Solvolysis cyclopropyl tosylate

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

The relative rates of concerted ring opening for substituted cyclopropyl tosylates can be explained on the basis of the selection rules and the principle of maximum orbital overlap. The relative rates of solvolysis for two different dimethylcyclopropyl tosylates are shown in the accompanying diagram ... 

The solvolysis product of alkyne-substituted 1-ethynyl cyclopropyl tosylates (6) appeared to be strongly dependent upon the nature of the substituent R of the triple bond. So, stabilization of the intermediate cyclopropyl cation (7), by delocalization of the positive charge over the three carbons of the mesomeric propargyl-allenyl system, entails a powerful electron-releasing substituent R effectively, when R = p-anisyl, the ring-opening of the cyclopropyl cation was totally suppressed (equation 5). ... 

The W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This issue was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 11 to 13. 

Due to the strain imposed by the three-membered ring, the cyclopropyl cation is not a stable intermediate and electrocyclic ring opening occurs readily. Therefore, in the solvolysis of cyclopropyl tosylate in acetic acid, allyl acetate is obtained rather than cyclopropyl acetate. Solvolysis reactions of other cyclopropyl halides, sulphonates, and diazotization of cyclopropylamine in aqueous solution also give the allylic products. 

SCHEME 2.11 Relative rates of solvolysis of some cyclopropyl tosylates. 

FIGURE 2.14 Solvolysis of disubstituted cyclopropyl tosylate via dis-in mode. 

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geomeby. In tosylate 1, the cyclopropane ring is locked into an orientation which affords a perpendicular arrangement. It reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geomeby, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate 4. ... 

They have postulated that the solvolysis of 3-substituted cyclobutyl tosylates 12 (X = Alkyl, Ar, Cl, OEt, SiR 3) proceeds through the initial formation of bicyclobutonium ion in the rate determining step, which rearranges stereospecifically to the cyclopropyl-... 

Although not competitive synthetically with the methods discussed above two interesting applications of ring closure reactions for the synthesis of homoadamantane derivatives have been reported. The first involves trans-annular cyclopropyl participation in the solvolysis of 48 acetolysis gives 3-homoadamantyl acetate as the only observed product (Eq. (32)) 112 Comparison of the acetolysis rate of 48 (2.14 x 10 4 sec"1,25°C) with that of exo-3-bicyclo[3.3.1]nonyl tosylate (5.82 x 10"s sec. 1,25°C) U3>114)... 

Similar results are obtained for the corresponding tosylates 281).In conforma-tionally nonrigid systems, on the other hand, where the preferred bisected (97) conformation may be adopted, cyclopropyl substituents enhance solvolysis rates by a factor of approximately 104 30S). Therefore, a free energy difference between 97 and 98 of about 10 kcal/mole (1.36 log 107) may be estimated for tertiary cyclopropylcarbinyl cations at 25 °C28°). 

On the other hand, solvolysis of the tosyl derivative of 1-cyclopropyl-cyclopropanol (112) in aqueous ethanol does not lead to ring enlarged products. 91 Apparently, the incipient cyclopropyl cation is better stabilized by conjugative interactions with another cyclopropyl system than with a vinyl group. 

The only products of solvolysis of the tosylate 60 from the 1 -ethynylcyclopropanol 9, with R = CH3 was the allyl derivatives 61 (R = CH3) from the ring opening of the cyclopropane ring, while unrearranged cyclopropanols (or derivatives) 62 were obtained in high yields when R = cyclopropyl or aryl, Eq. (18)13>. 

The solvolysis of j -cyclopropylethyl tosylate (128, R = c-Pr) has been reinvestigated and the rate relative to 128 (R = i-Pr) at 75°C is 0.92,0.96,1.1, and 3.4 in EtOH, HO Ac, HCO2H and CF3CH2OH, respectively . These low ratios and product studies using deuterium-labeled substrates indicated that solvent displacement predominated over cyclopropyl participation , but there was a relative enhancement of the latter for 1-methylcyclopropyP Optically active c-PrCH2CHDOTs underwent solvolysis with 17-18% retention, consistent with contribution from the route. ... 

The effect of a 1-cyclopropyl substituent on the rate of solvolysis of 2-adamantyl tosylates (129) has been compared to a variety of other 1-substituents. These rate effects were attributed to electronic substituent effects on formation of a bridged ion, but steric effects are probably also involved and a detailed interpretation of the conjugative effect of cyclopropyl is not straightforward. 

The initial report by Winstein and coworkers " in 1959 that cis-3-bicyclo[3.1.0]hexyl tosylate (108) undergoes solvolysis with cyclopropyl participation to give a homoaromatic trishomocyclopropenyl cation (109), opened up a large new area of... 

The proposal that cyclopropyl participation in solvolysis of the cis-3-bicyclo[3.1.0]hexyl tosylate (108) proceeds via a trishomocyclopropenyl cation intermediate (109) has been firmly established by product, deuterium scrambling and salt effect studies However, because the cyclopropyl participation in this system requires an unfavorable conformation, it is not apparent from simple kinetic studies. Thus, the cis-3-bicyclo[3.1.0]hexyl tosylate (108) has an acetolysis rate which is only nine times faster than that of the non-participating trans isomer, 110. However, when the 3-bicyclo[3.1.0]hexyl moiety is incorporated into a tricyclic or tetracyclic structure which locks it into a chair form, dramatically large rate enhancements are found. Some examples from the literature which are provided in the review by Haywood-Farmer are given below. These... 

This concerted ionization and disrotatory ring-opening of cyclopropyl substrates has been an area of continuing interest . It can be prohibited by steric or conjugative interactions . For instance, 1-cyclopropylcyclopropyl chloride (3a) or tosylate (3b) lead to unrearranged solvolysis products, i.e. 1-cyclopropylcyclopropanol (4), and to cyclopropylethyl ketone (5) arising from homoketonization (equation 4). 

Miscellaneous Rearrangements.—Solvolysis rates for cholesteryl tosylate in different solvents show an excellent correlation with rates for cyclopropyl-carbinyl tosylate, indicating similarity of mechanism. Two distinct correlation curves were obtained, depending upon solvent classification. Stereoelectronic restrictions in the cholesteryl cation permit only a single structure for charge dispersal between positions C-3, C-5, and C-6, but several non-classical structures seem plausible for the cyclopropylcarbinyl cation. It is suggested that two such structures, depending upon solvent character, may account for the observation of two correlation curves. 

The participation of the ring can also be levelled by introducing into the system a stronger stabilizing group which can accelerate the solvolysis of both exo and endo epimers. Schleyer used in this capacity the a-cyclopropyl group and studied the solvolysis of tosylates 269 and 270... 

Full details have now been published of the solvolysis of P-allenic tosylates, which gives rise to cyclopropyl ketones and methylenecyclobutanols as discussed in an earlier Report. The homoallenic participation may involve a cation of the bicyclobutonium type in addition to a cyclopropylvinyl cation. Similarly, a cyclopropylcarbinyl cation has been implicated in the solvolysis of the vinyl triflate (40) which gives rise to the cyclic products (41), (42), and (43 R = CHjCFg) in addition to acyclic derivatives (Scheme 4). The same cyclic products are formed in the solvolysis of the tosylate (43 R = Ts). 

The delocalized cation (785) is formed transiently on dissolving thujan-3-ol in strong acid but no evidence for cyclopropyl participation was found in the solvolysis of the tosylate (786). ... 

Attempted generation of a dicarbocation from (363 R = Me) leads only to (364) or ring-cleaved cations. However, (363 R = Ph) readily gives rise to (365) and thus the cyclopropane ring, of itself offers insufficient stabilization for these species. " Solvolysis of the tosylate (366 R = Me) results in almost quantitative cyclopropyl-allyl rearrangement. However, with (366 R = Ph or cyclopropyl) mesomeric stabilization of the cation intermediate results in ca. 90% of products derived from the unopened ion. ... 

---