A cationic CpRu/halogen/Brønsted acid hybrid catalyst (R cat) with the axially chiral Cl-Naph-PyCOOH ligand (naph = naphthyl, py = pyridine) is highly efficient for dehydrative cyclization of pyrroles to construct 1,2-fused-2-allylated pyrroles that, so far, have not been achieved. The mechanistic study has implied that hydrogen and halogen bonds play a key role for facilitating the R,S Ru-catalyzed reaction pathway via a σ-allyl intermediate and that the diastereomeric R,R Ru cat slowly gives the minor enantiomer via a π-allyl intermediate.

[Ru(II)Cp((R)-Cl-Naph-PyCOOH)]PF6 ((R)-1) catalyzes the dehydrative cyclization of (E)-hept-2-ene-1,7-diol (2) to 2-vinyltetrahydro-2H-pyran (3) with a 97:3 S/R enantiomer ratio. Complex (R)-1 is in equilibrium between two diastereomers (R,RRu)-1 (AR) and (R,SRu)-1 (AS). A difference of turn over efficiency between the AS and AR cycles is thought to be the origin of the high enantioselectivity. The AS gives a major enantiomeric product (S)-3, according to the results of detailed mechanistic investigation via i) X-ray crystallographic analysis of related complexes, ii) NMR experiments using allylic alcohol 2, OH-lacking 2-mimic 4, d-labeled (S)-4-1d, enantiomerically enriched hept-6-ene-1,5-diol (6) as branched isomer of 2, and OH-lacking 6-mimic 5, iii) substrate structure/reactivity and selectivity relationships, iv) deuterium-labeling experiment, v) kinetics via calorimetric analysis, and vi) ligand structure/reactivity and selectivity relationships. AS captures 2 via hydrogen and halogen bonds. Oxidative addition in an H2Oin mode leads to a macrocyclic σ-allyl intermediate. Here, an efficient nC(7)OH/π*C(3)=C(2) trans-annular (TA) interaction facilitates an SN2′ nucleophilic addition of OH in an OHTA manner to furnish (S)-3. Contrary to the AS, AR cannot capture 2 using the halogen bond and slowly operates to give (R)-3. A conventional π-allyl-complex-involved mechanism is ruled out by a contradiction in the result of ii) and iii).

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The asymmetric dehydrative intramolecular allylation reactions of furan and thiophene were performed using a cationic cyclopentadienyl-ruthenium (CpRu) complex of a chiral pyridine carboxylic acid, namely Cl-Naph-PyCOOH. Both furan and thiophene tethered with an allylic alcohol gave the corresponding bicyclic compounds in high yields and enantioselectivities using 0.1–5 mol% of catalyst. The reaction was found to proceed via a similar enantioface selection method mechanism to that previously reported by our group, which involved halogen and hydrogen bond formation, in addition to the generation of an intermediate σ-allyl complex.

Die Diels‐Alder‐Reaktion von trans‐ Pentadien‐1,3 (I) mit Acrylnitril (II) liefert in Abhängigkeit von der Polarität des verwendeten Lösungsmittels die vier isomeren Cycloaddukte (III)‐(VI).