Cyclobutadienes radical cations

Both valence isomers easily give off an electron the first vertical ionization energies amount to only 7.5 eV and 6.35 eV (29), and therefore, both compounds can be oxidized in solution using the selective one-electron oxidizing system AICI3/H2CCI2 (3). The result, however, is puzzling in both cases, the tetrakis-(tert. butyl) cyclobutadiene radical cation is identified by its ESR spectrum (30). 

Radiolysis of deca-2,8-diyne (57) results in an interesting cycloaddition, forming a cyclobutadiene radical cation (58 ) at 77 K without requiring annealing at higher... 

Radical cations (94 +) of simple alkynes (e.g., butyne) generated by y-irradia-tion in frozen matrices are stable at 77 K. Upon warming, changes in the ESR spectra support an interesting cycloaddition forming cyclobutadiene radical cations... 

For an example of the tetragonal E x (bi + bj) type problem we refer to the detailed ab initio calculations on the cyclobutadiene radical cation (C4H4 ) by Borden, Davidson and Feller [34]. The potential energy surface of this cation is found to ressemble the surface in Fig. 4 with rectangular minima and rhomboidal saddle points (i.e. E2 > 0). 

Figure 2. Calculated geometries of Jahn—Teller-distorted structures of cyclobutadiene radical cation. (Reprinted with permission from ref 15i. Copyright 1995 Elsevier Science.)...

V. Hrouda, T. Bally, P. Carsky, and P. Jungwirth, J. Phys. Chem. A, 101, 3918 (1997). The C4H/ Potential Energy Surface. 2. The Jahn-Teller Stabilization of Ionized Tetrahedrane and Its Rearrangement to Cyclobutadiene Radical Cation. 

T. Bally, S. Bernhard, S. Matzinger, J.-L. Roulin, G. N. Sastry, L. Truttmann, Z. Zhu, A. Marcinek, J. Adamus, R. Kaminski, J. Gebicki, F. Williams, G.-F. Chen, and M. P. Fulscher, The Radical Cation of i-yn-Tricyclooctadiene and Its Rearrangement Products, Chem. Eur. J. 2000, 6, 858. A combination of CASPT2 calculations of UV spectra and B3LYP and CCSD(T) calculations of PESs is used to identify the intermediates formed sequentially in the rearrangement of the radical cation of the syn dimer of cyclobutadiene and to understand why they differ from those formed from the anti dimer. 

Alkvnes do rot dimerize photochemically to give cyclobutadienes, but dimers are formed from arylalkynes under conditions of electron-transfer sensitization (2.105). These dimers arise from a reaction of the alkyne radical cation with ground-slate alkyne, followed by intramolecular electrophilic attack on the benzene ring. 

The third returns the reader to chemi-ionization of cyclopropane derivatives with excited state electron acceptors. In the particular, it has been observed that oxidation of l,6-dimethylbicyclo[4.1.0]hept-3-ene (46) and [4.4.l]propella-2,6-diene (47) result in 2,6-dimethylcycloheptatriene (48) and l,6-methano[10]annulene (49) (equations 28 and 29 respectively). The former compound is conceptually the deprotonation product of the dication wherein the C-C bond between the bridgehead carbons has been totally depleted of electrons. In this case, however, the dication is not expected to be as stable as in the above pyramidal or cyclobutadiene cases and indeed, the product is seemingly not formed via the dication route. The methanoannulene likewise does not arise from the tetracation of its precursor. Rather, proton and electron transfer reactions involving merely radical cations proceed to remove hydrogens sequentially. 

The novel four-center, two-electron delocalized a-bishomoaromatic species 182,183,188,190a, and 192 are representatives of a new class of 27i-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations (Fig. 5.11,193). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 194 having a conventional p-type delocalized electron structure (Fig. 5.11). 

Montellano et al., 1984). The cytochrome f -450-catalyzed oxidation of 2,3-bis(carbethoxy)-2,3-diazabicyclo[2.1.0]hex-5-ene, a known cyclobutadiene precursor, results in enzyme inactivation and eventual isolation of N-(2-cy-clobutenyOprotoporphyrin IX (Steams and Ortiz de Montellano, 1985). The formation of this adduct is readily rationalized by oxidative release of cyclobutadiene (or its radical cation) which binds to a nitrogen of the heme and then abstracts a hydrogen from a protein residue (Fig. 31). 

In cycloaddition reactions of tetra-t-butylcyclobutadiene with either dicyanoacetylene or tetracyanoethylene it is suggested that the initial step may be oxidation of the cyclobutadiene by the cyano compound to a radical cation which recombines with the radical anion of the cyano compound to provide the adduct [53],... 

Further examples for tetragonal point groups are the cyclic molecules cyclobutane (CB), cyclobutadiene (CBD), and their radical cations. Depending on whether CB is planar or puckered, the pertinent point group... 

Cation-radicals containing three electrons in the field of four atoms or anion-radicals with five electrons retained by four atoms represent a special group of multicentered ion-radicals. Thus, nonclassical, cyclically delocalized 3e/4C cation-radicals and 2e/4C (dication)-radicals of substituted cyclobutadienes are known (Allen and Tidwell 2001). The 3e/4N cation-radical and the 5e/4N anion-radical have also been discovered (Exner et al. 1998, 1999, 2000). The reactions in Scheme 3.31 illustrate structures of the 5e/4N anion-radical as well as the corresponding dianion and acety-lated products of the latter. 

Tetra(tert-butyl)tetrahedrane converts into tetra(tert-butyl)cyclobutadiene only when heated up to 140°C in vacuum. A barrier of 170 kJ mol separates these two isomers (Heilbronner et al. 1980). In the cation-radical state, the tetrahedrane structure converts into the cyclobutadiene structure without heating (Bock et al. 1980, Fox et al. 1982). From Scheme 6.34 it can be seen that by the action of aluminum chloride on methylene chloride, tetrahedrane forms the cation-radical of its isomer—the cyclobutadiene cation-radical and not the cation-radical of the same skeleton. The latter is more stable than the former because of more effective delocalization of the unpaired electron and positive... 

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