Homoallylic participation

Ihe homoallylic participation was discussed in a number of publications. Thus, LeBel et al. - s studied the acetolysis of 8- n- 414 and 8 nti-tosyloxy-bicyclo-[3,2,l]octenes-2 415 the 2.6 10 at 25 °C led the authors to suggest the 

The solvolysis rate of the tosylate 415 and the formation of the acetate 416 with the retained configuration corroborate the homoallylic assistance to the ionization and the formation of a nonclassical ion which can have one of the three structures  

419 solvolyzes about 10 times as fast as tosylate 414 (the negative inductive effect of the double bond, the smaller angle 1-8-5 in compound 414), compounds 415 and 

420 react at about equal rates this also confirms the homoallylic xsleration in the ionization of compound 415. As seen from the scheme, the classical ion from tosylate 434 reacts along different routes it rearranges into a nonclassical ion, into an allylic ion or r ts nonstereospedfically with solvents. 

Klumpp et al. report the following values for the anchimeric acceleration logarithm determined by Schleyer s method  

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For example, acetolysis of r/n /-6-tosyloxytricyclo[5.2.0.02 5]nona-3.8-diene proceeded smoothly at 35 C with stereospeeific rearrangement to c.Y0..mj-9-acetoxylricyclo[4.2.1,02 5]nona-3,7-diene (l).30 Interestingly, the rate of acetolysis of the substrate was considerably enhanced kre[25 C = 6.8 x 104) as compared with that of its bicyclic counterpart 2.30 An important conclusion from these studies is that the anchinteric assistances by cyclobutene in the form of homoallylic participation is effective in the stabilization of the carbocation intermediate. It was found in another study that cyclobutene is better than cyclobutane in terms of anchimeric assistance.31... 

Addition of Nf ion to J,2-epoxy-3 hutene (Eq. 000) provides on interesting illustration of 1,4-addition , sinoe there are isolated approximately equal proportion of 2-azido-3-buten-l-ot and 4-azido-2-butcn-l-ol,w Such homoallylic participation is a well-known phenomenon in many solvolytic processes,1878 and fteeros to suggest the existence of a earbonium ion-like intermediate in the present case. 

Figure 6.5 Orbital picture of homoallylic participation in the cholesteryl system.

The work on the cholesterol system stimulated investigation of other examples of homoallylic participation. Roberts found that exo-5- and endo-5-bicyclo[2.2.1]heptenyl (i.e., exo-5- and enrfo-5-norbornenyl) halides (47 and 48) both solvolyze in aqueous ethanol to give the same product (49) the exo compound (47) solvolyzes about ten times more rapidly than the endo compound (48). Roberts pointed out that backside homoallylic participation in ionization was possible in 47 but not in 48 (see Figure 6.6). Once 48 has ionized it can, in a second... 

Figures 6.6a and 6.7 show two kinds of homoallylic participation. We shall see below (p. 296) that other structures have also been proposed for this type of delocalized bonding.

There are strict geometrical requirements for homoallylic participation. For example, Bartlett and Rice found no indication of homoallylic participation on solvolysis of 53 in aqueous acids. Apparently the strain energy of bonding is greater than the stabilization so obtained.69... 

The importance for homoallylic participation of the exact position-of-lhe p orbitals of the double bond in relation to the developing p arhital at the-reartinn site li-also shown by the rate change attendant on puckering of the five-mem-bered ring in the series, 54, 55, and 56.70 In the lower homologs of these bi-... 

Note that if the bicyclobutonium ion were formed directly upoii ionization of an allylcarbinyl derivative, it would be a case of homoallylic participation in an acyclic system. In fact, the bicyclobutonium ion is similar to the carbonium ion proposed by Winstein for homoallylic participation in the 7-norbornenyl system— cf. Figures 6.7 and 6.8 and Structures 65 and 66. The difference between them is that 66 is more symmetrical. 

K. Stereoselective synthesis of cyclopropanes via homoallylic participation. Bull. Chem. Soc. Jpn. 1996, 69, 31-39. 

The fact that the rate enhancements observed for the homopropargylic participation are of the same order of magnitude of those for homoallylic participation may indicate that also in the case of the addition of carbonium ions, the rates of addition to the triple and double bond are similar. 

Inspection of models shows that the departure of a 3 a-tosylate anion cannot derive assistance from rear attack by the sr-electrons with ring A in its stable chair conformation (12). The alternative conformation (13) which might permit homoallylic participation [34] is highly strained and cannot compete with elimination of the m is-axial 4j8 proton to give cholesta-3,5-diene (14), the major reaction product (see p. 252 for further discussion). 

Homoallylic participation was first invoked by Shoppee to account for the high rates and the stereospecificity of 3/3-(415) and i-cholesteryl (416) interconversions339). The double bond in (415) is clearly restricted to participation by one end. We cannot distinguish, however, between a homoallylic and a cyclopropylcarbinyl cation as the intermediate [cf. (387) and (355)] because both would be chiral. The same limitations apply to the 3-cyclohexenyl system. The tosylate (417) acetolyzes slightly more slowly than cyclohexyl tosylate340). The products include the acetate (418) corresponding to the starting material, the bicyclic acetate (419), and (420) as a result of hydro-... 

The 5-norbomen-2-yl system (421) is relatively well understood. Homoallylic participation is weak so that the double bond actually causes a rate retardation342-. Participation is not detected by the tool of increasing electron demand (Section 7.2.2.)244,266. Yet the exo-tosylate (421), X=OTs, acetolyzes 7000 times faster than the mfo-tosylate (425), X=OTs, to give the tricyclic acetate (424) as the principal product (ca. 90%)343. The hypothetical homoallylic intermediate (422) is... 

Suppression of competing nucleophilic pathways, by changing from normal solvents to fluorinated alcohols, provided evidence for homoallylic participation (40). For solvolyses of cyclohexen-4-yl tosylate (equation 4), the substitution product (IV) is important, but the bicyclic product (V) is formed in significant quantities in HFIP (Table II). The stereochemistry of the substitution product (IV Table II) supports the interpretation that a displacement of solvent with inversion of stereochemistry (ks process) occurs in nucleophilic media, changing to a homoallylic (fcA) process with retention of stereochemistry in HFIP. In acetic acid, the fcA process is just beginning, and in formic acid, the ks and fcA processes occur about equally. 

The alternative explanation is that the role of homoallylic participation increases when near the leaving group there is an electronegative substituent. This appears to refer only to n- rather than to a-participation indeed, the rate ratio of acetolysis for tosylates 63 and 6/ is 336,000 while for 194 and 7 95 it is only 203. The increase in the homoallylic participation is suggested by the fact that the acetolysis of cis-endo-... 

Homoallylic participation can also be intensified by the effects of nonbonded steric interactions. De Puy et al. have shown tosylate 202 to solvolyze 270 times faster than 203 the tosylate 204 solvolyzes 3.2 10 times as fast as 205 does. The 100-fold difference in the accelerating effect of the double bond can be accounted for... 

These data show that the efficiency of homoallylic participation depends on the symmetry or asymmetry of the double-lxjnd location relative to the developing cation centre the participation is stronger in the compounds 209 and 194. Their relative distance effects the longer the syn-bridge, the less effective is the participation as in the >mpounds 209 and 421. A similar conclusion was also drawn by Hess who showed the rate of acetolysis of tosylate 422 to be 10 as high as that of the saturated analogue 419 but far lower than that of compound 209 (the logarithm of anchimeric participation for compound 422 is 8.3). 

Lambert has shown that the unsymmetrical homoallylic participation in the bicyclo[2,2,2]octene series is far more intensive than in the norbomene one this may be due to the smaller rigidity of the bicyclooctene skeleton in comparison with the bicycloheptene one... 

It should be noted that the trans-isomer 425 reacts 8 times as fast as the cis-isomer 423. This fact precludes the simultaneous detaching of both tosyloxy groups and the formation of a dication. The enhanced effect of unsymmetrical homoallylic participation is attributed to an electronegative substituent (OTs) which increases the demand for the double bond participation from the developing cation centre. 

The same is observed in the ions of type 524 with donor substituents at C (Fig. 13, b). As the stabilizing effect of the substituents in the latter ions decreases, the chemical shift of C increases faster than that of C in passing to the ions with considerable homoallylic stabilization the diemical shift of C decreases with a simultaneous increase in that of C. This points to the growing degree of a-participation in ions of type 524. The ratio of the degree of o-participation (which is reflaited by the chemical shifts of C atoms) to that of homoallylic participation (which is reflected by the differences between the chemical shifts of C and C ) is seen from the dependence in Fig. 13, c. 

The orientation required for this participation is not like the parallel Tc-arrangement of orbitals in a-cyclopropylcarbinyl cations, it rather contributes to the a-type interaction between the ionic centre and the sp -orbitals of cyclopropane. Here the situation is similar to the homoallylic participation of double bonds at long distances the a-overlap is more effective than the jt-overlap The fact that compound 565 reacts 10 as fast as the 7-anti-norbornenyl brosylate 567 points to a more effective... 

Macomber, 1969). The larger anchimeric assistance effect on rate is observed in the case of em-dimethyl substitution at C2. Bly and Koock (1969) estimated rate enhancements due to homoallenic participation of 8 3-58 X 10 for the 2,2-dimethyl brosylate (5a of Table 6) and as large as 1 7 x 10 in the case of 2,2,5,6-tetramethyl substituted brosylate in acetic acid at 75°. It is noteworthy that these rate enhancements are larger by factor of at least 10 than in the case of homoallylic participation in related compounds. 

Kinetic and product studies were reported to support the conclusion the homoallylic participation (see Volume 2) overwhelms R2O-3 participation in a medium-sized ring. This differs from the results in the acyclic system (42). While a weak anchimeric assistance was noted in the acetolysis of (108), the products obtained are explicable on the basis of the derived homoallylic cation (109). The products from (llO), however, may arise exclusively from (111). 

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