Makoto Yamashita's team first discovered a super alkaline alkyl aluminum carbene analogue

Previously, we introduced a dimethoxyanthracene stable potassium aluminate dimer [K {Al (non)}] 2 reported by Professor Simon Aldridge of Oxford University and Professor Jose M goicoechea on J am Chem SOC Generally, the oxidation state of Al in the compound is + 3, but the valence of Al in the dimer compound is + 1 Two amino groups in the anion part of 4,5-bis (2,6-diisopropylphenylamino) - 2,7-di-tert-butyl-9,9-dimethoxyanthracene are bonded to the Al (I) center At the same time, the oxygen in the pyran ring also has a coordination and stabilization effect on Al (I) The formed Al (I) anion has strong basicity and nucleophilic property It is easy to react with Br? Nsted Acid and electrophilic reagents of different carbon centers (such as meotf and Mei), activate H2 at room temperature, and even depolymerize into monomer after adding K + chelating agent ([2.2.2] cryptand) Benzene ring opening can be realized at room temperature ▲ [K {Al (non)}] 2 participates in many different reactions as a nucleophile (picture source: reference [1]) ▲ K {Al (non)} monomer realizes benzene ring opening (picture source: reference [3]) Recently, a team of Professor Makoto Yamashita from Nagoya University in Japan has successfully prepared a new potassium aluminate compound In this work, the synthesis, separation and characterization of alkyl Al (I) carbene analogues have been realized for the first time, and it has been found that compared with the previously designed Al (I) anion species, it is more alkaline and has excellent nucleophilic activity Relevant research results were published in nature chemistry, a subsidiary of nature ▲ picture source: reference [4] Carbene is a highly active species whose central carbon atom contains only six valence electrons The central carbon atom is covalently linked to two other substituent groups, and the other two electrons are not bonded If we use other main group elements to replace the central carbon atom of carbene, we can get a series of carbene analogues, such as nitroene and silylene shown in the figure below The central atoms of these active intermediates contain six valence electrons, which are affected by the number of electrons in the outer layer of the atoms of different main group elements For example, the carbene analogues of the central atoms of 13 groups of elements (B, Al, GA, etc.) are negatively charged, those of 14 groups of elements (C, Si, Ge, etc.) are electrically neutral, and those of 15 groups of elements (n, P, as, etc.) are positively charged, thus showing different electrical properties Reaction activity 13 group element carbene analogues (picture source: reference [4]) The stable carbene analogues of 13 groups are generally covalently linked with two nitrogen atoms by σ bond to form anion species At this time, the lone pair electrons of nitrogen atoms can also form π bond interaction with the unoccupied P orbit of the central atom, and the alkali metal anti Hengyang ions Li +, K + are introduced to stabilize them to obtain the corresponding monomer or polymer So far, four diamino substituted carbene analogues (I-IV in the figure above) have been successfully isolated In addition, a proper Lewis base can be added to coordinate with it to form octagonal stable structure The above mentioned potassium aluminate dimer compound [K {Al (non)}] 2 (XII) combines these two stabilization modes The oxygen of pyran ring in XII is Lewis basic, which plays an important role in the stabilization of Al (I) after coordination After the successful separation, its structure can also be characterized by X-ray crystal diffraction In addition to its high reactivity, XII can also be used to construct other Al Mg bonds and Al Au bond intermediates that are difficult to form in other reactions, and it will be fun to study It has also been assumed that the design center Al atom is covalently bonded to two carbon atoms as an alkyl Al (I) anion species However, α - C in this kind of species can not provide π bond stabilization to the central Al atom like N atom, so it is difficult to synthesize Previous studies have found that tetra (trimethylsilyl) butene can be used as a ligand to stabilize the six valence electron Silene (XIV) in the central Si atom and the seven valence electron phosphorus radical species (XV) in the central P atom The bis (trimethylsilyl) with large steric hindrance on both sides can avoid the dimerization of XIV and XV, and stabilize the species of Silene and phosphorus free radicals through its strong electron donating effect Therefore, the author thought of using this ligand to construct stable alkyl Al (I) carbene analogues The synthesis process of alkyl Al (I) carbene analogues is as follows: 1,1-Bis (trimethylsilyl) vinyl (1) is mixed with excessive lithium metal, Et2O is used as the solvent, and 1,4-bis-lithium-1,1,4 can be obtained, 4-tetra (trimethylsilyl) butane (2); further addition of ali3 to form the intermediate of iodoaluminate hetero five membered ring (3); 3 reduction of excess lithium to obtain the intermediate of diallyl alkane (4); further reduction of Na / K alloy at low temperature to 4, Al bond fracture to form toluene solvated potassium alkylaluminate (5) Synthesis of potassium alkylaluminate 5 (picture source: reference [4]) 5 is a red crystal, which is stable in toluene and can react with THF and benzene X-ray crystallography shows that the two α - C and K of the alkyl Al (I) species are in the same plane, in the form of plane triangle, and the Al atom is in the form of SP2 hybrid The length of the al-k bond is slightly longer than the sum of the covalent radii of Al and K, but the distance between Al and K in II and XII is much shorter than that in the previous diamino substituted Al (I) species The analysis of UV-Vis absorption spectrum shows that there is a maximum absorption peak at 309 nm and 468 nm for 5, which is similar to the case of Silene XIV, suggesting that there is indeed an unoccupied p orbital and a group of lone pair electrons in Al in 5 X-ray crystal diffraction and UV-Vis absorption spectrum analysis of ▲ 5 (picture source: reference [4]) They also combined the DFT calculation and X-ray crystal diffraction analysis data, and concluded that for the Al atom in 5, the P component of C-Al bond is higher, while the s component of al-k bond is higher The c-al-c bond angle is approximately 90 degrees, which is close to the n-al-n bond angle in V The highest occupied molecular orbital (HOMO) of 5 is an isolated pair of electrons of Al, and the lowest unoccupied molecular orbital (LUMO) is a π * orbital of toluene with K coordination The energy gap between homo and LUMO is significantly lower than that between II and XII, which means that the alkyl Al (I) anion species may have higher reactivity The theoretical analysis of NBO and aim shows that the al-k bond is highly polarized and has obvious ionic bond properties Molecular orbital analysis of ▲ 5 (picture source: reference [4]) Potassium alkylaluminate compound 5 shows strong basicity, which can deprotonate benzene by activating C-H bond at room temperature In contrast, XII needs to be able to activate benzene under heating conditions In addition, 5 is easy to be nucleophilic substituted with meotf and benzyl chloride, and can also be nucleophilic aromatic substituted with C6F6 ▲ 5 investigation of reaction activity (picture source: reference [4]) So far, the reaction activity of Al carbene analogues has set a new record and the alkalinity has risen to a new level We hope that further progress will be made in relevant fields reference material: [1] Jamie Hicks et al., (2019) Synthesis, Structure and Reaction Chemistry of a Nucleophilic Aluminyl Anion Nature, DOI: 10.1038/s41586-018-0037-y [2] Jamie Hicks et al., (2019) Reversible, Room-Temperature C-C Bond Activation of Benzene by an Isolable Metal Complex J Am Chem Soc., DOI: 10.1021/jacs.9b05925