By Wulff

Molecularly imprinted polymers (MIPs) can be prepared according to a number of approaches that are different in the way the template is linked to the functional monomer and subsequently to the polymeric binding sites (Fig. 6-1). Thus, the template can be linked and subsequently recognized by virtually any combination of cleavable covalent bonds, metal ion co-ordination or noncovalent bonds. The first example of molecular imprinting of organic network polymers introduced by Wulff was based on a covalent attachment strategy i.e. covalent monomer-template, covalent polymer-template [12]. 

The most successful approach in this reaction category has been the use of chiral boron Lewis acid catalysts, in the addition of ethyl diazoacetate to imines reported by Wulff (Scheme 1.33) [59-60]. 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Chemoselectivity plays an important role in the benzannulation reaction as five-membered rings such as indene or furan derivatives are potential side products. The branching point is again the rf-vinylcarbene complex D intermediate which maybe formed either as a (Z)- or an ( )-metallatriene the (E)-configuration is required for the cyclisation with the terminal double bond. (Z)-Metallatriene D, however, leads to the formation of furan derivatives H (Scheme 8). Studies on the formation of (E)- and (Z)-isomers discussing stereoelectronic effects have been undertaken by Wulff [17]. 

In an approach by Jung and Nishimura, the assembly of the dysidiolide decalin skeleton 54 was deemed possible via an intermolecular Diels-Alder reaction between cydohexene 52 and dienophile 53 [14]. Based on precedent established by Wulff et al. [15] [where Z = C(OCH3)=Cr(CO)5], the cycloaddition should give predominantly the exo isomer as shown (Scheme 19.12). However, all attempts to effect the cydoad-dition simply gave recovery of starting material. It was reasoned that steric hindrance was to blame. The steric hindrance assodated with the dienophile was decreased by replacing one of the methyl groups with another double bond in the... 

Attempts to determine whether the reaction proceeds through route A or B have been made by Garret et al. [35] and by Wulff and coworkers [36]. The approach followed by the former authors was to generate a... 

Molecular imprinting has recently attracted considerable attention as an approach to the preparation of polymers containing recognition sites with predetermined selectivity. The history and specifics of the imprinting technique pioneered by Wulff in the 1970s have been detailed in several excellent review articles [122-124]. Imprinted monoliths have also received attention as stationary phases for capillary electrochromatography. 

Prior to this report by Wulff, there had only been one previous example of a [2 + 2] cycloaddition to a complexed ketene system.47... 

O Connor proposed a mechanism involving deinsertion of carbon monoxide from the vinylketene complex 106 to form the new cobaltacyclobutene 109. The cobalt may then undergo a 1,3-shift to the carbonyl of the ester group to create the oxycobaltacycle 110, before deinsertion of the cobalt moiety forms the furan 108. Alternatively, 109 may rearrange to the vinyl-carbene 111, which then undergoes ester-carbonyl attack on the carbene carbon to form the zwitterionic species 112, which finally aromatizes to yield the furan 108. Notice that this latter postulate is identical to the final steps of the mechanism formulated by Wulff (see Section V,B) for the reaction between a cobalt carbene and an alkyne, in which a cobaltacyclobutene is a key intermediate.51... 

By the formation of copolymers. Alternate isotactic copolymers. . . M,M2M M2. .. of monomers CHA=CHB with CH2=CH2, or of CHA=CHA or CHA=CHB with CHD=CH2 or CHD=CHE are chiral (38, 268, 269). Several examples arc reported in 82-89 (Scheme 18). Chiral copolymers with more complex structure such as. .. M,M,M2M MiM2. . . may be obtained even from two vinyl monomers, as recently demonstrated by WulfF and Hohn (see 53) (93). The copolymers formed by monomer units, which (vide supra) themselves produce chiral homopolymers (270) are also chiral, unless accidental compensation occurs. 

In sharp contrast to the development of the [4 + 2] cycloaddition reactions of electron-rich 2-azadienes, reports dealing with the chemistry of electron-poor 2-azadienes remained unknown until a few years ago. In fact, the first cycloaddition of an electron-withdrawing substituted 2-azadiene was observed in 1986 by Wulff and Bohnke [86AG(E)90] while they were preparing dehydroaminoacid derivatives (Scheme 48). They isolated AL(arylmethylene)dehydroalanine methyl esters 208 by dehydration of the Schiff base of the serine methyl ester 207 and found that it dimerized through a [4 + 2] cycloaddition to give tetrahydropyridine derivative 209 in 56% yield as a sole diasteroisomer. 

Liq butane was proposed by Wulff(Ref 2) for use in expl mixts with a solid oxidizer, silica gel A1 powd... 

The means to determine the minimum-energy shape for a crystal of fixed volume was developed by Wulff (38), who showed that the equilibrium shape can be determined if the surface tension, y, at all crystallographic orientations is known. As illustrated in Fig. 2, on a polar y plot of the surface tension as a function of orientation, the inner envelope of the planes drawn perpendicular to and at the ends of the radius vectors gives the equilibrium shape of a crystal of constant volume. Faceting in the equilibrium crystal shape is due to cusps in the polar y plot. 

Scheme 4 Imprinting approach used by Wulff et al. for the preparation of a polymer capable of performing enantioselective synthesis. The l-DOPA (16) is reacted with salicylaldehyde (17) to give the corresponding Schiff s base (18) that reacts with borane (19) to generate the imprinting complex (20)...

Scheme 6 System used by Wulff to catalyse the hydrolysis of carbonate (32) imprinting the cavities with the TSA phosphonate (31)...

When these MIPs were first described in the 1970s and early 1980s by Wulff [1] and Mosbach [2], they were merely used as specific separation materials, for... 

More recently, a stereocontrolled approach to the synthesis of allocolchicinoids was disclosed by Wulff and co-workers using a diastereoselective benzannulation of chromium carbene complexes, Fig. (16) [104,105],... 

The synthesis of 11-deoxydaunomycinone by Wulff and Xu is presented in Scheme 39 and involves the benzannulation of the non-chelated o-methoxyphenyl complex (280) with the alkyne (281). ° The utilization of a t-butyl ester in the alkyne (281) makes possible a one-pot construction of the tetracyclic dione (282) via a tandem benzannulation-Friedel-Cr ts cyclization. After the reaction between the... 

WulfF construction — a way to obtain the equilibrium shape of a crystal introduced by Wulff in 1901. 

Covalently bound templates have been extensively studied from 1972 [27, 28] up to the present day by Wulff et al. [62-64] who were joined by Damen and Neckers (1980) [65,66], by Sarhan et al. (1982) [67-69] and by Shea and Thompson (1978) [70]. A variety of parameters have been investigated and their influence on the selectivity of binding and the application of these polymers as chiral stationary phases have been quantified. The parameters investigated are the degree of crosslinking [65,66,71-79] the structure of the crosslinker [71,72,... 

Surface-functionalised gels were also prepared by Wulff et al. using diimine silane monomers [37]. The researchers were interested in isolating the effect of functional group positioning in molecularly imprinted materials. By using a... 

Molecular recognition has drawn much attention with regard to separation chemistry. One of the promising approaches to create molecular-recognising materials is the molecular imprinting method, the concept of which was proposed by Wulff et al. about 20 years ago. The new methodology does not require a precise... 

The problem of the equilibrium shape of crystals was addressed 100 years ago by Wulff [74]. The problem was to find the shape that minimizes the surface energy for a given number of atoms. For an isotropic system the answer is obvious it corresponds to the shape that minimized the surface—that is, a sphere. For a real crystal, which presents an anisotropy of the surface energy, the problem is less trivial. Assuming that the equilibrium shape must be a polyhedron, Wulff showed that the minimum energy is obtained when the surface energy of a given face (n,) divided by the central distance to this face hi) is a constant ... 

The equilibrium shape of a macroscopic crystal is an old problem first addressed by Wulff [37], who showed the equilibrium shape at OK to be a polyhedron. At the equilibrium, the surface energy is given by the famous Wulff s theorem ... 

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