Para-quinodimethane

Paracyclophane-l, 5, 12, 16-tetrayne (37) was synthesized by Hopf via the reaction of dibromide 35 with Bu3SnSiMc3 (Scheme 10) [29]. The intermediacy of the novel cumulated para-quinodimethane 36 was invoked. Reaction of the ort/ o-substituted isomer 38 under the same conditions presumably gave the intermediate 39, which did not afford the corresponding orthocyclophane, but rather gave the highly strained 3,4-benzocyclooct-3-ene-l,5-diyne 40. 

Phenoxatellurin, in addition to the other phenoxachalcogens, forms a crystalhne complex, as donor, with 7,7,8,8-tetracyano-para-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 %-ti charge transfer type. 

Aldol reaction and alkylation of ketones at the a-position via enol silyl ethers (11) under neutral conditions are good examples [8]. Ortho-quinodimethane (15) is afforded at room temperature and undergoes [4 + 2] cycloaddition [10]. In contrast, the thermal ring opening of benzocyclobutane requires a much higher temperature (180-190°C) [11]. The fluorine-mediated Si—C activation protocol was the preferred method for the formation of tetrafluoro-para-quinodimethane (19), which then dimerizes to octafluoro-para-cyclophane (20) [12]. Azomethine ylide (22) can be generated from 21 byAgF catalysis and is used for heterocycles syntheses [13]. 

The intermolecular dimerization of ortho-quinodimethanes to [2.2]paracyclo-phanes is an established synthetic procedure [39]. However, with extended exo-cyclic double bond systems,para- or orf/zo-quinodimethanes yield cydophanes with alkyne groups in their bridges, such as 69, as products of an intermolecular cyclization [44]. Such para-quinodimethanes with cumulated double bonds are accessible via l,x elimination of bis-propargyl or benzyl bromides with Bu3SnSi-Me3/CsF [45]. Carbo- and heterocyclic unsaturated cydophanes have been pre-... 

Scheme 19. Synthesis of cydophanes via dimerization of para-quinodimethanes with cumulated exocyclic double bonds a) Bu3SnSiMe3/CsF, DMF, 6 h 0 °C, 12 h 20 °C [44]...

The highest (hyper)polarizability values appear for poly-para-quinodimethane. The polarizability for this polymer is 5-7 times larger than those for the other systems and the 2nd hyperpolarizability is more than 100 times larger. The HF (and thus MP2) method for the chain poly-para-quinodimethane gives unstable results. Apparently, this system has an allowed long-wavelength transition which makes it behave... 

Table 3.14 shows limit values of the optical parameters calculated according to Eqs. 3.41 and 3.42 for systems A, C, E, F, and H shown in Fig. 3.13. Polystilbenes (F) are combinations of model planar polyphenylenes and traus-polyenes. System H can also be represented as a combination of poly-para-quinodimethane and transpolyene. 

Based on the presented data, it can be concluded that the highest hyperpolarizability occurs in polymers based on para-quinodimethane. The results for system E, shown in the table, give an estimation of the lower bound due to the fact that, for the specific (y), a fast increase of the value with the number of the monomeric units is observed. Thus, one can expect even larger values for the linear poly-p-quinodimethane. 

Table 3.20 FCI hypercharges of [5-3] fulvalene without and with para-quinodimethane bridge...

The specific limit (y) for the push-pull variant of the poly-para-quinodimethane is found to be larger than that for the pure polymer. Clearly, the presence of the end... 

Scheme 2 Synthesis of ladder-type polyphenylenes using para- and meto-dibenzoyl benzene building blocks, and dehydrogenation of 3b to polymer 3 with para-quinodimethane repeat units. No control over the configuration at the bridging methylene carbons could be achieved...

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