Suprafacial shifts

A bonding interaction can be maintained only in the antarafacial mode. The 1,3-suprafacial shift of hydrogen is therefore forbidden by orbital symmetry considerations. The allowed... 

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

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... 

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. 

These are not common. An example of a suprafacial shift with inversion of configuration is the thermal rearrangements of bicyclo-hexene, studied by W.R. Roth and A. Friedrich (Tetrahedron Letters 1969, 2607). 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Take the most common of all these reactions, the [1,5] hydrogen shift, which illustrates a suprafacial shift, in which the hydrogen leaves the upper surface at C-l, and arrives on the upper surface at C-5. This can be drawn... 

Photochemically, a [1,3] suprafacial shift of hydrogen is allowed, and has been observed with several 1,3-dienes rearranging to the unconjugated 1,4-... 

Mislow s [2,3]-sigmatropic rearrangement of sulfoxides is more than a mechanistic curiosity, because the intermediate sulfenate 5.83 can be intercepted by a suitably thiophilic reagent, converting an enantiomerically enriched sulfoxide 5.82 into a comparably enriched rearranged allyl alcohol 5.84, with suprafacial shift 5.85 of the functionality. 

The chemical properties of 2,3-dihydro-1,4-diazepines are dominated by the presence of the diene portion of the molecules, either by electrophilic attack at the 1, 4 or 6 positions, or nucleophilic attack at positions 5 or 7. There have previously been no observations of the involvement of the saturated part of the molecule. It has now been demonstrated, however, that in vacuum flash pyrolysis there is a suprafacial 1,5 hydrogen shift of a proton from position 7 to position 3 . This has been confirmed by deutero-labelling studies when 7-deutero-2,3-dihydro-methyl-1,4-diazepine is converted into the 3-deutero-isomer. When the study is repeated with the bicyclic t/.s-2,3-cyclohexano derivative the /ram-isomer is isolated, confirminf a suprafacial shift mechanism... 

Depending on whether the starred carbon of 11 pivots so as to bond backside or frontside, there will be a 60° (82) or 120° (83) periodicity in the motion around the remaining carbons. The 60° pattern allows for raeemization in two ways if the starred carbon passes over the marker substituent M, the species develops a plane of symmetry if the starred carbon never passes over M, it still leads to dZ-pairs of nocaradiene or tropilidene. The 120° pattern does not allow for raeemization so that optical activity in the isomers of 11 would be preserved. According to the orbital symmetry rules (Fig. 20a), the [1,5] suprafacial shift is allowed the 120° pattern with retention should be the one observed, if the proposed mechanism is correct. 

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