Protecting group found for hydroxyls, aminos
CHICAGO Scientists at the University of Kansas, Lawrence, have devised a reagent to protect hydroxyl and amino groups dur ing multistep syntheses. Postdoctoral fel low student Binghe Wang of the Univer sity of Kansas School of Pharmacy told the Division of Organic Chemistry that the protecting group thus attached is re movable by mild reducing agents when no longer wanted. Such a reagent is useful because chemists want to selectively protect groups that are removable by many dif-
ferent mechanisms. Such a choice en ables them to apply and remove each group by design, while sparing protect ing groups that are still needed. The new reagent is a quinonebutanoic acid, 3-methyl-3-(3,5-dimethyl-2-benzoquinone)butanoic acid. In use, research ers would form amides or esters of the carboxyl group. To remove the group, the derivative would be shaken for a few minutes with dilute aqueous sodium dithionite. This procedure reduces the qui none to a hydroquinone. A hydroquinone hydroxyl group maneuvers around to displace the amine or alcohol from the carboxyl group to form a coumarin, spe cifically 3,4-dihydro-6-hydroxy-4,4^,7-tetramethylcoumarin. The coumarin can be recovered and reoxidized to the quinone reagent. Working with medicinal chemistry professor Ronald T. Borchardt and post doctoral fellow Siming Liu, Wang makes the reagent in three steps and in 70% overall yield from commercially avail able 2,6-dimethylbenzoquinone.
Protecting group removable by reducing agents
Stephen Stinson
Progress made in design of zeolite synthesis
UÏM.M.NI Researchers haven't yet achieved the ability to design a synthetic route that would make a specific zeolite. But Mark E. Davis, professor of chemical engineering at the California Institute of Technology, Pasadena, believes it won't be long. Davis explored the outlook for zeo lite synthesis for the Division of Colloid & Surface Chemistry. Organic struc ture-directing agents can be designed, he says, and the required molecular building blocks are available. What re mains is to combine these elements with the chemical compatibilities that lead to self-assembly.
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SCIENCE/TECHNOLOGY Zeolites and molecular sieves are crys talline alkali-aluminum silicates that can recognize, discriminate among, and or ganize molecules with differences in size smaller than 1 A. In the case of molecules with varying symmetry, zeolites may also be able to discriminate on the basis of shape. These abilities are commonly used in catalysis, separations technology, and chemical sensing. And zeolites serve as hosts to organize guest atoms and mole cules that provide composite materials with optoelectric and electrochemical properties. Hence, the goal in designing zeolitic materials is the capability to syn thesize zeolites having prescribed prop erties for specific purposes. Zeolite crystallization results from supramolecular self-assembly, and ig norance of the mechanisms involved is among the factors affecting the ability of researchers to design zeolitic synthe ses. Moreover, because there is no anal ogy with organic synthesis, the vast amount of information available on or ganic transformations provides little in sight for zeolites. Nevertheless, Davis says, great progress is being made in developing prescription syntheses. In synthesis of zeolites, the substanc es used, in addition to the inorganic materials themselves, can include cer tain organic molecules. These organics can be included in the synthesis mix tures and are ultimately retained in the zeolite structures. This phenomenon has sired much re search aimed at "templating" zeolites. The underlying idea is to enclose an or ganic molecule of specified shape and size within a zeolitic structure. When the zeolite has formed, the organic molecule can then be removed, leaving behind a zeolite with the desired cages and channels. To design a zeolite in this manner thus requires the design of an organic "template." The assumption, Davis says, is that there is a close correspon dence between the encaged organic molecule and the eventual size and shape of the zeolitic cages. However, he suggests that the correspondences achieved haven't been close enough yet to warrant describing the process as templating, especially when compared with analogous biological syntheses. Hence, the concept of structure-direc tion by organic templating is still open for debate and the appealing idea of templating may be wishful thinking.
Similarly, it is not clear if structuredirection ultimately can be used to de sign the pore architecture in zeolites. Information now available indicates that design of zeolite pore structures is feasible if pure-silica syntheses are used with water-soluble organics that do not decompose at synthesis condi tions. Letting the silicon structures grow in this way can be a lengthy pro cess, perhaps taking months. For prac tical application, faster times would be required. And the presence of heteroatoms that it would be desirable to add— aluminum or titanium, for example— could have an adverse effect on the structure-directing effects of the organ ic species. Davis notes that his group did re cently provide the first example of a zeolite whose pore system was formed by purposeful design of its structuredirecting agent. The approach used rig id polycyclic organocations that could direct the formation of multidimen sional pore systemso with pores in the size range of 5 to 8 A. In one case, a di-
quaternary cation with two fused fivemembered rings led to the formation of ZSM-12, a so-called one-dimensional system. Fusing a third, six-membered ring perpendicular to the five-membered rings in the cation led to two-di mensional SSZ-26. Ultimately, Davis says, the biggest handicap in zeolite design is basic ig norance of the mechanics of formation of the inorganic oxides. Attempts to synthesize pure silica zeolites from mo lecular building blocks are continuing. For obvious reasons, this approach has been termed //Lego-chemistry.,, In this regard, chemists are using spherosiloxanes as molecular building units because their polycyclic ring structures are found in many zeolites. In theory, a tremendous variety of structures can be produced from these basic units. However, success has been modest at best, Davis says, probably because of the lack of the basic knowl edge of mechanisms. These mecha nisms are now being studied. Joseph Haggin
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