Homo -sigmatropic hydrogen shift

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... 

Piers and Ruediger have smartly used this basic reaction (equation 168) for a synthesis of the sesquiterpenoid -himachalene ". However, loading the systems to be rearranged with alkyl groups can considerably decelerate the Cope process with the result of competing homo[l, 5]-sigmatropic hydrogen shift in the trans compounds (equation... 

A recently reported61 source of dimethylsilylene, vacuum flow pyrolysis of dimethyl-cis-propenylvinylsilane, which undergoes a 1,5-sigmatropic hydrogen shift (homo-retro-ene reaction) followed by an extrusion of dimethylsilylene, yields the same final isolable products as earlier sources and these again undoubtedly arise from the dimerization of dimethylsilylene to tetramethyldisilene. The thermal chemistry of tetramethyldisilene is complex and is discussed in Section III.B.l.b (equation 94). 

A frontier orbital analysis treats the 1,3-sigmatropic hydrogen shift in terms of the HOMO S of a hydrogen atom and an allyl radical. These are the hydrogen I5- and... 

Figure 12.27 The 1,5 hydrogen shift occurs easily. HOMO-LUMO requires suprafacial. Sigmatropic Migration with Inversion...

The stereochemistry of a sigmatropic hydride shift depends on the highest occupied molecular orbital (HOMO) of the T framework. In the transition state, a three-centered bond is required, which involves overlap between the s orbital of the hydrogen and lobes of p orbitals of the two terminal carbons. From the HOMO configuration of a seven-carbon allylic radical in the ground state, we see that the symmetry-allowed shift is in an ahtarafacial manner (see Figure 1). 

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... 

The second step, which yields vitamin D3, is a thennal [1,7] sigmatropic shift of a hydrogen atom. Based on the symmetry of the HOMO of a heptatrienyl system, the transfer of the hydrogen must occur antarafacially. Both the number of single bonds and the number of atoms provide sufficient ffexibihty for this reaction to occur. 

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