Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

Abstract

Chiral guests display kinetic stereoselective threading through chiral porphyrin cages if their chirality is located at the chain ends and not in the centers, supporting the previously reported entron effect of threading.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts

“…In a previous paper we showed that the methyl groups on the chiral carbon centers of the spacers in (R,R,R,R)-H 2 C* and (S,S,S,S)-H 2 C* are located inside the cavity. [18] As can be seen in the crystal structure of (R,R,R,R)-MnC* (Figure 2b) these methyl groups are now pushed out of the cavity, because the oxygen atom of DMF coordinates to the manganese center. Furthermore, the chiral centers do not induce any chiral twist in the relative orientation of the xylylene sidewalls of the glycoluril framework.…”

“…[18] They displayed similar but opposite Cotton effects in the circular dichroism (CD) spectra (Figure 2a), just like the chiral metal-free cages. [18] We tested two prochiral alkene substrates, the terminal conjugated alkene 7 and the alkyl bridged terminal alkene 17, under standard conditions, i. e. with excess tBuPy present. After reaction, the products were analyzed by chiral HPLC, which revealed that only racemic mixtures of products had been formed in isolated yields of 51 and 50 %, respectively.…”

“…Interestingly, the crystal structure contained a dimethylformamide (DMF) solvent molecule, which acted as a guest molecule in the cavity of ( R , R , R , R)‐ MnC *. In a previous paper we showed that the methyl groups on the chiral carbon centers of the spacers in ( R , R,R , R)‐ H 2 C* and ( S , S , S , S)‐ H 2 C* are located inside the cavity [18] . As can be seen in the crystal structure of ( R , R , R , R)‐ MnC* (Figure 2b) these methyl groups are now pushed out of the cavity, because the oxygen atom of DMF coordinates to the manganese center.…”

“…In this paper we report a mechanistic study, particularly using mass spectrometry, of alkene epoxidation by MnC and a new (chiral) manganese porphyrin cage MnC* , which has sterically encumbered linkers connecting the porphyrin to the glycoluril framework (Figure 1). [18] For our studies, we chose iodosylbenzene (PhIO) as the oxidant, as it is the most frequently used oxygen donor for the epoxidation of alkenes by manganese porphyrin catalysts [19] . In order to give the mechanistic studies a firm basis we studied in detail the epoxidation activities of MnC and MnC* and compared these with the activities of the model compounds manganese meso ‐tetraphenylporphyrin ( MnTPP) and manganese meso ‐(tetrakis‐2‐methoxyphenyl) porphyrin ( MnTMPP) , see Figure 1.…”

Recognition and Sensing of Chiral Organic Molecules by Chiral Porphyrinoids: A Review

“…Most recently, Elemans, Nolte and coworkers reported an unusual chiral macrocyclic porphyrin (Figure 19a, XXII) with a C2 symmetry for the kinetic enantiorecognition of chiral viologen molecules, illustrated in Figure 19b [82]. The CD spectra of the enantiomer pairs (R, R, R, R)-and (S, S, S, S)-XXII displayed mirror-image signals with bands centered at~415 nm (Figure 19c).…”