Radicals vinyl radical

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

Cyclization of vinyl radicals. Vinyl radicals formed by reduction of vinyl halides with tri-n-butyltin hydride (AIBN, hv) can undergo intramolecular cyclization to a suitably situated double bond. When either a five- or a six-membered ring can be formed, the former ring is favoured. When cyclization could lead to a six- or seven-membered ring, only the former is formed, as in the cyclization of 1 to 2. The cyclization of 3 to 4 is particularly interesting because the exocyclic double bond is formed at a predictable and useful position, and because a quaternary centetis formed without difficulty. 

In designing multicomponent coupling reactions, the nature of the individual components is obviously a key factor. Generally speaking, carbon radical species, such as alkyl radicals, aryl radicals, vinyl radicals, and acyl radicals are all classified as nucleophilic radicals, which exhibit high reactivity toward electron-deficient alkenes [2]. To give readers some ideas about this, kinetic results on the addition of tert-butyl and pivaloyl radicals are shown in Scheme 6.2. These radicals add to acrylonitrile with rate constants of 2.4 x 106 M-1 s 1 and 5 x 105 M-1 s-1 at... 

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

Radicals of the alkanes are referred to as alkyl radicals. There are two other important radicals they are the vinyl radical, which is produced when a hydrogen atom is removed from ethylene, and the phenyl radical, which results when a... 

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. 

Simple organic radicals such as the vinyl radical mentioned above were routinely treated using semi-empirical or rudimentary ab initio HF-LCAO techniques. 

Table 18.3 Hyperfine coupling constants/mT for the vinyl radical. ROHF calculations...

Two types of organometallic photoinitiators for free radical vinyl polymerization are considered (1) transi-... 

Mn(III) is able to oxidize many organic substrates via the free radical mechanism [32], The free radical species, generated during oxidation smoothly initiate vinyl polymerization [33-35]. Mn(III) interacts also with polymeric substrates to form effective systems leading to the formation of free radicals. These radicals are able to initiate vinyl polymerization and, consequently, grafting in the presence of vinyl monomers. 

Recently, in our laboratory the following novel Ce(IV) ion redox initiation systems have been investigated for vinyl radical polymerization. 

Stereochemistry of the vinyl bromide is inconsequential due to the high rate of inversion of the intermediate vinyl radical... 

Scheme 3. Selected vinyl radical cyclizations developed by Stork and coworkers.

Vinyl radicals can also participate in 6-exo cyclizations. In pioneering work, Stork and his group at Columbia University showed that stereoisomeric vinyl bromides 20 and 21 (see Scheme 3) can be converted to cyclohexene 22.7 The significance of this finding is twofold first, the stereochemistry of the vinyl bromide is inconsequential since both stereoisomers converge upon the same product and second, the radical cyclization process tolerates electrophilic methoxycarbonyl groups. The observation that the stereochemistry of the vinyl bromide is inconsequential is not surprising because the barrier for inversion of most vinyl radicals is very low.8 This important feature of vinyl radical cyclization chemistry is also exemplified in the conversion of vinyl bromide 23 to tricycle 24, the key step in Stork s synthesis of norseychellanone (25) (see Scheme 4).9 As in... 

Scheme 4. Stork s vinyl radical cyclization strategy for the synthesis of norseychellanone (25)...

Vinyl radical formation by intramolecular addition of a carbon-centered radical to an alkyne... 

The key features of Curran s productive and elegant tandem radical cyclization strategy are illustrated in a retrosynthetic analysis for hirsutene (1) (see Scheme 27). The final synthetic event was projected to be an intermolecular transfer of a hydrogen atom from tri-rc-butyltin hydride to the transitory tricyclic vinyl radical 131. The latter can then be traced to bicyclic tertiary radical 132 and thence to monocyclic primary radical 133 through successive hex-5-enyl-like radical cyclizations. It was anticipated that the initial radical 133 could be generated through the abstraction of the iodine atom from... 

Most radicals located on double bonds (e.g. 4, 5) or aromatic systems (e.g. 6) are a-radicals. The free spin is located in an orbital orthogonal to the it-bond system and it is not delocalized. The orbital of the vinyl radical (4) containing the free spin can be cis- or trans- with respect to substituents on the double bond. The barrier for isomerization of vinyl radicals can be significant with respect to the rate of reaction. 

CgHjCHiCHj), or the unsaturated monovalent vinyl radical (CH2 CH-) and then sulfonated. 

The reaction of organolithium reagents is rapid and complete in less than 1 min even at low temperature, while Grignard reagents require 12-24h at room temperature to react completely. Treatment of ( )- or (Z)-l-methylsulfonyl-2-phenylethylene with trialkylbo-ranes yields exclusively ( )-olefins via a vinyl radical intermediate14. 

In the thermal reaction of aliphatic and aromatic sulfonyl chlorides with acetylenes no adduct has been observed82. However, the light-catalyzed additions of sulfonyl iodides to acetylenes83 as well as the thermal addition of sulfonyl bromides to phenylacetylene84 to form 1 1 adducts have been shown to be stereoselective and to occur in good to excellent yields. The fact that the addition occurs in a trans manner forced the authors83,84 to suggest that chain transfer by the sulfonyl halide (k ) is much faster than isomerization of the intermediate vinyl radical (k2) (see Scheme 5). 

The reaction proceeds by a radical chain mechanism analogous to that outlined in equations 38-40. The observed stereoselectivity in the above addition indicates that reaction between the intermediate vinyl radical, ArS02CH=CR, and PhSeS02Ar must... 

The chain C, H2/i+i represented mainly by C4H9+ and C6Hi3+ is produced by Reactions 21 and 22 from the C2H3+ ion. The vinyl radical ion is also responsible for the CnH2n i+ series represented largely by C6Hn + and C8H15+. 

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