Potassium tert butoxide preferential

The preferential syn elimination of hydrogen fluoride from trans-I-bromo-2-fluorocyclohexane to give 1 -bromocyclohexene is achieved only when a strong base such as sodamide is used [55, 56] Potassium tert-butoxide causes elimination of hydrogen bromide to form 3-fluorocyclohexene [56] (equation 27)... 

Experimental conditions leading to good yields of olefins comprise heating the hydrazone or semicarbazone of the ketone with potassium tert-butoxide in anhydrous toluene/7]. The reason for the product difference is not known, but may be associated with preferential protonation at C(S) arising from the particular charge distribution and geometrical relationship in a specifically solvated ion-pair of the type (9), which would be favoured by the non-polar solvent. 

When a mixture of a cyclopropyl phenyl selenone and a ketone or an aldehyde is treated with potassium tert-butoxide in tetrahydrofuran, metalation of the selenone takes place preferentially and the metalloselenone formed immediately reacts with the carbonyl compound. Oxaspiropentanes are produced almost quantitatively when ketones are used whereas a mixture of oxaspiropentanes and cyclopropyl ketones are obtained when the reaction is carried out with aldehydes, for example, formation of 5 and... 

Butene is isomerized within minutes when treated with potassium tert-butoxide in dimethyl sulfoxide at 55° 8 in this and other cases it has been observed that the isomerization leads preferentially to the thermodynamically less stable cw -isomer. 

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