Hydrogen shift thermally allowed

Tike the thermal 1,3-hydrogen shift, a 1,7-hydrogen shift is allowed when antarafacial but forbidden when suprafacial. Because a tt system involving seven carbon atoms is more flexible than one involving only three carbon atoms, the geometrical restrictions on the antarafacial TS are not as severe as in the 1,3-case. For the conversion of Z,Z-l,3,5-octatriene to Z,Z, -2,4,6-octatriene, the E is 20.2 kcal/mol. 

Use the symmetry and nodal properties of the appropriate molecular orbitals to determine whether the thermal [1,7] and [1,9] hydrogen shifts are allowed suprafacially or antarafacially. 

The ground state electronic configuration of allyl radical is Pi -HOMO ( Pi) of this radical has opposite sign on the terminal lobes (Ci symmetry). Suprafacial [1,3] hydrogen shift under thermal condition is forbidden because there is no question of inversion at this atom, which is bonded to the carbon atom through its spherically symmetrical Ir-orbital. Antarafacial [1,3] hydrogen shift is allowed only by the principles of orbital symmetry. The transition state is a highly contorted species and the reaction is forbidden because of the steric inhibition involved in such a process. 

A concerted [1,5] shift of carbon is allowed suprafacially with retention of configuration as shown in Equation 5.34 [35]. A concerted [1,7] shift of hydrogen is thermally allowed if it can reach the opposite face of the %... 

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). 

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. 

Thermal 1,5-hydrogen shifts are thus allowed and, because of the symmetry of the T.S. (39), the H atom in the product (37, x = 1) will be on the same side of the common plane of the polyene s carbon atoms as it was in the starting material (36, x = 1) this is described as a suprafacial shift. This latter point would not be experimentally verifiable in the above example, but that thermal 1,5-shifts (which are quite common) do involve strictly suprafacial migration has been demonstrated in the compound (40). This is found, on heating, to yield a mixture of (41) and (42), which are produced by suprafacial shifts in the alternative conformations (40a) and (406), respectively ... 

The above selection rules, therefore, predict that [1, 5] hydrogen shifts in neutral polyenes would be thermally allowed and the reaction would be facile, but thermal [1, 3] and [1, 7] shifts must go by an antarafacial process and they will be difficult to attain because of the geometric strain. This is also confirmed by many experimental observations. Thus concerted uncatalysed [1, 3] hydrogen shifts have not been seen in the diene of the following type, [1, 5] shifts are well known. 

Attack by the carbene on a suitably placed aryl ring (Scheme 58152) gives an intermediate 164, which undergoes a thermally allowed suprafacial [l,5]-hydrogen shift to give the indene 165.151,152 The intramolecular nature of the hydride shift was established by lack of exchange with deuterium in the solvent.152... 

Thermal sigmatropic 1,5 hydrogen shifts are quite common in certain allene and diene systems, cis- 1,3-Dienes have the proper geometric arrangement to undergo a thermally allowed suprafacial hydrogen shift, cis-1,3-Hexadiene, for instance, gives... 

Annular prototropy is not of great importance for small heterocycles. However, it should be mentioned that 1-azirine (2) is much more stable than its antiaromatic 2-isomer (3). By analogy, antiaromaticity is certainly a key factor determing instability of lH-azepines which have never been observed. Thus, demethoxycarbonylation of methyl-3,6-di-r-butylazepine-l-carboxylate (46) by DBU gives a mixture of the corresponding 2H-, 3H-, and 4//-azepines in the approximate ratio 13 56 1 (Scheme 9) (94JCS(P1)1753). The distribution of the azepine isomers is proportional to their relative thermal stabilities as they interconvert via allowed 1,5-hydrogen shifts. 

Because in the ground state the HOMO is 4/3, the hydrogen shift is controlled by the symmetry of the vj 3 of the pentadienyl radical. The v) 3 has similar signs on the terminal lobes i.e. it is symmetrical. Thus, the [ 1,5] -hydrogen shift is thermally allowed and occurs in a suprafacial process. This involves a transition state in which the C-1 and C-5 orbitals overlap with Is hydrogen orbital. This shift is both symmetry allowed and geometrically favourable, as shown in Fig. 8.55. 

The dipolar intermediate 35 is pivotal in further reactions. The formation of 36 requires a 1,2-hydrogen shift and formally at least such a shift would be a thermally forbidden pericyclic reaction. An alternative to the forbidden process would involve a thermally allowed 1,5-shift to give, for example, 37... 

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