Electrophilic catalytic system Subject

Kinetic analysis of this system enables the data of Table 20 to be derived. The ratios k 3/k2 give a measure of the relative catalytic effectiveness of the cations since k2 is independent of the electrophile. The resulting order, Li+ > K+ > n-Bu4N+, is similar to that reported for the electrophile-assisted ionizations of p-methoxy-neophyl tosylate and the p-nitrobenzoate of spiro- [4,5 ] -deca-6,9-dien-8-ol in acetone (Winstein et al., 1964). The absence of these effects with the chloro compound is in accord with the observations that aromatic chloro derivatives are not usually subject to electrophilic catalysis of leaving group departure (Bemasconi, 1973). 

Both palladium and rhodium are effective catalysts, but the rhodium reactions seem to be more subject to steric effects. The reaction proceeds via a catalytic cycle similar to cross coupling, except that the formation of the diorgano(palladium) is brought about by electrophilic attack by the arylpalladium halide, rather than a transmetallation reaction, hence the need for electron-rich systems. The equivalent alkynylation can also be carried out using haloalkynes. ... 

The synthesis in Scheme 13.38 combines elements of a biomimetic-type polyene cyclization with a rearrangement similar to that just described in Scheme 13.37. The early stages of the synthesis culminate in the construction of the allylic alcohol in step B. In step C, it is epoxidized using the stereoselective VO(t-BuOOH) method (Section 12.2.1). This epoxidation provides the key intermediate for the polyene cyclization. The mild Lewis acid FeCla is used to promote the cyclization, which terminates in an electrophilic substitution of the methoxyphenyl substituent. Steps E and F convert the methoxyphenyl ring to a diene. This diene undergoes a Diels-Alder reaction in step G. After catalytic reduction of the double bond, the anhydride is subjected to an oxidative bis-decarboxylation (see Section 12.4.2 for discussion of this reaction). The resulting alkene is epoxidized, and the epoxide is reduced. A rearrangement is done at this point. The reaction is similar to that used in Scheme 13.37, except that it involves a saturated, rather than allylic, system. The final steps in the synthesis are those used in Schemes 13.33 and 13.34. 

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