Leaving group ability, table

Table 10.10 lists some leaving groups in approximate order of ability to leave. The order of leaving-group ability is about the same for SnI and Sn2 reactions. 

Allyl sulphones can be converted to dienes by alkylation and elimination of sulphinic acid under basic conditions (equation 64)105. Several vitamin A related polyenes have been synthesized following this two-step protocol (Table 10)106. The poor leaving-group ability of the arylsulphonyl group requires treatment with strong base for elimination. However, elimination of the allylsulphonyl group takes place readily under palladium catalysis (equation 65)107. Vinyl sulphones can be converted to dienes via Michael addition, alkylation with allyl halides and elimination of sulphinic acid sequence (equation 66)108. 

Another important site of structural variation in cephalosporins is C(3) (Table 5.4.J). Electron-withdrawing substituents at C(3) such as a Cl-atom or a MeO group increase base-catalyzed hydrolysis of cephalosporins by both resonance and inductive effects [92], For cephalosporins carrying 3-methylene-linked substituents with leaving group ability (e.g., acetate, thiol, or pyridine), it has been postulated that a concerted expulsion of the substituent facilitates the nucleophilic attack on the /3-lactam carbonyl group [104][105]. However, there are also arguments for a stepwise process in which the ex-... 

The data in Table 7 reveal that leaving-group ability and not carbonium ion stability is the key factor permitting intramolecular... 

The presence of four methyl groups at positions 4 and 5 of the dioxolan ring causes a larger decrease in 0- (Tables 12, 13) than in kH<. It is possible that in the hydroxide-ion catalysed reaction there is, superimposed on the steric effect, an electronic effect of the methyl groups which causes a decrease in the leaving group ability of the alkoxide ion due to their electronreleasing inductive effect. A similar effect is observed in the hydroxide-ion catalysed breakdown of benzaldehyde f-butyl and methyl hemiacetals for which / -( )/ -( ) = 37. 

A more complete picture of nucleophilicity and leaving group ability can be obtained from the data of Olmstead and Brauman (1977) and Tanaka et al. (1976). These results are compiled in Tables 4 and 5. 

Left-to-right across a row of the periodic table, basicity decreases so leaving group ability increases. 

Chapter 3 Reactions af Nucleophiles and Bases TABLE 3.1 Leaving Group Abilities... 

Table 1.5. Approximate order of leaving group ability...

Table 3 Measure of Leaving Group Ability in the Gas Phase ...

---