Benzene, hexasubstituted derivatives

Interestingly, cyclopropane and benzene have the same number of isomers for a given number of identical substituents ignoring optical isomers, cyclopropane and benzene have one type of monosubstituted derivative, three types of disubstituted, three types of trisubstituted, three types of tetrasubstituted and one type apiece of penta- and hexasubstitution. But does this seemingly accidental counting equivalence have any thermochemical consequences ... 

In 1974 Vogtle and Weber published a paper entitled Octapus Molecules 4). The paper describes a hexasubstituted benzene derivative (la) which shows remarkable phenomenological parallelisms to the mode of food capture by an octopus using its suction pads. Compound (7a), a water-insoluble oil, was found to be a powerful... 

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

Threefold donor-acceptor-substituted benzene derivatives like [109] (Ledoux et al., 1990) or [110] (Verbiest et al., 1994 Wolff et al., 1996b Wortmann et al., 1997 Wolff and Wortmann, 1998) show better performance for [109], only powder data and computational results are available. Both are of the hexasubstituted type, but strong intra- and inter-molecular hydrogen bonds provide for planarity. The discrepancy (Bredas et al., 1992) between the observation of a moderate powder SHG efficiency of [109] and the published (Cady and Larson, 1965) centrosymmetric crystal structure (PT) has been resolved. The powder consists of two polymorphs, with the second one adopting the close to optimal space group P3i (Voigt-Martin et al., 1996,1997). 

Monosubstituted alkynes may be included in this cyclization giving 1,2,4,7-, 1,2,4,6-, and 1,3,5,7-tetrasubstituted cyclooctatetraene derivatives [86]. A special case is the cyclotetramerization of 1 -phenylpropyne giving the octasubstituted C product besides the hexasubstituted benzene derivative [87] (eq. (30)). 

Irradiation of the biscyclopropenylmethane la in the presence of thioxanthone as a triplet sensitizer gave tetraphenylbicyclo[2.2.1]heptadiene 3a by bond relocation in an intermediate strained quadricyclane 2a.A similar cyclization occurred for the 1,2-biscyclo-propenylethane derivative lb, while in the case of 4 aromatization was observed either on photolysis in diethyl ether or thermolysis for 24 hours at 175 C. In the same manner, the sulfone 5 was converted into a hexasubstituted benzene. ... 

Reduction of 3-methyl-1,2-diphenylcyclopropenylium perchlorate (24) with a slurry of activated magnesium metal in diethyl ether afforded the hexasubstituted benzenes 25 and 26 together with derivatives of bi(cyclopropenyl) 27 and bicyclo[2.2.0]hexa-2,5-diene 28. ... 

The more complex [2 -f 2 + 2] cycloisomerization reaction of acetylene units is also catalyzed by transition metal-alkyne n complexation and can be readily utilized for the synthesis of a variety of polysubstimted benzene derivatives in a straightforward manner (10, 352, 353). Recently, this methodology has been applied to the cyclization of 15-membered, nitrogen-containing di- and triacetylenic macrocycles. Upon coordination with Pd(0) to the triacetylenic macrocycle at ambient temperature, the ti-coordinated Pd(0) complex results. Subsequent refluxing of this species in toluene promotes cycloisomerization to the hexasubstituted arene (354) (Scheme 28). The Rh(I) [e.g., RhCl(CO)(PPh3)2] complex also catalyzes these same transformations in high (>80%) yields. 

Crown ethers have recently been shown to form crystalline, strictly stoicheiometric addition compounds with hexasubstituted benzene derivatives, e.g. (94), which are suited for systematic investigations of intra- and inter-... 

Till now only few examples of discotic liquid crystalline polymers are known. In the first example described in literature (1),hexasubstituted triphenylene cores as discotic sidegroups, are linked to a polysiloxane backbone via a flexible alkyl spacer. Discotic main chain polymers (2,3) posses as disclike core benzene or tripenylene derivatives with the same high degree of substitution, i.e. six. 

It is noteworthy that the dichloro derivative 47 is a colorless compound (s ax 295 run) in contrast to 2,5-dianilinoterephthalic acid 17, (R = H) which is colored (Smax 435 run). Evidently the hexasubstituted central benzene ring of 47 prevents a coplanar arrangement and consequently an effective electronic interaction between the aniBno and carboxy groups. Cyclization of the terephthalic acid in PPA... 

The applicabihty of Turbo-Hauser A towards aromatic substrates is expertly proven by the susceptibility of the substituted aromatic ethyl 3-chlorobenzoate to undergo four consecutive direct magnesiations/electrophilic quenches to yield a hexasubstituted benzene derivative (Fig. 12) [30]. The second metallation requires careful choice of solvent in neat THF a competitive deprotonation occurs in a 90 10 ratio in favour of the desired product, which can be improved to a near quantitative level of 98.5 1.5 ratio by employing the less polar THF/Et20 (1 2) mixture. It should also be noted that the third and fourth metallations must be carried out at 50 °C, reflecting the greater sensitivity of the multi-functionalised benzenes. 

A multiple functionalization of a bisubstituted aromatic compound such as 68 can be achieved by successive magnesiation with TMPMgCl-LiCI (64) and quenching with various electrophiles, finally leading to hexasubstituted benzene derivatives such as 70-73 (Scheme 2-29). i... 

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