Leaving groups in electrophilic

Thus, not only should alkylmercuric iodides and dialkylmercurials have a more nucleophilic carbon compared with the others, but their respective -Hg-X functions should also be better leaving groups in electrophilic substitution reactions. This latter prophesy is supported in part by the results of Hughes et al. from their kinetic studies of mercury exchange reactions (20). 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally similar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different. The principal site of reaction in dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group. Pyrophosphate is a reasonably good leaving group in nucleophilic substitution reactions, especially when, as in dimethylallyl pyrophosphate, it is located at an allylic carbon. Isopentenyl pyrophosphate, on the other hand, does not have its leaving group attached to an allylic carbon and is far- less reactive than dimethylallyl pyrophosphate toward nucleophilic reagents. The principal site of reaction in isopentenyl pyrophosphate is the carbon-carbon double bond, which, like the double bonds of simple alkenes, is reactive toward electrophiles. 

Hydrogen as the Leaving Group in Simple Substitution Reactions A. Hydrogen as the Electrophile 11-1 Hydrogen Exchange Deuterio-de-hydrogenation or deuteriation... 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Non-heteroatom-substituted carbene complexes can also be generated by treatment of electrophilic transition metal complexes with ylides (e.g. diazoalkanes, phosphorus ylides, nucleophilic carbene complexes, etc. Section 3.1.3). Alkyl complexes with a leaving group in the a-position are formed as intermediates. These alkyl complexes can undergo spontaneous release of the leaving group to yield a carbene complex (Figure 3.2). 

Calb et al. have thoroughly investigated the use of allylic electrophiles containing heterocyclic leaving groups in regioselective allylic substitution (Scheme 8.7) [22]. 

Thus, the thiol sulfur may act as either a leaving group in car-tap or as an electrophile in bensultap on route to the formation of nereistoxin. Therefore, the number of nereistoxin analogs which may be prepared for insecticidal evaluation is very large. This includes the possibility of derivatives formed from the reaction between 2-dimethylamino-propane-l,3-dithiol and the chlorosulfenyl and chlorosulfinyl intermediates described in Figure 2. 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

The aryl sulfoxide moiety may serve as a good leaving group in the exchange reaction. Thus, 1-haloalkenyl sulfoxide 55 undergo the exchange at —78°C to give carbenoid compounds 56 which can be trapped by electrophiles or converted to acetylenes 57 (equation 39) °. Reaction of carbenoid 58 with lithium acetylides leads to the formation of enynes 59 (equation 40). ... 

The reaction of aryl electrophiles with organomagnesium compounds is known as Kumada or Kumada-Tamao-Corriu reaction. The most common leaving groups in the electrophile are halogen atoms and, among them, chlorine is the most wanted due to the good availability and the low price of aryl or heteroaryl chlorides. Unfortunately, the oxidative addition of a metal center to an aryl chloride is a difficult reaction and many efforts have been made to overcome existing limitations. 

As mentioned in Section II.B.5, the tert-butyldi phony I silyl group attached to the olefinic carbon is a poor electrofugal group and hence does not leave easily in electrophilic substitution reactions140. On the other hand, the epoxide 172, obtained from the corresponding vinylsilane having such a bulky silyl substituent, gives the carbonyl compound upon... 

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