Woodward—Hoffmann orbital symmetry

This is one of the observations that led to the Woodward-Hoffmann orbital symmetry rules.86... 

Woodward-Hoffmann orbital symmetry rules (Section 30.9) a series of rules for predicting the stereochemistry of pericyclic reactions. Even-electron species react thermally through either antarafacial or conrotatory pathways, whereas odd-electron species react thermally through either suprafacial or disrotatory pathways. 

The introduction of Valence Bond theory has motivated the search for structural regularities that can be interpreted by models of local electronic features, such as the powerful model of Valence Shell Electron Pair Repulsion [93,94] theory. Alternative approaches, based on Molecular Orbital theory, have led to the discovery of important rules, such as the Woodward-Hoffmann orbital symmetry rules [95] and the frontier orbital approach of Fukui [96,97], As a result of these advances and the spectacular successes of ab initio computations on molecular... 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

Woodward-Hoffmann orbital symmetry arguments and molecular orbital considerations allow for a distinction between the two possible paths for sigmatropic [1,7] suprafacial methyl migration (transition states 31 and 32), with 31 having the lower energy for the same reason... 

The Woodward-Hoffmann orbital symmetry rules are not limited in application to the neutral polyene systems that have been discussed up to this point. They also apply to charged systems, just as the Htickel aromaticity rule can be applied to charged ring systems. The conversion of a cyclopropyl cation to an allyl cation is the simplest... 

The ring-chain tautomerism between cyclobutene and butadiene is perhaps the most famUiar example of an allowed Woodward-Hoffmann process. This transformation invariably is discussed in every attempt to rationalize or teach the Woodward-Hoffmann orbital symmetry concepts. This popularity is due in large part to the existence of a geometrically well defined (and easily visualized) alternate, forbidden, electrocyclic pathway. Thus it is exceedingly simple to set up a nonaUowed strawman, the disrotatory ring opening, and... 

Woodward—Hoffmann orbital symmetry rules can be applied to the charged systems as well. The conversion of a cyclopropyl cation to an allylic cation is the simplest one, which involves only 27r-electrons (Figure 2.13). This is an electrocyclic reaction of (4n + 2) type (n = 0) and should, therefore, be a disrotatory process. 

In a sense, the shape and relative phase of molecular orbitals are a physical observable because these influence reaction mechanisms as described by the Woodward-Hoffmann orbital symmetry rules. One can see what individual MOs look like either by computing electron density or orbital maps, which is fairly easy with modern programs, or by manually examining the LCAO-MO coefficient matrix. To interpret this matrix, one needs to know how the molecule is oriented in the coordinate system (Table 4). Knowing this, the orbital lobes can be sketched in a way consistent with the signs of the coefficients (Figure 5). Such sketches show qualitatively what the MOs look like. In ac-... 

The kinetics of the reaction between iodine and bromine to form iodine monobromide in concentrated sulphuric acid (96% HaSOJ indicate a termolecular process, which is consistent with Woodward-Hoffmann orbital-symmetry considerations. The suggested transition state is cyclic (28). ... 

For Cp, it is impossible to distinguish between a Woodward-Hoffmann orbital symmetry-allowed 1,5 shift and a least-motion 1,2 shift because they are equivalent. In an ri -CyHy system, the two cases are distinguishable, Woodward-Hoffman giving a 1,4 and least motion a 1,2 shift. A similar analysis shows that (r -C7H7)Re(CO)5 follows a least motion and T] -C7H7SnMe3 a Woodward-Hoffmann path. 

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