Pendant arms

Kaden, Th. A. Syntheses and Metal Complexes of Aza-Macrocycles with Pendant Arms having Additional Ligating Groups. 121, 157-179 (1984). 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

Synthesis and metal complexes of azamacrocycles with pendant arms having additional ligating groups. T. A. Kaden, Top. Curr. Chem., 1984,121,157-179 (54). 

This section deals with the many POSS species that are not simple derivatives of the main compounds described in the sections above. For clarity, these compounds have been divided and listed in tables depending on the structure of the pendant arm. As there are a very large number of compounds of this type and many publications describing applications and properties of these compounds, the discussion has had to be limited to the most important ones. Some of these compounds have been reported only in patent literature and the synthetic and characterization data are included only if specifically described in the patent. This section also describes compounds in which not all eight pendant groups are the same. Many such compounds have been prepared but they are usually formed in complicated mixtures and are often not isolated as pure compounds. This highlights one of the problems in the synthesis of POSS derivatives, that is, the efficient synthesis of compounds in which several different pendant groups are present in well-defined positions. This is an area still in relative infancy but it will be seen below that there are useful syntheses available, especially for TsRyR compounds. 

The lithiated amino-benzamidinate Li[Me3SiNC(Ph)NCH2CH2NMe2] as well as its homolog containing a trimethylene pendant arm have been used to prepare... 

In a related study the synthesis of half-sandwich cyclopentadienyl titanium f-butylimido complexes containing pendant-arm-functionalized amidinate ligands... 

To mimic the square-pyramidal coordination of iron bleomycin, a series of iron (Il)complexes with pyridine-containing macrocycles 4 was synthesized and used for the epoxidation of alkenes with H2O2 (Scheme 4) [35]. These macrocycles bear an aminopropyl pendant arm and in presence of poorly coordinating acids like triflic acid a reversible dissociation of the arm is possible and the catalytic active species is formed. These complexes perform well in alkene epoxidations (66-89% yield with 90-98% selectivity in 5 min at room temperature). Furthermore, recyclable terpyridines 5 lead to highly active Fe -complexes, which show good to excellent results (up to 96% yield) for the epoxidation with oxone at room temperature (Scheme 4) [36]. 

Complexes with ligands bearing alkylpyridine pendant arms attached to an ethylenediamine framework were prepared, and their spectral and redox properties were studied (238), (239), and (240) (r = 0.10).222... 

An easily accessible hexadentate derivative of tacn, namely 1,4, 7-tris(u-aminobenzyl)- 1,4,7-triazacyclononane (tabtacn), forms a 1 1 complex [Cd(tabtacn)](C104)2 0.5 H20 Pc2 njPna2, Z = 4) with a distorted trigonal prismatic CdN6 core. The Cd—N bonds to the N donors in both the tacn moiety and in the aniline pendant arms are very similar in a rather narrow range (rav(Cd—N) 237 pm).178 Both solution and solid-state H, 13C, and 113Cd NMR spectra have been measured and analyzed with respect to details of the stereochemistry and to dynamics in solution. 

A general method for the preparation of 2-hydroxyalkyl-l,4,7-triazacyclodecane macrocycles 202, 203, 204 with a single pendant arm < 1999J(P 1)1211 > through reaction of epoxides has been achieved from the ortho amide derivative l,4,7-triazatricyclo[5.2.1.04 10]decane 40 (Scheme 30) <1999J(P1)1211>. 

Other macrocyclic ligands 218 with a single pendant arm have been prepared from l,4,7-triazacyclio[5.2.1.04 10]-decane 40 <1996ICA(246)343> by reacting with appropriate alkyl bromide/alkenyl bromide followed by base hydrolysis (Scheme 34). 

Other half-sandwich Cr complexes which show good activities for olefin polymerization include those with ether and thioether pendant arms (93) and (94) which show activities of 1,435gmmol-1 h-1 bar-1 and 2,010 gmmol-1 h 1 bar-1 respectively.252 The half-sandwich phosphine complex (95) affords a-olefins arising from chain transfer to aluminum,256,257 while the related boratabenzene chromium(III) complex (96) generates linear PF.258,259 Cationic species have also been investigated, and (97) polymerizes ethylene with an activity of 56 gmmol-1 h-1 bar-1.260-263... 

Many ligands of this category offer the prospect of inducing axial metal-ion coordination even for those cases where the pendant arms incorporate weak donor functions. Coordination will be enhanced simply because the donors are held near to the metal and hence their effective ... 

Stotter, 1979) in which single pendant arms incorporate functional groups such as -CH2CONH2 and -CH2CH2CH2NH2. The metal derivatives of these systems show a superficial resemblance to particular biochemical systems such as haemoglobin, in which the metal ion is surrounded by four nitrogen atoms in a square-planar fashion with a fifth donor, attached to another part of the molecule, occupying an axial coordination site. 

Rings with rigid pendant arms . The interesting ligand tris(2,3-dihy-droxybenzoyl)l,5,9-triazacyclotridecane (102), was synthesized specifically to act as a reagent for Fe(in) (Weitl Raymond, 1979). It is structurally quite closely related to enterobactin (103), the natural molecule used by E. coli to transport Fe(m) through its cell walls. The protonation and complexation equilibria of Fe(m) with (102) have been... 

Species such as (180) form a category of crowns incorporating an additional donor function which is not directly part of the macrocyclic ring. Many other derivatives have been synthesized in which additional donor functions are appended to the crown ring by means of arms . A selection of such pendant arm crowns is discussed in the following section. 

Other pendant arm systems. Many other pendant arm crowns have been synthesized - one system studied in some detail is given by (183) where R is a range of aliphatic derivatives incorporating further oxygen donors (Masuyama, Nakatsuji, Ikeda Okahara, 1981 Kaifer et al., 1982 Davidson et al., 1984). 

A new type of scorpionand in which the pendant arm has been equipped with a chromophore has been synthesized. The chromophore-containing pendant arm was then appended onto the macrocyclic framework by reaction of 233 with cyclam (fivefold excess) in boiling toluene (Scheme 58) <2004POL373>. 

The hydroxyethyl moiety, used as a pendant arm in DOTA-like ligands, also contains hydrogen that is in chemical exchange with bulk water. The CEST effect was observed when the complex was dissolved in acetonitrile. This functional group has so far not proved to be suitable for practical application as in aqueous solution the exchange is too fast to produce a CEST effect (159). 

Applications for Polyaza Macrocycles with Nitrogen-Attached Pendant Arms Kevin P. Wainwright... 

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