A semiempirical study on the ring-opening process for the cyclopropanone, 2,2-dimethylcyclopropanone,trans-2,3-di-tert-butylcyclopropanone, and spiro(bicyclo[2.2.1]heptane-2.1?-cyclopropan)-2?-one systems in solution

Abstract

ABSTRACT:The molecular mechanisms for the ring openings of cyclopropanone, 2,2-dimethylcyclopropanone, trans-2,3-di-tert-butylcyclopropanone, and Ž w x X . X spiro bicyclo 2.2.1 heptane-2.1-cyclopropan -2 -one systems were studied at the PM3 semiempirical level in the gas phase and including solvent effects. The behavior of the solvent polarity was considered by using the SCRF polarizable continuum method. Six solvents were selected: hexane, ether, tetrahydrofuran, pyridine, acetone, and acetonitrile. An extensive exploration of the potential energy surface using analytical gradient techniques allows the characterization of stationary points associated to the stereomutation conversion. Along a disrotatory ring-opening mechanism, cyclopropanone, 2,2-dimethylcyclopropanone and trans-2,3-di-tert-butylcyclopropanone are intraconverted Ž w x X via an oxyallyl intermediate. The epimeric forms of the spiro bicyclo 2.2.1 heptane-2.1-.X cyclopropan -2 -one intraconvert along two competitive pathways correspond to two-step processes by a disrotatory ring-opening mechanism. Two oxyallyl intermediates and four transition structures were obtained and the corresponding transition vectors are associated to the carbon᎐carbon bond-breaking process and the dihedral angle measuring the conrotatory movement of the plane defined by the three carbon atoms of the cyclopropanone ring. The oxyallyl intermediates and the transition structures for the four model systems present similar structures and energies and they are located on a ratherCorrespondence to: R. Castillo.

Rearrangements of Allene Oxide, Oxyallyl, and Cyclopropanone

“…The UB3LYP method with the 6-31G* basis set was used to locate the oxyallyl intermediate as well as a “disrotatory” transition structure ( 4 ) which were found to be similar to those found earlier with the CAS method (see Figure ) 12a. Energies relative to the energy of cyclopropanone are given in Table .…”

“…It is interesting to note that the DFT method gives an activation energy (Δ E °) of 29.6 kcal/mol that is in very close agreement with the experimental free energy of activation (29.1 kcal/mol) for the racemization of (+)- trans -2,3-di- tert -butylcyclopropanone in isooctane . Earlier CAS calculations gave a value of 21.9 kcal/mol for Δ E ° of this reaction 12a. However, the calculations based on CASPT2N/6-31G* and performed by these authors for cyclopropanone and oxyallyl gave a barrier of “about 28 kcal/mol”.…”

“…However, recent studies on cis -2,3-di- tert -butylcyclopropanone provide strong experimental evidence that oxyallyl is an intermediate rather than a transition structure . In addition the results of CASSCF(4,4)/6-31G* calculations on oxyallyl showed that indeed oxyallyl is an intermediate with C 2 v symmetry 12a. A transition structure with C s symmetry was also located on the potential surface which presumably links oxyallyl with the two cyclopropanones in a degenerate reaction, though no IRC calculations were performed to confirm this.…”

“…Early calculations were limited to the computation of the relative energies of allene oxide, oxyallyl, and cyclopropanone. With the exception of extended Hückel calculations, in all cases cyclopropanone was found to be more stable than allene oxide with oxyallyl significantly higher in energy than either 2 or 3 . − ,12a,− In Table the relative energies of 1 − 3 , including the results of the best calculations performed to date, are summarized.…”

“…Oxyallyl ( 1 ) has long been considered as a potential intermediate or transition structure in the rearrangement of allene oxide ( 2 ) to cyclopropanone ( 3 ) − and in the ring opening and closing reactions of substituted cyclopropanones that result in racemization and isomerization. − Early experimental evidence for the existence of allene oxide as a viable species was based on the characterization of its reaction products, since its apparent high reactivity precluded its isolation. However, in 1968 Camp and Greene reported that the epoxidation of 1,3-di- tert -butylallene gave the first isolable allene oxide, 1,3-di- tert -butylallene oxide, which when heated to 100 °C underwent rearrangement to trans -2,3-di- tert -butylcyclopropanone.…”