Sterically hindered tetrasubstituted alkenes

The regioselectivity of palladium-catalyzed additions of organoboronic acids to unsymmetrical alkynes is strongly dependent on steric and electronic effects (Scheme 12).62 Multi-component reaction has been reported for the synthesis of tetrasubstituted alkenes.58 The aryl group from an aryl iodide is generally added to the less hindered... 

In contrast, the a-peroxy lactones, also members of the dioxetane family, display a higher reactivity toward nucleophiles, in view of the inherent polarization of the peroxide bond by the carbonyl functionality. Consequently, the nucleophilic attack is expected to take place at the more sterically hindered but more electrophilic alkoxy-type oxygen atom of the peroxide bond. A recent detailed study of the oxidation of various di-, tri-and tetrasubstituted alkenes 6 with dimethyl a-peroxy lactone (7) revealed, however, much complexity, as illustrated in Scheme 7 for R = CH3, since cycloaddition (8), ene-reaction (9 and 10) and epoxidation (11) products were observed. In the presence of methanol, additionally the trapping products 12 and 13 were obtained, at the expense of the polyester 14. The preferred reaction mode is a sensitive function of the steric demand imposed by the attacking alkene nucleophile. 

The preferred reaction mode has been found to be sensitive to the structure of the alkene and the difference in the reactivity has been explained in terms of steric and stereoelectronic factors. Therefore, in the case of the sterically less hindered disubstituted cis alkenes, the pathway along the open 1,6-dipole D is favored (stereoelectronic control), while the more space-demanding disubstituted trans alkenes react by the epoxidation mode (steric control) a similar situation applies to trisubstituted versus tetrasubstituted alkenes. This remarkable but complex product dichotomy in the oxidation of alkenes... 

The following relative rates have been observed R2G—CH2 > RCEU-CH2 > (Z)-RCH= =CHR > (E> RCH—CHR. Only certain aryl-substituted alkenes, tetrasubstituted alkenes or sterically hindered alkenes... 

---