Shoko Kikkawa

Synthesis, Structures, and Properties of Hexapole Helicenes: Assembling Six [5]Helicene Substructures into Highly Twisted Aromatic Systems

Hexapole helicenes 1, which contain six [5]helicene substructures, were synthesized by Pd-catalyzed [2+2+2]cycloadditions of aryne precursor 6. Among the possible 20 stereoisomers, which include ten pairs of enantiomers, HH-1 was obtained selectively. Density functional theory (DFT) calculations identified HH-1 as the second most stable isomer that quantitatively isomerizes under thermal conditions into the most stable isomer (HH-2). Both enantiomers of HH-2 can be separated by chiral HPLC. Single-crystal X-ray diffraction analyses revealed a saddle-like structure for (P,M,P,P,M,P) HH-1 and a propeller-like structure for (P,M,P,M,P,M) HH-2. Because of the helical assembly and the resulting steric repulsion, the structure of HH-1 is significantly distorted and exhibits the largest twisting angle reported so far (up to 35.7° per benzene unit). Electrochemical studies and DFT calculations indicated a narrow HOMO-LUMO gap on account of the extended π-system. Kinetic studies of the isomerization from HH-1 to HH-2 and the racemization of enantiomerically pure HH-2 were conducted based on 1H NMR spectroscopy, HPLC analysis, and DFT calculations.

Synthesis and Shuttling Behavior of [2]Rotaxanes with a Pyrrole Moiety.

We synthesized [2]rotaxanes with a pyrrole moiety from a [2]rotaxane with a 1,3-diynyl moiety. The conversion of the 1,3-diynyl moiety of the axle component to the pyrrole moiety was accomplished by a Cu-mediated cycloaddition of anilines. The cycloaddition reaction was accelerated when the [2]rotaxane was used as the substrate. The effect of the structure of the pyrrole moiety on the rate of the shuttling was studied.

A palladium-catalyzed dehydrative N-benzylation/C–H benzylation cascade of 2-morpholinoanilines on water

A strategy for the palladium-catalyzed dehydrative tandem benzylation of 2-morpholinoanilines with benzyl alcohols has been developed. This cascade reaction is devised as a straightforward and efficient synthetic route for N-(1,2-diphenylethyl)-2-morpholinoanilines in moderate to good yields (50–81%). The dehydrative sp3 C–H bond benzylation proceeds chemoselectively at the benzylic position of N-benzyl-2-morpholinoaniline to form a new C(sp3)–C (sp3) bond. KIE experiments show that C–H bond activation is involved in the rate-determining step (KIE = 2.7). A Hammett study of 2-morpholinoanilines gives a negative ρ value, suggesting that there is a build-up of positive charge in the transition state. A “on water” protocol, which affords the corresponding desired products with water as the sole co-product, can be achieved under mild reaction conditions without the need for bases or other additives in an atom-economical process.

A borrowing hydrogen methodology: palladium-catalyzed dehydrative N-benzylation of 2-aminopyridines in water

We demonstrate a greener borrowing hydrogen methodology using the π-benzylpalladium system, which offers an efficient and environmentally friendly dehydrative N-monobenzylation of 2-aminopyridines with benzylic alcohols in the absence of base. The crossover experiment using benzyl-α,α-d2 alcohol and 3-methylbenzyl alcohol afforded H/D scrambled products, suggesting that the dehydrative N-benzylation in our catalytic system involves a borrowing hydrogen pathway. KIE experiments show that C–H bond cleavage at the benzylic position of benzyl alcohol is involved in the rate-determining step (KIE = 2.9). This simple base-free protocol can be achieved under mild conditions in an atom-economic process, affording the desired products in moderate to excellent yields.

Triple Helicene Cage: Three-Dimensional π-Conjugated Chiral Cage with Six [5]Helicene Units

Abstract A three‐dimensional π‐conjugated chiral cage with six [5]helicene units (a triple helicene cage) was synthesized for the first time. Taking advantage of the Yamamoto coupling reaction, the triflate‐substituted triple [5]helicene, a strained and preorganized precursor, was dimerized to afford the target compound. Single‐crystal X‐ray diffraction analysis revealed the unique structural features of the triple helicene cage: a cage‐shaped rigid structure with outer helical grooves and an inner chiral cavity. All‐P and all‐M enantiomers were separated successfully by HPLC over a chiral column and their chiroptical properties were characterized by circular dichroism spectra.

Synthesis and solvent vapor-induced transformations of crystals of 1D coordination polymers assembled with continuous void space

9,9′-Biacridine (L1) has a three-dimensional structure of perpendicularly connected acridine moieties with two coordination sites at either end of the molecules. Crystallization of L1 with silver(I) salts gives various 1D coordination polymers with structures that depend on the crystallization solvents and counter anions used. We synthesized six complexes: [Ag(L1)]n(BF4)n·2nTHF (1a·THF, THF = tetrahydrofuran), [Ag(L1)]n(ClO4)n·2nTHF (1b·THF), [Ag(L1)(CH3OH)]n(BF4)n·1.5n(furan) (1a·Furan), [Ag(L1)]n(BF4)n·nDCM (1a·DCM, DCM = dichloromethane), [Ag(L1)]n(ClO4)n·nDCM (1b·DCM), and [Ag2(L1)(CF3COO)2]n (1c). We also synthesized crystals of phenazine (L2) complexed with silver(I) cations which featured layered structures with no void space. In the crystals of 1a·THF and 1b·THF, a linear channel structure formed, which was filled with THF molecules; however, 1a·DCM and 1b·DCM featured no continuous channel structure and their void space was filled with DCM molecules. 1a·Furan had a zigzag-shaped channel filled with furan molecules. The crystal structures of the complexes were similar regardless of whether tetrafluoroborate or perchlorate anions were included in the structure. The crystal structure of 1c showed a ladder structure with chains of silver(I) cations and trifluoroacetate anions. A reversible crystal-to-crystal structural transformation was observed between the complexes of 1a·THF and 1a·DCM, which was investigated by single crystal and powder X-ray diffraction measurements. Our measurements suggested that solvent molecules in the crystal structure were replaced together with concerted rotation of the 1D chains of the coordination polymers, occurring when the crystals were exposed to the vapor of a different solvent.