Evans. Kim Breit Conformational Analysis: Cyclic Systems-2 Chem 206 Cyclobutane Cyclopentane 145-155° ax Eclipsing torsional strain overrides increased bond angle strain by I puckering 0=28° a Ring barrier to inversion is 1.45 kcal/mol C Envelope C2 Half-Chair CsEnvelope a Two lowest energy conformations(10 envelope and 10 half chair conformations favored by only 0.5 kcal/mol)in rapid conformational flux(pseudorotation) which causes the molecule to appear to have a single out-of-plane atom "bulge which rotates about the ring a Since there is no"natural"conformation of cyclopentane, the ring conforms to minimize interactions of any substituents present SeNven (MM2) LA single substituent prefers the equatorial position of the fiap of the envelope ■△G=1 kcal/mol favoring R= Me equatorial (barrier ca. 3. 4 kcal/mol, R= CH3) 1, 2 Disubstitution prefers 1, 3 Disubstitution prefers cis diequatorial to trans for steric/torsion ans by 0.58 kcal/mol for di-bromo cmpd reasons(alkyl groups) and Me 1,3 Disubstitution: Cis-1, 3-dimethyl ion prefers trans diequatorial to cyclopentane 0.5 kcal/mol more stable than trans nol for diacid (roughly equivalent to LA carbonyl or methylene prefers the planar position of the half-chair(barrier 1.15 kcal/mol for cyclopentanone)H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H Me Me H eq ax ax eq ax eq eq ax H H H H H H H H H H X (MM2) (MM2) X X Evans, Kim, Breit Chem 206 Cyclobutane j = 28 ° ■ Eclipsing torsional strain overrides increased bond angle strain by puckering. ■ Ring barrier to inversion is 1.45 kcal/mol. ■ G = 1 kcal/mol favoring R = Me equatorial ■ 1,3 Disubstitution prefers cis diequatorial to trans by 0.58 kcal/mol for di-bromo cmpd. ■ 1,2 Disubstitution prefers trans diequatorial to cis by 1.3 kcal/mol for diacid (roughly equivalent to the cyclohexyl analogue.) 145-155° ■ A single substituent prefers the equatorial position of the flap of the envelope (barrier ca. 3.4 kcal/mol, R = CH3 ). Cyclopentane C C2 Half-Chair sEnvelope ■ Two lowest energy conformations (10 envelope and 10 half chair conformations Cs favored by only 0.5 kcal/mol) in rapid conformational flux (pseudorotation) which causes the molecule to appear to have a single out-of-plane atom "bulge" which rotates about the ring. ■ Since there is no "natural" conformation of cyclopentane, the ring conforms to minimize interactions of any substituents present. ■ 1,2 Disubstitution prefers trans for steric/torsional reasons (alkyl groups) and dipole reasons (polar groups). CsEnvelope Conformational Analysis: Cyclic Systems-2 CsEnvelope ■ A carbonyl or methylene prefers the planar position of the half-chair (barrier 1.15 kcal/mol for cyclopentanone). ■ 1,3 Disubstitution: Cis-1,3-dimethyl cyclopentane 0.5 kcal/mol more stable than trans