In 1957, Canadian chemist A. G. Brook first reported the synthesis of silyl compounds. The representative reaction characteristic of silyl compounds is that they are prone to 1,2-Brook rearrangement to form carbon carbene, so they are also an important carbene precursor. Ge/Te acyl compounds are prone to homolysis of Ge/Te-C bonds to form acyl radicals. Boronyl compounds were once considered elusive intermediates in certain transformation reactions. In 2007, researchers first reported the synthesis of tricoordinate boronyl compounds using boron anions. Subsequent research groups have reported four-coordinate boranoyl compounds with different configurations, which are used as alternative raw materials for amide synthesis, and have shown excellent characteristics of rapidity, good selectivity and even compatibility with aqueous systems in the formation reaction of amide bonds. However, the aluminum acyl compounds of the same family usually appear in some literatures as a supporting role, and there is no systematic research report on their synthesis and reactivity so far.
The biggest difference between the aluminum anion and the traditional trivalent aluminum is that the aluminum anion has nucleophilicity. A research team envisions using the aluminum anion to react with acyl chloride to construct aluminum acyl compounds, hoping to provide a general synthetic method for the synthesis of aluminum acyl compounds.
In the reaction between aluminum anions and p-toluyl chloride, the color change of the reaction solution can be observed very quickly, and the detection can confirm the depletion of raw materials and the generation of new compounds. Product 2 was obtained by crystallization, and single bond structure analysis confirmed the formation of aluminoyl compounds. Compound 2 reacted further with LiOTf to obtain non-chain structure 3, and Cl ions were not replaced by OTf ions. Compound 2 can form compound 4 and by-product 5 under heating conditions, while the transformation at room temperature is relatively slow, which is also conducive to further exploring its reactivity. Compound 3 is less thermally stable than compound 2 and has a similar transition, but by-product 5 cannot be formed.
Theoretical calculations on the structures of compounds 3 and 4 show the HOMO orbitals mainly come from the nitrogen atoms of the ligands. In compound 3, the Al-C bond has the main contribution to HOMO-1/2; in compound 4, the Al-C bond has the main contribution to the HOMO-2/3 orbital. In compound 3, the LUMO orbital mainly comes from the π* orbital contribution of C=O and the benzene ring contribution; in compound 4, this part of the contribution corresponds to LUMO and LUMO+1. Further AIM-calculated structures indicated that the Al-C bonds in compounds 3 and 4 had weaker polarity.
Experiments were carried out on Lewis acid (TMSOTf), Lewis base (DMAP), ketone and imine compounds. TMSOTf can induce the ring expansion reaction of compounds 2 and 3 to generate compound 6; the reaction of DMAP with compounds 2, 3 and 4 can all give DMAP nitrogen ortho-functionalized products. The reactivity of benzophenone, cyclohexanone and imine compounds is mainly manifested by the nucleophilic behavior of unsaturated bonds. The researchers further experimented with the ability of aluminum acyl compounds to construct amides. Compound 10a is a substrate commonly used by boronyl compounds to construct amides, and 10a and compound 2 can quickly generate amides; substrate 10b and compound 2, which perform poorly in the reaction with boronyl compounds, can still generate the corresponding amides in higher yields. The researcher believes that the reaction mechanism of the formation of amides of aluminum acyl compounds is consistent with that of boronyl compounds, reflecting the electrophilicity of the carbonyl group in aluminum acyl compounds. Therefore, aluminum acyl compounds are nucleophilic and electrophilic amphoteric.
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Reference
- Alkali Chloride Adducts of Acyclic Acylaluminum: Synthesis, Structure, and Reactivity Studies.
Angew. Chem. Int. Ed., 2023
