Studies On The Selective Transformation Of Phenols To Oxygen-containing Heterocyclics

Phenols are abundant and easily available raw chemical materials that are widely applied to industry and human daily life.Studies on transformations from simply phenols to bio-active components,drugs as well as organic materials have attracted much attention to researchers owning to the higher attached-value.Because of the low oxidation potential,ring opening side products or C-C coupling as well as C-O coupling products often generated during the oxidative activation of phenols,which restricted the high selective transformation of phenols to target molecules.By controlling different oxidative conditions and reaction pathways,this paper focuse on exploring low-cost metal or metal-free procedures for the high selectivity transformation of phenols with simply coupling/cyclization substrates to prepare variety of oxygen-containing heterocyclics.The research contents come as follows:1.Preparation of benzoxazoles via aerobic oxidation of phenols with primary amines promoted by cooper.Using Cu powder as accelerator and NH4PF6 as additive,annulation reaction occures from simple phenols and primary amines under O2 and generate the benzoxazoles directely.This transformation is suitable for electron-rich phenols with 2,4-disubstitution and primary aminest.Control experiments show that benzoxazoles could be selectively transformed from the orth-oxidation products of phenols(biphenyldiol and o-quinone).GC-MS trapping experiment confirms that the possible intermediate generate from the reaction of biphenyldiol and amine.Further experiments illustrate that the stoichiometric addition of Cu is benefit for the transformation from biphenyldiol to benzoxazoles.2.Preparation of(Z)-1-(ary limino)naphtho[2,1-6]furan-2(1/H)-ones via Cu-catalyzed aerobic oxidation of naphthols with N-arylglycinates.Using Cu(OAc)2 as catalyst and HOAc as additive,annulation reaction occures from naphthols with N-arylglycinates and generat(Z)-1-(arylimino)naphtho[2,1-b]furan-2(1H)-ones under O2.This reaction has good functional group compatibility,which enable Me,OMe,SMe,NMe2 as well as halogen tolerant,whereas the stronge electron-withdrawing groups are unsuitable.The addition of acid additive is the key factor of this selective transformation,they can not only inhibit the over oxidation of phenols and N-arylglycinates,but accelerate the ester exchange procesure.X-ray diffraction confirms the Z-stereoselectivity of products,and the affection of ester exchange to the reaction is also been demonstrated.The essential of free N-H in N-arylglycinates is illustrated and a possible mechanism pathway is proposed.3.Preparation of(Z)-1-(arylimino)naphtho[2,1-b]furan-2(1H)-ones via naphthols participant three-component tandem cyclization.During this reaction,transition metal and catalyst are excluded,and the desired product could be obtained efficiently from the reaction of aromatic amines,naphthols and ethyl glyoxalate by using pivalic acid as additive and 5 A molecular sieves as moisture scavenger under O2.This transformation has an excellent functional-group tolerance,electron-donating groups such as Me,OMe,SMe and electron-withdraw groups such as halogen,CF3,NO2 are compatible in this catalytic system.Moreover,heterocyclic group such as pyridyl amine could also transform to the corresponding product.Ultraviolet absorption test of this type of products show that maximum absorption wavelength of partial products are in the visible area and the level of molar absorption coefficient reach up to 104.Based on previous experimentation,a plausible mechanism is proposed.4.Preparation of naphtho[2,1-b]furans via metal-free cyclization of naphthols with terminal alkynes.Using BF3 as Lewis acid catalyst and DDQ as oxidant,naphtho[2,1-b]furans can be efficiently prepared from easily available 2-naphthols and terminal alkynes.This reaction could enlarge to gram scale.Further functionalization halogen substituted product result in a more complicated molecule.A direct cross-coupling reaction pathway is exluded by the performance of deuterium labeling experiment,and a radical pathway is confirmed.O-radical generate from the oxidation of 2-naphthols is stabilized by the oxyphilic Lewis acid BF3 and then tautomerism to C-radical.By this means,the over oxidation of 2-naphthols is avoided,and the selective transformation of phenols is realized.A plausible mechanism is proposed based on these experimental results.5.Preparation of naphtho[2,1-b]furans via metal-free cyclization of naphthols with terminal alkenes.Using BF3 as Lewis acid catalyst and DDQ as oxidant,efficient synthesis of naphtho[2,1-b]furans can be realized from easily available 2-naphthols and terminal alkenes.Competiton experiments show that terminal alkenes have better reactivity in this type of reaction than terminal alkynes.Mechanism study shows that this reaction proceeds via tandem radical cyclization,dehydrogenative aromatization.