Woodward-Hoffmann’s rules

The Excited States of Poly(vinyl cinnamate) and their Photochemical Reactivities. The photochemical reaction of PVCi is believed to be Intermolecular crosslinking of the polymer chains by the formation of the four-membered ring from the central double bonds of cinnamoyl-oxy groups( ), and it is known that many experimental results from the photochemical reaction are interpreted well(6) if we can postulate that the reaction is the concerted cycloaddition according to the Woodward-Hoffmann s rule(lB). This means that the four-membered ring... 

The state function of ifo. The LE function of ethylene contributes mainly to S2(about 21 ). The result accords with the fact that the experimental data on the photochemical reaction of PVCi are explained on the bases of Woodward-Hoffmann s rule. CT configurations from the occupied MO(OMO) of the ground state of benzene to the unoccupied MO(UMO) of ethylene and the OMO of ethylene to the UMO of benzene increase 16.6 % and 4.6 respectively, compared with So.(Pig.6) These contributions decrease the bond order of the central double bond in S2. In S- the contributions of the LE of ethylene and CT are small... 

The state function of Ti. From the results from PPP calculations in Table 2, we can find out that the spatial part of the state function of Ti approximately corresponds to that of Sp and the contribution oftA Y and A shows the mixing of S. From the results from MIM methoatTable 3i, the contribution of the LE of ethylene extremely increases in T- compared with that in S2. The fact suggests that T- is highly photosensitive on the basis of the concerted cycloaddition following the Woodward-Hoffmann s rule. 

MIM method Table 3), the contribution of the LE of ethylene extremely increases in T- compared with that in S2. The fact suggests that T-) is highly photosensitive on the basis of the concerted cycloaddition following the Woodward-Hoffmann s rule. 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate) [131, 132], They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffmann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO (HOMO) of the grotmd state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the cmicerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2 [132]. 

Criteria and guidelines useful in network elucidation and supplementing the rules derived in this chapter include considerations of steric effects, molecularities of postulated reaction steps, and thermodynamic constraints as well as Tolman s 16- or 18-electron rule for reactions involving transition-metal complexes and the Woodward-Hoffmann exclusion rules based on the principle of conservation of molecular orbital symmetry. Auxiliary techniques that can be brought to bear include, among others, determinations of isomer distribution, isotope techniques, and spectrophotometry. 

The metal s role in cycloaddition processes can be extended beyond [2 -)-2]. The symmetry of the atomic orbital used by the metal to donate or withdraw the electron pair will be dictated by the Woodward-Hoffmann symmetry rules b. As in the [2- -2] case, the metal is looked upon as a pseudo-organic participant. When an orbital reacting in a suprafacial way is called for by the symmetry rule, a metal symmetric orbital is used. Similarly, when an antarafacial participant is called for, a metal antisymmetric orbital is used. In the [2 -f-2 -f-2] cycloaddition process, for example, where the ligand participants must react along the suprafacial mode, the... 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

The Woodward-Hoffmann rules have intellectual roots that can be traced back to Wigner-Witmer correlation rules (E. Wigner and E. E. Witmer, Z. Phys. 51 [1928], 859) and general correlation-diagram concepts (R. S. Mulliken, Rev. Mod. Phys. 4 [1932], 1) as employed, e.g., by K. F. Herzfeld, Rev. Mod. Phys. 41 (1949), 527. Alternative MO... 

Roald Hoffmann, a former coworker of R.B. Woodward and Nobel Prize as well for his contribution to the frontier orbital theory (the famous Woodward-Hoffmann rules concerning the conservation of molecular orbital symmetry), has also emphasised the artistic aspects of organic synthesis "The making of molecules puts chemistry very close to the arts. We create the objects that we or others then study or appreciate. That s exactly what writers, visual artists and composers do" [15a]. Nevertheless, Hoffmann also recognises the logic content of synthesis that "has inspired people to write computer programs to emulate the mind of a synthetic chemist, to suggest new syntheses". 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

The SC descriptions of the electronic mechanisms of the three six-electron pericyclic gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition offiilminic acid to ethyne [12], and the disrotatory electrocyclic ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman model [4-6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is very much easier to interpret and to visualize directly. 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

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