the chloride it is likely that the radical anion of the alkyne is produced, which allows nucleophilic intramolecular displacement of chloride (see Section II.A). 

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. 

Unambiguous evidence for the cathodic generation of the radical ions from certain alkynes (equation 75) was obtained by measuring their e.s.r. spectra- . Other workers have generated both radical anions and dianions (equation 75) by... 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

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). ... 

One of the most frequently eneountered reactions is that with proton sources, as observed with arenes (Birch reduction) [189], aldehydes [ 190], alkynes [2d], fullerenes [191], ketones [192] (even enantioselective protonation of ketyl radical anions [193]), nitriles [194], nitro [195] and nitroso compounds [196], and olefins [197]. Protons are often replaced as electrophiles by trialkylsilyl chloride [198],... 

---