Boronic acids templates

Of particular interest to combinatorial chemistry is the use of immobilised functionalised boronic acid templates which are capable of further transformations [20]. For instance, an aryl carboxylic acid 50 can be converted into the corresponding amide 51 (Scheme 12), whilst still being attached to the resin. Benzyl amine and butylamine were coupled efficiently to afford (after cleavage) the corresponding amides 52 in high yield (Scheme 12). 

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. 

Monocapped boronic acid adducts of Tc(III) /m(dioxime) (BATO) (X — Cl, Br dioxime = dimethylglioxime, cyclohexanedione dioxime R = CF13, C4Fl9) have been prepared by template synthesis starting with pertechnetate and stannous ion [NBu4][TcOCl4] or M2[TcXO] (M = NH4, X = C1, Br), yielding seven-coordinate... 

An interesting templated bis-functionalization was reported by Shinkai and co-workers who introduced two boronic acid groups regio- and diastereo-selectively into C6o by using saccharides or saccharide derivatives as imprinting templates.147-149 Thus, when D-threitol was used as the template, C2-symmetric cis-3 isomers fC-49 and fA-49,150 were obtained in a nearly. 72 28 ratio after removal of the saccharide templates (Scheme 1.6).149 Competitive complexation studies indicated that D-threitol-imprinted boronic acid (fC)-49 and L-threitol-imprinted (A)-49 preferentially rebind their original templates with up to 48% d.e.148 Exploration of other regio- and diastere-oselective double [4 + 2] cycloadditions between C6o and o-quinodimethanes... 

If we compare the value (—61.7°) for a polymer prepared from 1 (calculated for the molar content of 1), still containing the template, with the value —448.9° for the template monomer 1, it becomes apparent that the molar optical rotation value is considerably decreased as a result of polymerisation. This can have several causes, one being the influence of the polymer matrix. Its effect can be determined by removing the optically active template la. If the boronic acids are then converted with ethylene glycol to the corresponding achiral ester, the polymer shows a positive molar rotation [M] °546 = -1- 110.0°. Apparently, in P-E the imprints generated in the polymer make a positive contribution to the optical rotation value. Measuring the optical rotation in the solid phase allows the... 

The re-uptake of template in the imprinted polymer is relatively slow (consider the rates of formation of boronic esters listed above), but more than 90% of the free cavities can be reoccupied. Fortunately, in aqueous alkaline solution or in the presence of certain nitrogen bases (for example NH3 or piperidine) tetragonal boronic esters are formed (equation (b)), which equilibrate extremely rapidly with tetragonal boronic acid and diol [47,55]. In these cases the rate of equilibration... 

Other results involving liquid chromatography separations with imprinted metal oxides have been published in recent years. Norrlow et al. prepared surface imprinted silica gels functionalised with boronic acids that could form covalent linkages with riboses via boronate ester formation [43]. The gels were imprinted with templates containing two (nicotinamide adenine dinucleotide (NAD)) or four ribose units (bis-NAD 1, bis-NAD II). Columns were packed with the imprinted... 

Clathrochelate iron(II) tris-dioximates with hexadecylboron capping groups were synthesized by direct template condensation of acyclic and alicyclic dioximes with the corresponding re-Ci6H33B(OH)2 boronic acid (denoted as HdB(OH)2) on iron(II) ion in methanol ... 

The most successful approach in covalent molecular imprinting is probably the coupling of polymerizable boronic acids to hydroxyl groups present on the template to form boronate esters (Figure 2.2). This approach has, for example, been applied to the imprinting of sugars [57-59]. 

Figure 2.2 Imprinting of phenyl a-D-mannopyranoside using (4-vinylphenyl)boronic acid. Formation of monomer-template complex (1) polymerization (2) cleavage and extraction of template (3) and rebinding (4).

Covalent interactions (Fig. 7) this approach involves the formation of an easily cleavable functionalised monomer-template complex [61, 18]. The first example of an imprinted polymer network was prepared by Wulff s group and used the reversible formation of an ester bond between a diol and 4-vinylphenylboronic acid [15]. The reaction rates to reach equilibrium between the ester and boronic acid are comparable to those obtained with... 

D-Fructose Anthracene-boronic acid conjugate HEMA EDMA MeCN Using covalently bonded template new functional monomer fluorescent sensor [90]... 

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