Conrotatory, Woodward-Hoffmann rules

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 direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

The conrotatory stereochemistry fits the Woodward-Hoffmann rule, illustrated 4.81 as [dashed lines could equally have been drawn to make it [02a+w2J. 

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

Since the detailed calculation of these matrices is sufficiently described in the original literature [33, 58], it is possible to present directly the final results first for the case of concerted reactions for which there are two alternative reaction mechanisms, conrotatory and disrotatory. The first of these mechanisms is allowed by the Woodward-Hoffmann rules while the second one is forbidden. 

If we now look at the values of the above indices, it is possible to see that the prediction of the Woodward-Hoffmann rules is indeed confirmed since the greater values of the similarity index for the conrotatory reaction clearly imply, in keeping with the expectations of the least-motion principle, the lower electron reorganisation. If now the same formalism is applied to a stepwise reaction mechanism, the following values of the similarity indices result (Eq. 21). 

What made his proposal so convincing was that the stereochemistry of the endiandric acid D is just what you would expect from the requirements of the Woodward-Hoffmann rules. The first step from the precursor is an 87t electrocyclic reaction, and would therefore be conrotatory. 

The conrotatory ring opening predicted by correlation diagrams based on the Woodward-Hoffmann rules for cyclopropyl anions have their conterpart in the isoelectronic aziridines. The interconversions outlined below were investigated by Huisgen et and photochemical ring opening was shown to follow a... 

Apply the Woodward-Hoffmann rules to the electrocyclic reaction of hex-atriene to cyclohexadiene considering the appropriate Hiickel MO s. Determine whether the mechanism is conrotatory or disrotatory for both thermal and photochemical reactions. 

The Woodward-Hoffmann rules for electrocyclic reactions can also be formulated using the terms suprafacial and antarafacial (Table 4.3). A it system is said to react suprafacially in a pericyclic reaction when the bonds being made to the two termini of the it system are made to the same face of the 77 system. It reacts an-tarafacially when the bonds are made to opposite faces of the 7r system. In electrocyclic reactions, disrotatory reactions are suprafacial, and conrotatory reactions are antarafacial. 

In contrast to pentadienyl anions, heptatrienyl anions cyclize readily to cyclohepta-dienyl anions. The transformation of heptatrienyl anion to cycloheptadienyl anion proceeds with a half-life of 13 min at —13°C. The Woodward-Hoffmann rules predict that this would be a conrotatory closure. ... 

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