Quinodimethane type

Vanderzande showed that activation of the starting material 6 proceeds via 1,6-elimination of HX, and provided experimental evidence of radical chain propagation of the p-quinodimethane-type active monomer 8 [55-57]. However, while high-molecular-weight polymers form in the polar-protic... 

Although direct proof was not available until recently, chemical arguments, experimental indications, and the analogy to the aforementioned methods led to the assumption that the Gilch synthesis proceeds according to Scheme 29.3 p-quinodimethane-type species 11 probably are the active monomers, and... 

Benzylic-type ip -centres of heteroaromatic aldehydes (e.g., the methyl of indole 118) can be activated with chiral NHCs, via ort/io-quinodimethane-type intermediates, allowing cyclization with trifluoromethyl aryl ketones to give lactones (e.g., 119) in good yields and ee " ... 

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

Malpass, 1977). Diels-Alder type [2 + 4]-cycloadditions are possible with certain hetero-"ene components (J.R. Malpass, 1977 S.F. Martin, 1980) or with highly reactive o-quinodimethanes as diene components (W. Oppoizer, I978A). 

Bis-o-quinodimethanes have also been used to functionalize [60]-fullerene by Diels Alder reaction. An example is the preparation of main-chain polymers with incorporated [60]-fullerene units [48] illustrated in Scheme 2.20. Cycloaddition of bis-diene 50 generated in situ from bis-sulfone 49 with [60]-fullerene leads to an oligomer mixture 51. Another type of functionalization is based on the... 

An interesting example of the application of the theory is a prediction of a new route to polyamantane by polymerization of -quinodi-methane 121h The first step would be n-n overlapping interaction. The HO and LU of quinodimethane are indicated in Fig. 7.40 a. The mode of n HO-LU interaction and the possible structure of polyamantane derived therefrom (Type I polymer) can be seen in Fig. 7.40b. On the other hand, the direction of the hybridization change would be controlled by the a-n interaction. The nodal property of n HO and a LU of the monomeric unit are as shown in Fig. 7.40 c, so that the hybridized states of carbon atoms might change into the form illustrated in Fig. 7.40d to lead to the Type II polymer. 

Phenoxatellurin, in addition to the other phenoxachalcogens, forms a crystalline complex, as donor, with 7,7,8,8-tetracyano-pflrfl-quinodimethane (TCNQ) whose stoichiometry was found to be 1 1. The equilibrium content and the activation coefficients are relatively small, suggesting that the complex formed in dicloromethane solution is relatively weak, probably due to the n-n charge transfer type. 

Unstable dienes can also be generated in situ in the presence of a dienophile. Among the most useful examples of this type of diene are the quinodimethanes. These compounds are exceedingly reactive as dienes because the cycloaddition reestablishes a benzenoid ring and results in aromatic stabilization.37 38 39... 

All discussed Ksem s are related to the separately solvated members of a two step redox system. In solvents of low polarity the charged forms may form ion pairs. Especially prone to this association are anionic redox systems of Type C (RED + OX = 2 SEM ) since the often used gegenions K , Na and Li tend to form ion pairs with the anions. These exhibit special UV/VIS-, NMR- and ESR-spectra as well as g-values Dimeres of the type (SEM M )2 may also be formed, as demonstrated with the anion radicals of pyrazines ° heptafulvalene and tetracyano-quinodimethanes. Corresponding associations are reported for dian-... 

An alternative polymerization mechanism and polymer architecture has been proposed by Kirchhoff [1, 2, 3], Tan and Arnold [77], By this mechanism, polybenzocyclobutenes which do not contain reactive sites of unsaturation are proposed to polymerize by the 1,4 addition of the o-quinodimethane intermediates to give a substantially linear poly(o-xylylene) structure. Since the monomers all contain at least two benzocyclobutene units the net result of this reaction will to a first approximation be a ladder type polymer as shown in Fig. 17. The formation of a true ladder polymer however would require that all... 

The use of benzocyclobutene as the source of the diene in a Diels-Alder polymerization offers a unique solution to the problems described above. Benzocyclobutene containing monomers can be stored indefinitely at room temperature without concern for further advancement of the molecular weight. It is only when benzocyclobutene is heated to temperatures of approximately 200 °C that the reactive diene, o-quinodimethane, is formed at a significant rate and enters into reaction with the dienophile. The only requirement of the dienophile is that it must be stable at these temperatures and not undergo reaction with itself. The most common dienophiles that have been successfully used in the formation of polymers from AB type benzocyclobutene monomers have been acetylenes, olefins and maleimides. 

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- ... 

The annulated ring in type C carbonyl ylides 152, generated from oxi-ranes 151, can be heteroaromatic (furo- or thieno-), and the products 153 have good Diels-Alder reactivity, functioning as o-quinodimethane heteroanalogs (Scheme 46) [88AG(E)568 93CB975],... 

Quinodimethane derivatives of terthiophene and other oligomers showed an absorption maximum at 583-688 nm (02JOC6015 03OL1535). Bis(dicyanoethylene) oligothiophene derivatives have been characterized and the compound with four thienyl rings showed an absorption band at 790 nm (02JA12380). This type of compounds was used in thin film transistors (see below) (02JA4184). 

For fiirther reactions of this type see <95CC2025>. The synthesis of l,4-dihydrofurano[3,4-d]-3,2-oxathiine-2-oxides, precursors for nonclassical heteroaromatic o-quinodimethanes, and their application in the Diels-Alder reaction were reported <95CC2537>. The intramolecular Diels-Alder reaction with furans (e. g., 47) offers a rapid access to highly functionalized isoquinolines <95 JCS(P 1)2393>. 

This type of interaction determines above all the optical properties of lowdimensional organic conductors in the middle infrared region. It leads to activation of modes that are normally nonactive in the IR. For the tetra-cyano-p-quinodimethane (TCNQ), tetrathiafulvalene (TTF), and other symmetrical molecules, tt molecular orbital occupied by the radical electron are nondegenerate, so linear e-mv coupling is possible only for the totally symmetric (ag) modes. For example, TCNQ molecule has 54 normal modes, among which only 10 are the ag modes. They cover a range of frequencies from about 130 cm-1 to about 3050 cm-1 [16], while TTF molecules has seven totally symmetric modes of the range 250 cm-1 < toa < 3100 cm-1... 

Neutral imines also add to o-quinodimethanes. Kametani s group has reported several cycloadditions of the type shown in equation (16). 

The in situ-generated o-quinodimethane 51 underwent Diels-Alder reaction giving rise to the formation of both B- and C-rings with the correct relative stereochemistry in one step. Racemic estrone was then obtained in two steps in very good yield. A one-pot access to the ABCD-rings of diter-penes has also been described by ourselves [49]. In the following example, the -acetylenic- -ketoester 53 furnished through a diastereoselective one-pot sequence [ene type]/[2 + 2 + 2]/[4 + 2] the phyllocladane framework 56 in 42% yield (Scheme 26). 

A special case are the radical cations of o- and p-quinodimethane which have low-lying non-Koopmans excited states. Due to mixing between A- and B-type configurations (cf Figure 27) these states sometimes show up as additional weak bands (so-called satellite bands) in the PE spectra and thus represent rare examples of cases where the number of PE bands exceeds the number of occupied MOs in the range of tt-ionizations. In the case of o-quinodimethane and derivatives, the positions of these states were confirmed by the EA spectra of the corresponding radical cations whereas the same was not done for p-quinodimethanes. 

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