Cycloaddition 1,3-dipolar-electrophilic ring closur

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

The cycloadditions of (Ti -allyl)Fp complexes to alkenes proceed with high chemoselectivity. Only highly activated alkenes such as methylenemalonates, benzylidenemalononitrile, TCNE or I,2-di-cyano-4,5-dichloroquinone will participate in the reaction. With the less electrophilic a,p-unsaturated systems, cycloaddition can only be effected with Lewis acid activation. Thus the cycloadduct (6) is formed in reasonable yield (as a mixture of stereoisomers) from (3) and cyclohexenone using freshly sublimed AlBn (equation 4). The same reaction gives only a 8% yield with Aids. The exclusive formation of a ci.r-hydrindanone system can be explained by suprafacial attack of the enone on the allyl unit, which is then followed by another suprafacial ring closure of the dipolar intermediate, affording the thermodynamically preferi cyclic system (7a) and (7b). ... 

The cycloaddition of alkenylidenecyclopropanes 5 with chlorosulfonyl isocyanate is described via electrophilic attack at C4, disrotatory opening of the cyclopropane ring to form a dipolar intermediate 8, and ring closure by nucleophilic attack of nitrogen or oxygen on either end of the allyl cation portion of the intermediate 8 which gives adducts 9-12. [27t -I- 2tc] Products 13 were also obtained. 

This investigation opened the way to further two-step cycloadditions of the thiocarbonyl ylide 6.23 with other electrophilic alkenes, e.g., with l,2-dicyano-l,2-bis(trifluoromethyl)ethene (Huisgen et al., 1989). Both the latter dipolarophile and tetracyanoethene demonstrated that, besides the 1,3-dipolar addition product 6.28, the cyclic ketene imine 6.29 is formed in a reversible 1,7-ring closure (6-18, Huisgen et al., 1986b, 1989). 

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