Orbital Phase Environments

Keywords Chemical orbital theory. Electron delocalization. Frontier orbital. Orbital amplitude, Orbital energy, Orbital interaction. Orbital mixing rule, Orbital phase, Orbital phase continuity, Orbital phase environment. Orbital synunetry, Reactivity, Selectivity... 

Scheme 26 Endo-selectivity of the Diels-Alder reactions and orbital phase environments...

Scheme 12 Orbital phase environment in the EHels-Alder reactions of acetylenic aldehydes exo-selectivity...

The orbital mixing theory was developed by Inagaki and Fukui [1] to predict the direction of nonequivalent orbital extension of plane-asymmetric olefins and to understand the n facial selectivity. The orbital mixing rules were successfully apphed to understand diverse chemical phenomena [2] and to design n facial selective Diels-Alder reactions [28-34], The applications to the n facial selectivities of Diels-Alder reactions are reviewed by Ishida and Inagaki elesewhere in this volume. Ohwada [26, 27, 35, 36] proposed that the orbital phase relation between the reaction sites and the groups in their environment could control the n facial selectivities and review the orbital phase environments and the selectivities elsewhere in this volume. Here, we review applications of the orbital mixing rules to the n facial selectivities of reactions other than the Diels-Alder reactions. 

Deformation of symmetrical orbital extension of carbonyl or olefin compounds was proposed to be the origin of the facial selectivities. We illustrate the unsymmetrical orbital phase environment of % orbitals of carbonyl and olefin groups and facial selectivities in Fig. 1 [3, 4]. There are in-phase and out-of-phase combinations of... 

Fig. 1 Unsymmetrical orbital phase environment and preferential attack of reagents...

The SOI concept is akin to the unsymmetrization of orbitals. The only difference is in the sites of the subsidiary interactions, which occur between the non-reacting centers (positions 3 and 4 in Fig. 3a) in SOI and between the reacting and non-reacting centers (sites 2 and 3 in Fig. 3b) for the unsymmetrization of orbitals (Fig. 1). The orbital phase environment around the reaction centers is a general idea... 

In this review we will focus on the unsymmetrization of the orbital phase environment in the vicinity of reacting n systems, and its effect on facial selectivities. This idea can be applied to many kinds of recently observed facial selectivities, such as those involving ketones [10-21], olefins [22-31], dienes [32-46] and others [47-49]. 

Orbital Phase Environment Unsymmetrization of Carbonyl r Orbitals by Interaction with ji-a Orbitals... 

The carbonyl n face of the adamantan-2-one with an electron-withdrawing group at the 5-position is nnsynunetrized by interaction of the P o bonds antiperiplanar to the C-H bonds and to the C-R bond. The orbital phase environment of the carbonyl n orbital (7) is nnsynunetrized by the more electron-donating orbitals at the P-position, which is consistent with the observed syn preference. 

The unsymmetric n face of carbonyl groups is postulated to be attributable to orbital interactions between a o-fragment and a tt-fragment. Interactions between two 7t fragments in a carbonyl molecule can also lead to an unsymmetrical orbital phase environment [3]. 

This reviews contends that, throughout the known examples of facial selections, from classical to recently discovered ones, a key role is played by the unsymmetri-zation of the orbital phase environments of n reaction centers arising from first-order perturbation, that is, the unsymmetrization of the orbital phase environment of the relevant n orbitals. This asymmetry of the n orbitals, if it occurs along the trajectory of addition, is proposed to be generally involved in facial selection in sterically unbiased systems. Experimentally, carbonyl and related olefin compounds, which bear a similar structural motif, exhibit the same facial preference in most cases, particularly in the cases of adamantanes. This feature seems to be compatible with the Cieplak model. However, this is not always the case for other types of molecules, or in reactions such as Diels-Alder cycloaddition. In contrast, unsymmetrization of orbital phase environment, including SOI in Diels-Alder reactions, is a general concept as a contributor to facial selectivity. Other interpretations of facial selectivities have also been reviewed [174-180]. 

Keywords it-Facial selectivity, a/ir Interaction, CH/ir Interaction, Ciplak effect, Diels-AIder reaction, Electrostatic interaction, Orbital mixing rule. Orbital phase environment, Secondary orbital interaction, Steric repulsion, Torsional control... 

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