Air-Stable, Water-tolerant Organometallic Lewis Acids:Design,Synthesis, And Catalytic Application

Organometallic Lewis acid plays an important role in modern organic synthesis. During the past two decades the uninterrupted expansion of this field has continued. New Lewis acid research is targeting more versatile, more selective, more reactive, more stable, cheaper, and greener catalysts. Up to now, the development of organometallic Lewis acid has suffered from several shortcomings:(1) Partial organometallic Lewis acids are air-or moisture-sensitive, lower Lewis acidic, poor catalytic active and selective catalyst;(2) It is hard to achieve both properties of strong Lewis acidity and high stability in a same Lewis catalytic system;(3) The Lewis acid catalyst is high toxic or catalytic system is not environmental-friendly;(4) The metal adopted in some of organometallic complexes are too expansive to use in wide application;(5) most of organometallic Lewis acids are not versatile for many kinds of reactions. With the emphasized on the "green" and "sustainable" chemistry, how to design and synthesis of high catalytic efficient, cheaper, stable and versatile green organometallic Lewis acid is the key issue in modern organometallic Lewis acid catalysis chemistry.Based on the shortcomings of the organometallic Lewis acid chemistry and metallocence and organobismuth and organoantimony special features, by construction of stable C-M bond and incorporation of larger electronic-withdrawing group, a serial of air-stable, water-tolerant organometallic Lewis acid catalysts were synthesized and characterized by NMR, X-ray single crystal diffraction and TG-DSC technologies, etc to demonstrate their composition, structure and physical or chemical properties. Moreover, their applications in construction of carbon-heteroatom and carbon-carbon bond formation were also investigated. Some innovative results and conclusions were obtained as follows:1) By incorporation of larger electronic-withdrawing long chain perfluoroalkyl group ion to place the chloride in Cp2MCl2(M=Zr (1a)、Hf (1b)、Ti (1c)), air-stable and water-tolerant metallocene complex Lewis acid catalysts ([(CpZr(OH2)3)2(μ2-OH)2][C6F5SO3]4·6H2O (2a·6H2O),[(CpHf(OH2)3)2(μ2-OH)2][OSO2C8F17]4·4H2O·2THF (2b·4H2O·2THF)},Cp2M(OSO2C8F17)2·nH2O·THF (M=Zr (3a·3H2O·THF), Ti3b·(2H2O·THF)) and Cp2M(OSO2C4F9)2·nH2O (M=Zr (3c·2H2O), Ti (3d·2H2O))) were synthesized. These complexes can be stable in the open air for a month or a year and remain as dry crystal or white powders. The anions are packed around the complex cation in such a way that their oxygen atoms point towards the H2O ligands. The perfluoroalkyl group sides of the anion, on the other hand, are clustered together to form the hydrophobic domains. These complexes showed heat resistant and high solubility in polar organic solvent. Furthermore, they also showed high Lewis acidity in the same level as that of Sc(OTf)3, which can be used to promote carbon-carbon bond formation. Based on these complexes, we developed a versatile acylation method for alcohol, phenol, thiol and amine, which can be applied in biodiesel synthesis. Cp2Zr(OSO2C8F17)2can be also used in glycosylation to prepare bioactive molecules. Based on the Cp2Ti(OSO2CgF17)2/Zn dust catalytic system, we developed a simple, efficient and green synthetic method to prepare thio(seleno)ester, a-thio(seleno)carbonyl compounds and thio(seleno)ether. These metallocene Lewis acid catalysts also showed high catalytic efficient in allylation, Mukaiyama-aldol reaction, Friedel-Crafts acylation, Mannich reaction, and cyclotrimerization of ketones. In a word, the metallocene complexes can be used as versatile organometallic Lewis acid catalyst in various carbon-carbon bond and carbon-heteroatom formation reactions. On account of its stability, storability, and versatile ability, these complexes should find broad catalytic applications in organic synthesis.2) By design and synthesis of nitrogen-bridged ligands (4a-4c), the organobismuth chlorides RN(C6H4CH2)2BiCl (R=tBu (5a), c-C6H11(5b), Ph (5c)) with stable C-Bi bonds were synthesized. By incorporation of larger electronic-withdrawing counter anion to replace the chloride, air-stable and water-tolerant organobismuth complex Lewis acids c-C6H11N(C6H4CH2)2BiX (X=BF4(6a), OSO2C8F17(6b)) were synthesized. They showed high air-stability and Lewis acidity as well as heat resistant and can be applied as catalyst in Mannich reaction and allylation reaction. While the binuclear organobismuth complexes bridged by sulfur or oxygen atoms ([RN(C6H4CH2)2Bi]2X (X=O, R=tBu (8a), c-C6H11(8b), Ph(8c); X=S, R=tBu (9a), c-C6H11(9b), Ph (9c))) showed bimetallic cooperative catalytic activity in cyclic carbonate synthesis from epoxide and CO2compared with tBuN(C6H4CH2)2BiOMe (7a) and5a-5c.3) By adopting sulfur bridged pincer ligand (4d) with suitable Lewis basic site, an organobismuth chloride (5d) bearing stable C-Bi bonds was synthesized. By incorporation of larger electronic-withdrawing counter anion to place the chloride, air-stable and water-tolerant sulfur bridged organobismuth complex Lewis acids S(C6H4CH2)2Bi(OH2)X (X=ClO4(10a), BF4(10b), OSO2C4F9(10c), OSO2C8F17(10d)) were synthesized. With the exposed bismuth center acting as Lewis acid site and the uncoordinated lone pair electrons of sulfur as Lewis base site, the cationic organobismuth complexes work as bifunctional Lewis acid/base catalysts. These complexes showed high catalytic efficient and stereoselectivity in direct Mannich reaction and green synthesis of (E)-a, β-unsaturated ketones. Based on our green catalytic system, we developed a reaction-induced self-separation catalytic system, which showed facile separation of the catalyst and product. After that, the relationship between structure and catalytic activity as well as diastereoselectivity was uncovered. In summary, by adopting the sulfur atom as linking atom, the catalytic efficient and stereoselective of the organobismuth complex was significantly improved and Lewis acidic/basic bifunctional catalysts were obtained.4) By adopting nitrogen-bridged ligands (4a-4c) and changing the bismuth metal to antimony, air-stable5,6,7,12-tetrahydrodibenz[c,f)[1,5]azaantimocine framework were constructed, a serial of organoantimony chlorides (RN(C6H4CH2)2SbCl (R=tBu (11a), c-CeH11(11b), Ph (11e))) were successfully synthesized. By modification of counter anions, strong Lewis acidic organoantimony Lewis acid (RN(C6H4CH2)2BiX, R=c-CeH11, X=OSO2CF3(12a); R=Ph, BF4(12b), F (12c), Br (12d), I (12e)) were obtained. In one-pot three component Mannich reaction and ring-opening reaction of epoxide with amine, these complexes showed high catalytic activity and stereoselectivity, illustrating that by weakening of N-Sb atom bond strength, the long pair electrons of nitrogen atom can be also acted as Lewis basic site, implies that these organoantimony complexes may be acted as the self unquenched Lewis pairs. While the binuclear organoantimony complexes bridged by sulfur or oxygen atoms ([RN(C6H4CH2)2Sb]2O (R=tBu (14a), c-C6H11(14b), Ph (14c))) showed bimetallic cooperative catalytic activity in cyclic carbonate synthesis from expoxide and CO2compared with c-C6H11N(C6H4CH2)SbOMe (13a) and11a-11c.