Evans. Kim. breit Conformational Analysis: Cyclic Systems-3 Chem 206 Methylenecyclopentane and Cyclopentene Cyclohexane Energy Profile(kcal/mol) Strain trends Half-Chair a Decrease in eclipsing strain >人、△mmm) Relative to cyclohexane derivatives, those of cyclopentane prefer an sp center in the ring to minimize eclipsing interactions Reactions will proceed in such a as to favor the formation or retention of an exo double bond in the 5-ri the formation or retention of the exo double bond in the 6-ril Shechter H. J. Am. Chem 1954.76.467 Chair averted Chair k5 =23 H Brown. H. C. Ichikawa K. Tetrahedron 1957. 1. 221 △E=+657.0 drolyzes AE=+55 13 times faster than Conan J-Y: Natat, A: Prolet, D. Bull. Soc. Chim. Fr. 1976. 1935 The barrier: +10.7-11.5 E=0 95.5: 4.5 keto enol 76: 24 enol: keto Brown,HC,Brewster,JH:Shechter,H.JACS1954,76,467.H H H H O NaBH4 O O O OEt O O H H H H NaBH4 O OEt OH H H H H OH H OH H H H H O O "Reactions will proceed in such a manner as to favor the formation or retention of an exo double bond in the 5-ring and to avoid the formation or retention of the exo double bond in the 6-ring systems." Brown, H. C., Brewster, J. H.; Shechter, H. J. Am. Chem. Soc. 1954, 76, 467. Methylenecyclopentane and Cyclopentene hydrolyzes 13 times faster than Strain trends: > > ■ Decrease in eclipsing strain more than compensates for the increase in angle strain. Relative to cyclohexane derivatives, those of cyclopentane prefer an sp2 center in the ring to minimize eclipsing interactions. k6 k6 k5 = 23 95.5:4.5 keto:enol 76:24 enol:keto Brown, H. C., Brewster, J. H.; Shechter, H. JACS 1954, 76, 467. Brown, H. C.; Ichikawa, K. Tetrahedron 1957, 1, 221. Conan, J-Y.; Natat, A.; Priolet, D. Bull. Soc. Chim., Fr. 1976, 1935. » Examples: Chair Half-Chair Boat Twist Boat +5.5 10.7- 11.5 +1.0–1.5 Cyclohexane Energy Profile (kcal/mol) Inverted Chair Evans, Kim, Breit Conformational Analysis: Cyclic Systems-3 Chem 206 k5 E = 0 E = +5.5 E = +6.5-7.0 The barrier: +10.7-11.5 +5.5