Hindrance overcoming

Stereoselectivity. See Asymmetric induction Axial/equatorial-, Cis/trans-, Enantio-, Endo/exo- or Erythro/threo-Selectivity Inversion Retention definition (e.e.), 107 footnote Steric hindrance, overcoming of in acylations, 145 in aldol type reactions, 55-56 in corrin synthesis, 261-262 in Diels-Alder cyclizations, 86 in Michael type additions, 90 in oiefinations Barton olefination, 34-35 McMurry olefination, 41 Peterson olefination, 33 in syntheses of ce-hydrdoxy ketones, 52 Steric strain, due to bridges (Bredt s rule) effect on enolization, 276, 277, 296, 299 effect on f3-lactam stability, 311-315 —, due to crowding, release of in chlorophyll synthesis, 258-259 in metc-cyclophane rearrangement, 38, 338 in dodecahedrane synthesis, 336-337 in prismane synthesis, 330 in tetrahedrane synthesis, 330 —, due to small angles, release of, 79-80, 330-333, 337... 

Possible formaldehyde crosslinkers are paraformaldehyde [16,17,144,145], methylolurea mixtures such as urea-formaldehyde precondensate or formurea [145], or urea and phenol methylols with longer chains to overcome steric hindrance. 

On the basis of the above data it has been hypothesized that the conductivity of PFCM is due not to the contact between the filler particles but the current passes across the thin (less than 1 -2 microns) polymer interlayers. The conductivity arises when a spontaneous pressure exceeding the threshold value develops in the material. The overstresses apparently arise as a result of PP crystallization in the very narrow gaps between the filler particles [312], Since crystallization must strongly affect the macromolecular conformation whereas the narrowness of the gap and fixed position of molecules on the filler prevent it, the heat released in the process of crystallization must, in part, be spent to overcome this hindrance, whereby a local high pressure may arise in the gap. This effect is possible only where there are gaps of the size comparable with that of macromolecules. The small gap thickness will also hamper pressure relaxation, since the rate of flow from such a narrow clearance should be negligibly small. 

Most methods of separating molecules in solution use direct contact of immiscible fluids or a sohd and a fluid. These methods are helped by dispersion of one phase in the other, fluid phase, but they are hindered by the necessity for separating the dispersed phase. Fixed-bed adsorption processes overcome the hindrance by immobilizing the solid adsorbent, but at the cost of cyclic batch operation. Membrane processes trade direct contact for permanent separation of the two phases and offer possibilities for high selectivity. 

Sonolysis provokes electron-transfer reactions in which hindered phenols act as donors (Aleksandrov et al. 1995). Steric hindrance does not allow a donor and an acceptor to come closer and thus, prevents or significantly hampers overlapping of the corresponding orbitals. Acoustic field effect helps in overcoming this hindrance. 

To obtain semiconductors with magnetic properties, Fujiwara et al. (2003) developed several donor molecules containing a stable oxyl radical moiety based on the n-extended TTF framework. The TTF framework is too small to overcome the steric hindrance of the bulky nitroxyl radical and to accomplish strong intermolecular interaction indispensable for any electric conductivity. For this reason, a larger molecule was constructed and its cation-radical perchlorate was electrochemically prepared. This salt was obtained as black microcrystals of nonstoichiomteric ratio equal to 1 0.64. Being a semiconductor, the salt also manifests magnetic properties its structure is depicted in Scheme 8.13. 

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

A Friedel-Crafts-type reaction can be carried out with hexachlorofullerene and, e.g., benzene, fluorobenzene, anisol or toluene, but due to steric hindrance not with mesitylene [85, 86]. Upon stirring benzene solutions of CgoClg with ferric chloride for 20 min, CgoPh5Cl can be obtained (Scheme 9.12). Replacement of the sixth chlorine is prevented by steric hindrance. This hindrance can be overcome by addition of the more reactive toluene to CgoPhsCl to give C6oPh5(C6H4CH3) [86]. 

The ease of such reactions depends on two major factors the nucleophilicity of the nitrogen atom, dominated by its charge density, and the degree of steric hindrance. A minor factor is the juxtaposition of nitrogen lone pairs (the a-effect), which increases the reactivity at nitrogen in pyridazines, but not sufficiently to overcome the unfavorable electronic effect (see below). 

The overpotential 77 is required to overcome hindrance of the overall electrode reaction, which is usually composed of the sequence of partial reactions. There are four possible partial reactions and thus four types of rate control charge transfer, diffusion, chemical reaction, and crystallization. Charge-transfer reaction involves transfer of charge carriers, ions or electrons, across the double layer. This transfer occurs between the electrode and an ion, or molecule. The charge-transfer reaction is the only partial reaction directly affected by the electrode potential. Thus, the rate of charge-transfer reaction is determined by the electrode potential. 

Completely unprotected carbohydrate clusters have been successfully synthesized following this approach,94 overcoming the problems of steric hindrance inside the glycocluster and during the coupling reactions. 

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