Sihyun Ham

Orthoquinone monoketal chemistry. Experimental and density functional theory studies on orthoquinol acetate rearrangements

The non-dimerizing orthoquinone monoketal, 6-acetoxy-6-methoxy-3-methoxycarbonylcyclohexa-2,4-dienone, conveniently prepared from oxidative acetoxylation of its parent phenol with PhI(OAc)2 in CH2Cl2ue5f8AcOH (3:1), cleanly undergoes 1,3-acetoxy migrations in the presence of silica gel at room temperature to furnish a 60:40 product mixture conceivably derived from [3,5] and [3,3] sigmatropic rearrangements. Density functional theory calculations indicate that the [3,5] shift is pseudopericyclic, has a remarkably low activation energy of 20.1 kcal/mol, and is favored by 5.4 kcal/mol over the pericyclic [3,3] shift, in qualitative agreement with the experimental observations.

Multiphoton Infrared Initiated Thermal Reactions of Esters: Pseudopericyclic Eight-Centered cis-Elimination

Multiphoton infrared absorption from a focused, pulsed CO(2) laser was used to initiate gas-phase thermal reactions of cis- and trans-3-penten-2-yl acetate. By varying the helium buffer gas pressure, it was possible to deduce the product distribution from the initial unimolecular reactions, separate from secondary reactions in a thermal cascade. Thus, trans-3-penten-2-yl acetate gives 54 +/- 5% of beta-elimination to give trans-1,3-pentadiene, 40 +/- 3% of [3,3]-sigmatropic rearrangement to give cis-3-penten-2-yl acetate and 6 +/- 4% of cis-1,3-pentadiene. Similar irradiation of cis-3-penten-2-yl acetate gives 45 +/- 1% of beta-elimination to give cis-1,3-pentadiene, 32 +/- 2% of [3,3]-sigmatropic rearrangement to give trans-3-penten-2-yl acetate and 23 +/- 2% of trans-1,3-pentadiene. The latter process is an eight-centered delta-elimination, which is argued to be a pseudopericyclic reaction. Although beta-eliminations have been suggested to be pericyclic, B3LYP/6-31G(d,p), MP2 and MP4 calculations suggest that both beta- and delta-eliminations, as well as [3,3]-sigmatropic rearrangements of esters are primarily pseudopericyclic in character, as judged by both geometrical, energetic and transition state aromaticity (NICS) criteria. Small distortions from the ideal pseudopericyclic geometries are argued to reflect small pericyclic contributions. It is further argued that when both pericyclic and pseudopericyclic orbital topologies are allowed and geometrically feasible, the calculated transition state may be the result of proportional mixing of the two states; this offers an explanation of the range of pseudopericyclic and pericyclic characters found in related reactions.

An ab initio study of the reactivity of nitrosoketene with formaldehyde

Ab initio molecular orbital theory (RHF/6-31G*) was used to locate pseudopericyclic transition structures for the cycloaddition of formaldehyde to the Z and E conformations of nitrosoketene. The [3+2] pathway, with a calculated activation energy of only 3.5 kcal/mol, is predicted to be significantly favored over the [4+2] alternative.

Further ab initio studies on the reactivity of nitrosoketene

Abstract The cycloadditions of nitrosoketene with formaldehyde, acetone and 2-propenal were calculated using ab initio molecular orbital theory (MP2/6-31G ∗ ). The reactions proceed by the [3+2] pathway via a concerted, planar and pseudopericyclic transition state with a significantly lower barrier rather than by the alternative [4+2]. The transition state asynchronicity can be used to explain the substituent effects on the cycloaddition of ketones with nitrosoketene.