Three-Membered Unsaturated Rings

Phosphirene complexes are obtainable directly from the phosphirene and the appropriate metal carbonyl. 

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The mechanism of the formation of a three-membered unsaturated ring was clarified later, when this reaction was reexamined by the same authors in detail . They found that the reaction of dichlorogermylene-dioxane complex with one equivalent of f-BusSiNa in THF at —78 °C led to the formation of c ,frawi-l,2,3-trichloro-l,2,3-tris(tri-terf-butylsilyl)cyclotrigermene 7 in 98% yield (Scheme 4). The cis,trails conformation of 7 was established by NMR spectroscopy and X-ray analysis (Figure 3). Treatment of 7 with two equivalents of f-Bu3SiNa in TFIF at —78°C cleanly produced cyclotrigermene 5 (Scheme 4), which gives evidence that 7 is a precursor for 5. 

Three-membered strained ring systems constitute an attractive class of molecules as synthetic intermediates [34-36]. Among them the rigid, unsaturated cyclopropenes are the key to selective useful transformations [37,38]... 

Dioxiranes are three-membered cyclic ring peroxides that are expected to be very unstable owing to ring strain. They are effective oxygenating agents for epoxidations of olefins, allenes, polycyclic aromatic hydrocarbons, enols. and a, /i-unsaturated ketones for insertions of oxygen into X—H... 

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]. 

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... 

Apparent nucleophilic attack on large, fully unsaturated rings may occur by way of attack on a valence tautomer, such as the reaction of oxepin with azide ion. Attack on the oxanorcaradiene valence tautomer leads to ring opening of the three-membered ring, and formation of 5-azido-6-hydroxy-l,3-cyclohexadiene (Section 5.17.2.2.4). 

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. 

Structural isomerization of three-membered rings with exocyclic unsaturation (Scheme 21) has been found with aziridineimines (Section 5.04.3.13) and diaziridineimines (Section 5.08.3.2.4). This involves an acyclic intermediate, while large ring systems prefer to isomerize through bicyclic ones. The 1,2- to 1,3-diazepine conversion is an example (Scheme 22) (Section 5.18.2.2). 

Among unsaturated C—N—N three-membered rings only the 3H-diazirines (3) are known. IH-Diazirines with a C=N double bond were never obtained. Diazirines with one or two alkyl groups at carbon were prepared in many cases, aryldiazirines only in some cases. An important role is played by difluorodiazirine as well as by diazirines containing chlorine or bromine (9). 

Simple oxaziridines and diaziridines do not absorb in the near UV. Lack of absorption was one argument to distinguish between true three-membered ring structures and unsaturated open chain isomers like nitrones or hydrazones. 

The first member of the three-membered ring sulfones was synthesized about 70 years ago5, and its unsaturated analogue has been known for only 20 years6. Since the midsixties, an explosive expansion in the chemistry of some of these small- to middle-sized sulfone and sulfoxide heterocycles has taken place. 

The cydopropanation reaction of an unsaturated substrate is one of the most important strategies to access three-membered ring derivatives. The use of Fischer carbene complexes to perform this kind of cyclisation has become an important tool in organic synthesis [4]. In the next few sections the most significant features of this chemistry are briefly described. 

Three-membered rings can also be cleaved to unsaturated products in at least two other ways. (1) On pyrolysis, cyclopropanes can undergo contraction to propenes. In the simplest case, cyclopropane gives propene when heated to 400-500°C, The mechanism is generally regarded as involving a diradical... 

Diphosphorus Containing Unsaturated Three-Membered Rings Comparison of Carbon, Nitrogen and Phosphorus Chemistry... 

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