Radical alkynylations

Another approach to carrying out tin-free radical fragmentation processes, developed by Fuchs, utilizes trifluoromethyl sulfone, or triflone, derivatives. Fuchs first reported examples of free radical alkynylation reactions using acetylenic triflone 102 [62]. What is most remarkable about these reactions is that the radicals being alkynylated are formed from the cleavage of C-H bonds standard radical precursors are not required. For example, when tetrahydrofuran is mixed with triflone 102 at room temperature, alkynylation occurs a to the ether oxygen in 92% yield (Scheme 21). In this case, the radical chain process is most likely initiated by traces of peroxides in the THF. Similarly, unactivated alkanes such as cyclohexane will react with triflone 102 in good yield (83% for cyclohexane) when heated with a catalytic amount of AIBN. 

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°. 

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

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

Support-bound triazenes, which can be prepared from resin-bound secondary aliphatic amines and aromatic diazonium salts [455], undergo cleavage upon treatment with acids, leading to regeneration of the aromatic diazonium salts. In cross-linked polystyrene, these decompose to yield nitrogen and, preferentially, radical-derived products. If the acidolysis of polystyrene-bound triazenes is conducted in the presence of hydrogen-atom donors (e.g. THF), unsubstituted arenes can be obtained (Entries 8 and 9, Table 3.47). In the presence of alkenes or alkynes and Pd(OAc)2, the initially formed diazonium salts undergo Heck reaction to yield vinylated or alkynylated arenes (Entry 10, Table 3.47). Similarly, unsubstituted arenes can be obtained by oxida-... 

The cyclization of secondary alkyl radicals with a, fi-alkynyl esters 18 proceeded with high stereoselectivity to give predominantly (Z)-exocyclic alkenes at low temperature upon reaction with (TMS SiH (equation 54)79. On the other hand, the formation of (E)-exocyclic alkenes predominated with Bu3SnH, the E/Z ratio being 98 2 at 80 °C. It has been suggested that the main factor controlling the formation of these products is the ability of (TMS Si and Bu3Sn radicals to isomerize the product alkene. That is,... 

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