Functional groups electron-withdrawing effects

The cyanide ion plays an important role in this reaction, for it has three functions in addition to being a good nucleophile, its electron-withdrawing effect allows for the formation of the carbanion species by proton transfer, and it is a good leaving group. These features make the cyanide ion a specific catalyst for the benzoin condensation. 

As mentioned above, the reactivity of alkoxyallenes is governed by the influence of the ether function, which leads to the expected attack of electrophiles at the central carbon C-2 of the cumulene. However, the alkoxy group also activates the terminal double bond by its hyperconjugative electron-withdrawing effect and makes C-3 accessible for reactions with nucleophiles (Scheme 8.3). This feature is of particular importance for cyclizations leading to a variety of heterocyclic products. The relatively high CH-acidity at C-l of alkoxyallenes allows smooth lithiation and subsequent reaction with a variety of electrophiles. In certain cases, deprotonation at C-3 can also be achieved. 

Fluorination has an important indirect impact on hydrogen bonds, via neighbouring functions (hydroxyl, amine, carbonyl, hydrogen). The electron-withdrawing effect of fluorine atom and of fluoroalkyl groups (CF2, CF3, etc.) deeply modifies the pKg of neighbouring functions, and hence their character of hydrogen bond donors or acceptors (Table 1). 

The hydrogens on the methylene unit located between the two carbonyl functional groups are acidic due to the electron withdrawing effects of the carbonyl groups. Either or both of these hydrogens can be removed by reaction with strong bases. 

The emission band position is also influenced by the electron-withdrawing and electron-donating effect of the addend, respectively, and by the type of chemical functionalization (e.g., various size of the rings fused to the fullerene core). For example, it has been reported for three different fullerene derivatives that an enhanced electron-withdrawing effect causes an increasing red-shift of the emission band [108], Another example is a C6o derivative 12 (Fig. 10). The emission maxima appears at 705 nm, but if there are methoxy groups additionally attached to the anthryl moiety the emission maxima is shifted to 712 nm [92],... 

The nitro group, as one of the many functional groups with a strong electron-withdrawing effect, plays an extremely important role in the reactivity of the sugar molecule no matter where it is located. The anomeric nitrates introduced by Vasella [53] are particularly useful reactive synthons for introducing additional functionalities via formation of the C-C bond by Henry aldol condensation [54] followed by a radical denitration. 

The reaction proceeds in general stereospecifically as a cA-addition. Equation (9) shows exemplarily the addition of deutero-HCN (DCN 34) to hex-l-yne 33. The cA-position of deuterium and the cyano group is found in both branched (35) and linear (36) products. When dimethyl acetylenedicarboxylate is used as the substrate, the product of anti-addition is formed. This indicates a change in the mechanism as a result of the electron-withdrawing effect of the two functional groups in direct conjugation with the triple bond [62]. 

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