Hydrogen shift symmetry allowed

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. 

Analysis of a 1,7-hydrogen shift process indicates that the suprafacial hydrogen shift is symmetry forbidden in a thermal reaction. Photochemically, [1,7] suprafacial shift of hydrogen is symmetry allowed (Fig. 4.4) [1,2]. 

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. 

Cyclizations of doubly unsaturated diazo compounds containing a thiophene ring within rather than at the end of the diene system to yield thicnodiazepines have also been reported. Thus, thermolysis of the sodium salt 7 gives the l//-thieno[3,2-r/]-2,3-diazepine 9. The intermediate 8 rearranges to the more stable product 9 by a symmetry allowed [1,5] shift of hydrogen.14,1... 

Ar-Arylbenzimidoyl)tetrazoles t, prepared from JV-arylbenzimidoyl chlorides and 5-(dimethylamino)- or 5-aryltetrazole, give 3//-l,3,4-benzotriazepines 4 on thermolysis. It has been proposed that the reaction proceeds by way of the dipolar compounds 2, which undergo [l,7]-antarafacial 871-electrocyclization to 3. The process is completed by a symmetry-allowed [1.5]-hydrogen shift. Selected examples are given.349-350... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

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 ... 

So a [1, 5] or larger rearrangements suprafacial shift is symmetry allowed but a [1, 3] shift would be structurally prohibited, because geometrically it will not be feasible. This would require the hydrogen to migrate to the opposite side of the allyl system and this is sterically difficult. 

It resembles both a cycloaddition and a [1, 5] sigmatropic shift of hydrogen. It is a symmetry allowed process shown as follows ... 

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. 

Some of the initially formal singlet nitrene 52s can also cychze to isocarbazole 53. Isocarbazole 53 absorbs broadly in the visible (Xmax 430 nm) and decays to carbazole by a symmetry allowed 1,5-hydrogen shift with a time constant of 70 ns. ... 

In recent years some applications of retro-ene reactions in the generation of thio-carbonyls have appeared. The retro-ene reaction is a symmetry-allowed process which resembles both [2 + 4] cycloreversion and a [1,5] sigmatropic shift of hydrogen. It usually requires high temperatures, therefore FVT has become very widespread in these reactions. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

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. 

A frontier orbital analysis of the ene reaction is shown in Fig. 8.62. It displays many features of the [3,3]-sigmatropic shift. Thus, the hydrogen atom transfer from an allylic site to a double bond is seen to be symmetry allowed with respect to the HOMO of an... 

Practically, then, [1,3] and [1,5] sigmatropic reactions seem to be limited to supra shifts. A [1,3] supra shift of hydrogen is symmetry-forbidden since the s orbital of hydrogen would have to overlap p lobes of opposite phase, hyarogen cannot be bonded simultaneously to both carbons. A [1,5] supra shift of hydrogen, on the other hand, is symmetry-allowed. 

Both of these [1,51 hydrogen shifts occur by a symmetry-allowed suprafacial rearrangement, as illustrated in Figure 30.14. 

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