Four-Membered Unsaturated Rings

Four-membered ring complexes can be obtained from acetylenes and diphosphene complexes. 

FIGURE 8.36 A rraw -diphosphete complex with nickel. 

Some phosphynes dimerise to form diphosphetes which are stabilised by complex formation [19], 

There are usually alternative routes for the preparation of various isomers, for example, (8.320) for a trans isomer. 

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Scheme 17 Four-membered unsaturated rings with different numbers of a2,X3-P atoms...

Scheme 18 Four-membered unsaturated rings 22-24 with one and two a4,/.5- atoms...

Scheme 20 Four-membered unsaturated rings 26 with E O,...

Phosphacyclobutene P-oxides 64 are produced by the reaction of titanacyclobutenes with dichlorophenylphosphine <07CEJ4077>. The large instability of heterophosphete compounds 65, composed of a four-membered unsaturated ring with a pentavalent phosphorus atom and a heteroatom, has been identified and characterised by first-principle molecular orbital computations, at several levels of theory showing that strong electron-... 

Some exocyclic multiple-bond character may be influential at stabilizing the four-membered unsaturated rings the majority of known compounds contain a silicon substituent at the ring nitrogen atoms or a nitrogen substituent at the ring boron atoms. 

Pyrrole (two r s, one /) and imidazole are /ive-membered heterocycles, yet both have six tt electrons and are aromatic. In pyrrole, each of the four. sp2-hybridized carbons contributes one tt electron, and the sp2-hybridized nitrogen atom contributes the two from its lone pair, which occupies a p orbital (Figure 15.9). Imidazole, also shown in Figure 15.9, is an analog of pyrrole that has two nitrogen atoms in a five-membered, unsaturated ring. Both nitrogens are sp2-hybridized, but one is in a double bond and contributes only one electron to the aromatic tt system, while the other is not in a double bond and contributes two from its lone pair. 

Selenium diimides react with unsaturated organic compounds. (tBuN)2Se is an in situ reagent for allylic amination of olefins and acetylenes. Improved procedures for the diamination of 1,3-dienes have been developed using (RN)2Se (R = /wu-toluenesulfonyl, ort/zo-nitrobenzenesulfonyl).156 The reaction of A iV -dialkyl selenium diimides with fe(amino)stannylenes produces four-membered SnNSeN ring systems.157... 

The unsaturated four-membered P2N2 ring is also stabilised by the formation of diadducts with Lewis acids, which are surprisingly formed in the reaction of Ph2PCl with bis(trimethylsilyl)sulfur diimide with Gads or AICI3 [eqn... 

Another important class of cycloaddition reactions is the formation of oxetane rings between a photoexcited carbonyl compound and an unsaturated molecule. These reactions also occur probably through an exciplex although these exciplexes are non-fluorescent as they are formed from the triplet state of the ketone or aldehyde. The formation of the four-membered oxetane ring is an interesting example of a typical photochemical reaction... 

The structure of pyrimidine is shown below. It is a six-membered unsaturated ring compound containing four carbon and two nitrogen atoms. 

The reaction of the vinylcyclopropanedicarboxylate 301 with amines affords an allylic amine via the 7r-allylpalladium complex 302[50]. Similarly, three-membered ring A -tosyl-2-(l,3-butadienyl)aziridine (303) and the four-mem-bered ring azetidine 304 can be rearranged to the five- and six-membered ring unsaturated cyclic amines[183]. 

P-Hydroxy acids lose water, especially in the presence of an acid catalyst, to give a,P-unsaturated acids, and frequendy P,y-unsaturated acids. P-Hydroxy acids do not form lactones readily because of the difficulty of four-membered ring formation. The simplest P-lactone, P-propiolactone, can be made from ketene and formaldehyde in the presence of methyl borate but not from P-hydroxypropionic acid. P-Propiolactone [57-57-8] is a usehil intermediate for organic synthesis but caution should be exercised when handling this lactone because it is a known carcinogen. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

In the case of y,6-unsaturated acids, five-membered rings (y-lactones) are predominantly formed (as shown above note that Markovnikov s rale is followed), but six-membered and even four-membered lactones have also been made by this procedure. There is a gem- dimethyl effect that favors formation of 7-11 membered ring lactones by this procedure. ... 

Four-membered ring formation between unsaturated carbon bonds and carbonyl compounds is a photochemical reaction [25], This is an excited-state reaction in the delocalization band (Scheme 6). 

Three- and four-membered ring molecules of nitrogen and phosphorus are investigated (Scheme 32) [80]. The unsaturated four-membered ring molecule, tetracyclo-phosphene 79 is less strained than the saturated molecule, tetracyclophosphane 15... 

Disilenes are very reactive toward a variety of unsaturated bonds, such as C=C, C=C, C=N, C=N, C=0, C=S, N=N, and N=0, giving interesting four-membered ring compounds that are otherwise difficult to synthesize.3b Reactions using stable tetraaryl- or dialkyldiaryldisilenes and marginally stable tetra-t-butyldisilene (21), which is generated by photolysis of the corresponding cyclotrisilane, are summarized in Schemes 10 and 11, respectively. Some of them merit further comments. 

It will be seen that the enediolic system can theoretically be written in the isomeric 2-keto (II) or 3-keto (III) forms and these in turn are seen to be derived from the 2-keto and the 3-keto acids IV and V, respectively (compare with benzoin which reacts with iodine in an analogous fashion to L-ascorbic acid). Consequently the synthesis of L-ascorbic acid and of its analogs has consisted in devising methods for the formation of 2-keto or 3-keto hydroxy acids followed by their enolization and lactonization. Four main methods are available for the synthesis of analogs of L-ascorbic acid containing the characteristic five-membered unsaturated enediolic ring. 

Silyl substituted carbon-centred radicals, which are produced when adding RsSi to unsaturated bonds can participate in consecutive reactions other than cyclization. A simple example is given in Reaction (7.66) where the adduct of silyl radical to (3-pinene rearranged by opening the four-membered ring prior to H atom transfer [33,77],... 

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