Norbornyl tosylate, solvolysis

The first are relative rates for [14] - [17] obtained by Schleyer and coworkers and given below on the effects of substituents at the 6-position in augmenting solvolysis of 2-exo-norbornyl tosylates (Schleyer, 1972 Schleyer et al., 1965 Strang and Schleyer, 1968). 

This interpretation has not been universally accepted, one reason being that the product ratios do not decline in like manner to the rate ratios. Another factor is that, even if the rate ratios are taken to indicate the existence of some participation of the electrons in the bond between C-1 and C-6 in the solvolysis of exo-2-norbornyl tosyl-ate, this is far removed from proving the existence of the symmetrical and hence fully delocalized ion at the transition state or as an intermediate in the reaction. Hence, if one concludes that the exo- and cndo-derivatives of the restricted systems are solvolysing to essentially unbridged or classical ions, the extent of bridging to be deduced from high exojendo rate ratios found with less restricted systems is unclear. 

Less direct precedents are also available for the rearrangement of both exo-1,2-trimethylenenorbornane (32) and emfo-2,6-trimethylenenorbornane (33) to adamantane. Solvolysis of exo-1,6-trimethylene-exo-2-norbornyl tosylate in aqueous acetone at room temperature gives nearly quantitative yields of endo-2,6-trimethylene-ejco-2-norbornanol [Eq. (12)], which, when treated with sulfuric acid, produces 1-adamantanol43). 

However, both J. M. Harris and we, in independent studies, have now concluded that solvent participation is not a significant factor in the solvolysis of endo-norbornyl tosylate in solvents of moderate or low nucleophilicities42,43. Consequently, we can no longer account for the similarity in the behavior of secondary and tertiary 2-nor-bornyl derivatives in terms of the fortuitous presence of comparable solvent participation in enoto-norbornyl and carbon participation in exonorbornyl, both of which vanish in the tertiaries, resulting in essentially constant tertiary/secondary rate ratios40,41. ... 

The discovery by Winstein, Woodward, and coworkers of a 10 rate enhancement in the solvolysis of anfi-7-norbornenyl tosylate relative to that of 7-norbornyl tosylate led to extensive investigations of this ion. 

It is interesting that though C must formally bear a positive charge, no products from the attack on this atom have yet been isolated. It may be due to an increase in the steric strain brought about by introduction of a double bond into the product also, the migrating atom C is usually less substituted than C and C. Such an attack could probably occur if C were a tertiary carbon atom. Schleyerhowever, when studying the solvolysis of 6,6-dimethyl-2-norbornyl tosylates, showed that the formation of the tertiary carbocation is not yet sufficient to break the C —C bond. The results of Jones and Jones show that only a combination of two factors — the... 

Fig. 1. Free energy diagram for a the acetolysis of exo- and endo-norbornyl tosylate b the solvolysis of 2-p-anisyl-2-norbomyl p-nitrobenzoates in 80% aqueous acetone at 25 °C...

The value of m from the Winstein-Grunwald equation (for 2-endo-tosylate 0.69, i.e. between 2-adamantyl tosylate, 0.91, and isopropyl tosylate, m = 0.44)also points to some participation of the solvent. Finally, as shown by Nordlander et al. kjikj, = 30 for the solvolysis of 2-endo-norbornyl tosylate. If this ester was solvolyzed with considerable steric hindrance to ionization the solvent participation would be unusually great while the ratio observed coincides with that for trans-2-methylcyclopentyl tosylate (kjikj = 30). 

Lee has shown that in the acetolysis of — [1 — C]-cyclopentenyl-ethyl tosylate and 2-exo-[4 — C]-norbornyl tosylate k5/k3 is < 100 and <200, respectively. The distribution of the tag indicates that on solvolysis of exo-norbomyl brosylate the ion pair is internally returned. 

When acetolysing the tosylate 144 Gassman and Miles obtained the acetate 146 ( 90%) consequently, the reaction proceeds with retained configuration. This is explained by a nonclassical ion 145 3o -307) Another explanation for the solvolysis of 7-norbornyl tosylate is the fission of the S—O bond rather than that of C—O. 

Rate data for the acetolysis of norbornyl tosylates with substituents at the 3-, 5-, 6-, and 7-positions afford a linear log versus logkg plot with slope 1.11 0.08 (r = 0.98X indicating that the transition states derived from the exo- and ewdo-isomers are equally affected by inductive variations. Hence, it is concluded that o-delocali-zation in the transition state for exo-solvolysis is absent. Studies on the acetolysis rates of the cis,exo- and cis,endo-2,3-norhorna.ne ditosylates (82) and (83), the acetoxy-tosylates (84) and (85), and the norborn-5-ene ditosylates (86) and (87) have been reported. Whereas it was found that 7t-participation is enhanced by the presence of an adjacent electron-withdrawing group in (86) vs. (87) relative to the analogous mono-tosylates, no such response was found for the saturated systems (82) vs. (83) or (84) vs. (85) relative to 2-exo- and -endo-norbomyl tosylate. It is therefore concluded that a-participation is unimportant under these conditions of inductively enhanced electron demand. Essentially opposite conclusions have been reached from the results obtained on the solvolysis rates of the p-nitrobenzoates (88)—(91). In contrast... 

The most famous argument of modern times in organic chemistry concerned the question of delocalized intermediates produced by C—C O bond participation, and a fierce one it was—and still sometimes is. The reaction in question was the solvolysis (SnI) reaction of the 2-norbornyl tosylates, the tosylates of eico- and endo-bicyclo[2.2.1]heptan-2-ol (Fig. 21.48). 

Proposal No. 2. It was next proposed that the solvolysis of entfo-norbomyl tosylate is enhanced by large solvent participation comparable in magnitude to carbon participation in the exo isomer. Thus, one of the two interpretations considered, referring to the 2-Me/2-H reactivity ratios, was that, These = 10s values can be rationalized by the postulation of anchimeric assistance in the exo and solvent assistance in the endo secondary cases 40. This position was adopted and fully discussed by J. M. Harris and S. P. McManus in their interesting attempt to extrapolate from tertiary to secondary 2-norbornyl rates41. ... 

However, the relatively weak nucleophilic solvent assistance in the solvolysis of 2-endo-norbornyl sulphonate is corroborated, firstly, by the formation of about 8 % of optically active 2-exo-acetate 66 secondly, by a significant decrease in the solvolysis rate of 2-endo-tosylate on introducing 3-exo-substituents shielding the backside approach of solvent molecules... 

In contrast to Brown s assertions and in accord with Winstein s and Trifan s assumption, the solvolysis of these secondary systems proceeds with anchimeric acceleration. This is concluded from the following facts a) the exo endo rate ratio for 2-norbomyl systems is 10 -10 as the reaction rate of the endo isomer is not anomalous (see above), hende the exo isomer reacts at an elevated rate b) the rate of solvolysis of exo isomers is 10 to 10 times as high as that calculated according to the semiempirical scheme from only steric effects c) the ratio of the reaction rate of secondary 2-exo-norbomyl systems to the solvolysis rate of secondary cyclopentyl analogues is 100 times as great as that of tert-2-exo-norbomyl derivatives and tert-cyclopentyl analogues since tert-2-norbomyl derivatives are solvolyzed without anchimeric assistance, the factor of 100 characterizes tentatively the amount of anchimeric assistance in the secondary 2-exo-norbornyl systems d) exo- and endo-6-substituents decrease the solvolysis rate of 2-exo-norbomyl tosylate this cannot be accounted for without participation of the electrons of the 1,6 bond in the transition state their participation increases the non-bonded interaction due to a decrease in the C -C distance. 

The rate of solvolysis of 2-endo-norbornenyI sulphonate is lower than that of 2-endo-norbornyl sulphonate due to the polar effect of the transannular double bond. Many attempts have been reported to determine the deceleration of solvolysis caused only by the inductive effect of the homoallylic double bond This problem, however, is complicated by simultaneous action of the opposite a-participation effect. According to Wilt, a good way of determining the inductive effect of the homoallylic double bond may be to compare the reactivities of tosylates 190 and 191. In compound 190 the system geometry forbids the homoallylic delocalization and, hence, the tosylate 191 reacts 5 times as fast as 190 this can serve to measure the inductive effect of the homoallylic double bond... 

The solvolysis of a tricyclic isomer is accompanied by the greatest acceleration ever observed the rate of hydrolysis of tricyclic p-nitrobenzoate in 90% aqueous acetone exceeds that of bicyclic isomer 10 -fold while the increase in reaction rate in comparison with the endo-norbornyl analogue is 10 -fold. The rate of acetolysis of the 7-anti-norbomenyl tosylate 209 is 10 times as high as that of the saturated 7-norbomyl analogue 4. 

The synthesis of (+ )-sesquifenchene (450) and of ( )-epi-P-santalene (451) from the common intermediate (449), derived from endo-dicyclopentadiene, has been detailed. Conversion of (449) into the precursor of (450) makes use of the skeletal rearrangement that occurs during solvolysis of active 2-norbornyl esters. In a synthesis of (-H )-hinesol (452) and 10-epi-(-l-)-hinesol, interesting use has been made of a fragmentation reaction.The tosyl derivative (453), which was obtained in several steps from ( —)-P-pinene, was converted into the spiro[4,5]decane (455) on treatment with sodium hydride in DMSO the essential stereoelectronic changes are summarized in the intermediate (454). 

---