Orbital interactions opening

The finding of the cyclic orbital interactions in non-cyclic conjugation opens a way to systematic understanding and designing of molecules and reactions in a unified manner. Here, we apply the orbital phase theory to non-cychc interactions of bonds, groups, molecules, cationic, anionic, and radical centers, lone pairs, etc. 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

The comparison of their results with data for the open-chain compounds dimethyldithio-carbonate, (CH3S)2C=0 (14)102 and dimethyltrithiocarbonate (CH3S)2C=S (15)103 sheds light on the influence of cyclation on the structure and orbital interactions of thiocarbonyl compounds. A particularly important feature is the decrease in C=0 and C=S bond lengths upon cyclation. This follows from the reduction of the S—C(sp2)—S angles... 

A highly Z-selective olefination of a-oxy and a-amino ketones via ynolate anions has been reported (Scheme 9).43 The stereocontrol mechanism has been explained by (g) orbital interactions between the s orbital of the breaking C-O bond or n orbital of the enolate and the s orbital of the C-O or C-N bonds of the substituent in the ring opening of the /I-lactone enolate intermediates, and/or the chelation to lithium. 

Tetrasubstituted alkenes (214) were obtained with high Z selectivity (>99 1) by reaction of ynolates (211) with a-oxy- and a-amino-ketones (212 X = OR, NR2) (g) at room temperature. According to experimental and theoretical studies, the high Z selectivity is induced by orbital interactions in the ring opening of the /3-lactone enolate intermediate (213), rather than by the initially presumed chelation of the lithium atom.260... 

The pronounced difference in the relative stability can be understood as a manifestation of molecular orbital interactions. The SOMO of the cyclobutene ion has high orbital coefficients on the internal sp2 hybridized carbons (—CH=), whereas the butadiene SOMO has the highest coefficients in the terminal ( = CH2) carbons. Accordingly, substituents in the l-(and 2-) position will stabilize the closed ion. On the other hand, substituents in the 3-(and 4-) position will favor the ring opened ion and lower the barrier to ring opening. 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

The two exceptions to the ortho-para rule are the reactions between an X-substituted diene and an X-substituted dienophile. The frontier orbitals in Fig. 6.27, taken from Fig. 6.22, indicate that the preferred combination, whichever pair of frontier orbitals and whichever diene, 1-substituted or 2-substituted, is taken, will lead to the meta adduct. Neither frontier orbital interaction is between orbitals close in energy (> 11 eV in all combinations), with the result that the reaction can be expected to be very slow, and is in practice exceedingly rare. One example is the combination of the (vinylogous) 2-X-substituted diene 6.158, derived from the benzcyclobutene by electrocyclic ring-opening, and ethyl vinyl ether 6.159. Although not usefully regioselective, the reaction does take place, assisted by the aromatisation of the diene, and the meta adduct is the major product. 

Houk employed the MO model shown in Scheme 4.17 to explain substituent effects in the ring opening of cyclohexadienes. The key orbital interaction is... 

Most of the time, the addition is predominantly endo that is, the more bulky side of the alkene is under the ring, and this is probably true for open-chain dienes also. However, exceptions are known, and in many cases mixtures of exo and endo addition products are found. ° ° An imidazolidone catalyst was used to give a 1 1.3 mixture favoring the exo isomer in a reaction of conjugated aldehydes and cyclopentadiene. It has been argued that facial selectivity is not due to torsional angle decompression. Secondary orbital interactions. ° have been invoked, but this approach has been called into question. ° " There has been a direct evaluation of such interactions, however. The endo/exo ratio can be influenced by the nature of the solvent. ... 

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

V. Aquilanti, S. Cavalli, D. De Fazio, A. Volpi, A. Aguilar, X. Gimenez, and J. Maria Lucas, Hyperquantization algorithm II. Implementation for the F+H2 reaction dynamics including open-shell and spin-orbit interaction. J. Chem. Phys., 109 3805-3818,1998. 

Denmark and Keck provide important evidence of the synclinal model as the preferred open transition state, and it has been proposed that attractive forces of secondary orbital interactions of the HOMO at the site of the stannyl methylene with the LUMO of the carbonyl oxygen may introduce a stabilizing condibution in this arrangement.i Further evidence of inherent advantages in the synclinal transition state is acquired from the efforts of Nishigaichi. These results demonstrate stereospecificity in the allylation reactions of (E)- and (Z)-3,3-dialkyl-substituted allyfic stannanes with a number of simple aldehydes. 

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