Benzenes hexasubstituted

Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

Benzene with five hydrogens replaced are pentasubstituted, and when all six hydrogens are replaced, the benzene is hexasubstituted. 

A similar strategy has been applied by Mazik and coworkers. The receptor is usually titrated with increasing amounts of the sugar, to evidence binding, to extract the values of the binding constants and to get the stoichiometry of the complexes formed. Reverse titrations and extraction experiments can also be performed. The structures of the receptors described are based on different scaffolds the above mentioned hexasubstituted tripodal benzene moiety and 3,3, 5,5 -tetrasubstituted... 

Fig. 19 Suggested reaction mechanism for the construction of furopyrrolones (55) and hexasubstituted benzenes (57) based on the trisubstituted oxazole MCR...

In the last few years disc-like molecules have been shown to form liquid crystals (Chandrasekhar, 1994). Typical of them are hexasubstituted esters of benzene (I) and certain porphyrin esters (II) (see below). In the liquid crystalline state, the disc-like molecules are stacked aperiodically in columns (liquid-like), the different columns packing in a two-dimensional array (crystal-like). The phases have translational periodicity in two dimensions but liquid-like disorder in the third. In addition to the columnar phase(D), the disc-like molecules also exhibit a nematic phase (Nj,). A transition between D and phases has been reported. 

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 ... 

It can be used in nucleophilic -displacement reactions of unactivated aryl halides4 that originally were only possible in solutions in HMPT. An example is the reaction of C6C16 or C6F6 with the sodiu m salts of thiols to give hexakis(alkylthio)benzenes in >90% yield4, s. Indeed, hex akis(/ -naphthylthio)benzene (2) was prepared by reaction of hexachlorobenzene and the sodium salt of J -mercaptonaphthalene in 1 in 72% yield. Hexasubstituted ben zenes such as 2 are of interest because they can function as inclusion hosts related to clathrates of phenol.h... 

Alder reaction [89]. The latter compounds can be submitted again to a Diels-Alder reaction by treatment with maleimide and the resulting oxa-bridged intermediates, obtained as a mixture of diastereoisomers, can be transformed into hexasubstituted benzenes by thermal treatment. In these cases however, the possible asymmetric induction obtained during the cydoaddition is lost in the final part of the domino sequence. 

Catalytic applications of RCCo3(CO)9 complexes have been sought, and two such reactions have been reported. It was found that disubstituted acetylenes are trimerized to hexasubstituted benzenes when heated in the presence of alkylidynetricobalt nonacarbonyls (72) ... 

Under the conditions of Figure 12.12 acetophenone yields 1,3,5-triphenylbenzene. Lewis rather than Bronsted acids initiate the cyclodehydration of ketones to give aromatic compounds in an analogous fashion. Under influence of titanium tetrachloride and zinc chloride the steroid coprostanone, for example, leads to the formation of hexasubstituted benzene the 3x3 cyclohexane chairs that constitute the substituents of the resulting benzene ring are rigid... 

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... 

Ir(Cl)(N2)(PPh3)2] reacts with dialkyl acetylenedicarboxylates to form complex (225) via N2 displacement. The complex (225) may react with an alkyne at 383 K to yield the iradacyclopen-tadiene complex (226) and then hexasubstituted benzenes (Scheme 23).501... 

Co2(CO)8 and Hg[Co(CO)j2 (41), i7T-CMCo CO)2 191), Co CO)aNO 192), Co2(CO)8(RCaR ) (193), and Co4(CO)io(RC2R ) 48). The trimeri-zation reactions with symmetrically disubstituted alkynes, RC=CR, lead inevitably to hexasubstituted benzenes. Unsymmetrical alkynes RC=CR are trimerized generally to 1,2,4- rather than 1,3,5-trisubstituted benzenes. These cyclotrimerization reactions can be achieved readily by... 

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). 

Mislow et al. also studied the series of hexasubstituted benzenes 8a-8d.i24.i25 Jheir goal was to calculate the minimum energy conformers and to determine whether a concerted, geared rotation of all six substituents is the... 

Although catalytic preparations of cyclopentadienones with other metal systems are known, chemose-lectivity is often a problem. For example, the reactions of 2-butyne, 3-hexyne and diphenylacetylene with [(CO)2RhCl]2 at 80 °C give mixtures of hexasubstituted benzenes, metal-complexed, tetrasub-stituted cyclopentadienones and quinones. An exception is the preparation of the stable tetrakis(trifluo-romethyl)cyclopentadienone from the alkyne the unusual electronic properties of the product make this result lack generality. 

What two factors probably contribute to the very high melting points of these two hexasubstituted benzenes ... 

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)). 

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