Introduction and General Aspects

One of the first things to take serious note of when attempting a correlation of experimental aspects of conduction in CPs to conduction models is that to obtain a complete picture of conduction behavior, it does not suffice to merely measure DC conductivities. Rather, one must characterize the entire gamut of conduction behavior, i.e.  

Additionally, many supplementary measurements assist in characterizing conductivity behavior, and thus help in establishing models. These include, for instance, the complex microwave-region dielectric constants of the material, and Reflectance measurements from the Visible through the far-IR. 

It is noted at the outset in this chapter that the above experimental measurements or techniques for all aspects of conductivity are described in some detail elsewhere in this book (Chapters 11-12, Characterization Methods), and this chapter is confined to a consideration of conduction models or, occasionally, a brief synopsis of the measurement technique when this is found necessary for the discussion. 

We also note at the outset that while many models have been proposed for conduction in CPs, conduction has in fact been found to be a very complex phenomenon, and to date no single model is comprehensively accurate In varying degrees, the various models are, for instance, able to account for conduction behavior within a specific temperature range or doping range or dopant type, but then fail for other ranges or types, or for other CPs. In one case, a 3-d conduction model may be indicated, whereas in another case, it is not, and so on. 

Although a variety of measurement techniques have been applied, as noted above, one of the most important has been characterization of the conductivity (commonly DC, less commonly frequency dependent) of CPs as a function of temperature. This has been used in a very large number of studies to attempt to validate this or that conduction model, sometimes with a selective or incomplete representation of data. 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

Several reviews and books have already appeared on the hetero Diels-Alder reaction [3-23], The latest general overlook are the articles of Boger and Wein-reb in Comprehensive Organic Synthesis covering the literature until 1989. 

In this article we describe novel developments in the synthesis of heterocycles by hetero Diels-Alder reactions covering the literature from 1989. However, as a background and if neccessary for the understanding, also older publications will be presented. Due to the restriction of space only the most important and synthetically most useful dienes and dienophiles which are displayed in Table 1-1 and Table 1-2 will be discussed in this article. 

Clearly, an important feature will be the selectivity of these reactions. In this respect, the control of endo- and exo-selectivity using different Lewis acids, the induced diastereoselectivity with chiral heterobutadienes as well as chiral heterodienophiles and finally the use of chiral Lewis acids for the enantioselec-tive synthesis will be discussed. In recent time some attention has been paid to hetero Diels-Alder reactions in aqueous solutions and in the presence of inor- 

The reaction rate and the selectivity of hetero Diels-Alder reactions can also be influenced by applying high pressure. A large amount of knowledge has been 

Catalysis is of greatest relevance for chemical technology. It is assumed that about 90% of all chemical processes work with the help of at least one catalyst. It is further assumed that 80% of the added value of the chemical industry and about 20% of the world economy depend direcfly or indirectly on catalysis. The catalyst market (the value of traded catalysts) was about 10 in 2007, but at the same time the value of the goods produced by these catalysts was at least 100 times higher l x 10 Weitkamp and Glaeser, 2003]. A recent article forecasts that the value of traded catalysts will reach 17.2 billion in 2014 with an actual rise of 6% per year (Hydrocarbon Processing, 2011) 

Our world would look very different without the catalysts that have been developed over the last 100 years. For example, supplying food for about 6 billion people on earth would be impossible without the catalytic transformation of nitrogen from air into ammonia, as only the latter allows the production of fertilizers for food production on today s scale. Without refinery catalysts we would certainly have much higher energy prices and would run out of oil much earlier. 

One can calculate that the annual consumption of crude oil would be more than 400 Mio per year higher, due solely to the lower efficiency of our refineries without the catalysts used today (for comparison, annual crude oil consumption was about 3 X lO tons per year in 2005). Materials would be very different as many plastics cannot be produced without catalysts that promote the polymerization process or that are needed for the production of monomers. Incidentally, we should not forget that nature is also full of biocatalysts that accelerate important processes like photosynthesis or the metabolism in our bodies and thus provide the fundamentals of life on earth. 

Catalysis is of major socio-economic importance. To solve future problems connected with limited resources and energy, as well as environmental protection, there is no way around catalysis. In fact, we can regard catalysis as the key technology for the sustainable production of chemicals since efficient catalysis saves raw materials and energy and avoids waste formation. 

The term catalysis originates from the Greek word Karakiaia, which means to dissolve, to loosen, to unfix. Berzelius (1779-1848) introduced the term in 1836. Other pioneers of the concept of catalysis were Dobereiner (1780-1849), Mitscher-lich (1794—1863), and Liebig (1803-1873). These first catalyst researchers observed in many cases that two substances that do not show a tendency for reaction do in fact react quickly in contact with a third substance that is not consumed in the reaction. Wilhelm Ostwald (see box) made a very significant contribution to the modern physicochemical understanding of catalysis. 

An appreciation of the life of Dexter French has been published.  

A survey of regio-, stereo-, and chemo-selective reactions in carbohydrate chemistry includes discussion of phase transfer reactions in partial substitution reactions, selective halogenation 

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Wiesler U-M, Weil T, Mullen K (2001) Nanosized Polyphenylene Dendrimers. 212 1-40 Williams RM, Stocking EM, Sanz-Cervera JF (2000) Biosynthesis of Prenylated Alkaloids Derived from Tryptophan. 209 97-173 Wirth T (2000) Introduction and General Aspects. 208 1-5 Wirth T (2003) Introduction and General Aspects. 224 1-4... 

T. Wirth, Hypervalent Iodine Chemistry - Modem Developments in Organic Synthesis - Introduction and General Aspects, in Hypervalent Iodine Chemistry Modem Developments in Organic Synthesis Top. Curr. Chem., 2002, 224, 1. 

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