Vinyl radical intermediates

The chain is propagated by abstraction of iodine by the cyclized vinyl radical intermediate. 

Radical Diels-Alder reactions have been used mainly to synthesize polycyclic molecules. These reactions, like those that involve cations and anions as components, proceed quickly but generally do not give high yields. Thus, the tricyclic enone 14 is the result of an intramolecular Diels-Alder reaction of quenched vinyl radical intermediate 13 obtained by treating the iododienynone 12 with n-tributyltin hydride/2,2 -azobisisobutyronitrile (AIBN) [28] (Equation 1.11). 

The reaction of organolithium reagents is rapid and complete in less than 1 min even at low temperature, while Grignard reagents require 12-24 h at room temperature to react completely. Treatment of ( )- or (Z)-l-methylsulfonyl-2-phenylethyIene with trialkyIbo-ranes yields exclusively ( )-olefms via a vinyl radical intermediate... 

ESR and CIDNP studies intended to detect the radical intermediates failed [63], Conjugate addition of a vinylcuprate reagent to an enone takes place with retention of the vinyl geometry indicating that no vinyl radical intermediate is involved [64, 65], Kinetic isotope effects and substituent effects in cuprate addition to benzophenone indicate that C-C bond formation is rate-determining, which is not consistent with the involvement of a radical ion pair intermediate [66]. 

It is presumed that the stannyl radical, by addition to the terminal position of the triple bond in allenynes 132, or through bromine abstraction in bromovinyl and haloaryl allenes 134 and 136 gives the vinylic radical intermediates 138 and 142 in the propagation step, followed by cyclization toward the central carbon bond of the... 

The diastereomeric ratios of all these adducts are compiled in Table 7, and illustrate the impact of radical reactivity on stereocontrol. Vinylic radical intermediates formed from 49E and 49F cyclize with consistently lower selectivity than the corresponding alkyl radicals formed from 49D and 49E, which may reflect a shift to an earlier transition state, which could reasonably be expected for the more reactive vinylic radicals. 

In some cases vinylsamariums have been generated and used in THF. For example, Kim reported the reduction of a-bromoacrylamides and the addition of the resulting vinylsamarium to aldehydes and ketones (Scheme 5.69).119 The generation of vinylsamariums under these conditions may be possible due to an increased rate of reduction of the initially formed vinyl radical. Alternatively, the vinyl radical intermediate may abstract a hydrogen atom from THF, leading to a tetrahydrofuranylsamarium117 that deprotonates the acrylamide to give a vinylsamarium. 

A radical diphosphanylation of alkynes using tetraorganodiphosphanes as precursors for phosphanyl radicals has been applied to the synthesis of a doubly phosphinated diene 200 (Scheme 2.37)7 Tetraphenyldiphosphane was generated in situ from diphenylphosphane with an excess of chlorodiphenylphosphane in the presence of triethylamine. Addition of the phosphanyl radical to one C = C triple bond in the dialkyne 199 leads to vinyl radical 201, which undergoes a 5-exo cyclization (out of a Z-configured vinyl radical to minimize sterical hindrance in the cyclization) to give vinyl radical intermediate 202. The latter is trapped by a second phosphine moiety in a radical substitution step. The radical cyclization product is ultimately isolated as bis-phosphane sulfide 200 after treatment of the intermediate phosphane 203 with sulfur. 

One intramolecular radical addition to the bond has recently been described. Oxacyclopentane ring systems with an attached exocyclic phosphonomethylene functionality are formed in excellent yields (67%) under the thermal AIBN/(Me3Si)3SiH initiation conditions. The very bulky (Me SiljSill selectively transfers hydrogen to the (Z)-isomer of vinyl radical intermediate to avoid the easily envisioned steric repulsions encountered with the ( )-isomer to produce exclusively the (Z)-vinylphosphonate (Scheme 1.37). ... 

The chain is propagated by abstraction of iodine by the cyclized vinyl radical intermediate. This sequence of reactions benefits from the high reactivity of the intermediate alkenyl reaction in the iodine transfer step. 

When the triple bond is substituted with a trimethylsilyl or a phenyl group (e.g., 159), stereoselectivity is reversed and now kinetically controlled, with H-abstraction providing predominantly the (Z)-olefinic product 160 (Scheme 10-52). This difference in reactivity may be attributed to a linear vinyl radical intermediate which is conjugatively stabilized by the terminal substituent removing A -interactions. Now, unfavorable steric interactions between the H-donor and the allylic substituents are product-determining. 

Acetylene synthesis. 3,4-DiaIkyl-4-halo-2-pyrazoline-5-ones (1) are known to be converted into a, 3-unsaturated carboxylic acids (equation I), with the (El-isomer (2) often predominating. Kocienski et al. reasoned that this stereochemical preference could result from a vinyl radical intermediate (a). Indeed,... 

There has also been study of the structure of vinyl free radicals." Stereochemical results indicate that radicals formed at trigonal centers rapidly undergo interconversion with the geometric isomer. As a result, reactions proceeding through vinyl radical intermediates usually give rise to the same mixture from both the cis and trans precursors. 

As in the case of intramolecular HAS of arenes with aryl radicals (Scheme 9.4), also the vinyl radical intermediate can add to the arene in a 5-ipso attack or 6-cyclization, the product ratio dependent on tbe nature of the substituent on the arene under attack [47]. 

---