Radicals with alkynes

A novel [3 + 2] radical annulation of o-cyano-substituted aryl radicals with alkynes has been reported. The reactions occur by addition of the aryl radical to the alkynes... 

TABLE 6.15 High-Pressure Limiting Rate Constants (k,) for the Reaction of OH Radicals with Alkynes at 298 K ... 

Table 6. Rate constant data and Arrhenius parameters for the reaction of OH radicals with alKynes... 

RSO2, and so on. Addition of Ph3SiSH to terminal alkenes under radical conditions also leads to the primary thiol.With alkynes it is possible to add 1 or 2 mols of RSH. 

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 systematic route into non-fused derivatives appears to be from the reactivity of [S4][AsF6]2 and [Sg][AsF6]2 with alkynes.87 The equi-molar mixture of S42+ and Sg2+ appears to act as if it were S3+ although there is little evidence of this species in solution itself. The reactivity of this hypothetical S3+ radical appears to mimic that of the closed-shell SNS+ cation but with an additional electron in a ji orbital. Using this method Passmore has isolated 7 (R=CF3, R=C02Me). 

The key features of the catalytic cycle are trapping of the radical generated after cycliza-tion by an a,P-unsaturated carbonyl compound, reduction of the enol radical to give an enolate, and subsequent protonation of the titanocene alkoxide and enolate. The diaster-eoselectivity observed is essentially the same as that achieved in the simple cyclization reaction. An important point is that the tandem reactions can be carried out with alkynes as radical acceptors. The trapping of the formed vinyl radical with unsaturated carbonyl compounds occurs with very high stereoselectivity, as shown in Scheme 12.21. 

A range of addition reactions of (TMS)3GeH with alkynes, alkenes, ketones, azines, and quinones has been studied using EPR. In addition, synthetic studies of hydrogermylation of alkynes have shown that the reaction proceeds regio- and stereo-selectively, whereas reactions with alkenes do not take place (presumably owing to the reversibility of the germyl radical addition) (Scheme 29). 

From the Fischer rate study, it appears that primaiy ester-substituted radicals are not electrophilic but ambiphilic and the borderline between ambiphilic and electrophilic radicals is not at all clear. Consider our results68 (Scheme 16) on the atom transfer additions of ester-substituted radicals to alkynes (with the caution that it may be dangerous to compare yields in place of rate constants). The primary ester-substituted radical adds more efficiently to 1-heptyne but the tertiary ester-substituted radical prefers ethyl propiolate. 

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-... 

Atkinson, R., Aschmann, S.M., Goodman, M.A. (1987) Kinetics of the gas-phase reactions of N03 radicals with a series of alkynes, haloalkenes, and a, 3-unsaturated aldehydes. Int. J. Chem. Kinetics 19, 299-307. 

Synthesis of 12- and 13-membered sulfur-containing lactones by homolytic macrocyclization of mercaptoacetic esters and alkynes has been investigated [95S307]. Reaction of the mercaptoester 245 with alkynes using triethyl borane radical initiation gave the macrolactones 247 and 248 in low yield [95TL2293], Remote asymmetric induction is observed during the cyclization. 

We consider the anti process (144) as the normal, if ideal, course of addition. Closer to reality is the stepwise process. If we begin with the schemes (138) or (142) for substitution at an alkene carbon, and divert the first intermediates as in (160), we acquire a basic scheme for addition. Now, the attacks of nucleophiles, radicals and electrophiles on alkenes and of nucleophiles and radicals on alkynes produce Jrans-oriented intermediates, in which the memory of their parents is impressed. If > lcz, and 3 s and fc4 k5, the path to product should be stereoselective (stereoelectronic axiom 2). Indeed anti attack of the gegen... 

The introduction of tellurium into an organic substrate promotes functional groups transformations or presents structural features that can be used for synthetic purposes, if suitable methods to remove tellurium from the resulting structures are available. To date, four main strategies have been explored for this end, namely, the telluroxide elimination, the tellurium/metal exchange, the coupling of tellurides with organometallic species and with alkynes, and the reductive removal via free radicals. 

In Section 8-3B, we saw the effect of peroxides on the addition of HBr to alkenes. Peroxides catalyze a free-radical chain reaction that adds HBr across the double bond of an alkene in the anti-Markovnikov sense. A similar reaction occurs with alkynes, with HBr adding with anti-Markovnikov orientation. 

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