Hydrogen sigmatropic shifts

The evidence obtained clearly indicates that the above photorearrangements proceed by a mechanism involving a nitrile ylide intermediate since cycloadducts could be isolated when the irradiations were carried out in the presence of trapping agents. Intramolecular cycloaddition of the nitrile ylide followed by a 1,3-sigmatropic hydrogen shift of the initially formed five-membered ring readily accounts for the formation of the final product. 

Fig. 11.6. Classification of sigmatropic hydrogen shifts with respect to basis set orbitals.

Just like the Diels Alder reaction or the 1,5-sigmatropic hydrogen shift, the ene reaction is believed to proceed via a six-membered aromatic transition state. 

Both these 1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.12. In contrast with these thermal [L,51 sigmatropic hydrogen shifts, however, thermal [1,3 hydrogen shifts are unknown. Were they to occur, they would have to proceed by a strained antarafacial reaction pathway. 

The following rearrangement was devised and carried out to prove the stereochemistry of [1,5] sigmatropic hydrogen shifts. Explain how the observed result confirms the predictions of orbital symmetry. 

Detailed mechanistic studies101 indicate that benzazepines arc formed by 1,7-electrocyclization of a nitrile ylidc intermediate followed by a [l,5]-sigmatropic hydrogen shift in the initially formed 9a//-2-benzazepine. Similar products, e.g. 9, have been obtained by generating the nitrile ylide 8 from the imidoylchloride, e.g. 7. with potassium /ert-butoxide in tetrahydro-furan.101... 

Diels-Alder reactions of vinylpyrazoles 45 and 46 only occur with highly reactive dienophiles under severe conditions (8-10 atm, 120-140 °C, several days). MW irradiation in solvent-free conditions also has a beneficial effect [40b] on the reaction time (Scheme 4.11). The indazole 48, present in large amounts in the cycloaddition of 45 with dimethylacetylenedicarboxylate, is the result of an ene reaction of primary Diels-Alder adduct with a second molecule of dienophile followed by two [l,3]-sigmatropic hydrogen shifts [42]. The MW-assisted cycloaddition of 46 with the poorly reactive dienophile ethylphenyl-propiolate (Scheme 4.11) is significant under the classical thermal reaction conditions (140 °C, 6d) only polymerization or decomposition products were detected. 

This rearrangement, which has been called an enolene rearrangement, a homodienyl [1,5] sigmatropic hydrogen shift (see 18-29), and a [1,5] homo sigmatropic... 

In many cases the transformations may be more complex than indicated by Eqs. (9.89)-(9.100). An example of this is the photochemistry of cis,cis-1,3-cyclooctadiene [Eq. (9.94)].<169) A close examination of this reaction indicates that bicyclo[4.2.0]oct-7-ene is formed but in low relative yields during the initial reaction (see Table 9.9). In addition, the cis,trans-1,3-cyclooctadiene is formed and then consumed as the reaction proceeds. Fonken showed that the bicyclooctene initially formed, however, was not from thermal isomerization of the cis,trans-diene. Still a third reaction was the 1,3 sigmatropic hydrogen shift to form the cis, cis-1,4-cyclooctadiene ... 

Satake et al. reported further detailed studies on the chemistry of 2//-azepines highlighting a [1,5] sigmatropic hydrogen shift and also an unusual [1,5] sigmatropic propylthio shift on heating 9 to afford 10 and 11 respectively kinetic measurements were consistent with a... 

In what is one of the few examples of utilization of a higher order sigmatropic hydrogen shift in the synthesis of complex molecules, Eschenmoser, in studies directed toward the synthesis of Vitamin B12, found that an antarafacial [1,16] H-shift could be utilized to effect closure of secocorrin 63 to corrin 65 (Scheme 16)31. An intermediate biradical... 

A third symmetry-allowed transformation in which vinylallenes partidpate involves sigmatropic hydrogen shifts. As shown in Scheme 5.45, two types of these concerted processes have so far been observed. 

Lignans have been accessed using an intramolecular allenic Diels-Alder reaction (Scheme 19.21). Alkyne 109 was treated with potassium tert-butoxide at 60°C to afford a mixture of 112 and 113 via intermediates 110 and 111 [25]. Cydoadduct 112 could eventually be transferred to isomer 113 at room temperature by way of a second 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 oxidative deamination mediated by 3,5-di-t-butyl-o-quinone [257] could very well involve aC- 0 1,5-sigmatropic hydrogen shift within the Schiff base network. This process in essence accomplishes oxidation of the amine and reduction of the quinone. The interesting point is the strong donor oxygen forces the nitrogen atom into an acceptor role during the reaction. 

Electrocyclization of dienyldiazoalkanes, e.g. (129), provides a general route to 3H-1,2-diazepines, e.g. (131). In the example shown the eight-ir-electron cyclization is followed by a 1,5-sigmatropic hydrogen shift in (130) (83CC1003). Such hydrogen shifts are rapid at room temperature and the isomer ratio reflects thermodynamic stability. 

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