Thermally allowed sigmatropic process

The [1,2]-Wittig rearrangement proceeds via a radical-pair dissociation-recombination mechanism, while the [2,3]-Wittig rearrangement is a concerted, thermally allowed sigmatropic process proceeding via an envelope-like transition state in which the substituents are pseudo-equatorial. 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

Sigmatropic shift is thermally allowed suprafacial process of Site and 2oe. The thermal rearrangements of aryl ethers 154 to 155, l,2-(l,3-butadienyl) cyclohexyl enolates 156 to 157 and hydrazobenzene 158 to p-benzidine 159 are illustrative... 

H-Azepines 1 undergo a temperature-dependent dimerization process. At low temperatures a kinetically controlled, thermally allowed [6 + 4] 7t-cycloaddition takes place to give the un-symmetrical e.w-adducts, e.g. 2.231-248-249 At higher temperatures (100-200°C) the symmetrical, thermodynamically favored [6 + 6] rc-adducts, e.g. 3, are produced. These [6 + 6] adducts probably arise by a radical process, since a concerted [6 + 6] tt-cycloaddition is forbidden on orbital symmetry grounds, as is a thermal [l,3]-sigmatropic C2 —CIO shift of the unsym-metrical [6 + 4] 7t-dimer. 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

The [1,5] shift would be a suprafacial reaction. The [1,7] shift must be antarafacial in the v component to be thermally allowed. The proton, located at carbon A in 6-20, moves from the bottom side of the six-mem-bered ring to the top side of the w system at the other end. In general, the antarafacial process, necessary for a thermal [1,7] sigmatropic shift, occurs with ease, especially when compared to the antarafacial process that would be required for a thermal [1,3] sigmatropic shift. 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

Since the Diels-Alder cycloadditions used to prepare various dihy-drothiazine imines generally produced mixtures of sulfur epimers, it was desirable to find a procedure to also convert the unreactive adducts to the vicinal diamines. It was discovered that adducts 60 rearranged thermally by a novel [2,3]-sigmatropic process to afford thiadiazolidines 62 stereoselectively (Scheme 1-XV). Reduction of 62 with sodium borohy-dride yielded the -threo vicinal diamine derivative 57. Similarly, unreactive adducts 61 cleanly gave thiadiazolidines 63 on heating, which could be converted to E-erythro vicinal diamines 59. This methodology thus allows both sulfur epimers from ( , )-2,4-hexadiene to be converted to the -threo product, and the epimers from ( ,Z)-2,4-hexadiene to be used to prepare the -erythro series of compounds. 

The TS for [3,3]-sigmatropic rearrangements can be considered to be two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic orbital array results and the process is thermally allowed. Usually a chairlike TS is involved but a boatlike conformation is also possible. ... 

Because organoboranes and organoalanes form relatively stable intermediate complexes with various substrates as a prelude to final product formation, it seems permissible to try to extend the scope of the Woodward-Hoffmann principle to the reorganization pathways of such complexes. Thus, it would be useful, for example, to consider whether the chemical behavior of an allylic aluminum system complexed with a ketone (3) might resemble the thermally allowed [3, 3] sigmatropic rearrangement (4). The value of viewing the collapse of such complexes as potential pericyclic processes will become evident in Section IV,C, where the interplay of kinetic versus thermodynamic control on ketone insertions into carbon-metal bonds is discussed. 

The thermal [1,5] sigmatropic hydrogen shift is an allowed suprafacial process, which has been investigated at the SCF level in 1,3-pentadiene, in -hydroxyacrolein and in cyclopentadiene. ... 

Sigmatropic shift is thermally allowed process as it involves 10 e (4n-n2) process and TS is Huckel-type and aromatic. The ammonium salt 152a on treatment with a base gives ammonium ylide 152, which on [4,5] -shift affords 153. 

Sigmatropic shift is thermally allowed process and it involves a Huckel-type TS of 18e (16a and 2oe). For example, fcw[4-(2-furyl)-phenyl] diazane 162 gives hydrochloride salt of 5,5 -fcw(4-amino phenyl)-2,2 -bifuryl 163 in high yield in acidic solution [144]. 

Stereochemical outcomes of 1, 5-sigmatropic shifts witn retention in cyclic systems can be analysed in a thermally allowed suprafacial sigmatropic process by the application of FMO method. Let us illustrate this by the example of thermolysis of hypothetical norcaradiene system. 

But, (1, 4]-sigmatropic shifts are sterically prohibited. Thermal (1, 4]-shifts are thermally allowed through suprafacial mode of migration this process involves inversion at the migrating centre. 

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 formation of the oxepin is reasonably explained by an electrocyclic ring opening of rearranged epoxide 299 in a thermal reaction. As mentioned above, two routes to 299 are possible. If the rearrangement is concerted, a 1,5-sigmatropic reaction with inversion of the reaction center (oxygen) in 299 is photochemically allowed. It is possible to separate a nonconcerted process... 

Thermal [ 1,5] sigmatropic rearrangements are allowed and are quite common. This process is responsible for the interconversion of the methylcyclopentadiene isomers, which proceeds with a half-life of about 1 h at 20°C. 

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. 

We noted in Chapter 15 that, for the most part, the orbital symmetry rules are not directly applicable to photochemistry. However, some photochemical reactions of simple tt systems do give products that are consistent with expectations based on orbital symmetry, although this does not prove that these are concerted, pericyclic processes, The photochemical selection rules for pericyclic reactions are opposite of those for thermal pericyclic reactions. For example, there are many examples of [1,3] and [1,7] sigmatropic shifts that appear to go by the photochemically "allowed" suprafacial-suprafacial pathway Eqs. 16.22 and 16.23 show two (recall that the thermal reactions would be suprafacial-antarafacial). These reactions occur upon direct irradation, while sensitized photolysis produces products more consistent with biradical-type reactions. 

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

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