Cyclization Woodward-Hoffmann Rules

Scheme 2. Thermally allowed 8 n electron and 6 k electron electro-cyclizations (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). 

The cyclization of conjugated polyenes and the inverse reaction were those processes which provided superb materials 142> leading to the Woodward-Hoffmann rule 51>. 

Similarly, electrocyclic reactions7 follow the Woodward-Hoffmann rules and proceed in either a con- or disrotatory manner. For example, the diene 5 cyclizes to the cyclobutene 6 or 7, which are enantiomers. No diastereomers with R1/Ri trams geometry are formed. 

Among the many types of reactions which may be treated by Woodward-Hoffmann rules, cyclizations in which open-chain olefins are converted, either thermally or photochemically, to cyclic species are especially important and can serve well to illustrate the principles involved in this type of analysis. We... 

Whether the reaction is supra- or antarafacial ought to be reflected in the relative stereochemistry of the cyclized products—and indeed it is. This reaction gives solely the diastereoisomer on the left, with the methyl groups syn—clear proof that the reaction is suprafacial. This is a difficult result to explain without the enlightenment provided by the Woodward-Hoffmann rules ... 

Some thermal cyclization reactions of allene derivatives in their own crystals are described. Intramolecular cyclization of diallene derivatives within their crystals proceeds stereoselectively according to the Woodward-Hoffmann rule. For example, heating of colorless crystals of meso- 19 and rac-diallene 23 at 150 °C for 1 h gave the in,out 22 and a 1 1 mixture of in,in 26 and out,out 27 cyclization products, respectively, in quantitative yields (Scheme 2.2.1) [14]. Since no liquid state was observed during the cyclization reactions, the reaction in the crystal is a real crystal-to-crystal reaction. An X-ray analysis showed that 19 and 23 have an s-trans configuration in the crystal [15], In order to cyclize to 22, and to 26 and 27, 19 and 23 should first isomerize to their s-cis-isomers 20 and 24, respectively, in the crystal. The conformational change, from the s-trans form to the s-cis form, requires... 

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. ... 

This cycloaddition is thermally forbidden under the Woodward-Hoffmann rules, but can be achieved by the transition metal catalysed coupling of two dienes (Scheme 11.55). The transformation of butadiene into a variety of products, including cyclooctadiene 11.163, has been known for many years. The [4 + 4] cycloaddition reaction has been applied in an intramolecular fashion to form eight-membered rings (Scheme 11.56). In the presence of a source of nickel(O) and phosphine ligands, tetraene 11.164 cyclizes to the bicyclic product 11.165, mainly as the cu-isomer. The ratio of isomers is dependent on the phosphine. A stereogenic centre present in the tether between the two dienes can result in useful diastereoseleetivity, once again dependent on the phosphines. This chemistry has been used to synthesize asteriscanolide 11.166, which contains a substituted cyclooctane, by intramolecular cycloaddition of tetraene 11.168 to... 

Electrocyclization provides an efficient way to synthesize a large variety of heterocyclic compounds. Among the many photocyclization reactions of aromatic compounds, electrocyclization is among the most frequently studied. An example of this is that illustrated in Scheme 1. This involves cyclization between the arene moieties of the protonated azobenzene 1, which reacts from the jtJt state reaction and is controlled by the Woodward-Hoffmann rules. Six rr-electrons are involved in the conrotatory cyclization. The reaction was not observed when unprotonated azobenzene was irradiated because in this case, the ntt state is populated. Since the photochemical electrocyclization is reversible, a consecutive trapping reaction is needed to obtain the final products in good yields. In the present case, as in many other reactions, a rearomatization step via oxidation took place to afford the final product 2. In the same way, the imine 3 can be cyclized to the phenanthridine derivative 4. In this case, the oxidation of the... 

The classes of cyclization reactions are important, not because we have a compulsive Victorian desire to classify everything, but because which class a reaction falls into determines whether or not it is likely to work. Not all cyclizations are successful, even though they may look fine on paper The guidelines that describe which reactions will work are known as Baldwin s rules they are not really rules in the Woodward-Hoffmann sense of the term, but more empirical observations backed up by some sound stereoelectronic reasoning. To emphasize this, the rules are couched in terms of favoured and disfavoured , rather than allowed and forbidden . We will deal with the rules step by step and then summarize them in a table at the end. 

The open-ring isomer of DTE has parallel and antiparallel conformations which are in dynamic equilibrium. The closing process follows the rules according to Woodward-Hoffmann, in which the photocyclization occurs, via a con-rotatory mechanism, only from an antiparallel conformation of the two thienyl rings. Eor unsubstituted DTE derivatives, the ratio of molecules in the parallel and antiparallel conformations is close to 1 1 and the cyclization quantum yield therefore cannot exceed 0.5. The photoreversion is significantly less efficient than the photocyclization, in most cases the quantum yield values typically not exceeding 0.1. 

The cyclization of fully conjugated polyenes containing 2n + 2 jr-electrons (equation 1) was termed electrocydie by Woodward and Hoffmann, who showed that the steric course of such reactions was governed by the rules of orbital symmetryI. 3. 

The symmetry-based selection rules for cyclizations and other rearrangements and reactions of oiganic molecules developed by Woodward and Hoffmann are covered in detail in their book ... 

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