Sigmatropic processes

Scheme 11.2. Generalized Selection Rules for Sigmatropic Processes...

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. 

Mechanistic investigation in this case has shown that there is an equilibrium between an alkenyl silane and an allylic silane by a rapid 3,3-sigmatropic process. The cyclization occurs through the more reactive allylic silane. 

The allene moiety of the products 70b, 72 and 75 is in each case chiral and, furthermore, an additional chiral center is created in 72a,b and 75b,e-g, thereby leading to the possible formation of diastereomers. However, the concerted nature of such sigmatropic processes should result in suprafacial migrations and formation of the racemate of only one diastereomer in each case, as shown for 74 — 75 in Scheme 7.10. High stereoselectivity can really be found for the reaction of (fc)-71a and 74b,e,f, but not for other examples of type 71 and for 74g, which lead to mixtures of diastereomers. 

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

The mechanistic basis of sigmatropic rearrangements was introduced in Chapter 11 of Part A. The sigmatropic process that is most widely applied in synthesis is the [3,3] sigmatropic rearrangement. The principles of orbital symmetry establish that concerted... 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry restrictions in the same way that cycloadditions and sigmatropic processes are. 

Dimethyl-4//-pyrazoles, bearing a 2-propenyl or a 2-propynyl group in the 4-position, rearrange by [3,3]-sigmatropic processes.11 13 If a 4-methyl or 4-ethyl substituent is also present (80 R = H or Me), two competing pathways are observed (Scheme 20). 

In contrast to 77, thermal rearrangement (o-dichlorobenzene solvent) of 4,4-dialIyl-3,5-diphenyI-4//-pyrazole gave only l,4-dialIyl-3,5-diphenyl-pyrazole (93%), suggesting a [3,3]-sigmatropic process rather than a free radical mechanism.13... 

Since allyl sulfoxides may quite easily undergo racemization at the sulfur atom via a reversible [2,3] sigmatropic process, the configurationally more stable chiral allylic phosphine oxides were also investigated.201 Compounds (184) and (185), prepared as a 1 1 mixture from allylphosphonyl dichloride and (-)-ephedrine, were shown to add to cycloalkenones with reasonably high diastereoselectivities. Ozono-lysis of the initially formed 1,4-adducts affords the respective optically active ketoaldehydes (Scheme 67). With a / /-isopropyl-substituted derivative even higher selectivities (88-98% ee) could be obtained. 

A new type of asymmetric induction in this area has been achieved in our group. The need for absolute stereocontrol with the aid of a removable chiral centre, and the observation that the sulfinyl group, recently introduced for Diels-Alder reactions, has not yet been introduced in substrates prone to [3.3] sigmatropic processes, were our motivations. The requisite racemic ketene dithioacetals bearing a sulfinyl group have been prepared and their rearrangement was shown [202] to proceed at ambient temperature. The asymmetric induction was extremely effective, with diastereoselectivi-ties ranging from 93 7 to 99 1, in favour of the (2S,SS) isomer. 

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