Substituents of different sizes

If the substitution pattern is ideal, then we can try varying the substituents themselves. Substituents of different sizes and electronic properties are usually tried to see if steric and electronic factors have any effect on activity. It may be that activity is improved by having a more electron withdrawing substituent, in which case a nitro substituent might be tried in place of a chloro substituent. 

The pressure effect on phptochemical cycloadditions was also investigated in a very elaborate procedure [58]. For this purpose an optical cell with sapphire windows and a laser containing a Xe/Vi/Hi gas mixture was used. Irradiation of cy-clopentenone (111) with substituents of different size at C-3 ranging from hydrogen to methyl, ipropyl and t-butyl, with cyclopentene (112) and 3,3-dimethylbutene (115) gave the diastereomeric cycloadducts 113-117 (Scheme 8.27). 

Different types of lateral substituents can be incorporated by monomers with substituents of different size, flexibility, and polarity. In addition, the number of substituents and the symmetry of the substitution can result in a different positional isomerism along the polymer backbone (Fig. 11). For example, in polyesters derived from mono-substituted monomers, the substituents can point towards each other (syn) and away from each other (anti), resulting in a random... 

Figure 4.2. Yeast reduction of ketones with substituents of differing sizes.

For the Ireland-Claisen variant, Wilcox and Babston [25] reported kinetic data in order to quantify the influence of aUcyl substituents of different size at the 1- and 5-position on the rate of the rearrangement (Schemes 11.16 to 11.19). This study also considered the effect of the vinyl ether double-bond configuration on the rate of the rearrangement. 

Comparison of substituents of similar size but different polarity (CH3, Br, NO2 and CN) with substituents of different size (H, CH3, Cl and Br) demonstrated that the size of the substituent rather than the polarity was the important factor, with transition temperatures (Tg, and isotropiz-ation temperature) decreasing with increasing substituent size in a homologous series of monosubstituted polymers. This indicated that the predominant steric effect of the substituent was to increase the separation of the mesogenic units and reduce the molecular packing... 

Miyamoto and Yamazaki elaborated a general synthetic method to spiro-substituted [l,2,4]triazolo[l,5-c]pyrimi-dines bearing alicyclic rings of different sizes and bearing different (amino and sulfano) substituents in position 5 <1997JHC871>. 13C NMR chemical shifts of the ring carbon atoms of some selected derivatives are compiled in Table 4. 

Should the two substituents be different, and especially of different sizes, then the simple reasoning used above with two methyl substituents will need adapting the larger substituent will prefer to be equatorial. Where we have three or more substituents. 

In Figures 2a and b L and S are two substituents having different sizes (L > S) in 2b Z is a substituent that is larger than H. The space available for accommodating the substituent(s) of the olefinic double... 

Scheme 29 Diaza crown ethers 51-53 of different sizes and with different terminal substituents...

Formazanes, whose general structure is given by 227, form chelates with metal-cycles of different sizes depending on the nature of substituents R1, R3, and R5... 

The stereochemistry of the reactions of vinyl cations with nucleophiles is predictably different depending on their geometry, (a) High stereospecificity is expected from bridged ions (trans addition to acetylenes and retention of configuration in substitution reactions of vinyl derivatives) and is experimentally observed in the case of thiirenium ions, (b) From free linear cations with two j3 substituents of equal size, complete racemization is expected and is fully verified in the substitution products from l,2-dianisyl-2-phenylvinylbromide (section II,C,2). 

Rings of different sizes are readily distinguished by NMR methods, as both the 119Sn chemical shift and V(ll9Sn-C— H) values (to both endo-and exocyclic substituents) are sensitive to the bond angles at tin (15,16). Typically, 6 and 7 have 8 9Sn values of 0 and — 74 ppm, respectively (12). Further NMR data can be found in the compendia of data now available (17-19). 

Acyclic Substrates. The situation in acyclic systems is much more complicated than in cyclic ones as high stereocontrol for additions to prostereogenic double bonds [reactions (47a,b), Scheme 16] or carbonyl groups [reaction (48), Scheme 16] can only be achieved if the molecule adopts a definite reactive conformation in which one of the two diastereofaces is efficiently shielded by the steric effects of the substituents (Scheme 16) [41]. This means that these substituents have to be of different sizes and may be classified as small (S), medium... 

In ring closures to rings of different sizes, rates decrease in the order of C3 > C5 > C4 > Cg. Alkyl substituents at either end of the double bond decrease the cyclization rate. For systems with different numbers of methyl substituents at the a-carbon, the unusual reactivity order 1° < 2° > 3° is observed. 

Although the structures of cyclopolysilanes can be similar to the carbon analogs, the methods of synthesis are not. The majority of the systems studied result from the selfcondensation of R2SiCl2 or RR SiCl2 which provide cycles of different sizes and with identical substituents at each silicon center. Of course, it is possible to cocondense two different dichlorosilanes, however then not only are rings of different sizes produced but isomers within each ring size are generated265 and separation can be a real problem. Clearly this area of catenated silicon derivatives would benefit from other reaction methods. 

For a reasonable reaction rate it was necessary to introduce an electron-withdrawing group at position 2 of the 1-oxa-l,3-butadiene. In order to investigate the influence of this substituent on the simple and induced diastereoselectivity, groups of different size and electron-withdrawing strength were used. Thus, a methyl ester, a menthyl ester as a chiral functionality, a trichloro, a chlorodifluoro... 

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