Synthesis Of 3,1-benzoxazines Via Dirhodium/Xantphos Catalysis

Transition-metal catalysis has become a powerful tool to construct various chemical bonds.As research continues,the chemical reactions building one chemical bond at a time can no longer meet the current needs,thus chemists further focus on a convenient,fast,environmentally friendly,and economical method to construct multiple chemical bonds at once.Since the discovery in the 1990s,a highly active ylide,derived from nucleophiles(X-H,X=O,N,S)and metal carbenes which generated from the transition-metal-mediated decomposition of diazo compound,can be efficiently trapped by electrophilic reagent.This method based on electrophilic trapping of active ylides to construct multiple chemical bonds has become one of the hot topics in chemistry field.Rhodium complexes are excellent transition metal catalysts.They have high activity in hydroformylation,carbon-carbon bond activation,carbon-hydrogen bond activation,allyl oxidation,cyclopropanation and other reactions.In the traditional understanding,dirhodium(Ⅱ)complexes are bimetallic compounds containing one metal-metal bond,four bridge ligands,and two axial empty coordination orbitals.Due to the metal-metal bond,electronic alteration in one metal atom would affect the electrophilic properties of the other Rh atom.Usually,chemists adjust the electrophilicity of the rhodium atom via altering the bridging ligand,thus changing the catalytic properties.Some recent studies showed that the simple axial coordination of the binuclear rhodium(Ⅱ)dimer can result in completely different activity and selectivity from the original catalyst.This discovery provides us a new strategy to tune the structure and catalytic properties of rhodium(Ⅱ)dimers without changing the bridge ligands.Heterocyclic compounds containing a benzo[3,1]-xazines core are important structural motifs for numerous pharmacological compounds and biologically active molecules including antihypertensive,anti-diabetic,anti-inflammatory,antimicrobial and anticonvulsants drug,antitumor central nervous system(CNS)activity of inhibitors and diuretics.However,the synthesis of heterocyclic compounds containing functionalized benzo[3,1]-xazines remains a challenge and few synthesis methods has been reported.Therefore,it is still highly significant and desirable to find an efficient synthetic method with a good tolerance for various functional groups under mild reaction conditions.In this paper,the axial ligand modification of the dirhodium(Ⅱ)dimers has been studied and its activity and selectivity was changed in diazo reaction,providing a series of functionalized benzo[3,1]-xazines.With Rh2(OAc)4(5 mol%)and Xantphos ligands(6 mol%)as catalyst,Cs2CO3(1.5 eq.)as the base then stirring in DCM under 50℃for 12 h,the reaction gave moderate to excellent yield(48-83%)with good functional group compatibility.