Chiral Salen/Salan Metal Complexes:Synthesis,Characterization And Their Catalytic Behavior

In the field of chiral synthesis and asymmetric catalysis,chiral salen ligands and related catalysts occupy a pivotal position,as well as the salan analogues with similar structure were also received more and more attentions.The design and development of highly efficient and selective ligands play a pivotal role in metal-catalyzed asymmetric reactions.Bulky substituents on the salen ligands and salan analogues significantly impact the enantioselectivity of the corresponding catalysts.Taking into consideration of the great significance of the carbon dioxide utilization and the carbon dioxide copolymers as well as the importance of the?-hydroxy nitro compounds,we have synthesized some series of new chiral salen metal and salan metal complexes with bulky substituents and investigated them as catalysts in stereoselective copolymerization of epoxides with CO₂ as well as asymmetric Henry reaction based on previous work experience.The details are described as below:A series of new chiral salen Co X complexes with bulky substituents(2,3-dimethylbut-2-yl,cumyl and 1,1-diphenylethyl)at the 3,3?-positions of the salicylidene moieties 1a-1i were synthesized and investigated as catalysts in copolymerization of CO₂/propylene oxide(PO)as well as sequential copolymerization of succinic anhydride/PO and CO₂/PO.The effect of reaction conditions were examined by using complex 1b as a representative catalyst for the copolymerization of CO₂/PO.It was found that catalysts with different ligands and axial coordinating groups show remarkable difference in catalytic properties.The catalysts with relatively small R groups show relatively high catalytic activity and produce copolymers with relatively high molecular weight.High molecular weight PPCs(M_w>100 kg/mol)were prepared with 1d/bis(triphenylphosphoranylidene)ammonium chloride(PPNCl)and1d/PPNDNP(DNP=2,4-dinitrophenoxide anion)catalyst systems.While the catalysts with relatively bulky R groups show relatively high stereo-selectivity.With the catalyst system of 1f/4-(dimethylamino)pyridine(DMAP),a high kinetic resolution coefficient(K_rel)of 10.6 for the insertion of(S)-PO over(R)-PO was obtained at 25°C.Catalysts containing an axial coordinating group of DNP show relatively high stereo-selectivity.And catalyst systems empolying DMAP as cocatlyst show relatively low catalytic activity but high stereo-selectivity in comparison to the ones observed in the systems with PPNCl or PPNDNP as cocatlyst.Stereoregular block copolymers poly(propylene succinate-block-carbonate)(polyester-block-polycarbonate)were obtained from the sequential copolymerization via catalytic kinetic resolution by using 1f/PPNDNP catalyst system.A series of new chiral salen Cr X complexes with bulky substituents(cumyl,1,1-diphenylethyl and trityl)at the 3,3?-positions of the salicylidene moieties 2a-2f were synthesized and investigated as catalysts in asymmetric copolymerization of cyclohexene oxide(CHO)with CO₂.With PPNCl or PPNDNP,these salen Cr X complexes show moderate catalytic activity toward the copolymerization and yield alternating poly(cyclohexene carbonate)s(PCHCs)with number-average molecular weights of 6.9-10.8 kg/mol.Comparing with the axial coordinating group of the catalyst and the anion of the cocatalyst,the different ligands of the catalysts show remarkable effect on catalytic performance.The salen Cr X complexe with 1,1-diphenylethyl groups2b/PPNCl was found to show moderate stereo-selectivity toward the copolymerization producing optically active PCHC at 19%ee.Using salen Cr X catalyst with the bulkiest trityl groups as catalyst,great decreases on both activity and stereo-selectivity were observed.Several new chiral salan ligands L^fH2-L^hH2 as well as new salan Ti Cl₂ and salen Ti Cl₂complexes 3a-3f with bulky substituents(cumyl,1,1-diphenylethyl and trityl)at the ortho-position of the aryloxide moieties were synthesized and evaluated as catalysts in the cyclohexene oxide(CHO)/carbon dioxide cyclization reaction,CHO/phthalic anhydride(PA)and CHO/succinic anhydride(SA)ring-opening copolymerization reactions.In combination with PPNCl,these salan Ti Cl₂ and salen Ti Cl₂complexes 3a-3f show moderate catalytic activity toward these reactions at high temperature and yield cis-cyclic cyclohexene carbonate(CHC),poly(cyclohexene phthalate)(PCHP)and poly(cyclohexene succinate)(PCHS),respectively.For the coupling reaction of CHO/carbon dioxide,salan Ti Cl₂ show better performance than salen Ti Cl₂ and complexes with bulkier substituents show higher catalytic activity.Salen Ti Cl₂ catalyst with bulkier substituents show lower catalytic activity toward the copolymerization while salan Ti Cl₂ catalyst with bulkier substituents show higher catalytic activity.The in-situ generated Cu(II)complexes from the chiral salan ligands with bulky substituents(cumyl,1,1-diphenylethyl and trityl)L^fH2-L^hH2 and Cu(OAc)₂·H₂O were evaluated as catalysts in asymmetric Henry reaction.Substituents on the aryloxide moieties of these ligands were found to show remarkable effect on the enantioselectivity.Cu(II)complex generated from the ligand with 1,1-diphenylethyl groups L^gH2 and Cu(OAc)₂·H₂O was found to show good catalytic performance in the presence of TEA in THF at-20°C.The catalyst systems were examined with 16different aldehydes and the corresponding products were obtained in good yields(up to94%)with high ee values(up to 95%).The absolute configuration of the products was S as determined by comparison the retention times with literature data.Nitrobenzaldehydes undergo reaction in absence of the base resulted in higher ee value of the products.In addation,the reactions ran with bulkier ligand L^hH2 lead to products with a higher ee value.Diastereoselective reactions of 6 examples using nitroethane as the nucleophile afford syn-?-nitroalcohols in good yields with good dr(syn/anti up to11.5:1)and high ee values(92%-96%).The absolute configuration of the products was1S,2S as determined by comparison with literature data.