P-Quinodimethanes

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

A. Issaris, D. Vanderzande, and J. Gelan, Polymerization of a p-quinodimethane derivative to a precursor of poly (p-phenylene vinylene) — indications for a free radical mechanism, Polymer, 38 2571-2574, 1997. 

A [2.2]paracydophane was prepared by dimerization of p-quinodimethane 232, which was obtained by a [4+ 2]-cycloaddition reaction of bisallene with DMAD [184], This sequence represents one of the most general approaches to functionalized para-cyclophanes. 

Attempts to detect a thermally populated triplet state ( A ) of 8 by ESR spectroscopy were unsuccessful. This was attributed to the high reactivity of the diradical, which presumably easily abstracts hydrogen atoms in hydrocarbon matrices (to form p-quinodimethane), even at very low temperatures. In this context, the triplet state of /7-phenylenebis(phenylmethylene) has been observed. Apparently, substitution of the carbenic hydrogens of 8 by phenyl groups confers sufficient stability (thermodynamic and perhaps kinetic) to the biradical, which allows its observation. According to variable-temperature ESR spectroscopy, the triplet state of /7-phenylenebis(phenylmethylene) is thermally populated and the singlet state lies 0.5-1 kcal/mol lower in energy [76-79]. 

Tetracyano ligands have been used to bridge between four Ru(NH3)5 moieties. The complexes [ Ru(NH3)5 4(/i-L)] + (L = tetracyanoethene, tetracyano-p-quinodimethane, 1,2,4,5-tetracyano-benzene, 2,3,5,6-tetracyanopyrazine) exhibit intense, long-wavelength electronic absorptions. Oxidation to [ Ru(NH3)5 4(yU-L)] °" " and reduction to [ Ru(NH3)5 4(//-L)] + and [ Ru(NH3)5 4-(/i-L)] + can readily be achieved. The species are fully delocalized with partially reduced ligands or partially oxidized Ru centers. Treatment of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato] cobalt(II) or [5,10,15,20-tetrakis(4-cyano-2,6,-dimethylphenyl)porphyrinato]cobalt(II) with [Ru-(NH3)5(0S02CF3)] introduces cyano-bound pendant Ru (NH3)5 groups to the porphyrinato complexes. ... 

Photoinduced oxidation of Cjq has been achieved by electron transfer from excited to a strong electron acceptor such as p-chloranil [72, 73], p-benzoquinone [73], tetracyano-p-quinodimethane (TCNQ) or tetracyanoethylene (TONE). This electron transfer proceeds efficiently only by addition of promoters such as Sc(OTf)3 or triflic acid, both of which strongly enhance the electron-transfer process [72, 73]. Another possibility to produce the cation is the electron transfer from to the singlet excited state of a strong electron acceptor such as N-methylacridinium hexafluorophosphate (NMA ) [74, 75], triphenylpyriliumtetrafluoroborate (TPP" )... 

Chemical oxidation of the TTF groups in compounds 34 and 35 has been achieved by reaction with an excess of iodine in dichloromethane solution, leading to new low-energy absorptions in the UV/visible spectra which are diagnostic of TTF cation radicals the broad absorption at = 830 nm for the iodide salt of 35 suggests the formation of aggregated TTF species. A charge transfer complex formed by 35 and tetracyano-p-quinodimethane (TCNQ) has been isolated as an insoluble black powder. The stoichiometry is (35), (TCNQ)3 (i.e. 8 TTF units 3... 

Since the discovery in 1973 of metal-like conductivity in the charge transfer salt tetrathiafulvalene-tetracyano-p-quinodimethane (TTF-TCNQ, 1,-2), a host of new materials have been prepared displaying this interesting property. Widespread research on these materials has led to an improved understanding of the physics underlying the organic metallic state, and to a succession of molecular modifications which have enhanced these properties. ... 

Bulk crystalline radical ion salts and electron donor-electron acceptor charge transfer complexes have been shown to have room temperature d.c. conductivities up to 500 Scm-1 [457, 720, 721]. Tetrathiafiilvalene (TTF), tetraselenoful-valene (TST), and bis-ethyldithiotetrathiafulvalene (BEDT-TTF) have been the most commonly used electron donors, while tetracyano p-quinodimethane (TCNQ) and nickel 4,5-dimercapto-l,3-dithiol-2-thione Ni(dmit)2 have been the most commonly utilized electron acceptors (see Table 8). Metallic behavior in charge transfer complexes is believed to originate in the facile electron movements in the partially filled bands and in the interaction of the electrons with the vibrations of the atomic lattice (phonons). Lowering the temperature causes fewer lattice vibrations and increases the intermolecular orbital overlap and, hence, the conductivity. The good correlation obtained between the position of the maximum of the charge transfer absorption band (proportional to... 

Two types of even AH have been recognized. Those which can be represented as classical polyenes (in which all the atoms are linked in pairs by double bonds) have been described as Kekule hydrocarbons. Benzene is a Kekule hydrocarbon and so are the o- and p-quinodimethanes (469 and 470). >0- ... 

Values are for the 1 1 complex of 1,10-phenanthroline with 7,7,8,8-tetracyano-p-quinodimethane. 

There has been some interest in the electrical conductivity and magnetic properties of salts of 1,10-phenanthroline and 1-alkyl-1,10-phenanthrolinium cations with 7,7,8,8-tetracyano-p-quinodimethane and its anion radical297-302 and related species.303... 

Charge transfer complexes between various phenanthrolines and tetracyano-p-quinodimethane have been patented as inks and copying materials because of their high color.300... 

The tetrabutylammonium salts of the bis chelates are semiconductors but exhibit enhanced conducting properties after doping with iodine. The corresponding salts which are formed when Bi N is replaced by TTF+ or TSeF+ have higher conductivities than the analogous dithiolene complexes with magnitudes similar to that of TTF-TCNQ (TTF = tetrathiafulvalene, TSeF = tetraselenofulvalene, TCNQ = tetracyano-p-quinodimethane). 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

Compared with the typical photodimerization of condensed aromatics such as anthracenes, benzene is inert to photodimerization even if two benzene rings are connected intramolecularly. Photoirradiation of higher strained [2.2]paracyclo-phane (312) with a low-pressure mercury lamp at 196°C gave rise to photocleavage to give p-quinodimethane (313) without any photodimerized product [311] (Scheme 89). 

Earlier, I criticized the Cooper pair being philosophically unacceptable because the proposed pair is not seen elsewhere outside of the BCS superconductivity theory. The same measurement must be placed against the Covalon-Conduction theory proposed here. We shall show therefore, that a number of physical property changes observed in [TCNQ-TTF] (tetracyano-p-quinodimethane-tetrathiofulvalene), an organic compound, as a function of temperature can be interpreted and understood in terms of Covalon-conduction . 

Other functional polymers were prepared for which the counteranions (BF4, PF6-,. ..) were replaced by the tetracyano-p-quinodimethane radical... 

Treating 113 (easily available from 45 by reaction of its dilithio derivative with formaldehyde and treatment of the resulting diol with phosphorus dibromide) with the so-called Mori reagent [72], a trimethylsilyl(tri-n-butyl)stannane/cesium fluoride combination that has been used very successfully in many debromination reactions [73], results in the production of the [6.6]paracyclophane 115 in acceptable yield. Since p-xylylene (p-quinodimethane) is known to dimerize to [2.2]paracyclophane, it may well be that the bis(cumulene) 114 is formed as an intermediate in this reaction. Since a stepwise process is also conceivable, further clarification of the mechanism is obviously desirable (Scheme 25). 

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