Intramolecular bridging

Amino and sulfur analogues of pyrazolones also yield the aromatic quaternary salt (231 X = NH or S). If the pyrazole bears a substituent with a second pyridine-like nitrogen atom, an intramolecular bridge can be formed by reaction with a dihalogenoalkane. Thus pyrazol-I -ylpyridines react with 1,2-dibromoethane to form (233) (81JHC9). 

Figure 37. Synthesis of the first pretzelane 96 by intramolecular bridging of the two sulfonamide groups bearing macrocycles of [2]catenane 79.

Figure 39. Intramolecular bridging of the wheel and the thread of rotaxane 80m yields [l]rotaxane 100.

The pretzel-shaped molecule 96 (the first pretzelane ) was synthesized by intramolecular bridging of the two sulfonamide units of 79 with a bifunctionalized podand-like chain [54]. Again the enantioseparation of 96 was accomplished with a baseline quality separation and a large separation factor (a=5.20). Preparative separation of the enantiomers enabled the detection of the circular dichrogram of 96 (Figure 48). The optical rotation values of 79 and 96 were both determined to be [aD] = 168° (Troger base [aD] = 281°). 

Carbon-13 chemical shifts of transition metal carbonyls [471, 473] decrease as one proceeds down a given group of the periodic table, as demonstrated for the triad Cr(CO)6, Mo(CO)e, and W(CO)e in Table 4.71. Substitution of CO by cyclopentadienide and other organic ligands deshields the carbonyl carbon nuclei. This is exemplified by iron pentacarbonyl (211 ppm) in comparison to cyclopentadienylirondicarbonyl-dimer, which displays one avaraged carbonyl signal ( 240 ppm) at room temperature [488] due to rapid cis-trans isomerization and intramolecular bridged-terminal carbonyl interconversion ... 

The formation of dihydroxo-bridged species may be considered to take place via stepwise formation of a monohydroxo-bridged species with subsequent intramolecular bridge formation. Thermodynamic... 

A large body of thermodynamic as well as kinetic data for intramolecular bridge-cleavage and -formation reactions has been accumulated during the last two decades. These reactions are usually very fast and reversible, and in this sense are related to the preequilibria observed during the hydrolysis of bidentate ligands. 

Intramolecular bridging can also be used to suppress the P state selectively. This has dramatic photophysical consequences DCS-B34 shows fluorescence quantum yields of about 80% and no sign of intramolecular fluoresrence quenching [35] but some indication of a temperature-dependent A E equilibrium. The high quantum yield of this compound shows that the TICT emission is allowed in this case. The same can be concluded for DCS from the sizeable quantum yields at the lifetime maximum near — 90 °C in ethanol [35]. A possible explanation are narrow minima in kf or f (case II of Fig. 14). First results for compounds like DM-DS-B34 sterically confined to the neighbourhood of a perpendicular conformation show a large reduction in fluorescence quantum yield with respect to the unconfined case (DS-B34). 

Fig. 4 illustrates the complex coaeervation for systems 1 and 4. In system one, the complex is composed of two different polymer structures intermolecularly cross-linked (Fig. 4A), whereas in system 2, the complex is formed by the inter- and/or intramolecular bridging of a single polymer structure (Fig. 4B). Specific examples for the two complex types are sodium alginate (polyanion) with chitosan (polycation), and sodium alginate (polyanion)/calcium (divalent counterion), respectively.

The metalated ethers with EWG = acyl are simply etiolates and azaenolates. These allow for intramolecular bridging via the Li+ cation (Scheme 11). Agreement with the experiments would be attained if such bridging was favored on the endo face of the five-membered transition state (70). Indeed, the atom H-2 in the enolate is less sterically hindered in e/ido-(70) vj. exo-(71). In (70), the H-2 is directed towards a small hydrogen atom, while it would be confronted with the larger alkyl group R in (71). 

Intramolecular bridging is also observed. For functionalized dienes (e.g., RCH=CHCOCH=CHR), ring closures occur with the formation of thiacyclohexanones, while the related bis alkenylketones result in the formation of the corresponding thiacyc-lohexadienones . Simple thiophene derivatives are formed by insertion of diynes into SCI2 . Similar chemistry is reported for Se halides, and tricyclo thia- and selena ethers are obtained from sequential addition of SCI2 and SeCb to cyclooctatetraene . 

Chloromethyl groups are an obvious starting point for the introduction of further functions, e.g. via the Arbuzov reaction. Bidentate A-donor groups were introduced by nucleophilic substitution with suitable diamines . Especially interesting is the intramolecular bridging of adjacent phenolic units by reaction with bis-nucleophiles. Thus, derivatives with C2 symmetry have been obtained from chloromethylated calix[4]arenes , while a... 

Intermolecular H bonding can be recognised in the H NMR spectrum by the fact that the shifts due to the protons concerned depend very strongly on the concentration, as the simple case of methanol (36) demonstrates (Fig. 2.22a) solvation with tetrachloromethane as a solvent breaks down the H bridging increasingly with dilution of the solution the OH shift decreases in proportion to this. In contrast, the shift of the H signal of an intramolecular bridging proton remains almost unaffected if the solution is diluted as illustrated in the example of hexafluoroacetylacetone (37), which is 100% enolised (Fig. 2.22b). 

Giles N, Forrest A, GabrieUi B (2003) 14-3-3 acts as an intramolecular bridge to regulate cdc25B localization and activity. J Biol Chem 278 28580-28587... 

An emerging synthetic theme in the chemistry of coordination polymers that contain metal-metal bonds is to use diphosphines, diisocyanides, and diacetylides to link units together that contain metal-metal bonds. Harvey and co-workers have had notable success in this area using metal-metal units intramolecularly bridged by the bidentate dmb (1,8-diisocyano-p-menthane) ligand or intermolecularly connected by the dmb ligand.25,26,27... 

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