Tiaraed Pillar[5/6] Arenes:Design,Syntheses,and X-ray Single Crystal Structural Analyses

Since the emergence of pillar[5]arenes（P[5]s）on the stage of macrocyclic chemistry,they have attracted extensive research interests owing to their facile synthesis and high symmetry.In the past decades,a variety of functionalization patterns on P[5]s have been successfully demonstrated.Among all substitution patterns,the C₅-symmetric rim-differentiated P[5]s,whose upper and lower rim are decorated with two different substituents,used to be synthesized by the cyclization of paraformaldehyde and asymmetric-substituted 1,4-dialkoxybenzenes in the presence of Lewis acids.As a result of the statistical generation of four constitutional isomers in the cyclization process,rim-differentiated P[5]accounts only for 1/16 among all P[5]products obtained in the some pot.The bottleneck has been overcome by the“pre-oriented”protocol,which has improved the synthesis of rim-differentiated P[5]both in selectivity and isolation.However,similar rim-differentiation pattern on P[6]s has not been reported.The separation and identification of eight potential constitutional isomers of asymmetrically-substituted P[6]s remain challenging tasks.In this thesis,assorted rim-differentiated P[5]s with long alkyl phosphate chains was synthesized.Under optimized pre-oriented protocol,in addition to rim-differentiated P[5],the P[6]analogue,was also obtained with an isolation yield of 6%.Finally,a new class of macrocyclic molecules,tiara[6]arenes（T[6]s）,was be created.At first,the phosphate rim-differentiated P[5]s with long alkyl chain were successfully synthesized based on the pre-oriented protocol.Next,the pre-oriented protocol was further optimized using penta-benzyloxy P[5]as the model compound.Different temperatures,concentrations and water contents were explored during the cyclization process.The results revealed that the cyclization reactions performed in lower concentration and anhydrous conditions showed significant improvement on the yield of penta-benzyloxy P[5].At the same time,rim-differentiated P[6],hexa-benzyloxy P[6],was isolated with 6%yield and fully characterized by NMR,MS and X-ray crystallography.Interestingly,the X-ray structure revealed that the hexa-benzyloxy P[6]scaffold adopts a 1,3,5-alternate conformation to reduce steric hindrance and allow tight packing in the solid state.Instead of being C₆-symmetric,the hexa-benzyloxy P[6]molecule belongs to the S₆ point group.Subsequently,T[6]s,the new macrocyclic molecules composed of six phenols linked by methylene bridges at their 2,5 positions,which is not accessible by condensation reaction of paraformaldehyde and phenol directly,can be synthesized by taking advantage of the formation of rim-differentiated P[6]s,followed by selective dehydrogenation of one rim.Moreover,the X-ray crystallography of the intermediate hexa-hydroxy P[6]and final T[6]revealed the formation of extensive hydrogen-bonded networks between these macrocycles with THF and H₂O,respectively.