Brosylate

The acetolyses of both ero-2-norbomyl brosylate and e do-2-norbomyl brosylate produce exclusively exo-2-norbomyl acetate. The exo-brosylate is more reactive than the endo isomer by a factor of 350. Furthermore, enantiomerically enriched exo-brosylate gave completely racemic ero-acetate, and the endo-brosylate gave acetate that was at least 93% racemic. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

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. 

When bicyclo[2.2.2]octyl brosylate was solvolyzed in acetic acid containing sodium acetate, the products were a mixture of bicyclo[2.2.2]octyl acetate and bicyclo[3.2.1]octyl acetate, each of which was optically active. The formation of bicyclo[2.2.2]octyl acetate was found to proceed with 82 15% retention of configuration, a result which is in... 

The compound 2-octyl brosylate was labeled at the sulfone oxygen with 0 and solvolyzed. The unreacted brosylate recovered at various stages of solvolysis had the 0 considerably, though not completely, scrambled ... 

The fact that acetolysis of both 4-methoxy-l-pentyl brosylate (18) and 5-methoxy-2-pentyl brosylate (19) gave the same mixture of products is further evidence for... 

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... 

Sulfonic esters are most frequently prepared by treatment of the corresponding halides with alcohols in the presence of a base. The method is much used for the conversion of alcohols to tosylates, brosylates, and similar sulfonic esters. Both R and R may be alkyl or aryl. The base is often pyridine, which functions as a nucleophilic catalyst, as in the similar alcoholysis of carboxylic acyl halides (10-21). Primary alcohols react the most rapidly, and it is often possible to sulfonate selectively a primary OH group in a molecule that also contains secondary or tertiary OH groups. The reaction with sulfonamides has been much less frequently used and is limited to N,N-disubstituted sulfonamides that is, R" may not be hydrogen. However, within these limits it is a useful reaction. The nucleophile in this case is actually R 0 . However, R" may be hydrogen (as well as alkyl) if the nucleophile is a phenol, so that the product is RS020Ar. Acidic catalysts are used in this case. Sulfonic acids have been converted directly to sulfonates by treatment with triethyl or trimethyl orthoformate HC(OR)3, without catalyst or solvent and with a trialkyl phosphite P(OR)3. ... 

In the brosylate acetolysis sets, conjugative stabilization of the transition state is geometrically excluded in the syn form, but is anticipated in the anti form (13). That is, on structural grounds, is expected to be the parameter of choice for the syn set and for the anti set. The available data (for the OMe, Me, Cl and NO2 substituents) do indeed conform to this expectation ... 

Formation of rearranged products in the solvolysis of homopropargyl systems need not involve triple-bond participation and vinyl cations in all instances. Ward and Sherman investigated the formolysis of 4-phenyl-1-butyn-l-yl brosylate, 57 (80). At 80°C in the presence of one equivalent of pyridine, they observed formation of phenyl cyclopropyl ketone, 58, and... 

A vinyl cation is probably an intermediate in the acetolysis of 6-phenyl-5-hexynyl brosylate, 86. At 80°, despite the inductive effect of the triple bond, the rate of acetolysis of 86 is comparable to that of the saturated analog and yields, besides the acyclic acetate 87, 36% of the rearranged acetate 88 (83). The exclusive formation of the five-membered ring rearranged product with none of... 

Bly and co-workers (93-95) have studied the solvolysis of neopentyl-type homoallenic brosylates, 126. The rates of acetolysis and other pertinent R3 CHj... 

Summary of Kinetic Data on the Acetolysis of Neopentyl-Type Homoallenic Brosylates 126 at 75°C... 

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... 

Essentially similar results and conclusions were obtained by Peterson and Indelicato (158) in the solvolysis of the corresponding tosylates and brosylates, 171 b (R = p-CHj Cfi H4 or p-BrCg H4 ) and 171 c (R = p-CHj 5 H4, p-BrCg H4 ), in 50% aqueous methanol at 130°. In this case, the trans isomer was found to react at a rate 10 times that of the cis isomer. Furthermore, the trans isomer gave 95% 2-butyne and 5% 2-butanone, whereas the cis isomer gave 72% 2-butyne and 28% 2-butanone as products. Also, as expected (vide supra) for a unimolecular solvolysis reaction, the cis brosylate reacts at a rate four times that of the corresponding tosylate. 

An interesting case in point is the acetolysis of 3-aryl-2-butyl p-bromobenzenesulphonates or brosylates (25), for which the Hammett plot is shown in Fig. 13.6. The lower right-hand side of the... 

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 ... 

Support for the suggestion that Fig. 13.6 involves a change in actual reaction pathway is provided by acetolysis of the threo diastereoisomer (31) of the brosylate. Acetolysis leads to two different distinguishable, diastereoisomers whose relative proportion will depend on how much of the total reaction proceeds by external nucleophilic attack via the SN2 pathway (erythro product, 32), and how much by internal nucleophilic attack via a cyclic phenonium ion intermediate (threo product, 33) ... 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

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