Despite many advances in recent years, crystal engineering remains a risky venture. A successful outcome requires manipulation of the noncovalent bonding and properties such as size, shape, repulsion, attraction, polarity, and chirality. In this Account, we describe the interplay of crystal engineering and synthetic organic chemistry required to develop the family of helical tubuland diol hosts, the members of which exhibit a wide range of tube dimensions and inclusion properties. Certain alicyclic dialcohols crystallize with a hydrogen-bonded network structure, termed the helical tubuland lattice, in space group P3121 (or its enantiomorph P3221). Double helices of diol molecules surround parallel tubes that contain guest molecules, which are included on the basis of size and shape rather than functional group. The crystal structure of (diol)3·(chloroacetic acid)1.2 is illustrative. These chiral helical tubulate lattice inclusion compounds are formed when the racemic host diol is allowed to crystallize from solution. Complete enantiomer separation occurs during this process, producing a 1:1 mixture of pure (+)- and pure (?)-crystals (a conglomerate). The challenge of creating this family of compounds required the development of much synthetic chemistry, in particular new pathways to alicyclic ring systems with specific substitution patterns. It was also necessary to understand and control the supramolecular properties of the diol molecules. What makes the original compound tick, and why did it behave in this remarkable manner, when most of its structural neighbors crystallize totally differently? The synthesis of new helical tubuland diols requires not just preparation of a new molecular structure but also a transplant of the original unchanged hydrogen-bonding supramolecular synthon. Synthesis of the specific crystal space group is necessary. This was achieved by defining structural characteristics, termed molecular determinants, which are essential for the helical tubuland structure to occur. If these requirements were met, then the target molecule had a high probability of success. This investigation has close conceptual parallels with the search for pharmacophore properties of bioactive molecules. In both situations, parts of a molecule with little or no chemical reactivity may actually play vital supramolecular roles. The review illustrates how crystal engineering is based on specific supramolecular properties that can be uncovered and then exploited by synthetic chemists.
School of Chemistry, The University of New South Wales, UNSW Sydney, NSW 2052, Australia
The article examines the position of the University of Cape Town (UCT) on academic freedom and institutional autonomy. It also provides information on the Mafeje Affair, in which its 40th anniversary was celebrated through a public symposium held at UCT in South Africa in August 2008. The concept of a social policy or racial segregation, as compared to liberal universities was also discussed, as well as their differences in policies and practices. The article also offers a narrative account of the UCT’s decision-making process on the appointment of Archie Mafeje as the senior lecturer in the Department of Social Anthropology.
Rhodes University, Grahamstown
The burgeoning field of nanoscience has stimulated an intense interest in properties that depend on particle size. For transition metal particles, one important property that depends on size is catalytic reactivity, in which bonds are broken or formed on the surface of the particles. Decreased particle size may increase, decrease, or have no effect on the reaction rates of a given catalytic system. This Account formulates a molecular theory of the structure sensitivity of catalytic reactions based on the computed activation energies of corresponding elementary reaction steps on transition metal surfaces. Recent progress in computational catalysis, surface science, and nanochemistry has significantly improved our theoretical understanding of particle-dependent reactivity changes in heterogeneous catalytic systems. Reactions that involve the cleavage or formation of molecular ?-bonds, as in CO or N2, must be distinguished from reactions that involve the activation of ?-bonds, such as CH bonds in methane. The activation of molecular ?-bonds requires a reaction center with a unique configuration of several metal atoms and step-edge sites, which can physically not be present on transition metal particles less than 2 nm. This is called class I surface sensitivity, and the rate of reaction will sharply decrease when particle size decreases below a critical size. The activation of ? chemical bonds, in which the activation proceeds at a single metal atom, displays a markedly different size relationship. In this case, the dependence of reaction rate on coordinative unsaturation of reactive surface atoms is large in the forward direction of the reaction, but the activation energy of the reverse recombination reaction will not change. Dissociative adsorption with cleavage of a CH bond is strongly affected by the presence of surface atoms at the particle edges. This is class II surface sensitivity, and the rate will increase with decreasing particle size. Reverse reactions such as hydrogenation typically show particle-size-independent behavior. The rate-limiting step for these class III reactions is the recombination of an adsorbed hydrogen atom with the surface alkyl intermediate and the formation of a ?-type bond. Herein is our molecular theory explaining the three classes of structure sensitivity. We describe how reactions with rates that are independent of particle size and reactions with a positive correlation between size and rate are in fact complementary phenomena. The elucidation of a complete theory explaining the size dependence of transition metal catalysts will assist in the rational design of new catalytic systems and accelerate the evolution of the field of nanotechnology.
Rutger A. Van Santen
Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, P.O Box 513, 5600 MB Eindhoven, Eindhoven University of Technology, The Netherlands
The article discusses the construction of the World Cup stadiums in Cape Town and Durban. It also explores on the interconnection of different factors that influence the decision-making process of policy makers in South Africa. It also offers information based on media sources and documents from the South Africa’s 2010 World Cup Local Organising Committee (LOC). It is cited that the political economy of world football, which is dominated by institutions, media and corporations, influences the processes of decision-making and policy-making in the local context.
Department of History, Michigan State University,
As master craftsmen, modern synthetic chemists are challenged to achieve remarkable feats of efficiency and elegance toward molecular targets. The nature of this pursuit necessitates the collection of synthetic repertoires that are tried and true. With methodologies and pathways increasingly scrutinized, the adept chemist must seek out propitious tools to incorporate into the arsenal. With this in mind, this Account highlights the versatility of alkoxyallenes as precursors to valuable heterocyclic building blocks for such efforts as natural product synthesis. Accessed by the etherification of either propargyl alcohols or propargylic halides, alkoxyallenes are obtained after base-catalyzed isomerizations of the propargylic ethers. A host of umpolung synthons are available through this scheme after metalation, generating C3 nucleophiles synthetically equivalent to vital anionic and zwitterionic synthons. Reactions with a diverse set of heteroatomic electrophiles yield carbohydrates, spiroketals, alkaloids, and heteroaromatics via [3 + 2] or [3 + 3] cyclizations. By employing lithiated alkoxyallenes into transformation routes, the natural product chemist can utilize this methodology as a viable resource in stereoselective synthesis. A survey of our own utilization of alkoxyallenes along synthetic pathways toward natural product targets reveals their suitability for generating advantageous precursors. A set of four stereoisomeric 2,6-dideoxyhexoses were stereoselectively obtained after an initial lithiated alkoxyallene and lactaldehyde cyclization, followed by the oxidative ring opening of the dihydrofurans. Through the addition of a lithiated alkoxyallene to a functionalized benzaldehyde, an essential spiroketal diastereomer was rapidly achieved in a few steps. We greatly benefitted from alkoxyallenes in the construction of complex nitrogen-containing synthetic targets, whether pyrrolidine alkaloids, substituted imidazole derivatives, or functionalized pyridines. A pinnacle example of their utility came from the coupling of alkoxyallenes to nitrones affording 1,2-oxazines, which served as a gateway to an array of novel polyfunctionalized compounds such as aminopolyols, hydroxylated pyrrolidines, or carbohydrate mimetics. Alkoxyallenes have proven themselves to be powerful C3 building blocks toward complex molecular targets, revealing novel pathways to a variety of desirable highly functionalized heterocycles. In our view, the full extent of their synthetic utility has yet to be truly realized.
Malte Brasholz?Hans-Ulrich Reissig?Reinhold Zimmer
Freie Universitt Berlin, Institut fr Chemie and Biochemie, Takustrasse 3, D-14195 Berlin, Germany
The article discusses the gendered respectability, which was built into a labour process of shop work in South Africa. It offers a brief history of the interactions between white women and black service workers in the late 1960s and early 1970s. It is said that the National Union of Distributive Workers (NUDW) had protected the rights and reputability of white women workers. It is cited that the NUDW had raised public claims to employers and to their white customers, to defend for the legitimacy of women’s status as workers.
University of the Witwatersrand,
The article describes and analyses the conflicts over land, authority and natural resources among the non-governmental organisations (NGOs) and the ethnic groups in West Caprivi in Namibia. It also offers information on the tribalism, tradition and ethnicity in South Africa, and also presents and analysis on the multi-layered political nature of identities. The ethnic groups, namely Khwe and Mbukushu, were also discussed, as well as their contested authority in relation to the state and how the people of each group were constructed.
Taylor, Julie J.1
Department of International Development, University of Oxford,
Unlike normal chemical bonds, hydrogen bonds (H-bonds) characteristically feature binding energies and contact distances that do not simply depend on the donor (D) and acceptor (:A) nature. Instead, their chemical context can lead to large variations even for a same donor?acceptor couple. As a striking example, the weak HO?H···OH2 bond in neutral water changes, in acidic or basic medium, to the 6-fold stronger and 15% shorter [H2O···H···OH2]+ or [HO···H···OH]? bonds. This surprising behavior, sometimes called the H-bond puzzle, practically prevents prediction of H-bond strengths from the properties of the interacting molecules. Explaining this puzzle has been the main research interest of our laboratory in the last 20 years. Our first contribution was the proposal of RAHB (resonance-assisted H-bond), a new type of strong H-bond where donor and acceptor are linked by a short ?-conjugated fragment. The RAHB discovery prompted new studies on strong H-bonds, finally leading to a general H-bond classification in six classes, called the six chemical leitmotifs, four of which include all known types of strong bonds. These studies attested to the covalent nature of the strong H-bond showing, by a formal valence-bond treatment, that weak H-bonds are basically electrostatic while stronger ones are mixtures of electrostatic and covalent contributions. The covalent component gradually increases as the difference of donor?acceptor proton affinities, ?PA, or acidic constants, ?pKa, approaches zero. At this limit, the strong and symmetrical D···H···A bonds formed can be viewed as true three-center-four-electron covalent bonds. These results emphasize the role PA/pKa equalization plays in strengthening the H-bond, a hypothesis often invoked in the past but never fully verified. In this Account, this hypothesis is reconsidered by using a new instrument, the pKa slide rule, a bar chart that reports in separate scales the pKa’s of the D?H proton donors and :A proton acceptors most frequently involved in D?H···:A bond formation. Allowing the two scales to shift so to bring selected donor and acceptor molecules into coincidence, the ruler permits graphical evaluation of ?pKa and then empirical appreciation of the D?H···:A bond strength according to the pKa equalization principle. Reliability of pKa slide rule predictions has been verified by extensive comparison with two classical sources of H-bond strengths: (i) the gas-phase dissociation enthalpies of charged [X···H···X]? and [X···H···X]+ bonds derived from the thermodynamic NIST Database and (ii) the geometries of more than 9500 H-bonds retrieved from the Cambridge Structural Database. The results attest that the pKa slide rule provides a reliable solution for the long-standing problem of H-bond-strength prediction and represents an efficient and practical tool for making such predictions directly accessible to all scientists.
Paola Gilli?Loretta Pretto?Valerio Bertolasi?Gastone Gilli
Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Universitdi Ferrara, I-44100 Ferrara, Italy