Nucleofugality

Tnmethylpentafluorophenylsilane reacts with ketones in the presence of cyamde ion to form the silyl eiher denved from the alkoxide created by the addition of the pentafluorophenyl amon to the carbonyl carbon [89] The nucleofugal character of the pentafluorophenyl group is significantly greater than that of the added cyamde ion so that the addihon proceeds under nearly neutral condmons (equation 73)... 

In this reaction I" is the nucleophile, and Br" is called the leaving group (or nucleofuge). Beyond this, the classification symbolism may include a designation of the molecularity of the reaction. Molecularity is the number of reactant molecules included in the transition state. The above reaction is an 8 2 reaction, because both reactants are present in the transition state. On the other hand, this substitution... 

Just as with cefpimazole, cefepime (54) has a quaternary nitrogen substituent at C-3 whose electron withdrawing character and excellent nucleofugic properties increases the reactivity of the... 

Sulfonic acids themselves are unfit for electrophilic transfer of sulfonyl groups because of the poor nucleofugality of the hydroxide anion. However, the high acidity obviously leads to an equilibrium between the acids and their anhydrides and water, from which water can be removed either by special reaction conditions (i.e., azeotropic distillation with appropriate solvents) or chemically with anhydride forming agents316 (equation 63). sulfonic acid anhydride sulfonylations are compiled in Table 10. 

However, in some circumstances, an arylsulphonyl group is a sufficiently good nucleofuge that an elimination reaction is possible. This has actually been known for some 60 years,... 

Another approach to an oxidative desulphonylation reaction is to oxidize an a-sulphonyl carbanion with an oxidizing agent that is also nucleofugal. An example of this was presented by Little and Sun Ok Myong203 who used a molybdenum peroxide complex (M0O5.Pyridine.HMPA) as the oxidant. However, this reagent is expensive and... 

Step 2. Only one bond is broken in this step and no bonds are formed. (The movement of a pair of unshared electrons into the C-C bond, forming a double bond, is not designated by any symbol. In this system, bond multiplicity changes are understood without being specified.) Thus the symbol is D. The broken bond is between a core atom (C) and a nucleofuge (Cl), so the designation is D. ... 

In nucleophilic substitution the attacking reagent (the nucleophile) brings an electron pair to the substrate, using this pair to form the new bond, and the leaving group (the nucleofuge) comes away with an electron pair ... 

When a molecule has in an allylic position a nucleofuge capable of giving the SnI reaction, it is possible for the nucleophile to attack at the y position instead of the a position. This is called the SnI mechanism and has been demonstrated on 2-buten-l-ol and 3-buten-2-ol, both of which gave 100% allylic rearrangement when treated... 

Ideally, Y should measure only the ionizing power of the solvent, and should not reflect any backside attack by a solvent molecule in helping the nucleofuge to leave (nucleophilic assistance k, p. 411). Actually, there is evidence that many solvents... 

In the case above, 100 is protonated in the last step by the acid HA, but if the acid is omitted and a suitable nucleofuge is present, it may leave, resulting in a cyclo-pentene. In these cases the reagent is an allylic anion, but similar 3 + 2 cycloadditions involving allylic cations have also been reported. 

Effect on Reactivity. We refer to the carbon containing the nucleofuge (X) as the a carbon and to the carbon that loses the positive species as the p carbon. Groups attached to the a or P carbons can exert at least four kinds of influence ... 

CF3S02CR R S02CHR R (anywhere from one to four alkyl groups can be put in), which, when treated with base, gives R R C=CR R. The nucleofuge here is the CF3S0 ion. 

In many cases it is not certain whether the nucleofuge X is lost first, creating an intermediate nitrene or nitrenium ion, or whether migration and loss of the nucleofuge are simultaneous, as shown above.It is likely that both possibilities can exist, depending on the substrate and reaction conditions. 

In some cases the rate-determining step has been shown to be the addition of peracid to the substrate (see, for example, Ogata, Y Sawaki, Y. J. Org. Chem., 1972,37, 2953). Even in these cases it is still highly probable that migration is concerted with departure of the nucleofuge. 

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