Chinese scientists realized the ring shrinkage reaction through the aromatic relay drive strategy

On December 13, 2022, Beijing time, the team of Professor Xia Haiping of Southern University of Science and Technology/Xiamen University and Associate Professor Lin Yumei of Xiamen University published an article entitled “Ring Contraction of Metallacyclobutadiene to Metallacyclopropene Driven by π- and σ-Aromaticity Relay” in the journal Nature Synthesis Research results.

This achievement proposes a relay drive strategy based on π- and σ-aromatics, realizes the first case of metal heterocyclic butadiene to metal heterocyclic propylene ring shrinkage reaction, reveals the reaction mechanism and driving force by capturing reaction intermediates and DFT theoretical calculations, provides new ideas for the application of aromatity in synthetic chemistry, and also inspires chemists to explore new modes of application of aromatic drive in organic chemical synthesis.

The corresponding authors of the paper are Xia Haiping and Lin Yumei; The first authors are Zhuo Kaiyue and Liu Yanan.

Aromatity is the cornerstone of aromatic chemistry. Traditional π aromatization is characterized by a high degree of delocalization of π electrons in a circular closed conjugate system. σ-aromatization is characterized by the delocalization of σ-electrons, a theory pioneered by Dewar in 1984 to explain the abnormal magnetic behavior of cyclopropane. π-aromativity is an important driver for the synthesis of aromatic compounds, and reactions driven by σ-aromatity are rare. At the same time, the formation and conversion of metal heterocycloids is a key step in alkyne metathesis, polymerization reaction, and C-H bond activation, which has important value in organic synthesis and catalytic applications. Given the high ring tension, metal heterogeneous rings tend to undergo ring-opening or ring-expanding reactions such as rearrangement and addition, and ring shrinkage reactions are less common because they usually lead to increased ring tension.

Figure 1: Ring shrinkage reactions driven by π- and σ-aromatic relays: metal heterocyclic butadiene → metal heterocyclic propylene.

In this work, the team of Professor Haiping Xia (Southern University of Science and Technology/Xiamen University)/Associate Professor Lin Yumei (Xiamen University) designed and synthesized chlorine-substituted metal heterocyclic butadiene and pentylene compounds β based on the unique physical and chemical properties of the D-orbital and the p-orbital of the main group atoms to form Dπ-Pπ conjugation due to the unique physical and chemical properties of the D orbital of the metal and the P orbital of the main group atom2a-2c, which undergoes a ring shrinkage reaction under acidic conditions to form metal heterocyclic propylene and pentyl ene compounds3a-3c(Figure 2). In previous studies, the ring shrinkage process of metal heterocyclic butadiene was only speculated as an intermediate for mechanism research.

Figure 2: Study of quaternary cyclic compounds 2a–2c retracted into ternary cyclic compounds 3a–3c.

To further understand the shrinkage mechanism, the authors performed DFT calculations. The results suggest that the reaction may undergo a step-by-step mechanism of first opening the loop and closing the loop later, i.e2aThe medium-metal heterocyclic butadiene is attacked by protons, and the four-membered ring is opened to form a vinyl carbene intermediate with 16 electrons in the center of the metal4a, and subsequently, deprotonation closes the loop to produce an energy-favorable end product3a(Figure 3).

Figure 3: Mechanistic exploration: DFT calculation.

Through the regulation of acid equivalent and species, the reaction intermediate was successfully captured under the condition of tetrafluoroborate ether4A(Figure 4). Theoretical calculations reveal that the influence of acid on the ring shrinkage reaction lies in different intermediates4Aand4aThe difference in energy, and the conversion of intermediates into the final product3aThe energy barrier of the process. That is, under the conditions of tetrafluoroborate ether, intermediates4ACompared to trifluoroacetic acid under conditions4aHas lower energy and the transition state experienced by the next closing loopTS2The energy barrier is higher, the conversion rate is slower at room temperature, and it is thermodynamically more advantageous, which is an intermediate4AThe capture of offers the possibility. Gibbs free energy curve indicates compounds2a4A3aThe process energy continues to decrease, and it is speculated that this transformation process is related to the aromatic stabilization energy.

Figure 4: Control experiment and separation of key intermediate 4A.

Subsequently, the authors used a variety of aromatic criteria for model compounds2a’4A’and3a’The aromatization is studied. The nuclear independent chemical shift (NICS) calculation, induced current density (ACID), isobond reaction and other criteria for splitting orbits are consistently shown2a’π-anti-aromatization of heterocyclic butadiene, intermediates4A’π – aromatic and3a’The σ-aromatic characteristics of medium-metal heterocyclic propylene (Figure 5) reveal the driving force of the step-by-step mechanism of “opening the loop first-then closing the ring”: the opening of the π-aromatic driving four-membered ring releases the antiaromatic; Then, driven by the generated ternary ring σ-aromatic, the ring is closed again, and metal heterocyclic propylene and pentylene with π- and σ-double aromatic stability are generated, which expands the aromatic skeleton and further decreases the system energy.

Figure 5: Evaluation of the aromativity of model compounds 2a’, 4A’ and 3a’ by theoretical calculation. Image source: Nature Synthesis

The carbon dragon complex has unique properties due to the π conjugation of the d-orbital participation of the transition metal. The team recently developed a new catalyst system based on the carbon dragon skeleton (J. Am. Chem. Soc. 2022, 144, 2301–2310); A carbon dragon complex with absorption spectra up to NIR-II was designed and synthesized (Angew. Chem. Int. Ed. 2022, 61, e202211734), which further demonstrates the potential value of the new structural primitive of the carbon dragon skeleton, shows the new application mode of aromatic driving in organic chemical synthesis, and brings new opportunities for the development of aromatic chemistry. (Source: Science Network)

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