Good leaving group

Imidazole can be A -allylated. The A -glycosylimidazole 299 is prepared by regiospecific amination at the anomeric center with retention of configuration. Phenoxy is a good leaving group in this reaction[181]. Heterocyclic amines such as the purine base 300 are easily allylatedfl 82]. 

Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are structurally sim liar—both contain a double bond and a pyrophosphate ester unit—but the chemical reactivity expressed by each is different The principal site of reaction m dimethylallyl pyrophosphate is the carbon that bears the pyrophosphate group Pyrophosphate is a reasonably good leaving group m 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 m isopentenyl pyrophosphate is the carbon-carbon double bond which like the double bonds of simple alkenes is reactive toward electrophiles... 

Fig. 11. Reaction of ionized coupler and oxidized developer (Dev ) to produce the intermediate leuco dye. If X is a good leaving group, the reaction proceeds spontaneously to dye, and the coupler is said to be two-equivalent. If oxidation by a second molecule of oxidized developer is required, the...

SuIfona.tlon, The sulfonic acid group is used extensively in the dyes industry for its water-solubilizing properties, and for its ability to act as a good leaving group in nucleophilic substitutions. It is used almost exclusively for these purposes since it has only a minor effect on the color of a dye. 

Af-Linked azole rings behave as good leaving groups, the more so the more nitrogen atoms contained in the ring (cf. Section 4.02.3.12.4). 

The preparation of esters can be classified into two main categories (1) carboxy-late activation with a good leaving group and (2) nucleophilic displacement of a caiboxylate on an alkyl halide or sulfonate. The latter approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondaiy halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). ... 

The latter is an exttemely reactive species. Trifluoroacetate is a good leaving group and facilitates cleavage of the O—Br bond. The acyl hypohalites are also the active halogenating species in solutions of the hypohalous acids in carboxylic acids, where they exist in equilibrium. 

Acid chlorides, which have a very good leaving group (low basicity and nucleo-philicity), may react in this manner. 

Much less work has been done on the arylation of enamino ketones. The enamino ketone (49) has been allowed to react with 2,4,6-trinitrochloro-benzene. In this instance the latter reacts as the acid chloride of an acid the anion of which is a good leaving group (46). This type of reaction will be discussed in Section IV.A. 

Strong acids promote SnI substitution reactions by converting an electron-rich ( basic ) atom on the substrate into a good leaving group, e.g., for substitution reactions of tert-butyl derivatives. 

---