A series of eight thermal cheletropic decarbonylations show dramatic differences in reaction pathways and in activation energies depending on the molecular orbital topology, as calculated by using ab initio molecular orbital theory (MP2(FC)/6-31G* optimized geometries and MP4/D95** + ZPE single point energies). The decarbonylations of 3-cyclopentenone (1) and bicyclo[2.2.1]hepta-2,5-diene-7-one (3) are pericyclic, orbital symmetry allowed reactions, but it is argued that the decarbonylation of cyclopropanone (9), although formally orbital symmetry allowed, lacks an energy of concert and thus is “effectively forbidden”. The carbon monoxide produced from 1 is predicted to be formed vibrationally cool and rotationally hot. Fragmentations of 2,3-furandione (5) and 2,3-pyrroledione (7) are pseudopericyclic reactions with two orbital disconnections, proceed via planar transition structures, and have activation energies that are much lower than expected for pericyclic reactions of comparable exothermicity. It will be an experimental challenge to determine if the carbon monoxide product from each of these is formed with little vibrational or rotational excitation as predicted. Fragmentations of 3H-furan-2-one (11), 3-cyclopentene-1,2-dione (13), and 3-methylene-3H-furan-2-one (15) each have a single disconnection. Strong bonding at the orbital disconnection in the transition structure tends to lower the barrier and give the reaction more pseudopericyclic character.

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It has been proposed that all pseudopericyclic reactions are allowed, regardless of the number of electrons involved. 5-Oxo-2,4-pentadienal (1) is a vinylog of formylketene, and thus its reactions provide a test of this proposal. Ab initio (MP4(full, SDQ)/D95**//MP2/6-31G* + ZPE) calculations were carried out on all the conformations and several reactions of 1, as well as related systems. One of the eight possible conformations of 1 (zZz1) does not exist, but closes without a barrier, via a pseudopericyclic pathway to pyran-2-one (3). IR spectra of 1 in Ar matrices are consistent with three conformations (zZe1, eZz1, and eZe1). Calculated rotational barriers from zZe1 and eZz1 to 3 are consistent with observed kinetics for the of decay of 1. The structures of s-Z- and s-E-3-hydroxy-1,2-propadien-1-one (18 and 19) and the vinylogous cumulene 16 are predicted to have strongly bent geometries. The barrier to internal proton transfer from 16 is only 0.8 kcal/mol, while the barrier from 18 is 23.7 kcal/mol. Barrier heights for these and other pseudopericyclic reactions are shown to correlate with (1) the nucleophilicity and electrophilicity of the reacting centers, (2) the exothermicity of the reaction, and (3) deviations from the ideal geometries for the reaction. Significantly, the barrier heights do not correlate with the number of electrons involved in the reaction.

Ring openings of 2-furylcarbene (5a), 2-pyrrolylcarbene (5b), 1-cyclopenta-1,3-dienylcarbene (5c), and of 11, as well as of the 3-and five-membered ring vinylogs (7a, 7c, 9a, 9c) were investigated at the B3LYP/6-31G(d,p) level of theory. The reactions of 5a, 5c, and 11 were also studied at the G2(MP2) level. This work provides the first comparison of pseudopericyclic and coarctate orbital topologies in concerted reactions. The ring openings of 5a, 5b, 11 and their vinylogs have pseudopericyclic (but not coarctate) topologies, while the ring openings of 5c and vinylogs are pericyclic and can be described as coarctate. The transition structure geometry for the ring opening of 5c is not well described by MP2 theory. The G2(MP2) barrier for this ring opening is estimated to be 6.4 kcal/mol, significantly higher than that calculated by Sun and Wong. The ring opening is an eight-electron, Mo ¨bius, conrotatory process, not the six-electron, Huckel process described by Herges in his original work describing coarctate orbital topology. The vinylogous reactions of 7c and 9c are disrotatory. The triplet photochemistry of 3a and 3b, as studied by Nakatani et al., was modeled by 6a and 12, respectively. The difference between the two is attributed to the greater stability of the triplet carbene 3 11 as compared to 3 syn-5.

Acetylketene (1) was generated by flash pyrolysis of 2,2,6-trimethyl-4H-1,3-dioxin-4-one (6). The selectivities of 1 toward a number of representative functional groups were measured for the first time in a series of competitive trapping reactions. The trend in reactivities toward 1 follows the general order amines > alcohols >> aldehydes approximately ketones and can be rationalized by considering both the nucleophilicity and the electrophilicity of the reacting species. Alcohols show significant selectivity based on steric hindrance, with MeOH approximately 1 degrees > 2 degrees > 3 degrees. These selectivities are consistent with the activation energies and the pseudopericyclic transition structure previously calculated for the addition of water to formylketene. The results, presented here, of ab initio transition structure calculations for the addition of ammonia to formylketene are qualitatively consistent with the experimental trends as well. N-Propylacetacetimidoylketene (2) was generated by the solution pyrolysis of tert-butyl N-propyl-3-amino-2-butenoate (9a) and showed similar selectivity toward alcohols as opposed to ketones and similar steric discrimination toward alcohols. This is again in agreement with previous ab initio calculations. Taken together, these experimental trends in the reactivities of both 1 and 2 toward a variety of reagents provide strong, although indirect support for the planar, pseudopericyclic transition structures for these reactions which are predicted by ab initio calculations.

The reactions of vinylketene (1a), imidoylketene (1b), and formylketene (1c) with formaldimine (2) were studied at the B3LYP/6-31G* level. For the cycloadditions of these conjugated ketenes with 2, several possible pathways to both [4 + 2] and [2 + 2] products were examined. The lowest energy [2 + 2] pathways are, in most cases, calculated to be stepwise, forming the products via rate-determining conrotatory electrocyclization of zwitterionic intermediates. However, concerted transition structures analogous to the ketene plus ethene [2 + 2] cycloaddition reaction were also located; the existence of multiple transition states offers a resolution to a long-standing controversy regarding the mechanism of ketene plus imine cycloadditions. Both stepwise and concerted [4 + 2] pathways were calculated for 2b and for 2c; both these pathways are pseudopericyclic. The inherently low barriers associated with pseudopericyclic transition states provide an explanation of the experimental preference for [4 + 2] cycloadditions of alpha-oxoketenes and predict [4 + 2] cycloadditions should also be favored for imidoylketenes. For a vinylketene constrained to a Z-geometry, the concerted [4 + 2] cycloaddition is also predicted to be the lowest energy pathway. An explanation is offered for the unusual thermal equilibration from a six-membered ring (3d) to a four-membered ring (4d) observed by Sato et al. Transition structures for facile pseudopericyclic 1,3- and 1,5-hydrogen shifts in the zwitterions were also calculated.

Allowed by orbital symmetry rules, the [3,5] sigmatropic ester rearrangement of ester 1 even has a lower barrier than a competing [3,3] rearrangement, according to ab initio calculations. This surprising result is a consequence of the lack of orbital overlap in the pseudopericyclic transition state.