7r-dimerization

The dimerisation energy for derivatives of 2 (ca. 35 kJ mol-1) is considerable, particularly in relation to the strength of intermolecular forces and some persistence is required in order to isolate derivatives of 2 which do not form 7T —7r dimers in the solid state. A survey of the monomeric derivatives has been published recently.26 Since the spin density distribution in 2 is rather insensitive to chemical tuning, approaches to inhibit dimerisation rely exclusively on structural modifications, which affect the nature of the intermolecular forces. Inclusion of sterically demanding groups, such as 13, 14 and 15 has proved partially successful (in the case of the diradical 14 one ring is involved in formation of a dimer, while the other retains its open shell character). 

In situ EPR spectra were taken from the paramagnetic sexithiophene radical cation and anion and allowed the determination of the spin density distributions. In accordance with the postulated 7r-dimerization, for the radical cation the intensity of the EPR signal decreases substantially with decreasing temperature and indicates a spin pairing to the 7r-dimer. This fact easily explains the puzzling observation that... 

It is commonly accepted that in solution a-6T is oxidized in two steps, first to the radical cation, then to the dication. The cationic state is believed to exist as a 7r-dimer in which two radical cations couple via their 7r-systems and which has a nearly spin-less state [292-297]. This u-dimer forms as easier as longer the chain length and as lower the temperature is due to its exothermic building process. ESR measurements show that the mono-oxidized oligothiophene is a (nearly) free radical with an intense and narrow Lorentzian signal centered at g = 2.0023-2.0025 for unsubstituted [298] as well as alkyl-substituted a-nT [293], The dication formation is only possible for a-6T and longer oligomers, q-4T and a-5T either do not react or dimerize to octi- and decithiophene, respectively [299]. (For a review on results obtained in solution, compare [250].)... 

There is no evidence of 7r-dimer formation or dimerization to an oUgomer with double chain length as in solution, even not for a-2T and a-3T. After one day the original spectra of the undoped materials are revealed. Apparently the radical cations remain nearly immobile on the surface. 

The structure of the 6 1 compound has three perylene species at general positions in the asymmetric unit and disordered ClOj (Figure 2.7). Each perylene species belongs to a 7r-dimer whose counterpart is generated by the nearest inversion centre. The inter-planar distances within the dimers are 3.38(2), 3.44(2) and 3.46(3) A, with similar overlap patterns. The dimers do not stack, as they are isolated in the crystal and tilted towards each other with dihedral angles of 102°, 84° and 103°. 

Two monomeric and dimeric 2-substituied 7r-allylic complexes (548 and 549) are obtained by treatment of allene with PdCl2(PhCN)2. They are formed by the nucleophilic attack at the central carbon of allene[493, 494],... 

An active catalytic species in the dimerization reaction is Pd(0) complex, which forms the bis-7r-allylpalladium complex 3, The formation of 1,3,7-octa-triene (7) is understood by the elimination of/5-hydrogen from the intermediate complex 1 to give 4 and its reductive elimination. In telomer formation, a nucleophile reacts with butadiene to form the dimeric telomers in which the nucleophile is introduced mainly at the terminal position to form the 1-substituted 2,7-octadiene 5. As a minor product, the isomeric 3-substituted 1,7-octadiene 6 is formed[13,14]. The dimerization carried out in MeOD produces l-methoxy-6-deuterio-2,7-octadiene (10) as a main product 15]. This result suggests that the telomers are formed by the 1,6- and 3,6-additions of MeO and D to the intermediate complexes I and 2. 

Dimerization is the main path. However, trimerization to form 1.3,6,10-dodecatetraene (15) takes place with certain Pd complexes in the absence of a phosphine ligand. The reaction in benzene at 50 C using 7r-allylpalladium acetate as a catalyst yielded 1,3,6,10-dodecatetraene (15) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 h[ 19,20]. 1,3,7-Octatriene (7) is dimerized to 1,5,7,10.15-hexadecapentaene (16) with 70% selectivity by using bis-rr-allylpalladium. On the other hand. 9-allyl-l,4,6.12-tridecatetraene (17) is formed as the main product when PI13P is added in a 1 1. ratio[21]. 

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

Others) with (+)-DIOP for asymmetric induction. The 7r-allylpalladium chloride dimer 118 was mixed with the hgand (ratio 1 2) in THF at the stated temperature, after which a solution of diethyl sodiomalonate in THF is added (Scheme 34). 119 was obtained with an optical purity of up to 20%. 

Dimeric via OH-tt interactions Monomeric, intramolecular OH-ir interaction Monomeric, intramolecular OH 7r interaction Monomeric, intramolecular OH- it interaction Intramolecular OH-N only Dimers, intermolecular OH - N only Dimers, intermolecular OH---N only Chains, intermolecular OH -N only... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

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