Host-guest catalysis

Platinum-catalyzed hydrolytic amidation of unactivated nitriles was reported by Cobley and coauthors. The platinum(ii) complex, [(Me2PO- H- PMe2)PtH (PMe2OH)], efficiently catalyzes the direct conversion of unactivated nitriles into N-substituted amides with both primary and secondary amines. Possible mechanisms for this reaction are discussed and evidence for initial amidine formation is reported. Isolated yields vary from 51 to 89% [25]. 

An interesting extension to this finding resulted when 2-aminoethanol was reacted with propionitrile. Initial formation of amidine 9 occurs, with subsequent ring-closure giving the final product, 2-ethyl-2-oxazoline 10. 

Biological systems have provided much of the inspiration for the development of supramolecular chemistry (see Chapters 1 and 6). One ofthe main challenges in this area is to mimic the enzymatic systems and to understand the assembly processes involved. These natural systems have an extremely high selectivity and catalytic efficiency. Although there are many successful results in this area [26], a complete understanding of these systems is still lacking. 

The first examples of the so-called supramolecular catalysis are based on bioinspired molecular recognition, which is an essential attribute of biochemical systems. Structures such as receptors, antibodies, and enzymes can all recognize a feature that is important for their specific functions, often in the presence of species of quite similar structure. The ability to discriminate depends exclusively on the structural properties of these biological macromolecules. Recent progress in bioor-ganic chemistry has shown that many of these functions can be incorporated into smaller, synthetically more accessible structures as model systems [27]. 

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

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The use of host-guest catalysis in industrial applications would seem further away, as even the construction of a host by supramolecular means still remains costly. In terms of space-time yields heterogeneous counterparts may be more attractive, for instance (periodic) mesoporous (organo) silicas (for PMOs [99]). Highly selective processes that do not occur otherwise may find their way to applications. The capacity of a host to distinguish between guests (substrates) is as yet not a useful property for industrial applications as mixtures are not commonly used as feedstock (apart from... 

FIGURE 8 Molecular clips equipped with phosphines or phosphites used for host-guest catalysis in rhodium-catalyzed hydrogenation and hydroformylation. At the bottom are depicted the peculiar tandem cyclizations, found only for allyldihydr-oxybenzenes in the host molecule as the catalyst (37,32). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

The regioselectivity in the hydroformylation of monohydroxyben-zenes was the same for the host catalyst and a triphenylphosphine complex of rhodium (with a molar product ratio linear branched = 2 1) (35). No isomerization was observed. Rates of the conversion of monohydrox-ybenzenes when host-guest catalysis is employed are lower than when a triphenylphosphine complex of rhodium is used. In contrast, dihydroxy substrates, which are more strongly boimd to the host, react at higher rates, with an initial increase by a factor of 4 relative to catalysis by the bare rhodium complex. Dihydroxybenzenes also gave the selectivities to linear products (linear branched molar product ratio > 20 1, see below). At 30% conversion, product inhibition took place. 

As enzymes encapsulate multiple functionalities within their catalytic cavity, they have also served as a major source of inspiration for the fields of biomimetic chemistry and supramolecular catalysis. Early mechanistic theories about how enzymes work have prompted scientists from various fields to explore similar approaches for synthetic systems. One of these approaches is host-guest catalysis, where one or more substrates are bound in a cavity next to the catalytically active site. 

Calix[n]arenes in action Useful host—guest catalysis in organic chemistry 12COC949. 

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