Reactions involving three-membered-ring intermediates

4 REACTIONS INVOLVING THREE-MEMBERED-RING INTERMEDIATES  

When treated with base, a-halogenosulphones, possessing-y hydrogen atoms, undergo a 1,3-elimination to give an episulphone, which rapidly extrudes 

Using an alternative route, Bordwell et have prepared a number of 

The treatment of a-halogenoketones with nucleophilic bases gives rearranged carboxylic acids or their derivatives. For substrates lacking y hydrogen atoms, a iemi-benzilic acid rearrangement is generally accepted 

The cyclopropanone ring opens so as to give the more stable carbanion, which is rapidly protonated by the solvent . Elimination of halide ion is first-order in both the substrate and the base. 

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In this section, we have seen the formation of three-membered ring intermediates when a neighbouring group participates in a reaction. We will now look at some other reactions involving three-membered rings, but this time ones that exist in either the starting material or the product, rather than just as an intermediate. 

Allyltrimethylsilane reacts rapidly with PTAD to generate a 1,4-dipolar species (230) leading to three products, including (232), different from those expected from the usual ene reaction (81JOC614). In the formation of the pyrazolo[ 1,2-a]-s-triazole (232), a three-membered ring intermediate (231), a co-contributor to the resonance hybrid along with (230), may be involved. A direct transformation of (230) to (232) appears to be rather difficult, since it includes transformation of a secondary carbonium ion to a primary one. 

The reported proposed sequence also offers two additional alternative mechanisms for the cyclodimerization of BCP (3), involving either intermediate 463 or 464 [6a, 13b]. However, they appear less likely, requiring successive three-membered ring fissions and formations. Alternatively, a thermally allowed concerted [jt2s + rt2a -I- pericyclic reaction involving the Walsh type molecular orbital of cyclopropane [124] has been proposed (Fig. 4) [13b]. 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Conversely, the presumed heterolytic C-N bond separation should involve a prior protonation step since this reaction takes place only under strong acid catalysis when it is run at room temperature. In the absence of acid, however, the same process may be thermally induced. There are two potential sites for proton attachment The aziridine nitrogen and the carbonyl oxygen. Both—as protonated species—are suitable to initiate fragmentation of the three-membered ring by way of intermediates VII and VIII, respectively (see Scheme 14.2). While evidence supporting the existence of VIII is available from proton nmr spectral analysis of a V-acrylaziridine in superacid media, other researchers ... 

The three-, four-and five-membered cyclic sulfones have some interesting reactions. Three-membered ring sulfones, called thiirane-1,1-dioxides or episulfones, can be prepared by reaction of a diazoalkane, e.g. (160), with sulfur dioxide (Scheme 63). The reaction affords a mixture of the cis-episulfone (161) and trans-episulfone (162), and both isomers can be isolated by fractional crystallisation at low temperature. Another method of preparation of episulfones is by treatment of a diazoalkane with a sulfonyl chloride (containing a-hydrogen atoms) and a tertiary amine (Scheme 64). Both these syntheses involve the formation of the highly reactive sulfene intermediate (163) (see Chapter 7, p. 114) episulfones on warming eliminate sulfur dioxide to form the alkene (164) as indicated in Scheme 64.7... 

The thermal and photochemical reactions of aziridines and azirines primarily involve opening of the strained three-membered ring. Aziridines produce azomethine ylides, while 2//-azirines produce nitrile ylides or nitrenes. Typically, these intermediates react further in cycloaddition processes or other rearrangements. 

The reaction of 1,1-diacetylcyclpropane with hydroxylamine proceeded in a similar fashion and gave the corresponding isoxazoles. Both reactions are understood to involve a spiro-intermediate which is attacked by the nucleophile and opens the three-membered ring. ... 

Streith and Nastasi have reviewed the photoreactions of three-membered rings. A study of the photo-ring-opening reactions of the azirines (125) has been reported. A CIDNP study of photoelectron transfer from cis- and trans-, 2-diphenylcyclopropane to chloranil has been carried out. The evidence collected from this study indicates that the intermediate involved is the radical cation (126), since no polarized rearrangement product was observed. The failure to observe reaction is in marked contrast to the behaviour when the cyclopropane is irradiated in the presence of 1,4-dicyanonaphthalene. ... 

Aryl participation is more common than simple alkene participation. Again jt-electrons are involved, but the reaction is now electrophihc aromatic substitution with a delocalized intermediate, often called a phenonium ion. A phenonium ion is symmetrical that can be attacked on either atom in the three-membered ring to give the same product (Scheme 2.40). 

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