Symmetry control

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Suggest mechanisms for the following reactions. Classify the orbital symmetry-controlled process as clearly as you can with respect to type. 

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

Orbital symmetry control of subsequent ring opening could account for isomerization at only one of the double bonds. Taking ij/ as the controlling frontier orbital, it can be seen that a concerted return to ip2 to rotation at only one terminus of the diene ... 

Cheletropic reactions include both addition and elimination reaction. The number of elimination reactions that have been studied in detail is not large but there is sufficient information to establish that orbital symmetry controls are operating. 

Chlorination of olefins has also been achieved with SbCls in chlorinated solvents, which gives with mono-olefins vicinal dichloroalkanes by a syn addition. A concerted mechanism was initially proposed68 to rationalize this stereochemical behavior and the unexpectedly large amount of c -l,4-dichloro-2-butene found in the reaction of butadiene. In this case, however, because of orbital symmetry control it has been suggested that the addition occurs in an antarafacial direction69. 

Dyotropic rearrangements are uncatalyzed concerted dihydrogen exchange reactions, another class of orbital symmetry controlled processes, which involve the simultaneous migration of two cr-bonds. These conversions can be both thermal and photochemical. They can be subdivided into two types (1) reactions in which two migrating cr-bonds interchange their positions (equation 78), and (2) reactions without such positional interchange (equation 79)91,92. 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

Again it has to be noted that the frontier orbitals participating in such a valence isomerization are delocalized over the whole molecule [22]. This has consequences for the orbital symmetry and, thereby, a prior analogy with comparable processes involving 6 t-electrons only is not given. However, compared with smaller Jt-systems the selection rules for orbital symmetry controlled processes in fullerenes seem to be less restrictive, since a large number of tt-orbitals with small energy separation are available. Calculations at the AM 1 and PM3 level show that the photocycKzation... 

Thus far, in the alkaloid series discussed, the nitrogen atom has always been part of the core of the alkaloid strucmre, rather than acting in a dipolarophilic manner in the cycloaddition of the carbonyl ylide. Recently, Padwa et al. (117) addressed this deficiency by conducting model studies to synthesize the core of ribasine, an alkaloid containing the indanobenzazepine skeleton with a bridging ether moiety (Scheme 4.57). Padwa found that indeed it was possible to use a C = N 7i-bond as the dipolarophile. In the first generation, a substimted benzylidene imine (219) was added after formation of the putative carbonyl ylide from diazoketone 218. The result was formation of both the endo and exo adduct with the endo adduct favored in an 8 1 ratio. This indicates that the endo transition state was shghtly favored as dictated by symmetry controlled HOMO—LUMO interactions. 

The thermolysis of cycloalkenes is often a more facile process than for the cycloalkenes. Cyclobutene undergoes thermolysis at 175 °C and yields butadiene in an orbital symmetry controlled reaction as shown by stereochemical studies of... 

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