Cyclopropanes acetolysis

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. ... 

Byproducts of this rearrangement are cyclobutenes, cyclopropane derivatives and allenic alcohols. The ratio of these products depends on the substitution of the substrate and on the reaction conditions. For example, 3-methyl-5-tosyloxypenta-l,2-diene (3) gives 75% of 1-methyl-2-methylenecyclobutanol (4) upon hydrolysis with water and calcium carbonate at 100 °C, while acetolysis with acetic acid/sodium acetate at 80 °C, and subsequent treatment with lithium aluminum hydride, provides only 37% of the cyclobutanol.12... 

TABLE 7. Relative rates of acetolysis of cyclopropanes . Reprinted with permission from J. Am. Chem. Soc., 107, 1003. Copyright (1985) American Chemical Society... 

TABLE 8. Relative rates and deuterium isotope effects for acetolysis of some cyclopropanes... 

In a different biscyclopropylcarbinyl system encompassing a five-membered ring, the quadricyclyl system (100), the effect of the second cyclopropane ring is not so pronounced The relative rates given are for ) -naphthalenesulfonate (0) NS) acetolysis at 25 °C. Richey and Buckley have suggested as an explanation that the... 

Chenier and coworkers " have studied the acetolysis of 4-nortricyclyl tosylate (119) and found it to be 10 slower than the bridgehead norbornyl tosylate (118). They argue that a substantial portion of the bridgehead reactivity difference is due to inductive electron withdrawal by the cyclopropane ring. 

This area has been reviewed in detail by Haywood-Farmer and no major new literature reports on the subject have appeared. There is kinetic evidence that some cyclopropyl participation does occur in 5,6-cyclopropyl-fused exo-norbornyl and related type systems where the cyclopropane is also exo. This is seen in the exo/endo rate ratio of 1130 for acetolysis of the exo-cyclopropyl isomer shown below. The exo/endo rate ratio of 385 for the emio-cyclopropyl isomer is almost identical to that of 350 for the... 

The effect of cyclopropane participation in the solvolysis of c/s-bicyclo [3. 1.0] hex-3-yl tosylate 582) is much less dramatic than in the rigid systems mentioned above441 The rate of acetolysis of 582) is not increased over that cyclopentyl tosylate (584), although 582) solvolyzes 40 times faster than the tram isomer (583). 

Scrambling of deuterium in (595), the major product of acetolysis, clearly indicates participation of only one cyclopropane ring446. The trishomocyclopropenium intermediate (594) is preferred to the pentahomocyclopentadienylium ion (596), in accordance with Winstein s concept of homoaromaticity319 ... 

Acetolysis of 19-mesyloxy-4-androstene-3,17-dione-3-thioketal under buffered conditions yields the steroidal 3-eno[3,4-b]dithiane 18 in 73% yield. In the absence of buffer, acetolysis leads exclusively to 19-acetoxy-4-androstene-3,17-dione 3-dithioacetal in 65% yield. Subsequent desulfurization of 18 with Raney nickel affords 5f ,19-cycloandrost-3-en-17-one in 81 % yield.The rearrangement of 3-eno[3,4-6]dithiane constitutes a homoallylic cyclopropanation accompanied by sulfur migration. 

As seen from Table 11, the kinetically controlled products of the reaction between the methyl-substituted cations 545 and 547 are mainly tertiary, rather than secondary, acetates but in every case the products have only a cis-configuration. Upon acetolysis of 6,6-dideuterated monomethyl-substituted cis-tosylate 544 the tertiary acetate contains, judging by the PMR spectra, two H atoms in the methylene group of the cyclopropane ring, and the secondary — one H atom on the average, in full conformity with the structure of the intermediate ion 545. On the other hand, the secondary acetate formed on solvolysis of the trans-epimer contains two D atoms in the cyclopropane group. Thus in this case, either, no conversion is observed of a classical ion into a nonclassical one in the trans-epimer solvolysis. 

The acetolysis of the exo-8-anti-tricyclo[3,2,l,0 ]octyl tosylate 554 proceeds three times slower than that of the nonreactive 7-norbomyl tosylate 4 In compound 554 the p-like orbitals of and involved in the formation of the C —bond are remote from the 8-position where the cation centre arises but the face participation of the cyclopropane fragment seem to be insignificant. At the same time the endo isomer 555 solvolyzes 10 times as fast as the exo-epimer 554 in this case the above-mentioned orbitals are directed to the 8-position (edge participation). The hydrolysis of this tosylate in 70 % aqueous acetone results in an alcohol mixture containing 99.9% of a rearranged alcohol — endo-3-tricyclo[5,l,-0,0 ]octanol 556 — and 0.1 % of alcohol 557 which retains the parent skeleton and configuration. 

Since in the above examples the solvolysis of cyclopropane derivatives is accompanied by rearrangement to less strained structures, the comparison of the reactivities of compounds 392 and 567 cannot answer the question what is the relative efficiency of the participating double bond and cyclopropane ring respectively. It would be more reasonable to compare the acetolysis of the compounds 568 and 569 because their reactions are not accompanied by rearrangements... 

To elucidate this question Coates investigated the acetolysis of 9-tosyloxy-pentacyclo[4,3,0,0 0 . 0 ]nonane 583. In this molecule the homocyclopropyl rearran ment results in a structure identical with the parent one. Thus, a decrease in the steric strain cannot contribute to the solvolysis rate of compound 583. In the molecule 583 the syn- and anti-positions relative to the leaving group are occupied by cyclopropane rinp, so one can study the intramolecular competition between syn- and anti-participation. If both cyclopropane rings take part in the rearrange-... 

The acceleration caused by the participation of the cyclobutane ring (10 ) upon solvolysis of compound 650 amounts to about a third of the anchimeric effect of the cyclopropane ring in the compound 565. The calculated extent of bond bending for cyclobutane is also about a third of bond bending for cyclopropane. Similar data have also been obtained in the acetolysis of ojmpound 651. 

Details have appeared of the synthesis of (150) and (151) and the acetolysis of their tosylates, which both give an approximately 2 1 ratio of syn- and anti-acetates whereas the related secondary alkyl tosylates solvolyse with very high retention of configuration. From this comparison it is concluded that o-bridging, and not steric or strain effects inherent in the framework, is the cause of the high stereospecificity with the secondary tosylates. A further study of solvolytic conversion of (152) into (155) shows that rates correlate better with ct rather than a constants for various aryl groups (p = —1.44, which is fairly low). This suggests that the cyclopropane... 

Bridgehead olefins (834 n = 3 or 4) are formed in the solvolysis of the dibromo-propellanes (833 n = 3 or 4) by loss of halide anion, electrocyclic cleavage of the cyclopropane ring, and solvent capture/ The products isolated depend upon the precise reaction conditions in the case of (833 n = 3) acetolysis affords (835 53%) and (836 18 %) provided that water is not present. The details of the mechanism of the Ag" "-assisted solvolysis of (833 n = 4) have been elucidated with the aid of labelling. " ... 

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