Regioselectivity dimethyl trisubstituted alkenes

For a series of geminal dimethyl trisubstituted alkenes, the same trend of regioselectivity was earlier recognized by Thomas and Pawlak as well as Rautenstrauch and coworkers . By examining molecular models and assuming that the reaction is concerted, Thomas and coworkers proposed that the difference in regioselectivity could be due to the different conformations and steric repulsions in the transition states. 

In the reaction with geminal dimethyl trisubstituted alkene 104 (Scheme 27), the methylene hydrogen atoms showed little or negligible reactivity depending on the cation of the zeolite [140], These results demonstrate clearly that regioselectivity within zeolites is different to that found in solution. 

Regioselectivity with geminal dimethyl and diethyl trisubstituted alkenes... 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

Dibromoborane-dimethyl sulfide exhibits regioselectivity in hydroboration of alkenes which is comparable to that exhibited by dialkylboranes. Excess alkene should be avoided in reactions of trisubstituted ethylenes, otherwise hydrogen bromide liberated during work-up may add to the excess ethylene and cause problems. 

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