Photochemical sigmatropic hydrogen shift

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

Photochemical cyclization of l-(lV-methylanilino)cycloalkenes gives primarily trans-fused hexahydrocarbazole derivatives 170 (68JA5329). This cyclization may involve a conrotatory electrocyclization of divinylamines, followed by a suprafacial [l,4]-sigmatropic hydrogen shift (70CC531). The intermediates in the initial cyclization are believed to be cyclic azomethine ylide 1,3-dipoles 169, but no direct observation of the participation of such dipoles was made. All attempts to capture dipoles 169 by cycloaddition with maleic anhydride or furan were unsuccessful (71JA2918). 

Interest in these reactions has continued until today because the course and the stereochemistry of many chemical reactions could now be explained and predicted by simple theoretical concepts based on the properties and interactions of the molecular orbitals involved. Pericyclic reactions have found wide application in synthesis, including that of natural products5. Pericyclic reactions involving sigmatropic hydrogen shifts are also observed in biological systems in nature the photochemical and thermal interconversions of compounds related to the vitamin D complex constitute important examples6-8. 

NMR studies in a CDCl3 acetic acid solution clearly demonstrated an equilibrium between the dimers and the corresponding N-imines. Based on this observation, a mechanism for the formation of the 1,2-benzodiazepines via the diaziridines and o-quinonoid 2H-1,2-benzodiazepine intermediates is proposed. Considering their aromatic stability, the latter would be expected to undergo a photochemically allowed [l,7]-sigmatropic hydrogen shift to give the observed product (Eq. 42). 

In cycloheptatriene, an antarafacial [1,7] hydrogen shift is impossible. Consequently, [1,7] hydrogen shifts within this system must be initiated photochemically. For example, the interconversion of isomers of l,4-di(cycloheptatrienyl)benzene involves the sequence of thermal [1,5] and photochemical [1,7] sigmatropic hydrogen shifts (Scheme 3.8). 

From cholesterol 4.18) biosynthesis leads through truncation of the side-chain and other modifications to mammalian steroid hormones, e.g. oestrone 4.22) [11, 33]. Vitamin D3 4.24) is obtainable by photochemically mediated electrocyclic ring-opening of 7-dehydrocholesterol 4.23), followed by a thermal 1,7-sigmatropic hydrogen shift (Scheme 4.7) [34, 35]. The ecdysone insect moulting... 

Fig. 4.1 Orbital interactions in fhmnal and photochemical reactions of [l,3]-sigmatropic hydrogen shift...

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

The [1,6] shifts in pentadienyl anions have rarely been observed. Thermal [1,6] sigmatropic hydrogen shifts occur in the acyclic systems, but not in the analogous cyclic ions. This indicates that an antarafacial interaction on the TT-system is necessary to the migration, as is predicted by the theory. Further confirmation for these results comes from the finding that the cyclic pentadienyl anion is photochemically isomerized by a [1,6] hydrogen shift the favourable [ 2s +, 6J interaction is available to the excited state process. 

Sandmeyer reaction, 306 Sandwich compoimds, 275 Sawhorse projections, 7 Saytzev elimination, 249, 256 Schiff bases, 221 Schmidt rearrangement, 122 Selectivity, 156, 169, 326 a, 362 a, 370 372 aj,385 a bonds, 6 o complexes, 41,131 Sigmatropic rearrangements, 352-357 antarafacial, 353 carbon shifts, 354 hydrogen shifts, 352 orbital symmetry in, 352 photochemical, 354 suprafadal, 353 thermal, 353... 

The outcome of all this for photochemical sigmatropic shifts is that those most commonly observed are of order (1.3) or (1.7) these involve 4 or 8 electrons, respectively, and occur in a suprafacial manner. Examples of photochemical 1.3-shifts of hydrogen are found for monoalkenes (2.25) and for conjugated dienes 2.26). In the case of dienes a 1,3-shift is favoured over a 1,5-shift, because for the latter to occur photochemically it would have to take place in an antarafacial manner. Note that in both examples the direction of... 

The [1,5] sigmatropic shift is intramolecular and the maintenance of bonding interactions explains why [1,5] hydrogen shifts are possible and [1,3] hydrogen shifts are exceedingly rare. But we have not yet been able to deal with the dependence of the course of this reaction on the mode of energy input—thermal or photochemical. 

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