Leaving group properties

A reaction which is related to hydroxy-de-diazoniations is the formation of aryl trifluoromethylsulfonic esters (aryl triflates, ArOSC CFs) which became widely used reagents because of their leaving-group properties. The classical method of synthesis by esterification of phenols with trifluoromethane-sulfonic anhydride or -sulfonyl halide is, however, not applicable for the preparation of aryl triflates bearing a (free)... 

As pointed out in Sects. 2 and 3, the leaving group properties of X can often be changed drastically by substitution or by protonation or deprotonation (as can the redox potentials). 

The addition of a-deprotonated alkyl halides to alkenes or carbonyl compounds can, because of the good leaving-group properties of halides, also lead to formation of cyclopropanes [292] or epoxides [187, 304, 306, 310], respectively. Because of the inherent instability of a-halo organometallic compounds, these intermediates should be handled carefully and on a small scale only. The ketone produced by the last reaction in Scheme 5.34 is probably formed by Oppenauer oxidation of the intermediate alcohol by the excess benzaldehyde [310],... 

The reciprocal [67] mechanism for ring-opening is H+-catalysis, in which the oxygen at is protonated, resulting in enhancement in the nucleofugal leaving-group properties of the alkoxy function, e.g. 

As the leaving group properties of X are enhanced (equivalent to reducing the pXa values of the conjugate acids H-X) compared with X = OH or OR (Schemes 1-7), the rates of / -elimination increase considerably such that support of heterolysis by deprotonation of Ca-OH or by protonation of X to achieve complete reaction tends to become unnecessary (see later). 

Such compounds should continue to be prominent amongst the most synthetically accessible and practical when seeking good leaving-group properties at one or even several sites of a complex. 

Another example is the conversion of cyclohexene (164) to the bicyclic sultone (165) (Scheme 67). This allows the synthesis of sultones with partially determined stereochemistry. Both reactions depend on the high leaving group properties of the methanesulfoxy anion. Unsaturated 8-sultones like (166) can be obtained by 1,4-cydoaddition of sulfenes (generated in situ) to a,(3-unsaturated carbonyl compounds (Scheme 68). 

Due to their good leaving-group properties, all (a-haloalkyl)cyclopropanes except (a-fluoroal-kyl)cyclopropanes are readily substituted by a carbon nucleophile under the correct conditions. Fairly clean reactions have been achieved with sodium cyanide and potassium cy-anide under homogeneous and phase-transfer conditions. Carbon nucleophiles generated by hydrogen abstraction under basic conditions can result in the formation of considerable amounts of byproducts, but successful reactions have been reported, particularly when an intramolecular substitution reaction oc-curs. It is also noteworthy that a phenyl group can be attacked under similar conditions. ... 

The leaving group properties of the cyclopentadienyl group (Cp) have been employed in the reaction of lithium acetylides LiC=CR (R = Ph, SiMe3) with an in situ prepared solution of CpAgPRj (R = Ph, Me) according to equation 1952. 

Pd and Ni are the most widely used catalysts for the hydrogenolysis of C-O bonds. The rates of hydrogenolysis increase in the order OH < OR << OAr < OCOR. This order corresponds to the leaving-group properties of these moieties, which is characteristic of S -type reactions. The stereochemistry of the reaction depends mainly on the metal. Inversion of the configuration of carbon is characteristic of Pd whereas retention is usually observed with Ni [2,11]. 

Leaving group properties also influence the chemistry. l-Methyl-2-nitroimidazole substituted with bromomethyl at the 5-position eliminated bromide unimolecularly in < 1ms, whereas in the analogous compound with bromide replaced by salicylate linked through the carboxylate moiety there was a marked reduction in elimination rate [145]. However, measurements of the stoichiometry of release indicated elimination from the 4-electron reduction product, the 2-hydroxyaminoimidazole, did occur. The quantitative basis for radiolytic radical production is a valuable feature of this type of study, since it is indicative of the release mechanism. 

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