Alkynyl radicals

Table I summarizes the available ESR data for Group IV alkenyl or alkynyl radical anions.

Group IV Alkenyl and Alkynyl Radical Anion ESR Data... 

We will discuss the preferred geometries and the MO descriptions of carbon radicals and the corresponding carbenium ions or carbanions in two parts. In the first part, we will examine carbon radicals, carbenium ions, and carbanions with three substituents on the carbon atom. The second part treats the analogous species with a divalent central C atom. Things like alkynyl radicals and cations are not really important players in organic chemistry and won t be discussed. Alkynyl anions, however, are extremely important, but will be covered later. 

An immediate consequence of the different ease with which Cy,-element bonds dissociate is that in radical substitution reactions, alkyl radicals are more easily formed. Vinyl and aryl radicals are less common, but can be generated productively. Alkynyl radicals do not appear at all in radical substitution reactions. In the following, we therefore limit ourselves to a discussion of substitution reactions that take place via radicals of the general structure R1 R2R3C. ... 

Cyclopenta-fused pyridines 48 have been synthesized through a cascade initiated by intermolecular addition of C radicals to the C = N triple bond in vinylisonitriles. The reaction in Scheme 2.8 shows addition of the nucleophilic alkynyl radical 49 to the carbon end of the isonitrile group in 46 to give vinyl radical 50, which undergoes a... 

The synthesis of conjugated diynes via the Glaser coupling reaction " is the classical method for homocoupling of terminal alkynes. The coupling reaction is catalyzed by CuCl or Cu(OAc)2 in the presence of an oxidant and ammonium chloride or pyridine to yield symmetrically substituted diynes. " The oxidative dimerization appears to proceed via removal of the acetylenic proton, formation of an alkynyl radical, and its dimerization. 

Vinylidene and carbyne complexes also offer a rich chemistry following electron transfer, and a comprehensive review on that topic is available in the literature.As an example, oxidation of vinylidene complexes generates highly reactive radical cations. These may undergo a host of different follow-up reactions and they are summarised in Scheme 6.10. Possible follow reactions include dimerisation by direct Cp—Cp homocoupling to dinuclear butanediyli-dyne complexes M " =C—CRH—CRH—deprotonation to 17 valence-electron alkynyl radicals M —C=CR, which subsequently dimerise to the corresponding bis(vinylidenes) (= 1,3-butadiene-1,4-diylidene derivatives) M = C = CR-CR = C = C Mf and CH-bond homolysis. The latter... 

While the transition to a ferromagnet has not been observed for 22, this strategy appears to be a promising method for exploring the ordering of spins in molecular solids in general and alkynyl radicals in particular [35]. 

Unsubstituted isoxazolinones 60 were chlorinated and then converted into 1-chloroalkynes 61 upon treatment with NaNO and FeS04 in ACOH/H2O <01CC1894>. The nitrous acid mediated cleavage of isoxazolidinones and the intermolecular radical addition of xanthates were used as key steps in the synthesis of alkynes 64 which correspond to the formal adducts of an alkynyl radical to the alkene 62 <01CC1304>. 

Scheme 3.15 The cross-coupling of o-aminoalkyl radical with alkynyl radical [61]...

Early studies concentrated mostly on phenylethynyl halides and were interpreted in terms of radical intermediates. Specific attempts to induce ionic photobehavior from the 1-halo-l-hexynes 209 in polar media, including the use of low temperature and viscous solvents, afforded almost exclusively radical-derived products. This is consistent with the high ionization potentials of 1-alkynyl radicals. 

In a reaction closely related to the latter, pyranylidene derivatives are obtained by the intermolecular radical coupling of alkynyl- or alkenylcarbene complexes and epoxides. Good diastereoselectivities are observed when cyclic epoxides are used. Moreover, the best results are reached by the generation of the alkyl radical using titanocene monochloride dimer [90] (Scheme 43). 

At present, the chemisty of selenophenes and tellurophenes is a relatively scantily studied area. Nevertheless, a number of new valuable contributions dealing with their chemistry have emerged. Electrophilic cyclization of l-(l-alkynyl)-2-(methylseleno)arenes provides a route to a variety of 2,3-disubstituted benzo[fe]selenophenes, as illustrated by the preparation of the system 88. Other useful electrophiles for similar reactions are E or NBS <06JOC2307>. Similar chemistry has also been employed in preparation of 2,3-disubstituted benzo[f>]selenophenes on solid phase <06JCC163>. In addition, syntheses of 2,3-dihydroselenolo[2,3- >]pyridines have been achieved using radical chemistry <06OBC466>. 

Reaction of dpp-bian with Mg in THF for 30 min reflux gives complex 87 (Ar = 2,6-diisopropylphenyl) which undergoes oxidative addition via m-bond metathesis with PhC=CH to give the black alkynyl amido complex 88. The insertion reaction of 88 with Ph2CO in EtzO yields complex 89. Unexpectedly, hydrogen abstraction to give the radical anion occurs simultaneously with ketone insertion.268... 

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],... 

Cobaloxime(I), electrochemically regenerated from chloro(pyridine)-cobaloxime (III) (232), has been employed as a mediator in the reductive cleavage of the C—Br bond of 2-bromoalkyl 2-alkynyl ethers (253), giving (254) through radical trapping ofthe internal olefin (Scheme 95) [390]. An interesting feature of the radical cyclization (253) (254) is the reaction in methanol, unlike the trialkyltin hydride-promoted radical reactions that need an aprotic nonpolar solvent. An improved procedure for the electroreductive radical cyclization of (253) has been attained by the combined use of cobaloxime(III) (232) and a zinc plate as a sacrificial anode in an undivided cell [391]. The procedure is advantageous in terms of the turnover of the catalyst and the convenience of the operation. 

Cyclization of secondary alkyl radicals can occur with a, (S-alkynyl esters, such as 12, and proceeds with high stereoselectivity to give predominantly (Z)-exocyclic alkenes at low temperature upon reaction with (TMS)3SiH (Reaction 7.17) [28]. 

One-electron reduction of complexes traws-[RuCl(=C=C=CR2)(dppe)2][PF6] (R=Ph, Me) with cobaltocene provides highly reactive radicals traws-[RuCl (C=CC R2)(dppe)2] which, in the presence of Ph3SnH, can be trapped by Id-transfer yielding alkynyl compounds frans-[RuQ(C=CCHR2)(dppe)2] (Figure 2.14) [81]. 

It was also reported that treatment of -alkynyl iodides 17 and 18, having a triple bond activated by conjugation either with an aromatic ring or a double bond, with zinc dust in THF resulted in the formation of the cyclic products 19 and 20 respectively (equation 8)20. However, their formation was ascribed to a zinc-induced radical cyclization process due to the failure to detect any open-chain organozinc species prior to cyclization as well as unsuccessful attempts to efficiently functionalize any alkenylzinc species that would have been normally expected from an anionic pathway20. 

Alkynyl Group IV compounds of type IV (M = C, Si, Ge) may be reduced using any of the alkali metals in THF to give radical anions which, with the exception of [PhC=CGeMe3]" Li+, are stable at -90°. 

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