Pentadienyl radical cations

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... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

For each of the following processes, predict whether it will proceed in a conrotatory or a disrotatory manner (i) Thermal electrocyclic reaction ofthepentadienyl cation, (ii) thermal electrocyclic reaction of the pentadienyl radical, and (iii) photochemical electrocyclic reaction of the pentadienyl anion. 

Ernst has recently reviewed structural and spectroscopic features of pentadienyl compounds. He discusses experimental and theoretical studies of pentadienyl anions, radicals, and cations and also the structures of transition metal pentadienyl complexes (21). He has also written an account of his own work in this field (22). 

Thus, Hine (1966a) used PLNM successfully to rationalise the sites of attack on conjugated reactive intermediates (cations, radicals and anions). The data is puzzling since the thermodynamically less stable non-conjugated isomers predominate protonation of the cyclohexadienyl anion, for example, yields predominantly cyclohexa-1,4-diene. The PLNM rationalisation of this result is set out in Scheme 14 in terms of the resonance structures of the pentadienyl anion fragment. 

C. Experimental and Theoretical Studies of Pentadienyl Anions, Radicals, and Cations... 

It should also be noted that even when coordinated to transition metals, formal pentadienyl cations can undergo ring closure reactions108, as can various heteroatom analogs109. Finally, transformations involving pentadienyl and cyclopentenyl radicals have also been observed110, In. ... 

However, another aspect of these methyl groups pertains to whether or not they serve to stabilize or destabilize a pentadienyl anion. Of course, it is clearly recognized that a stabilizing influence would ensue for pentadienyl cations and radicals. Results of the MNDO study indicate that methylation at an active (i.e., 1,5, or especially 3) position of a pentadienyl anion is accompanied by a stabilizing effect of ca. 1 kcal/mol, while methylation at a 2 or 4 site is destabilizing, by ca. 2. kcal/mol188. For the pentadienyl cation, methylation of the 1, 2, or 3 positions always leads to stabilization, by ca. 6, 1, and 4.5 kcal/mol, respectively. 

By using the orbitals drawn for Problem 6.24, indicate the electronic configuration for the pentadienyl cation, radical, and anion. Label the HOMO and LUMO for each case. 

The pentadienyl cation has 47r electrons the radical and the anion have 5tt and 6tt electrons, respectively. To obtain the electronic configuration, fill in the MOs in order, starting with the orbital of lowest energy. 

Sketch the molecular orbitals for the pentadienyl system in order of ascending energy (see Figures 14-2 and 14-7). Indicate how many electrons are present, and in which orbitals, for (a) the radical (b) the cation (c) the anion (see Figures 14-3 and 14-7). Draw all reasonable resonance forms for any one of these three species. 

It has been found that unbranched conjugated cations, anions and free radicals possess odd number of carbon atoms. Two such simplest systems are allylic systems and 2, 4-pentadienyl systems. One important characterstic of... 

Fig. 2.15. HOMO and LUMO of 2,4>pentadienyl cation, free radical and anion.

Reaction of the 1-methyl derivative of (LXXXVI) with metallic zinc produces the bis(iron tricarbonyl) deriArative of tra i-trfree radical intermediate, also involves geometrical inversion 32, 25). The cyclic cation (LXXI) reacts with zinc to give a dimeric product in which geometrical inversion cannot have occurred 42). 

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