Steric screening

The remarkable thermal stability of the di-ferf-butyl-substituted thiepin 6, due to steric screening, has been demonstrated by its synthesis in an analogous manner to the carbocation rearrangement which provides dibenzothiepins. Under acidic conditions, methanesulfonic acid was eliminated from Ihiopyran 5 to give the thiepin via cationic rearrangement.18... 

The bulky N-anthracenylmethyl group clearly plays a key role in the efficient stereoselective alkylation. The steric screening would be provided by the N-anthracenylmethyl group and the tight ion pair 28 from the ammonium cation (N+) and the enolate (0 ) would be stereoselectively alkylated as shown in Figure 3.1241 Furthermore, removal of the quinoline ring proved to be not useful to attain the good enantioselectivity.1261... 

Pure benz(c)phenanthrene, mp. 67—68° and 2,1 l-dimethylbenz(c)phenanthrene, mp. 129—130°, can be simply obtained hydrolyzing 148a) or 148 b) by potassium hydroxide in triglycol at 180° and decarboxylation of the dicarboxylic acids in boiling quinoline in the presence of copper powder, giving a 91% or 70% yield, respectively32 The sterically screened carbonitrile groups in 148 d) and 148 e) cannot be saponified. 

The kinetic preference for 3 could be attributed to steric screening of the internal methylene protons near the tertiary carbon center through their interaction with the large planar porphyrin molecule. 

The reaction of trialkyl alanes with tertiary alcohols is worth mentioning in this connection, as only the first alkyl group is split off. The resulting dialkyl-alkoxy alanes are particularly unreactive because of steric screening of the AIO2AI bridge. They are also astonishingly stable to air oxidation (106, 107). 

Further examples of the elaboration of chiral -substituted acids [e.g. (14)] using Michael additions to sterically screened alkenoates also derived from camphor have been reported.1 An alternative to this is the addition of nucleophiles to proline derivatives (16) depending upon the precise way the reactions are carried out, either... 

An alternative way to exploit the steric screening principle in the elaboration of chiral a-hydroxy-esters is by asymmetric alkylations of the dioxolanones (140) and (141) [R=H] derived in a... 

This view is also supported by the fact that on esterifying the group, the tendency to form molecular associations is completely lost. In terms of the information on page 63 there are two reasons for this. First, by steric screening, the effect of the dipole field of the COOH group is diminished and, in the absence of a mobile H atom, any quantum mechanical... 

Free radicals are neutral or charged particles with one or more uncoupled electrorrs. Unlike usual (short-living) radicals, stable ones (long-living) are characteristic of paramagnetic substances whose chemical particles possess strong delocalized uncoupled electrons and sterically screened reactivity centers. This is the very catrse of the high stability of many classes of nitroxyl radicals of aromatic, fatty-aromatic and heterocyclic series, and ion radicals and their complexes. 

The asymmetric induction originates on the steric screening of the complexed nucleophile anion provided by the chiral tetrahedral ammonium cation [193], From this point of view, enantioselective phase-transfer catalysis can be regarded as a particular case of asymmetric countercation-directed catalysis. In 2008, Ooi... 

The cluster 3 is unexpectedly stable. One reason for this may be the steric screening of the four coordinatively unsaturated Ni atoms (Ni2, Ni4, Ni5, and... 

The structure determination of 7 (Fig. 3-50) revealed that the Ni-Ni distances in the Nig cube are comparable to those in 3. The Ni5-Ni9 bond length of 268.2 pm lies in the normd range of Ni-Ni single bonds. Since Ni9 is not bound to any other ligands, it can only be the steric screening by the six immediate phenyl groups that makes 7 is stabil. The nonbonding Ni C(Ph) contacts of 326-380 pm make interactions between the n systems and Ni9 improbable. It is pos-... 

The existence of the coordinatively unsaturated cluster 3 leads one to consider whether the synthesis of clusters with still larger vacant coordination sites might be possible. 8ince these vacant sites arise as a direct consequence of the steric screening of the metal atoms by peripheral ligands, several possible synthetic routes immediately suggest themselves ... 

It is assumed that ligand L blocks one metal valence orbital otherwise involved in the decomposition reaction and in combination with R produces an effective steric screening of the metal. Rationalization of these ideas can easily be made on basis of the Lauher-Hoffmann bonding scheme for bent metallocene derivatives [21]. 

High diastereoselectivities are secured when one side of the substrate is sterically screened. Lithium acetylide approaches 3-methoxyoestra-3,5-dien-17-one (213) exclusively from the a-face to afford, after deprotection, the orally administrable progestogen norethindrone in up to 78% yield (Scheme 1-156). The same happens with androsta-1,4-dien-17-one. ... 

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