Of electrocyclic

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

An orbital correlation diagram can be constructed by examining the symmetry of the reactant and product orbitals with respect to this plane. The orbitals are classified by symmetry with respect to this plane in Fig. 11.9. For the reactants ethylene and butadiene, the classifications are the same as for the consideration of electrocyclic reactions on p. 610. An additional feature must be taken into account in the case of cyclohexene. The cyclohexene orbitals tr, t72. < i> and are called symmetry-adapted orbitals. We might be inclined to think of the a and a orbitals as localized between specific pairs of carbon... 

A striking illustration of the relationship between orbital symmetry considerations and the outcome of photochemical reactions can be found in the stereochemistry of electrocyclic reactions. In Chapter 11, the distinction between the conrotatory and the disrotatory mode of reaction as a function of the number of electrons in the system was... 

The possibility of electrocyclic valent tautomerism of the dithiins 55 with the product of their ring opening, but-2-ene-l,4-dithione (56), has been considered (85KGS1443 96T12677). However, all the experimental data provide evidence for the dithiyne form 55 (85KGS1443 96T12677). 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Thermal and photochemical electrocyclic reactions always take place with opposite stereochemistry because the symmetries of the frontier orbitals are always different. Table 30.1 gives some simple rules that make it possible to predict the stereochemistry of electrocyclic reactions. 

Although the above-mentioned electrocyclization reactions were well studied prior to the discovery of the endiandric acids, their utilization in the total synthesis of complex molecules had not been demonstrated. The endiandric acids, therefore, offered an irresistible opportunity to explore the utility of electrocyclization reactions in synthesis. The successful studies disclosed below demonstrate that these reactions can provide concise solutions to the challenge presented by complex polycyclic frameworks. 

Through a display of a series of electrocyclization reactions, the Nicolaou group demonstrated the biomimetic , one-step synthesis of the endiandric acids involving the cascade of reactions proposed by Black. The polyunsaturated compounds 37 and 38 (Scheme 7) were designed for their relative stability and potential to serve as... 

A number of electrocyclic reactions have been carried out with systems of other... 

The ew face selectivities are still recent topics of electrocyclic reactions [39] and transition metal catalyzed reactions [40-47],... 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

The theory of electrocyclic reactions has had a dramatic impact on synthetic and physical organic chemistry. It will not be possible to develop the theory in detail here, and the interested reader is referred to... 

Tabue 9.8. Characterization of the Transition State of Electrocyclic Reactions ... 

To date, only a few examples have been identified of electrocyclic transformations and ene reactions as the initiating event in a domino process. 

SCHEME 22 Activation and reaction volumes of electrocyclic rearrangements... 

A set of electrocyclic ring closures is the subject of recent controversy because their mechanism lies in the borderline between pericyclic and pseudopericyclic reactions [123-127], The mechanisms were clarified by means of ELF analyses [121,122]. As shown in Figure 28.4, connected patterns (C) are... 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

The mode of reaction of electrocyclic reactions is shown in Table 8.1, where the total number of electrons (N) is given as a multiple (4n) or not a multiple (4n + 2) of four. 

The basis of electrocyclic reactions can be considered in terms of the molecular orbitals involved. By considering the phasing of the molecular orbitals it is possible to say whether a reaction can proceed (only orbitals of the same phase can overlap and bond) and to predict the stereochemistry of the reaction. This approach is called the frontier orbital model. 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

The concepts of electron-transfer catalysis and so-called hole-catalysis [1] are closely related. It is now generally accepted that many organic reactions that are slow for the neutral reaction system proceed very much more easily in the radical cation. Although hole-catalysis is now well documented experimentally [2], there is surprisingly little mention of the corresponding reductive process, in which a reaction is accelerated by addition of an electron to the reacting system. Although the concept of electron-catalysis is not as well known as hole-catalysis, there are experimental examples of electrocyclic reactions that proceed rapidly in the radical anion, but slowly or not at all in the neutral system [3], For reasons that will be outlined below, we can expect that, in many cases, difficult or forbidden closed-shell reactions will be very much easier if an unpaired electron is introduced into the system by one-electron oxidation or reduction. Thus, if a neutral reaction A - B proceeds slowly or not at all, the radical cation (A" -> B" ) or radical anion (A" B" ) may be facile... 

Conjugated heterocumulenes generated in situ by an aza-Wittig reaction are also capable of electrocyclic-ring closure with a subsequent 1,3-H shift. This principle, applied for the first time by Saito et al. on butadiene iminophosphorane 51 (Scheme 28) by treatment with isocyanate and isothiocya-... 

Diazabicyclo[3,3,0]octa-2,6-dienes of type 391 undergo a sequence of electrocyclic reactions upon thermolysis and photolysis (82JPC338) to yield 4,5-diazaocta-l,3,5,7-tetraenes (393), probably via 392 (79CB2620). The influence of the substitution pattern on the conditions necessary for initiation of the reaction and on the valence isomeric equilibria have been studied (86JPC5552). 

A new benzannulation methodology was developed in order to overcome the limitations of electrocyclic ring closure of divinylindoles. The cyclization is achieved via an allene-mediated electrocyclic reaction of 2,3-difunctionalized indoles. This method is more efficient for the synthesis of highly substituted 2-methyl carbazole alkaloids (559). The 3-alkenyl-2-propargylindole 557, a precursor for the allene intermediate, was prepared from 2-formylindole over several steps using simple functional group transformations (536,537) (Scheme 5.20). 

In the same year, Hibino et al. reported a total synthesis of furostifoline (224) employing a new type of electrocyclic reaction (636). This cyclization proceeds through a 2-alkenyl-3-allenylindole intermediate, which is derived from 2-(fur-3-yl)-3-propargyUndole 1128. Compound 1128 was prepared starting from 2-chloroindole-3-carbaldehyde (891), furan-3-boronic acid (1124), and ethynylmagnesium bromide. 

Figure 12.2. Three classifications of pericyclic reactions, with examples of thermally allowed reactions. Cheletropic is a special case of electrocyclic.

A number of electrocyclic reactions have been carried out with systems of other sizes, e.g., conversion of the 1,3,5,7-octatetraene 97 to the cyclooctatriene 98.383 The stereochem-... 

The concept of electrocyclic ring opening of a 1,2-dihydro six-membered heterocycles is familiar from the numerous examples found after nucleophilic attack, especially on cationic rings. Similar reactions occur with isolated 1,2-dihydro derivatives. Dihydropyridines can undergo isomerization by electrocyclic ring opening (see Equation 10). 1 -Vinyl- 1,2-dihydropyridines in a somewhat similar sequence yield pyridines via azacycloocatrienes (Equation 11). 

---