Graftable chiral ligands for surface organometallic materials: calixarenes bearing asymmetric centers directly attached to the lower rim

Abstract

A family of chiral lower-rim substituted calixarene ligands 1-14 is synthesized and characterized as prospective ligands for synthesis of asymmetric active sites for heterogeneous catalysis and adsorption using a surface organometallic approach. The ligands possess asymmetric centers directly attached to lower-rim oxygens; the asymmetric centers are tailored with sterically bulky naphthyl and electron withdrawing CF 3 substituents. 1 H and 13 C NMR spectroscopies demonstrate that mono-and 1,3-di-alkylated calixarenes synthesized using this procedure adopt the cone-shaped conformation, which is stabilized by multiple hydrogen bonds at the lower rim. Smaller pendant groups and polar solvents increase the dialkylation/monoalkylation ratio. The reaction proceeds with full inversion of configuration as established via single crystal X-ray diffraction of an N-Cbzprotected aminocyclopentoxy-modified calixarene. The calixarenes synthesized by this approach are characterized using circular dichroism (CD) spectroscopy and ab initio modeling, which is used to identify the minimal molecular fragment responsible for the observed lowest energy Cotton effect in the CD spectrum. Factors that influence the intramolecular induction of asymmetry throughout the calixarene scaffold are investigated by systematically varying substituent steric bulk, connectivity of the asymmetric center to the calixarene core, and the extent of hydrogen bonding at the calixarene lower rim. The asymmetry of the calixarene core is quantified using 1 H NMR spectroscopic shifts of aromatic meta and methylene bridge hydrogens and supported using ab initio calculations. The results demonstrate that calixarene core asymmetry is most strongly induced for rigid calixarene cores-an attribute that is expected to be preserved upon metal complexation and anchoring of these prospective ligands.

Postsynthetic Modification of Gold Nanoparticles with Calix[4]arene Enantiomers: Origin of Chiral Surface Plasmon Resonance

“…Measured Δδ’s of diastereotopic hydrogens are listed in Table and reflect the transfer of asymmetry from the chiral center throughout the calixarene skeleton. Compared with previously reported Δδ values of 0.08−0.21 for ΔδArH unsub and 0.06 − 0.21 for ΔδArH sub for aromatic hydrogens of calixarene phenyl rings whose lower rim oxygen is directly connected to an asymmetric center, the Δδ values of 0.05−0.09 for ΔδArH unsub and 0.06−0.07 for ΔδArH sub for 1b , 1c , and 2b are lower; , however, these Δδ values nonetheless clearly demonstrate transfer of asymmetry to hydrogens of the calixarene core. This asymmetry transfer is rather insensitive to the bulkiness of substituents bonded to nitrogen (Cbz vs Boc-t vs H).…”

“…In addition, in order to eliminate effect (i) in the paragraph above as a possibility, postsynthetic modification of existing gold nanoparticles is employed. However, because of the proven transmission of asymmetry from the chiral center on the lower rim to its immediate surroundings, effect (iii) should be facilitated in 2a and 2b by the short distance between the metal surface and the chiral center, and should be disfavored when the distance between the metal surface and the chiral center is large, as in previously reported chirally modified nanoparticles where chiral signatures in the CD spectrum were not observed …”

“…The Mitsunobu reaction is a convenient method that is widely used in synthetic organic chemistry . The high stereoselectivity of the reaction has been beneficial in the functionalization of the calixarene lower rim with sugars and chiral alkylaryl groups . Using this reaction with chiral cyclic amino alcohols, an absolute configuration of the product has been determined, and the mechanism of this reaction has been investigated as well .…”

“…The high stereoselectivity of the reaction has been beneficial in the functionalization of the calixarene lower rim with sugars and chiral alkylaryl groups . Using this reaction with chiral cyclic amino alcohols, an absolute configuration of the product has been determined, and the mechanism of this reaction has been investigated as well . In this manuscript, we extend this protocol to the preparation of calixarene amine derivatives possessing two chiral centers tethered from a calixarene core, which is used for preparing chirally modified gold nanoparticles.…”

Immobilization of Cryptophane Derivatives onto SiO2/Au and Au Substrates

“…Even though a few methods for the deposition of crypto­phanes on supports have been previously proposed, , no deep study of the immobilization of a crypto­phane monolayer onto solid substrates has been reported in the literature. On the other hand, a lot of articles report the construction of self-assembled monolayers (SAMs) of receptors for molecular recognition of chemical or biological species. − Such sensor arrays have been obtained by assembling different receptors, generally calixarene or cyclodextrin derivatives, on silica or gold surfaces. Self-assembly of systems on silica surfaces was performed using organosilane chemistry, − whereas SAMs on gold were generally formed by the adsorption of thiol, thioether, or disulfide groups. − …”