Theoretical and experimental studies of some unusual potential energy surfaces and pseudopericyclic reactions

Transition states are central to our understanding of the mechanism of reactions; especially in organic chemistry. This dissertation is aimed at understanding some unique transition states and energy surfaces. Both theoretical and experimental methods are used in this dissertation.

Tetrazines undergo inverse electron demand Diels-Alder reactions. UV and stopped-flow spectrophotometric studies were undertaken to study the rate of Diels-Alder cycloadditions with N,N-dipropynylamine. For the cycloaddition of 3,6-diphenyl-1,2,4,5-tetrazine the ?G‡ is 19.2 ± 1.0 kcal/mol and for 3,6-dimethyl dicarboxylate -1,2,4,5-tetrazine, ?G‡ is 11.5 ± 1.2 kcal/mol. These results were consistent with the DFT (density functional theory) calculations carried out at (B3LYP/6-31G(d,p) + ZPVE) level. Study of these revealed two interesting features of the calculated energy surfaces. First, there might be no barrier for the loss of nitrogen from the bicyclic intermediate; which in turn makes it a transition state. Thus there are two sequential transition states; one for the addition of a dienophile to tetrazine and the other for the loss of nitrogen from the bicyclic structure. This study extends Berson’s correlation of activation energy with reaction energy in pericyclic reactions to significantly lower barriers. Second, the additions of a dienophile to the pyridazines lead to an interesting and unique energy surface, which we termed "a corner".

An extension of sequential transition states to energy surfaces for chemical reactions that might have transition structures or states that are monkey saddles or higher order saddles (three or more valleys converging at a point) were also studied. At the transition structure, such a surface has second derivatives of the energy that are equal to zero and thus two frequencies that are equal to zero. These do not have the imaginary frequency associated with a one-dimensional saddle point. The Diels-Alder reactions between nitrogen and tetrazines were studied as possible reactions with monkey saddle transition states. Computationally, we have shown that these reactions go through a monkey saddle transition states at some levels of theory. For 12-annulene, a D3 conformation is calculated to be a monkey saddle transition structure at the BH&HLYP/3-21G level. For the addition of iodide to COI2, the C3v structure is calculated to be a monkey saddle transition state at the RHF/3-21G level.

A theoretical study (B3LYP and G3MP2B3) of the dimerization of thioformylketene was performed. Four pathways, two [4 + 2] pathways and one [4 + 4] pathway with thioformylketene and one [4 + 2] pathway involving thioformylketene and thietone were also considered. Interestingly, the [4 + 4] pathway with thioformylketene had the lowest barrier among all dimerizations. The geometry of the transition state is unusual, with the forming bonds in the plane of the ketene. This reaction is best interpreted as pseudopericyclic reaction.

The electrocyclic ring closure of a vinylogous form of an ?-oxoketenes ((6Z)-6-(3-oxoallylidene)cyclohexa-2,4-dienone) was studied theoretically at B3LYP/6-31G(d,p) level. The dienone cyclizes without a barrier. Constrained optimization, NBO (natural bonding orbital) analysis, NICS (nucleus independent chemical shift) values were determined to understand and classify the nature of the reaction as either a pericyclic / pseudopericyclic or possibly both.

Finally, generation and trapping of ?-oxoketenes using novel synthetic methods were also attempted.