Sigmatropic shift reactions

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

Preparations of 5-oxo-2,8-dihydro-577-dibenzo[, < ]thiocin 23 and the corresponding3 -methylthiocinium hexachloro-antimonate 6 have been described (Scheme 6) <2002J(P1)2704>. The methylthiocinium salt 6 has been employed in a [2,3]-sigmatropic shift reaction (Section 14.03.6). 

Although for the [l,7]-sigmatropic shift reaction of CHT the semiempirical calculations yield a reaction mechanism that is in good qualitative agreement with all experimental observations, the quantitative residts, and in particidar the value of 23 kcal moh for the excited-state barrier, require redetermination on the basis of more reliable ab initio methods. For details, see Section 6.6. 

In sigmatropic shift reactions, one portion of a molecule detaches from one atom and reattaches at another position of the same molecule. The transition state is best described by an atom (or group of atoms) moving across a polyenyl radical. Once again, an analysis of the symmetry of the HOMO of the polyenyl radical is the key to understanding. A bonding interaction must be achieved at the arrival point for the reaction to be allowed. 

Another type of reaction to which qualitative MO theory has been applied is the sigmatropic shift reaction, where a hydrogen migrates from one carbon to another and simultaneously a shift in the double bond system occurs. An example is given in Fig. 14-26. 

Based on the orbital relations discussed in the text and extensions of these relations to other cases, formulate generalized verbal rules (Woodward-Hoffmann rules) for cycloadditions and sigmatropic shift reactions. 

The table below presents relevant data for the sigmatropic-shift reaction of hexa-1,5-diene, which is illustrative of pericychc reactions. Substituents can cause changes in the reaction energy and/or in the bond order at the transition state, rfi. Select reaction series where linear free-energy relationships (LFER) can be established. 

Retro-ene type [2 r + In + 2a] and [3,3]-sigmatropic shift reactions involving the substituent groups R in heteroatom-substituted cyanates and thiocyanates RX-YCN and the isomeric isocyanates and isothiocyanates of the type RX-NCY (X = CR2, NR, O, or S, Y = O or S) have been investigated computationally (Scheme 3). ... 

Hiickel-type systems (such as Hilcfcel pericyclic reactions and suprafacial sigmatropic shifts) obey the same rules as for sigma electron. The rationale for this observation is clear If the overlap between adjacent p-electron orbitals is positive along the reaction coordinate, only the peraiutational mechanism can... 

The equihbtium composition is normally ca 80% vitamin D and 20% previtamin D. This reaction is an intramolecular 1 7 Ft sigmatropic shift... 

Recently, the 1,3-sigmatropic shift in 2-cyanopyrrole was studied by using the SINDOl semiempirical method (90JOC2288). This study showed that the reaction occurred via a transition and that some biradical intermediates are probably involved in the reaction. 

Another pathway for the aromatization of the cr -adducts was found in the reactions of 3-pyrrolidino-l,2,4-triazine 4-oxide 81 with amines. Thus the treatment of 1,2,4-triazine 4-oxide 81 with ammonia leads to 5-amino-1,2,4-triazine 4-oxides 54—products of the telesubstitution reaction. In this case the cr -adduct 82 formed by the addition of ammonia at position 5 of the heterocycle undergoes a [l,5]sigmatropic shift resulting in 3,4-dihydro-1,2,4-triazine 83, which loses a molecule of pyrrolidine to yield the product 54. This mechanism was supported by the isolation of the key intermediates for the first time in such reactions—the products of the sigmatropic shift in the open-chain tautomeric form of tiiazahexa-triene 84. The structure of the latter was established by NMR spectroscopy and X-ray analysis. In spite of its open-chain character, 84 can be easily aromatized by refluxing in ethanol to form the same product 54 (99TL6099). 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

---