Homo- -sigmatropic rearrangement

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... 

The reactions of 1,2,3-triazolium 1-imide (277) with a range of alkene and alkyne dipolarophiles give rise to a variety of new ring systems (Scheme 54). Compounds (276) and (278) are obtained from (277) by reaction with acrylonitrile and DMAD, respectively. These reactions are tandem 1,3-dipolar (endo) cycloadditions and sigmatropic rearrangements which are regio- and stereospecific <90JCS(Pl)2537>. Kinetic and mechanistic studies show that these reactions are dipole-HOMO controlled. The second-order rate constants are insensitive to solvent polarity, the reaction indicates... 

The approach of the two orbitals on the same side of the surface is called supra , that on opposite sides is called antara . The above rules, 1 and 2, also apply in the case of sigmatropic rearrangements, since the changes of orbital symmetries are the same on going from a HOMO to a LUMO or from a chain of 4n to a chain of 4n + 2 tt electrons. 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

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. 

At first, the asymmetric [2,3]sigmatropic rearrangement of allylic sulfur ylides is surveyed. The first successful asymmetric [2,3]sigmatropic rearrangement involving sulfur ylides by Trost and Hammen appeared in 1973 [44]. The treatment of optically active sulfonium salt afforded the corresponding homo-allylic sulfide with 94% enantiomeric excess, in which the chiral information has been transferred from the sulfur atom to the carbon atom with excellent control (Scheme 28). Trost and Biddlecom also reported another approach to generate... 

The first step in both cases is an ene reaction with the Se=0 bond (pp. 1270-1). The electrophilic selenium attacks the less substituted end (largest HOMO coefficient) of the alkene and a proton is removed from the methyl group trans to the main chain. Then a [2,3]-sigmatropic rearrangement puts the double bond back where it was (trans selectively) and functionalizes the old methyl group with an oxygen atom. 

A thermal [1,3] sigmatropic rearrangement is allowed only if one component is antarafacial, but a photochemical [1,3] sigmatropic rearrangement is expected to be allowed when it proceeds suprafacially with respect to both components. The stereochemical requirement changes because under photochemical conditions, the HOMO of the three-atom component is tf/2 (symmetric), not i/q (antisymmetric). 

The prefix homo (meaning one extra atom, interrupting conjugation, cf. homoaromaticity) has frequently been applied to sigmatropic rearrangements, but it is misleading. 

Let s look at the similar six-electron (three arrow) thermal 1,5 hydrogen sigmatropic rearrangement. We expect the ends of the pi system to change phase as we add two more electrons. This means the rules will reverse and the allowed process will be suprafacial. Figure 12.27 shows the HOMO-LUMO prediction of suprafacial. 

Substituted 5,5-dimethyltetrahydrofuran 8 is obtained by reaction of homo-allylic alcohol 7 with a catalytic amount of lri(OTf)i (0.1 equiv.) and aldehyde (0.1 equiv.) (Scheme 8.123). When the reaction is conducted with an equimolar amount of aldehyde and catalytic In(OTf)3 (0.1 equiv), compound 9 is formed selectively. A tandem 2-oxonia [3,3]-sigmatropic rearrangement/cyclization mechanism is postulated [166]. 

The product is the cage amine 97. The azadiene must cycloadd to the nearer of the two unactivated alkenes in the conformation 95 (mechanism not shown for clarity). Though the regio-selectivity is what we should expect from the HOMO of the dienophile and the LUMO of the protonated 2-azadiene, the short tether (two saturated carbon atoms) is probably responsible. It is certainly responsible for the stereochemistry as the tether delivers the dienophile to the top face of the diene.16 The product 96 cyclises by addition of the remaining alkene to the immonium ion to give 97. We shall see more of these tandem sequences later in the sections on sigmatropic rearrangements. 

In obtaining all-tram bis-homo-farnesol (7), needed for the synthesis of Cig-JH, Still and co-workers (1979) made use of their former finding (Still and Mitra, 1978) on the [2,3]-sigmatropic rearrangement of alkoxyorganolithium reagents, which provides an efficient method for the preparation of Z-homoallylic alcohols. 

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