Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов

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Scheme 1.2 Current synthetic routes to TPP.

      This method not only affords TPP readily but is also useful for preparing its derivatives with diverse structures. For example, by using mono‐ or disubstituted benzoin, di‐ or tetrasubstituted TPP derivatives can be easily obtained. The reaction is less affected by the electronic effect of substituents. However, if the substituents are bulky, the crude products are difficult to purify by recrystallization. It is worth noting that two isomers are usually formed by using monosubstituted benzoin in the reaction. Such behavior is similar to that in the preparation of disubstituted TPE derivatives by the McMurry coupling reaction [44]. The presence of isomerization in TPP derivatives is proved by the fact that four resonance signals exist around 148 ppm in their ¹³C NMR spectra due to the four different chemical environments of carbon atoms in the pyrazine ring [33].

Tamaddon used SnCl₂·2H₂O as a catalyst to synthesize α‐amino ketones with benzoin and aniline as starting materials under solvent‐free conditions at either 80 °C or microwave irradiation [45]. Although the yields of reaction are high (up to 83%), the undesired byproduct of 1,4‐dibenzyl‐2,3,5,6‐tetraphenyl‐1,4‐dihydropyrazine is formed as examined by the NMR analysis, which is possibly due to the self‐condensation of the in situ formed α‐amino ketones. Further investigation indicated that under the same catalytic condition, a [2 + 1 + 1 + 2] four‐component reaction of benzoin and ammonium acetate can generate TPP in a high yield of 90% (Scheme 1.2, Route A, Condition 2). It provides a convenient strategy to prepare tetrasubstituted tetraphenylpyrazines in high yields. The substituents on benzoin are widely alternative. For example, by using methoxyl‐, methyl‐, bromine‐, chlorine‐, and fluorine‐substituted benzoins, the reactions can proceed smoothly. However, as the electron‐donating property of substituents increases, the yield of reactions slightly decreases.

      Subsequently, Tamaddon found that the multicomponent reaction can be carried out without SnCl₂·2H₂O as catalyst (Scheme 1.2, Route A, Condition 3) [46]. That is, SnCl₂·2H₂O is not necessary in the reactions. By directly heating the solid mixture of benzoin and ammonium acetate at 80 °C, the product can be obtained in a very high yield. Overall, this reaction is very close to the original one reported by Laurent, Erdmann, Japp, and Wilson, except that the ammonium chloride has been replaced by ammonium acetate. This optimal reaction condition was confirmed by the author after evaluation of the influence of ammonium salt, solvent, and reaction temperature on the reaction efficiency. The yields remain high in the reaction of benzoin derivatives regardless of the electron effect of the substituents. The reaction mechanism was similar to the previous one. Ammonium acetate releases the ammonia by heating to react with benzoin to generate an intermediate. Because the ammonium acetate is subtly excess, few acetic acid is formed, which can hardly react with the intermediate to produce 2‐methyl‐4,5‐dipenylglyoxaline. On the other hand, the solid‐state reaction allows the full contact of reactant with air. Both the factors may decide a high yield of reaction.

Khafizova reported a new one‐pot synthesis of tetrasubstituted TPP derivatives based on the reaction of nitriles with EtAlCl₂ catalyzed by metallic Mg and Cp₂TiCl₂ (Scheme 1.2, Route B) [47]. The reaction is efficient, and the product can be obtained in good yields of 60–90%. The catalytic system of Ti and Zr complexes such as Cp₂TiCl₂, Ti(PrⁱO)₄, TiCl₄, Cp₂Ti(PMe₃)₂, Ti(acac)₂Cl₂, Ti(Net₂)₄, and Cp₂ZrCl₂ is investigated to examine the influence on the reaction. Only Cp₂TiCl₂ and Ti(Pr_iO)₄ display superior catalytic activity and selectivity. Besides, the reaction has a wide universality of nitriles. For example, by using the starting materials of 2‐methyl‐, 3‐methyl‐, 4‐methyl‐, 4‐isopropyl‐, 3,5‐dimethyl‐, and 4‐methoxy‐substituted benzonitriles, different TPP derivatives with the tetrasubstituents can be obtained efficiently. The probable mechanism of this reaction was also given. Cp₂TiCl₂ was first reduced to the coordinatively unsaturated Cp₂Ti(II) complex by activated Mg. Then, two nitriles can coordinate with the Cp₂Ti(II) complex to form titanium‐nitrile π‐complexes, followed by transforming to diazatitanacyclopentadiene intermediate A. Subsequently, two additional nitrile molecules are inserted into the active Ti–N bonds of the intermediate, which gives rise to ring expansion to form the unsaturated tetraaza derivative B. The excess EtAlCl₂ reacts with B to replace the metal center to generate intermediate C, СКАЧАТЬ