|dc.description.abstract||Several organic reactions that could potentially involve coarctate transition states
were investigated by a combination of experimental and theoretical studies.
In the thermal fragmentation of Ã¢ÂÂ-1,3,4-oxadiazolines, the mechanism supported by
kinetic isotope effects and theoretical calculations is a three-step process that does not
demonstrate any special stabilization in coarctate transition states. Rather than
undergoing a direct coarctate conversion to product, the mechanism avoids coarctate
steps. The last step is a concerted coarctate reaction, but being concerted may be viewed
as being enforced by the necessity to avoid high-energy intermediates.
In the deoxygenation of epoxides with dichlorocarbene, the stabilization from the
transition state aromaticity is not great enough to compete with the preference for
asynchronous bonding changes. KIEs and calculations suggested that the reaction occurs
in a concerted manner but with a highly asynchronous early transition state with much
more CÃÂ±-O bond breaking than CÃÂ²-O bond breaking. In the Shi epoxidation, a large ÃÂ²-olefinic 13C isotope effect and small ÃÂ±-carbon
isotope effect indicated an asynchronous transition state with more advanced formation
of the C-O bond to the ÃÂ²-olefinic carbon. The calculated lowest-energy transition
structures are generally those in which the differential formation of the incipient C-O
bonds, the "asynchronicity," resembles that of an unhindered model, and the imposition
of greater or less asynchronicity leads to higher barriers. In reactions of cis-disubstituted
and terminal alkenes using Shi's oxazolidinone catalyst, the asynchronicity of the
epoxidation transition state leads to increased steric interaction with the oxazolidinone
when a ÃÂ-conjugating substituent is distal to the oxazolidinone but decreased steric
interaction when the ÃÂ-conjugating substituent is proximal to the oxazolidinone.
Dynamic effects were studied in Diels-Alder reaction between acrolein and methyl
vinyl ketone. This reaction yields two products in a ratio of 3.0 ÃÂ± 0.5. Theoretical studies
shows that only one transition structure is involved in the formation of both.
Quasiclassical trajectory calculations on an MP2 surface give a prediction of a product
ratio of 45:14 (3.2:1), which is in good agreement with the experimental observation.||en_US