STEREOCHEMISTRY AND MECHANISM OF THE REVERSE ENE REACTION OF CIS-2-ALKYL-1-ALKENYLCYCLOBUTANES - STEREOELECTRONIC CONTROL IN A SYSTEM SHOWING MARGINAL ENERGETIC BENEFIT OF CONCERT

In the temperature range 243.8-267.5-degrees-C, the racemic cyclobutanes (1RS,2RS,1'SR)-1-(1-methoxyethyl)-2-vinylcyclobutane (16b) and (1SR,2SR,1'SR)-1-(1-methoxyethyl)-2-vinylcyclobutane (17b) each undergo a sigmatropic hydrogen shift (reverse ene reaction) amounting to about 18% of the total pyrolysis product, in addition to four other unimolecular processes. The other four reactions are [2 + 2]-cycloreversion, epimerization, double epimerization, and sigmatropic carbon 1,3-rearrangement. Overall disappearance of reactant occurs with first-order kinetics. The activation parameters determined for 16b are E(a) = 47.8 +/- 2.1 kcal/mol and log A = 14.8 +/- 0.9, and for 17b they are E(a) = 48.6 +/- 2.2 kcal/mol and log A = 15.2 +/- 0.9 (A, s-1). In the reverse ene reactions, vinyl derivatives 16b and 17b yield (2E,6Z)-2-methoxyocta-2,6-diene ((E,Z)-18) and (2Z,6Z)-2-methoxyocta-2,6-diene ((Z,Z)-18), respectively, with stereospecificities of 95 and 91%. These are minimum values because of the competing interconversion of the reactant cyclobutanes 16b and 17b. Correction for this gives a stereospecificity of about 220:1 for the 6Z reverse ene product formed directly from 16b and about 35:1 for that from 17b. This demonstrates high stereospecificity at the double bond of the product derived from the migration origin. Secondary beta-deuterium kinetic isotope effects (DELTA-DELTA-G), measured at 517 K) for [2 + 2]-cycloreversion, epimerization, and sigmatropic carbon 1,3-rearrangement of 10-20 (+/- 120) and 80-180 (+/- 240) cal/mol were measured for 16b and 17b. Primary isotope effects for reverse ene hydrogen shift were 980 +/- 125 and 1760 +/- 290 cal/mol, respectively. The stereochemistry at the terminus of migration can be determined in the corresponding cyclobutanes bearing a propenyl instead of a vinyl group. The enantiomerically enriched isotopically labeled cyclobutane (1R,2R,1'S)-(-)-1-(1-methoxyethyl)-2-(2-deuterio-1 (E)-propenyl)cyclobutane ((-)-37) having 93.2% ee was prepared in nine steps from (S)-(-)-3-butyn-2-ol. The reverse ene reaction accounts for roughly 5% of the total product in the pyrolysis of the propenyl derivative (-)-37-d at 239.3-degrees-C. The principal reverse ene product is (2E,6Z)-8(S)-deuterio-2-methoxynona-2,6-diene (45), formed with roughly 13:1 stereospecificity compared to the next most prevalent double bond isomer. The product 8(S)-45 vas chemically degraded to (S)-2-deuteriopropanoic acid, which was subsequently converted to the corresponding propanoate ester 49/50 with (R)-(+)-methyl mandelate. Analysis of the diastereomeric excess of this ester by H-1 and H-2 NMR indicates that 8(S)-45 is formed from (1R,2R,1'S)-(-)-37-d with 68.9 +/- 4.9% (H-1 NMR) or 80.5 +/- 3.3% (H-2 NMR) transfer of stereogenicity. Taking into account potential stereochemical contaminants, the actual stereogenicity transfer may be as high as 100% but not lower than 64%. This suffices to show that a suprafacial hydrogen transfer dominates the retro ene reaction in the propenyl case and probably also in the vinyl cases. The results from 16b, 17b, and 37 are interpreted in terms of a dominant concerted mechanism for the reverse ene reactions of these compounds. They are consistent with a transition-state structure in which the alkenyl moiety is endo with respect to the ring and the breaking C-H bond orbital is aligned with the breaking C-C ring bond. This is the sole geometry consistent with predictions based on the conservation of orbital symmetry and orbital overlap control.