Organosilicon compounds are widely used in organic synthesis, material science and pharmaceutical chemistry. For example, flusilazole, thiabendazole and flusilathrin are organosilicon compounds. Therefore, it is of great significance to develop efficient synthesis methods to achieve functional organosilicon compounds. Recently, Zhao Dongbing's research group of Nankai University realized the functionalization of silicomethyl C? H bond by ingenious design, using 1,5-palladium migration from aryl to alkyl, and completed the efficient synthesis of alkenylsilane, benzylsilane and boroxylated silane (Figure 1). It is worth mentioning that although metal transfer reaction is widely used as a C? H bond activation strategy, no previous literature has reported 1,5-palladium transfer from aryl to alkyl.
Fig. 1. 1,5-palladium transfer reaction experiment silicomethylation. Image source: angelw. Chem. Int. ed
At first, the reaction of (8-bromonaphthalene-1-yl) trimethylsilane and n-benzylidene-4-methylbenzenesulfonyl hydrazide was taken as the model substrate. The optimum reaction system was 5 mol% pd2dba3, liotbu, sphos and toluene as solvent. Alkenylsilane was obtained in 87% yield at 100 ° C for 24 hours. Then, under the optimal conditions, the substrate expansion of the alkenylation reaction between bromosilane 1 and n-p-toluenesulphonylhydrazone 2 was carried out. First of all, the author found that all kinds of n-p-toluenesulphonylhydrazone can react with (8-bromonaphthalene-1-yl) trimethylsilane to effectively convert it into useful vinyl silane. Furthermore, the author also found that the 1,5-transfer / carbene coupling reactions can take place smoothly when various silanes with silicomethyl groups are used as substrates, resulting in a variety of alkenylsilanes (Fig. 2).
Fig. 2. Substrate development of C-H bond alkylation of silicon methyl. Image source: angelw. Chem. Int. ed
On this basis, the author tried to transform the nucleophilic reagent into arylboric acid in the reaction process. They hope that after 1,5-palladium migration to form C? PD species, the substrate will undergo transition metallization and reduction elimination, so as to realize the synthesis of various benzylsilanes. It is gratifying that when they transform the alkali under the above optimal conditions into cesium carbonate, the series reaction can take place smoothly, and the substrate has good universality and high yield (Fig. 3).
Fig. 3. C-H bond arylation of silicomethyl. Image source: angelw. Chem. Int. ed
Finally, according to their understanding of the reaction mechanism, b2pin was used as the boration reagent to further realize the boration reaction of silicomethyl C? H bond, thus obtaining a series of borated silane reagents (Fig. 4).
Figure 4. C-H bond boration of silicomethyl. Image source: angelw. Chem. Int. ed
On the basis of substrate expansion, the author further proves the universality of the alkenylsilane and benzylsilane obtained in this paper as synthesis intermediates through various further transformations, highlighting the importance of their established methods (Figure 5).
Figure 5. Derivatization reaction. Image source: angelw. Chem. Int. ed
Subsequently, the author also explored the reaction mechanism through a series of control experiments (Fig. 6). In the experiment of mechanism exploration, it was found that when the carbon like substrate of silane reacts with phenylboronic acid, there is no transfer process from aryl group to alkyl 1,5-palladium. The results show that the silicon atom in the α - position of methyl plays a key role in triggering the migration of 1,5-palladium. In addition, when the deuterium compounds are reacted with phenylboric acid under standard conditions, it is found that the deuterium atom is 100% regioselectively transferred to the 8-Position of 1-naphthyl. This indicates that the transfer of H-atom in the reaction is probably an intramolecular transfer process.
Figure 6. Mechanism exploration experiment. Image source: angelw. Chem. Int. ed
Finally, according to the literature and mechanism experiments, the author proposed a reasonable catalytic cycle (Figure 7). They tend to think that proton transfer is a cooperative process.
Figure 7. Prediction mechanism. Image source: angelw. Chem. Int. ed
To sum up, the author reported for the first time the unique migration process of 1,5-palladium from aryl group to alkyl group, and realized the remote C? H-linked alkenylation, aromatization and boration of silylmethyl group. It is further proved that the α effect of silicon atom may be the key to trigger the 1,5-pd / H transfer. This will provide more opportunities for further experiments of C? H bond functionalization by 1,5-palladium transport pathway.
This research achievement was published in the recent angew. Chem. Int. ed., researcher Zhao Dongbing of Nankai University is the corresponding author of the paper, master's student Han Jielian is the first author of the paper, master Qin Ying and undergraduate Ju Chengwei are the second and third authors of the paper respectively. This work is supported by NSFC and Nankai University.
Original text (scan or long press QR code, and then go to the original page after identification): diligent synthesis of vinyl, benzyl, and borylsilanes: aryl to alkyl 1,5 ‐ palladium migration / coupling sequencesjie ‐ Lian Han, Ying Qin, Cheng ‐ Wei Ju, Dongbing zhaoangle. Chem. Int. ed., 2020, DOI: 10.1002/anie.201914740
Tutor introduction
Zhao Dongbing https://www.x-mol.com/university/factory/50103
The copyright of this article belongs to x-mol (x-mol.com), which is not allowed to be reproduced without permission! Welcome readers and friends to share to friends circle or Weibo!
Long press the following figure to identify the QR code in the figure, and pay attention to us easily!
Click "read the original" to view all journals in the field of chemistry? Materials