Radicals 4-pentenyl

It was proposed that the transition state requires approach of the radical directly above the site of attack and perpendicular to the plane containing the carbon-carbon double bond. An examination of molecular models shows that for the 3-butenyl and 4-pentenyl radicals (16, =1,2) such a transition state can only be reasonably achieved in < Xf>-cyclization (i.e. 16—> 15). With the 5-hexcnyl and 6-heptenyl radicals (16, w=3,4), the transition state for exo-cyclization (16- 15) is more easily achieved than that for enc/o-cyclization (i.e. 16 — 17). 

Nevertheless, in spite of this observation, it was found that the parent system, the perfluoro-4-pentenyl radical, 24, failed to cyclize even when Et3SiH was employed as the hydrogen atom transfer agent [ 140]. Either the equilibrium between 24 and the cyclized radical must be very unfavorable or the rate constant for 24 s cyclization is less than lx 104 s 1 at 30°C. Thus the lack of cyclization of 24 could be due to either thermodynamic or kinetic factors. 

An interesting aspect of fluorinated 4-pentenyl radicals that distinguishes them from their hydrocarbon counterparts is their ability to cyclize to form four-membered rings. As mentioned in Sect. 5.3.2, Piccardi and his coworkers reported in 1971 that C2F5I underwent free radical addition to 3,3,4,4-tetra-fluoro-l,5-hexadiene to form a four-membered ring product [173]. Subsequently they observed similar results in the addition of CC14 [323]. 

Cekovic, Z. Saidc, R. Radical cyclization reactions. Cydopropane ring formation by 3-exo-cycbzation of 5-phenylfhio-3-pentenyl radicals. Tetrahedron Lett. 1990, 31, 6085—6088. 

Baker, J. M. Dolbier, W. R. Density functional theory calculations of the effect of fluorine substitution on the cyclobutylcarbinyl to 4-pentenyl radical rearrangement. J. Org. 

Park, S.-U. Varick, T. R. Newcomb, M. Acceleration of the 4-exo radical cyclization to a synthetically useful rate. Cyclization of the 2,2-di-methyl-5-cyano-4-pentenyl radical. Tetrahedron Lett. 1990, 31, 2975-2978. 

A major focus of our research program has been the development of synthetically useful radical-mediated methodologies in recent years. Our interest in this area was partly initiated by asking how to generate five- and six-membered ring radicals from acyclic radical precursors. This problem has been unsolved in radical chemistry due to a disfavored 5-endo cyclization in pentenyl radical cyclizations. This simple but intriguing curiosity led us to study radical reactions of yV-aziridinylimines. The outcome of this research is the development of a novel consecutive carbon-carbon bond formation approach, which has tremendous synthetic potential. This review provides a full account of radical cyclizations of V-aziridinylimines and their applications to sesquiterpene natural products. 

In addition to these two problems in heptenyl radical cycliza-tions, competitive 1,5-hydrogen transfer may occur if there are accessible allylic hydrogens. Furthermore, in pentenyl radical cyclizations the formation of cyclobutane rings is very slow and the reverse reaction is greatly favored, whereas the formation of cyclopentane rings by 5-endo cyclization is disfavored. Therefore, despite the synthetic usefulness of radical cyclization processes, the cyclization pathway is mainly limited to 5-exo cyclization along with the much less efficient 6-exo and 6-endo cyclizations. 

The appearance of free radicals R- in this process has been proved by de Boer54). Photolyzing 1 -cyclopropyl- 1-nitroso-ethane 27 he obtained aminyloxide 30. In this way it is shown that the initially formed 1-cyclopropylethyl radical 28 by ring opening converts to A3-pentenyl radical 29 which then adds to unchanged 27. 

Chatgialialoglu and coworkers applied carbonylative cyclization to a unique synthesis of polyketones from 1,4-c -polybutadiene and CO [30]. This polymer contains a unit each of cyclopentanone and cyclohexanone as well as an unreacted olefin unit. Carbonylative 6-endo cyclization which leads to a selective formation of cyclohexanones is also possible using 4-pentenyl radical precursors having a substituent at the 4-position [31]. Scheme 7 illustrates such an example. 

The 4-pentenyl radical can undergo 4-exo or 5-endo cyclization. UB3LYP/6-3IG calculations find a preference of 1.8 kcal/mol for the 5-endo TS. The angle to approach to the double bond is found to be about 89°... 

Critical energies for transfo of H atoms in alkyl radicals have been determined by studies with diemically activated pentyl and pentenyl radicals formed by addition of H atoms to pentenes and pentynes. For the 1,4 transfer... 

Beckwith ALJ, Easton JC, Lawrence T, Serelis AK (1983) Reactions of methyl-substituted 5-hexenyl and 4-pentenyl radicals. Aust J Chem 36 545-556... 

Pentenyl radicals produced by reaction 30 are also hot radicals. The correctness of this statement will be comprehended from the... 

Next, the cyclization of a hot 4-pentenyl radical, i.e., reaction 33 was postulated. 

The next discussion concerns the formation of propylene, 1-butene and butadiene which are the other main primary products of the reaction. The hot allyl (or 4-pentenyl) radical generated in reaction 29 (or 30) may abstract hydrogen from ethylene to form propylene (or 1-pentene) and a vinyl radical. From the highly endothermic nature of the vinyl radical formation, the postulation of a hot radical is again reasonable in this step. A similar reaction of a cyclopentyl radical with ethylene, i.e., + C=C... 

C=C, was excluded from the whole scheme by the following reasons. Firstly, no cyclopentane was detected experimentally. Secondly, the cyclopentyl radical may have released its energy in the course of cyclization reaction, reaction 33. A hot 4-pentenyl radical itself may partly be quenched by reaction 36. 

Vinyl radical generated in reaction 32 undergoes reaction 37, which is considered to be a path for formation of C4 products. Another, presumably a main, reaction course for C4 product formations is the addition of ethylene to a hot 4-pentenyl radical, i.e., reaction 38, accompanied with hydrogen shifts (or isomeriza-tions) as exemplified in reactions 39- 42, and with 3-scission in reactions 43- 46. It is ambiguous whether reactions 45 and 46 should be replaced by reaction 47 because no ethane or propane was detected in the experiments. 

Figure 10 illustrates the heat contents of key radicals such as reported in the literature (23,26,28,33). Activation energies obtained in the present investigation for reactions starting from allyl radical plus ethylene to cyclopentene and 1-pentene formation fit the diagram consistently. This figure strongly supports the conclusions that it is impossible to produce any linear (cyclized) C5 radicals from stable cyclopentyl (4-pentenyl) radicals, and that, in the case of the reaction of an allyl radical with ethylene, it is possible to produce both cyclized and linear C5 compounds at the same time. 

For the methyl pentenyl radical systems, the most favorable path to diolefin formation involves allylic carbon-carbon bond scission ... 

In the case of cis and trans 3-niethyl-2-pentene, hydrogen addition leading to the tertiary 3-methyl pentenyl radical is favored ... 

In the case of 2-methy1-2-pentene, hydrogen addition again gives predominantly the tertiary methyl pentenyl radical ... 

---