Boron templates

The use of a boron atom as a covalent template for intramolecularizing the Diels-Alder reaction has been relatively little investigated [20-22]. However Narasaka et al. have reported the use of a boronate template to control regio- and stereoselectivity in an IMDA reaction between anthrone 50 and methyl 4-hydroxybut-2-enoate 3 [20a]. Formation of the boronate tether was achieved by simply heating an equimolar mixture of phenylboronic acid with the diene and dienophile at reflux in pyridine with azeotropic removal of water. After 5 h, a single cycloadduct 51 was obtained in 81% yield. The tether was readily removed oxidatively affording the corresponding diol 52 in excellent yield (Scheme 10-19). 

Macrocyclic Lactams.— Two spermidine alkaloids have been prepared by the reaction of spermidine itself with a thiazolidine 2-thione diamide derivative under high-dilution conditions the two regioisomers produced were separated by chromatography.An interesting boron-template cyclization of the amino-ester (173) to the lactam (174) has been used to prepare several members of the celacinnine group of natural products (Scheme 85). ... 

A high-yield synthesis of the macrocyclic spermidine alkaloids (97) involves the boron-template cyclization of the aminoborane intermediate (98), and... 

The method involves successive ring expansion of smaller heterocyclic units. Another paper describes" boron-templated cyclizations of small units for the construction of macrocyclic lactams [e.g. (219)]. The fact that the reactions occur... 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Figure 74 The polyhexenyldecaborane (128) used in the production of nanostructured boron carbide materials by nanoscale templating methods utilizing porous alumina templates.

In most cases the symmetric boron-capped clathrochelates have been synthesized by template condensation on a metal ion matrix as, for instance, is shown in Scheme 68 <2005MI3>. 

The SSZ-33 zeolite was synthesized using the template molecule derived from 8-keto tricycle [5.2.1.0] decane. The boron containing SSZ-33 was converted to Al-SSZ-33 by a one-step reflux in 1 M aluminum nitrate. The SSZ-35 zeolite was prepared with the template camphorquinone. SSZ-35 zeolite was then subjected to three-fold ion exchange in ammonium nitrate. 

Examples of template procedures in which the ring-closing condensation involves reaction at a centre other than a donor atom have been documented. In an early synthesis of this type, the reaction of bis-(dimethylglyoximato)nickel(n) (62) with boron trifluoride was demonstrated to yield the corresponding complex of the N4-donor macrocycle (63) (Schrauzer, 1962 Umland Thierig, 1962). In this reaction the proton of each bridging oxime linkage is replaced by a BF2+ moiety. X-ray... 

Fig. 2. Concept of molecular imprinting - the covalent approach. 1. Derivatization of sugar template with p-vinylphenyl boronate. 2. Polymerization in the presence of a cross-linker...

The use of a p-vinylphenyl boronate as functional monomer to be covalently linked with a diol-template [2] is demonstrated in Fig. 2. Following polymerization in the presence of a cross-linker, the template has to be extracted from the polymer network. This requires breaking the covalent bond. During the application of covalently imprinted materials, the target molecules have to reform such bonds in order to be retained. Both making and breaking the bonds is at best a time-consuming process. 

The zeolite (3, synthesized using hydrothermal method, was kindly provided by zeolite plant of NanKai University and labeled as TP-(3. It was calcined at 550 C for 9 h in an oxygen flow to decompose the template. The zeolite (3 after calcination, with a Si/Al ratio of 15.20 (chemical analysis value), was labeled as dTP-p and used as the parent material for boronation. 

Replacement of the hydrogen bonds in oxime complexes with boron bridges leads to macrocyclic complexes such as those from dimethyl glyoxime (equation 59).201,202 This kinetic template technique has been used for the encapsulation of metals inside cage ligands.203 204... 

Wulff and his collaborators reported, in 1989, the preparation of imprinted polymers able to perform enantioselective synthesis [11, 12]. The imprinting complex was prepared by reacting 3,4-di-hydroxy-phenyl-alanine methyl-ester (l-DOPA methyl ester) (16) with the 4-vinyl-salicylaldehyde (17) to form the corresponding Schiff s base (18), which was further reacted with the 4-vinyl-phenyl-boronic acid (19) to afford to the corresponding ester (20) (Scheme 4). The imprinting complex obtained was then polymerised and the template removed. The resulting polymers were incubated with sodium glycinate to allow formation... 

Attachment of the alkene monomers to the template (Fig. 6.5), which must be reversible - readily formed and readily broken - to permit removal of the template after polymerisation, can generally be accomplished in two ways covalently or non-covalently. While the latter interactions (ionic, hydrophobic, n-n, hydrogen bonding) can easily be reversed, there is less scope for reversible covalent linkages. One of these is the formation of boronic esters - from boronic acid units of the monomers and OH groups of sugar templates. 

In contrast, use of metalloid elements, such as silicon, tin antimony or boron, which can form weak covalent bonds with oxygen, nitrogen or sulfur substituents during the course of the reaction, results in templated products that may be obtained metal-free by simple hydrolysis. These covalent template reactions (the M—X bond is essentially covalent in these cases) also have the advantage that the... 

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