Three-membered

Figure 5.13 shows the way in which the molecules are visualised, their chemical symbol, and the names of the first three members of the series. The carbon atom has four bonds that can join with either one or more carbon atoms (a unique-property) or with atoms of other elements, such as hydrogen. Hydrogen has only one bond, and can therefore join with only one other atom. 

Some reactions require the bonds being broken or made in a reaction to be aligned with other parts ti- or free electrons) of a molecule. These requirements are called stereoelectronic effects. Figure 3-6f shows that the bromide ion has to open a bro-monium ion by an anti attack in order that the new bond is formed concomitantly with the breaking of one bond of the three-membered ring. 

Figure 7-2. Strain energies [kj/mol] of three-membered ring systems.

Three membered rings are kineticahy easy to form but are rather unstable. Some conventional methods work but are rather capricious. Tliis obvious discoimection on cyclopropyl ketones turns out to be all right ... 

Analysis We must consider two alternative disconnections of the three-membered ring ... 

A more general route to three-membered rings is based on a new type of disconnection the removal of an atom... 

You might reasonably have said stereochemistry, but the best answer is the three membered ring. Which three carbene discormections might we consider (See frames 276-288 if you ve forgotten all tliis ). 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

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]. 

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

The chief reason why ethylenebromonium ion m spite of its strained three membered ring IS more stable than 2 bromoethyl cation is that both carbons and bromine have octets of electrons whereas one carbon has only six electrons m the carbocation... 

You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

Three membered rings that contain oxygen are called epoxides At one time epox ides were named as oxides of alkenes Ethylene oxide and propylene oxide for exam pie are the common names of two industrially important epoxides... 

Unlike most ethers epoxides (compounds m which the C—O—C unit forms a three membered ring) are very reactive substances The principles of nucleophilic sub stitution are important m understanding the preparation and properties of epoxides... 

Incorporating an oxygen atom into a three membered nng requires its bond angle to be seriously distorted from the normal tetrahedral value In ethylene oxide for exam pie the bond angle at oxygen is 61 5°... 

Thus epoxides like cyclopropanes have significant angle strain They tend to undergo reactions that open the three membered nng by cleaving one of the carbon-oxygen bonds... 

Section 16 11 Epoxides are much more reactive than ethers especially m reactions that lead to cleavage of their three membered nng... 

Although epoxides are always considered to have their oxygen atom as part of a three membered ring the prefix epoxy in the lUPAC system of nomenclature can be used to denote a cyclic ether of vanous sizes Thus... 

We have seen in Section 4.1.4 that = n and that S2 = i, so we can immediately exclude from chirality any molecule having a plane of symmetry or a centre of inversion. The condition that a chiral molecule may not have a plane of symmetry or a centre of inversion is sufficient in nearly all cases to decide whether a molecule is chiral. We have to go to a rather unusual molecule, such as the tetrafluorospiropentane, shown in Figure 4.8, to find a case where there is no a or i element of symmetry but there is a higher-fold S element. In this molecule the two three-membered carbon rings are mutually perpendicular, and the pairs of fluorine atoms on each end of the molecule are trans to each other. There is an 54 axis, as shown in Figure 4.8, but no a or i element, and therefore the molecule is not chiral. 

Methane, ethane, and propane are the first three members of the alkane hydrocarbon series having the composition, Selected properties of these... 

Ediyleneimine (a2indine, a2acyclopropane) is the smallest cyclic itnine consisting of a three-membered A/-heterocyclic ring (n = 2) ... 

A2iridines (X = H) can be alkylated on the nitrogen, with retention of the three-membered ring, by reaction with aUphatic and aromatic haUdes in the presence of base (2,154). The reaction can also be carried out, in some cases with very good yields, under phase-transfer conditions using 30% NaOH and optionally an organic solvent (155). If the haUdes do not react readily, the alkaU metal salts (X = Na) of the corresponding ayiridine can be used (156—158) to form, for example, triethyleneiminemethane [23974-29-0].. 

At temperatures >300° C, substituted pyrroHdines can be obtained by reaction of substituted a2iridines (R = CH, C2H ) and conjugated olefins X = CN, CO2CH2, CH=CH2) with C—C cleavage in the three-membered ring (214—216). 

Tertiary amines have been shown to react with isocyanates ia an analogous fashion to form ureas (41—43). Similarly, a2iridines (three-membered rings containing nitrogen) are found to react with isocyanates to yield cycHc ureas. Tertiary amines have also been shown to form labile dipolar 1 1 adducts with isocyanates reminiscent of salt formation. In contrast, formaldehyde acetal aminals form iasertion products with sulfonyl isocyanates (44,45). 

Dioxiranes are three-membered cychc ring peroxides that ate expected to... 

Chemical stabilization involves removing the concentration of surface hydroxyls and surface defects, such as metastable three-membered rings, below a critical level so that the surface is not stressed by rehydroxylation in use. Thermal stabilization involves reducing the surface area sufficiently to enable the material to be used at a given temperature without reversible stmctural changes. The mechanisms of thermal and chemical stabilization are interrelated because of the extreme effects that surface hydroxyls and chemisorbed water have on stmctural changes. Full densification of gels, such as the... 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Epoxy Resins. Epoxy resins (qv) are used to cross-link other resins with amine, hydroxyl, and carboxyHc acid (or anhydride) groups. The epoxy group, properly called an oxkane, is a cycHc three-membered ether group. By far the most widely used epoxy resins in coatings are bisphenol A (BPA) (4,4 -(l-methylethyHdene)bisphenol) [80-05-7] epoxy resins. 

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