Analysis of sigmatropic rearrangements

For the analysis of these sigmatropic reactions, correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements. 

However, the [1,5]-sigmatropic rearrangement is a concerted suprafacial reaction because the HOMO and LUMO (of migrating group, which is H, and of polyene component, pentadienyl) can interact in a suprafacial process (Fig. 8.53). The six electrons involved are considered to occupy the hydrogen HOMO (two) and the (two) and F2 (two) of pentadienyl. The LUMO of pentadienyl is Fs. 

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. 

In an excited state (i.e. photochemically) the HOMO is now ij 4 and it is antisymmet-rical. Therefore, [1,5]-suprafacial migration of H is no longer possible i.e. it is symmetry forbidden. The photochemical reaction will follow the antarafacial route, which is 

However, [l,3]-H-atom shift takes place photochemically. The HOMO of allyl system for an excited state is i] 3 (v) 3 1 2 3 ) which is symmetrical i.e. now it has 

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Correlation diagram method is not suitable for the analysis of sigmatropic-rearrangements because only transition state but not reactants or products possess molecular symmetry elements. Methods for the analysis of this type of reactions are discussed in forthcomming discussion. 

Analysis of sigmatropic rearrangement is also possible by PMO method and conclusion about their feasibility are same as by other methods. For instance, 1, 3-sigmatropic suprafacial shift occurs via transition state with 0 nodes and 4 electrons (antiaromatic) and hence it is thermally forbidden. But [1, 3]-antarafacial sigma migration occurs through a transition state with 1 node and four electrons (aromatic T.S.) and is therefore thermally allowed. 

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

In their discussion of sigmatropic rearrangements, Woodward and Hoffmann state For the analysis of these reactions correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molecular symmetry elements. [1, p. 114] This is something of an overstatement. For example, they could hardly have meant it to apply to 1,5-hexadiene, which is no less symmetrical than any transition state that can be assumed for its Cope rearrangement. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

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... 

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

A single report on 1,4-oxathiocines has been found in the literature published since 1995. The 2-styryl-l,3-oxathilane 215 with the methyl diazoacetate, catalyzed by Rh2(OAc)4, produced a complex mixture of products. 1,4-Oxathiocine 217 and 1,3-oxathiolane 218 were isolated from the mixture in low yield and their stereochemistry was assigned by NMR analysis (Scheme 25) <2006T3610>. The proposed mechanism of this transformation may involve formation of the ylide 216, which presumably underwent [2,3]-sigmatropic rearrangement into 217, or Stevens rearrangement into 218 (cf. Scheme 24 (Section 14.06.6.6) and Scheme 26 (Section 14.06.8)). 

For sigmatropic rearrangement of order (i,j) in which both i and j are greater than 1, the migrating group of the above analysis consists of more than one atom, and appropriate... 

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