Cyclohexyl tosylate

Conversely, processes which convert carbons to sfp- carbons are more favorable for five-membered than for six-membered rings. This can be illustrated by the data for acetolysis of cyclopentyl versus cyclohexyl tosylate. The former proceeds with an enthalpy of activation about 3kcal/mol less than the latter." A molecular mechanics analysis found that the difference was largely accounted for by the relief of torsional strain in the cyclopentyl case." Notice that there is an angle-strain effect which is operating in the opposite direction, since there will be some resistance to the expansion of the bond angle at the reaction center to 120° in the cyclopentyl ring. 

Base-promoted E2 elimination involves simultaneous loss of H+ and X from neighboring carbons. Applying this rule to 2-methyl cyclohexyl tosylate suggests that two different products might form, but the actual situation is more complicated. One tosylate isomer gives only one of the two possible alkenes, while the other gives both. 

Winstein investigated the kinetics and products of 3-/J-cholesteryl substitutions further. He found that under certain conditions 3-/J-cholesteryl tosylate (or chloride) is acetolyzed to the cholesteryl z -acetate (Equation 6.30), and that this reaction is 100 times faster than the solvolysis of cyclohexyl tosylate. Moreover,... 

Explain the following observation The trifluoroacetoxy group on trans-2-triflueroacetoxycyclohexyl tosylate (5) is 30 times more rate-retarding (relative to cyclohexyl tosylate) in strongly ionizing trifluoroacetic acid than in formic acid. 

For example, Figure 7.3 shows a plot of log kE vs. log ks for cyclohexyl tosylate with a number of soft bases.90... 

Figure 7.3 Relationship between elimination and substitution rates of cyclohexyl tosylates with soft bases. From A. J. Parker, M. Ruane, G. Biale, and S. Winstein, Tetrahedron Lett., 2113 (1968). Reprinted by permission of Pergamon Press.

From the effect of changing solvents on rates, it is apparent that an E2C transition state is loose—that is, both base and leaving group are solvated ions. For example, elimination of toluenesulfonic acid from cyclohexyl tosylate by Cl- proceeds only approximately 50 times faster in acetone than in methanol. Compare this with the rate enhancement of about 106 when the Sw2 reaction of CH3OTs is transferred from protic to aprotic solvent.93 (See also Section 4.3.) This indicates that the double bond must be highly developed in the transition state,... 

Until recently it was thought that silicon substituents beyond the /-position have no effect on reactivity71. Fessenden and coworkers72 found that the rates of ethanolysis of els- and /raw.s-4-(trirncthylsilyl)cyclohexyl tosylates were almost identical to those of the cis- and fraws-4-tert-butylcyclohexyl tosylates. 

The kinetic influence of a 5-silicon substituent on the solvolysis of cyclohexyl tosylates is insignificant. Fessenden and coworkers101 found virtually identical rates of ethanolysis of cis and trans 263 to those of the t-butyl reference compounds 264. However, no evidence for a rate-determining kc solvolysis was provided. 

By similar treatments, the effects of many substituents and combinations of substituents were evaluated, and found to give a reasonably linear correlation with reaction rates when plotted by the Hammett-Taft method. The value of —3.3, determined graphically, is close to the value of — 3.49 reported for solvolysis of substituted cyclohexyl tosylates. Although the general validity of the multiple-path treatment has still to be demonstrated, it may be noted that similar calculations, using an attenuation factor of 0.51, produced a reasonable correlation of substituent effects at C(3> and C(i7) with rates of bromine addition to the 5,6-ethylenic bond [17]. 

In a study of the reduction of primary neopentyl and cycloalkyl mesylates and tosylates, two groups showed that LiBEtaH in THF is superior to LAH and L-selectride in both rate and selectivity for reduction versus elimination. While cyclohexyl tosylate still gave 20% alkene, elimination was not seen for the cyclo-pentyl, -heptyl and -octyl esters. The lower rate for neopentyl systems was used to advantage in the selective reduction of (5) to (6) at 25 °C. In a more complex system, LiBEtaH reduction of (7 =... 

Another system where long distance cyclopropyl participation seems to be occurring is in the spiro tosylate 145 shown below which forms homoadamantyl acetate (146) as the only product and also is 5000 times more reactive than cyclohexyl tosylate. 

Compared with unsubstituted cyclohexyl tosylate, the 2-acetoxycyclohexyl tosylates show the following relative reactivities toward acetolysis ... 

Reaction of the cis tosylate is much slower than that of cyclohexyl tosylate, and this we can readily understand powerful electron-withdrawal by acetoxy slows down formation of the carbonium ion in the SnI process. Reaction of the trans tosylate, although much faster than that of its diastereomer, is still somewhat slower than that of cyclohexyl tosylate. But should not the anchimerically assisted reaction be much faster tlian the unassisted reaction of the unsubstituted tosylate The answer is, not necessarily. We must not forget the electronic effect of the acetoxy substituent. Although SN2-like, attack by acetoxy has considerable SnI character (see Sec. 17.15) deactivation by electron withdrawal tends to offset activation by anchimeric assistance. The cis tosylate is electronically similar to the trans, and is a much better standard by which to measure anchimeric assistance. (This point will be discussed further in the next section.)... 

The competitive elimination (ET) and substitution (iSn2) reactions of cyclohexyl tosylate with triphenylphosphine have been examined. Triphenylphosphine is considered to be representative of neutral weak bases which have good nucleophilic afiinity for carbon, but it is a poor reagent for elimination when compared with anionic weak bases that are also good carbon nucleophiles. The reaction of triphenylphosphine with cyclohexyl bromide occurs with almost complete substitution. Tertiary phosphines react with fluorosulphonyl isocyanate and with isothiocyanates to form the zwitterionic adducts (56) and (57). 

Three-center SN2 displacement and anti-eliminations from unsaturates are obvious examples of the coplanarity principle. DePuy et al. (1965) noted that when anti eliminations cannot have coplanar reacting centers the syn coplanar-transition state may become more favorable. Syn bimolecular eliminations had been noted in various systems previously, e.g. haloethenes (Miller, 1961), but these were generally slower than anti eliminations. There were, however, syn bimolecular eliminations whose rates approached that of the anti form or exceeded it. The relative rates of elimination of 2-phenylcyclopentyl and cyclohexyl tosylates with t-butoxide in t-butyl alcohol at 50° are as follows syn-cyclopentyl, 3 awii-cyclopentyl, 26 syw-cyclohexyl, 0 and anii -cyclohexyl, 2. In the cyclopentyl system in which the torsional angle (r) between the leaving groups approaches zero the syn rate is close to the anti rate. In the cyclohexyl system in which t of the stable form is ca. 60° the syn rate is 0. LeBel et al. (1964), report that in the reactions of t-butoxide with the 2,3-dihalobornanes, 92-95,... 

Cyclohexyl tosylate 7-Norbomyl tosylate 5-exo-Norbomenyl tosylate 5-endo-Norbomenyl tosylate 7-anti-Norbomenyl tosylate 7-syn-Norbomenyl tosylate 2-endo-Norbomyl tosylate... 

Cyclohexyl p-toluenesulfonate Cyclohexyl tosylate. See Cyclohexyl 4-toluenesulfonate 3-Cyclohexyl-5,6-trimethyleneuracil. See Lenacil... 

---