Radicals cyclopentyl

Cycloalkanes may be pyrolized in a manner similar to that for alicyclic alkanes. Cyclopentane, for instance, yields methane, ethane, propane, ethylene, propylene, cyclopentadiene, and hydrogen at 575°C. Analogous to cracking of alicyclic alkanes, the reaction proceeds by abstraction of a hydrogen atom followed by p scission. The cyclopentyl radical may undergo successive hydrogen abstractions to form cyclopentadiene. 

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m -1 cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M -1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts max to 270 nm with extinction coefficients of 20,000-40,000 M 1 cm 1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

The pent-l-en-5-yl radical does not cyclize in any of the matrices. Non-terminal addition would involve very severe steric factors and necessitate the formation of a cyclobutyl structure which would be highly strained. Addition to the terminal end to form the cyclopentyl radical might be expected to occur but steric models again show that approach in the plane of the 77-orbital is impossible and that only approach in the nodal plane can occur. 

Our next idea was to utilize diazo compounds as radical acceptors. The radical cyclization of diazo compound 9 would initially give intermediate 11 which should release nitrogen gas to generate cyclopentyl radical 12 (Scheme 3). However, the diazo compounds were very labile to the tributyltin radical and the radical reaction of diazo compounds under normal radical conditions (BujSnH/AIBN) gave tributyltin addition products with the release of nitrogen gas. Alkyl... 

More recently, Hasebe and Tsuchiya found that the photochemical decomposition of oxime esters provides a reasonably efficient method of alkylating pyridine when the latter is employed as the solvent <86TL3239>. The reaction gives all three regioisomers and the proportion of each was influenced to some extent by the nature of the radical intermediate. Thus, while the homobenzyl radical gave the C4 addition product as a minor constituent, both cyclopentyl radical and cyclohexyl radical gave C2 and C4 addition products in almost equal proportion (Scheme 8). 

In 1960 s, CIDEP was less popular than CIDNP because CIDEP did need much faster measuring techniques than CIDNP. This is due to much faster relaxation times (usually less than 1 /r s) of polarised electron spins than those (usually a few second for protons) of nuclear spins. In 1968, Smaller et al. [2] observed a population inversion for the cyclopentyl radical with a 2-MHz ESR apparatus coupled with a 15 MeV electron beam with pulse duration of 0.5 -4.0 /z s. The response time of the system corresponded to a time constant of 1.6/z s. In 1970, Atkins et al. [3] obtained the photo-CIDEP for the ketyl radical from benzophenone in paraffin solvents with a 2-MHz ESR apparatus coupled with a 20-ns laser flash. Under favorable chemical conditions, Wong and Wan [4] demonstrated that the photo-CIDEP for some semiquinone radicals in alcohol solvents could be observed with a commercial ESR spectrometer having a 100-kHz modulation unit and a custom-designed rotating sector giving light pulses. 

Mechanism of cyclopentyl radical decomposition Homolysis of C-C5H9 radicals involves the sequence [95]... 

The stereoselectivity in addition and abstraction reactions of cyclopentyl radicals has been reviewed recently3. It has been concluded that /f-substituents at the radical, as well as the alkene substituents, have a large influence on the selectivity, however only small solvent effects have been found. 

Cyclopentyl radicals substituted in the /1-position relative to the radical center are formed during the solvomercuration/reductive alkylation reaction of cyclopentene34. The organomer-curial produced in the first solvomercuration step is reduced by sodium borohydride and yields free cyclopentyl radicals in a radical chain mechanism. Addition of alkenes can then occur tram or cis to the / -alkoxy substituent introduced during the solvomercuration step. The adduct radical is finally trapped by hydrogen transfer from mercury hydrides to yield the tram- and ris-addition products, The transicis ratio depends markedly on the alkene employed and it appears that the addition of less reactive alkenes occurs with higher trans selectivity. In reactions of highly substituted alkenes, this reactivity control is compensated for by steric effects. Therefore, only the fnms-addition product is observed in reactions of tetraethyl ethenetetracarboxylate. The choice of alcohol employed in the solvomercuration step has, however, only a small influence on the stereoselectivity. 

Cyclopentyl radicals flanked with /1-substituents on both sides are formed in reactions of 3-alkyl-2-phenylselenocyclopentanones36. After photochemical initiation, the cyclopentyl radical is formed through abstraction of the phenylseleno group. Addition to tributyl(2-propenyl)stannane occurs preferentially trans to the / -alkyl substituent and consecutive elimination of the stannyl radical gives the final allylation product. It has also been reported37 that addition of the /1-methyl cyclopentyl radical to 2-propenenitrile occurs with d.r. (transjeis) 92 8, hence it seems likely that the carbonyl group adjacent to the radical center reduces the selectivity. 

Heterocyclic cyclopentyl radicals formed in the solvomercuration/reductivc alkylation reaction of dihydrofuran give products with tram- selectivity in a slightly higher ratio than the corresponding carbocyclic analogs34. This is attributed to anomeric effects, which lead to a more pronounced axial orientation of the /J-alkoxy substituent in the tetrahydropyranyl radical. 

Heterocyclic ring systems annulated to cyclopentyl radicals can be considered as two simultaneously present substituents, cis-Annulated ring systems in the /i.y-position to the radical center are found to give tram selectivities which are larger than when a single -substituent is present. Bicyclic radicals with an overall bend shape are often exclusively attacked by alkenes from the convex face. 

Cyclohexyl radicals differ from the smaller cyclobutyl and cyclopentyl radicals in their much clearer distinction between equatorial and axial positions. The stereoselectivity should, therefore, be discussed with particular reference to the equatorial/axial selectivity37. 

Similar lo the effect of ring oxygen atoms is the influence of a carbonyl group adjacent to the radical center 6. In allylation reactions of 2-oxocyclohexyl radicals, the formation of an axial product is slightly enhanced compared to the corresponding cyclohexyl radicals. This is in contrast lo the situation in cyclopentyl radicals, where the introduction of adjacent carbonyl groups leads to a lowering of cisjtrans selectivity in allylation reactions (see this section Cyclopentyl Radicals),... 

Annulated ring systems have as /1,7-substituents, when compared to annulated cyclopentyl radical systems, a stronger effect on the stereoselectivity than the corresponding combination of acyclic substituents. In all cases, attack tram to the /J.y-m-annulated ring is preferred. The stereoselectivity depends, furthermore, on additional substituents at the radical and the alkene, but it appears that the reactions of cyclohexyl radicals proceed less selectively than their cyclopentyl analogs. One frequently used route to these systems is sequential cyclization/ addi-tion reactions, in which the primary radical cyclizes to form the bicyclic ring system, followed by intermolecular addition to an alkene45,47 74. 

Electron transfer to 2-(3-butenyl)cyclopentanone using zinc-chlorotrimethylsilane generates a radical that cyclizes stereoselectively in a 5-exo mode to cfv-annulated cyclopentanols31. Similar to the cyclizations of the 2-(3-butenyl)cyclopentyl radical, a strong preference for the 1,5-ris-substituted diastereomer is observed. 

The effect of /(-substituents on the stereoselective trapping of cyclopentyl radicals has been studied in reduction reactions of organomercurialsla-16. /(-Hydroxy or /J-methoxy substituents lead mainly to the /rau.s-addition product as a result of steric effects. 

When the radical center is located at the ring junction of a ring system annulated to a cyclopentyl radical, the annulated ring serves as an a-, as well as a f)-, or y-substituent. This is the case in furanose radicals, in which the isopropylidene group serves as a second cyclopentyl ring22. The attack of dodecanethiol as hydrogen donor occurs in such a way that the cis ring system is formed exclusively. 

Cyclopentyl radicals annulated to cyclohexyl rings yield somewhat lower selectivities28, ... 

Trapping of secondary, polycyclic cyclopentyl radicals with deuterium donors has been investigated for only a few examples35 36. Selectivities seem to be lower compared to cyclopentyl radicals bearing a third non-hydrogen substituent at the radical center. 

Another class of cyclopentyl radicals, in which high stereoselectivity in hydrogen-abstraction reactions is observed, are bridged systems. The 2-norbornyl radical, without15-16 and with37 substituents at C-2 has been investigated in detail. In the reduction of 2-norbornylmercury halides or acetates with sodium borodeuteride the intermediate 2-norbornyl radical is trapped from the toco-side predominantly. 

The contribution of the transition state position is usually not discussed as a main factor in controlling the stereochemistry of a radical reaction. However, its importance was recently raised in several publications. Moreover, the reactivity-selectivity principle proposed by Giese to rationalize the influence of the radical trap on the stereochemical outcome of 2-substituted cyclopentyl radicals [39] could also be considered as an influence of the transition state position (Scheme 15) in early transition states (reactive olefins such as fumarodinitrile) the reagent is far away from the radical center and the face discrimination is low. With less reactive olefins such as styrene, a later transition state is occurring and the product stability starts to influence the stereoehemical outcome. Therefore, the most stable trans di-substituted cyclopentanes are produced with a higher degree of stereocontrol. 

The Arrhenius parameters for the reactions of cyclopentyl radicals with HI and I2 were obtained from kinetic data for the reverse reactions combined with thermodynamic estimates of the equilibrium constants. 

The year under review has seen a number of applications of e.s.r. spectroscopy to confirm the formation of radicals postulated in a variety of reactions. Thus, in the reaction of t-butoxyl radicals with triethyl phosphite in cyclopentane solution, both phosphoranyl radicals and cyclopentyl radicals were detected (Scheme 1). By comparison with... 

Physical Properties. - The E.S.R. spectra of sulphonamidyl (26) and nitroxide (27 R Me,Bu, ,cyclopentyl) radicals were recorded and analysed. The N and (J hyperfine splitting constants of both [and 2 other sulphonamidyls] were in agreement with a n/N electronic ground state including a planar geometry around the N radical centre. The possible geometries around nitroxide N and further conformational implications were discussed. ... 

Wolf S, Agosta WC (1981) Stereoselectivity in the cyclization of 2-(bet3-enyl) cyclopentyl radical. J Chem Res Synop 78-79... 

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