Brosylate, leaving group

Attack ty acetate at C-1 of C-2 would be equally likely and would result in equal amounts of the enantiomeric acetates. The acetate ester would be exo because reaction must occur from the direction opposite the bridging interaction. The nonclassical ion can be formed directly only from the exo-brosylate because it has the proper anti relationship between the C(l)—C(6) bond and the leaving group. The bridged ion can be formed from the endo-brosylate only after an unassisted ionization. This would explain the rate difference between the exo and endo isomers. 

Although halides are common leaving groups in nucleophilic substitution for synthetic purposes, it is often more convenient to use alcohols. Since OH does not leave from ordinary alcohols, it must be converted to a group that does leave. One way is protonation, mentioned above. Another is conversion to a reactive ester, most commonly a sulfonic ester. The sulfonic ester groups tosylate, brosylate, nosylate, and mesylate are better leaving groups than... 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

What we might expect as a pathway for this reaction would be simple Sn2 displacement (p. 98) of the good leaving group— brosylate anion—by acetate anion ... 

The most frequently encountered representative of the KSOsO" close of leaving groups is probably the toluene-p-eulfonote anion (commonly abbreviated to toeylate ), but others are occasionally employed aa well. The latter include the methylaulfonatc ( mesylate ) end the p-bromobenzenesulfbaate ( brosylate ) ankme. 

The rate of substitution of the endo-brosylate 2,18 was considered normal, since its reactivity is comparable to that of cyclohexyl brosylate. Ionization of the exo-brosylate 2.17 is assisted by the neighbouring C1-C6 bonding electrons participation with the expulsion of the leaving group. The non-classical carbocation 2.20 is formed as an intermediate in which positive charge residing on Cl is delocalized on C2 as well (Scheme 2.15). 

The absence of the migration of C in the brosylate 28, as well as upon solvolysis of unsubstituted 2-endo-norbomyl brosylate 23 confirms that the C —C bond participates before the endo leaving group is completely separated from the molecule. 

Brown s approach was to show that the properties ascribed to the nonclassical ion could be duplicated in systems involving classical carbocations. Since the rate enhancement of solvolysis of the exo isomer was an important part of the argument for (7-bond participation, he also analyzed the relative reactivities of various systems. He argued that the cxo-norbomyl system should be compared to the cyclopentyl system rather than to cyclohexyl brosylate. The torsional relationship of the leaving group and adjacent substituents is eclipsed in norbomyl sulfonates, and strain is relieved on ionization. Cyclohexyl brosylate, in contrast, is completely staggered in the ground state, and so no strain relief accompanies ionization. In the cyclopentyl... 

---