Orbital mixing reactions

The orbital mixing rules are described in detail and shown to be powerful for understanding and designing selective reactions in Chapter Orbital Mixing Rules and applied in chapter ji-Facial Selectivities of Diels-Alder reactions . 

According to the frontier orbital theory, a bond preferentially forms between the atoms with the largest frontier orbital amplitudes (Sect. 3.4 in the Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). This is applicable for the regioselectivities of Diels-Alder reactions [15]. The orbital mixing rules are shown here to be useful to understand and design the regioselectivities. 

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. 

The orbital mixing rules and their chemical consequences have been ongoing for more than 30 years and have provided impact on the studies of molecular properties and chemical reactions [53-56],... 

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

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... 

Inagaki, Fujimoto and Fukui demonstrated that ir-facial selectivity in the Diels-Alder reaction of 5-acetoxy- and 5-chloro-l,3-cyclopentadienes, 1 and 2, can be explained in terms of deformation of a frontier molecular orbital FMO [2], The orbital mixing rule was proposed to predict the nonequivalent orbital deformation due to asymmetric perturbation of the substituent orbital (Chapter Orbital Mixing Rules by Inagaki in this volume). 

The cyclopentadiene having substituents of C CR and C N are typical examples of the former case ,i homo)- Theoretical calculation showed that the diene 13 is expected to react with highly syn tr-facial selectivity [15], Experimental smdy of the selectivity in the reactions of the pentamethylcyclopentadiene derivatives 18-21 is particularly of interest (Scheme 11). Exclusive formation of syn attack product in the reaction of the diene 18 is well consistent with the prediction [16]. In the case of the dienes 19-21, the efficiency of the orbital mixing mediated by or orbital was dependent on the conformation of the substituents. However, the selectivity observed was well consistent with the theory. In the reactions of the dienes 19 and 20 where considerable formation of syn... 

Halterman et al. reported that 5-aryl-5-phenylcyclopentadienes 23-25 reacted with dienophiles to favor the reactions on the anti side of the more electron rich aromatic system [19]. The orbital mixing rule failed to predict this selectivity, since orbital mixing is expected to take place mainly by mediation of the JtAr-HOMo of more electron rich aromatic system (Scheme 13). Destabilization due to the orbital phase environment or stabilization due to Cieplak effects can be responsible for the selectivity (See Sects. 2.1.2 and 2.1.3). 

They pointed out that the results are consistent with an explanation based on their steric hindrance, although the orbital mixing rule already predicted the deformation of the FMO of 85 to favor the reaction at the syn side [2]. 

The syn addition mode was also confirmed by ab intio calculation of the reaction between thiophene 1-oxide 99 and ethylene. They stated that the selectivity can be explained by the orbital mixing rule (Scheme 51). The ir-HOMO of the diene part of 99 is modified by an out-of-phase combination with the low lying n-orbital of... 

They reported that the DFT calculations of 114 at the B3LYP/6-31G level showed that the ji-HOMO lobes at the a-position are slightly greater for the syn-n-face than for the anti face. The deformation is well consistent with the prediction by the orbital mixing rule. However, the situation becomes the reverse for the Jt-LUMO lobes, which are slightly greater at the anti than the syn-n-face. They concluded that the iyn-Jt-facial selectivity of the normal-electron-demand Diels-Alder reactions... 

The second fixation reaction that has been the subject of theoretical study is the nickel(0)-catalyzed coupling reaction of C02 with acetylene (equation 1), which was theoretically investigated with the ab initio SD-CI method [25]. The theoretical calculations clearly showed that if the nickel(O) moiety was eliminated from the reaction system, the activation barrier increased very much, and that the C-C bond formation between C02 and acetylene was accelerated by the charge-transfer from the nickel(O) d orbital to the orbital resulting from the combination between n orbitals of acetylene and CO2. Actually, the HOMO contour map of Ni(PH3)(C02)(C2H2) clearly displays this orbital mixing in the transition state. 

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