Concerted sigmatropic process

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 latter will consequently undergo the usual concerted sigmatropic process finally resulting in the cycloheptadiene derivative. Due to the higher temperature needs and to the participation of radical intermediates, side reactions are more likely to occur in the trans series. 

The [1,2]-Meisenheimer rearrangement most likely proceeds via a hemolytic dissociation-recombination mechanism, whereas the [2,3]-Meisenheimer rearrangement is a concerted sigmatropic process that goes through a five-membered envelopelike transition state. 

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. 

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. 

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. 

The usually high stereoselectivity on formation of a new double bond or a stereogenic center is due to the stereodifferentiating ability of envelope-like transition states of this concerted suprafacial process. The envelope conformation of cyclopentane serves as a model for the transition state of the [2,3] sigmatropic rearrangement6,7. 

Sigmatropic rearrangement is termed the 1,3-shift of an allylic atom pair ——Y with a concomitant 1,2-shift of an electron pair from Y to X (Scheme 1). It is symmetry allowed as a concerted suprafacial 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. 

The mechanism of these copper catalyzed isomerizations is not fully understood. A concerted sigmatropic 1,3-shift as Well as a biradical process have been proposed but a decision between these two possibilities could not be made 85). Kinetic measurements at least have shown that the reactions are not monomolecular and that the reaction rates depend on the catalyst concentration. 

In general, assuming a concerted suprafacial process, four stcrcochemically distinct pathways (A) - for C-H bond formation by [1,5] sigmatropic hydrogen shifts in cis-alkyl vinyl-allenes have to be considered as shown for the reactions 8 to 9 (R2 < R1). 

A careful study of the mechanism of the oxy-Cope rearrangement of 1,5-diene alkoxides has provided a neat synthesis of ery//jro-juvabione (68) (Scheme 6). This work has shown that the [3,3] sigmatropic process proceeds in a concerted fashion predominantly via a chair transition state. 

Complex, Multi-step, or Non-concerted Pericyclic Processes.—Pyrolysis of labelled tricyclo[8,2,2,0 ]tetradeca-3(8),ll,13-triene (264 -n = D) at 180°C led to the formation of unlabelled 1,2-dimethylenecyclohexane, [ H jbenzene, 2,3-dideuterio-butadiene, and tetralin labelled at the four aromatic ring positions. The labelling pattern suggests that the 1,2-dimethylenecyclohexane and benzene derive from a common precursor, as do the butadiene and tetralin. The suggested reaction pathway is shown in Scheme 3 and involves preliminary [1,3] sigmatropic or biradical rearrangement of (264) into (265), which in turn is transformed into (266) by intramolecular [4 +2] cycloaddition retro-[4 +2] addition is considered to convert... 

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. 

The Claisen rearrangement [5], the intramolecular reaction of allyl enol ethers 1 to y,<5-unsaturated carbonyl compounds 3, has become a valuable tool for organic synthesis [6]. The sigmatropic process allows a significant alteration of the molecular framework within a single step. The concerted mechanism involves a highly organized transition state 2 that often directs the stereochemical course in the reaction of substituted derivatives and enables the simultaneous formation of two asymmetric centers (Scheme 1). 

Another significant concerted rearrangement exhibits the [2,3]-sigmatropic reactivity pattern. The most well-developed of these reactions are rearrangements of nitrogen and sulfur ylides and rearrangements of allyl sulfoxides. One requirement for a [2,3]-sigmatropic process is that the atom at the allylic position... 

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