Ring formation cryptands

The one-pot synthesis of polymers via Ugi-4CR can only be performed if either two bifunctional AA-type components or one AB-type component are used. Moreover, highly concentrated reaction mixtures are needed because otherwise ring formations occur, as thoroughly investigated by Wessjohann and coworkers. Here, the directed synthesis of macrocycles or cryptands under classic or pseudo high dilution conditions, or even without dilution exploiting metal template effects were demonstrated [66, 67]. 

On the other hand, anions formed by the ring cleavage of octaphenylcyclotetrasilane may be very unreactive due to steric hindrances as well as to the formation of the tight silyl ion pair with alkali metals. We have attempted to decrease these interactions by using cryptands in order to better solvate the alkali metals and form the loose,... 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

Scheme 3.27 represents a case when the odd-bond formation governs the geometry of an anion-radical (Chona et al. 2004). The substrate was reduced on an alkali mirror in the presence of a cryptand, giving the anion-radical, which is not coordinated to the counterion. Such a species undergoes internal rotation to adopt a conformation in which the two phosphine rings occur to be bound with a weak but real one-electron bond. 

The larger-ringed macrocycles of (53a-d) form binuclear complexes with alkali metal, alkaline earth, silver(I) and lead(II) cations.68,192 The two 18-membered rings are large enough to allow for cations as large as Rb+ to be incorporated within their cavity, with a net result of increasing the metal-metal separation. Thus, crystal structure data for the disodium complex of (53a) indicate the sodium ions to be 6.40 A apart,193,194 compared to a 3.88 A separation found for the aforementioned disilver complex of (52a). Heteronuclear complex formation has also been observed, e.g. with both Ag+ and Pb2+ incorporated in the same cryptand.192... 

The three Schiff-base bibracchial lariat ethers H2l7a-c derivatives have been designed for their ability to form a cryptand-like cavity upon reaction with Lnm ions, the size of which can be tuned by varying the number of -CH2-CH2-O- units in the macrocycle. The two 2-salicylaldiminobenzyl pendant arms fold in such a way that jt-jt interactions between aromatic rings result in the formation of a cryptand-like cavity, as demonstrated by the X-ray structure of the Cem complex with 17b depicted on fig. 25 (Gonzales-Lorenzo et al., 2003). The triplet state of the di-anionic receptor is located at 18750 cm-1 (0-phonon component measured on the Gd111 complex) so that sensitization of the NIR luminescence is not optimum nevertheless, Ndm emission could be detected. 

Further studies on the reduction of bisphosphinine 55 showed that the existence of a rigid structure is not a prerequisite to the formation of a one electron P-P bond [38], Combination of EPR spectroscopy and DFT calculations indicates that a loose P-P bond (3.479 vs. 5.8 A in the neutral molecule) is formed upon reduction with one electron (compound 56). Additional experiments carried out with (or without) cryptands at various temperatures showed that the formation of a tight ion-pair can stabilize compound 57 in which the odd electron is exclusively localized on a single phosphinine ring (Scheme 12). 

Bicvclic Receptors. Cryptands are known to form more iq)ecific and stable complexes with metal cation than coronands (23) sev d mono anthiaciio-cryptands were designed in order to iicapsulate some cations in a given potion (aa). A consequence of the presence of two bridgdiead tertiary amines in the proximity of the anthracene ring, is the easy formation of intramolecular exciplexes which, in polar solvents, usually lead to fluorescence quoiching. This is the case of A33 (see formula Ann, n = 3). 

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