1,2, 3-Trisubstituted benzene derivatives

A recent synthesis of mellein involves a Diels-Alder reaction, which gives the expected 1,2,3-trisubstituted benzene derivative... 

This mechanism is supported by the transformation of preformed metallacyclo-pentadienes with alkynes,73,76-78 and labeling experiments79 that excluded the involvement of cyclobutadiene intermediates. It also accounts for the observation that terminal alkynes yield 1,2,4- (and 1,3,5-) trisubstituted benzene derivatives as the main product but not 1,2,3 derivatives. In contrast with this picture in cyclotrimerization with PdCl2-based catalysts, stepwise linear insertion of alkynes takes place without the involvement of palladacyclopentadiene.80... 

Retention of isomers. We have examined the effect of alkyl groups on the retention behaviour, in dependence on the chain length, branching and the relative positions in di- and trisubstituted benzene derivatives. The experimental data are summarized in Table I. 

The general mechanistic picture presented above applies to a number of synthetically useful systems including the first to be discovered, in which trimerization of acetylene to benzene, and propargyl alcohol to a 1 1 mixture of 1,2,4- and 1,3,5-trisubstituted benzene derivatives was catalyzed by (Ph3P)2Ni(CO)2. Phosphine nickel carbonyls are usually much more highly regioselective for 1,2,4-... 

A few of the trisubstituted benzene derivatives that have been described for autoxidative dyeing are shown here ... 

Another type of benzannulation, which might be related to the benzannulation of conjugated enynes, is the Pd-catalyzed cyclotrimerization of 1,3-diynes (Scheme 1,3-Diynes cyclotrimerize in the presence of a Pd catalyst to give 1,3.5-trisubstituted benzene derivatives. In this reaction, too, the regioselectivity is perfectly controlled as shown in Scheme 4, and other products were never isolated. The reaction of various diynes is summarized in Table 7. Some functional groups such as olefins and ether linkages may be present in the side chain. An example for the cross-cyclotrimerization of a diyne and an aUcyne has also been reported (Scheme... 

Conjugated enynes cyclodimerize in the presence of Pd(0) catalysts to give di- or trisubstituted benzene derivatives with high regioselectivity. This reaction could be carried out in an intramolecular fashion to give [njparacyclophanes. 

Fig. 10 CD-curves (o-band only) of the homochirally trisubstituted benzene derivatives 11 and of analogue 12 with only one such chiral group and two achiral ones.

A trisubstituted benzene derivative possessing one bromine and two methoxy substituents exhibits three aromatic proton resonances 8 6.40, 6.46, and 7.41. What is the identity of the compound ... 

The present host-guest complexation provides a synthetically very simple route to a great variety of face-to-face porphyrin-quinone architectures. The present method may exert it s maximal usefulness when one considers to construct a suramolecular device for charge separation using a functional quinone such as 7 (Scheme 2). This quinone is functionalized by a molecular wire. All you have to do is prepare a trisubstituted benzene derivative 7. If you follow the covalent approach, you have to prepare a pentasubstituted derivative 8, where the substituent X is essential as the site of double coupling with a suitably disubstituted porphyrin derivative. 

Vogtle et al. reported the first example of a photoswitching dendrimer [94] with six azobenzene moieties attached to a derivative of 1,3,5-trisubstituted benzene as the central core. Irradiation of the all (F)-isomer at 313 nm led to a photostationary equilibrium where most of the azobenzene units were switched to the (Z)-configuration. Conversely, irradiation of this species again at a lower energy frequency (436 nm) led to a second photostationary equilibrium where the (F)-form was dominant however, it was not proven as to how many azobenzene units isomerized after irradiation. 

Transition metal-catalyzed intermolecular [2 + 2 + 2] cyclotrimerization of alkynes to benzene derivatives has been extensively studied. In this section, the focus is on the cyclo-trimerizations of the substrates bearing three independent unsaturated bond components. The key issue with this type of process usually involves the challenge of controlling regioselectivity [1—1]. However, 1,3,5-trisubstituted benzene 44 can be obtained as the sole product in good yield when 3-butyn-2-one 43 is used as the substrate for the cyclotrimerization catalyzed by Rh2(pfb)4 (pfb=perfluorobutyrate) in the presence of EtsSiH under a CO atmosphere (Eq. 11) [30]. 

Mechanistically, this unusual multicomponent trimerization can be rationalized as a sequence of a Pd-catalyzed alkyne dimerization [154,155] giving rise to the regioselective formation of the enyne 218, which undergoes subsequent Pd-catalyzed [4 + 2]-benzannulation [158,159] with a butadiyne as an enynophile to furnish the benzene 216 via an allenyl-Pd species 219 (Scheme 90). Upon submitting an electron-deficient alkyne together with a terminal alkyne in equimolar amounts to the sequence, the unsymmetri-cal alkyne dimerization gives a trisubstituted enyne to set the stage for the formation of pentasubstituted benzene derivatives 217. 

The metallacyclic compounds react with protic reagents to yield the corresponding substituted 1,3-butadienes and the iodination reactions give the 1,4-diiodo 1,3-butadiene derivatives (Scheme 122). Reactions with different unsaturated organic molecules have also been investigated. They catalyze the cyclotrimerization of a range of alkynes. Terminal alkynes with small substituents produce the 1,2,4-trisubstituted benzene, preferentially, in an exothermic reaction. The more bulky substrates BuTl CH and Me3SiC=CH react more slowly and only the symmetrical 1,3,5-isomer is produced (Scheme 122).174... 

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