Ionic Iron(Ⅲ) Complexes Containing An Imidazolinium Cation:Synthesis,Characterization And Application Inthe Esterification Of Sp~3 C-H Bonds

In this paper,15 ionic iron（Ⅲ）complexes containing an imidazolinium cation have been synthesized in high yields.The catalytic behaviors of these ionic iron（Ⅲ）complexes have been investigated in the oxidative esterification of the sp³ C-H bonds with carboxylic acids.1.Nine imidazolinium salts 1a-1i were synthesized according to the methods reported previously.Among them,1i is a novel imidazolinium salt.New methods for the synthesis of 1b,1e and 1h have been afforded.2.Reactions of imidazolinium salts with anhydrous FeCl₃ or FeBr₃ afforded the corresponding ionic iron（Ⅲ）complexes 2a-2g and 3a-3h in high yields.All of these ionic iron（Ⅲ）complexes have been characterized by elemental analysis,Raman spectroscopy and electrospray ionization mass spectrometry.Complex 2a,2b,3a,3b,3c and 3h have also been characterized by X-ray crystallography.3.The catalytic activity of 2a-2g and 3a-3h in the oxidative esterification of the benzylic sp³ C-H bonds with carboxylic acids has been studied.It was found that the modification of the N-substituent on the imidazoline ring as well as the halide ion of these complexes has a significent effect on their catalytic activity,wherein a sterically bulky N-substituent together with a bromide ion are of benefit to a higher catalytic activity.As a result,complex 3a showed high catalytic activities and a broad substrate scope for the oxidative esterification of the benzylic sp³ C-H bonds with carboxylic acids.This is the first example of iron-catalyzed oxidative esterification of the benzylic sp³ C-H bonds with simple carboxylic acids.Comparing with the previous report,complex 3a might be among the most efficient catalyst for the oxidative esterification of benzylic sp³ C–H bonds to benzyl esters,providing almost quantitative yields of the desired product with a 5 mol%loading.In general,10-20 mol%or higher loadings of iron-based catalysts were usually required to achieve satisfactory yields for various direct functionalizations of benzylic sp³C-H bonds reported in literatures.Moreover complex 3a could catalyze the esterification of the primary benzylic sp³ C-H bonds and the study involved the functionalization of primary benzylic C-H bonds is relatively rare.4.It was found complex 3a showed high catalytic activity and broad substrate scope for the oxidative esterification of the allylic sp³ C-H bonds with carboxylic acids.As far as we know,iron-catalyzed allylic sp³ C–H bonds functionalization is still highly desired but challenging.This is the first general and efficient iron-catalyzed esterification of allylic sp³C–H bonds with carboxylic acids.A variety of allylic esters were synthesized in good to excellent yields using 3a as a catalyst.5.The first efficient iron-catalyzed esterification of sp³ C–H bonds adjacent a nitrile group with carboxylic acids using 3a as a catalyst was achieved.A series of cyanomethyl carboxylic esters were synthesized in good yields.6.It was found complex 2b showed high catalytic activity and broad substrate scope for the oxidative esterification of the sp³ C-H bonds in aryl methyl ethers with carboxylic acids,which provides a useful alternative to other methodologies for the synthesis of hemiacetal esters.7.The preliminary study of reaction mechanism has been carried out.Based on our experimental results and studies reported previously,two plausible mechanistic pathways were proposed.