1- Methyl-5-hexenyl radical

Giese and Kretzschmar7j found the rate of addition of hexenyl radicals to methyl acrylate increased 2-fold between aqueous tetrahydrofuran and aqueous ethanol, Salikhov and Fischer74 reported that the rate constant for /-butyl radical addition to acrylonitrile increased 3.6-fold between tetradecane and acetonitrile. Bednarek et al75 found that the relative reactivity of S vs MMA towards phenyl radicals was ca 20% greater in ketone solvents than it was in aromatic solvents. 

Another major influence on the direction of cyclization is the presence of substituents. Attack at a less hindered position is favored by both steric effects and the stabilizing effect that most substituents have on a radical center. These have been examined by DFT (UB3LYP/6-31+G ) calculations, and the results for 5-hexenyl radicals are shown in Figure 10.14. For the unsubstituted system, the 5-exo chair TS is favored over the 6-endo chair by 2.7kcal/mol. A 5-methyl substituent disfavors the 5-exo relative to the 6-endo mode by 0.7kcal/mol, whereas a 6-methyl substituent increases the preference for the 5-exo TS to 3.3 kcal/mol.322... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The reaction (equation 76) of the hexenyl radical 47 forming cyclopentyl-methyl radical was discovered independently in several laboratories and has been of pervasive utility in both synthetic and mechanistic studyThe competition between formation of cyclopentylcarbinyl and cyclohexyl radicals favors the former even though the latter is more stable, and this kinetic preference is explained by more favourable transition state interaction. The effects of substituents on the double bond, heteroatoms in the chain, and many other factors on the partitioning between these two paths have been examined. In the gas phase above 300°C, methylcyclopentane has been observed to form cyclohexane via isomerization of cyclopentylmethyl radicals into the more stable cyclohexyl radicals. ... 

A single fluorine substituent at C-5 (as in radical 13) leads to a significant, 11-fold decrease in rate constant. This decrease no doubt derives largely from the steric effect which would be expected from any substituent at the 5-position. A methyl substituent, for example, gives rise to a 45-fold decrease in cyclization rate [164]. Interestingly, whereas the presence of a 5-methyl substituent causes endo-cyclization to become preferred (63%), the cyclization of 5-fluoro-5-hexenyl radical remains exo-specific within our NMR analytical methodology ( 4%). 

The reaction of Ph3MLi (M = Ge, Sn) or Me3SnNa with 6-bromo-l-heptene gave the expected 6-Mi4-l-heptene as the major product, but also (2-methylcyclopentyl)methyl derived metal-14 compound (equation 92)137. An intermediate l-methyl-5-hexenyl radical was proposed, but its participation was not clearly established. Distannylation of haloalkylpropene was also described (equation 93)138. 

Because of the rather fast cyclization 131 - 132 (kj = 1 10s s"1 at 25 °C)90) this reaction or the cyclization of the related l-methyl-5-hexenyl radical 134 91) and the o-(3-butenyl)phenyl radical 135 92> is often used as a radical clock in reactions possibly passing through radical intermediates 93... 

Similarly, fused and spiro cyclopropane systems 31 and 33 can also be synthesized by the reaction of appropriate cycloalkenyl cobaloximes 30 and 32 with free radical precursors such as toluenesulfonyl iodide (Scheme 11). The thermal and photochemical reactions of hexenyl cobaloximes 34 with a large excess of CCI4 gives mainly the pentachloroheptane 35 (path A). On the other hand, the photochemical reactions in the presence of low concentration of CCI4 gives mainly the cyclopentyl methyl chloride 36a through homolysis of the C-Co bond followed by cyclization of the hexenyl radical and chlorine atom abstraction (path B). However,... 

Cyclization of 5-hexenyl radicals has been compared computationally (BLYP/31-I-G ) with 2-oxo-5-hexenyl radicals." Several methyl-substituted analogs... 

The most important process (which accounts for most of the uses of radical cychzations in synthesis) is the selective 5-exo-cyclization of the 5-hexenyl radical to give the cyclopentyl methyl radical. This occurs even though the alternative - a 6-endo cyclization to give a more stable, cyclohexyl radical - is thermodynamically more favorable. Thus, the 5-ex o-cycHzation proceeds under kinetic control. The preference for 5-ex o-cyclization is explained by an early transition state with little product character. The transition state is a strain-free chair-like arrangement, which nicely accommodates the stereoelectronically required attack angle on the alkene. This model also nicely explains the stereochemical outcome of the cycHzation reaction. Assuming that substituents prefer to adopt pseudo-equatorial positions in the chair-Uke transition state, we see why ... 

The 2-methyl-5-hexenyl radical (and the 4-methyl-5-hexenyl radical) cyclizes to give predominantly the trans-dimethyl product. 

The 3-methyl-5-hexenyl radical cycHzes to give mostly the ds-dimethyl product. 

The l-methyl-5-hexenyl radical also gives predominantly the ds-product, due to a combination of steric and stereoelectronic factors. 5-exoCydization proceeds with... 

Cyclization of the 5-methyl-5-hexenyl radical (1) provides a 24 1 mixture of II and III. Explain the reversed regioselectivity in the cyclization of9 —> 10. (LasonoIide-3)... 

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