Rearrangements, sigmatropic hydrogen shifts

In addition to the migration of hydrogen atoms in sigmatropic rearrangements, alkyl shifts also take place. A large number of such reactions occur... 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

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. 

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... 

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... 

A variety of sigmatropic rearrangements through six-electron transition states are known. In contrast to the rare [l,3]-migrations, [l,5]-shifts of hydrogen in dienes, suprafacially allowed, occur readily. The experiment outlined in Equation 12.88 confirms the predicted stereochemistry.139 The authors estimated the... 

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