Pentadienyl radical

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

Draw three resonance forms for the pentadienyl radical. A radical is a substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot ( )-... 

Thus, the three resonance forms for the pentadienyl radical are ... 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

A common biologically active radical is the pentadienyl radical, RCHCHCHCHCHR, where the carbons form a long chain, with R and R, which can be a number of different organic groups, at each end. Draw three resonance structures for this compound that maintain carbon s valence of four. 

Allylic radical are relatively stable, and the pentadienyl radical is particularly stable. In such molecules, (E), E), (E),(Z), and (Z),(Z) stereoisomers can form. It has been calculated that (Z),(Z)-pentadienyl radical is 5.6 kcal mol less stable than the ( ),( )-pentadienyl radical. ° It is noted that vinyl radical have (E) and (Z) forms and the inversion barrier from one to the other increases as the electronegativity of substituents increase. Enolate radicals are also known. ... 

Consideration of the feasibility of these shifts as concerted processes, i.e. via cyclic transition states, requires as usual a consideration of the symmetry of the orbitals involved. A model related to the transition state can be constructed by the device of assuming that the C—H a bond that is migrating can be broken down into a hydrogen Is orbital and a carbon 2p orbital. For the case where x = 1 in (36), the T.S. can then be considered as being made up from a pentadienyl radical (38), with a hydrogen atom (one electron in a Is orbital) migrating between the terminal carbon atoms of its Site system (i.e. a 6e system overall is involved) ... 

By analogy with the categories of pericyclic reactions we have already considered, the feasibility of the migration will then be decided by the relative phase of the terminal lobes, i.e. the symmetry, of the HOMO of the pentadienyl radical (38). As this is a 5ne system, its... 

We naturally exclude here the cyclopropenyl, cyclopentadienyl and cycloheptatrienyl radicals, all of which can also be recognized as cyclic C H species much as we did not include in our discussion the enthalpies of formation of allyl and pentadienyl radical as part of our analysis of polyenes such as butadiene and hexatriene. 

The same method was used to determine the electron affinities of pentadienyl radical (0.91 eV) and heptatrienyl radical (1.27 eV)35. The corresponding bond dissociation... 

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

In the pentadienyl radical, predict the distribution of the unpaired electron (a) from the resonance model, and (b) from the molecular orbital model. 

Typical examples are unsaturated fatty acids (Ferreri et al. 1999 Sprinz et al. 2000, 2001 Adhikari et al. 2001). The equilibrium constants for the oleic and linoleic systems are in the order of 10 dm3 mol1 and the reverse reaction in the order of 106 s"1 (Sprinz et al. 2000 and Sprinz, pers. comm.). In polyunsaturated fatty acids, such isomerizations could, in principle, also occur by an H-abstrac-tion/H-donation mechanism as discussed above. However, the rate of H-dona-tion of RSH to the pentadienylic radicals must be verylow (see above), and isomerization has been considered to occur only by the addition/elimination pathway (Sprinz et al. 2000). With the nucleobases, any thiyl addition can only be detected when the short-lived adduct is trapped by a fast reaction (Chap. 10.10). 

Zils R (2000) Direct determination of the equilibrium constant and thermodynamic parameters in the reaction of pentadienyl radicals with O2. CR Acad Sci Ser lie Chim 3 667-674 Zils R, Inomata S, Imamura A, Washida N (2001) Determination of the equilibrium constant and thermodynamic parameters for the reaction of pentadienyl radicals with 02.J Phys Chem 105 1277-1282... 

Nevertheless, in the case of stable R radicals, like pentadienyl radicals, reaction (3) is reversible (6,7) ... 

Write down the simple Hiickel Fock matrices (in each case using a, (3 and 0, and 0,-1 and 0) for (1) the pentadienyl radical (2) the cyclopentadienyl radical (3) trimethylenemethane, C(CH2)3 (4) trimethylenecyclopropane (5) 3-methylene-l, 4-pentadiene. 

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