Reaction control hydrogen-bonding templates

A3 deals with polymerization of multimonomer in which two different types of groups are connected with one template by covalent bonding. In this case, two types of units A and B with double bonds are deposited onto one template. It is worth noticing that the order of units is controlled by process of synthesis of multimonomer, not by copolymerization process, as in conventional copolymerization. Point B deals with the case in which at least one of the comonomers interacts with the template due to strong intermolecular forces. In particular B1 shows the reaction of one comonomer which is free (i.e., it has no affinity to the template) why the second comonomer A is bound (for instance, hydrogen bonding with the template). B2 represents the reaction of two comonomers adsorbed onto two different templates, B3 shows the reaction of two comonomers connected with the same template. 

Although the mechanism of the base-induced formation of calixarenes has been studied in some detail, the reaction pathways remain uncertain. The most intuitively reasonable proposal is that the immediate precursor of any particular calixarene, regardless of size, is the linear oligomer carrying the requisite number of aryl residues. Another proposal, however, postulates that calix[8]arenes, for example, arise from intermolecularly hydrogen-bonded dimers (hemicalixarenes) formed from a pair of crescent-shaped, intramolecularly hydrogen-bonded linear tetramers. Calix[4]arenes, formed under considerably more strenuous conditions, have been postulated to be the result not of direct cyclization of the linear tetramer but of reversion of the calix[8]arene. The cyclic octamer is viewed as the product of kinetic control, and the cyclic tetramer is viewed as the product of thermodynamic control. The particular efficacy of KOH and RbOH for the formation of calix[6]arenes suggests that the hexamer is the product of template control. 

Apart from providing hydrogen bond donors, coordinated protic NHC ligands can be deprotonated at the ring-nitrogen atom followed by reaction with an electrophile. Thus, complexes of types B and C are easily mono- or dialkylated. Such alkylation reactions can be utilized for the template-controlled synthesis of linear or macrocyclic homodonor (E) or heterodonor (F) ligands (Scheme 9.1) [15d]. 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

There are few addition reactions to a,/J-disubstituted enoyl systems 151 that proceed in good yield and are able to control the absolute and relative stereochemistry of both new stereocenters. This is a consequence of problematic A1,3 interactions in either rotamer when traditional templates such as oxazolidinone are used to relieve A1,3 strain the C - C bond of the enoyl group twists, breaking conjugation which results in diminished reactivity and selectivity [111-124], Sibi et al. recently demonstrated that intermolecular radical addition to a,/J-disubstituted substrates followed by hydrogen atom transfer proceeds with high diastereo- and enantioselectivity (151 -> 152 or 153, Scheme 40). 

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