Bridged substitution

The parent with the most substituents cited as prefixes (bridging substitution, e.g. 2,3-O-methylene, is regarded as multiple substitution for this purpose). 

Figure 7.19 Several ligands and gadolinium(III) complexes used in MRI imaging. (A) DTPA ligand (B) [Gd(DTPA)(H20)]2, Magnevist (C) MS-264-L, ethylene-bridge-substituted DTPA (D) BOPTA, acetate-substituted DTPA (E) [Gd(B0PTA)(H20)]2, MultiHance (F) AngioMARK . (Adapted with permission from Chart 1 of Caravan, P. Ellison, J. J. McMurry, T. J. Lauffer, R. B. Chem. Rev., 1999, 99(9), 2293-2352. Copyright 1999, American Chemical Society.)...

Irradiation of [2.2]paracyclophane, under different conditions (various solvents, light sources of different wavelength, addition of photosensitizers) always leads only to open-chain cleavage products of 2. The counterpart of 775, the polycyclic equinene (77(5), could not be detected 22>. Cram and Delton 96> even ruled out the intermediate occurrence of 116 analogs in the photo-racemization of a number of optically active nuclear- and bridge-substituted [2.2]paracyclophanes. 

Figure 16 Newman projections of (5) conformers of methylene bridge-substituted salenH-, derivatives...

Unbridged, bridged, substituted, and half-sandwich complexes have been used as metallocenes for ethylene polymerization (Figs. 1 and 2). To compare the activities and molecular masses, the polymerizations are carried out under the same conditions (30°C, 2 bar ethylene pressure, with toluene as a solvent) (105). Table IV shows the polymerization behavior of various met-allocene/alumoxane catalysts. Generally, zirconium-containing catalysts are... 

The nucleophilic addition of a hydride to the exocyclic double bond of fulvenes, using LiBEt3H as the hydride transfer reagent, resulted in the formation of the appropriately substituted lithium cyclopentadienide intermediates, which is insoluble under the reaction conditions chosen and can be isolated for purification purposes. Two equivalents of the substituted lithium cyclopentadienide undergo a transmetallation reaction when reacted with 1 mol equivalent of titanium tetrachloride in THF under reflux to give the appropriate non-bridged substituted titanocene dichloride in overall yields of up to 77% as seen in Scheme 2. 

Thus, the initially formed 4-CBgH]3- can react with PhPCl2 to form a bridge-substituted species which can then undergo dehydrohalogenation with loss of PSH+C1- to yield the RP inserted species. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

Porphyrin inner proton exchange. Ribo and co-workersreported a study of the effect of steric factors due to bridge substitution on the modulation of proton exchange rates in a water soluble porphyrin, 161. The authors utilized both HSQC and... 

The mechanism of formation of 1 and 2 has yet to be determined however, the simple scheme shown in Figure 4 can be used to rationalize the formation of both species in the reaction. Thus, the initially formed 4-CBgHi3 can react with PhPCl2 to form a bridge-substituted species which can then undergo dehydrohalogenation with loss of PSH+Cl- to yield the RP inserted species. 

Thermal synthesis of methylene bridged, substituted phenols in the absence of a catalyst occurs in a melt with paraformaldehyde at temperatures between 150-200°C [33]. A representative reaction is 4,4 -(l-methyl ethylidene)bisphenol reacting with paraformaldehyde at 200°C to form novolacs with the structure drawn in equation 12 [33]. The acidity of the phenolic group is probably sufficient to catalyze the formation of these special novolacs, all of which have a yellow color. These products are prepolymers for the synthesis of epoxy resins [63]. 

These noncovalent interactions introduce geometric restrictions that most probably affect the k values. Ground state absorption properties show that the chromophores are weakly coupled when compared to reference monomers comprised of the bridge-substituted donor or acceptor. Highly efficient ET rates, ranging from 70 to 13 ps have been observed for ET from the zinc donor to the free base acceptor. Almost no competition with hole transfer, due to excitation of the free base, was observed, except for dyad 35 in Figure 13.28. Excited state lifetimes for the individual chromophores and ET quantum yields are given in Table 13.3. 

Staining (amines) phenyl naphthylamines. 4,4-dialkyl or dialkoxydiphenyl-amines. N,N-dialkyl or diaryl-p-phenylenediamines staining (phenols) styrenated phenols, substituted phenols, ct- or p-bridged substituted phenols... 

Cocrystalfization of 4-benzylresorcinol (4-bn-res) with l,4-bis[2-(4-pyridyl)ethenyl]benzene (1,4-bpeb) afforded a solid wherein two sets of C=C bonds were positioned for a double photodimerization (Figure 7). UV irradiation afforded the corresponding bridge-substituted [2.2]para-cyclophane (tpcp) stereospecifically and in quantitative yield. A similar approach was later reported by Brun-klaus and coworkers to direct the reactivity of a F-substituted diene in the form of p-di-[2-(4-pyridyl)ethenyl]-2-fluorobenzene (1,4-bpef). Cocrystallization of 1,4-bpef with 2,4-dihydroxybenzaldehyde produced a four-component assembly that reacted to give the corresponding F-substituted [2.2]paracyclophane in 60% yield. 

Rate of Diels-Alder reaction influence of bridge substitution 41-45... 

BH2-S(H)-BH2-H-], i-mercaptodiborane(6), is also observed as a product from the reaction between H2S and BgHg. The NMR spectrum exhibits a triplet of doublets at 6 = -22.1 ppm, J(B,Ht)=140 Hz, J(B,H ) = 40 Hz, and is consistent with a bridged substituted diborane(6). SCF calculations as described above indicate that the reaction... 

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