Electrocyclic reactions general examples

Similarities between mass spectral and thermal fragmentations are particularly common in certain reaction types. Electrocyclic reactions, for example, are frequently similar in the two processes. The thermal process has in general a higher stereoselectivity (because of the higher aromaticity in even-electron systems). Retro-Diels-Alder reactions are typical examples for the similarity of the two processes. Internal displacement reactions may also be similar in the two processes, mainly in the case of internal radical displacements. The relationship between mass spectra and thermal fragmentation is complex, and it is useful to discuss it for separate classes of compounds. 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

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. 

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... 

Pericyclic reactions in general and electrocyclic reactions in particular, for example the ring opening of cyclobutene to 1,3 butadiene as shown in Figure 8.6, have been central to physical organic chemistry and its interpretation by orbital theories of electronic structme. " ... 

Although 2,8, etc., electron electrocyclic reactions are less common, they do exist, so it is useful to summarize the stereochemical outcomes quite generally (Table 18.1). You don t need to memorize the whole of this table—if you remember just one entry, and the fact that 4h and 4 + 2 are different, and thermal and photochemical are different, then you have it all. Some examples of electrocyclic reactions are given in Figure 18.34. 

Because of condition (iii) all pericyclic reactions may formally be regarded as cyclo-addition processes or their retrogressions, but it is generally more useful to divide pericyclic reactions into a number of more distinct reaction series. These are electrocyclic reactions (e.g. Equations 3.3 and 3.4), cycloaddition reactions (e,g. Equations 3.5 and 3,6), sigmatropic reactions (e.g. Equations 3.7 and 3.8), cheletropic reactions (e.g. Equations 3.9 and 3.10), group transfers (e.g. Equation 3.11), and eliminations (e.g. Equations 3.12 and 3,13). Examples in other categories are less numerous, and will not be considered in this volume. 

Electrocyclization of 1,4-dienes is an efficient process when a heteroatom with a lone pair of electrons is placed in the 3-position, as in 77 (Scheme 20)38. Photoexcitation of these systems generally results in efficient formation of a C—C bond via 6e conrotatory cyclization to afford the ylide 78. These reactive intermediates can undergo a variety of processes, including H-transfer (via a suprafacial 1,4-H transfer) to 79 or oxidation to 80. In a spectacular example of reaction, and the potential it holds for complex molecule synthesis, Dittami and coworkers found that the zwitterion formed by photolysis of divinyl ether 81 could be efficiently trapped in an intramolecular [3 + 2] cycloaddition by the... 

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