Theoretical studies of electronic effects on structure and reactivity of highly unsaturated organic systems

Abstract

The thesis entitled “Theoretical Studies of Electronic Effects on Structure and Reactivity of Highly Unsaturated Organic Systems” involves the elucidation of molecular and electronic structures, energetics, and reactivity patterns in organic molecules containing multiple unsaturations. Various computational methods-ranging from semiempirical approaches to ab initio and DFT procedures-have been employed. The systems examined are uniformly of considerable experimental interest. The first chapter provides a general introduction about the importance of molecular and electronic structures and their significance in understanding energetics and reactivity. The use of computational procedures for deriving useful insights is highlighted. A brief summary of the methodologies employed in this work, including some technical details, is provided. In Chapter II, the possibility of designing planar cyclooctatetraene (COT) derivatives is explored. Usually, the planar form represents a high?energy transition state associated with ring inversion and bond?shift isomerization. The classical angle?strain effect, as well as the electronic factors associated with the ??framework, is modulated to derive new substrates (such as 1 and 2) with potential for planarization. Fusion with one or more four?membered rings reduces the barrier for planoid distortion. The relative energies of various minima and saddle points have been characterized at the B3LYP/6?31G* // HF/3?21G level. Incorporation of an additional double bond in the smaller ring leads to a highly stabilized planar structure with a 10?? aromatic periphery. In the third chapter, [0_n]paracyclophanes (n = 5 and 6), which are of topical interest in molecular recognition and fullerene chemistry, have been investigated. The extent of ??conjugation, the rotational barrier on twisting a single phenyl ring, and the aromatic character in each ring as well as in the whole molecule have been evaluated. [0?]Paracyclophane is predicted to have a quinonoid structure, whereas [0?]Paracyclophane possesses benzenoid character. Approximate band structures have been derived for larger cycles of [0?]paracyclophanes. The energetics of three competing low?energy isomers of the smaller fullerene C?? (D?h, D?d, and C?v forms) in their singlet and triplet states have been computed. The most stable structure is predicted to be the D?h isomer with a triplet ground state of ³A?u symmetry. This electronic structure accounts for the “covalent” interactions noted in solid C??. Dimerization is calculated to be energetically favorable. Important changes in vibrational and electronic spectra resulting from dimerization are predicted. In the next chapter, a simple method (AM1/PECI = 8) is identified for studying relative rates of Bergman cyclization, based on comparisons with experimental data and high?level ab initio calculations. It is demonstrated that the computational procedure reproduces several experimentally observed trends in complex substrates. For example, variations in rates upon changing the terminal distance within the enediyne moiety and modifications in the hybridization at a remote center are correctly reproduced. The unexpected rate reduction recently observed upon rehybridization is also consistent with the energetics obtained from the semiempirical procedure with limited configuration interaction (CI). Interestingly, a remarkable dependence of Bergman cyclization rate on the orientation of an oxo?substituent at a remote sp³ center is predicted. In the fifth chapter, electronic effects on the mode of cyclization of enediyne radical cations (3–6) have been investigated. The influence of various substituents on the preferred cyclization mode has also been examined. Experimentally, some enediyne radical cations undergo 1,5?cyclization rather than the classical Bergman (1,6) cyclization. To understand the electronic origins of this behavior, computations on both cyclization pathways for the parent enediyne radical cation 3 were performed. The state symmetries of the reactants, products, and transition states are found to control the preferred mode of cyclization. Calculations on substituted enediyne radical cations 4–6 reveal a reversal in frontier orbital sequence, rationalizing the likely cyclization mode. In the sixth chapter, the effect of remote substituents on ??facial selectivity in cycloadditions involving rigid polycyclic dienes (7–9) has been unraveled using AM1 calculations with various model and realistic dienophiles. Transition states for attack from both ??faces of the diene have been located precisely for all substrates. Computed results are generally in good agreement with experimental observations. Subtle changes in functionality within the polycyclic diene lead to remarkable variations in facial selectivity of different dienophiles. The observed trends are attributed to direct through?space interactions between remote substituents and the approaching reagent via three distinct modes:

Electrostatic interactions Hydrogen bonding Stabilizing orbital interactions

It is suggested that such stereoelectronic effects need to be considered more generally when interpreting ??facial selectivity.