Macrocyclic Ligands - Web del Profesor

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6 MACROCYCLIC LIGANDS and bis-macrocycles are known. 11 N-Alkylation, the more common of the functionalization routes, is usually achieved by alkylation or acylation of the amino nitrogens using a variety of agents such as chloroacetic acid, ethylene oxide, acrylonitrile, and formaldehyde. 11 Routes for selective alkylations are known, including the use of selective protection and deprotection techniques. 45 Bis-macrocycles (10), another extension of the concept of functionalized macrocycles, can be made by several different methods. One of the more commonly used is the condensation of two precursor chains already joined by the linker. 23 Bis-macrocycles also can be formed from cyclam (1) asa by-product bis-macrocycle in low yield (24). 46 NH NH HN HN (24) NH NH 3.1.2 Template-Mediated Syntheses HN HN Metal ion template mediation in macrocyclic synthesis has been a part of the field since its inception, its importance having been realized early in the development of this area. Two specific roles for the metal ion in template reactions have been proposed. These are, in turn, kinetic and thermodynamic in origin. 47 In the kinetic template effect, the arrangement of ligands already coordinated to the metal ion provides control in a subsequent condensation during which the macrocycle is formed. The thermodynamic effect serves to promote stabilization of a structure which would not be favored in the absence of a metal ion. Schiff base condensations tend to be dependent on this latter type of template effect. Some of the more routine and general synthetic procedures will be described here. A more in-depth treatment can be found in a review by Curtis, with particular emphasis on general methods as well as modifications of preformed macrocycles. 48 Carbonyl compounds are commonly used precursors for the polyaza macrocycles, as in the classic synthesis of the Curtis ligand from the condensation of acetone with Ni(en)3 (Scheme 2). 3 2,6-Diacetylpyridine has provided the Ni H2 N N H 2 3 O N HN Ni Scheme 2 NH N + HN HN Ni N N Me N Me O O + H2N NH2 N H small M n+ large M n+ Scheme 3 Me N Me N M N Me N Me N M N Me NH HN precursor for a number of pyridine-derived imine macrocycles (Scheme 3), where smaller metal ions as templates tend to implement the formation of 1:1 condensates, while larger metal ions allow for 2:2 stoichiometries. 49 Schiff base condensations can be considered as another variation of the carbonyl condensations (Scheme 4). 50 The expanded porphyrins (8) and(9) 21,22 and the compartmental ligands (16) 28 are usually synthesized by Schiff base condensations. In some instances the macrocyclic analog cannot be obtained via other methods. This is true for the self-condensation of o-aminobenzaldehyde, which can yield both tridentate (TRI) and tetradentate (TAAB) macrocycles (25) and(26). 51 N N N (25) N N N (26) Template-assisted condensations of amines with formaldehyde yield a wide variety of macrocyclic products. Sepulchrates (7) can be synthesized from the template-assisted condensation of [Co(en)3] 3+ with formaldehyde and ammonia under basic conditions. 20 Primary aldehydes other than formaldehyde have also been used. 52 Caged metal ion complexes in which the metal ion is used as a template are normally N N N H M N N Me
N N CHO H2N + CHO H2N M n+ extremely inert, so much so that removal of the metal is often impossible. Nonmetal template syntheses of polyaza cages have also been reported. 53 A number of interesting variations utilizing the template-assisted condensation of formaldehyde and amines have also resulted in structurally new macrocycles such as (27). 54 Me N NH HN N N N (27) 3.2 Polythia, Polyphospha, and Polyarsa Macrocycles One of the reasons for the relative ‘late-blooming’ of the thioether macrocycles can be found in synthetic difficulties. While the polyaza and polyoxa macrocycles can often utilize template effects in controlling the critical condensations, polythia condensations are more limited in this area. In general, these macrocycles are made from condensation of the appropriate polythiane with a dibromoalkane (Scheme 5). 55 Synthetic procedures and yields have been greatly enhanced by the addition of high dilution techniques. 56,57 A cage-like sulfur macrocycle has been reported as an analog of the nitrogencontaining sepulchrates (28). 58 Mixed nitrogen–sulfur cages can also be obtained. 58 S S N S S N (28) N S S Scheme 4 N N N M M N N N Ph N N S S SNa NaS + Br Br P P Ni PH HP Ph Ph + Br Br Ph MACROCYCLIC LIGANDS 7 Scheme 5 Scheme 6 Ph S S S S P P Ph Ni Ph P P Ph Polyphospha macrocycles can be made via template condensations of coordinated polyphosphine ligands and dibromoalkanes (Scheme 6). 59,60 Polyarsa macrocycles can be made by the reaction of lithiated polyarsanes with a dichloroalkane (Scheme 7). 26,60 3.3 Mixed Donor Macrocycles Simple mixed donor macrocycles, such as aza–oxa, aza–thia, oxa–thia, and analogous phospha and arsa analogs are generally achieved via combinations of the routes used for synthesis of the ‘pure’ donor analogs. Since the possibilities are so extensive they will not be treated here, but are found elsewhere. 16,60 New mixed donor phosphorus techniques have been devised for phospha–thia and phospha–aza macrocycles. 61,62
Page 1 and 2: Macrocyclic Ligands Kristin Bowman-
Page 3 and 4: 2.1.5 Bis-Macrocycles Bis-macrocycl
Page 5: Me 2.4.4 Calixarenes O Me OMe OMe M
Page 9 and 10: HO O O OH + Cl O O Cl Scheme 9 Sphe
Page 11 and 12: to six, the complex stability decre
Page 13 and 14: NH NH NH NH NH NH (46) HN HN HN HN
Page 15 and 16: mechanism is the assumption that co
Page 17 and 18: their guests more strongly and are,
Page 19 and 20: 48. N. F. Curtis, in ‘Comprehensi

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