Synthetic problem, defining

Asymmetrical nitrido Tc(V) complexes (simply defined as heterocomplexes) are defined as coordination compounds in which two different bidentate ligands are bound to the same Tc=N group, and are represented by the general formula [Tc(N)(L)(L )]"+/0/". The attempt to develop a high-yield synthesis of these types of complexes may first appear to be prevented by basic chemical considerations. Actually, it is reasonable to expect that the reaction of two different bidentate ligands, A and B, with the same Tc=N group would always yield a statistical mixture of symmetrical and asymmetrical complexes, namely [Tc(N)(A)2], [Tc(N)(B)2] and [Tc(N)(A)(B)]. However, the peculiar properties of mixed 7r-acceptor-7r-donor ligands offered the route to the solution of this synthetic problem. The key approach can be outlined as follows. 

The purpose of this review is to provide a summary (through to the end of 1988) of the uncatalyzed reactions of type I and type III allyl organometallics with C X electrophiles. Most of the examples involve aldehydes and ketones, but the reactions of allyl organometallics with imines are also covered. Because the focus of this review is on selectivity and synthetic efficiency, this review is not intended to be as comprehensive as an Organic Reactions chapter or a Chemical Reviews article. Rather, we have attempted to define and illustrate the factors that influence stereoselectivity, to provide access to the most pertinent literature, and, most importantly, to provide a basis for selection of an allyl organometallic reagent for application in specific synthetic problems. 

This is illustrated by a simple synthetic problem in which bromohydrin 2 is prepared from cyclohexanone (1), which requires only functional group transformations. The synthetic relationship between 1 and 2 is represented by the diagram 2=> 1. This representation is meant to show that the bromohydrin can be formed from the ketone by an as yet unknown number of synthetic steps. Corey and co-workersi 2 called this representation a transform and used it to define retrosynthetic relationships (sec. 1.2). If we analyze the 2 => 1... 

In general, it is far more difficult to synthesize miktoarm star polymers than regular stars having identical arms because there are strict requirements in terms of multistep quantitative reactions that correspond to introducing different arms. In addition, the isolation of intermediate polymers is often required to obtain pure products. Although several successful examples of well-defined p-star polymers have been reported, most of them are composed of less than three different arms. Only a few examples of miktoarm star-branched polymers with four different arms have been synthesized to date. For this difficult synthetic problem, general and versatile methodologies for the synthesis of multiarmed and multicomponent p-star polymers have been vitally desired. 

One of the most interesting aspects of the synthetic problem in the case of longifolene, as also with other bridged polycyclic structures, is the extraordinary variety and diversity of pathways by which the carbon network may be assembled. The importance of exhaustive analysis of the topological properties of the carbon network to define the range of possible precursors from which the desired skeleton can be produced by establishing one or two connecting bonds has been discussed in detail by Corey (Ref. 1). 

The strategies explored and defined in the various examples presented open a way for wider application of microwave chemistry in industry. The most important problem for chemists today (in particular, drug discovery chemists) is to scale-up microwave chemistry reactions for a large variety of synthetic reactions with minimal optimization of the procedures for scale-up. At the moment, there is a growing demand from industry to scale-up microwave-assisted chemical reactions, which is pushing the major suppliers of microwave reactors to develop new systems. In the next few years, these new systems will evolve to enable reproducible and routine kilogram-scale microwave-assisted synthesis. 

This section deals with the many POSS species that are not simple derivatives of the main compounds described in the sections above. For clarity, these compounds have been divided and listed in tables depending on the structure of the pendant arm. As there are a very large number of compounds of this type and many publications describing applications and properties of these compounds, the discussion has had to be limited to the most important ones. Some of these compounds have been reported only in patent literature and the synthetic and characterization data are included only if specifically described in the patent. This section also describes compounds in which not all eight pendant groups are the same. Many such compounds have been prepared but they are usually formed in complicated mixtures and are often not isolated as pure compounds. This highlights one of the problems in the synthesis of POSS derivatives, that is, the efficient synthesis of compounds in which several different pendant groups are present in well-defined positions. This is an area still in relative infancy but it will be seen below that there are useful syntheses available, especially for TsRyR compounds. 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

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