Phenalene anion

The hydrocarbon phenalene is the precursor of both a highly stabibzed anion and a highly stabilized cation. The Huckel MO diagram is shown in Fig. 9.7. The single orbital at the nonbonding level is the LUMO in the cation and the HOMO in the anion. The stabilization energy calculated for both would be the same and is 0.41/3 by the HMO comparison. ... 

The orbital coefficients for the MO of energy a for the phenalenyl system described in Fig. 9.7 are as shown below. Predict the general appearance of the NMR spectra of the anion and cation derived from phenalene. 

Not all fused systems can be fully aromatic. Thus for phenalene (37) there is no way double bonds can be distributed so that each carbon has one single and one double bond. ° However, phenalene is acidic and reacts with potassium methoxide to give the corresponding anion (38), which is completely aromatic. So are the corresponding radical and cation, in which the resonance energies are the same (see p. 55). ° ... 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

Figure 6. First derivative electron spin resonance spectrum of radical anion formed spontaneously from phenalene in dimethyl sulfoxide (80% )-tert-butyl...

Two laboratories simultaneously reported the synthe.sis of the valence isomer of pleiadicne (14), namely naphtho[l,8]tricyclo[4.l.0.0 - ]hcptcne (13), by the reaction of the lithium anion of phenalene (II) with methylene chloride and n-butyllithium at low temperatures. A mixture of (13) and (14) is obtained in the ratio of 4 1. The new hydrocarbon (13, m.p. 76-78°) is surprisingly stable, but when heated at 150° in cyclohexane... 

The pK of phenalene ionizing to its anion 4 (Case Study 4.1) is about 20. The first absorption band of phenalene lies at vq o 2.9 pm Predict the acidity constant of phenalene in the lowest excited singlet state. [p fa = — 1]... 

The H-NMR and C-NMR spectra of both the anion and cation derived from phenalene have been observed. The HMO pattern for phenalene is given below. 

The pK for conversion of phenalene to its anion is 19. The cation is estimated to have a pXr+ of about 0-2." Several methods for generating the phenalenyl cation have been developed." ... 

Phenalene attracts considerable attention from chemists because of its ability to generate an anion, neutral radical, and cation, all of which are aromatic and stable in solution [6]. This molecule is prepared by the reduction of phen-alenone. Phenalenone, synthesized by the method of Fieser and Hershberg [7], when stirred overnight with excess of 9-BBN in THF at room temperature affords [8] phenalene (Eq. 25.10) in net yields of -90%. Phenalene is sensitive to silica gel, so Florisil, a much more acidic absorbent than silica gel, has been used to get the satisfactory purification. 

The for conversion of phenalene to its anion is 19. The cation is estimated to have a plphenalenyl cation have been developed. Because the center carbon is part of the conjugated system, the Huckel rule, which applies only to monocyclic conjugated systems, caimot be applied to just the peripheral conjugation. The nature of the phenalenyl system is considered further in Problem 12 at the end of this chapter. 

The cyclization also yields an aromatic phenalenyl anion moiety. This was demonstrated by the high-field shift of carbon atoms C8, C9, and ClOa (and their symmetrically equivalent atoms) and hydrogen atoms H8 and H9, which are part of the periphery of phenalene. This conclusion was supported by the similarity between the carbon chemical shifts of the phenalene moiety of 44b and 48b and those of the 2-methylphenalenyl anion 50 [128]. 

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