Highly active and selective: A supported Ru5PtSn nanoparticle cluster (the picture shows an axial projection of a tomogram), prepared from the carbonyl cluster [PtRu5(CO)15(μ‐SnPh2)(μ6‐C)], is an excellent catalyst in the single‐step hydrogenation of dimethyl terephthalate to cyclohexanedimethanol under mild conditions (100 °C, 20 bar H2).

Helical chiral 2-aminopyridinium ions were designed as a significantly more acidic (active) dual hydrogen-bonding catalyst than commonly used (thio)urea-based systems. The helicene framework was specifically utilized to position an inherently chiral barrier on the hydrogen-bonding side of the catalyst. The catalyst reactivity and enantioselectivity were successfully demonstrated in additions of 4,7-dihydroindoles to nitroalkenes (0.5-2 mol % catalyst loadings, up to 98:2 er).

The design, synthesis, and study of new catalyst structures have had an enormous impact on chemical synthesis, and continue to be a central challenge in asymmetric catalysis.[1]We recently described that a 2-aminopyridinium ion might be a promising catalaphore [2] for the design of new asymmetric hydrogen-bond donor catalysts. [3] In that connection, we became interested in 1-aza [6]helicene [4] 1 as a chiraphore [2] because a first-order analysis of the crystal structure of an analogous 1,16-diaza[6]helicene [5] suggests that its pyridine ring is well-desymmetrized in terms of both top-from-bottom and left-from-right differentiations. To our knowledge, the application of 1 and analogous helical chiral pyridines [5][6][7] in asymmetric catalysis has not been studied, even though 1 has been known in the literature since 1975.[4d] In this context, we were prompted to develop an efficient synthesis of 1-azahelicenes, which allows systematic structural variation-important for the elucidation of the relationship between catalyst structure, reactivity, and selectivity-and to exploit them as chiraphores. In view of the utility of helical chiral pyridines such as 1, it occurred to us that the corresponding pyridine N-oxides might prove to be effective asymmetric catalysts.[8] Herein, we describe the scalable synthesis of 1-azahelicenes and the structural characterization of the corresponding N-oxides, and we apply this new family of compounds to the catalytic enantioselective desymmetrization of meso epoxides (see Table 1). This study provides the first report of the application of azahelicenes in asymmetric catalysis. [9] An examination of the structure of 1 suggests that the chiral environment in the vicinity of the nitrogen atom can be tuned by structural modification at cabon atoms 11-16. Therefore, we devised a convergent synthetic route to 1 in which benzoquinoline unit 2 and C11-C16 unit 3 could be expeditiously united (Scheme 1). This strategy would allow ready access to the necessary 1-azahelicene derivatives by simply replacing 3 with its readily available structural analogues, such as 9 and 12 (Scheme 2). Preparation of key unit 8 starts from commercially available pyridine 4 and phosphonium salt 5, which was readily synthesized in three steps from commercially available 2-bromo-4-methyl benzaldehyde. The highly Z-selective Wittig reaction [6b, 10] of 4 and 5 and subsequent Stille-Kelly reaction [5,11] provided benzoquinoline 6. The catalytic C À H functionalization method developed by Sanford and co-workers [12] readily converted 6 into 7 from which 8 was obtained in an ordinary way. The second sequence of the highly Z-selective Wittig reaction and the Stille-Kelly reaction of 8 with 9, 11, or 12 provided 1-azahelicenes 10, 1, or 13, respectively. The scalability of this Scheme 1. Synthesis design.Scheme 2. Syntheses of 1-azahelicenes: a) NaHMDS, DMF, 78 %; b) [PdCl 2 (Ph 3 P) 2 ], (Me 3 Sn-) 2 , PhMe, 77 %; c) Pd(II) catalyst, [12] NBS, CH 3 CN, 84 %; d) benzoyl peroxide, NBS, PhH, 71 %; e) 2-nitropropane,...

Self-assembling nanoparticles of amphiphilic polymers can transport hydrophobic molecules across hydrophilic media and, as a result, can be valuable delivery vehicles for a diversity of biomedical applications. Strategies to monitor their dynamics noninvasively and in real time are, therefore, essential to investigate their translocation within soft matrices and, possibly, rationalize the mechanisms responsible for their diffusion in biological media. In this context, we designed molecular guests with photoactivatable fluorescence for these supramolecular hosts and demonstrated that the activation of the fluorescent cargo, under optical control, permits the tracking of the nanocarrier translocation across hydrogel matrices with the sequential acquisition of fluorescence images. In addition, the mild illumination conditions sufficient to implement these operating principles permit fluorescence activation within developing Drosophila melanogaster embryos and enable the monitoring of the loaded nanocarriers for long periods of time with no cytotoxic effects and no noticeable influence on embryogenesis. These photoresponsive compounds combine a borondipyrromethene (BODIPY) chromophore and a photocleavable oxazine within their covalent skeleton. Under illumination at an appropriate activation wavelength, the oxazine ring cleaves irreversibly to bring the adjacent BODIPY fragment in conjugation with an indole heterocycle. This structural transformation shifts bathochromically the BODIPY absorption and permits the selective excitation of the photochemical product with concomitant fluorescence. In fact, these operating principles allow the photoactivation of BODIPY fluorescence with large brightness and infinite contrast. Thus, our innovative structural design translates into activatable fluorophores with excellent photochemical and photophysical properties as well as provides access to a general mechanism for the real-time tracking of supramolecular nanocarriers in hydrophilic matrices.

Supramolecular complexation behavior of cucurbiturils with paramagnetic nitroxide spin probes was examined by (1)H NMR, X-ray diffraction studies of crystals, computation, and EPR. Both cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) form a 1:1 complex with 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide (CAT1). The structure of the complex in the solid state was inferred by X-ray diffraction studies and in the gas phase by computation (B3LYP/6-31G(d)). Whereas ESI-MS data provided evidence for the existence of the complex in solution, indirect evidence was obtained through (1)H NMR studies with a structural diamagnetic analogue, 4-(N,N,N-trimethylammonium)-2,2,6,6-tetramethyl-N-methylpiperidine iodide (DCAT1). The EPR spectrum of the CAT1@CB7 complex consisting of three lines suggested that probe CAT1 is associated with host CB7 such that the nitroxide part is exposed to water. The spectral pattern was independent of the concentration of the complex and the presence of salt such as NaCl. The most interesting observation was made with CB8 as the host. In this case, in addition to the expected three-line spectrum, an additional spectrum consisting of seven lines was recorded. The contribution of the seven-line spectrum to the total spectrum was dependent on the concentration of the complex and added salt (NaCl) to the aqueous solution. The coupling constant for the seven-line spectrum for (14)N-substituted CAT1 is 5 G, and that for the four-line spectrum for (15)N-substituted CAT1 is 7.15 G. The only manner by which we could reproduce the observed spectra by simulation for both (14)N- and (15)N-substituted CAT1@CB8 was by assuming a spin exchange among three nitroxide radicals. To account for this observation, we hypothesize that three CAT1 molecules included within CB8 interact in such a way that there is an association of three supramolecules of CAT1@CB8 (i.e., [CAT1@CB8](3)) in a triangular geometry that leads to spin exchange between the three radical centers. We have established, with the help of 13 additional examples, that this is a general phenomenon. We are in the process of understanding this unusual phenomenon.