Cyclocholates are efficiently and rapidly synthesised from suitable monomers by transesterification under thermodynamic, equilibrating conditions using a potassium methoxidexrown ether complex.
Die Synthese molekularer organischer Verbindungen ist traditionell durch die Anwendung kinetisch kontrollierter Reaktionen dominiert, [1] die zu irreversiblen (starken) kovalenten Bindungen f¸hren. Dabei werden die Reagentien (oder Katalysatoren) und die Reaktionsbedingungen in Hinblick auf eine gute Ausbeute eines einzigen Produkts sorgf‰ltig ausgew‰hlt. Im Wesentlichen ist ein Reaktionsweg er-w¸nscht, der zur energetisch bevorzugten Bildung eines bestimmten Produkts f¸hrt und die Bildung anderer, uner-w¸nschter Produkte vermeidet; so wird, wie in Abbildung 1 Angew. Chem. 2002, 114, 938 ± 993 Stuart J. Rowan wurde 1969 in Edinburgh, Schottland, geboren. Er erhielt 1991 seinen B.Sc.-Titel an der University of Glasgow. Dort fertigte er auch seine Doktorarbeit mit dem Titel πSupramolecular Crystal Engineering of Inclusion Compounds™ bei Dr. David D. MacNicol an (Ph.D.-Titel 1995). 1994 ging er nach Cambridge zu Prof. Jeremy K. M. Sanders zur Erforschung von dynamischen kombinatorischen Bibliotheken mit dem Schwerpunkt Umesterungsreaktionen. 1996 wurde er zum Research Associate of Girton College in Cambridge ernannt. 1998 f¸hrte ihn sein Weg an die University of California in Los Angeles, wo er als Postdoc bei Prof. J. Fraser Stoddart seine Arbeiten fortsetzte. Dort entwickelte er zahlreiche neue Methoden zum Aufbau von mechanisch verkn¸pften Verbindungen, speziell durch Anwendung der dynamischen kovalenten Chemie, und Stoppern. 1999 wurde er zum Assistant Professor am Department of Macromolecular Science an der Case Western Reserve University in Cleveland, Ohio, ernannt. Seine Forschungsinteressen konzentrieren sich auf das Potential der dynamischen (kovalenten und nichtkovalenten) Chemie, auf den Aufbau und die Eigenschaften polymerer Stoffe und auf die Entwicklung neuer Synthesemethoden zum Aufbau komplexer polymerer Architekturen. Stuart J. Cantrill wurde 1974 in Lichfield, England, geboren. 1993 begann er an der University of Birmingham mit dem Studium der Chemie und der Bioorganischen Chemie und erhielt dort 1996 seinen B.Sc.-Titel. Seine Doktorarbeit fertigte er bei Prof. J. Fraser Stoddart an (University of Birmingham (M.Phil.-Titel)/University of California in Los Angeles). Der Schwerpunkt seiner Arbeit lag in Forschungen zur Wechselwirkung zwischen sekund‰ren Dialkylammoniumionen und Kronenethern, wobei speziell selbstkomplement‰re πdaisy chain™-Systeme untersucht wurden. 2001 erlang er an der UCLA den Ph.D.-Titel. Gegenw‰rtig forscht er als Postdoc am California Institute of Technology in der Arbeitsgruppe von Prof. Robert H. Grubbs. Graham Cousins wurde 1975 in Southampton, Gro˚britannien, geboren. Er erhielt seinen B.Sc.-Titel in Chemie 1997 an der University of Birmingham. Anschlie˚end fertigte er an der University of Cambridge bei Prof. Jeremy Sanders seine Doktorarbeit an und erhielt dort 2000 seinen Ph.D.-Titel. Sein Hauptforschungsinteresse gilt der Supramolekularen Chemie und der Kombinatorischen Chemie. In Birmingham erforschte er w‰hrend seiner Studienzeit bei Prof. J. Fraser Stoddart die mol...
The reversible nature of the imine bond formation in CDCl(3) solution has been exploited to exchange substituted for unsubstituted m-phenylenediamine (MPD) units in hemicarcerand octaimines. Moreover, acid-catalyzed imine exchange has been shown to provide a novel mechanism whereby ferrocene (Fc) can be released as an entrapped guest from the hemicarceplex C(2)B(4)&crcldt;Fc dissolved in CDCl(3) to give the hemicarcerand C(2)B(4) when excess of both MPD and trifluoroacetic acid are present.
Dipyrido[24]crown-8 (DP24C8) has been synthesized and shown to form [2]pseudorotaxanes spontaneously with dibenzylammonium ions. These complexes, which have been demonstrated by (1)H NMR spectroscopy to form faster in solution than when the macrocyclic polyether is dibenzo[24]crown-8 (DB24C8), are also stronger than their DB24C8 counterparts. One of the [2]pseudorotaxanes has been used to construct a [2]rotaxane (see above) comprising a dumbbell-shaped component based on a dibenzylammonium ion which is encircled by a DP24C8 macrocycle and terminated by (triphenylphosphonium)methyl stoppers.
By utilizing the dynamic nature of imine bonds, it is possible to construct [2]rotaxanes from a ring and a preformed dumbbell under thermodynamic control. These dynamic [2]rotaxanes, which exhibit reversible supramolecular-like behavior in the presence of appropriate catalysts, can be "fixed" by reduction of their imine bonds.Although thermodynamic control dominates the chemistry of noncovalently bonded systems and allows spontaneous access to intricate supramolecular entities, with a few notable and spectacular exceptions, 1 many molecular aggregates are difficult to characterize in solution because of a lack of sufficient cooperativity between their noncovalent bonding arrays. The prospect of being able to synthesize more robust analogues of these noncovalently bonded systems is an appealing one. In a recent Letter, 2 we outlined the synthesis of a rotaxane utilizing thermodynamically controlled reversible covalent chemistry involving imine bond formation. In this Letter, we describe the construction of two dynamic rotaxanes in which the ring component is cyclobis(paraquatp-phenylene) 3 (CBPQT 4+ ), with its π-electron-deficient bipyridinium units, and a dumbbell component which contains either a π-electron-rich 1,5-dioxynaphthalene or 1,4-dioxybenzene ring system centrally located within a polyether chain, terminated by large aromatic stoppers containing imine bonds produced as a result of either imine formation or exchange. 4,5 We show how these two rotaxanes can be equilibrated in the presence of an excess of the appropriate dumbbell component. Furthermore, we also demonstrated that we can "fix" the dynamic [2]rotaxanes by reduction of the imine bonds present in their dumbbell components.The syntheses of the dynamic dumbbells 5NP/5BZ in both the 1,5-dioxynaphthalene (NP) and 1,4-dioxybenzene (BZ) series are outlined in Scheme 1. The account given here
Thermodynamic control operates in the synthesis of a [2]rotaxane based upon the dibenzylammonium ion/crown ether recognition motif. When dibenzo[24]crown-8 is added to an acetonitrile solution containing a diimine dumbbell-like component, the dynamic nature of the system (i.e., imine hydrolysis/reformation) offers the ring component access to the NH2 + center, allowing the self-assembly of the corresponding “dynamic” [2]rotaxane to occur. The “fixing” of this [2]rotaxane can be achieved upon reduction of the imine bonds, affording a kinetically inert [2]rotaxane.
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