Bulky groups

Apart from these simple silanes, derivatives witli aromatic groups at different places in tire chain have also been investigated [136, 137], It was found tliat tire average tilt angle of tliese molecules depends on tire specific functional entities contained in tire chains. It is likely tliat apart from packing considerations—important for bulky groups, for example—otlier factors also influence tire resulting tilt. 

The effect of ligands on the endo-exo selectivity of Lewis-acid catalysed Diels-Alder reactions has received little attention. Interestingly, Yamamoto et al." reported an aluminium catalyst that produces mainly exo Diels-Alder adduct. The endo-approach of the diene, which is normally preferred, is blocked by a bulky group in the ligand. 

Alkyllithium bases are generally less suitable for deprotofiation of compounds with strongly electron-withdrawing groups such as C=0, COOR and CsN. In these cases lithium dialkylamides, especially those with bulky groups (isopropyl, cyclohexyl), are the reagents of choice. They are very easily obtained from butyllithium and the dialkylamine in the desired solvent. 

If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

FIGURE I.I6 Isomers of biphenyl compounds with bulky groups attached at the ortho positions. 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Risaliti et al. (22), have shown that in the addition of the electrophilic olefins to the enamines of cyclohexanone, the formation of the less substituted enamine is favored when a bulky group is present at the electrophilic carbon atom. For instance, the reaction of (8-nitrostyrene with the morpholine enamine of cyclohexanone gave only the trisubstituted isomer (30) with the substituent in the axial orientation (23). The product on hydrolysis led to the ketone (31) to which erythro configuration was assigned on the grounds illustrated in Scheme 3 (24). 

This bulky group was developed as a fluorescent, acid-labile protective group for oligonucleotide synthesis. It has properties very similar to the DMTr group except that it can be detected down to 10" ° M on TLC plates with 360-nm ultraviolet light. ... 

Stable stannaethenes, >C=Sn<,d25 ) and stannaphosphenes, >Sn=P<,d3o have been reported and these, again, exploit the use of bulky groups to prevent oligomerization. 

There are indications that bulky groups in the 7- or 8-position decrease the absorption in the 405 m/x region (and hence they can presumably increase hydration in the anion, a somewhat surprising result). 

Steric hindrance to activation by carboaromatic nitro groups is well-known, but there seems to be no analogy in the chemistry of azines. The lone-pair of azine-nitrogen has a steric effect comparable " to, somewhat greater than, or somewhat less " than a hydrogen atom. It is not certain whether bulky groups such as i-butyl produce a steric distortion of the lone-pair orbital and whether activation or deactivation results. 

If the chifdl duxilidry in Eq 4 96 is modified by changing MeO into more bulky groups such dS trityl (Tr or r-butyldimethylsilyl (TBS group, an improved asymmetric nitro-olefinddon of ct-alkyl- /- and 5-lactones is possible fEq 4 97) ... 

The reaction of an alcohol with an acid chloride is strongly affected by steric hindrance. Bulky groups on either partner slow down the reaction considerably, resulting in a reactivity order among alcohols of primary > secondary > tertiary. As a result, it s often possible to esterify an unhindered alcohol selectively in the presence of a more hindered one. This can be important in complex syntheses... 

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