Steric 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. 

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). 

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. 

Apart from overcoming coulombic repulsions, 8 2 reactions also proceed with inversion in the face of steric hindrance. By comparison, the stereochemical result of unimolecular nucleophilic substitution SN1 is variable. In fact, nucleophilic substitutions at carbon with retention invariably follow other than SN2 paths. In its broad outlines, the Hughes-Ingold approach swept away the confusions of the period 1895-1933 and has not ceased to stimulate and provoke ideas in the area of substitution reactions. Surprisingly enough, the theoretical foundations of the SN2 process require reexamination and modification, as we shall see. 

The ability of a /i-olefin to copolymerise with ethylene in the presence of Ziegler-Natta catalysts arises from minimisation of steric interactions at the catalytic active site by ethylene units the steric hindrance, which prevents homopropagation of the /1-olefin, is overcome when the /<-olefin monomer is... 

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