Phenalenyl radical

So, the true relevance of the phenalenyl radical to graphene edge chemistry and physics resides in the fact that, as graphene layers grow by condensation/dehydro-genation processes, and rather than trapping n radicals as a consequence of edge sat-... 

A graph with an odd number of points is non-Kekulean by definition. No benzenoid hydrocarbon molecule or radical corresponding to non-Kekulean graph has ever been synthesized. The phenalene skeleton, XHIa, is the smallest non-Kekulean benzenoid. However, it is regrettably true that even phenalenyl radical is... 

Fukui, K., K. Sato, D. Shiomi, T. Takui, K. Itoh, K. Gotoh, T. Kubo, K. Yamamoto, K. Nakasuji, and A. Naito. 1999. Electronic structure of a stable phenalenyl radical in crystalline state as studied by SQUID measurements, cw-ESR, and 13C CP/MAS NMR spectroscopy. Synth. Metals 103 2257-2258. 

Why does a stable bis-allyl analog exist on the Cope reaction surface of 27 In the prototype Diels-Alder reaction of 1,5-hexadiene, the possible bis-allyl intermediate (i.e., two isolated allyl radicals) is about 26 kcal mol higher in energy than the Cope transition state. Only with significant radical stabilization might one expect a bis-aUyl intermediate to occur. One can consider 28 as composed of two bridged phenalenyl radicals (29), which affords a rather dramatic stabilization of each radical. 

Odd-Alternate Polynuclear Aromatic Hydrocarbon Radicals. Substantial evidence supports the contention that the stable free radicals formed during the pyrolysis of polynuclear aromatic compounds are odd-alternate hydrocarbon radicals. As an example, the phenalenyl radical (5) is formed during pyrolysis of a number of organic compounds including acenaphthylene (3) and dihydronaphthalene (4) (24) (see Scheme III). The... 

Scheme HI. Formation of the phenalenyl radical during pyrolysis of acenaphthylene and dihydronaphthalene.

Examples of monocation dimer formation among aromatic hydrocarbons have been confined mostly to alternant hydrocarbons, and the dimer can be regarded as an association of two closed shell molecules which have lost an electron. Recently Paskovich and Reddoch (1972) made a new class of monocation dimers, in which an electron is missing from two associated open shell molecules. Oxidation of phenalene by oxygen led to the phenalenyl radical and, it is thought, to the phenalenyl cation, association of which gave the monocation dimer (93). 

E2.6 The ENDOR spectrum of the phenalenyl radical in fluid solution is due to... 

Figure 1 First-derivative spectra obtained by continuous-wave techniques for the phenalenyl radical (A) ESR, (B) ENDOR, (C) special TRIPLE resonance, (D) general TRIPLE resonance, and (E) ENDOR-induced ESR. The arrows indicate the positions of the RF pumping frequencies there are several possibilities in (D). The RF pumping frequency in (E) is set in the same position as shown in (B). (Reprinted from Kurreck H, Kirste B, and Lubitz W (1988) Electron Nuclear Double Resonance Spectroscopy of Radicals In Solution. Weinheim VCH Publishers.)...

Fig. 3 Molecular orbital diagram and resonance structures of phenalenyl radical 4...

The phenalenyl radical 4 represents the most fundamental and widely explored member in this family. Unlike typical stable neutral radicals such as TEMPO and a-nitronylnitroxide derivatives with a spin-localized nature, 4 is characterized by a planar, rigid structure with the spin spread over the whole molecular skeleton (Fig. 3). The resonance structures of 4 show that the spin density is predominantly at its oc positions, while the spin at the peripheral positions (jS positions) is much smaller, which can be explained by the spin polarization effect. In addition, 4 exhibits a high amphoteric redox ability with thermodynamically stable cation, neutral radical and anion species. All of these features of 4 lead to new insights in the field of physical chemistry. 

As shown in Scheme 1, the synthesis of fert-butyl phenalenyl radical 10 started from 2,7-di-ferf-butylnaphthalene in ten steps. Bromination of 2,1-di-tert-butylnaphthalene gave 6 in high yield, which was converted into aldehyde by lithiation followed by reaction with DMF. Successive Reformatsky reaction afforded ester 7, which upon reduction, hydrolysis, and Friedel-Crafts acylation reaction gave the phenalanone 8. The key intermediate 9 was then obtained as pale yellow crystals by reduction of 8 and subsequent dehydration. Oxidation of 9 with p-chloranil in degassed toluene led to a blue neutral radical solution while similar treatment in hexane gave deep blue needles. This crystal showed high stability in the absence of air, while changing into phenalanone derivatives and other byproducts in 1 week in air. 

Phenalenyl radical derivatives with other substituents in the periphery, such as alkoxy [12], hydroxyl, amino [13, 14], and N-S-N groups [15], have been prepared and studied. Among them, an interesting example was a perchlorophenalenyl radical 16 with aU a-positions and -positions substituted by chlorine atom. The molecule was firstly prepared by Haddon et al. in 1987 [16] and the X-ray crystal structure was obtained in 2001 [17]. The synthetic route is depicted in Scheme 2, chlorination of acenaphthene 11 affording a mixture of perchloroacenaphthene 12 and perchloroacenaphthylene 13, and conversion of 13 from 12 can be realized... 

Scheme 2 Synthetic route and single-crystal structure of perchlorinated phenalenyl radical...

Fig. 5 (a) Structures of phenalenyl radicals with disulfide bridges (b) single-crystal structures of 17 [18-20], Reprinted with permission from [19], Copyright 2007 American Chemical Society... 

The first synthesis and isolation of an a type nitrogen-containing phenalenyl radical, a 2,5,8-tri-ferf-butyl-l,3-diazaphenalenyl 29, was achieved in 2002. The synthesis involved reduction of compound 26 followed by condensation with fBuCHO and dehydrogenation catalyzed by Pd/C. Oxidation with Pb02 and recrystallization led to a green crystal of 29. The crystal exhibited increased stabihty in air due to heteroatomic modification [21] (Scheme 3). 

Small D, Zaitsev V, Jung Y, Rosokha SV, Head-Gordon M, Kochi JK (2004) Intermolecular Ti-Xo-Ti bonding between stacked aromatic dyads. Experimental and theoretical binding energies and near-IR optical transitions for phenalenyl radical/radical versus radical/cation dimerizations. J Am Chem Soc 126 13850-13858... 

Zaitsev V, Rosokha SV, Head-Gordon M, Kochi JK (2006) Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered n- and (T-bonds. J Org Chem 71 520-526... 

Suzuki S, Morita Y, Fukui K, Sato K, Shiomi D, Takui T, Nakasuji K (2006) Aromaticity on the pancake-bonded dimer of neutral phenalenyl radical as studied by MS and NMR spectroscopies and NICS analysis. J Am Chem Soc 128 2530-2531... 

Beer L, Mandal SK, Reed RW, Oakley RT, Tham FS, Donnadieu B, Haddon RC (2007) The first electronically stabilized phenalenyl radical effect of substituents on solution chemistry and solid-state structure. Cryst Growth Des 7 802-809... 

Beer L, Reed RW, Robertson CM, Oakley RT, Tham FS, Haddon RC (2008) Tetrathio-phenalenyl radical and its disulfide-bridged dimer. Org Lett 10 3121-3123... 

Nakasuji K, Kubo T (2004) Multi-stage amphoteric redox hydrocarbons based on a phenalenyl radical. Bull Chem Soc Jpn 77 1791-1801... 

Fig. 19. Spontaneous self-associations of phenalenyl radicals forming the cr-dimer.

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