Polar reagents substituents

When a positively charged substituent such as the trimethylam-monio group is anywhere on the ring, but most effectively when it is ortho to the leaving group, it can favorably affect the entropy of activation with anionic nucleophiles and accelerate reaction. A recent example of reagent-substituent interaction is the electrophilic substitution of 2-carboxybiphenyl, nitration (non-polar solvent) of which occurs only at the 2 -position and not the 4 -position and has been postulated to be due to the interaction of the nitronium ion with the carboxyl group. 

Fluorine is well known for its ability to fluonnate seleeti vely terttary carbon-h drogen bonds that are slightly activated by polar substituents Hypofluorite reagents also react with carbon-hydrogen bonds, both secondary and tertiary but tertiary bonds react better [6]... 

The good correlation found between the reactivities of 2-chloro-3-cyano-5-nitropyridine and the polar effects of the substituents on the aniline reagent has enabled Chapman and his co-workers to illustrate the steric requirements of the reagent by including in their studies... 

The conversion of a nitrile R —CN into a ketone R —CO—R demonstrates that polarized multiple bonds other than C=0 also react with Grignard reagents, and that such reactions are synthetically useful. Esters 22 and acid chlorides can react subsequently with two equivalents of RMgX the initially formed tetravalent product from the first addition reaction can decompose to a ketone that is still reactive, and reacts with a second RMgX. The final product 23 then contains two substituents R, coming from the Grignard reagent ... 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

The stereoinductive effect of the a-substituent is often more powerful when this substituent is a polar group such as an alkoxy or amino gronp. In this instance as well, minimization of syn-pentane interactions with 3-monosnbstituted reagents is important, and the selectivity can be amplified if it acts in concert with other effects. Known models inclnde the stereoelectronic preference... 

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